CIRIA C514 - Grouting for Ground Engineering

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CIRIA

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Cover photograph: Grouting eqUipment for London Underground's Jubilee Line Extension, Westminster (courtesy WS Atkins). Printed and bound in Great Britain by The Basingstoke Press (75) Ltd, Basingstoke, Hampshire.

London, 2000

CIRIA C514

Grouting for ground • • engineering

C G Rawlings E E Hellawell W M Kilkenny

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Relationship between injection pressure, width of aperture and water intake (after Ewert, 1992)

Vertical discontinuities are generally easier to fill as they act as venting paths for the displaced water. Horizontally orientated discontinuities are usually more difficult to grout properly and higher injection pressures are generally required. Further problems with horizontal discontinuities are the formation of weak grout depositions caused by fingering of the grout and grout bleed. Hydraulic uplift of the rock mass should be considered when determining a suitable grout injection pressure. It is usually undesirable in projects requiring a reduction in rock permeability to open existing or new fissures. The mass strength of the rock generally determines the critical pressure at which hydraulic deformation occurs (although the fracture pattern and rock weight also influence thiS) and the grout injection pressure should not exceed this value. In weak rock formations at shallower depths containing fine fissures, the required injection pressure for grout penetration may be greater than the critical hydrofracture pressure. In this case rock grouting will not be successful. The injection stage length also affects the injection pressure. In weak rock, short stages are preferable because these allow higher pressure in the lower injection stages than if the whole stage was grouted in one. This is also the case for descending-stage rock injection near the surface. The stage lengths can then be increased with increased depth. Where hydrofracture or grout leakage is a significant risk, eg on a grout curtain for a dam, a concrete grout cap is often employed. In certain projects, such as grouting beneath an existing structure such as a gravity concrete dam, the effect of high injection pressures on the structure should be considered. An initial injection pressure of 10-20 kPa (0.1-0.2 bar) per metre depth below ground

level (measured at the top of the hole) can be used and adjusted, depending upon the competence of the rock and results of hydrofracture tests obtained in the field. For inclined holes the pressure corresponding to the distance measured along the hole is generally satisfactory (Houlsby, 1990). However, in difficult conditions (weak, weathered rock) the inclination of the hole may have to be taken into consideration and the lower vertical criterion used.

82

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6.8.3

Trials Full-scale grouting trials are carried out at some sites to evaluate the grouting process and obtain information on the most appropriate drilling technique, grout properties, injection technique and equipment. Generally, the main requirement is to determine the benefit of grouting. The trials should be extensive, ie include enough groutholes to enable reasonable assessment using a range of grout mixes and injection techniques and an injection layout bracketing to the likely final design. Houlsby (1990) recommends at least 24 groutholes and assessment by comparison of groundwater flow rates before and after grout injection. Trials are costly but may reduce the uncertainty within the project, enable the design to be adapted and help improve efficiency.

6.8.4

Design considerations for a grout curtain In dam grouting, the location of the curtain in relation to the dam depends on considerations including the site geology, the dam design and the proposed construction method and programme. The depth and extent of the grout curtain are usually determined using judgement and experience. It also depends upon the site geology, dam materials, dam geometry, the acceptable seepage loss and practical limits of treatment. Ideally, the curtain should reach an impermeable layer or a depth with an acceptable permeability (Figure 6.5). To obtain a significant seepage reduction in rock, with relatively uniform permeability, the grout curtain should extend through most of the stratum depth.

A Dam Grout curtain

Permeable Impermeable

B

with depth

.. Permeable

Figure 6.5

CIRIA C514

t

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Depth of a grout curtain: a) to an impermeable layer; b) when rock permeability reduces with depth; and c) across a permeable stratum

83

6.9

SITE OPERATIONAL REQUIREMENTS In rock grouting, it is important to design the injection process to minimise the disturbance of surface blocks and the surface break -out of high pressure grout. Breakouts of grout can occur either at the surface or in underground openings such as tunnel drives. These may be caused by distinct discontinuities reaching an exposed surface, where weathered materials or weak rock are dislodged by the grout or where concrete structures have been inappropriately placed. Caulking (driving pliable material and wedges into discontinuities) is often employed to stop these leaks. Alternatively, the leaking area may be covered with a coating that binds the surface and can hold against normal grout injection. A grout cap may also be used. In cases such as grout curtains under dams, grouting may be carried out under partially completed structures so that appropriate grouting pressures can be used. Grouting at high pressures can dislodge or move unstable blocks of rock. The site investigation should identify potentially unstable blocks, eg where a surface block sits on a downward-sloping joint. Such blocks should be removed before grout injection. Occasionally underground rock movements are monitored, but these rarely affect the grouting process. Rock movement is usually indicated by a sudden increase in grout take and a sudden loss of pressure in the hole. At this point the control valve for the system should be closed and checks and observations made to determine the cause. A key indicator is grout leakage in the vicinity of potentially unstable rock. Instruments have been developed to monitor surface rock movements. However, it is often difficult to determine appropriate positions to install these. Low-pressure grout injection and an appropriate procedure should be used near the surface if the surface material is very weak. This may lead to an increased number of grout injections to penetrate all the ground with grout.

6.10

MONITORING

6.10.1

Grouting parameters It is important to continually monitor the grouting parameters during injection and thus adapt the grouting parameters to the conditions encountered. The development of accurate electronic monitoring techniques combined with visual graphs displaying the grouting parameters during grout injection enables a quick response to any problem (eg rock movement). The accurate location and depth of grout injection, the duration of grout injection, grout consumption, injection pressure and visual observations should be recorded continuously and reported daily. Typical monitoring for a rock grouting project includes: •

pressure transducers (located as close to the injection point as possible)

•

flow recorders (one per injection line)

•

graphic display showing the injection pressure and flow rate on a time base

•

visual inspection of the site during the grouting process for signs of surface rock movement and grout break-out

•

ongoing assessment of the grout mix parameters.

Specific requirements for rock grouting includes the use of Lugeon tests for estimation of the permeability of the grouted formation.

84

CIRIA C514

The grout intensity number (GIN) has recently been introduced as a limiting operational parameter for grouting applications. This provides a fixed predetermined relationship between grout pressure and flow rates and so is more suited to automated computer control. There is potential danger in carrying out automated process control without parallel visual inspection for break-out and heave. The GIN is obtained by multiplying the injected grout volume (in litres) by the grouting pressure (in bar). It is defined per metre of borehole as the specific "injection energy". Damage to the ground or a structure is related not only to the grouting pressure, but also to the surface on which this pressure acts. The surface area depends upon the volume of grout injected but not yet set. Generally, GIN values range between 200 bar lim and 2000 bar lim.

6.10.2

Validation Following grout treatment, pumping tests should be performed to assess the success of the project. Such trials are designed to evaluate whether the treatment has reduced the rock permeability to fulfil the project objectives. A comparison of the design values with a back analysis of grout consumption against porosity can be useful in assessing the project design and may aid further grouting projects. In many rock grouting projects (eg dam cutoff), the long-term performance of the grouted ground should be observed. A badly designed or poorly executed grouting project may lead to the grout being gradually washed away by seepage. Regular surveillance of seepage flows and hydrostatic pressures combined with data from instrumentation may be required.

CIRIA C514

85

6.11

CASE HISTORIES

6.11.1

Case history 6.1 :

Rock grouting at Kalvasos Dam, Cyprus (1983-1985)

Case description Grout technique

Rock grouting (curtain and blanket)

Geology

Weathered lava, diabase

Grout

Cement-bentonite (various water:china ratios)

Specification

Method

Control tests

Permeability, injection pressures and injection quantities

Validation tests

Rock permeability

Contractor

Joannou and Paraskevaides and Medcon Ltd

Consulting engineer

Rofe, Kennard and Lapworth and Wallace Evans and Partners

Reason for grouting

To form a grout curtain in rock beneath a new 58 m-high, 500 m-Iong rockfill dam.

Ground conditions The dam is situated on weathered lava, 1.5 krn upstream from the geological boundary between sedimentary and igneous rocks.

Criteria for grouting The criteria for grouting was to achieve a reduction in permeability to about 1 Lugeon in the grout curtain zone and infill any voids in the upper levels of foundation near the core contact. A flexible approach was adopted allowing the grouting layout, extent and pressures to be continuously adapted to the conditions encountered.

Types and methods of grouting The grout curtain was installed by pressure injection as a single line curtain (Figure 6.6). Primary holes were drilled 8 m apart and grouted in two descending stages of 3 m. Injection pressures were measured at the top of the hole. These were defined in kPa as 15 times the depth to the midpoint ofthe stage in metres or as specified by the engineer. At least two hours were allowed between stages. Most primary holes were 21 m deep, but holes at 32 m centres were taken to 27 m for investigatory purposes. Grout takes were generally very low. Standard five-stage water (Lugeon-Packer) tests were carried out using successive increments of depth in the order of 3 m and allowing 10 minutes of steady flow for each stage of pressure. The five-stage water tests were modified to oneor three-stage when it became apparent that the results were very low or nil. Similarly, the length of the hole tested was increased when flows were obviously low. Curtain grouting continued with secondary holes drilled to 9 m depth between primaries giving a 4 m spacing and with a limited number of tertiary and quaternary holes at 2 m and 1 m spacings, respectively. The grout curtain was continued into both abutments as a wing curtain to limit seepage through the abutments.

86

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Blanket grouting refers to the formation of a horizontal treatment zone using conventional grouting techniques. The blanket grouting was carried out over a central zone about 5 m deep and 20 m wide (see Figure 6.6). Primary holes were put down at 8 m spacings and secondary holes were spaced at 4 m centres. Tertiary and quaternary holes were required at a few locations. Grouting was carried out using a grouting pressure of up to 50 kPa, but many of the holes refused to take any grout. A cement-bentonite grout was used, generally in proportions of9:1 by weight. An average of 4.91 kg per linear m of drillhole was used in the left abutment curtain, which was three times the average for the whole grout curtain. The average for the right wing grout curtain was 0.09 kg per linear m. Overall, only about one-third of the amount of grouting allowed for in the design was carried out.

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CIRIA C514

Section of Kalvasos Dam, Cyprus, showing grout cap, blanket grouting and curtain grouting

87

6.11.2

Case history 6.2:

Rock grouting of Hvalfjordur Tunnel, Iceland

Case description Groutingtec~que

Rock grouting

Geology

Basalt, scoria and sedimentary (volcanic) beds

Grout

Cement-microcement and two-component polyurethane

Specification

Performance

Control tests

Groundwater inflow measurements

Validation tests

Groundwater inflow measurements

Contractor

Fossvirki

Reason for grouting

To reduce the water leakage into a subsea tunnel.

Ground conditions The bedrock consisted of gently dipping, primarily basaltic, lava flows interbedded with subordinate acidic rocks and relatively thin sedimentary beds (mixed face conditions). The sedimentary beds lying between the lava flows are commonly thin, red, fine-grained and tuffaceous.

Criteria for grouting Water leakages detected during probe drilling ahead of the tunnel face were measured by a collector in each hole. Stable inflow of 7 Umin or more from one probe hole or 14 Umin or more from four probe holes were set as the criteria for grouting. A further criterion for grouting was that a stable gross inflow into the tunnel per km should not exceed 300 Umin. Types and method of grouting Four (to six) probe holes with a length of 20-30 m were drilled ahead of the tunnel face to check groundwater and ground conditions. A minimum overlap of 6 m of the probe holes was used. Water leakage was measured from the probe holes.

In general cement grout was used with water:cement ratios of 3:1-0.5:1. Microcement was used in very thin water-bearing features. Grouting was started using relatively thin mixes and the cement content increased with time. Additives included an accelerator (2 per cent), and super plasticiser (1.25 per cent). Where large cavities were found sand was added to the grout mix (50-100 kg/m3 ). A two-component polyurethane grout was used for treating zones with heavy waterinflows equal to or more than (600 Umin per km length of tunnel). Figure 6.7 shows the grouting for mixed face rock conditions.

88

CIRIA C514

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Hydrofracture grouting

7.1

PHYSICAL PRINCIPLES Hydrofracture involves the deliberate fracturing of the ground (or opening up of preexisting fractures) by injection of grout or water under pressure (Figure 7.1). The fluid pressure causes tensile failure within the ground and a new fracture is initiated or an existing closed discontinuity opened. Grout fills the discontinuity and the fissure propagates in the direction of minimum resistance. In theory, this is the direction of major principal stress, but in practice it is usually controlled by anisotropic factors in the ground, eg bedding in soils, fracture orientation in rocks. For subsequent injections, the stress state of the ground may have altered and the ground characteristics been modified by the previous injection phases; later fissures may propagate in a different direction. In over-consolidated soils, the initial fracture is generally horizontal. Subsequent grout injections tend either to produce fractures in the same direction as the initial fracture, or the reinjected grout thickens and extends the existing fractures. The result of hydro fracture is a network of grout-filled planes and lenses throughout the ground.

Figure 7.1

Hydrofracture

Ground heterogeneities affect the location of these lenses and the grout penetration. Grout favours more open material, leaving other areas untreated. Hydrofracture may access more permeable zones remote from an injection point by way of the system opened up by hydrofracture itself. Such treatment of heterogeneous ground reduces its overall ground permeability and may possibly yield an increase in ground strength. Ground heave occurs during hydrofracture. This can be used to lift overlying structures, as in slab jacking. The technique is also used to compensate for settlement caused by volume loss (often associated with tunnelling), when it is called compensation grouting (see Section 10). In this case grouting is initiated just as ground movements begin, with the aim of maintaining the ground surface or overlying structures at their original level. It is important to differentiate between controlled (intentional) heave or ground jacking and uncontrolled (unintentional) heave, which might damage overlying structures. Hydrofracture also describes the technique of creating fractures within the ground to act as pathways from which to permeate fine-grained soils, ie an enlarged access area for grout to enter the soil at a rate controlled by soil permeability. Thus the flow rate down the pipe is enhanced.

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Previous page is blank

91

7.2

APPLICATIONS OF HVDROFRACTURE GROUTING Hydrofracture grouting can be applied to many ground conditions, but is most often used to treat fine-grained soils. Applications of hydrofracture grouting include: •

reducing the permeability of the soil, eg dam core repair

•

halting the settlements of buildings

•

restoring levels of settled ground or buildings

•

accessing zones that can be permeated, eg dam cutoffs

•

compensating for movement induced by underground works.

For conventional applications, the technique can be more economical than alternatives because higher injection rates and pressures can be employed. The speed of operation is dictated by the purpose of the hydrofracture project, however; eg high grout injection rates may result in uncontrolled uplift, which can be dangerous. The ability of hydrofracture grouting to cause ground movements, notably heave, can be used in applications such as relevelling of failed foundations, or compensation grouting to prevent tunnel volume loss causing undue building settlements or distortions. In these applications, although high break-out pressures may be necessary, injection volumes are normally small; thus ground movement occurs local to the point of injection and can be controlled with little risk of unwanted uplift.

7.3

LIMITATIONS SPECIFIC TO HVDROFRACTURE GROUTING Hydrofracture grouting requires specialised knowledge and experience. Specific problems of the technique include: •

uncertainty in the position and orientation of hydrofracture

•

difficulty in forecasting or determining the amount and thickness of grout within the fissures

•

lack of control over the extent and frequency of hydrofracture.

Predicting the grout injection parameters accurately is also very difficult. This is because of the lack of correlations between injection quantities, injection pressure and the many fluctuating parameters. Trials of the grouting process are therefore important to establish empirical relationships between relevant parameters for a specific site. The success of a hydro fracture grouting project depends on the "dynamic" response, in real time, from the analysis of results from monitoring equipment and grout injection as the grouting works proceed.

7.4

DESCRIPTION OF TECHNIQUES USED A packer system (normally a double packer), often as part of a tube-a-manchette (TaM), is generally used for hydrofracture grouting. The advantages of this system are:

92

1.

The injection points can be revisited.

2.

It provides an injection borehole that remains stable over the duration of the project.

3.

The packer system provides a good seal to ensure that hydrofracture occurs at the required position (so far as the ground conditions allow).

CIRIA C514

Using the tube-a manchette technique, the packers are located either side of the chosen sleeve and inflated, using water or air under pressure, to seal that sleeve and injection port. Grout is injected into the gap between the packers. The injection pressure is increased until grout break-out occurs as the overburden pressure is exceeded (unless some specified limiting pressure is reached). In stabilisation applications, water may be used for the initial fluid injection to open the ground and to optimise the subsequent penetration of the grout. The tube-a-manchette rubber sleeve is lifted off the tube as grout break-out occurs, allowing injection to continue and relaxes back on the tube once this is complete. The tube-a-manchette thus provides a reusable injection facility. After initial injection, the injection pressure will usually (but not always) drop. Pumping proceeds until a termination criterion is achieved (Figure 7.2), which is generally one of the following: •

grout refusal at the maximum permitted pressure

•

minimum pumping rate occurring at the maximum permitted pressure

•

predetermined grout volume is injected.

Successive injections can be made at the same injection point (using tube-a-manchette), provided enough time has elapsed to allow the grout to sufficiently harden.

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Record of hydrofracture injection (after Raabe and Esters, 1990)

Hydrofracture grouting can cause ground heave. Controlled heave is used to relevel structures. Uncontrolled or unintentional heave can lead to damage to overlying and adjacent structures or services. In designing a hydrofracture grouting project the orientation of fractures and overburden pressure should be considered, as maximum heave occurs from near-horizontal fractures in the ground with a low overburden pressure. Control of heave relies on adequate monitoring (and interpretation) of ground movements and adjustment of grout injection parameters, particularly viscosity and grout volume limits. The following is a typical sequence of site work for a hydrofracture grouting project.

CIRIA C514

1.

Preliminary survey of structures and ground levels (as applicable).

2.

Installation of grouting system.

3.

Injection trials.

4.

Controlled hydrofracture grouting (including appropriate iterations).

5.

Final survey of structures and ground levels (as applicable).

93

7.5

,--_.-.

-

GROUTING MATERIAL Cement or chemical grouts are used for hydrofracture grouting. The former is often employed for filling fractures, particularly in the conditioning or pre-treatment stage. Recent research (Chin, 1996) has shown that fractures propagate at about 20--40 rn/s through soil and the propagation speed is related to the shear modulus and strength of the soil. Limiting the extent of the fractures is difficult, but can be achieved by careful design of the grout rheology and the quantity of grout pumped per injection. Low-viscosity grouts form many fme hydrofractures. These grouts reduce pressure dissipation along a fracture, allowing the grout to propagate further. Water is sometimes used to open up the ground before grout injection. Higher-viscosity grouts are used to expand a few major fractures. Admixtures to reduce water content are beneficial in conjunction with a short bleed time to reduce period between injection phases and impede the grout flowing away from the treatment zone.

7.6

PLANT AND EQUIPMENT Tubes-a-manchette are generally used for hydrofracture grouting projects and are particularly recommended for use in poor ground or where added control is required for optimum results. The tube-a-manchette pipes are normally plastic, although steel pipes may be used for high-pressure applications. Larger-diameter pipes are more often used for particulate grouts than chemical grouts in order to reduce pipe resistances. The grout pumps should be capable of comfortably generating the high pressures required for hydrofracture (in the range 400--6000 kPa, ie 4--60 bars, depending upon the ground conditions). Reciprocating ram or piston pumps are generally used. Grout lines and fittings that operate safely at these elevated pressures are required.

7.7

INFORMATION REQUIRED FOR SELECTION AND DESIGN The following information is important in the selection and design of a hydrofracture grouting project (details are obtained via desk studies, site investigation, visual observations, field and laboratory tests):

7.8

•

site geology and geotechnical parameters (including principal stress directions)

•

soil or rock permeability

•

hydrogeological conditions

•

allowable settlement or deformation of structures above and adjacent to the site

•

adjacent structures, services and buried structures (and their condition)

•

site access for plant.

, BASIS OF DESIGN The design of a hydrofracture grouting project is based upon project objectives and uses results from site investigation, tests and trials, and perhaps numerical modelling and risk assessment. In addition, during the grouting work every grouthole is a further test, providing more information to allow ongoing development of the design and refinement of the conceptual grouting model.

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CIRIAC514

-

------

As in all grouting projects, the approach and degree of sophistication can be varied to suit requirements. The choice of grouting system depends on the scope of work, limitations of the location and the required result. The grouting system should be capable of injecting the grout at the required pressure in the required location and avoid grout break-outs (eg at the surface). Generally, the tube-a-manchette technique gives the best results for hydrofracture projects for all ground conditions.

7.8.1

Layout of injection points The position of the grouting zone and layout of injection points depends on the project, method of injection and access. In projects involving the stabilisation or raising of structures, deep grout injection usually causes more uniform, controlled heave. This is due to the larger buffer zone between the grout and the structure. Grout injection from the surface is usually more cost-effective. This is not always possible and for deep injection points or where surface access is restricted, access shafts are required. In tunnelling applications, drilling is occasionally carried out from the tunnel rather than the surface. For controlled and accurate grout injection, it is recommended that vertical holes are set out in a simple, regular pattern. These might have to be inclined to avoid foundations or for certain orientations of discontinuities. For tunnelling applications, this might not be possible or economic and a fan of holes may be required. The injection hole lengths typically vary from 20 m to 80 m. The longer the hole, the greater the potential deviation from the desired hole position (typically ± 3 per cent horizontally, ± 1 per cent vertically). Additional groutholes may be required to allow for drilling errors and to ensure that required areas of ground are treated. The design should also allow for some redundancy in case of loss of injection holes or injection points (eg due to a blocked hole).

7.8.2

Injection parameters The grout pressure required to cause hydrofracture in homogenous soils can be calculated from the in-situ stress state of the soil (ie overburden pressure). Accurate predictions are difficult in heterogenous ground. Field tests (eg water pressure tests) are therefore recommended to determine empirical relationships to evaluate the required grout injection pressure for hydrofracture and set limiting injection parameters. During grouting, these values can be refined through observations and records from previous injections. Closer to the surface, lower injection pressures are used (to avoid break-out to surface and unwanted heave). At greater depths, higher injection pressures can be used, achieving greater grout penetration. The number of injection phases is determined from the project requirements, the stress state of the ground, grout properties, injection parameters and layout of injection points. In practice, the complexities of the process and heterogeneities within the ground mean that the design is continually refined from observations of key parameters during injection.

7.8.3

Trials For larger grouting projects it is advisable to conduct a trial of the grouting process in the actual ground conditions (for example, the trial works at Redcross Way, Linney and Essler, 1994). The trial was used to evaluate and modify preliminary grouting parameters and thus aid the design and selection of grout equipment, grout mix, grout

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95

injection parameters, grouthole layout and injection sequence. Throughout trials all grouting parameters are monitored and pre- and post-trial tests of soil permeability and strength determine the effectiveness of the grouting technique in fulfilling the project requirements. Trials should be broadly based rather than being simply a trial run of one preliminary design.

7.9

SITE OPERATIONAL REQUIREMENTS Hydrofracture involves the injection of grout at high pressure, so it is important to consider the suitability of all pressure equipment and pipe connections. The use of excessive pressure or lack of control over injection quantities can cause unwanted lifting of foundations of buildings or services, the ground surface or to the breakout of pressurised grout. Uncontrolled hydrofracture grouting is potentially damaging and dangerous. A risk analysis should be completed as standard procedure for all projects. Its aim is to evaluate the potential hazards of the grouting process. In tunnelling, increased stresses and displacements on any tunnel or shaft lining should be considered. Quality control is required in all aspects of the project. Table 7.1 outlines a typical quality control procedure. Table 7.1

Quality control for hydrofracture grouting

Treatment phase Before treatment

Quality control Survey of structures, services and ground level (as required) Trials of hydrofracture grouting Tests for hydrofracture pressure

During treatment

Check the location of boreholes and injection points is correct Monitor the grout mix on a routine basis Measure flow rate and injection pressures Monitor ground and adjacent structures for heave or settlement Calibrate all equipment

After treatment

In-situ pumping, Lugeon and penetrometer tests as required. Laboratory tests on samples, check boreholes

7.10

MONITORING Real-time data control is essential during hydrofracture grouting. Ongoing assessment of grout injection and treatment parameters throughout the treatment process allows design modification based on observations and provides data for the performance assessment.

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7.10.1

Grouting parameters During the project all aspects of the drilling phase should be checked and monitored. This provides extra site information to aid the grouting engineer and may lead to the detection of soil irregularities that may hinder the grouting process. The following grout parameters should be monitored across the site, throughout the grouting process: •

injection pressures

•

grout flow rates

•

grout mix characteristics (including the fluid properties, materials stock control and the grout set properties)

•

displacement of particular features by the grouting process.

All hydrofracture grouting projects require real-time control of injection parameters, ie pressure and volume. Automatic recording of data is recommended for larger projects and should be coupled with an automatic cut-out when limiting criteria (eg maximum injection pressure, maximum volume per injection) are reached. The data should be shown on a visual display. The nature of the project and the risk of structural damage due to ground heave determines the requirements for other monitoring survey and equipment. In projects involving controlled uplift or compensation, movement-sensitive instruments may be required and be attached to structures to be raised or protected (see Section 10, Compensation grouting).

7.10.2

Validation In a hydrofracture grouting project a post-treatment analysis is usually required to assess the success of the grouting. Tests to determine the change in ground parameters (eg the decrease in ground permeability or increase in mass strength) or the controlled ground movements enable the success of the grouting to be quantified. Other information may also be required, depending upon the project objectives and the performance specification, including:

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•

pore water pressures (using piezometers)

•

soil permeability (via pumping tests)

•

soil strength (via in-situ penetrometer tests)

•

soil stress (via in-situ pressuremeter tests)

•

soil specimens obtained from between grout lenses (including laboratory shear tests if time permits).

97

7.11

CASE HISTORIES

7.11.1

Case history 7.1:

Hydrofracture during tunnelling on the Vienna Metro, Austria, 1987-1993

Case description Grouting technique

Hydrofracture

Geology

Made ground over granular soil overlying clays and clayey silt

Grout

Cement-limestone suspension with bentonite. Water: cement ratio 0.5-1.0

Specification

Perfonnance

Control tests

Grout properties

Validation tests

Settlement and angular distortion monitoring during grouting; heave control

Contractor

Keller Grundbau, Vienna, Austria (Porr, Ziiblin, Wibeba, Griin and Bilfmger, Hochtief, Rella) Stuag

Reason for grouting

Compensate for ground defonnations due to NATM tunnelling operations. Settlement reduction by soil fracture grouting.

Ground conditions Made ground (2-3 m thick) overlying granular Quaternary stratum (4 m thick), which is underlain by Tertiary clays and clayey silts. The metro tunnels are located within the Tertiary clays and clayey silts. Groundwater levels are in the granular Quaternary stratum.

Criteria for grouting Hydrofracture grouting was required to limit settlements and angular distortions caused by the excavation of a tunnel. A particularly sensitive five-storey brick office building (constructed around 1900) lay within the predicted settlement trough. The maximum settlements and angular distortions allowed during the tunnel excavation were set at 40 mm and 1:1000 respectively.

Types and methods of grouting Three shafts 5.5 m in diameter and 13 m deep were excavated around the office building and fan-shaped grouting holes (tube-a-manchette) installed to fonn the grouted mat (Figures 7.3 and 10.3). This mat covered the main part of the expected settlement trough. The grouting zone extended over 2700 m2, covering about one-half of the area of the office building. The distance between the lower and upper drilling level was 1.6 m and the distance apart of the injection pipes at the end of the boreholes ranged from 1.4 to 1.8 m. To prevent any grout breaking through into the tunnel, grouting was not permitted within 10 m of the tunnel excavation zone. The main grouting work was carried out more than 10 m behind the tunnel face, although some grouting was carried out in front of the face. On a few occasions an angular distortion of 1: 1000 occurred around the face of the tunnel. The tunnelling excavation works were then stopped and the face sealed with reinforced shotcrete before grouting within the 10m safety distance from the tunnel face. Grouting pressures were generally 1-3 MPa (10--30 bar) although pressures up to 6 MPa (60 bar) were required to achieve additional ground fractures.

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START AND SUPPLY SHAFT FOR TUNNELLING ACTIVITIES

WATER LEVEL INDICATOR -----10111 ~

OUTLINE OF THE OFFICE BUILDING

o !

Figure 7.3

25m !

Plan view of the tunnel and the hydro fracture grouting area (after Potoschnik, 1992)

An electronic monitoring system was used recording settlements, differential settlements and angular distortions from 36 water level gauges positioned in the basement of the office building. Levelling stations were also established in the building walls and these were regularly surveyed. Sliding micrometers installed at several tunnel cross-sections provided additional information on the ground behaviour during the tunnel driving and grouting phases.

Lowering the groundwater table before the driving of the shafts caused angular distortions of 1:3000. Settlements also occurred as a result of the shaft driving and the drilling of the grouting mat. These settlements were offset by a pre-grouting phase used to infill any voids. Compression of the ground from grouting however, affected the primary shotcrete support for the tunnel. Settlements exceeded the 40 mm maximum value because of the initial settlements occurring during dewatering. However, the angular distortion was kept within 1:1000 during tunnel excavation. A reduction of about 50 per cent of settlements was achieved by hydrofracture grouting.

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7.11.2

Case history 7.2:

Hydrofracture during the Channel Tunnel marine drives, UK

Case description Grouting technique

Hydrofracture of rock

Geology

Lower Chalk Marl

Grout

Silacsol T grout. A silica-based grout incorporating a calcium reagent and fine-grained inert filler

Contractual

Perfonnance specification (max in-situ penneability after grouting of 2 Lugeons)

Preliminary tests:

Trial carried out

Contractor

TML and Stent Soletanche

Reason for grouting:

To optimise tunnelling progress in the marine running tunnels through water-bearing fissured Lower Chalk Marl, 5 km from the UK coast.

Ground conditions The tunnelling horizon for the Channel Tunnel is the fissured Lower Chalk Marl. The marine service tunnel (MST) commenced at a chainage of 19 823 m and began to encounter difficult ground conditions around chainage 20 140 m: where increased frequency of jointing and water ingress with considerable overbreak made tunnel advance difficult. The marine running tunnels would be higher in the chalk succession than the MST and were nearer to the seabed. Criteria for grouting The primary perfonnance specification was defmed as the requirement to achieve a maximum in-situ penneability of 3 Lugeons in the ground after treatment. Types and methods of grouting A programme of sideways probes to examine penneabilities at the higher level of the running tunnel crown gave values of 1-29 Lugeons. The trial involved injecting of the ground in the crown zone of the MST and excavating a heading in that ground to examine the effectiveness of the treatment. Thirty holes were drilled (at 3 m spacing) and grouted in stages up to depths of 15 ffi. Controlled fracture was required for effective injection and tubes-a-manchette were found to give improved control when compared to open-hole grouting. This treatment resulted in a penneability reduction from 4.7-14.5 Lugeons before treatment to 0-3.9 Lugeons post treatment. No inherent improvement in the adhesion of the joints in the fissured chalk was observed. The driving of the Marine Running Tunnel North (MRTN) began at chainage 19300 m in February 1989 and encountered adverse conditions around chainage 20 200 m, including high flows of saline water. This section required cast iron linings instead of the planned concrete segments. To reduce penneability and saline water flows into the tunnel, a length of 700 m needed ground treatment. This would reduce plant maintenance times (which had been dramatically increased by the saline water) and possibly improve the stability of the excavations by reducing lubrication along joint surfaces. The grouting was designed to treat a 3 m annulus above the tunnel crown. A fan-shaped configuration of 3 m spacing, drilled perpendicular to the MST over the 700 m length,

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with groutholes between 17.8 m and 20.5 m long using a tube-i'l-manchette system, was chosen. This allowed the individual grouting of 500 mm lengths and provided flexibility in the working procedures as the drilling and grouting processes could be operated independently (see Figure 7.4). An optimum injection of 65 litres per 500 mm length at a pressure of 25 bars was specified with a set time of 30-40 minutes.

RUNNING TUNNEL

SERVICE TUNNEL 3.Om

Figure 7.4

8.J6m 00

Grout treatment - schematic cross-section (after Crawley and Pollard, 1992)

For this work, the contractor used Silacsol T, a silicate-based grout incorporating a calcium reagent with a fine-grained inert filler designed for penetration (low-viscosity) of finely fissured rocks. The calcium reacts with the silica to form a hydrated calcium silicate crystalline grout, which provides greater strength and reduced permeability compared to standard silica grouts and improved durability with time. The grout programme was designed to treat a total volume of 80 000 m 3 of fissured chalk. Materials storage and workshop facilities were located at the surface on the Shakespeare Cliff lower site. Since space was at a premium all operations were contained on three specially designed trains for drilling, grouting and supply. In view of the rapid set of 30-40 minutes, the grout components, silica liquor, hydrated lime, filler, water, had to be held separately underground before mixing. Drilling and tube-i'l-manchette installation began on the north side, working towards France, followed by grouting (approximately one week behind), and finally control hole drilling and testing. At the midpoint of the operation, the works switched to the south side, again working towards France. Water testing was carried out after a minimum of four days and indicated values of 0.08-1.4 Lugeons after grouting. Figures 7.5a and b show an example of trial pressure and grouting volumes graphs. The result of the hydrofracture grouting project can be summarised as:

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a.

water inflow was reduced to a value approaching nil (with the single exception of one high inflow between chainage 22 950 m and 23 000 m in MRTN)

b.

logging of the face indicated Silacsol T grout in the upper portion of the tunnel and visible in joints in and at axis level

c.

the decrease in permeability and consequent reduction in water inflow gave a marked improvement to the general ground stability in the treated zone.

101

Final Pressure Graph ( Bars )

0-5 6 - 10

Cl

C2

RUNNING ruNNEL

RUNNING ruNNEL

Grouting Volume Graph

( Utres / Sleeve )

C2

Cl

RUNNING ruNNEL

Figure 7.5

102

RUNNING ruNNEL

a) trial pressure graph; b) grouting volume (after Crawley and Pollard graph, 1992)

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8

Ground compaction

8.1

PHYSICAL PRINCIPLES Ground compaction is a technique in which a stiff grout is injected into the ground to compact soil or very weak rock. This improves the stability, stiffness and bearing capacity of the ground. Compaction grouting has maximum effect in the weakest zones of the soil mass. Hence, problem soils can be treated and the minimum soil density increased. Ground compaction describes a variety of techniques including compaction, intrusion and squeeze grouting.

8.1.1

Compaction grouting Compaction grouting describes the injection of very stiff paste or mortar into the ground to displace and compact the soil materials in-situ. The grout remains homogeneous and in soil forms solid bodies of broadly spherical or cylindrical form, depending on the grouting procedure adopted. These grout masses grow with continued grout injection (Figure 8.1). Eventually, the increase in the size of the grout mass overcomes the natural soil compressibility and causes controlled ground movements. While the injection of grout through the end of a casing is straightforward, the actual mechanism of compaction grouting is complex. The expanding grout mass causes a complex system of radial and tangential stress to develop in the ground. A zone of major disruption, shearing and plastic deformation occurs immediately adjacent to the grout bulb, where the soil density may sometimes reduce. Loose granular soils are displaced plastically. In clay soils there may be very small elastic movements leading to increased pore water pressures. In time these excess pore water pressures dissipate, leading to consolidation. In layered or mixed soils a combination of both occurs. This grouting technique is used in a wide range of ground materials and is particularly applicable in granular soils.

Stiff mortar

Figure 8.1

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Compaction grouting

103

8.1.2

Intrusion grouting Intrusion grouting or pressure fIltration grouting describes a grouting technique that is a combination of compaction and hydrofracture grouting. Ground circumstances and grout rheology enable a compaction effect to be achieved in a limited number of hydrofractures. The technique involves the injection of very thick, viscous, particulate grout into the soil. Pressure fIltration (bleed) causes the water to be squeezed out of the grout and into the soil pores in permeable soils (in clays this may lead to hydrofracture). This causes an increase in grout viscosity and prevents the grout permeating into the ground. The grout forces a cavity within the soil to expand, compacting and displacing the adjacent soil (Figure 8.2). Localised fracturing of the soil may also occur.

"/

Figure 8.2

Intrusion grouting

Intrusion grouting is often focused at a natural boundary in the ground or at the interface of a structure with the underlying or surrounding soil. The technique is normally carried out in loose granular soils or at an interface between clayey and granular soils. It can also be carried out in clay soil resulting in hydrofracture or squeeze grouting. Care should be taken to prevent the development of high pore pressures as a result of the compression of the surrounding soils.

8.1.3

Squeeze grouting Squeeze grouting involves the injection of moderately thick particulate grouts into fractures (natural or created via hydrofracture) to compact or consolidate surrounding ground. The process is used in ground with sufficient overburden pressure to enable the application of high grout injection pressures without causing extensive hydrofracture. It is most applicable to soils and rocks in which the pores or fissures are too fme for permeation or fissure grouting respectively. The technique is fairly specialised and only used in special circumstances, eg deep tunnels in finely fractured rock.

8.2

APPLICATIONS OF COMPACTION GROUTING Compaction grouting has many applications and is particularly appropriate for correcting settlement problems. The technique's advantage lies in the way it can be used in constricted spaces or on sites where access is difficult. The stiff pastes or grouts used should remain close to the injection point and thus the grout location is controlled. Treatment zones, even at significant depths, can be targeted.

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CIRIA C514

Compaction grouting is used to:

8.3

•

remedy settlement problems (of shallow or deep foundations)

•

prevent liquefaction and seismic problems

•

compensate for settlements from soft ground tunnelling (see Section 10)

•

treat ground with solution feature problems (Figure 8.3)

•

prevent or help remedy the failure of underground pipes and culverts.

LIMITATIONS SPECIFIC TO COMPACTION GROUTING Compaction grouting can be used in most granular soils, but the technique is not appropriate for clay soils as displacement occurs. Clay soils would not be compacted as they are saturated, and the grout intrusion would result in displacement of the soil. The results of a compaction grouting project are hard to assess using existing geotechnical testing techniques. For example, after treatment, the soil density throughout the site varies in relation to distance from the grout bulbs, intrusions or fractures. Specialist equipment is required for mixing, pumping and injecting the low-slump paste. In addition, the monitoring equipment, ie flow meters and pressure transducers, need to be carefully calibrated. High-solids grouts are prepared and injected with heavy-duty pressure grouting equipment, but abrasion and wear can be a problem. Hydrofracture can occur if errors are made in the selection of paste and grouting method. Compaction grouting requires drilling to be carried out throughout the grouting process. If the problem requires prolonged grouting, compaction grouting can be expensive in comparison with other grouting methods.

---------------------~]§~~

4

Jr::-=landfill

loose grovel zone

Figure 8.3

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subsidence threat liner system

missed void

grouted void

Compaction grout soil blanket to prevent liner subsidence (after Schmertmann and Henry, 1992)

105

8.4

DESCRIPTION OF TECHNIQUES USED An outline of the compaction grouting process is given in Figure 8.4. This shows two

methodologies. Choice of these depends upon how the compaction grouting process is anticipated to work. Compaction grout is either injected to form discrete inclusions or injected in overlapping stages in a grouthole to form a column of grout. Ascending or descending stage grouting injections are generally used. The choice of injection technique largely depends upon the project requirements and site conditions. For improving the soil density, descending stage injections are generally preferred as these treat the upper layers of soil first and therefore reduce heave. Ascending-stage injections are more cost-effective, however, and are often used if a large depth of ground is being treated.

Predominantly by inclusion of rigid grout bodies within soil moss

Grout.

Grout.

monitoring pressure. volume injected and surface movement

Compare assessed actual displaced volume with required volume

monitoring. volume injected. pressure and surface movement

Not OK

Finish

Figure 8.4

8.4.1

Assess each inclusion in comparison to torget inclusion geometry

Dimension an additional inclusion o augment/replac the inadequate inclusion Not OK

~ove on/finish

The compaction grouting process

Descending-stage grout injection The following describes a typical procedure for top-downwards injection:

106

1.

Drill the grout hole (minimum SO mm diameter) to the top of the first injection stage.

2.

Cement the casing into the hole.

3.

Extend the grouthole to the base of the first injection stage.

4.

Inject grout at the predetermined pressure and rate and leave to set.

S.

Later (usually the next day), extend the grouthole and begin the second injection phase.

6.

Continue with subsequent grout injection phases at suitable time intervals until a grout column, or series of grout bulbs are created (Figure 8.S).

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a)

b)

Casing Discrete mortar

bulb

I

I

_--.!-I_-,-

_Competent soil

I

Figure 8.5

,

'

hr

rock-L

,

Typical descending-stage grouting: a) discrete bulbs; or b) columns of grout (after Warner, 1982)

This technique has the advantage that the strength and density of the upper layers are increased first. This increases the restraint capability of the upper layers and greater injection pressures can be used (Warner, 1982). Additional drilling is required at each injection stage, which generally makes this technique more expensive than ascendingstage injection.

8.4.2

Ascending-stage grouting In ascending-stage injection, the grouthole is drilled in one phase to the deepest injection point and the casing inserted. Obtaining a positive seal at the grouthole base is important, but often difficult. Either the grout is injected in discrete phases or the casing is lifted incrementally to coincide with the grout injections to form a columnar grout body. Ascending-stage injection is generally more economical than descending injection as there is only one drilling phase. The soil above the grout injection zone is untreated, however, and injection pressures might have to be reduced near the ground surface or where the overburden pressure is low, to prevent heave. More injection points might be needed near the ground surface to offset the lower injection pressures used.

8.5

GROUTING MATERIAL The ASCE defines a compaction paste as having a slump of less than 25 mm (ASCE, 1980). This may be too restrictive, however, and target ranges of 25-50 mm commonly used. The overriding consideration in the choice of paste is whether it can withstand the required pumping pressure, without losing fluid and fluidity. Higher slumps (> 50 mm) lead to fracturing of the ground and the formation of mortar lenses rather than bulbs. An ideal slump is probably 30 mm, with a tolerance of ± 20 mm.

are

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107

Dense particulate pastes are used, usually composed of well-graded sand and silt, often with an anti-bleed additive. The two most important considerations for the paste mix are: 1.

Grading of the granular component. A sand that is too coarse, will cause excessive pressure filtration of the grout under high pressures. Warner (1982) states that the ideal paste comprises sand that: •

is a natural rounded material

•

100 per cent of which passes through a 2.4 mm sieve

•

has no more than 20 per cent grains finer than 50 1lIll.

The above parameters are only an example of a paste. The addition of silt-sized particles (eg PFA) is advised and some practitioners insist that this should be at least 15 per cent of the granular material. Bentonite is occasionally added to improve the pumpability of the grout. This has the disadvantage of increasing the plasticity of the grout and may cause hydrofracture. 2.

Maintenance of stiff paste consistency. The grout mix should be checked frequently and the standard concrete slump test (BS 1881) should be performed regularly to monitor the stiffness of the paste.

8.6

PLANT AND EQUIPMENT Specialised equipment is required for compaction grouting due to the very stiff pastes used. The mixer should be capable of uniformly mixing the low-slump, stiff paste in sufficient quantities. Examples include concrete paddle mixers, pug mixers or screw mixers. Alternatively, if the granular component of the paste is pre-mixed, an ordinary drum mixer may be appropriate. The pump required for compaction grouting should: •

be able to handle low-slump paste

•

be able to work at high pressures (4000-7000 kPa, 40-70 bar) without producing

peaks or pressure surges •

have a pumping rate range of up to 20 m3/h, adjustable throughout the range while pumping

•

include a pump hopper fitted with a force-feed mechanism

•

include an appropriate flowmeter

•

include an in-line pressure recorder at the injection point.

Reciprocal piston-type grout pumps are often used for compaction grouting. These pumps cause pressure surges at the pump piston. Line damping and friction usually eliminate this surge before it reaches the injection point. However, pumps with a long stroke or large volume per stroke should be avoided as these encourage hydro fracture by "peaky" flow and pressure characteristics. Paste is usually transferred from the pump to the injection point by pipes of 38-50 mm diameter. Sharp bends and standard pipe fittings should be avoided to prevent blockage of the grout flow. The stiff, frictional paste used in compaction grouting causes high pressure losses within the injection pipe, so this should be as short as practicable.

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8.7

INFORMATION REQUIRED FOR SELECTION AND DESIGN Information required for the selection and design of compaction grouting projects is similar to that for all grouting projects, ie:

8.8

•

site geology and geotechnical parameters (particularly soil type, density and structure) including their variation and distribution

•

soil permeabilities, their variation and distribution

•

timescale for the application (ie permanent or temporary)

•

permitted ground displacements to be counteracted or achieved

•

site access.

BASIS OF DESIGN A flexible approach in design and implementation is required in compaction grouting. General specifications for injection parameters, layout and grout mix should be specified and then refined using results from trials and observations during the grouting process.

8.8.1

Layout of injection points The spacing and location of the injection holes depends upon the soils to be treated, the depth, the project requirements and judgement. Analysis and field observations indicate that the size of the grout compaction zone depends upon: •

the restraining pressure of the soil

•

the weight of the structure above the grout bulb

•

the surface area of the grout bulb

•

the grout pressure at the bulb (Graf, 1992).

In most cases, the horizontal spacing of the injection points is greater in zones with a high overburden pressure (ie deep injection points or beneath a structural load), as higher grout injection volumes and pressures can be used. Injection into loose soils generally produces a large grout bulb and the required horizontal spacing is thus greater than for denser soils. A grout injection grid and sequence of injections are often designed with primary, secondary and tertiary injection phases (as required). Normally, the primary injection points are positioned on a regular grid and injected first. The secondary injection points are positioned midway between these to compact the ground further. In certain locations a further tertiary injection phase might be required. The injection points are positioned midway between the secondary and primary injection points (Figure 8.6).

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109

12m

\.

12m

.\

Legend

o

o

o Figure8.S

8.8.2

Primary Injection Secondary Injection Tertiary Injection

Compaction grouting injection grid (after Baez and Henry, 1993)

Injection parameters Initial grout injections and trials give indications of the optimal grout injection rate and volume. This is the fastest flow rate that can be continuously maintained without causing a rapid pressure increase. The grout pressure should be monitored throughout the injection process generally at both the pump and point of injection. This gives information on the soil conditions and any injection problems. For example, a sudden loss of pressure could show: •

a breakthrough of grout into a void, subsurface structure or service

•

loss of lateral restraint (eg provided by a retaining wall)

•

impending surface disturbance.

It is usual to set criteria for when the pumping of grout should cease. Pumping may stop

when the first of the following is observed:

8.8.3

•

the pressure drops when pumping at a constant rate, indicating that the shear strength of the soil has been overcome

•

surface heave occurs (unless settlement recovery is an objective)

•

the predetermined maximum quantity of grout has been injected (this rarely occurs)

•

grout refusal occurs (Graf, 1992).

Trials For large projects, a trial of the grouting process in the actual ground conditions is recommended. This should evaluate, and allow modification of, the preliminary grouting parameters, grout, layout and injection sequence. The trials should be broadly based and not simply to test one design. The trial should also show whether the compaction grouting is the most appropriate technique for the project.

8.9

SITE OPERATIONAL REQUIREMENTS Compaction grouting demands the use of high-pressure grout injection, so appropriate pumping equipment, hoses, valves and monitoring devices are required. High pressures may also cause grout break-out in the area around the injection point. A build-up of back

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CIRIA C514

pressure should be avoided while injecting pastes or mortars because this might cause the injection pipe or casing to be forced out of the borehole. Compaction grouting can cause unplanned ground heave. The stability of adjacent structures should be evaluated, and ground and structural movements monitored.

8.10

MONITORING

8.10.1

Grouting parameters Observational techniques have to be used throughout the compaction grouting process to refine the design and injection parameters. A carefully planned monitoring system is an important part of the grouting process. Continuous monitoring of injection parameters, particularly the pressure build-up and grout pumping rates, together with precise surveys and the monitoring of surface and structural movements, are recommended. The pore water pressures between the injection points should be monitored using piezometers. All instrumentation should be calibrated carefully. Pressure gauges are particularly prone to failure and need to be checked frequently. The grout take at each injection point should be observed and plotted. This provides an initial indication on the success of the project, identifies zones of loose soil and areas in which further grout injection may be required (Figure 8.7). A check should be kept on the materials used, which should be compared to pump stroke counts and mixer records. Data should generally be recorded automatically, presented visually in real time and stored on computer disk. A manual back -up is important.

.."." ..•. "

"

•••• •••••• ••••

30

• • • -• •

I.,

-

·- - ..... -

4.· ••• •

••••• 4

. •••• 4

• . . . ·4 ~ • • • •

....

4 • • • • • 4~

u

c

.E 15

'"

•••••

-

i· .;.

(5

·4

• • • • • 4•

•. • . • 4.

••••• • ••• ·4

Scale

•

••••• • 4

o

0.:.:. o

6

·11 12

1.5m

18

Distance {m}

Figure 8.7

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Grout takes at injection points evaluated in terms of the equivalent scaled grout column diameter at Pinopolis West Dam (after Baez and Henry, 1993)

111

8.10.2

Validation In-situ tests, performed at intervals in the grouting project, help the effectiveness of the treatment to be determined and injection parameters to be adjusted. Post-treatment evaluations of the grouting project are important, particularly as this treatment process can produce highly variable results across a site. It is recommended that SPT and CPT data at selected locations (eg quarter and half spacing of injection points) be collected and compared with values obtained before treatment (Figure 8.8).

Tip Resistance (bars) 50

0

100

150

10

12

\

I I

....--.. E .........

14

,

"

,... ,, ,,

Silt with sand interlayers

\

> ( \

1

~PI=5-17)

\

Sand and silty sand. medium to fine grained

I

(I)

u

-0

( {

16

~

~

(f)

\

"0

c:

~

\ I

18

0

0

(5-15% Fines)

f f

~

c.:> ~

46-99% Fines)

)

20

J

Q3

,.- /

co I

.J:: -+-'

a.. 22 Q)

0

"~ "

24

Sandy grovel and grovel

- - - Before treatment - - - I-week after treatment ------- 18-months after treatment

26

(0) FigureS.S

112

(b)

Mean CPT tip resistance before and after treatment using compaction grouting at a building site in Sacramento (Boulanger and Hayden, 1995)

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8.11

CASE HISTORIES

8.11.1

Case history 8.1 :

Ground compaction of solution features in Norwich, Norfolk

Case description Grouting technique

Compaction

Geology

Infill sands, gravels and chalk

Grout

Cement mortar with sand PFA and bentonite

Contract

JCT Conditions of Contract

Specification

Performance

Control tests

Grout properties, injection pressures, injection quantities, uplift monitoring

Validations tests

Plate load and SPT

Designer

Ove Arup and Partners

Reason for grouting:

To facilitate the construction of a shopping centre with basement on a site founded on granular marine deposits overlying chalk containing solution features.

Ground conditions The site geology comprised made ground overlying granular marine deposits (Norwich Crag) and Upper Chalk containing solution features.

Criteria for grouting The foundations (4 x 4 m pad footings) were required to carry typical column loads of 5000-7000 kN with a limiting differential settlement of 20 mm for adjacent columns on a 7.6 m grid. The grouting was therefore designed to treat the solution features to enable the foundations to satisfy the above criteria.

Types and methods of grouting Two methods were used. Bulkfilllow-pressure grouting was used to fill small voids and stabilise the ground. Compaction grouting was used to treat the larger solution features. It was designed to improve the bearing capacity of the chalk by overall densifying and stiffening any loose infill material, and to fill voids. Initially the grout mix consisted of sand, PFA, cement and bentonite in the ratio 6:2: I :0.1 giving a slump of 70-100 mm. The grout mix was later modified to a mix ratio of the above components of 6: 1: 1:0.1 with a target slump of 60 mm. This was a less fluid grout, with a higher shear resistance to try to reduce the grout takes. The extent of the feature was determined and the most appropriate grouting technique chosen. Triangular or square grids of groutholes at 3 m spacing were designed that extended beyond the chalk-feature interface. It was decided to grout from the perimeter holes towards the centre of the solution feature and drill and grout each hole before moving on to the next hole in the sequence. The grout was mixed in a standard reversing drum mixer with a batch capacity of 700 I and transferred to the 500 I hopper of a Schwing pump. Grout was gravity fed from the

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hopper through 75 mm internal diameter reinforced flexible hoses and steel pipes to the grouthole casing. The grouting process was controlled by the pressure measured at the injection hole. This was monitored using pressure gauges on special lengths of pipe. Grout volumes were measured using a manual stroke counter on the pump. A rotating laser level, with five targets positioned 1.5-2.5 m away, was used to monitor ground heave. Ascending-stage grouting was carried out, initially in 0.5 m stage lengths, to 3.5 m below ground level. (The stage length was later adjusted to 1.0 m in chalk, remaining at 0.5 m in the features.) For each stage length, grout injection continued until either the grout injection pressure reached the target injection pressure (initially 4 MPa (40 bars) later reduced to 3 MPa (30 bars) in feature material and 2 MPa (20 bars) in chalk) or ground heave was detected. Other events that stopped grout injection were the casing being forced out of the ground or grout break-out, though these rarely occurred. The performance specification was based on plate loading tests and SPT N values. Two plate loading tests were performed on treated ground to determine the load settlement behaviour up to a bearing pressure of 210 kPa An acceptable settlement was set at 5 mm after 24 h hold at 105 kPa.

SPT testing performance criteria were set at: •

minimum SPT N for the infill of 10

•

average SPT N for the infill of 15.

Table 8.1

Compaction grouting of six (out of 11) solution features (after Francescon and Twine 1994)

Type of grout mix

Typical grout slump (mm)

Solution feature

No of holes

Drilled depth (m)

Pumped volume (m3)

Volume! hole (m3)

Injection pressure in solution feature (bar)

A

33

15

186

5.6

0.49

100

B

16

15

111

7.0

0.60

100/60

C

7

18

103

14.7

1.00

100

D

15

18.5 -21

98

6.5

0.43

1 2

55 50-70

E

34

17.5

340

10.0

0.71

1 and 2

55

F

15

21.5

69

4.6

0.26

2

55

Grout mix: sand:pfa:cement:bentonite, 1

=6:2: 1:0.1, 2 =6: 1: 1:0.1

The drilling, grouting and testing of the compaction grouting at six of the features is shown in Table 8.1. Figure 8.9 shows the surveyed extent of the surface of one feature (identified by "scraping" the formation in the vicinity of the feature), grouthole locations, the length of feature material obtained within the grouthole and grout take in each hole. Grout takes within the chalk were found to be very high; this led to concern that the limiting injection pressure may have been excessive for chalk, inducing hydrofracture or cavity expansion. Grout was found to be present in vertical and horizontal laminae. It did not remain as an expanding body about the injection point.

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$4.5

Surveyed extent of solution feature surface.

Legend + SPT Borehole Length of feature material observed within grouthole $ None (grouthole totally within chalk)

$ 10m

oI

Figure 8.9

5m I

Plan of solution feature A (from Francescon and Twine, 1994)

The compaction grouting project cost £500 000. This was for 245 holes of average depth of 17 m at a cost of approximately £2000 per hole. The project showed the usefulness of compaction grouting in specialist ground treatment situations. It did however highlight aspects of this grouting process, particularly relating to the influence of parameters and design on hydrofracture, that need more investigation.

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9

Jet grouting

9.1

PHYSICAL PRINCIPLES Jet grouting is a specialist geotechnical process invented in the UK and developed in Japan. The natural soil structure is eroded by high-pressure water or grout jets and the soil residue is, by design, mixed with the grout, partly mixed, or removed and replaced with fresh cementitious grout. Solid, relatively impervious, often interlocking grout columns or panels are produced, which can: •

improve the physical characteristics of the ground, ie reduce permeability, increase strength and stiffness

•

act as barriers to fluid flow

•

fix contaminants within the grout -soil matrix.

Jet grout columns are usually formed by rotating and lifting the jetting monitor as the grout is injected. The jet grout columns are commonly terminated before they reach the ground surface. The required volume of grouted soil can then be obtained by overlapping numerous columns. Panels or wings of grout may also be formed by withdrawing the jetting monitor with little or no rotation. The technique is very versatile and can treat many soil types, including weak clays, sands, sludges, stiff clays and laminated soils. Problem soils and strata can be targeted, eg grouted structures can be produced by jetting between discrete levels to treat critical zones.

9.2

APPLICATIONS Jet grouting is a very adaptable ground treatment technique that can be used for a wide variety of applications for either temporary or permanent works. Large plant is not usually required and thus the technique can be used in areas of restricted access or limited head space (eg Kingston Bridge, Glasgow; Carruthers et ai, 1994). Applications include:

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•

providing foundations for structures to be erected

•

underpinning existing foundations and structures (Figure 9.1a)

•

vertical and horizontal barriers to fluid flow (Figure 9.1b)

•

formation of retaining or support walls

•

solidification or containment of contaminants within the ground (Figure 9.1c)

•

stabilisation of embankments (Figure 9.1d)

•

remedial works to dam cutoffs, dam curtains or cofferdams

•

break-ins or break-outs for tunnels.

Previous page is blank

117

a)

b)

Piling platform level :!"_6_~

_______ _ ~~~~~~~~

Basement +lm

London Cloy

11'

22'

30'

d)

c)

Jet grouted columns

Figure 9.1

9.3

Applications of jet grouting: a) underpinning of foundations (after Wheeler, 1996); b) cutoff wall; c) leachate barrier; and d) stabilisation of an embankment (after Keller Colcrete Ltd)

LIMITATIONS OF JET GROUTING There are three main limitations of jet grouting.

9.4

1.

It is important to maintain good control of the jet grouting process in regard to spoil return so as to minimise or eliminate ground heave. Excessive ground heave can damage underground services or adjacent structures or foundations.

2.

Spoil returns can become blocked; if jetting persists, hydro fracture of the ground may occur.

3.

Large quantities of slurried spoil can be produced, eg ajet grouting unit typically generates up to 50 m 3 of spoil per shift. Of this up to about 33 per cent by volume would be solid material, depending on the grout and jet grouting process selected.

DESCRIPTION OF TECHNIQUES USED The first stage in jet grouting is to drill an injection hole (normally 100-200 mm in diameter). The jetting pipe is usually part of the drilling equipment and is placed at the required depth for the base of the cemented soil column (Figure 9.2). In difficult ground conditions the borehole may be pre-drilled.

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2. Jetting commences with the grouting pipes positioned at the maximum depth and proceeds by withdrawing at a t:=~~=~~ steady rate.

1. A small diameter borehole is drilled to the maximum depth required.

3. The upper air-shrouded high pressure water jet erodes a column of soil and grout is simultaneously placed in the cavity by the lower jet. Waste soil, air and grout escape to the surface via the annular space between the bore and the grouting pipes. Cemented soil is formed from the remaining grout and soil.

4. Once the cemented soil column mass has been formed over the depth range of interest, the rig can be moved to another L - _........-+~_--' column position.

GROUT COLUMNS Figure 9.2

Procedure for forming

a jet-grouted column using the triple system

A high-pressure fluid jet of either grout, or water and grout, erodes the soil around the borehole and mixes with, or replaces, the eroded soiL Lifting and rotating the grout monitor produces a column of cement or soil and cement. The jet-grouting rig is then moved to another location and the process repeated. Adjacent grout elements often intersect to form interlocking cement or soil-cement elements.

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There are three jet grouting systems in use (Figure 9.3): Single jet process

This is the simplest technique. The grout is injected into the ground at high pressure (60 MPa, 600 bars) from one or several 5-10 mm-diameter nozzles on a hollow-stem grout pipe. The grout jet erodes the soil and the grout mixes with the spoil. Excess soil and grout slurry is forced up the annulus between the grout injection pipe and the borehole casing to the surface.

Double jet process

This is an extension of the single system process in which the grout jet is shrouded in compressed air at typically 0.2-1.5 MPa (2-15 bars). A greater volume of soil is treated. The air shroud is found to improve the cutting ability of the jet.

Triple jet process

This system uses a water jet, shrouded in air to erode the soil and a separate nozzle for grout injection. A triple concentric pipe is required to inject the air, water and grout at typical pressures of 0.2-1.5, 4 and 0.5-3 MPa (2-15, 40 and 5-30 bars) respectively.

Woter----.

0) Single

Figure 9.3

Air~

Air----,

Gro~

Grout

eb

b) Double

c) Triple

The main variants of the jet grouting process (after Bell and Burke, 1994)

Table 9.1 provides a simple comparison of the three main systems used to date. A further double process has recently been introduced. This can be referred to as the double water system to distinguish it from the process above. It is very similar to the triple process, but without the use of the air shroud. With ongoing development and refinement of jetting pumps and related equipment, greater jet energies are now possible. This is increasing the application of the double jet process and as a result, the use of the triple jet process is becoming less prevalent. A single injection process results primarily in injection and mixing of the grout with the soil. Increased spoil is produced using the double and triple injection systems. This increased spoil indicates that these are part mixing and part replacement processes.

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Table 9.1

Comparison of the different jet grouting systems

Single

Double

Triple

Coarse granular soils

Coarse granular soils and weak rock and some clay soils

Any granular soil, soft to firm or stiff clay soils, mixed alluvium and some weak rocks

Intermediate

Largest, because of penetration of compressed air-shrouded jet

Typical soil treated

Weak weathered rock

Column diameter

Smallest, as energy absorbed Typically 0.5--{).6 m

Typically 1.0-2.0 m Action

High-pressure grout injected directly into the ground

High-pressure grout with air shroud injected directly into the ground

High-pressure water jet with air shroud for erosion; grout injection at low pressures

Ground movement

Blockage of spoil return could lead to the build-up of high pressures, hydrofracture may occur in silts and clays, resulting in ground movements

As for single

Air leakages may occur and cause the erosion of weak soil layers

Grout

Grout

30-50MPa (300--500 bar)

Grout

0.5-3 MPa (5-30 bar)

Air

0.2-1.5 MPa (2-15 bar)

Air

0.2-1.5 MPa (2-15 bar)

Water

up to 50 MPa (500 bar)

Pressures

Cost

30-50MPa (300--500 bar)

Usually less effluent to dispose of Less plant and equipment

Blockage of grout flush returns reduced by air lifting the spoil. However, deep unlined holes through soft clay may close, increasing the chance of blockage

Intermediate: more plant than single; more coluIIUls than triple

More plant than other systems

Intermediate

The water jet permits a less viscous, better-flowing grout

More columns required may result in higher overall cost Other

Weaker coluIIUls are produced Technically inferior to double and triple systems

Fewer coluIIUls required may result in lower overall cost

The diameter of the jet grouted column depends upon the jet process and the: •

soil type and erodibility

•

pressure and injection rate of jet

•

rotation and lift speed.

The strength of the final column is a function of the:

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•

grout mix characteristics

•

degree of soil and grout mixing

•

type of soil.

121

-

._-_.-

The following is a typical sequence for jet grouting operations. 1.

Preliminary survey of structures and ground levels.

2.

Previous applications of the technique in similar ground reviewed.

3.

Field trials used to: - confIrm the feasibility of the technique and performance requirements - obtain more site data - study the most appropriate geometry for grouting operations.

9.5

4.

Controlled grout injection.

5.

Minor settlements and the condition of adjacent structures continuously monitored throughout the grouting process.

6.

The performance of the grout elements checked after installation.

GROUTING MATERIAL In jet grouting, the process breaks down the soil structure and mixes grout with the soil particles, or replaces them by grout. There are no geotechnical restraints on the size of grout particle, so expensive chemical grouts rarely need to be used.

Most commonly used are simple cementitious grouts consisting of cement only or cement with bentonite or other fIllers. Water:solids ratios usually range from 0.5 to 1.5. Apart from cost, factors detennining grout selection are the intended strength, permeability or stiffness of the ground following treatment. It is important in this regard to appreciate that these properties in treated ground will not be the same as those for the neat grout, owing to the partial mixing and replacement that takes place.

9.6

PLANT AND EQUIPMENT In addition to the grout mixing and pumping plant, jet grouting requires the following equipment:

•

drilling rig

•

jet grouting rig

•

high-pressure fluid lines.

Jet grouting uses a modifIed drill, with a jet grouting monitor mounted at the tip of the drill string. This modifIed drill creates the hole, and the cutting fluid and grout are injected through valves in the monitor. Where hard strata or foundations are encountered a conventional drill is used to pre-drill the initial grouthole. Ideally, the jet-grouted element is constructed in one injection phase. Where this is not possible because of height restrictions or large grout element depths, the grout element should be designed with as few injection phases as possible and adequate overlap of adjacent grout sections. The injection system dictates the equipment used. Single-phase injection plant consists of a hollow-stem pipe with one or more injection nozzles positioned at the end. For double-phase injection, the grout is contained in a pipe and the compressed air fills the gap between this pipe and the outer pipe wall. The grout with the air shroud is injected via a double nozzle. Triple-phase injection uses a triple concentric pipe. This contains the water pipe positioned within the air pipe and surrounded by the grout pipe. In this case the grout is injected via a separate nozzle below the air-shrouded water jet. The diameter of the jet nozzle varies greatly, but needs careful consideration as fluid

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pressure builds up if the diameter is too small. Conversely too large a diameter results in high flow rates, reduced injection pressures and small-diameter jet columns. High-pressure pumping equipment is needed to attain the required jet energies. Table 9.1 lists typical jet pressures.

9.7

INFORMATION REQUIRED FOR SELECTION AND DESIGN Before design and selection of this technique, it is necessary to obtain information on: •

geology and geotechnical properties of the soil and rock

•

adjacent structures, services, buried structures

•

disposal requirements for fluids and spoil return

•

hydrogeological conditions

•

allowable settlement or deformation of structures above and adjacent to the grouting

•

site access for plant.

According to the draft publication the European Standard for the Execution of Jet Grouting Works (CENrrC 288/WG7), particular attention should be given to:

• •

9.8

• •

cemented layers or lenses

•

aquifer conditions

• •

high hydraulic gradients

• • • • • •

density of granular layers

•

fum or stiff clay layers or lenses swelling soils presence of artesian or confined

high organic content

large voids or high permeability.

highly sensitive or quick clays position of the water table aggressive soil or water cobbles and boulders chemical wastes or deposits

BASIS OF DESIGN Table 9.1 outlines the relative merits of each of the three injection processes for jet grouting. Project requirements and contractual preferences often determine the grouting technique chosen.

9.8.1

Layout of injection points The diameter of any jet-grouted column or thickness of a grout panel depends on the grouting method used and the ground conditions. The grout element size can be estimated, but will be affected by heterogeneities within the soil. Tolerances on the grout element dimensions should be set by adjusting the injection parameters according to the ground conditions. The required dimensions of the fmal grout elements determine the arrangement of columns (or panels) and in particular the overlap of adjacent grout elements. An example is the minimum grout thickness of a wall or cutoff. Certain projects, eg foundation construction, may require the whole area to be infilled with cement. This requires considerable overlap of adjacent grout columns.

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The strength of any jet grout element depends upon the technique used to apply the process, the jetting or grout mix parameters, and the control system. The soil parameters, conditions and variability also affect the grout element strength. Ground heterogeneities may vary the dimensions and therefore strength of the grout element. Jet grouting usually involves the formation of overlapping grout elements. This requires careful planning of the grouting sequence to allow a column (or panel) time to cure before beginning construction of adjacent elements.

9.8.2

Injection parameters There is little available theory to determine the working parameters for jet grouting techniques. The specialist contractor often bases the choice of grout mix, injection pressures, withdrawal rates and rotation on practical experience. Data from field trials and, for particularly onerous conditions, laboratory tests, might aid these decisions. Many jet-grouting parameters are interrelated. For example, the jet grouting column diameter depends on soil conditions, injection technique and the energy of the grout jet.

9.8.3

Trials A trial of jet grouting in similar ground is advisable. This should be designed to give the maximum information and confirm critical parameters (eg rate of extraction to produce the specified column diameter). Ideally, several grout elements should be constructed then the soil excavated to allow visual inspection of the grouted element. Trials give the opportunity to determine the optimum grout mix, rate of extraction, likely quantity of spoil and most appropriate jet-grouting injection technique (single, double or triple).

9.9

SITE OPERATIONAL REQUIREMENTS Jet grouting involves the use of high-pressure equipment. This requires care with pumping and grout injection equipment and necessitates the use of appropriate valves, pipes, fittings and monitoring devices. Spoil disposal should be considered as part of the treatment design and should be specifically identified in the method statement. The spoil should be handled efficiently to keep the working platform clean. Possible solutions include recycling in whole or part, disposal in bulk off site or separation of liquid from solid spoil on site before disposal. The final choice depends on site-specific factors and cost. Ground disturbance in jet grouting is generally minimal, but sometimes in fine-grained soil swelling occurs, resulting in considerable heave (500-1000 mm). The influence of heave on the surrounding ground and adjacent structures should always be considered. Clayey soils are particularly prone to heave when jet grouting close to retaining walls, tunnels or services. The development and incidence of local increases in lateral pressures should be taken into account.

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9.10

MONITORING

9.10.1

Grouting parameters Success in jet grouting critically depends upon control oftwo elements. 1.

The injection parameters The density of the injected grout, flow rate, fluid pressures (measured at the drill head), rotation and withdrawal rate should be continuously monitored throughout the process.

2.

The spoil return This is visually inspected, its density monitored and, where appropriate, compression tests performed on representative samples.

The inclination of the grout string and the quality and properties of the grout might need to be assessed too. The latter may include measuring daily the density, viscosity, bleeding and setting time and performing material tests on the cement and grout mix. A record of all grout injection parameters should be printed out and retained as a quality record. These data allow graphical interpretation of the grouting process. As with all grouting processes, it is important to check the monitoring instrumentation regularly. Monitoring equipment and survey may be required to assess ground heave and any movements of overlying and adjacent buildings during the grouting process in critical conditions. Section 10.10 discusses the appropriate monitoring equipment.

9.10.2

Validation Validation of a jet-grouting project involves continued monitoring of the injection process and post-treatment investigations. The latter involves determining the geometry and physical properties of the grouted element. Ideally, visual inspection and direct measuring are the best techniques to determine the geometry of the grout element. They are not usually practical, however, as they require excavation to the grout column or panel. Visual inspection is a key element for assessing the results of field trials, particularly for evaluating jet grouting in previously unknown or untried ground conditions. Alternatively, information on the cross-section and length of a grout element can be obtained using core samples from bores drilled parallel and inclined to the element axis as well as at the junction of the columns. It is important to consider problems relating to borehole deviations and core recovery when interpreting these results. Several types of tests are available to determine the mechanical properties of the grout element. The designer should determine the number and type of tests required. Available techniques include in-situ penetrometer tests and shear and compression tests on representative grouted cores. It is important that the hardening time for the cement grout is considered in all tests. Pumping or falling or constant head permeability tests are used to determine the permeability of the grouted structure in-situ. The permeability of the grout material is also obtained from laboratory permeameter tests on representative core samples or spoil return samples. Alternatively, the effectiveness of the jet-grouted element can be assessed by piezometers, installed either side of the cutoff structure.

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9.11

CASE HISTORIES

9.11.1

Case history 9.1 :

Jet grouting at Parkshot House, Richmond, Surrey, UK

Case description Grouting technique

Jet

Geology

Thames Gravel over London Clay

Grout

Cement (water:cement ratio = 0.6)

Contract

JCT contract

Specification

Performance

Control tests

Grout properties, injection pressures, injection quantities

Validations tests

By excavation

Contractor

Keller Ltd

Reason for grouting

Jet grouting was used to form an interlocking gravity retaining wall and to underpin existing buildings, enabling the construction of a deep basement at Parkshot House in Richmond, Surrey.

Ground conditions The ground comprised Thames Gravels underlain by London Clay. Criteria for grouting Interlocking, inclined 1.2 m-diameter columns were designed to act as a gravity retaining wall with foundation support beneath the footings of the existing buildings (Figure 9.4). Drilling sequence 1 Primary 2 Secondary :3 Tertiary

Plan view

Section A - B

A

Figure 9.4

126

B

Jet-grouted columns used to underpin the building (after Wheeler, 1996)

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Types and methods of grouting A trial of the jet grouting was initially carried out in the middle of the site to determine the grouting parameters and the optimum spacing of grout columns. Three or four rows of 1.2 m nominal diameter interlocking columns were installed at inclinations from Q vertical to 30 from vertical. The grout was injected from a piling platform at a level of 6 m forming columns that penetrated 0.5 m into the London Clay to form a seal. The triple injection grouting process was used, with a 90 mm-diameter drill string. An air-shrouded water jet, injected at pressures up to 4 MPa (40 bar) was used to break up the soil while the grout was injected through a separate nozzle. A rotating laser and target system was used to monitor vertical and horizontal movements throughout the grouting process and thus detect any heave affecting the adjacent buildings. Following the construction of the grout retaining wall, the basement was excavated up to the grout columns. This grouting technique maximised the available basement space and enabled rapid basement construction, as the excavation was unhindered by props.

9.11.2

Case history 9.2:

Jet grouting of Kingston Bridge, Glasgow

Case description Grouting technique

Jet

Geology

Alluvial deposits of sands and gravels overlying sandstone and siltstone

Grout

Cement (water:cement ratio = 0.53)

Contract

ICE modified contract

Specification

Performance

Control tests

Grout properties, injection pressures, injection quantities, trial columns

Validations tests

Core samples taken

Designer

Keller Ltd

Reason for grouting

Major refurbishment of Kingston Bridge became apparent when it was discovered that the main north piers were out of plumb by 165 mm, the main span had flattened by 300 mm and the quay wall in front of the north west pier foundation was bulging by up to 200 mm. Detailed investigation identified the need for ground improvement and provision of extra lateral support for the north and south quay wall.

Ground conditions The site geology was identified as approximately 30 m of alluvial sand overlying Carboniferous sandstone bedrock. Behind the quay the upper layers consisted of 4-7 m of fill containing ash, brick, concrete and rubble with timber obstructions. In front of the quay the sand had been dredged and was overlain by very soft, slightly clayey, sandy silt.

Criteria for grouting The overriding requirement of the remedial measures was not to reduce the factor of safety against failure of the quay wall. In addition, the work was to be carried out without interruption to the traffic flow across the bridge.

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Types and methods of grouting Jet grouting was determined to be the most suitable technique to improve the stability of the pier foundation. This was primarily because: •

it utilises small drillholes (required as the ground contains numerous obstructions, eg ties, timber piles)

•

grout columns can be formed at an angle underneath the pier foundation

•

grout columns can be formed to a depth of 17 m with limited headroom (7-10 m).

In addition to the jet grouting, rock bunds were placed from the river through the silt on to the underlying sand to form a resisting wedge against the quay wall. The construction sequence was set in five stages. 1.

Formation of the jet grout columns at a distance> 3 m from the quay wall.

2.

Positioning of Stage A of the rock bund (Figure 9.5).

3.

Grouting of any voids between the two timber sheet pile walls.

4.

Positioning of Stage B and C of the rock bund.

5.

Formation of the remainder of the jet grout block.

~ STAGE A ROCKFILL

~ STAGE B ROCKFILL ~ STAGE C ROCKFILL •

STAGE 1 JET GROUT BLOCK

D

STAGE 2 JET GROUT BLOCK

HW

SZOD SZLW

SILT LEVEL SAND LEVEL

I

I I

Figure 9.5

128

I I

Construction sequence of stabilisation works (after Coutts et ai, 1994)

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Particular problems to the jet grouting process were:

1.

The maintenance of passive resistance from the river silt at all times.

2.

Potential blockage of spoil returns causing an instantaneous pressure rise within the fluid grout and imposing a horizontal force on the quay wall.

3.

The enlargement of any cavities within the soil leading to soil or masonry collapse or grout leakage into the river.

The jet grout specification stated that the minimum cube unconfined compressive strength of the grouted block was 5 MN/m2 after 28 days with a minimum mass density 3 of 1800 kg/m • Cores of the block were obtained from five 63 mm holes drilled at 30° to the vertical. The specification also gave limits for the number of columns that could be grouted in an area within 72 hours as this prevented large adjacent volumes of soil being temporarily weakened. The quay wall was constantly monitored throughout the remedial treatment using rotational lasers, inclinometers, electrolevels and precise survey techniques, allowing ongoing modification of the construction sequence and method. Although no particular allowable limits for movements were specified, initial guidelines for heave and outward deflection of the quay wall were set at 5 mm and 25 mm respectively. Of key importance was the rate of movement and timing of movement in relation to construction operations so that these operations could be slowed or stopped well before visible evidence of movement. Column layout was complex because of the many obstructions within the site, eg tie bars, anchor piles, driven H piles, a sewer outfall. In general, aIm triangular grid was chosen with some local variations.

Table 9.2

Results of tests (after Coutts et ai, 1994)

Column

2

8

7

4

5

6

3

9

Water:cement ratio

0.53

0.53

0.53

0.5

0.53

0.52

0.52

0.56

0.56

Lift (m/min)

0.25

0.15

0.20

0.20

0.20

0.15

0.25

0.25

0.25

1.37

1.66

1.25

1.4

1.4

1.65

1.41

1.3

1.25

13.7

7.9

Diameter observed (m)

1.34

1.38 1.42

Slurry strength 28 days from jetting (MN/m2)

8.56

18.07

Core strengths (MN/m2)

22.5

12.5

19.5

6.0

11.0

9.0

15.0

8.8

14.5

15.5

8.6

10.95

12.5

23.5

20.0

20.0

16.0

21.0

5.0

8.0

18.0

23.0

7.0

7.5

A trial was conducted of the jet grout process on land adjacent to the treatment area. Eight columns were jetted with variable grouting parameters, ie rate of extraction, grout flow and composition. The grouting process was monitored and following treatment a cofferdam constructed. This allowed excavation to 3 m depth and examination of the columns (Figure 9.6). Table 9.2 shows the results of tests on the columns, indicating that the contract requirements were generally achieved.

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Construction of the main remedial works began. Electrolevel measurements were continually recorded, and if allowable levels were exceeded the jetting was stopped. After some time, the grouting caused some movement of the quay wall. Accordingly, the construction sequence was modified such that 50 per cent of the vertical columns were constructed following the positioning of the restraining rock bund. In addition, some of the columns were constructed in two phases with the lower half jetted and allowed to harden prior to completion of the column. There were no technical problems with the grouting process. The main difficulty was the frequency with which obstructions were encountered. In total 628 columns were grouted, involving 5858 mjetting and using a total weight of 3500 t of grout. No unacceptable movements took place throughout the period of the works.

Figure 9.6 Trial jet grouting site after excavation exposing jet grout columns

130

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10

Compensation grouting

10.1

PHYSICAL PRINCIPLES During the construction of tunnels or excavations, stress relief occurs in the surrounding ground. This causes volume loss towards the tunnel face or into the excavation and associated surface ground movements and structural settlements. Compensation grouting is used to counteract this volume loss, reduce the ground movement and prevent or limit the settlement of structures and the ground (Figure 10.1). It is a procedure that uses a number of grouting techniques (hydrofracture, permeation, compaction and intrusion grouting) to inject grout into the stress-relieved zone or between this zone and the overlying or adjacent structures. Controlled settlement2>,

r

Compensation grouting

Figure 10.1

\

\

~ "'-

~-~--~----Settlement in absence of compensation grouting

\

-

Compensation grouting during tunnelling (from Mairet ai, 1995)

In practice, compensation grouting most frequently takes the form of a succession of minor jacking movements while volume loss accrues. This is because during the initial phase of compensation grouting the reinstatement of the minor principal stress is required from the grouting pressure. However, if the grouting is carried out after the settlement occurs jacking is required, as shown in Figure ID.2. Jacking requires recompaction and stress application in excess of the major principal stress. Pre-empting settlement by monitoring the ground and injecting (conditioning) before the structure is affected is often preferable but is not always possible.

In Europe, compensation grouting refers only to the process of grout injection for the avoidance or limitation of ground movement. This process is also called compaction grouting in the USA. Compensation grouting should also be distinguished from slab jacking or grout jacking. These are processes to correct or reverse movement that has already occurred.

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131

Q = qA

~ 't _---~ ~~~~~--+, ,,

.....

~

I

\

9-~ /

\ \ \ \

0;

I

_1---- ,, I

\

---

I

\

I

\

\

~

U-o;

--- ---

\I-

->'

BASEMENT EXCAVATION

COMPENSATION REQUIRES ONLY REINSTATEMENT OF MINOR PRINCIPAL STRESS

JACKING REQUIRES RECOMPACTION AND STRESS APPUCATION IN EXCESS OF MAJOR PRINCIPAL STRESS.

...... ...... SHEAR STRESS

SHEAR STRESS

......

"-

"-

'\

\

\

\

,

\

REQUIRED COMPENSATION PRESSURE

cr, (q)

DIRECT STRESS-

0;

cr, (q)

AXED

REQUIRED JFACNKING STRESS I o FRACTURE

0; ACTIVE 0; RECOMPACTED DIRECT STRESS-

()

STRESS CHANGE DUE TO DEFORMATION

Jj

»

()

.....

(J1 ~

Figure 10.2

Grouting pressures - settlement compensation and grout jacking

STRESS CHANGES DURING COMPACTION I JACKING (EXCEPTING HYOROFRACTURE)

DIRECT STRESS-

I cr, (JACKING)

10.2

APPLICATIONS OF COMPENSATION GROUTING The defmition of compensation grouting above shows that its applications relate to ground excavations. The main uses of the technique are compensating for: •

settlements caused by soft ground tunnelling (Figure 10.3) or other subsurface excavation

•

the lateral deformation of retaining walls.

o,

10

5

15m

]

I

Scale Court Yord

Ground Level

~i: Sand/Grovel

Office Building

::: Groundwater Level ;:::,

~\

Grouting Mot

South - Silt/Cloy

Cross Section of the Platform Tunnel

Figure 10.3

10.3

Cross-section of the platform tunnel of the Vienna Metro showing grouting locations (after Pototschnik, 1992)

LIMITATIONS SPECIFIC TO COMPENSATION GROUTING Compensation grouting can be expensive. It is more costly than other forms of grouting or methods that produce the same effects. The need for many injection points can be a problem in urban areas. Furthermore, compensation grouting can increase the pressure on the excavation or tunnel face and linings.

10.4

DESCRIPTION OF TECHNIQUES USED Compensation grouting is a procedure that uses one or all of the following processes: •

permeation grouting (for conditioning the ground before compensation grouting)

•

hydrofracture grouting (Section 7)

•

intrusion grouting (Section 8)

•

compaction grouting (Section 8).

The choice of grouting process depends upon the ground conditions, scope of the work, site limitations and required results.

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133

A general (simplified) sequence of operations for a compensation grouting project is: 1.

Perform a detailed geological and site survey. A detailed assessment of the ground conditions is vital to determine the stability of the excavation and the effect of the excavation on overlying and adjacent surface and subsurface structures. The survey should also include a detailed assessment of the condition of structures likely to be affected by the excavation and to determine locations of structural foundations or services in relation to the planned excavation.

2.

Risk analysis and determination of acceptable limits of settlement movement, heave and angular distortion.

3.

Determine a suitable compensation grouting technique.

4.

Perform trials of the grouting process. Trials of the grouting technique are recommended in soils and in stress states similar to those on the site. These provide valuable information on suitable grouting parameters and ground response to treatment.

5.

Install the grouting system and monitoring equipment.

6.

Pre-treatment of the ground (as required). Once the grouting system has been installed, an initial grout injection may follow to condition and tighten the ground. This pre-treatment is designed to leave the ground in such a condition that further grout injection will produce an immediate ground response. In some circumstances it may be sufficient itself to mitigate settlement of vulnerable structures.

7.

Initiate tunnelling or excavation and grouting when necessary. Grouting is initiated in anticipation of the development of settlement or when predetermined surface or subsurface settlement limits are exceeded. It is preferable to commence grouting on initiation of settlement. The rate of settlement, direction of settlement and whether it is increasing or decreasing also needs to be taken into account. The control sequence for compensation grouting in the case of the latter is outlined in Figure lOA.

8.

Final survey of structures and ground levels (as required).

No

Figure 10.4

134

Compensation grouting process (after Mairet ai, 1995)

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10.5

GROUTING MATERIAL See relevant sections: 5.5 - permeation grouting 7.5 - hydro fracture grouting 8.5 - compaction and intrusion grouting. (Slump limits for grouts commonly used in compensation grouting are up to 150/200 mm.)

10.6

PLANT AND EQUIPMENT For plant and equipment see the following sections: 5.6 - permeation grouting 7.6 - hydrofracture grouting 8.6 - compaction/intrusion grouting. Computer-controlled systems are rapidly being developed for use in compensation grouting processes. Typical systems allow a maximum grout pressure and minimum flow rate to be set for each injection point. Audible and visual warnings are given if any of the parameters reach predetermined maximum levels. These computer systems also provide on-screen displays of the injection parameters at each injection point. This enables the operator to respond quickly to any observations and manually adjust injection parameters. The system can also be linked to other software monitoring ground and structural movements. Computerised control systems considerably improve the quality control on a grouting project. Detailed records of injection parameters, different injection phases, grout mix, date, duration of injection are obtained and presentation of large amounts of data, in a manner compatible with rapid comprehension, is possible (Table 10.1). Table 10.1

Shaft

Hole

Sleeve

Episode

Extract of compensation grouting daily report on JLE Contract 102 (courtesy of AMEC GEOCISA) Line no

Start time

End time

Av flow IImin

Av pressure bars

End pressure bars)

Episode Total vol vol (I)

(I)

4/1

0016

26

11

02:35:57

02:43:53

15.6

16.6

12.4

121

1330

4/1

0020

32

14

02:46:16

02:53:41

16.8

17.5

13.4

121

1700

4/1

0060

18

11

02:26:58

02:30:15

16.1

26.6

30.5

53

677

4/1

0062

14

10

00:50:27

00:54:41

14.0

27.1

28.2

61

671

4/1

0062

19

11

00:59:06

01:01:32

14.5

35.6

53.0

36

693

4/1

0065

13

12

01:05:33

01:10:40

15.6

18.9

17.3

81

1008

Note: The maximum pressure is computer-controlled, so it is preset.

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135

10.7

INFORMATION REQUIRED FOR SELECTION AND DESIGN In addition to the information required for the particular grouting technique (outlined in the relevant sections) information is required for compensation grouting projects:

10.8

•

rate of tunnel excavation or excavation works

•

estimate of tunnel face loss

•

settlement contour plan noting width of settlement trough and points of inflexion

•

ground distortion permitted (including allowable settlement due to tunnelling or excavation)

•

structural distortion permitted

•

area of buildings requiring protection

•

geometry of tunnels and building foundations

•

ground profile.

BASIS OF DESIGN The choice of grouting process between hydro fracture, intrusion or compaction grouting or a combination of grouting processes generally depends upon the ground conditions, scope of the work, site limitations and required results. In general, two principles apply. 1.

In stiff clays and more competent soils, hydrofracture grouting is likely to be the most useful method, although care is necessary, both in the design of the rheology of the grout and the injection procedure followed.

2.

In granular soils, either intrusion or compaction grouting is generally preferred. The former, when carried out through tubes-a-manchette, is the more flexible method, as drilling and installation of TaMs can be completely divorced from grouting.

Compensation grouting is carried out when there is the potential for excavation to cause damage. A risk assessment is particularly important for compensation grouting projects, therefore, and should be considered at the design stage of the project. Numerical modelling is often carried out in compensation grouting projects to give an overview of the process and aid understanding of the possible behaviour mechanisms. Results from modelling have been used to predict with reasonable success the stresses acting on concrete tunnel linings or structures. It is nevertheless difficult for models to simulate the rapid dissipation of grout away from the injection points and the flow of grouts through the soil matrix. Models to determine design parameters and to predict ground movements for current grouting projects should be used with caution until more comprehensive field data allows calibration.

10.8.1

Layout of injection points The layout of injection points and injection parameters are interrelated and are generally determined by the project, soil conditions and structural restraints. The key parameters used for design are the initial and final soil conditions, grout mix, achievable injection pressures, stability of adjacent buildings, site access and cost. A large spacing between points may reduce drilling costs at the expense of increased grout injection pressures and volumes and the control and uniformity of the treatment or ground response. Recent advances in computer software have led to the development of packages designed to generate optimum three-dimensional grout treatment layouts. The user

136

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enters data about the soil layers and uses a mouse to define the zones where grouting is required and any obstacles. During project execution the computer can produce instruction sheets for each drill unit detailing the location, inclination and length of the drillhole (Harris and Cotet, 1994). The following basic principles should be observed in the choice of the zones where grout injection is to take place.

10.8.2

1.

In general, the grouting zone should be as near to the source of the volume loss as is compatible with the stability of the excavation, any temporary works or tunnel lining.

2.

Observation of subsurface movement during grout injection is important if the objective is prevention, as opposed to recovery, of settlement at or near the surface.

3.

All compensation grouting exercises should allow for an initial phase of injection (pre-treatment) before excavation. This "tightens" the ground and gives some insight to the ground and structural response.

4.

A thorough understanding of how the structures are likely to interact with soil movements is required in the design of grouting arrays. For highly concentrated loads it may be necessary to create a raft immediately under the foundation by other grouting methods, to spread the load and provide a sufficient area to enable compensation grouting to be carried out.

Grouting parameters The grouting parameters are grout mix and injection pressure and volume. The choice of these parameters depends upon the grouting technique selected. See relevant sections: 5.8 - permeation grouting 7.8 - hydrofracture grouting 8.8 - compaction and intrusion grouting.

10.8.3

Trials Trials of the grouting process are important for evaluating and optimising grouting parameters, techniques and layout of injection points. They are of particular value in relating the amount and rate of uplift to injection parameters and thus aid the control of the compensation grouting process.

10.8.4

Settlement or uplift values and rates Settlement damage occurs because of differential settlements and angular distortion. Empirical methods are used to predict the surface settlement caused by tunnelling or excavations. In greenfield sites, a normal or Gaussian settlement trough is usually assumed for the settlement from tunnelling. Building interactions, however, generally widen the settlement trough. Individual building assessments are carried out in zones of greatest settlements involving the definition of relevant building span lengths and calculation of horizontal bending and shear strains. The maximum combined tensile strain indicates the building damage classification (BRE 1990). The uplift values and rate of uplift are determined by the stability of the adjacent and overlying buildings or the settlement of the ground caused by the excavation. Preliminary grout injection parameters are designed and then altered based on observation, in order not to exceed limiting values for uplift.

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10.9

SITE OPERATIONAL REQUIREMENTS A performance specification, in which limits on settlement and angular distortion of structures or the ground are specified for management of ground movements, is the most common contractual arrangement for compensation grouting projects. Site operational requirements are usually designed based on this limiting parameter.

Box 10.1

Example of performance specification

In the Jubilee Line Extension project, the acceptable settlement of adjacent structures was determined based upon the settlement that would produce a maximum tensile strain of 0.15 per cent in the structure. This is consistent with slight damage (Boscardin and Cording, 1989). Acceptable limit curves were produced relating the maximum settlement to the length of wall affected. These translated to simpler site performance limits of 15 mm settlement as an "amber" limit, 25 mm as the "red" limit and an angular distortion of 1 in 1000.

If the grouting procedure is not rigorously controlled, compensation grouting projects have the potential to cause large ground movements (heave or settlement). The stability of adjacent buildings should therefore be considered and acceptable settlement or uplift values determined. Throughout the grouting process, the settlement of structures and the ground should be observed and kept within the predetermined values. In compensation grouting, high-pressure grout injection near excavations may increase the stress on the temporary works.

10.10

MONITORING The key to a successful compensation grouting project is a well-designed monitoring system. All grouting operations should be carefully controlled and respond rapidly to monitored structural and ground movements. Advances in computer software and electronic sensing devices enable continued logging and recording of such data. Modem links may be utilised to transmit data rapidly. The use of different monitoring techniques to determine a parameter enables readings to be validated, eg surveying and electrolevels. In addition, a degree of redundancy should be included in the design to take account of instrumentation failure (an inherent problem in any monitoring exercise). A manual system back-up is also desirable. Usually, information is required on the following parameters: 1.

Settlements (surface and subsurface) An appropriate distribution of surface and subsurface monitoring along with precise surveying techniques is generally required to determine the settlement of the ground and structures during the excavation and grouting phases of the project. It is important that the results be presented in real time and in a way that is easily understood, eg graphically. Equipment and techniques are briefly outlined below.

2.

Injection position During the grouting process it is important to accurately determine the precise position, orientation and deviation of the groutholes and the exact location of injection points. This can be obtained using an accurate survey and instrumentation such as a Reflex Maxibor or Multibore, for inclined and vertical boreholes.

3.

Injection parameters The grout injection parameters at each injection point should be monitored throughout the grouting process and the information recorded and displayed on a

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screen. Trigger or perfonnance specifications can be entered into the data-logging computer software and alarms sounded if these values are exceeded. 4.

Excavation parameters For compensation grouting used on a tunnelling project, infonnation should be obtained on the defonnation of the tunnel lining, face movement and progress of the tunnel-boring machine.

Careful calibration of all instrumentation is essential and manual readings should also be taken, including manual precise levelling to provide datum reference points.

10.10.1

Subsurface monitoring Ground monitoring is vital in compensation grouting to detect the progression and propagation of settlement from the ground loss to the ground surface. The grouting contractor can then prevent surface settlement by responding rapidly to instrumentation measurements and intercepting and compensating the settlement loss. Instrumentation is required within the ground to determine: •

ground displacements (including the rate of change)

•

total stress changes

•

pore pressure changes

due to the tunnelling or excavation activities. Devices available to measure these parameters are: •

magnetic or sleeved

•

hydraulic gauges

•

rod extensometers

•

piezometers

•

inclinometers

•

total stress cells

•

settlement pins

•

electrolevels.

Siting instrumentation is important and varies according to the type of instrumentation, the parameter required, the excavation, surface buildings and existing underground structures and services. Electrolevels can be placed within boreholes or attached to existing underground structures. Extensometers, piezometers and total stress cells are positioned within the ground. (Total stress cells are hard to calibrate and use accurately.) The interval between readings and the time required to interpret results is important.

10.10.2

Surface monitoring Structures at risk from the tunnelling activities are surveyed and their condition assessed before tunnelling. The client is generally concerned with obligations to property owners and often requires abundant structural and surface instrumentation. Ideally, however, the surface monitoring should target sensitive buildings and be designed to complement the ground instrumentation to provide information to the grouting contractor. The following equipment is available to monitor surface settlements: •

precise survey and levelling

•

holographic prism

•

rotating laser

•

hydraulic gauges

•

electro levels

It is also prudent to have a robust back-up system.

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10.11

CASE HISTORIES

10.11.1

Case history 10.1 :

Compensation grouting at Waterloo station, London

Case description Grouting technique

Compensation

Geology

Alluvial clays overlying Thames Gravels and London Clay

Grout

Intrusion grout 13: 1 pulverised fuel ash:cement mix, 0.4 per cent bentonite by weight and a water:solids ratio of 0.4 (by weight)

Specification

Performance - incremental trigger levels

Contractor

Bachy (UK) Ltd

Reason for grouting

Compensation grouting was required to protect two sensitive masonry structures (Victory Arch and the Waterloo and City Line, WCL) during the construction of a new escalator tunnel and associated passageways at Waterloo station.

Ground conditions The geology was made ground and soft to firm alluvial clays, overlying about 4 m of medium-dense to dense Thames Gravels, underlain by stiff London Clay. In-situ constant head permeability tests undertaken in the Thames Gravels indicated permeabilities of 10-3 to 10-5 rn/s. The groundwater table was just above the top of the Thames Gravels at 1 m below Ordnance Datum_

Criteria for grouting Compensation grouting was undertaken in the ground between the tunnels and overlying structures to limit settlements to acceptably small levels. A conservative approach was adopted to determine an acceptable magnitude of movement of the structures, because the consequences of damage were serious, particularly in the case of the WCL tunnels. The contractual arrangements were designed so that compensation grouting would be used, when necessary, throughout the tunnelling period~ jacking up of the structure was kept to minimum. The specification defined incremental trigger levels at which compensation grouting had to be undertaken. If the instrumentation detected movements of 1 rom of the structures or 5 rom of the ground relative to the structures (as measured byextensometers) since the previous compensation grouting operation, the contractor was required to grout. Grouting was also permitted before the trigger levels were exceeded, if necessary, to ensure compliance with the overall performance criteria. In practice this only rarely occurred.

Types and methods of grouting The outline scheme specified for the compensation grouting was compaction grouting within 4-5 m thickness of gravels beneath the Victory Arch and a combination of intrusion grouting in gravels and hydro fracture grouting in the London Clay beneath WCL. As an alternative, Bachy (UK) Ltd proposed a form of intrusion grouting beneath Victory Arch as well as beneath WCL. A grouting trial was carried out to demonstrate the viability of the alternative proposal. The intrusion grout mix was selected on the basis of the results of the trial.

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CIRIA C514

Box 10.2

Instrumentation used for compensation grouting at Waterloo station

During the construction of an escalator tunnel at Waterloo station, where compensation grouting was used, the following instrumentation was installed to monitor ground and structural movements (Harris and Cottet, 1994): •

electrolevel sensors at 1 m centres in an inclined 20 m borehole and horizontal 33 m borehole, both positioned 1 m above the crown of the enlarged tunnel

•

29 sleeved rod extensometers through the invert of the existing underground tunnel at 3 m centres

•

eight magnetic extensometers in Victory Arch area

•

electrolevels on Victory Arch and Elizabeth House.

In addition, at 3 m intervals along each of the existing underground tunnel walls, the following instrumentation was installed: •

electrolevels attached to horizontal beams on the tunnel wall at 3 m centres

•

single electrolevels attached to vertical beams to measure the out-of-plane tilt of the wall

•

hydraulic manometer gauges to measure differential settlement of the tunnel walls.

Precise levelling was also undertaken at regular intervals. (Figure 10.5 details the arrangement of some of the monitoring instrumentation.)

Waterloo and City Une • Extensomelres -- Hydraulic gouges • Vertical and horizontal electrolevel beams plus levelling points Tubes a manchettes E~zabeth

o b.

Figure 10.5

House Electrolevel tillrnetres Levelling points

ond~

Waterloo -\''r--T''v-City Une Tunnels

o,

"

"

10

Scale (metres)

Plan of tUbe-a-manchette and instrumentation during compensation grouting at Waterloo station (after Harris and Cottet, 1994)

Injection points were installed with a maximum spacing of 1.5 m (see Figure 10.4). A computer-controlled injection system was used, which allowed a maximum grout volume, a maximum grout pressure and a minimum flow rate to be pre-programmed for each injection. The Thames Gravels were preconditioned (ie tightened) before tunnelling by injecting grout from approximately 50 per cent of the injection points. Most of the compensation grouting was carried out within the gravels with limited hydrofracture grouting within the London Clay. Within the gravels two types of grouting were carried out: high-pressure and low-pressure. The low-pressure grouting was carried out in the zones of chemically treated gravels or at the interface between the foundations and the gravels to make sure that no void had formed.

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Comprehensive instrumentation, monitoring and interpretation were recognised as being of fundamental importance to the success of the technique. The instrumentation was installed in the ground and on the structures. The grouting operations were to be strictly controlled in response to observed ground and structure movements. Figure 10.5 shows a plan of tube-a-manchette and instrumentation used during the compensation grouting and Box 10.2 gives details of the instrumentation used. Examination of the monitoring data at face value gave a confusing and contradictory picture and considerable time was expended on interpretation of the data. It was necessary to provide a degree of redundancy in the instrumentation system so that the displacements indicated by one set of instruments could be cross-checked to allow identification of consistent trends in the data. Grouting from within WCL could only be carried out at night. The following daily procedure was generally adopted: 06.00

report produced on data-logged instrumentation readings

11.00

results of manually monitored instrumentation and previous night's grouting records supplied; meeting to discuss performance

15.00

grouting proposal submitted by the contractor.

The procedure's timing was modified to reflect the rate of advance of the tunnelling operations. (Table 10.1 gives an extract of the compensation grouting daily report). On completion of the pilot tunnel construction, the maximum settlement of the structure was 4 mm. On completion of the enlargements to the 7.5 m-, 8.25 m- and 6.5 mdiameter tunnels, the total settlement had reached about 7-8 mm. Construction of the passageways and openings resulted in the maximum total settlement of the Victory Arch and the centre wall of the WCL increasing to about 12 mm and 16 mm, respectively. Figure 10.6 gives a comparison of settlements caused by the tunnelling above the centre line of the new tunnels with the predicted greenfield site potential settlement. The maximum relative rotations at the end of construction were 1:1250 and 1:2250 for the WCL and the Victory Arch respectively (ie below the specified limit of 1:1000). No cracking of either structure was observed. The following conclusions were made:

142

•

the monitoring system and trigger levels determined when grout injections were necessary during the tunnel construction, such that minimal controlled heave of the structures occurred.

•

key elements in the success of the compensation grouting were: the comprehensive system of instrumentation detailed electronic monitoring detailed interpretation of data rapid data processing of the movements of both the structures and the ground, which allowed the grouting strategy to be refmed continually in response to the observed behaviour good communication between grouting and monitoring crews, enabling precise control over grouting on an injection-by-injection basis.

CIRIA C514

Waterloo and City Line tunnels

Mode ground/ olluvium

Ground Level

Elizabeth House

Made ground

horizontal beams with eledrolevels

:t

vertical beams with electro levels

U

hydraulic settlement gauges

o Sc:ote of ........ 10 !wwI Iaaaez! !wwI

New tunnels Distance

0

5

15

10

20

25

30

m

\ E 20 E

\

Measured settlements of structures due to tunneling alone

\

45

40

35

/7

50

-----

/'

.,

E

UJ

\

/'

/'

//-----'--/./

,

\

E.,

60

-

/'

/'

\..

......

Figure 10.6

CIRIA C514

/'

/ , " '-

-- -

/'

;,/' ---~ -........... Predicted

-

."..--.

\ \

80

/' , /

/'

\ "E ., 40

60

55

greenfield site potential settlements

Comparison of settlements due to tunnelling above the centre line of the new tunnels with predicted greenfield site potential settlement (after Harris and Cottet, 1994)

143

10.10.2

Case history 10.2:

Compensation grouting on the Jubilee Line Extension (Contract 102), London

Case description Grouting technique

Compensation

Geology

Made ground overlying Terrace Gravels overlying London Clay

Grout

Permeation grouting Phase 1 cement-bentonite Phase 2 silicate-ester chemical grout Compensation grouting Fluid cement-bentonite grouts, thicker flyash-cementbentonite "mortar" pastes with low bleed capacities and high resistance to pressure filtration

Preliminary tests

Risk assessments

Contractor

Balfour Beatty/Amec, Amec/Geocisa

Reason for grouting

Protection of sensitive buildings in the area associated with the Westminster and Waterloo station works and along the Jubilee Line tunnel route from Green Park to Waterloo and to facilitate shaft sinking.

Ground conditions Made ground overlying Terrace Gravels overlying London Clay. Criteria for grouting An assessment of likely volume loss resulting from the tunnelling operations was made and then a detailed grouting proposal prepared to compensate for the predicted settlements.

Types and methods of grouting Several types of mixing and pumping systems were used, including a grouting manifold pumping system developed by Amec/Geocisa for permeation grouting. This equipment allowed up to 12 sleeve ports to be injected simultaneously and at controlled rates of 1-80 l/min. The activity at each sleeve port was controlled and monitored independently with real-time injection pressures and flow rates for each line displayed on the digital gauge. Metering pumps combined with static "stream" mixers supplied the chemical grout to the grouting manifold. More than 90 000 m of grout injection tubes-a-manchette were installed. Figure 10.7 shows tube-a-manchette arrays under the Westminster area installed from five temporary grouting shafts.

144

CIRIA C514

Figure 10.7

Tube-a-manchette arrays under the Westminster area installed from five temporary grouting shafts

Grouting was carried out from 22 purpose-built temporary shafts of 4.5 m diameter and 20 m deep. Up to 65 groutholes per shaft could be drilled into the ground without undermining the integrity of the shaft lining. The drilling equipment was supported on intermediate decks for installation of the plastic pipes for low-pressure permeation grouting at about 6 m below the ground surface and the steel pipes for high-pressure compaction and hydrofracture grouting at 16 m below the ground surface. Ground treatment work was continuous. Permeation grouting was carried out in the upper water-bearing gravels to assist shaft sinking and safeguard tunnel excavation in areas where the clay cover above the tunnel is less than 6 m thick. The grouting would also help to provide homogeneous ground between the tunnel and the surface to prevent differential settlement beneath buildings. The blanket of relatively impermeable ground had a width equal to two tunnel diameters and was injected in a two-phased programme. The initial injection of cement-bentonite grout was designed to fill the coarse pores in the gravel. This was followed by a second injection of silicate-ester chemical grout designed to treat the finer-grained soils (110 I of water, 80 I of sodium silicate liquor and 10 I of ester hardening agent). The grout used to compensate for volume loss ranged from fluid cement-bentonite grouts, which can travel several tens of metres in thin veins, to thicker flyash-cementbentonite "mortar" pastes with low bleed capacities and high resistance to pressure filtration, which form thicker wedges in the clay. These were injected at typical flow rates 2-10 l/min. Up to about one-half of the total estimated compensation grout volume was injected during preconditioning phase. Exclusion zones over NATM work to protect the face and the "green" strength of the shotcrete shells of 8 m ahead to 6 m behind the face were put into force. The instrumentation equipment included more than 2000 racks of electrolevels and 7000 monitoring points. Settlement, differential settlement and angular distortions and tilt were monitored and the incoming data received at a central monitoring station.

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11

Information needs and contractual framework

If grouting is to achieve its objectives cost-effectively, it is important that those with the necessary knowledge and experience make the technical decisions, that the contractual framework encourages this and that neither narrow commercial advantage nor an overrigid commercial framework prevent it. Where grouting may have been considered unsuccessful in the past, it can often be attributed to one or more of the following:

11.1

•

unrealistic aspirations for the grouting leading to an inappropriate grouting concept

•

poor definition of contractual interfaces and lines of communication

•

inadequate ground investigation data relating to the treatment zone

•

inappropriate grout or grouting techniques

•

inadequate method specification

•

unattainable performance specification.

PARTIES INVOLVED As with other forms of ground treatment, grouting is almost always an ancillary activity to the main works and is often treated as a domestic subcontract. Very occasionally grouting forms the main works, eg in the remediation of an existing structure. The parties involved in the procurement and execution of grouting operations are those common to the construction of new works: •

client/owner or operator

•

consulting engineer or specialist subconsultant

•

main contractor for the works

•

specialist grouting contractor

•

planning supervisor.

Knowledge and expertise of grouting will be variously distributed between these parties. The specialist grouting contractor will generally know most about processes. While the powers, duties and legal responsibilities of the parties will ultimately be defined by the conditions of contract, the entire system of procurement is important if a quality product is to be obtained. The system of procurement should include: •

placing and definition of responsibility

•

definition of the contract work

•

method of tender, tender review and acceptance criteria.

Procurement should be driven by the information and contractual needs of each of the parties before, during and after grouting.

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147

11.2

ROLES AND INFORMATION NEEDS

11.2.1

Clients Clients for grouting may span the complete spectrum from being informed and experienced in grouting, perhaps with their own engineering department, to having no engineering knowledge. The latter might only be concerned with the end result, provided there are assurances of reliability together with insurance against third-party claims for damage or nuisance. In reality, through technical advisers, the client will be involved in many of the most central activities in grouting. Prior to the contract the client will have commissioned appropriate site investigation for inclusion in the tender documents. The client will have to choose those invited to tender and make the subsequent award between alternate tendered schemes. During the contract, the client will be responsible for payments and referral of disputes to the engineer or to arbitration. In the ICE Conditions o/Contract 5th and 6th edition the specialist grouting contractor will be "hidden" from the client and the client becomes heavily dependent on the grouting knowledge of the engineer. This may not always be sufficient, and the appointment of a subconsultant or direct negotiations with the specialist grouting contractor may be to the benefit of all parties concerned. The client should evaluate directly or through an engineer or other adviser the following points. 1.

The extent of grouting advised and whether all alternatives have been considered.

2.

The influence of grouting at nominated locations on adjacent services and structures and the public both during and after construction.

3.

The legal authorisation and wayleaves necessary for execution of grouting work.

4.

The efficiency and durability of the proposed method to meet design life maintenance requirements.

5.

The scope of the site investigation proposed to enable an appropriate grouting method to be determined and reliably priced.

6.

The probabilities and consequences which should be attached to under-performance of grouting during or after construction.

7.

The relevance of the knowledge and experience of any likely grouting contractor for the particular project tasks.

8.

The necessity of preliminary grouting trials.

9.

The impact of grouting, including trials, on programme.

10. The choice of a method or a performance specification. 11. The scope of instrumentation and who should be responsible for monitoring during and after the contract. 12. The level of supervision of the grouting process to check compliance with any specification and control cost variations.

148

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11.2.2

Consulting engineer or specialist subconsultant In the ICE Conditions of Contract, 5th and 6th editions, the role of the engineer or specialist subconsultant, by virtue of their detailed and expert knowledge, is to advise the client, to provide a detailed design specification and bill of quantities and prepare tender documents before the contract award and then to supervise the work during construction. The engineer has to produce options and recommendations for the consideration of the client under many of the points in Section 11.2.1. To discharge this responsibility properly, the engineer requires the fullest knowledge of the client's objectives, the detailed nature of the main project, the timescale for completion, all physical and time constraints, a comprehensive understanding of ground and groundwater conditions and sound technical knowledge of grouting techniques and their limitations with, preferably, some recent experience of their use on similar projects. The engineer should assess the following:

11.2.3

•

the scope of the ground investigation for execution of the grouting works

•

whether to use grouting

•

pre-condition surveys of existing developments

•

the merits of preliminary trials and testing

•

whether to use a method or performance specification

•

the overall design including the purpose, type of grouting, location and geometry of treated zones

•

consideration of temporary phases of construction

•

directions on sequence of working

•

evaluation of preliminary trials and tests (jointly with contractor)

•

identification of key items in the design and subsequent advice to all parties

•

the need for instrumentation, schedule of type, reliability, accuracy of instruments, frequency of measurement and interpretation

•

allowable movement limits on adjacent structures and services

•

supervision of the works and definition of quality requirements

•

the impact of the design health and safety issues.

Main contractor The main contractor has an obligation to complete the works in accordance with the drawings, specification and bill of quantities. The main contractor may have specialist grouting companies within the organisation but will probably seek to find a specialist subcontractor on the basis of capability and price from the market. A budget figure for grouting may be submitted in the main tender and a serious search not begun until after the tender award. The main contractor will have obligations with regard to the grouting operations - substantially access and enabling works and programme integration - and will assume primary responsibility within the contract for the grouting during and after construction. The main contractor's information needs centre initially on the extent to which the grouting works have been designed and specified. The more detailed the specification, the easier it is to identify suitable subcontractors and receive comparable prices.

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149

The main contractor should assess the following points.

11.2.4

1.

Whether the ground investigation data are adequate for grouting purposes and, if not, what supplementary test data or trials may be required.

2.

Whether the grouting proposition can be carried out as envisaged by the engineer and whether the performance criteria specified can be met.

3.

Details of any preliminary trials and the nature of any acceptance criteria.

4.

The possibility of substituting an alternate grouting process or radically different strategy.

5.

Details of the grouting process so as to produce method statements and the working sequence.

6.

Details of ground or structural monitoring and quality control measures during grouting.

The specialist subcontractor The specialist grouting subcontractor's information needs relate to the constraints dictated by physical circumstances and those imposed by other parties. The specialist should assess:

11.2.5

•

the practicality of the proposals

•

the adequacy of the site investigation report

•

details of adjacent services and structures including condition surveys

•

whether adequate baseline instrumentation data exists

•

the role and responsibility in design and monitoring

•

tunnelling technique, rate of tunnelling advance and expected face loss

•

programme constraints on grouting

•

physical access

•

the possibility of substituting a different grouting process.

The planning supervisor Early in the procurement sequence, the Construction (Design and Management) Regulations 1994 requires a monitoring engineer - the planning supervisor - and the preparation by the designer of a scheme based upon risk assessment and hazard analysis. Hazards should be identified, risks assessed and control measures and emergency procedures developed. The preferred scheme and design rationale is then set out in a health and safety plan prepared by the planning supervisor with input from the designer This plan is included in the tender documentation and accepted or modified by the successful contractor. The planning supervisor has to check that the designer's duties are fulfilled by: •

identifying health and safety issues

•

mitigating hazards through design

•

providing information about residual risks, which would not be obvious to others in the chain of design and construction.

Case study guidance for the designers is available in CIRIA Report 145 (CIRIA, 1995).

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The pre-tender health and safety plan should be sufficiently developed to form part of the tender documentation. It will then: •

make plain the health and safety issues specific to the project

•

note where and when the principal risks are likely to occur and alert tenderers to the hazards, thereby enabling all to take these into account when tendering and to plan safe systems of work

•

provide clients with part of the framework within which they can judge that they are selecting competent and properly resourced contractors by assessment of their responses to the plan, and thus

•

eliminate contractors who have failed to demonstrate that they would plan and provide resources for work to be done safely.

Appendix 4 to the Approved Code of Practice (ACOP) (Health and Safety Executive, 1995) describes the areas on which the health and safety plan gives information. Much of the information required might already have been provided elsewhere in the tender documentation. The ACOP makes it clear that the health and safety plan is to be aligned to the scale and complexity of the project in hand. There is no policy intention that health and safety plans should duplicate information from elsewhere in the tender documents that can simply be referred to.

11.3

RESPONSIBILITIES OF THE PARTIES There is a substantial degree of shared and interrelated responsibility among the various parties, particularly in the areas of design and monitoring. In the conventional contract, the specialist grouting subcontractor is at the base of any project organogram. While the cost of the grouting works might form only be a small percentage of the main project, poor grouting practice, as inadequate ground investigation, can have disproportionate and sometimes disastrous consequences. There is, therefore, an argument for considering how the specialist grouting subcontractor could be brought into the design process for the main works in a more formal manner. Figure 11.1 shows a decision tree setting out the responsibilities and flow of information of all parties in a conventional project with grouting works. This acknowledges present conventional contractual arrangements, including proposals for improved interfaces, with the engineer responsible for the design and the specialist grouting contractor as a domestic subcontractor. The figure is broadly consistent with the list of technical, contractual activities and roles in a typical grouting contract given in the draft European Standard Execution o/Special Geotechnical Work; Jet Grouting. Ground investigation The engineer is responsible for scoping the ground investigation and the client for procuring it. It is advisable to seek input from specialist grouting subcontractors if supplementary investigations are to be avoided at a later point in procurement of the contract when time is of the essence. This is often achieved by informal communication. Design of the grouting process The concept design - essentially the decision to grout and the selection of the method is usually, but not invariably, the responsibility of the client or engineer. The grouting layout and detailed procedure is, however, the delegated responsibility of the specialist contractor. Instruction to all parties on design criteria, including consideration of temporary phases of execution and constraints on sequence of working, and the basis of shared decisions are important design responsibilities of the engineer.

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151

ENGINEER! SUB-CONSULTANT

CLIENT

Provision of project details, adjacent services and structures

MAIN CONTRACTOR

SPECIALIST GROUTING CONTRACTOR

Definition of task for which grouting works are considered

.-----------i

Provision of site investigation data to enable selection of the most Acquisition of all legal appropriate grouting technique or authorisations ~_ _ _ _ _ _ _ _~_~__t:ec:h~n:iq:u=es:.-r_ _-i~--------~

Consultations on scope of geotechnical investigation

Arrange

additional ground investigation

Preliminary planning and design of grouting programme.

Consultation on techniques, and their potential, access requirements and scope of instrumentation

[-------1 ~-------------~ Consultation design and timing of trial

No

Assess need for trial and when it should be carried out - as a separate work package to the main grouting works or to be included as part of the main grouting works

Yes

No

Arrange for grouting trial to be carried out if a separate work package to the main contract.

Input and presence during trial \4+-----'

Appointed for trial

Input Confirm grouting process

Figure 11.1

152

Decision tree setting out the responsibilities and flow of information of all parties in a conventional project with grouting works

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CLIENT

SPECIALIST GROUTING CONTRACTOR

1

:

I

MAIN CONTRACTOR

ENGINEER! SUB-CONSULTANT

Specification for grouting works - Method or Performance specification?

Input on information requirements

Input

,

Define working sequence if appropriate

\

Input

:

t Identify key items in design

Specification for Instrumentation and Monitoring

Input on additional or separate provisions for monitoring of grouting process

I

: sthere adequate information to enable a competitive tender?

Arrange to No obtain additional information

Yes

Choose appropriate CondmonsofContract

,

I I

Agree tender list and invite tenders for main contract

:

-- - - - - - - - - - - - - - - -

i

i

Appointment of I - - specialist grouting contractor

Tender review and appointment of Main Contractor

~

Overall supervision of works

Evaluation of grout trials

Execution of in-contract grout trials and testing

I ..

..

Execution of Grouting Ivvorks including monitoring of process parameters

l

-

Interactive Monitoring and Reporting

Figure 11.1

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Decision tree setting out the responsibilities and flow of information of all parties in a conventional project with grouting works (contd)

153

Specification The grouting specification, including the necessity for preliminary trials and acceptance tests, is usually the responsibility of the engineer, but the location and geometry of grouted zones may be chosen by the engineer or the main and specialist contractor. Both method and performance specifications are used on grouting contracts, although performance specifications are more common. The forms of specification are discussed more fully in Section 11.4.

Monitoring The design and scheduling of instrumentation for passive monitoring is normally done by the engineer or the "designer" in a design-and-build contract. Instrumentation for interactive monitoring during grouting is carried out usually by the grouting specialist. Readings are taken by the engineer, the main contractor and/or the grouting specialist during the contract duration, with separate provisions made for post-contract readings if appropriate.

Supervision Supervision of the works, including definition of quality, is usually the responsibility of the engineer, with both the main and subcontractor responsible for the quality of materials and workmanship and meeting any performance specification.

Alternative contractual arrangements This report proposes an alternative set of contractual arrangements (Figure 11.2), which introduces the specialist grouting subcontractor at an earlier stage. This is more in line with the strategic thinking of the Quality Liaison Group and the Latham Report (1994).

Option A Conceptual, pre-tender stage Client/specialist grouting contractor

Contract

Contract stage Client/main contractor

Main contract

Main contractor/specialist grouting contractor*

*

Subcontract

Could be different to pre-tender stage

Option B Conceptual, pre-tender stage Client/specialist grouting contractor

Contract

Contract stage Main contractor

Contract

Specialist grouting contractor*

Contract

Client

*

Same as pre-tender stage

Figure 11.2

154

Alternative contractual arrangements

CIRIA C514

In Option A, the client appoints the specialist grouting contractor early in the conceptual and pre-tender stage under a separate contract. The client would then let the main contract for the works to a main contractor. The latter would, in the conventional manner, engage a specialist grouting subcontractor (which mayor may not be the organisation employed in the fIrst stage). This might be suitable for straightforward grouting contracts.

In Option B, a more radical proposal is advanced. During the concept and pre-tender stage, there is an initial contract between the client and the specialist grouting contractor, as in Option A. The separate contracts then continue between the client and main contractor and the client and the specialist subcontractor during the contract stage. This may be preferable for pre-contract grouting and technically diffIcult contracts.

11.4

METHOD AND PERFORMANCE SPECIFICATION Traditionally, method specifIcations are used on fIssure and permeation grouting contracts for dams, shafts, and around underground excavations. Where the purpose of the grouting is to minimise settlement or structural movement, as in compaction or compensation grouting, performance specifIcations are more common. If a method specifIcation is to be used, input from a specialist grouting contractor is always advisable. If a performance specifIcation is chosen, this can be drawn up by the

engineer in discussion with the client. However, the corollary of this action is that the specialist grouting contract has to be a design-and-construct package, which introduces an important design element too late in the overall project procurement. The temptation to produce hybrid specifIcations (which might introduce incompatible method and performance criteria) should be resisted. Method specification As a minimum, a method specifIcation should include:

CIRIA C514

•

purpose of treatment

•

zone to be treated

•

grouting technique

•

grouting system, eg tube-a-manchette

•

layout of groutholes

•

drilling procedure

•

grout type

•

required grout properties

•

grouting pressure limit

•

grouting volume limit per episode

•

maximum settlement rate to be accommodated

•

anticipated pumping rates

•

refusal criteria including minimum pumping rates

•

grouting sequence

•

anticipated grout take (as percentage of treated ground)

•

quality control methodology including data capture and report.

155

Performance specification As a minimwn, a performance specification should include the items listed below. Where the aim is to limit structural movements: •

purpose of treatment

•

identification of all sensitive structures and services

•

initiation settlement limits, typically 0-5 rom or rate of displacement

•

settlement limit, typically 15-25 mm

•

angular distortion 1:1000-1:1500

•

requirement for precise levelling or electrolevels.

Unnecessarily rigorous criteria should be avoided. Trigger values may be specified during construction that introduce further contingency grouting, eg during the secondary settlement stage when the primary influence of the cause of ground movement is over. Where the aim is to reduce permeability: •

purpose of treatment

•

zone to be treated

•

pre-treatment permeability (if available)

•

acceptable limits to post treatment permeability

•

acceptance test regime.

Where the aim is to increase shear strength or stiffness:

11.5

•

purpose of treatment

•

zone to be treated

•

pre-treatment strength or stiffness, if available

•

acceptable limit to post-treatment strength or stiffness

•

acceptance test regime.

METHOD OF MEASUREMENT AND PAYMENT Grouting contracts can be divided essentially into two activities: drilling for groutholes or driving injection pipes; and injection of grout materials. Grouting contracts may be paid as a lump sum or per unit volume of grout injected, or on a cost-plus basis or through a schedule of rates with measurement, or a bill of quantities. Lump sum contracts should be contemplated only for small and demonstrably straightforward contracts. Cost-plus is probably the most equitable basis; but most clients would be reluctant to accept this form from the start. ill certain circumstances, eg emergency work, no prior evaluation may be possible and cost-plus may be unavoidable. As the full extent of grouting is rarely known at contract award, the preferred method is for a bill of quantities with additional rates, if appropriate, for extra work. Grouting contracts are usually measured using Civil Engineering Standard Method of Measurement (CESMM, 1991). ill that publication, grouting items are dispersed under Class C - Geotechnical and other Specialist Processes, Class T - Grouting carried out from within tunnels and Class Y - Grouting carried out within sewers.

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12

Recommended good practice

This elRIA publication covers current practice, predominantly in the United Kingdom, in the use of grouting for ground engineering. The various aspects of grouting for ground engineering have been described in this document and the important issues identified. This section summarises good practice.

12.1

12.2

GROUTING CONCEPTS, PRINCIPLES AND GROUTING SYSTEM 1.

The grouting treatment technique should be designed specifically for each project.

2.

The extent and objectives of the grouting operations should be clearly defined at the outset.

3.

A comprehensive site appraisal of ground and ground water conditions should be carried out. An accurate assessment of permeability and its variation is very important. The limitations imposed by environmental constraints and the influence of grouting on adjoining structures and services should be considered.

4.

A grouting trial should be included as part ofthe site appraisal (where appropriate).

5.

The grout placement technique should be appropriate to the ground conditions and the aims of the grouting works.

6.

An observational approach should be used when carrying out grouting works, ie all observations are compared with the design parameters and assumptions. If the observations differ significantly from the design, the reason for the deviation should be investigated and the design or grout placement criteria adjusted to these conditions.

7.

The use of combinations of grouts and/or techniques should be considered where the ground is heterogeneous.

8.

Be aware of the limitations of grouting techniques used in ground engineering, eg borderline applications usually cost more and achieve least.

GROUT TYPES, MIX DESIGN AND CONTROL TESTING There are three main recommendations.

CIRIA C514

1.

The grout materials should be appropriate to the grouting technique and the purpose of grouting and they should be mixed according to manufacturers' recommendations or design trials.

2.

The quality control testing of grouts should be at two levels: an extensive testing programme at a certified laboratory, and a restricted on-site testing programme.

3.

The use of grout materials should comply with health and safety, and environmental legislation.

157

12.3

PLANT, EQUIPMENT AND CONTROL SYSTEMS The main recommendations are as below.

12.4

1.

Drilling and driving equipment should be selected so that they will not adversely affect the success of the subsequent grouting operations.

2.

The flushing medium for drilling should be selected so that it does not have a detrimental effect on the permeability of the ground.

3.

Grouting equipment and grout delivery injection methods should be compatible with the grouting techniques being used.

4.

Storage, handling and delivery procedures should recognise that grout components are sensitive to changes in temperature and humidity as well as ageing.

5.

The grouting works design should specify the control criteria and the necessary tests to be carried out to verify that the objectives of the grouting works are met.

6.

The designers should participate in the decision-making process when changes are made in the monitoring, control and testing equipment during the grouting works.

7.

Use real-time monitoring by computer systems ofthe grouting works, neighbouring structures and services but ensure there is a manual back-up for key monitoring aspects and that the data generated is reviewed at regular intervals.

GROUTING PROCESSES - PERMEATION There are two main recommendations for permeation grouting.

12.5

1.

The ground permeability and particle size distribution and their variation should be characterised as accurately as possible to aid design, application and the success of the grouting process.

2.

Reliable monitoring techniques should be used for grout injection parameters with visual output enabling a quick response by the grouting specialist to any problem or ground variations.

GROUTING PROCESSES - ROCK GROUTING In addition to the recommendations given for permeation grouting, the following are recommended for rock grouting.

158

1.

A thorough assessment of the ground and the nature of the discontinuities (structure, pattern and aperture) should be obtained.

2.

The injection pressure should be as high as possible for the rock formation to enable maximum grout penetration.

3.

Before and after rock grouting a water test should be carried out for each grouting stage to assess the reduction in permeability.

4.

In weak rock short stages are preferable as these allow higher pressures in the lower injection stages than if a whole stage was grouted in one.

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12.6

GROUTING PROCESSES - HYDROFRACTURE The following are recommended for hydrofracture grouting in ground engineering.

12.7

1.

Field tests should be carried out to determine empirical relationships and limiting values for the grout injection parameters, eg hydrofracture pressure.

2.

Tubes-a-manchette are recommended for grout injection, as they enable injection points to be revisited and provide a stable borehole with isolated and well-sealed injection points.

GROUTING PROCESSES - GROUND COMPACTION The main recommendations for ground compaction are as below.

12.8

1.

Grout-take should be monitored and used to identify zones of loose soil and areas where further injection is required.

2.

Continuous monitoring should be carried out of injection parameters, particularly the pressure build-up and grout pumping rate, together with precise survey and monitoring of surface and structural movements.

3.

In-situ CPT or SPT tests should be carried out before, during and after grout treatment to evaluate the effectiveness of the grouting programme.

GROUTING PROCESS - JET GROUTING The following recommendations are made.

12.9

1.

Spoil disposal should be considered during the design process.

2.

The grout injection parameters should be continuously monitored and spoil return inspected and tested throughout the grouting process.

3.

Double or triple jet grouting injection systems are recommended since better results are obtained compared to the single injection system.

4.

The jet-grouted element should be constructed in one injection phase.

COMPENSATION GROUTING Compensation grouting may use several grouting processes, including hydrofracture, permeation and compaction grouting. Recommendations for these grouting processes are given earlier in this section, to which may be added the following points.

CIRIA C514

1.

Ground monitoring should form an integral part of compensation grouting as the ground monitoring initially detects ground movement and volume loss can then be compensated before it reaches the ground surface as settlement.

2.

Computerised control systems should be used by the operator for large and complex compensation grouting projects to process data from grouting and monitoring equipment and enable rapid understanding (in real time) of the grouting process.

3.

A performance specification is recommended that specifies trigger and action values for settlement and angular distortions.

159

12.10

INFORMATION NEEDS AND CONTRACTUAL FRAMEWORK If grouting is to achieve its objectives, it is necessary that at all stages technical

decisions are taken by those with the necessary knowledge and experience and that the contractual framework encourages this. The following reconunendations apply. 1.

The system of procurement should include definition and placing of responsibility, definition of the contract work; and method of tender, tender review and acceptance criteria.

2.

A contractual framework should include a degree of shared responsibility among the parties, particularly in areas of design, monitoring, and adaptability during execution of the grouting works.

3.

The specialist grouting contractor or specialist adviser should be involved from project inception to completion of grouting. Where grouting is integral to the success of a project partneringlalliancing should be considered.

4.

Lump sum contracts should not be used.

5.

The preferred method of measurement is bill of quantities with rates for additional work.

Table 12.1

Checklist of common issues to be addressed when grouting is proposed and factors affecting selection of grouting for ground improvement

Checklist Definition of the purpose of grouting for ground improvement

160

2

Adequacy of information on ground and groundwater conditions to enable preliminary assessment of whether grouting is a feasible technique for ground improvement, ie use of permeation, rock, hydrofracture, compaction or jet grouting

3

Adequacy of information on the site, neighbouring structures and limitations imposed by environmental considerations to enable application of grouting techniques, equipment, plant etc

4

Adequacy of information on the ground to enable selection of grout type, materials etc

5

Need for grouting trials

6

Adequacy of information to enable the selection of grout placement, drilling system/flushing media, drilling pattern and drillhole design, grouting sequences, grouting pressure

7

Selection of adequate supervision, monitoring and control

8

Control testing of grout mix both on site and in an off-site laboratory

9

Health and safety matters

10

Grout placement with respect to drilling and driving equipment; grout preparation - storage, batching and mixing; control tests; pumping and delivery; injection methods; monitoring and control etc

11

Information and contractual framework with respect to: • • •

parties involved roles and informational needs responsibilities of the parties

• •

method or performance specification method of measurement and payment.

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13

References

AFfES (1991) Recommendations on grouting for underground works Tunnelling and Underground Space Technology, V016, No 4, pp 383--461 ASCE (AMERlCAN SOCIETY OF CIVIL ENGINEERS) (1980) Preliminary Glossary of Terms Relating to Grouting ASCE Journal of Geotechnical Engineers, Vol 106, No GTI, July, pp 803-815 ASCE (AMERlCAN SOCIETY OF CIVIL ENGINEERS) (1982) Proceedings of the ASCE Conference on Grouting in Geotechnical Engineering, New Orleans, ASCE, New York ASCE (AMERlCAN SOCIETY OF CIVIL ENGINEERS) (1985) Issues in Dam Grouting Geotechnical Engineering Division, ASCE, New York, 1985 ASCE (AMERlCAN SOCIETY OF CIVIL ENGINEERS) (1992) Proceedings of ASCE Speciality Conference; Grouting, Soil Improvement and Geosynthetics, New Orleans ASCE, New York, 1992 BAEZ ,J I and HENRY, J F (1993) Reduction of Liquefaction Potential by Compaction Grouting at Pinopolis West Dam, SC In: Proceeding of Geotechnical Practice in Dam Rehabilitation ASCE Geotechnical Special Publication No 35, pp 493-506 BELL, A L and BURKE, G K (1994) The compressive strength of ground treated using triple system jet grouting In: Proceedings of Conference on Grouting in the Ground, ICE, London, November 25-26, pp 525-538 BOSCARDIN, M D and CORDING, E J (1989) Building response to excavation induced settlement Journal of Geotechnical Engineering, ASCE, Vol 115, No 1 BOULANGER, R W and HAYDEN, R F (1995) Aspects of compaction grouting of liquefiable soil Journal of Geotechnical Engineering, ASCE, Vol 121, No 12, P 844 BRITISH STANDARDS INSTITUTION (1983) Method for determination of slump BS 1881, Part 102 BRITISH STANDARDS INSTITUTION (1989) Code of Practice for Site Investigation BS 5930:1989, BSI, London BRlTISH STANDARDS INSTITUTION (1993) Glossary of Building and Civil Engineering Terms

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161

BROOKS, S A and JOHNSON, D (1994) Grout Standards and Specifications (Cementitious Grouts) Society of Chemistry and Industry, Grouts and Grouting BUILDING RESEARCH ESTABLISHMENT (1990) Assessment of damage in low-rise buildings with particular reference to progressive foundation movement BRE Digest 251, August CAMBEFORT, H (1977) The principles and applications of grouting Quarterly Journal of Engineering Geology, Vol 10, Part 2, pp 57-95 CARON, C (1982) The State of Grouting in the 1980s - Background Talk In: Proceedings ASCE Conference on Grouting in Geotechnical Engineering, New Orleans, pp 346-358 CARRUTHERS, D, COUTTS, D, McGOWN, A and GREENWOOD, D (1994) Background to the design of the quay wall stabilisation works at Kingston Bridge, Glasgow In: Proceedings of Conference on Grouting in the Ground, ICE, London, November 25-26, pp 417-432 CHIN, C Y (1996) An experimental study of hydrofracture in soils PhD thesis, University of Cambridge CIRIA (1995) CDM Regulations - case study guidance for designers: an interim report Construction Industry Research and Information Association, Report 145, London CONSTRUCTION (DESIGN AND MANAGEMENT) REGULATIONS (1994) Statutory Instrument 1994/3104 HMSO, London, 1994 THE CONTROL OF SUBSTANCES HAZARDOUS TO HEALTH REGULATIONS (1988) SI 1988/1657 as amended by SI 1991/2431 HMSO, London, 1988 COUTTS, D, HUTCHINSON, D E and ESSLER, R D (1994) Specification, planning and construction of quay wall stabilisation works at Kingston Bridge, Glasgow In: Proceedings of Conference on Grouting in the Ground, ICE, London, November, 2526, pp 433-453 CRAWLEY, J D and POLLARD, C (1992) Ground treatment to improve tunnel progress on the Channel Tunnel marine drives Ground Engineering, Vol 25, No 1, January/February, pp 27-35

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CROCKFORD, R M, ESSLER, R D and REED, B A (1994) Injection grouting to overcome artesian water pressure at Esholt Sewage Works In: Proceedings of the Conference on Grouting in the Ground, ICE, London, November 25-26, pp 1-14 DEERE, D U and LOMBARDI, G (1985) Grout Slurries - Thick or Thin In: Baker, W H (ed) Issues in Dam Grouting ASCE Geotechnical Engineering Division, New Orleans, pp 279-300 THE ENVIRONMENTAL PROTECTION ACT (1990) HMSO, London, 1990 ESSLER, R D (1995) Applications of jet grouting in Civil Engineering Engineering Geology of Construction Geological Society Special Publication No 10, pp 85-93 EUROPEAN STANDARD FOR GROUTING, Final Draft (1996) Prepared by Working Group (WG6) of the Technical Committee on the Execution of Special Geotechnical Works (TC 288), February EWERT, F K (1992) The individual groutability of rocks In: Water Power and Dam Construction, January, pp 23-30, London FRANCESCON, M and TWINE, D (1994) Treatment of solution features in Upper Chalk by compaction grouting In: Proceedings of the Conference on Grouting in the Ground, ICE, London, November 25-26, pp 327-349 GRAF, E D (1992) Compaction Grout, 1992 In: Proceedings of the Conference on Grouting, Soil Improvement and Geosynthesis, New Orleans, ASCE Special Publication No 30, pp 275-287 HARRIS, D L, MAIR, R J, TAYLOR, R N and HENDERSON, T 0 (1994) Observations of ground and structure movements for compensation grouting during tunnel construction at Waterloo station. Geotechnique, Vol 44, No 4, pp 691-713 HARRIS, M and HERBERT, S (1994) Contaminated Land; Investigation, Assessment and Remediation ICE Design and Practice Guide HARRIS, R R Wand COTET, P (1994) Evaluation of computer control and analysis of grouting parameters during grouting works In: Proceedings of the Conference on Grouting in the Ground, ICE, London, November 25-26, pp 237-245

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HEALTH and SAFETY EXECUTIVE (1994) Managing construction for health and safety: Construction (Design and Management) Regulations Approved Code of Practice (1994) HSE, L54 HEALY, P R and HEAD, J M (1984) Construction over abandoned mineworkings Consbuction Industry Research and Information Association, Special Publication 32 HEWLETI, P C AND HUTCHINSON, M T (1983) Quantifying Chemical Grout Performance and Potential Toxicity In: Proceedings of the Eighth European Conference on Soil Mechanics and Foundation Engineering, Helsinki, Vol 1, pp 361-369 HOULSBY, A C (1976) Routine Interpretation of the Lugeon W ater-Test Quarterly Journal of Engineering Geology, Vol 9, The Geological Society, London, pp 303-313 HOULSBY, A C (1985) Cement Grouting: Water minimising practices In: Issues in Dam Grouting Geotechnical Engineering Division, ASCE, New York HOULSBY, A C (1990) Construction and Design of Cement Grouting: A Guide to Grouting in Rock Foundations Wiley InterScience, New York INSTITUTION OF CIVIL ENGINEERS (1991) Civil Engineering Standard Method of Measurement 3rd edition, Thomas Telford, London INSTITUTION OF CIVIL ENGINEERS (1994) Grouting in the Ground In: Bell. A L (ed) Proceedings of the conference on Grouting in the Ground, ICE, London, November 25-26,658 pp. 1992 INSTITUTION OF CIVIL ENGINEERS (1995) Conditions of contract and forms of tender, agreement and bond for use in connection with works of civil engineering construction Fifth edition, 1973. revised 1979; reprinted with amendments 1986 Sixth edition, 1991; reprinted with amendments 1995 ISCHY, E and GLOSSOP. R (1962) An Introduction to Alluvial Grouting In: Proceedings Institution of Civil Engineers, Session 1961-1962, London, Vol 21. Paper No 6598, March, pp 449-474 ISO 6707/1 (1989) Building and Civil Engineering Vocabulary, Part 1 General Terms International Organisation for Standardization

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KAROL, R H (1996) Chemical Grouting Marcel Dekker Inc, 2nd edition, New York LATHAM, M (1994) Constructing the team: Joint review ()fprocurement and contractual arrangements in the UK construction industry HMSO, London LINNEY, L F and ESSLER, R D (1994) Compensation grouting trial works at Redcross Way, London In: Proceedings of the Conference on Grouting in the Ground, ICE, London, November 25-26, pp 313-349 LINNEY, L F and FRIEDMAN, M (1996) Protection of buildings from tunnelling induced settlement using permeation grouting In: Mair and Taylor (eds) Geotechnical Aspects of Underground Construction in Soft Ground, Balkema LITTLEJOHN, G S, INGLE, J and DADASBILGE, K (1984) Improvement in Base resistance of Large Diameter Piles Founded in Silty Sand In: Proceedings of the Eighth European Conference on Soil Mechanics and Foundation Engineering, Helsinki, Vol 1, pp 153-156 MAIR, R J, RANKIN, W J and ESSLER, R D (1995) Compensation grouting In: Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, 113, January, pp 55-57 POTOTSCHNlK, M J (1992) Settlement Reduction by Soil Fracture Grouting In: Proceedings of the Conference on Grouting, Soil Improvement and Geosynthetics, New Orleans, ASCE Special Publication No 30, pp 398-409 RAABE, E W and ESTERS, K (1990) Soil fracturing techniques for terminating settlements and restoring levels of buildings and structures Ground Engineering, May, pp 33-45 RAFFLE, J F and GREENWOOD, D A (1961) The Relation Between the Rheological Characteristics of Grouts and Their Capacity to Permeate Soil In: Proceedings of the 5th I nternational Conference on Soil Mechanics and Foundation Engineering, Paris, Vol 2, pp 789-793 RANKIN, W J (1996) Recent developments in compensation grouting Tunnels and Tunnelling, May, pp 36-38 SCHMERTMANN, J H, BAKER, W, GUPTA, R and KESSLER, K (1986) CPTIDMT QC of ground modification at a power plant In: Proceedings of In-situ '86, Special Conference, ASCE, New York, pp 985-1001

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SCHMERTMANN, J H and HENRY, J F (1992) A Design Theory for Compaction Grouting In: Proceedings of the Conference on Grouting, Soil Improvement and Geosynthetics, New Orleans, ASCE Special Publication No 30, pp 215-229 WARNER, J (1982) Compaction grout: rheology vs. effectiveness In: Proceedings of the Conference on Grouting, Soil Improvement and Geosynthesis, New Orleans, ASCE Special Publication No 30, pp 229-239 WATER RESOURCES ACT (1991) HMSO, London, 1991 WHEELER, P (1996) Richmond's jet set Ground Engineering, January/February, pp 18-19 WRIGHT, R H, LITTLEJOHN, G S, and SOLA, P (1996) Ground Treatment at Westminster, London A VA News, March, Minneapolis, USA YOGI, M (1996) Strainer grouting: step down grouting with Strainer tube In: Proceedings of IS-Tokyo '96, Second International Conference on Ground Improvement Geosystems, Grouting and Deep Mixing Tokyo, Balkema, Voll, pp 717-720

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CIRIA Core Programme members February 2000 Alfred McAlpine Construction Ltd

IMC Consulting Engineers Ltd

AMEC Pic

Institution of Civil Engineers

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BAA pic

Keller Ground Engineering

Bachy Soletanche Limited

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London Underground Limited

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British Nuclear Fuels Ltd

Miller Civil Engineering Ltd

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carillion

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Mott MacDonald Group Ltd

Cementitious Slag Makers Association

National Power PLC

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Davis Langdon & Everest

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Department of the Environment, Transport and the Regions

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Dudley Engineering Consultancy

Scott Wilson

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Environment Agency

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Southern Water Services Ltd

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Taylor Woodrow Construction Holdings Ltd

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Highways Agency, DETR

Wardell Armstrong

HJT Consulting Engineers

WS Atkins Consultants Limited

HR Wallingford Ltd

Yorkshire Water Services Limited

Hyder Consulting Limited

~' I?.'!;M: R TRANSPORT

REGIONS

The Construction Directorate of the DETR supports the programme of innovation and research to improve the construction industry's performance and to promote more sustainable construction. Its main aims are to improve quality and value for money from construction, for both commercial and domestic customers, and to improve construction methods and procedures.

The aim of this book is to improve understanding of grouting techniques and thereby to encourage their proper use. After explaining the main grouting techniques and the principles behind them, the book explores concepts such as groutability, the classification and chemistry of grouts. and grouting plant and equipment. Permeation. rock grouting, hydrofracture, ground compaction, jet grouting and compensation grouting are examined in more detail, illustrated throughout with case studies. The principles and applications of each technique are described, the plant and equipment detailed. typical injection-hole layouts provided. and information given on monitoring and site operational requirements. The book also covers typical contractual relationships between the various parties in a grouting contract. and sets out their responsibilities. It concludes with a number of recommendations for improving grouting practice. This book will help all those involved in the procurement and use of grouting in ground engineering. including geotechnical and civil engineers. contractors. consultants and clients.

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II .ill

ISBN 0 860 I 7 514 6