Bearing Piles and Groups

University of Bolton School of Built Environment and Engineering BEARING PILES AND GROUPS CONTENTS: 1. Introduction 2. Types of bearing pile 3. Pile types in more detail. 4. Load capacity of the pile shaft 5. Load capacity of the soil. a. Capacity by calculation from soil properties i. Ultimate capacity 1. General principles 2. Piles in cohesive soils 3. Piles in non-cohesive soils 4. Time effects ii. Working capacity, factors of safety b. Capacity from driving formulae 6. Settlement of Piles 7. Pile testing a. Load testing b. Indirect testing methods 8. Tolerances, spacing, pile caps and ground beams 9. Piles in tension 10. Downdrag (negative skin friction) 11. Laterally-loaded piles 12. Capacity of pile groups. NB: These notes do not cover retaining walls formed by sheet piles or diaphragm walls 1

Introduction

The function of a bearing pile is to transfer loads to lower levels of the ground which are capable of sustaining the load, with an adequate factor of safety and without settling at the working load by an amount detrimental to the supported structure Bearing piles are used: Where adequate bearing soil is at low depth Where loading is uneven, thus making the use of a raft unadvisable In shrinkable clay soils, where loads can be transferred to below the zone of shrinkage Piles are normally used in compression. Sometimes piles have to carry tension, as shown in the diagrams of a piled quay on the next page. Generally the lateral load capacity of piles is much less than their capacity for tension or compression.

Bearing Piles and Groups – October 2010

Quay

Ship Impact

Pile in Tension

Quay

Ship Pull

Pile in Compression

Pile in Compression

Pile in Compression

Pile in Tensionn

Illustrations of Tension Piles used to Sustain Lateral Forces 2

Types of Bearing Pile

Plies can be classified: A By method of installation: Bored piles Driven piles. B By the way that the soil is moved to make way for the pile: Displacement Piles Replacement Piles C By the way that they carry load : Friction piles End-bearing piles Combined friction and end-bearing piles. D By material and installation Steel piles Pre-cast concrete piles Cast-in-situ concrete piles Timber piles. 3

Pile types in more Detail

Displacement Piles The pile is driven, jacked or vibrated into the ground. The soil is displaced outwards but is not actually removed: Displacement Piles: Pre-formed These are driven by heavy hammer blows until the required “set” is achieved. (ie number of blows for 100mm penetration). Various devices (pile frames, hanging leaders, suspended hammer guides) are used to keep the pile upright (or raked) and to align the hammer with the top of the pile. Pile hammers may be “drop hammer”, “single-acting hammer” or “double acting hammer”. The first of these is a simple weight, the other two are specially designed machines, normally hydraulically powered, that can deliver blows much faster than a drop hammer. To prevent damage to the top of the pile, a cushion or helmet is used. Bearing Piles and Groups – October 2010

Water-jetting is sometimes used to aid pile penetration, and vibration is and alternative driving method in granular soils. Where sufficient dead-weight or reaction is available, piles can be jacked into place. Driven Pre-Formed Piles Steel

Pre-Cast Concrete

Notes H piles Tube piles Box piles Long Piles

H O

Small displacement

Made on site: heavily reinforced or prestressed to withstand handling and driving Modular jointed piles West “Hardrive” Johnson “Herkules” Timber Limited length A special case is the screw pile, in which the helical head is both the boring device and the loadbearing element. These are mainly used in sand, and are found under Victorian seaside piers. Displacement Piles: Driven Cast-in-Place A casing with a closed end is “bottom driven” into the ground to a set or a pre-determined depth. The casing may be temporary or permanent. Temporary casing. The casing is normally steel, and the closed end is a plug of dry concrete or gravel which is driven out at the required depth. The empty casing can then be inspected from the top. The pile is filled with concrete and the casing is carefully withdrawn. The correct driving and extraction of the casing is a skilled job. Permanent casing. The casing is usually a stack of pre-cast concrete tubes, sometimes a steel tube. The closed end is a steel shoe. After driving the empty casing can be inspected from the top. The casing is then filled with concrete. Replacement Piles can be divided into Bored and cast-in-place piles Drilled-in Tubular Piles. Replacement Pile: Bored and Cast-in-Place By Mechanical Auger. In stable ground, an unsupported hole can be drilled with a mechanical auger. In suitable ground an enlarged base, or under-ream can be formed. A light reinforcement cage, if required, is placed in the hole, followed by concrete. In water-bearing soils support is required to the sides of the hole. This is provided by temporary casings (often only for the top part of the PILE) or by bentonite slurry, or by some combination of these. If concrete is placed under water or under bentonite slurry, it must be fed to the bottom using a tremie pipe. Thus delivers the fresh concrete below the surface of the concrete that is already poured, so the concrete never falls free through the water. Care is needed to lift the tremie pipe so that it remains within the concrete at all times, and failure to achieve correct pouring may result in necking or waisting of the pile.

Bearing Piles and Groups – October 2010

By Continuous Flight Auger. The CFA is drilled into the ground to the correct depth. A cement-sand mortar is then pumped down the hollow stem of the auger to fill the void as the auger is slowly withdrawn while still rotating. The sides of the bore are first supported by the auger and the soil, then by the mortar as it is pumped in. If required, a cage of reinforcement can be pushed into the top of the fluid mortar (max length about 12m).. By Percussion Rig. For small and medium diameter piles. A conventional cable percussion rig (similar to those used for site investigation borings) is used. Appropriate granular soil shells or clay cutters are used, with a casing. The hole is filled with concrete and the casing is withdrawn. Although this is a labour-intensive technique, it can be used in cramped conditions and where headroom is limited. Replacement Piles: Drilled-in Tubular Piles or Caission Piles These have a robust permanent casing, normally steel, which is pushed or drilled into the ground. The soil inside the casing is removed by grabbing, augering or other methods. Large machines can install 1m diameter casings, but the technique can be expanded to almost any size with complete bridge foundations being formed in a single caisson that is driven down by deadweight. Classically large caissons were kept dry by compressed air as men worked to excavate the soil inside, but the hazards of compressed-air working make this technique unacceptable today. Note that modern piling machines are versatile, and can drive many different types of pile. A Categorisation of Pile Types Pile Types

Displacement

Replacement

Driven Cast-in-Situ Concrete

Permanent Casing

Pre-Formed

Temporary Casing

Concrete

Normal Reinforced Concrete

Type Pre-Formed Displacement Piles

Materials Steel Timber Pre-Cast Concrete

Unsupported

Timber

Pre-Stressed Concrete

Box

Steel

Tune

CFA

Supported

Permanent Casing

H Section

Advantages Inspected for quality and soundness before driving. Not liable to squeezing or necking. Construction not affected by groundwater. Can be left protruding (useful for marine applications) Can withstand high bending and tensile stresses. Can be driven in long lengths

Screw

Temporary Support

Casing

Drilling Mud or Water

Disadvantages Unjointed types cannot easily be varied in length. May break or bend during driving. Uneconomic if the design is governed by driving stresses rather than working stresses, Noise and vibration during driving. Displacement of soil may damage adjacent installations. Cannot be driven in low headroom.

Bearing Piles and Groups – October 2010

Driven Castin-Place Piles

Concrete

Length easily adjusted Groundwater can be excluded by a driven closed end. Enlarged base possible. Design governed by working conditions. Noise and vibration reduced by internal hammer.

Bored and Cast-in-Place Piles

Concrete

Length easily adjusted Removed soil can be inspected for comparison with design data. Very large bases can be formed in favourable ground. Drilling tools can break up boulders and other obstructions. Pile is designed to working stresses. Very long lengths possible. Little noise and vibration during construction. No ground heave.

Choice of Pile Materials

Timber

Cheap and easy to handle Durable below water table. Suits a range of pile types Resistant to aggressive conditions Resistant to reasonably hard driving.

Concrete

Steel

High strength Easy to handle Can be driven hard. Little ground displacement

Where temporary tubes are used, necking is possible. Concrete cannot be inspected after installation. Length may be limited if tubes are to be extracted. Displacement of soil may damage adjacent installations. Noise and vibration during driving. Pile liable to squeezing and necking in soft ground. Special techniques needed for concreting in water-bearing ground. Concrete cannot be inspected after installation. Enlarged bases cannot be formed in collapsible soil. Cannot easily be extended above ground. Boring may cause loss of ground and settlement at adjacent structures.

Decays above water table Limited lengths Unsuitable for heavy loads Pre-cast piles can be damaged by very hard driving. Cast-in-place piles can suffer from necking or from poor concrete quality. Expensive May require protection from corrosion.

Bearing Piles and Groups – October 2010

4

Load Capacity of the Pile Shaft

The axial load capacity of the pile shaft is not usually critical. Although the pile is a slender column, buckling will be prevented by the upper layers of soil unless these are very weak (cu < 20 kN/m2) Working stresses (calculated as axial force/area) on piles should not exceed: Concrete piles: fcu/4 fcu is the cube strength of the concrete, typically 35 N/mm2 for cast-in-place piles or 45 N/mm2 for pre-cast piles. Steel piles py/3 py is the yield strength of the steel, typically 275 N/mm2 Driven piles have to resist installation damage, and so pre-cast concrete piles may need to be stronger than is required for the permanent load. 5

Load Capacity of the Soil:

5a

Capacity by Calculation from Soil Properties

5(a)(i) Ultimate Load Capacity 5(a)(i)(1)

General Principles

The Ultimate Load Capacity of a pile is: Shaft Resistance plus Base Resistance Qu = Qs + Qb

where Qu = ultimate total resistance, kN Qs = ultimate shaft resistance, kN Qb = ultimate base resistance, kN

Qs =

where

sAs

Qb = qbAb

= ultimate skin friction or cohesion, kN/m2 ( s will vary down the pile.) As = area of shaft = dL (d = shaft diameter, L = shaft length) s

where qb = end bearing resistance kN/m2 Ab = base cross sectional area = D2/4 (D = base diameter)

Typical amounts of movement to develop the full resistances are: 1% to 2% of diameter for skin friction 10% to 20% of diameter for end bearing. It is common to apply different factors of safety to the two resistances. In layered soil, the skin friction resistance is the sum of the resistances in the various layers. Skin friction is ignored for the depth of any under-ream.

Bearing Piles and Groups – October 2010

End bearing resistance depends on the soil at the base of the pile. If this soil is underlain by a layer of weaker soil less than 10 pile diameters below the pile base, the end bearing resistance is reduced (see box below). Qstrong = base resistance in strong soil Qweak = base resistance in weak soil. H = depth of strong soil beneath the pile base D = diameter of pile base.

Q

Qb = Qweak + (Qstrong – Qweak) H/10D but not more than Qstrong Base Capacity in Strong Soil above a Layer of Weak Soil (after Meyerhoff)

5(a)(i)(2)

Qstrong

Pile Base

Strong Soil H

10 x Pile Diameter Weak Soil

Qweak Depth

Piles in Cohesive Soils

If cohesion does not vary with depth Qs =

ca dL

where cu = cohesive strength of clay (kN/m2) is an adhesion factor (not more than 1) L = length of pile in clay (m) d = pile diameter (m)

If the cohesion varies with depth, we replace this by Qs = ( cu dL) or d

( cuL)

Now add the base resistance to get

Ultimate Load Capacity of a Single Pile in Clay Qu = cuNcAb + d ( cuL) Now we have the tricky task of determining values for all the bits of the formula for Qu. Parameter Commentary cu The undrained shear strength of the clay. This may vary with depth. If the clay is fissured, laboratory cu values should be reduced to represent the fissured shear strength. Skempton (1966) suggested reduction factors: fissured clay, d < 0.9m. reduction factor 0.8 fissured clay, d > 0.9m. reduction factor 0.75

Nc Ab

Under Eurocode 7 cU is modified by a materials factor m of 1.5 to 1.8 for both the shaft and base. If pile depth in bearing stratum > 5 base diameter, Nc = 9 This is easy. Ab = D2/4, where D = base diameter OK? Bearing Piles and Groups – October 2010

d

Shaft diameter Symbol indicates adding The cohesion between the pile and the clay may be less than cu (See Note 1). The factor takes account of this. Ways of finding : 1. Simple Values for bored piles Soft and firm clays = 1.0 London clay – long piles = 0.45 London clay – short piles = 0.30 Underreamed piles = 0.30

(Vesic 1977) (Skempton 1959) ( .. ) (Tomlinson 1995)

2. Tomlinson (driven piles) See Barnes Figures 10.3 (short piles) or 10.4 (long piles). 3. Weltman and Healy (driven piles in boulder clays) See Barnes Figure 10.5 for predictions of based on cu 4. Semple and Rigden (driven piles) For values based on the soil strength ratio cu/ v and the length/diameter ratio L/d, see Barnes Figure 10.7. Note that Semple's pF corresponds to the used here.

cu L

See Note 2 on maximum adhesion values. See above. See also Note 2. Length of pile shaft. Divide the pile shaft into suitable lengths and calculate average friction over each length.

Note 1 Softening of the clay may be caused by disturbance during piling. swelling in the unconfined pile bore, groundwater, water from in-situ concrete Note 2 - Maximum Adhesion Values Suggested maximum values for adhesion cu are Most clays 100 kN/m2 (Skempton, Vesic) Glacial Clays 70 kN/m2 (Weltman and Healy)

Bearing Piles and Groups – October 2010

Under-Reamed Piles. The diameter D used for calculating the base resistance is greater than the diameter d of the shaft. Skin friction on the under-ream is ignored: (see diagram).

d Shaft resistance used

Shaft resistance ignored Base resistance used D Under –Reamed Pile

Design Charts for Piles in Cohesive Soils

Bearing Piles and Groups – October 2010

Design Charts for Piles in Cohesive Soils

Bearing Piles and Groups – October 2010

5(a)(i)(3)

Piles in Non-Cohesive Soils

If friction did not vary with depth fs = ultimate skin friction of sand (kN/m2) d = pile diameter (m) L = length of pile in sand (m)

Qs = fs dL

Assuming: A vertical effective stress A coefficient Ks, so that horizontal stress A coefficient of friction between the pile and the soil Then the ultimate skin friction fs

= 'v kN/m2 = 'vK s = tan( ) = 'vKs tan( )

The vertical stress, and so the friction, will increase with depth, so we replace the formula for Qs by Qs = ( 'vKs tan( ) dL), where we add the resistances for the different depths. Normally d will be constant, so we can write Qs = d ( 'vKs tan( )L), Now add the base resistance to get

Ultimate Load Capacity of a Single Pile in Sand Qu = Abq0Nq + d ( 'vKs tan( )L) Now we have to determine values for all the bits of the formula for Qu. Parameter Ab q0 Nq

d 'v Ks tan( ) Kstan(

L

Commentary Ab = D2/4, where D = base diameter Vertical effective pressure in the soil beside the base of the pile.(but see Note 1 on Critical Depth) Bearing capacity factor (see Note 2). Values from Beresantzev are shown in Barnes Figure 10.8. The value of ' after installation should be used: see Note 3. Shaft diameter Symbol indicates adding Effective Vertical Stress (but see Note 1 on Critical Depth) Coefficient of Horizontal Effective Stress Coefficient of friction between the pile and the soil. As an alternative to determining Ks and tan( ) separately, Poulos has published a direct correlation between ' and Kstan( ). See Barnes Figure 10.12 Length of pile shaft. Divide the pile shaft into suitable lengths and calculate average friction over each length.

Bearing Piles and Groups – October 2010

Note 1 - Critical Depth. The equations for Qb and Qs suggest that skin friction and bearing capacity increase without limit as the depth increases. Field tests show that both reach peak values at a "Critical Depth" zc of between 10 and 20 diameters. Barnes Figure 10.10 suggests values of Critical Depth zc based on the value of ' after installation. See note 3. Note 2 - Bearing Capacity Factor. The Nq values used here are equivalent to Terzaghi's (Bearing Capacity Factor) (Depth Factor) Note 3: Values of before and after installation. The initial value of the soil can be estimated from site data (SPT or qc tests) using Barnes Figure 10.11. Note that the SPT or qc values should be sampled to below the pile base. will change as the pile is installed. Values of after installation can be estimated from this table (Poulos 1980) Value of after installation ( I = value before installation) Bored Piles Driven Piles For finding Nq ( I + 40 )/2 I-3 For finding zc 0.75 I + 10 For finding K0tan( ) I Design Charts for Piles in Non-Cohesive Soils

Bearing Piles and Groups – October 2010

Design Charts for Piles in Non-Cohesive Soils

Bearing Piles and Groups – October 2010

5(a)(i)(4)

Time Effects

Driving piles into soft clays increases the pore-water pressure and so reduces the effective stress. As the excess pore water pressure dissipates, the bearing capacity of the pile increases. Stiff clays are cracked and heaved by the driving, and vibrations form an enlarged hole which fills with groundwater. Some strengthening occurs as the disturbed soil stiffens. The diagram below suggests that it may take between one day and a year for the bearing capacity to stabilise.

Diagram from Vesic/Tomlinson

Gain of Carrying Capacity with Time: Driven Piles in Soft to Stiff Clay (after Vesic, figure from Tomlinson 1995)

Bearing Piles and Groups – October 2010

5(a)(ii) Working Load Capacity, Factors of Safety According to the Code BS8004, “The main function of bearing piles is to transfer the load to lower levels of round which are capable of sustaining the load with an adequate factor of safety and without settling at working load by an amount detrimental to the structure they support” It suggests Working load for single pile = Ultimate Bearing Capacity

FoS

where FoS is normally 2 to 3 The ultimate bearing capacity may be taken to be that load applied to the head of the pile which causes the head of the pile to settle 10% of the pile diameter Tomlinson (1987) suggested that for piles up to 600mm dia, if an overall FoS of 2.5 is adopted, then the settlement of the pile under working load is unlikely to exceed 10mm. In the case of large bored piles, particularly ones with enlarge bases, it is advisable to take into account the different resistance/settlement relationships of the shaft and base when calculating the working load by applying different load factors to the calculated ultimate resistances of the shaft and base. Burland et al (1966) suggested that, for bored piles in London Clay, the allowable working load, Qa, is the smaller of : Qa = Qult = Qs + Qb 2 2

or

Qa = Qs + Qb 3

where Qs = Ultimate shaft resistance Qb = Ultimate base resistance The first expression tends to govern design for straight sided piles, the second for large underreamed piles. Where there is less certainty about ground conditions, loads and pile construction effects, higher factors of safety should be applied. EC7 also gives separate factors for base and shaft resistance: EC7: Partial Safety Factors b, for Base Resistance Driven Piles 1.3 Bored Piles 1.6 CFA Piles 1.45

s for Shaft Resistance 1.3 1.3 1.3

t for Total Resistance 1.3 1.5 1.4

5(b) Capacity from Driving Formulae An accepted in-situ test for soil is the Standard Penetration Test (SPT) which measures the number of blows needed to drive a standard cone 300mm. By analogy, it is suggested that the capacity of a driven pile can be determined by its set, the number of blows needed to drive it through a certain distance. In effect, the installation of the pile IS the site investigation. Bearing Piles and Groups – October 2010

For design purposes, the relationship is stated the other way round: “For a driven pile of ...... diameter, the safe working load will be ........ kN if it is driven to a set of ...... blows per 100mm” Intuitively, there must be some rationality behind this approach. A pile which is hard to drive is likely have a higher bearing capacity than a pile which is easy to drive. For piles which are driven through a weak layer of soil to become endbearing on a stronger stratum at greater depth, the approach makes more sense than trying to predict pile lengths from a limited site investigation. However the approach has drawbacks. First we look at two common driving formulae. The Sanders Driving Formula. This is based on the idea simple idea that the energy in the hammer blow is absorbed by the pile moving through the soil. Assume The required ultimate load capacity of the pile is U kN The weight of the hammer is W kN The height of drop of the hammer is h metres. Then each hammer blow delivers W h kNm of energy. When the pile has sufficient ultimate capacity, each blow will push the pile S = Wh/U metres into the soil. The formula U = Wh/S was published by Sanders in about 1850. Example:

Required safe working load on pile = 30 tonnes Required factor of safety = 2.5 3 tonne hammer, W = 30kN 200mm drop, h = 0.2m

So

Required ultimate load capacity 30 x 2.5 = 75 tonnes , U =750 kN Set per blow = 30 0.2/750 = 0.008m. Set for 10 blows = 80mm

The Hiley Driving Formula. The Hiley Formula takes a slightly more sophisticated approach and assumes: 1. Only a certain percentage (usually 50% to 70%) of the hammer energy is usefully delivered to the pile. Say this efficiency is n (n = 0.5 to 0.7) 2. Some of the energy causes elastic deformation of the soil and pile. This elastic deformation is called the temporary compression and about half is recovered after the blow. Say this temporary compression is c metres. From this we get the Hiley Driving Formula, also known as the Danish Formula: U = nWh/(S + c/2) or S = nWh/U – c/2 In the example, taking n = 0.6 (60% efficient) and c = 0.005m (5mm) Set per blow = nWh/U – c/2 = 0.6 30 0.2/750 – 0.005/2 = 0.0023m. Set for 10 blows = 2.3mm Bearing Piles and Groups – October 2010

There are no reliable ways of estimating n and c. Terzaghi and Peck state that “no satisfactory relation exists between the capacity of the piles as determined by load tests and as calculated from the formula”, and that for 2% of piles installed using the formula the factor of safety may be as low as 1.2 or as high as 30. The formulas are unreliable because in all piles except those driven to bedrock, and especially those in soft cohesive soils, strength increases after driving. Piles in permeable soil should not be tested until 2 to 3 days after driving; in impermeable soils a delay of a month may be required. For friction piles in soft clay, the driving resistance is almost constant with depth and application of a driving formula would suggest that the ultimate load capacity does not increase with depth. In fact the ultimate load capacity increases almost in direct proportion to the depth, so the driving formulae are of no use. To quote Terzaghi and Peck again, “in Shanghi and New Orleans ….. no experienced engineer would even consider using a pile formula”. 6

Settlement of Piles

As noted above, ultimate skin friction resistance may be developed at settlements of 1% to 2% of pile diameter, and ultimate end-bearing resistance at 10% to 20% of pile base diameter. As the working load is likely to be 1/2 to 1/3 of the ultimate load, these figures do not assist in predicting working load settlements. If the soil behaviour is assumed elastic, then settlement can be predicted using this equation from Tomlinson (equation 7.15)

WS 2

WB

L AS EP

WB 4 AB

Load carried by shaft Load carried by base

B(1 2 ) IP EB WS WB

The first term gives the elastic shortening of the pile: Pile Length L Pile Shaft Area As Pile Elastic Modulus EP The second term gives the settlement at the base: Area of pile base Pile base diameter Soil Elastic Modulus Soil Poisson's Ratio Influence factor

AB (equal to AS unless the pile is under-reamed) B EB ) beneath base ) IP (related to L/B)

For most piles and soils ( between 0 and 0.25, L/B >5), the equation can simplified to:

WS 2

WB

L AS EP

WB 2 AB

B EB Bearing Piles and Groups – October 2010

More sophisticated methods are available which take account of shaft slip before base failure. Similarly the effect of layered soils can be included. The settlement of a pile in a group is likely to be greater than the settlement of an isolated pile carrying the same load. See later notes. 7

Pile Testing

Once a pile is installed it can be tested, directly or indirectly, to determine its load capacity. Direct load tests are much more reliable than indirect tests, but much more expensive. There should be a reasonable delay between installation and testing, to allow the soil strength to develop. Suitable delays are: Piles in granular soil: 2 days Piles in silt or clay: 1 month 7(a) Direct Load Testing The two main types of test are: i) Constant Rate of Penetration Test (CRP) ii) Maintained Load Test (MLT) Both these tests are normally carried out on preliminary test piles, and sometimes on a proportion (perhaps two per hundred) of the working piles. Constant Rate Penetration Test The compressive force is increased to cause the pile to penetrate the soil at a constant rate until failure occurs. The ultimate load is therefore determined and the factor of safety with respect to the design working load calculated. Typical penetration rates are 0.75 mm/min for piles in clay, and 1.5 mm/min for piles in sand. (BS8004) The "failure load" for the pile is the lesser of: the load at which settlement increases without significant increase of load the load which produces a settlement of pile diameter/10 The pile will pass the test if its failure load is at least the specified value. This test does not give a good indication of the settlement of the pile under long-term service load. Maintained Load Test Results are obtained by measuring deflections of the pile as loads are added and relieved. A typical loading sequence for a "proof load" test is: 0, 0.5WL, 0, 0.75WL, 0, 1.0WL, 0, 1.25WL, 0, 1.5WL, 0. Typical Maintained Load tests are shown in this diagram.

Bearing Piles and Groups – October 2010

Kentledge

Beam

Beam Jack Load Cell

Jack Load Cell

Deflection Gauge

Deflection Gauge Tension Pile

Tension Pile

Test Pile

Kentledge Test

Test Pile

Tension Pile Test

In both cases it is important that the deflection gauge is supported on an independent frame, not on the main beam, so that the gauge gives true readings of pile penetration. The criteria for passing the test may be specified as one or more of: maximum penetration at working load. maximum penetration at 150% of working load. maximum permanent penetration after the load is removed. In determining these criteria, the specifying engineer will take account of the effect of settlement on the structure that will be built on the piles. This test does not necessarily determine the ultimate load capacity of the pile. Determining the Design Bearing Resistance of a Pile from Load Tests. A "proof load" test confirms the safety of a pile that has already been designed, but is not itself a design tool. If the test is continued until "failure" (penetration = 10% of pile diameter), then the result can be used to determine a Design Bearing Resistance for the pile. EC7 defines an allowance factor to convert failure loads from tests to characteristic failure loads. Characteristic Bearing Resistance Load = Test Failure Load / Values of depend on the number of tests carried out: EC7: Number of tests 1 2 More then 2 1.5 1.35 1.3 value to be used on the average test strength 1.5 1.25 1.1 to be used on the minimum test strength Further partial safety factors ( factors) are used to derive the Design Ultimate Bearing Resistance: Design Ultimate Bearing Resistance = Characteristic Bearing Resistance / Bearing Piles and Groups – October 2010

If the skin friction and end bearing components can be determined separately, either by purpose-designed load tests or by calculation, then different factors can be used for each. If it is not possible to distinguish between the two, then a single factor is used. EC7: Partial Safety Factors Driven Piles Bored Piles CFA Piles

b,

for Base Resistance 1.3 1.6 1.45

for Shaft Resistance 1.3 1.3 1.3 s

for Total Resistance 1.3 1.5 1.4 t

7(b) Indirect Testing Methods, Integrity Tests Refer to CIRIA Report 144 - `Integrity Testing in Piling Practice` (1997) Turner MJ Testing techniques can be classified into `direct` and `indirect` techniques. Direct Examination Techniques: Visual examination – during or after installation Drilling, boring or probing – alongside, or into the pile Indirect Examination Techniques: Internal – utilising drillholes or pre-formed ducts within the pile, and includes sonic logging and nuclear techniques External – from top or side of exposed pile, and includes integrity and resistivity techniques Remote – alongside the pile where access to the pile head is not available, and includes parallel seismic techniques The following charts, taken from the CIRIA 144, give a useful overview of the above techniques, and the suitability of the techniques to detect particular pile construction defects. CIRIA 144 Figures 12. and 13.

Bearing Piles and Groups – October 2010

CIRIA 144 Figure 1.3

Bearing Piles and Groups – October 2010

CIRIA 144 Figures 1.4, 1.5 and 1.6

Bearing Piles and Groups – October 2010

CIRIA 144 Table 1.1

Bearing Piles and Groups – October 2010

Low Strain Integrity Tests `Sonic Echo` Testing The most commonly used integrity test is the `sonic echo` test, a form of low strain integrity test. The pile head is struck by a light, hand-held hammer and a shock wave propagates down the pile at a constant velocity – if the pile is homogeneous. The wave will be reflected at any change of impedance, and the greater the change of impedance, the greater the proportion of the wave that will be reflected. Changes of impedance may be caused by: the pile toe inclusions within the pile cracks or pile joints dimensional changes variations in concrete quality overlapping of reinforcement (in unusually heavily reinforced piles) variations in soil stiffness

CIRIA 144

A typical signal response is shown below

CIRIA 144

L = ct/2 where

The length L to the point of reflection is given by:

c is the velocity of propagation of the wave through the pile (typically 4000m/s in sound concrete) t is the total time for the wave to travel to the point of reflection and return to the pile head

Bearing Piles and Groups – October 2010

8

Tolerances, Pile Spacing, Pile Caps and Ground Beams

Tolerances It is difficult to locate piles precisely. The contractor will be given a specification which states acceptable tolerances. eg. The pile must be placed within 75mm of the location shown on the drawings and be within a slope of 1/100 from vertical. If the top of the pile is not at ground level, the tolerance will be affected by both limits. eg. If the top of the pile is to be 1m below ground level, the tolerance at the top of the pile will be 75 + 1000/100 = 85 mm. The structure must be designed to be safe wherever the piles are within these tolerances. If the piles are found to be outside these tolerances, the Contractor will be obliged to pay for changes. The structure supported by the pile (usually a pile cap or ground beam) must be large enough so that the pile will always be under the structure so long as its position is within the given tolerance. On small works, the structure is designed to extend 75mm beyond the nominal edge of the piles, on larger work this is increased to 150mm. Pile Spacing. A pile in a group will have a lower load capacity than an isolated pile. To reduce interference between piles, the centre-to-centre distances should be at least: Friction Piles: 3 x pile diameter End-bearing Piles 2 x pile diameter. For groups of more than 4 piles the capacity may be reduced unless the pile spacing is much larger: see later notes. Columns on Single Piles

N

Single piles should be designed for the axial load (N) plus a bending moment due to tolerances (M = N*e). The lateral stability of this arrangement is suspect, and so it is seldom used except on very large piles. Multi-Pile Groups It is good practice to support each column on at least two and preferably three piles. This evens out the effects of badly-positioned piles, gives some protection against a single weak pile, and removes from the pile any bending moment due to eccentricity of the load. The piles will be connected by a pile cap which carries the column. The pile cap is usually quite rigid (thickness = approx half of the pile spacing), so differences between the settlement of the piles is evened out.

e A Column putting an Eccentric Load onto a Single Pile

A Square Column on a Square Pile Cap supported by four Piles

To support loadbearing walls, it is more convenient to use a ground beam, which spans between the piles and carries the wall. Ground beams are often also used to support the ground floor slab. A Small Building supported on Nine Piles and Ground Beams Bearing Piles and Groups – October 2010

Stability Pile caps or connected ground beams give adequate stability to the top of the pile (ie prevent the top of the pile from moving sideways). If the piles are required to resist significant lateral loads (eg wind load in high buildings, mooring forces in quaysides) then the piles should be specifically designed for this or raked piles should be used. See later notes on the design of piles for lateral load. 9

Piles in Tension

Piles in tension are used to resist uplift or overturning, and raking tension piles are used to resist horizontal loads. Clearly piles cannot resist tension by end bearing (though an underream can be used to develop more strength) so the tension force must be carried by skin friction. Tomlinson suggests that skin friction capacity in tension may be only 50% of the compression capacity. In addition: Short-term load (eg wind loads) will be sustained more easily than long-term sustained loads. A pull-out test made soon after installation may be a poor guide to long-term strength. Despite this, designs should be confirmed by pull-out tests (which are much cheaper than similar tests on compression piles). As with compression piles, the tension capacity of a pile in a group may be less than the capacity of a similar isolated pile. Tension piles are also known as ground anchors. 10

Downdrag (Negative Skin Friction)

Applies when piles are constructed through recently placed unconsolidated fill, or soils that may consolidate after the pile is placed.. In addition to the working load on the head, a downdrag force is transmitted by `negative skin friction`. If the fill is placed over compressible material, negative skin friction may increase through consolidation. Tomlinson (1995) states that, for mobilisation of maximum negative skin friction, the soil (or fill) must move downwards relative to the pile by around 1% x pile dia. The unit negative skin friction force at any depth can be estimated from the equation Fsneg = 0 where 0 = effective overburden pressure = factor = 0.3 for piles up to 15m long = 0.2 for piles up to 40m long = 0.1 for piles up to 60m long (Tomlinson 1995) The negative skin friction becomes and additional load on the pile, so FoS = Ultimate bearing capacity Working Load + neg. skin friction In extreme cases (long piles through unconsolidated fill) it may be worth sleeving the piles, surrounding them with a soft asphalt coating that will limit the downdrag. This is expensive, and it is difficult to be sure that the coating will survive the driving process, so it is more usual to take account of the extra load in the design of the loadbearing part of the pile.

Bearing Piles and Groups – October 2010

11

Laterally-Loaded Piles

If it is necessary to use piles to resist lateral loads, they must be designed for the shear forces and bending moments that will develop. Laterally loaded piles should be checked for strength under factored loads and for serviceability under working loads. Reference: “Design of laterally-loaded piles” CIRIA Report 103,1984. 12

Capacity of Pile Groups.

Bearing piles are seldom used alone because of overturning and eccentricity effects. Instead groups of piles are often used, with thick reinforced pile caps to spread loads between the piles. Centre-to-centre distances s for piles are normally taken as three diameters for friction piles and two diameters (or two pile base diameters if under-reamed) for end-bearing piles. If the pile spacing is more than necessary, a large and expensive pile cap will be needed. If the piles are too close, the soil between the piles will be disturbed and the load-bearing capacity reduced. The stressed zone around a single pile is much smaller than the stressed zone around and beneath a pile group. As a result: Group capacity is not greatly dependant on installation method. A compressible layer beneath a pile group may produce more settlement than it would beneath a single pile. It follows that: The capacity of a group of N piles may not be N times the capacity of one pile. Tests on a single pile may not adequately predict the performance of a pile group. Model piles in clay tested by Whittaker (1957) showed that Under a rigid pile cap, the piles do not all carry the same load. Outer piles carry more load than inner piles (this effect is reversed in sand). See figure 10.15 (Barnes, 1995) If the efficiency of a pile in a pile group is defined as: =

average load per pile at failure of group failure load of a single isolated pile

the efficiency decreases as the pile spacing decreases. See figure 10.16 (Barnes 1995)

Bearing Piles and Groups – October 2010

Figure 10.15

Figure 10.16

Ultimate Capacity The results above apply only to Whitaker’s experiments. For a generally-applicable rule: Pile Group Capacity is the lesser of: 1. the sum of the failure loads of the individual piles Pult = n(Qs + Qb) 2. the bearing capacity (including side friction) of a block of soil defined by the perimeter of the pile group. Pult = cuNcscdcBgLg + 2(Bg + Lg)Lc If the bottom of the pile cap is in contact with the supporting soil, then the first of these can be increased by the failure capacity of this contact surface. 1. The sum of the failure loads of the individual piles plus the failure bearing capacity of the reminder of the pile cap. Pult = n(Qs + Qb) + cuNcscdc(BcLc – nAp)

Pile Cap

Pile Cap

Individual Pile Failure

Block Failure End

Bearing Piles and Groups – October 2010

Laterally-Loaded Piles Ultimate lateral resistance Near the surface, passive pressures may be developed. At depth, local flow of soil around the pile limits the lateral resistance. Broms simplification for piles in clay For a pile with a diameter D, ignore the passive pressure down to a depth of 1.5D. Then take a limiting lateral pressure of 9cu.

LH

D

e B L A Ha

1.5D LH Ha 9(cu/ m) D

Providing the pile is strong enough, the whole pile will rotate about the point A. Normally the pile is not strong enough and the horizontal force is limited by the moment capacity of the pile at B. Example Calculate the ultimate moment capacity required for a long pile of 600 mm diameter if a lateral force of 100 kN (short-term) is to be applied 1 m above ground level. Assume the clay has cu = 100 kN/m2. Partial safety factor on loads = 1.6 Partial safety factor on cu = 1.6 Design load = 1.6*100 = 160 kN Ignore top 1.5*0.6 = 0.9 m. Resistance = 9*(100/1.6)*0.6 = 337 kN/m

Bearing Piles and Groups – October 2010

Depth to resist lateral force = 160/337 = 0.475 m

Bearing Piles and Groups – October 2010

160 kN

0.6 m

1.0 m 0.475 m

0.9 m 160 kN 337 kN/m

Depth to point of maximum moment = 1.0 + 0.9 + 0.475 = 2.375 m Maximum moment = 160*2.375 - 160*0.475/2 = 342 kNm Note: Serviceability should also be checked because the passive pressures develop only with substantial movements.

Bearing Piles and Groups – October 2010

Example to show the implications of design to EC7. In the following case, design the length of pile required i) to BS 8004, with an overall FoS of 2.5 ii) to EC7, to the ULS, assuming case C to be critical. Details : Pile dia 450 mm Dead load 550 kN Variable load 300 kN Ground conditions : 0 – 5m 5 – 30m

Clay Clay

Cu = 90 kN/m2 Cu = 120 kN/m2

Assume the top metre of the clay does not support load Adhesion factor = 0.45, bearing capacity factor Nc = 9 Revision Sheet 1.

Pile Design

a) A closed end steel tubular pile, 0.6m dia, is driven into stiff clay with a penetration of 35m. The undrained shear strength of the clay is 130 kN/m2 and the submerged unit weight is 13 kN/m3. Assuming a bearing capacity factor, Nc, of 9, determine the allowable pile working load assuming an overall factor of safety of 2.5. (2414 kN) Relevant charts and expressions are given on Fig. Q6(a) and (b) (9 marks) With reference to Figs Q6(a) and (b), explain why (i) (ii)

2.

the peak adhesion factor p reduces as the soil strength ratio increases the length factor F reduces as the length/diameter ratio increases (5 marks)

A 750 mm dia bored pile is to support a dead load of 900 kN and a variable load of 300 kN. Ground conditions involve two layers of clay. The upper layer is 8m thick and has an undrained shear strength of 50 kN/m2. The lower layer is of considerable thickness and has an undrained shear strength of 120 kN/m2. The top metre of the shaft does not support any load. The adhesion factor is 1.0 for the upper clay layer and 0.5 for the lower clay. The bearing capacity factor, Nc, is 9 Determine the required pile length : (i)

in accordance with BS 8004 assuming factors of safety of 1.5 and 3.0 are applied to the shaft load and base load respectively (8 marks)

Bearing Piles and Groups – October 2010

(ii)

in accordance with Eurocode 7, to the ultimate limit state, assuming case C to be critical. Relevant clauses and tables from EC 7 are provided (8 marks)

(i) (ii)

13.2 m 17.2m

Bearing Piles and Groups – October 2010

3.

A precast concrete pile, 0.4m dia., is to be driven through a deposit of stiff clay 6m thick and into a thick deposit of dense sand. The water table lies at 2m below ground level. Properties of the soils and relevant parameters are as follows :

CLAY Undrained shear strength increases from 90 kN/m2 at ground level to 126 kN/m2 at the base of the deposit. Bulk unit weight

b

= 21 kN/m3

Adhesion factor = 0.35 SAND Angle of friction Bulk unit weight

= 37 b

= 19 kN/m3

kstan = 1.5 For a working load of 1600 kN, determine the length of pile required assuming: (i) (ii) (iii) (iv)

no adhesion occurs over the top metre of the pile the factor of safety on adhesion in the clay is 2.0 the factor of safety on skin friction in the sand is 2.5 and on the base it is 3.0 there is a critical depth within the sand only, and measured below the top of the sand (Use the Meyerhof curve for determination)

Design charts, tables and expressions are given in Fig Q5 (15 marks) (13.5m)

Bearing Piles and Groups – October 2010

Elastic Pile Settlement Example: 12m long concrete pile, diameter 450mm Working load = 600 kN Load is carried 65% by skin friction and 35% by end bearing E value of soil beneath base is estimated to 250 MN/m2 E value for concrete is estimated to be 10,000 MN/m2 Using equation:

WS 2

WB

L AS EP

WB 2 AB

B EB

WS = 0.65 600 = 390 kN, WB = 0.35 600 = 210 kN AS = AB = 0.452/4 = 0.16 m2

390 210 2

12 0.16 10,000 103

210 0.45 2 0.16 250 103

0.003 0.0012 0.004m, or 4mm

Bearing Piles and Groups – October 2010

Example of Pile Capacity from Static Test: to EC7 Three CFA piles tested. Estimate that 80% of capacity is in shaft friction and 20% in end bearing. Use the EC7 method to determine the Design Ultimate Bearing Capacity a pile similar to those tested. Static Load Test Results: Pile Number Test Load at Failure

1 145 kN

2 120 kN

Calculation: Average test strength Minimum strength From the table of values Characteristic Bearing Resistance is the lesser of: 127/1.3 115/1.1

3 115 kN

= 127 kN = 115 kN

= 98 kN (adopt this) = 105 kN

Estimate of shaft friction resistance is 0.8 98 Estimate of end-bearing resistance is 0.2 98

= 78 kN = 20 kN

Ultimate Design Bearing Resistance = 78/1.45 + 20/1.3

= 69 kN

Bearing Piles and Groups – October 2010