GT18R1A1_Design Sizing Construction of Segments Lining

ASSOCIATION FRANÇAISE DES TUNNELS ET DE L’ESPACE SOUTERRAIN Organization member of the AFTES www.aftes.asso.fr

AFTES Recommendations The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) GT18R1A1

AFTES RECOMMENDATIONS FOR

THE DESIGN, SIZING AND CONSTRUCTION OF PRECAST CONCRETE SEGMENTS INSTALLED AT THE REAR OF A TUNNEL BORING MACHINE (TBM) A.F.T.E.S. will be pleased to receive any suggestions concerning these recommendations

Version 1 - 1997 - approved by the Technical Committee on 13/11/1997

Translated in 1999

Text presented by M. Pascal GUEDON, SIMECSOL Working Group leader, with the collaboration of : Messrs. AUTUORI Philippe, BOUYGUES - BACHTANIK Bruno, MINISTERE DE L'EQUIPEMENT, BARTHES Henri, A.F.T.E.S. - BERNARD Simon - BONNA - BILLANGEON Rémi, SPIE BATIGNOLLES BOCHON Alain, SNCF - CHANTRON Laurent, CETu - CHARDIN Daniel, SOGEA - DARDARD Bruno, SNCF HUEBER Jean, SETEC - LABONNE Hubert, INDUSTRIELLE DU BETON - LEOGANE Jean Paul, RATP PETIT François, CAMPENON BERNARD SGE - SAMAMA Laurent, SCETAUROUTE - TAQUET Bernard, EDF - CNEH VAN DUC Tri, CAMPENON BERNARD SGE A.F.T.E.S. reading panel : Messrs. GUILLAUME Jean, RAZEL - LAUNAY Jean, DUMEZ - GTM - LECA Eric, SCETAUROUTE MAUROY Fabien, SYSTRA - NIQUET Jean-Jacques, SOCIETE DU CANAL DE PROVENCE SCHWENZFEIER André, CETu

CONTENTS Pages

1 - GENERAL 1.1 - Purpose of recommendations 1.2 - Scope of application of recommendations 2 - HISTORICAL REMINDER 3 - TUNNEL LINING DESIGN 3.1 - Introduction 3.2 - Basic data required to design a tunnel lining 3.3 - Lining functions 3.3.1 - Functions associated with operating constraints 3.3.2 - Functions associated with construction constraints 3.4 - Description of the concept 3.4.1 - General 3.4.2 - General aspects of tunnel lining design 3.4.3 - Tapering of rings 3.4.4 - Length of rings 3.4.5 - Composition of a lining ring 3.4.6 - Segment geometry 3.4.7 - Nature of lining materials 3.5 - Lining installed within the area enclosing the TBM 3.5.1 - Ring design principle 3.5.2 - Composition of rings 3.5.3 - Contact surfaces

Pages

3.5.4 3.5.5 3.5.6 3.5.7

210 210 210 211 211 211 211 211

3.6

3.7

212 212 212 212 212 213 213 213 213

3.8

3.9 214 214 214 216

- Waterproofing gaskets - Segment assembly systems - Connector inserts, pockets - Gaskets for distributing loads at segment contact joints 3.5.8 - Back grouting behind ring extrados - Lining installed outside the area occupied by the TBM 3.6.1 - Ring design principle 3.6.2 - Advantages and drawbacks - Specific aspects of water conveyance pressure tunnels 3.7.1 - Hydrogeological reminders 3.7.2 - Tunnel lining structural behaviour 3.7.3 - Roughness of segment-lined tunnels - Construction tolerances 3.8.1 - Specification 3.8.2 - Identification of main criteria contributing to tolerance specification 3.8.3 - Accuracy - Durability 3.9.1 - Segment concrete 3.9.2 - Steel reinforcing bars 3.9.3 - Waterproofing gaskets 3.9.4 - Connector inserts

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM)

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Pages

3.10 - Economic considerations 4 - TUNNEL LINING DESIGN 4.1 - Main parameters influencing sizing 4.1.1 - Implementation conditions 4.1.2 - Parameters for analysing ring stresses 4.2 - Design assumptions 4.2.1 - Regulations and references 4.2.2 - Material properties 4.2.3 - Nature of actions and loadings 4.2.4 - Combined actions 4.2.5 - Sizing criteria 4.3 - Determination of stresses in the tunnel lining 4.3.1 - Introduction 4.3.2 - Hyperstatic reaction method 4.3.3 - Composite solid method 4.3.4 - Adaptation of analysis methods to a segments lining and to TBM-based excavation 4.3.5 - Parameters which can be integrated in the different methods of analysis 4.4 - Proof of concrete and reinforcement 4.4.1 - Choice of segment wall thickness 4.4.2 - Circumferential reinforcement (hoops) 4.4.3 - Longitudinal reinforcing bars (arranged parallel to the tunnel axis)

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Pages

5 - DESIGN OF ASSEMBLY SYSTEMS 5.1 - Design assumptions for bolts and anchor bolts 5.1.1 - Regulations 5.1.2 - Nature of actions and loadings 5.1.3 - Combined actions - Design stresses 5.2 - Proof of assembly and pick-up components using materials other than steel 5.2.1 - Introduction 5.2.2 - Actions to be considered 5.2.3 - Combined actions - Stresses 5.2.4 - Behaviour of materials and assemblies - Tests 5.2.5 - Conclusions 6 - TRANSITION AND ANCILLARY WORKS 6.1 - Design of ancillary works 6.2 - Construction of transition and ancillary works 7 - INSTRUMENTATION 7.1 - Aims 7.2 - Monitoring methods REFERENCES ANNEX : TUNNEL LINING CONSTRUCTION PRECASTING AND INSTALLATION

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FOREWORD

T

he present text is aimed first and foremost at the different active parties (Owners, Owner's Representatives and Engineers, Consulting Engineers, Contractors) working in the field of TBM-based mechanized tunnel driving.

The prime aim of the present document is not only to avoid certain past mistakes in the design, sizing and construction of precast concrete segmental linings installed at the rear of a TBM, but also to contribute to extending know-how in these areas based on the experience gained over the last decades by the various parties practising of this technique. It is hoped that this text will stimulate the wish of all those concerned to make progress in relation to the technical aspect of this type of lining and technology involved.

I - GENERAL

1.2 - Scope of application of recommendations

1.1 - Purpose of recommendations The aim of the present recommendations is to provide guidelines for the design, sizing and construction of precast concrete segments linings installed at the rear of a TBM. In par ticular, they are intended to update and complement past recommendations presented by A.F.T.E.S. Working Group 7 (Tunnel Suppor t and Lining) entitled "récommandations sur les revÍtements préfabriqués des tunnels circulaires creusés au tunnelier" (recommendations for prefabricated linings of TBM-driven circular tunnels) (Tunnels et Ouvrages Souterrains (T.O.S.) Special Issue 05-88). They are also based on other past recommendations published by A.F.T.E.S.

concrete tunnel linings, these recommendations recall in a section covering design: • the functions which linings must fulfil,

The present recommendations deal exclusively with the case of precast concrete segments linings. Thus, tunnel lining designs based on using other materials, such as cast iron or steel, or having recourse to a mix of these materials do not fall within the scope of these recommendations and call for a specific recommendation drafting project. On the other hand, the expression "installed at the rear of a TBM" does not limit the scope of application of this text to only linings installed within the TBM shield tail; it also includes linings installed outside the area occupied by the shield tail, such as linings formed from expanded segments. Thus, following a brief historical reminder describing the emergence of precast

• the different elements forming tunnel linings and their roles, stressing, in particular, the important points to be complied with to ensure satisfactory behaviour of the structure during construction and throughout its life. The document subsequently reports on the sizing aspect of linings in relation to which it is impor tant to recall immediately the essential osmosis, in the general sense (design, analysis, construction) of the word, which must prevail between TBM and lining designers. This process will result in the avoidance of many sources of malfunction liable to lead, in some cases, to cer tain design inconsistencies at times detrimental to the long-term performance of the structure.

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Thus, the following aspects will be addressed in this section of the recommendations: • the main parameters influencing sizing;

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• design assumptions (regulations and recommendations, types of materials, actions and combined actions, sizing criteria); • methods available to the design engineer for analysing soil-str ucture interacting stresses and where the refinement of the calculations should be adapted to: - the design levels implemented (preliminar y studies, design studies, construction studies), - the nature of the problems encountered (sensitivity of the site to ground deformations, closeness of other structures, etc.); • cation of the different structural elements forming the lining. Moreover, the recommendations draw attention to the essential control of certain unusual design aspects, such as transition between the lining and different types of underground structures (stations, terminals, addits, shafts, pipes, etc.), often causing problems which are awkward to deal with. Finally, consideration is given to the particular aspects of monitoring and instrumentation of this type of structure. An annex specific to tunnel lining construction provides a review of the recommendations advocated to ensure total conformity between the structure engineered at design stage and the implemented finished product from segment casting stage to segment erection within the tunnel. This summary of the content of the recommendations reveals the full range of the areas affecting the design and construction of precast concrete segments linings installed at the rear of a TBM. It also highlights the special care which must be applied to every stage of the project when working towards completion of a quality finished structure whilst complying with production- and automation/robotization-related demands imposed by a concept of this type.

2 - HISTORICAL REMINDER Up to 1930, TBM-driven tunnels were mainly lined using cast iron segments. Thereafter, precast concrete segments tunnel linings started to appear, mainly in Great Britain, for small diameter tunnels (1.5 to 3 m) driven in London clay for use as sewers.

Since that period, several hundred kilometres of generally small diameter tunnels driven in the London area have been lined with concrete segments of various shapes and types; they are often ribbed, in other words their shape stems from that of cast iron segments. It should be noted that, most of the time, these underground structures were built in very low permeability ground in which the excavated periphery offered short-term stability (London clay).

3.2 - Basic data required to design a tunnel lining

In time, British manufacturers offered a whole range of standard off-the-shelf tunnel lining segments covering a wide range of diameters (1.5 to 6 m internal diameters). One of the significant features of these segments was their small size and reduced weight (100 to 400 kg per segment), which resulted in a large number of ring elements for the largest diameter tunnels (12 segments per ring for a diameter of the order of 6 m).

• its operating life;

Prior to designing any tunnel lining, it is essential that Owner s, Owner's Representatives and Engineers specify the aims and constraints which the planned tunnel structure must satisfy: • its function(s): rail or road transpor t, water or air conveyance, power or data conveyance, storage, etc.;

3.1 - Introduction

• the operating constraints: - geometrical criteria (clearance, route, construction tolerances, etc.), - type and location of all permanent facilities (benches, inver t slab, wall recesses, branches, hangers, connector inser ts, wall pockets, suppor t systems for intermediate floors and ventilation ducts, etc.), - roughness criteria for the permanent works compatible with projected water or air flows (precast lining possibly combined with and internal cast-in-place lining), - water tightness criteria (acceptable seepage flows both from outside to inside and conversely for water conveyance tunnels), - fire resistance criteria, - possible requirements in relation to steel reinforcement equipotential; • environmental constraints: - geology, hydrogeology, - aggressivity of surrounding ground, - site urbanization (limitation of ground settlements, etc.), - presence of nearby underground structures (existing or future, if known), - seismicity;

It is essential to state that there is no unique design for a segmental lining.

• structural sizing criteria resulting especially from the above-mentioned constraints:

Since 1965, major development in the use of concrete segments linings in Europe (Germany, Belgium, Austria, France) and Japan is notewor thy, in parallel with the development of TBMs for excavating large diameter tunnels (approximately 5 to 10 m) in soft and water-bearing ground. Specifically, mechanized erectors, larger size segments with ver y low precasting tolerances and elastomeric gaskets capable of guaranteeing lining water tightness even in heavily water-bearing ground, have appeared.

3 - TUNNEL LINING DESIGN

Very often, its design is based on the experience and skill acquired by Consulting Engineers and Contractors on past projects. Consequently, the purpose of this section is to review the main factors entering into the design of this type of lining and to draw attention to cer tain vital engineering aspects, of which a perfect command is required. It cannot recommend a single type of lining design to reader because too many interdependent factors come into play. On the contrary, over-precise recommendations, which do not integrate all the parameters, could prejudice construction of a quality structure.

- regulations, standards and recommendations to be applied, - actions and combined actions to be considered.

3.3 - Lining functions 3.3.1 - Functions associated with operating constraints During tunnel operation, the segmental lining may be required to fulfil the following functions, which depend entirely on the pre-established aims of the Owner, the Owner's Representative and the Engineer :

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) • To act as permanent tunnel lining/support compatible with the various environmental constraints;

3.4 - Description of the concept

• To act as an envelope intended to ensure permanent compliance with tunnel operating clearance(s);

3.4.1 - General

• To ensure imperviousness with respect to:

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- water inflows from the enclosing ground, - possible fluids flowing freely or under pressure within the structure. It should be recalled that structural imperviousness depends on the ability of the lining elements to oppose the passage of a fluid (water, etc.), both from outside to inside and conversely, within the leakage flow limits specified for its operation; • To ensure air or water conveyance depending on whether the structure is likely to have to ensure the flow of air (piston relief) or water respectively; • To provide support for permanent service mobile and fixed equipment.

Segment

An impor tant criterion for tunnel lining design lies in the requirement or not to interface driving and lining installation functions for the purpose of ensuring throughout the tunnelling period: Ri ng

• total continuity of tunnel support; • total control of water inflows.

Figure 1 : Lining comprising precast segment rings

Continuity is thus initially provided by the TBM shell itself (the shield tail with its rear seal) and thereafter by the lining, incorporating its water tight gaskets, installed inside the shield tail.

• either parallel plane surfaces, as in the case of so-called straight rings; • or out-of-parallel plane surfaces, as in the case of so-called tapered rings.

It goes without saying that these design options depend entirely on the geological and hydrogeological conditions of the surrounding hydrogeological environment through which the tunnel passes.

Depending on the arrangement retained, the latter ring geometry allows the lining to best adapt itself to curvature in the horizontal and vertical alignments of the tunnel or to correct accidental deviations caused by the TBM.

3.4.2 - General aspects of tunnel lining design

3.3.2 - Functions associated with construction constraints During the construction phase of the tunnel, the segmental lining may be required to fulfil some of the following functions resulting from both construction and environmental requirements: • To provide to the tunnel:

3.4.3 - Tapering of rings

A precast concrete lining for a TBM-driven tunnel generally comprises a sequence of rings placed side-by-side.These rings are divided into sectors and each of these elementary units is called a segment (see figure 1).

Ring taper "p" is defined as the difference between the maximum and the minimum lengths of the ring and must be dimensioned to ensure that design curves are complied with and to allow TBM deviations to be taken up. It can attain several centimetres (see figure 2).

The transverse faces of the rings (see figure 2) can be formed by:

- either immediate suppor t, mainly when drawing in the TBM shield tail in soft ground, - or deferred support, when the lining is installed outside the TBM shield tail; the peripher y of the excavation is stable in the shor t-term within an enclosing soil-rock mass of sufficiently low permeability to allow work to be carried out in satisfactory conditions, without having recour se to immediate continuous support; • To provide protection against water inflows, when tunnel driving is undertaken in water-bearing ground;

Straight

Straight ring

• To provide longitudinal support allowing the TBM to:

Curved

Plan view

Left or right tapered trapezoidal ring

Straight

Curved

- penetrate the ground, - if necessary, exert confinement pressure at the excavation face to ensure its stability and that the hydrostatic pressure applied to the TBM cutterhead is taken up; • To support the back-up equipment and construction plant required for carrying out the work; • To ensure evacuation of drainage water.

Universal tapered ring

Plan view

Figure 2 : Sequence of rings

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM)

- tunnel diameter

Although often used in the past, today their use is almost exclusively confined to structures such as shafts. They require an larger excavation diameter than solid segments for the same sectional area and inertia.The size of the hollows allows implementation of an assembly system based on straight bolts, which requires greater rearward clearance during erection than other assembly systems.

- alignment design (horizontal and vertical radii of curvature),

3.4.7 - Nature of lining materials

Development of techniques (control of supply to the workface) tends to favour the use of the universal tapered ring.

➝

Segment

3.4.4 - Length of rings The ring length may depend on:

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• operation-linked criteria:

- limitation of the length of gasket sealing material and thus of the risk of a defect in tunnel watertightness;

The main constituent materials of a tunnel lining are:

• construction-based criteria:

- cement,

- optimization of driving (mucking) and lining installation cycles, - size of rings (impact on design of TBM thrust mechanisms: stroke of thrust cylinders, etc.), - weight of ring segments (impact on yardand tunnel-based segment handling equipment). Ring length is generally between 0.60 m and 2.00 m. 3.4.5 - Composition of a lining ring The number of segments comprising a ring varies widely from one tunnel project to another and is subject to the following constraints: • operating constraints: - limitation of the number of segment contact joints, therefore of the risk of a defect in tunnel watertightness, - limitation of head losses due to seepage of internal fluids;

• concrete, containing: Figure 3 : Typical cross-section of a ring

- aggregates, - admixtures • reinforcing steel.

3.4.6 - Segment geometry

3.4.7.1 - Cement

The geometr y of a segment is essentially linked to the type of ring assembly system retained.

Preference should be given to using additive-free rapid-hardening cements, whose durability is unaffected or little affected by steam curing.

The following shapes can be distinguished: • "Solid" segments: These are the most frequently used today; almost their full wall thickness contributes to the strength of the ring.They incorporate small size pockets allowing assembly of the different parts of the ring (by anchor bolts, cur ved bolts, plugs, etc.; see § 3.5.5 Segment assembly systems); •"Hollow" or "ribbed" segments:

For this type of application, use of standard CPA-CEM I-type cement is therefore preferred to the following cement types: - CPJ - CEM II - CLC - CEM V - CLK - CEM III - CHF - CEM III. However, the latter types can be considered for tunnel linings in aggressive ground

Figure 5 : "Hollow" and "ribbed" segments (Caracas Underground - Lines 1 and 2)

• construction constraints: - weight of segments (impact on formwork stripping operations, handling, yard storage, ring erection using erector arm), - size of ring elements (transport from precast yard, supply to the workface), - impacts on concreting conditions (curvature), - segment behaviour under TBM thr ust (limitation of risk that cracks will appear under temporar y stresses resulting from bearing defects between rings), - positioning of TBM thrust mechanisms. - ring-to-ring assembly constraints (layout of ring assembly devices).

Figure 4 : "Solid" segments (Lille Underground - lines 1 bis - section B)

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) 3.5 - Lining installed within the area enclosing the TBM

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Lining wall thickness (cm)

The various operating and environmental constraints referred to above ver y often impose erection of the lining rings under cover of the TBM in the rear par t of its shield tail. Different points entering into the design of this type of lining are enlarged upon in the following sections. 3.5.1 - Ring design principle As already referred to above in Section 3.4, this lining design can require the adoption of:

Lining internal diameter (m) final final final final

lining lining lining lining

-

open face TBM compressed air TBM slurry pressure TBM earth pressure TBM

temporary temporary temporary temporary

lining lining lining lining

-

open face TBM compressed air TBM slurry pressure TBM earth pressure TBM

Figure 6 gives an idea of the increase in segmental lining wall thickness with respect to the tunnel internal diameter.

conditions based on special mix design and production if necessary. Use of CPA - PMES-type cement may be recommended for certain applications. 3.4.7.2 - Aggregates

In general, the nature of fine and coarse aggregates used depends on the available quarries in the area where the tunnel is to be built. Aggregate sizes must suit perfectly the geometrical accuracy of the segments, formwork recesses, reinforcement arrangements and possible connector inserts. The proper ties of these materials will be specified on the basis of physical and chemical analyses and examinations. In particular, aggregates will be sought which are frostresistant, unreactive, sound, free of fines, non-absorbent, non-brittle and hard. Sands should preferably contain (mineral) filler. Continuous aggregate grading should be specified to ensure good workability when placing the fresh concrete in the moulds. 3.4.7.3 - Admixtures

In cases in which aggregates lack fines, use of additional fly-ash or fillers (e.g. limestonebased materials) are recommended. The origin of such products should of course be checked.

The use of standardized water-reducing superplasticizers is recommended to obtain increased workability to achieve higher strength. N.B. In France, studies are being conducted on the fire resistance of silica fumes within the framework of the BHP 2000 national project. 3.4.7.4 - Reinforcement

Grades of steel used for segment reinforcing cages must comply with applicable standards. The most commonly used steels are weldable hot-rolled or cold-worked Fe E 500 and Fe E 235 grades. Should hot-rolled steel, generally featuring a large quantity of mill scale, be used, care must be taken to remove this scale before welding (during straightening of coiled steel or by shot blasting steel bars). 3.4.7.5 - Reinforcing fibres

Use of metal fibres, exclusively or in addition to conventional reinforcement, has been experimented in cer tain works. In France, studies are in progress for the purpose drawing up relevant design r ules, within the framework of the national metal fibre reinforced concrete project (BEFIM).

• either straight rings to be used for the straight sections and tapered rings (or tapered wedges) to be used specially for curved sections of the tunnel route and/or for correcting TBM deviations. • or universal tapered rings to be used on a systematic basis, including for straight sections of the tunnel route; the tapers of one ring compensate for those of another ring, thereby cancelling out the overall tapering effect. It should be noted that this second type of design, besides being the most frequently used, implies fabricating specific moulds for each segment (the amount of taper being different from one segment to the another). 3.5.2 - Composition of rings In general, division of a lining ring into segments depends on the ring erection technology retained. 3.5.2.1 - Rectangular and trapezoidal segments

This design does not generally allow the excavation cycle to be restar ted until the ring has been completely erected. The gap available between the extrados of the ring being erected and the intrados of the shield tail is usually small and ring closure is very often ensured by a longitudinal key, which requires additional forward space to allow insertion of this final segment. To partially satisfy this constraint, use of a key segment of "trapezoidal shape" in plan (see figure 7) is very often resorted to. Consequently, the geometr y of the segments adjacent to this ring part will have to be special to suit that of the key: these adjacent elements are called counter segments.

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Key

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SECTION AA

Direction of advance

Key

Figure 7 : Longitudinally inserted key segment Figure 8 : Rectangular and trapezoidal segments - perspective view

Thus, in this type of design, a lining ring generally comprises: • rectangular virtually identical (apart from possible taper) standard segments, whose number can vary from one project to another (see figure 8);

DIRECTION OF TBM ADVANCE

Key segment

• two counter segments; Standard segment

• one key segment.

Counter segment

Assembly of these ring elements is often under taken by means of bolts or anchor bolts (see § 3.5.5 - Segment assembly systems). In general, contact faces between segments are offset longitudinally both to prevent defects in watertightness at the corners and to maintain a certain pressure on the segments previously placed to prevent them from loosening completely when the TBM thrust cylinders are retracted (see figure 9).

Segments are numbered in accordance with their order of placement

In some cases, these contact faces can be aligned (see figure 10); this configuration can be adopted in particular :

DIRECTION OF TBM ADVANCE

• at future openings (entrances to crosstunnels, etc.) to be provided in the lining; • in areas in which alignment corrections are made essential (special care must then be taken in relation to measures to be adopted to guarantee water tightness at segment corners); • if defects in water tightness at segment corners are not a concern (systematic alignment of contact faces).

Figure 9 : Rectangular and trapezoidal segments longitudinal offsetting of contact faces between segments

Key segment Standard segment

Counter segment

Segments are numbered in accordance with their order of placement Figure 10 : Rectangular and trapezoidal segments longitudinal alignment of contact faces between segments

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Two different ring designs can be considered with this type of rectangular or trapezoidal segment:

front of the existing pockets in the previously erected ring in order that they can then be driven in hard.

• Universal segment

The segment being erected is pushed longitudinally on the projecting section without any real oppor tunity for crosswise movement. For this reason and for the purpose of maintaining gradual transverse compression of the radial gasket section (between segments) whilst sliding the segment longitudinally, standard segment geometr y is designed in the shape of a parallelogram.

As its name suggests, this design requires only one set of rings.

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The key segment can then be erected in any angular position.

Thus, this type of lining ring usually comprises (see figure 13): Key segment at bottom of ring

• parallelogrammic standard segments , whose number varies from one project to another ;

Key segment at top of ring

•one reversed key segment;

Figure 11 : Sequence of universal rings

•one key segment. • "Left" and "right" rings This type of design necessitates resorting to two sets of rings, to which the following constraints apply: - ring taper, - key and counter segments with specific geometry, - key positions limited to the upper semicircumference of the ring. If necessary, "left" and "right" rings can be transformed into universal rings (correction of tunnel alignment deviations, etc.).

Configuration 1, shown in figure 14, requires segments to be erected in the same order of placement from one ring to another. Under the pressure exerted by the longitudinally oriented waterproofing gaskets (between segments of the same ring), the rings can be gradually subjected to disruptive rotation in the absence of transverse bolts. In time, this rotation can lead to a discrepancy of several centimetres between the position of the rings and that of the TBM thrust cylinder ram pads. To overcome this gradual rotation, a solution based on alternate segment erection with respect to the first ring part placed is recommended (see figure 15). 3.5.2.3 - Trapezoidal segments

This ring design may allow tunnel excavation and lining erection operations to be

carried out simultaneously (stroke of thrust cylinders adapted to two ring lengths). In general, a ring is broken down into an even number of trapezoidal segments. Half the segments are "counter" type, i.e. wider on the side of the previously placed ring. The other half are "key" type, i.e. narrower on the side of the previously placed ring (see figure 16). Once in place, the counter segments provide suppor t for the thrust cylinders, thereby allowing the TBM to advance again. During this time, ring erection can continue with the key segments, as long as the shield tail seal is not crossed. Subsequently, rear support for the thrust cylinders is transferred to the key segments without halting TBM penetration. This continuous penetration method requires the following problems to be overcome: • placement of "key" segments between the "counter" segments; • total thrust can be mobilized using only some of the thrust cylinders or continuous penetration is limited to certain favourable ground conditions requiring reduced thrust; • more difficult guidance. 3.5.3 - Contact surfaces 3.5.3.1 - Circumferential or transverse contact joints Contact joints between adjacent rings can be required to bear : • compressive (possibly eccentric) loads resulting from the longitudinal thrust of the TBM;

Figure 12 : Sequence of "left" and "right" rings - key systematically positioned above the horizontal diameter

Contractors' final choice of one or the other type of rings is very often based on practices tested at length on different projects. 3.5.2.2 - Parallelogrammic and trapezoidal segments

This ring design is associated with the use of plugs incorporated in the lining wall at the contact face between successive rings (transverse contact face) (see § 3.5.5 Segment assembly systems). When erected, segments are fitted with these projecting plugs, which are lined up in

Figure 13 : Parallelogrammic and trapezoidal segments - Perspective view

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) • shear forces due to differential deformations between adjacent rings associated with:

DIRECTION OF TBM ADVANCE

- offsetting of contact faces between rings (see figure 9), shear being transmitted by permanent ring assembly systems (bolts, anchor bolts, plugs, tenons, etc.),

Reverse key type segment

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- non-uniform load distributions resulting from the ground or from neighbouring structures.

Key type segment

Segments are numbered in accordance with their order of placement

Obviously, assembly systems offer different capacities for opposing potential displacement of the ring parts with respect to each other (out-of-flushness).

Figure 14 : Parallelogrammic and trapezoidal segments - Configuration 1

DIRECTION OF TBM ADVANCE

The final geometry of these contact joints and their possible additional equipment must be selected in relation to the purpose of the tunnel (wastewater collector, water conveyance, rail or road tunnel, etc.) and must be compatible with the out-of-flushness tolerance.

Reverse key type segment

Key type segment

•forces resulting from segments overhanging (accidentally or otherwise) during ring assembly.

Segments are numbered in accordance with their order of placement

These contact joints usually fall under one of the following types:

Figure 15 : Parallelogrammic and trapezoidal segments - Configuration 2

DIRECTION OF TBM ADVANCE

a) plane contact joints This contact principle is shown diagrammatically in figure 17. Key type segment

Depending on the relative intensity of the forces described above, radial slippage can occur leading to out-of-flushness of one segment with respect to an adjacent one. Addition of mechanical systems (see § 3.5.5.) can help to limit the extent of this phenomenon.

Reverse key type segment

Segments are numbered in accordance with their order of placement

Out-of-flushness can be acceptable or unacceptable depending on the purpose of the tunnel (temporary or permanent, air or water conveyance function to be fulfilled). When it is unacceptable, it may be possible to turn to combined geometr y contact joints of a form allowing transfer of shear forces.

Figure 16 : Trapezoidal segments

b) combined geometry contact joints

excavation

This type of contact joint, shown in figure 18, is less common than

back grouting extrados

waterproofing gasket

excavation back grouting boss

extrados waterproofing gasket

intrados

plane contact joint

intrados

boss Figure 17 : Plane contact joint

Figure 18 : Example of combined geometry contact joint

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 217 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) excavation

excavation back grouting

extrados

back grouting

waterproofing gasket

extrados

waterproofing gasket

convex face

concave face

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

plane contact joint intrados

intrados Figure 19 : Example of plane contact joint

Figure 20 : Example of concave-convex cylindrical contact joint

the one above because it is difficult to reconcile erection tolerances with the load transfer efficiency which could be assumed from such a system. Moreover, there is a danger that very high local stresses will develop in the load transfer zones (boss, tenon and mor tise, etc.) and reinforcement of these zones, by means of reinforcing bars, is often delicate simply because of the contact joint geometry. In this type of design, it is thus essential to study carefully the geometr y of such contact joints. Finally, normal loads are necessarily concentrated on reduced surfaces which must be capable of sustaining such loads.

The final geometry of these contact joints must therefore be guided by the following aims: • to allow correct centring of stresses; • to limit the danger of segment out-of-flushness, which both generates disruptive loads and can be detrimental to the purpose of the tunnel (e.g. air or water conveyance).

• concave-convex cylindrical contact joints (see figure 20):

These contact joints are usually one of the following types:

• convex-convex cylindrical contact surfaces (see figure 21).

a) Plane contact joints

c) Other contact joints (see figure 22).

3.5.3.2 - Radial or longitudinal contact joints

A mechanical assembly system is generally incorporated; it contributes to maintaining erection accuracy by preventing, in particular, gradual drift in both segment alignment and intersegment contact.

This type of contact joint, shown in figure 19, is the most commonly used because, in general, it is sufficient for transferring the forces applied to the rings.

The effects of both the surrounding ground conditions and back grouting cause these contact joints between segments of the same ring to be subjected to: • compressive loads; • bending forces: It should be noted that bending forces are reduced in the immediate vicinity of the radial contact joint. The inertia of this zone is effectively lowered with respect to that of the standard section; • transverse shear forces.

b) Cylindrical contact joints When ring stresses are too high to consider plane contact surfaces, joints are often designed with cylindrical surfaces. Through plasticizing the concrete, the contact surface widens gradually in relation to the load and centres it. These contact joints can be of different types:

The radius of curvature of the concave surface may be greater than that of the convex surface in cases when rotation of ring parts in contact is expected or, conversely, these radii of cur vature can be essentially the same (the system is then intended to provide shear strength only);

Incorporation of a guide rod can be adopted in some cases. 3.5.3.3 - Flanks

Given the intensity of compressive stresses often applied to the contact surfaces, whether they be transverse or longitudinal, experience gained from past projects leads to the recommendation that ver y special care should be taken in designing segment flanks in order to limit breakage at their edges to a minimum. It should be borne in mind that these fractures, which are often observed in the segment intrados and repaired by simply restoring, can also affect the extrados of lining ring par ts and can lead to local damage which is difficult to repair and is prejudicial excavation

excavation

back grouting back grouting extrados

extrados convex face

guide rod

convex face

intrados

waterproofing gasket

intrados

Figure 21 : Example of convex-convex cylindrical contact joint

Figure 22 : Example of contact joint incorporating a guide rod

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) - risk of accident to personnel in charge of segment mould-based operations (sharp projecting edges for forming chamfers). To avoid these risks, it is preferable to provide either sharp edges or, if this chamfer is retained, to incorporate a foam rubber (or similar) seal over its full length; this construction-based provision will thereby solve the first problem but not the second. All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

3.5.4 - Waterproofing gaskets Figure 23 : Spalling and cracking of concrete cover

to the durability of the wor ks (water ingress, corrosion of reinforcement, etc.). Whilst it is advantageous to provide relative large flanks in the contact zones in order to overcome the above problems, it should be ensured that the stresses acting on the contact surfaces remain acceptable, including under unfavourable construction configurations (eccentricity of load from thrust cylinders especially in cur ved alignments, erection tolerances, etc.). Detailed geometrical design of these flanks should be undertaken in parallel with that of the reinforcement in order to guarantee the highest possible strength at these particularly highly stressed segment sections. Similarly, in cases in which lining waterproofing is to be provided by means of compressible gasket sections, it should be ensured that the groove receiving the gasket is positioned sufficiently far away from the extrados to avoid the segment edges breaking off when the system is compressed under load. Moreover, whilst it has been the practice to provide a chamfer around the extrados edges of both transverse and longitudinal contact joints, this may give rise to certain drawbacks such as: - risk of defective imper viousness of the TBM tail seal with respect to the back grouting product, water in the surrounding ground or slurry from the forward chamber (in cases involving hydraulic confinement);

It should be recalled that, when an waterproofing function is sought from a segmental lining, this can be fulfilled by: • the segments themselves, for which it is important to limit in particular : - the mass porosity, - cracking associated with temporar y or permanent stresses, - defects involving formation of the groove receiving the waterproofing gasket; • waterproofing gasket positioned between the segments. Properties of the latter are described in the "recommandations pour les profilés d'étanchéité entre voussoirs" (recommendations for intersegment waterproofing gasket) presented by A.F.T.E.S. Working Group 9 (see T.O.S. Issue 116, March-April 1993). The remainder of this description applies to so-called "conventional" waterproofing gaskets retained for the design of standard tunnel projects involving low to medium overburden. N.B. These imper vious systems are not transposable to other tunnel projects involving very high overburden (e.g. major Alpine crossings). Research work (International Eureka Contun) is in fact being conducted concerning the design of TBMs and tunnel linings suited to the special constraints imposed by such projects. On account of the very high pressures liable to be exerted on these linings (loads induced by both ground and water), thoughts are very naturally tending towards seeking a reduction in their rigidity by incorporating a degree of deformability in par ticular at their waterproofing gaskets.

3.5.4.1 - Compressible gasket sections

a) Properties It should be recalled that these are elastomeric gaskets, which have been designed and manufactured for fitting to precast concrete lining segments (see figure 24). Watertigntness is ensured by compressing them during erection and maintaining this compression throughout the life of the structure. During construction, the compressive load is applied by the TBM thrust cylinders or segment erector and is temporarily maintained by the ring building system. Watertigntness of gasket sections is guaranteed for a permanent hydrostatic pressure laid down in the project specifications. b) Construction configurations In general, the gasket is fitted into a groove formed in the segment faces; it is positioned several centimetres from the segment extrados and fitted around the full perimeter of the segment. Gasket section dimensions must be compatible with erection tolerances and take into account ring out-of-roundness. In the specific case of water conveyance pressure tunnels, combined behaviour of the ground / lining must be analysed before possibly modifying the position of the imper vious gasket within the lining wall (conventional well tested approach). 3.5.4.2 - Water-expansive gasket a) Properties It is recalled that these are elastomeric waterproofing gasket with water-expansive properties, i.e. they swell in the presence of water.These cycles can alternate during the life of the works. If necessar y, initial water tigntness can be achieved by compression. The presence of water then triggers swelling of the waterexpansive material, which allows the applied hydrostatic pressure to be resisted. Watertightness of gasket is guaranteed for a permanent hydrostatic pressure laid down in the project specifications.

Water-expansive parts

Neutral parts Figure 24 : Examples of compressible waterproofing gaskets

Figure 25 : Examples of water-expansive waterproofing gasket

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) In some cases, these gasket can be reinforced by "neutral" (non water-expansive) parts (see figure 25).

Double thickness gasket system

Single thickness gasket system

b) Construction configurations The gasket is positioned on the segment sides several centimetres from its extrados.

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There are two types of construction configuration (see figure 26). In the double thickness system, gasket are fitted around the full perimeter of each segment. In the single thickness system, gasket are fitted around half the segment perimeter with a return of several centimetres at diagonally opposing corners. As in the case of compressible sections, waterproofing gasket dimensions must be compatible with erection tolerances and take into account ring out-of-roundness.

Direction of advance Figure 26 : Layout principle for waterproofing gaskets

the combined section is fitted around the full perimeter of each segment and positioned several centimetres from its extrados. 3.5.5 - Segment assembly systems 3.5.5.1 - Purpose of assembly systems

3.5.4.3 - Combined gaskets

a) Principle These are products which combine both types of gaskets described above. The compressible gasket represents the basic component and the water-expansive gasket is usually fitted into the groove formed in the former (see figure 27). This composite product allows the waterproofing system offered by either the compressible or the water-expansive gasket to be complemented. b) Construction configurations As in the case of the compressible gasket, Removal of strip forming the groove

Assembly mechanisms implemented at circumferential (transverse) and radial (longitudinal) contact joints are aimed at: • maintaining sufficient erection accuracy by preventing gradual cumulative out-of-flushness between segments and gaps at contact joints; • keeping waterproofing gasket compressed in the short-term, during construction, and even in the long-term, during tunnel operation, especially in the vicinity of stations; • ensuring segment stability at ring building stage, even when there no load is exerted by the TBM thrust cylinders. However, measures might be adopted to ensure this stability without necessarily resorting to the use of assembly systems (e.g. a thrust cylinder anti-retract system); • ensuring segments are kept in their relative positions (guidance role) in the specific case of water conveyance tunnels, for which linings are required to "breathe" during tunnel filling and emptying cycles.

State of compressible section whilst "stripping" the groove

"Dovetailed" water-expansive section

Final state with "dovetailed" water-expansive section

Figure 27 : Example of combined waterproofing gaskets

In general, longitudinal assembly systems are regular ly spaced along each transver se contact joint (between consecutive rings).

When they exist, assembly systems between segments in the same ring generally comprise between one and three units. In standard rings, longitudinal and transverse assembly systems are usually only essential during construction (except when wanting to take advantage of the contribution to rigidity of adjacent rings whose radial contact joints are then combined). As a general rule therefore, these assembly systems can be removed when the TBM is more than 200 metres away and all additional grouting operations have been completed. In the vicinity of stations, longitudinal assembly systems are usually necessar y during tunnel operation to keep the waterproofing gaskets compressed. They are therefore kept in place over a minimum tunnel length of two or three diameters. The durability of all permanent assembly systems must be the same as that of the structure itself. 3.5.5.2 - Bolted assemblies

Bolts or threaded rods are fixed from pockets provided on the intrados side of the lining. These bolts are generally of two types: • straight bolts fixed from hollows formed in the intrados of segments: - bearing directly on the concrete (see figures 28 and 29),

Their number varies from one project to another depending on:

- bearing on steel plates inserted in the segments (see figure 30);

• the forces to be balanced;

• curved bolts allowing the volume of the hollows to be significantly reduced (see figure 31).

• the desired possibilities for relative rotation of a ring with respect to the last one installed (considering the constraints imposed by the design, such as offsetting of longitudinal contact joints, small deviations in alignment, etc.).

3.5.5.3 - Inclined socket bolted assemblies

This assembly system allows a reduction in the number of pockets in the intrados of

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 220 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Washer

Extrados

Extrados

Intrados Bolt Washer

Nut Washer

Pocket Nut

Pocket

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Intrados Washer Threaded Nut rod

Pocket

Figure 28 : Example of removable assembly using straight bolts

Pocket

Steel plate

Extrados

Figure 29 : Example of permanent assembly (straight bolts previously inserted into one of the segments before placing the adjacent segment)

3.5.5.4 - Assemblies using plugs, studs or other derivatives

Compared with the assemblies described above, such systems (see figures 33 and 34) can offer a number of advantages such as:

Intrados Nut Washer

Connector insert Bolt

Figure 30 : Example of steel plate assembly using short bolts

Plug Back grouting

Plug

• no pockets in the intrados of segments, thereby providing the lining with improved air or water conveyance properties; • reinforcement of ring par ts is often easier ;

Intrados Pockets Segment being placed Ring erected

• simplified ring erection operations; Curved bolt Extrados

• good centring of rings with respect to each other (reduced out-of-flushness); • high shear strength (with some types of plug);

Plug Back grouting

• greater safety of personnel (no human intervention inside the ring). Intrados Nut Washer

Pocket

Segment being placed

Bolt head

Ring erected

Figure 31 : Example of assembly using curved bolts Socket Extrados

Excavation

Extrados

Apar t from the fact that most of these devices cannot be removed, they can furthermore inhibit cer tain degrees of freedom, which may result in excessive lining stresses. When designing and sizing these assemblies, it is therefore essential to evaluate as closely as possible all the loads to which such connections are likely to be subjected (e.g. dissymmetry of thrust cylinder loads, overhanging of segments, differential ring

Excavation

Extrados

Intrados

Segment being placed

Pockets Ring erected

Figure 33 : Examples of assembly using plugs

Back grouting Intrados

Segment being placed

Pocket Socket Anchor bolt bolt Washer head Ring erected

Excavation Extrados

Figure 32 : Example of assembly using inclined socket bolts

the segments.This design also allows tightening up operations to be carried out under cover of a fully erected ring and no longer under segments only held in place by the erector arm and the TBM thrust cylinders; this system thereby ensures increased safety of personnel (see figure 32).

Intrados

Segment being placed

Ring erected

Figure 34 : Examples of assembly using pins

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 221 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) out-of-roundness, waterproofing gasket crushing and distortion loads, local stresses capable of reducing the assembly capacity of the system, tendency for key ejection, etc.) in order to prevent the appearance of disturbances concentrated at these connections, which could adverse affect the durability of the structure. 3.5.6 - Connector inserts, pockets

• temporary functions: - handling, erection (e.g. for picking up units using "grippers" or "pins"), - assembly (holes and boxes for inserting or fixing assembly mechanisms, etc.), - grouting (holes through the segment wall), - precutting (e.g. to facilitate later formation of openings at in-line structures).

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• permanent functions Pockets, as well as most connector inserts fitted to lining segments, represent sources of reduced strength of the reinforced concrete ring elements. Their potential impact on segment structural behaviour (including during construction stages preceding ring erection) as well as on the purpose of the tunnel must, therefore, be closely examined. 3.5.6.1 - Connector inserts

In general, connector inserts can fulfil two sets of functions: • temporary functions : - handling, erection (socket for segment pick-up bolts, etc.), - adjustment, - assembly (sockets for bolts, etc.), - mating (pre-grouted sockets, etc.)

- traceability (recess for identification of characteristic zones), - building in of specific equipment, - instrumentation (concrete control blocks, etc.). These pockets can sometimes lead to structural weakening of the segments concerned and very often complicated detailing of ring par t reinforcement (local discontinuity of reinforcing bars, etc.). It is thus important to limit their number and size to a minimum. Moreover, in the case of designs involving high air- and water conveyance-based constraints, their presence on the segment intrados requires the adoption of often costly special measures (sealing of "boxes", internal second lining, etc.). 3.5.7 - Stuffings for distributing loads at segment contact joints

- grouting; • permanent functions:

- inspection.

Incorporation of stuffings may be called for to distribute TBM thrust loads over ring interfaces (circumferential or transverse contact joints), whilst "smoothing out" as much as possible inaccuracies resulting from both segment precasting tolerances (often ver y low) and erection tolerances during ring building, or to "channel" these loads towards specially reinforced sections of the segments.

It should be stressed that their positioning within the segments sometimes calls for alterations in segment reinforcement detailing in order to:

These millimetre thick stuffings must provide sufficient surface area to fulfil their function and, on the contrary, must not be a source of "load concentration" through

- traceability (identification of characteristic zones, etc.), - fixing (pre-grouted sockets for supporting service equipment or floors, etc.), - mating, connection, - instrumentation (vibrating wire extensometers, total pressure measuring cells, etc.),

• comply with concrete coverage requirements; • avoid zones occupied by the connector inserts;

dimensional underestimation (see figure 35) or an unsuitable layout. Materials used can be very different depending on design philosophies retained. They can be of low stiffness and even behave like "flowing" (e.g. bitumen-based) materials or, on the other hand, they can be relatively stiff (e.g. hard Isorel™). 3.5.8 - Back grouting behind ring extrados 3.5.8.1 - Purpose of back grouting

The purpose of these grouting operations is to fill the annular gap between the lining extrados and the TBM-excavated ground profile. The grouting material therefore fulfils several functions: • in the short term: - it ensures efficient blocking of the lining against the enclosing ground to reduce the danger of ring displacement especially during TBM thrusting and passage of the back-up equipment, when support is essential, - it minimizes surrounding ground deformations likely to cause disorders both above and below ground (especially in urban and/or sensitive environments), - in the case of pressurized face TBMs, it provides good control of confinement pressure (especially when using compressed air) by ensuring imperviousness at the back of the machine; • in the long term: - it ensures the most uniform bond between the lining and the ground and therefore offers effective distribution of confinement loads at this interface; an essential condition for guaranteeing the durability of the structure, especially when it is a pressure tunnel at operating stage, - in cer tain ver y special cases, it fulfils a "drainage" function (granular matrix).

Stuffings for distributing loads in contact with segments

• reinforce the concrete immediately in contact with these connector inserts (local stresses, pulling out, etc.). 3.5.6.2 - Pockets

In general, pockets can fulfil two sets of functions: Figure 35 : Example of distribution of stuffings for spreading TBM thrust loads over a segment flank

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) 3.5.8.2 - Nature of infilling material

Except for materials with a granular matrix, the composition of an infilling material will determine:

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• its rheology: the infilling material must, on the one hand, be sufficiently fluid to facilitate its placement and to fill completely the annular gap and, on the other hand, be sufficiently firm to avoid leakage through the shield tail seals and segment waterproofing gaskets, as well as to avoid seepage flows along the shield tail towards the front of the TBM. Moreover, its setting time must suit the construction conditions; •its short- and long-term structural properties: the infilling material must suit the ground and construction conditions (convergence of ground, rate of penetration, etc.); its modulus of deformation and compressive strength must be sufficient to prevent lining out-of-roundness. There are currently two main types of grouting material: •active material: cement-based grout to which fly-ash, sand, filler, bentonite, lime and admixtures such as water-reducing plasticizer, retarder or accelerator may be added; • inert material: cement-free material comprising a mixture of bentonite, polymers, filler and sand with the possible addition of a plasticizer. The material will be termed "semi-inert" if lime or fly-ash is added. In the case of sufficiently stable enclosing ground, infilling can be carried out in two stages: • primary infilling with a fine gravel "matrix" to ensure blocking of the lining in step with TBM penetration; • secondary grouting to improve bond between the lining and the ground, which is often deferred and independent from TBM penetration. 3.5.8.3 - Back grouting implementation

As detailed in Section 3.5.8.1, back grouting is carried out in step with TBM advance for reasons of efficiency. Two implementation processes are commonly used:

• grouting through the lining by means of holes incorporated in the segment structure (grouting operation generally not controlled by the TBM); • continuous grouting at the rear end of the shield tail through integrated grout pipes arranged longitudinally within the tail (grouting operation controlled by TBM). In general, the latter process provides greater control of ground deformations. Grouting pressures to be implemented are determined in relation to the type of infilling material, geological and hydrogeological conditions (ground loads and stiffnesses, hydrostatic pressure), lining strength and construction aims sought (formation of ground-based suppor t, control of ground deformations, etc.). During constr uction, it is impor tant to monitor continuously the grouting pressures as well as the volume of material back grouted. 3.5.8.4 - Additional grouting

Should the primar y back grouting prove insufficient for the purpose of the structure, resorting to additional grouting through the lining may be contemplated. 3.5.8.5 - Grouting quality control

In general, grouting quality control is preferable to damaging the lining by taking core samples. However, the latter do allow the quality of annular gap infilling to be ascertained, although they should obviously be limited to a minimum.

3.6 - Lining installed outside the area occupied by the TBM It may be advantageous to erect segments outside the area enclosing the TBM when ground stability permits and water inflows are naturally low. 3.6.1 - Ring design principle In this case, the ring is built outside the shield tail after placing the invert segment; ring stability is only ensured after longitudinal driving in of the key segment which, due to its trapezoidal shape, expands the ring thereby pushing the segments against the surrounding ground. The length of the key in the longitudinal direction of the tunnel is less than the length of the ring enabling it to be driven in to a maximum between the two counter

Figure 36 : Example of expanded ring

segments; the longitudinal travel of this key can vary slightly depending on the ground and the erected configuration of the first segments. 2 to 3 cm thick pads feature on the extrados of the segments allowing both a good bond with the surrounding ground and the possibility of light back grouting. Other ring closure systems have also been implemented on some projects: double key, spring line jacking system. 3.6.2 - Advantages and drawbacks The advantages of this expended ring solution are: • straightforwardness of implementation; • the simplicity of the structure (no waterproofing gaskets); • speed of progress in good ground, when very regular excavation can be maintained; • segment erection is very regular and follows the excavated profile because segment contact surfaces are cylindrical; • saving due to no intersegment bolting. However, there are also quite a number of drawbacks to this solution: • ring building and the TBM guiding system are not interrelated (no rear cylinders); • ring geometry does not allow them to be deviated one way or the other ; shims have to be slipped between rings to follow correctly the excavation; • "Wingdings" contact faces between rings are no impervious and it is impossible to undertake pressure back grouting to fill in properly the gap left around the extrados pads;

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 223 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) • nothing at all can be done to ensure any degree of water tightness if water inflows are incompatible with tunnel operation; in this case, a waterproofing membrane and a cast in-situ lining should be implemented if possible;

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• segment erection becomes highly problematical when the ground becomes unstable during excavation or under the action of the grippers (overbreak and block falls); • a void at the crown must be mortar-filled because the key segment is shorter than the ring length in the longitudinal direction. This solution can be advantageous as long as, using the same TBM, it allows segment erection inside the shield tail when difficult ground is to be penetrated or water inflows occur.

3.7 - Specific aspects of water conveyance pressure tunnels 3.7.1 - Hydrogeological reminders Knowledge of groundwater pressures within the soil-rock mass is important when a tunnel is to be pressurized. This involves, not only very conventional installation and reading at regular intervals of piezometers, but also monitoring the regime of surrounding springs. The reader is referred to the text entitled "recommandations pour le choix des paramètres et essais géotechniques utiles à la conception, au dimensionnement et à l'exécution des ouvrages creusés en souterrain" (recommendations for the design, the sizing and the construction of underground excavated str uctures) presented by A.F.T.E.S. Working Group 7 in T.O.S. Issue 123, June 1994. Once the pre-excavation hydrogeological conditions have been established, the influence of tunnel excavation on behaviour of the groundwater table(s) must be determined. Tunnel-driving in fact often causes a collapse of piezometer levels and therefore alter s significantly the hydrogeological conditions in the vicinity of the works. 3.7.2 - Tunnel lining structural behaviour 3.7.2.1 - Geotechnical aspects

Behaviour of a segment ring therefore depends on the state of equilibrium of the following forces, the first two tending to close intersegment contact joints and the third tending to open them:

• confinement pressure; • external hydrostatic pressure; • internal fluid pressure (including hydraulic transient pressures, in some cases). It should be noted that the geotechnical parameters (E, c, CARSPECIAUX 102 \f "Symbol" ) of the surrounding ground play a preponderant role in the relevant states of equilibrium and, as a result, they must be known for each geological formation crossed. In general, cases of adverse loading with respect to opening of intersegment contact joints are obtained for moderate overburden associated with low geotechnical parameters. Analysis of overall structural behaviour must also take into account the presence of back grouting which blocks the lining rings within the surrounding ground. 3.7.2.2 - Functional aspects

When the zones which will tend to open the lining rings have been localized, the hydromechanical behaviour of the tunnel lining structure will then depend on: • the presence of bolting and its sizing; • the degree of contact joint opening with respect to compression of the incorporated waterproofing gaskets. In general, take-up of tensile loads by bolts can only be justified in the shor t-term, except if suitable anti-corrosive material or protection is provided.

However, tests carried out have allowed the following main results to be brought to light: • application of the Colebrook formula is possible with a segments lining; • loss of head can therefore be calculated from an equivalent roughness Ks; • equivalent roughness can be related to the absolute value (s) for the average outof-flushness between segment rings; • for 0.60 m long rings and for tunnels with diameters less than 3.60 m, the following formula has been established: Ks (mm) = 0.3 + 60 X (lsl)2/1 where s (positive or negative "step") and l (length of unit) are expressed in mm. N.B.: it is advisable to use this formula with care because it has been established from a limited number of cases and for 0.60 m long concrete segments of various types (smooth or incorporating pockets); • Unlike the behaviour of cast-in-place tunnel linings, smoothing by deposits of the many out-of-flush locations will tend to reduce the equivalent roughness of a segment-lined tunnel. However, estimating not only this reduction but also that of bore sectional area associated with the presence of these deposits (reduction in conveyance) remains a delicate task for the engineer.

3.8 - Construction tolerances

Consequently, elimination of bolting in a tunnel subject to a tendency for its lining contact joints to open would appear desirable. Structural waterproofing is only then ensured if the lining gaskets open only partially with respect to their compressive strain. A factor of safety of 2 is desirable to allow possible self-adjustment of the segment rings during loading / unloading (tunnel pressurizing / depressurizing) cycles.

3.8.1 - Specification

If such conditions cannot be ensured and total watertightness is required, an additional waterproofing system is then necessary (cast-in-place concrete ring, membrane or any other suitable process).

Very often, these tunnel operation-related tolerances are complemented by other tolerances, in this case associated with lining implementation to ensure correct structural behaviour, as well as the required quality of finish and imperviousness.

3.7.3 - Roughness of segmentlined tunnels Relatively few loss of head measurements have been taken on segment-lined wastewater collectors due to the complex nature of the means to be implemented.

Construction tolerances for segments forming the permanent lining of an underground structure must be specified on the basis of general service criteria and must be laid down in the project specifications. Moreover, they can vary depending on the lining design retained (expanded segments, bolted segments, etc.).

3.8.2 - Identification of main criteria contributing to tolerance specification 3.8.2.1 - Criteria related to tunnel function

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Function-related criteria essentially comprise:

• ring length (taking into account its possible taper);

• tunnel internal sectional area

• flatness of intersegment contact surfaces within the same ring and between rings;

- aggressive chemical agents contained in the ground, in the groundwater and in the liquid conveyed,

• roughness;

- micro-organisms,

• layout and geometry of pockets provided (waterproofing gasket grooves, recesses for plugs or pins, etc.);

- sulphate,

The accuracy sought must satisfy the general profile of the tunnel bore over its whole route;

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• operating clearances For this and for the last point, these criteria influence specification of tolerances concerning the tunnel construction axis with respect to the design theoretical axis; • water and air conveyance flows It is important that project specifications lay down geometrical criteria (surface roughness, out-of-flushness, etc.) allowing acceptable structural head losses; • layout of equipment fixings or pockets. 3.8.2.2 - Tolerances associated with segment implementation

These tolerances can be broken down in terms of the following two construction stages: • casting: - mould geometry, - bending and fixing of reinforcing steel; • installation: - ring building, - lining deformation under the action of back grouting and surrounding ground.

• layout of connector inserts (pick-up sockets, bolts, connectors, etc.);

All criteria defined above must be considered in order to specify tolerances in the construction axis with respect to the structural design axis. The required means should then be implemented to ensure compliance with tolerances for the segments forming the relevant ring. Successive combinations of these tolerances mean that dimensions must be kept within small variations at mould stripping. Geometrical proper ties foreseen at construction study stage should be reproduced to obtain contact faces between segments or rings which are capable of transferring loads between ring par ts and ensuring proper watertightness. It is therefore essential to identify clearly the segments sections requiring par ticular levels of geometrical accuracy and to quantify them.

- condensation, - frost, - salts,

• clearances allowed for assembly devices;

- fire, etc.

• dimensions of reinforcing cages and layout of the different reinforcing bars to ensure:

The use of admixtures influencing the strength of concrete mixes must be checked to ensure satisfactory performance in relation to the various forms of attack mentioned above.

- suitable corrosion protection, - proper operation of parts with respect to contact pressures, - efficient edge reinforcement. It is very difficult to recommend the exact tolerance values to be complied with on a finished product because they depend on numerous parameters such as the overall dimensions of the structure to be built, the method of erecting segments and their shape. To achieve the levels of accuracy sought, it is clear that concrete shrinkage and temperature are parameters which must be considered, especially during segment inspection at the precast plant.

3.9 - Durability

3.8.3 - Accuracy

- hydrocarbons,

3.9.1 - Segment concrete Segment concrete durability depends on the purpose of the tunnel and may be associated with the following criteria: • compactness of concrete mixes; • concrete mix proportions: - fine aggregate, - coarse aggregate, - cement, - admixtures, - water, all require physical chemical analysis of their constituent materials; • active alkaline balance; • permeability;

The main criteria to be examined are:

• environment-related internal and external forms of attack:

• general dimensions of the assembled ring;

- temperature,

Selection criteria referred above must take into account the requirements of both the contract specifications and current standards. Specific impervious treatment (mineralization, impregnation, etc.) can be applied to the extrados of segments to provide protection against particular forms of attack. 3.9.2 - Steel reinforcing bars Durability of steel bars used for segment reinforcement cages is related to the permeability of the encasing concrete and depth of concrete cover, as well as to the internal and external aggressive environments mentioned above. The choice of cement types and their contents influence passivation of steel reinforcing bars. Chemical composition and surface condition of these steel bars must ensure good weldability for reinforcing cage fabrication. Depth of concrete cover to reinforcement must be specified with respect to conditions of structural exposure laid down in the project specifications. It may vary depending on application zone for the stresses encountered and the level of protection sought in relation to the relevant forms of attack (intrados fire resistance, inter segment contact, TBM thrust cylinder bearing surface, etc.). In general, these depths of concrete cover can vary from approximately 20 mm (ironbanded reinforced zones) to approximately 30 mm (standard intrados and extrados zones). In some cases, steel protection systems can be applied:

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 225 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) • painting; • metallization (galvanization, cataphoresis, metal-coating); • epoxy-coating; • cathodic protection (electrical links).

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3.9.3 - Waterproofing gaskets In material terms, durability in gasket watertightness is associated with the same criteria as those listed above for short- or longterm concrete attack. Once these criteria have been checked, the rate of long-term relaxation of the waterproofing system should be ensured.

tions must allow standardization in favour of one or two possible ring lengths. Creation of several ring lengths on a project effectively multiplies the number of mould and reinforcing cage types, leading to more complicated and therefore more expensive precasting, control and handling. These considerations are also linked to the type of TBM used (existing machine possibly modified for the project, new TBM designed and built for the project). Reuse of TBMs and segment moulds undoubtedly provides substantial savings on similar projects, even if servicing, adaptation and overhaul is always required.

Gluing of gaskets is only carried out to ensure that they remain in their grooves during segment handling, storage and erection. Adhesive used must be compatible with the type of gasket.

Thus, on projects such as:

3.9.4 - Connector inserts

• wastewater collectors, etc.,

Connector inser ts must be durable and neutral with respect to their environment. In all cases, they must comply with safety standards in force.

it would appear desirable to retain geometrical and equipment standards on a project basis at national level.This would allow production capacities to be increased and costs to be reduced. These standards could be based on the "récommandations sur la standardisation des profils des tunnels circulaires" (Recommendations for the standardization of circular tunnel profiles) presented by A.F.T.E.S. Working Group 11 (see T.O.S. Issue 88, July-August 1988): this principle is already in hand in several European Community countries and in those under Anglo-saxon influence.

They must be positioned in accordance with values of concrete cover to reinforcement. They must cause neither structural weaknesses nor preferential water seepage or electrical flux paths.

3.10 - Economic considerations Right from the start of the project, tunnel lining design must integrate construction methods and take into account factor s involving its adaptation to the tunnel construction location (influence on materials, labour, management and supervision, etc.) with both engineering optimization and cost-saving in mind. From the beginning, it is important to determine the type of ring best suited to the project (universal or "left"/"right" rings), the most appropriate ring length, the taper and the number of types of ring required for the project. In general, the universal ring is the most economical from the segment fabrication point of view (less moulds required, smaller precast plant area, less types of segment reinforcing cage). Analysis of the tightest curves on a project alignment compared with the straight sec-

• mass transit railway tunnels, especially for the Val system; • rail and high-speed rail tunnels; • road tunnels;

4 - TUNNEL LINING DESIGN 4.1 - Main parameters influencing sizing 4.1.1 - Implementation conditions 4.1.1.1. - From segment precasting to workface supply

Between casting in the precast plant and supply to the tunnel workface in view of erection at the TBM, segments are subjected to a series of operations which can induce appreciable stresses in some. Whilst it is difficult to describe all these operations, which depend on a process varying from one project to another, a large number of operations never theless recur on a systematic basis. For example, this is the case for the segment turning stages

after mould stripping (both when casting horizontally - cf. conditions annexed to the present recommendations - and vertically), for the handling stages from precast plant to preliminary storage then storage areas (e.g. using a lifting beam fitted with grippers, suction pads or slings, etc.), for the storage stages involving segment stacking and insertion of timber blocks between units, for the stages of removal from storage and unloading on site, or for the stage involving segment supply to the workface (by trailer or railcar). The impact of each of these stages must be subjected to a reinforced concrete design check in terms of internal stresses induced in the segments. These design calculations must consider not only the possible dynamic effects of handling (e.g. placing a segment on a stack during lifting or storage stages) and implementation tolerances (e.g. accuracy of intersegment block positioning at the storage area), but also the true age of the concrete (and thus its characteristic strength) when carr ying out the relevant operation. In most cases, the process only involves design checking because the sectional areas of reinforcement and concrete proper ties for the segments are most often designed for tunnel service or TBM-excavation stages (thrust cylinder applied loads). However, certain cases can become dimensionally critical and lead to either improving shor tterm concrete proper ties or increasing reinforcement sectional areas, or to more suitable redesigning of cer tain process stages. 4.1.1.2 - Ring building (erection and bolting)

Assembly of segments behind the TBM is carried out using an erector. During erection, segments are subjected to a number of loads such as: • the load applied by the segment pick-up and lifting system (dead weight of the segments modified by a dynamic coefficient); • loads applied to compress waterproofing gaskets; • possible bumping impact loads; • loads associated with accidental impacts during approach; • loads applied by the assembly systems retained (bolts, anchor bolts or plugs). Although impact loads are very difficult to estimate and thus to integrate in the lining design, the segments must nevertheless be designed to withstand other types of load.

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All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Special care must be given to checking sections near connector inserts (unsupported thrusts, local stresses, risks of bursting), which often result in special construction requirements or local additional reinforcement.

strength, rock mass characteristic parameters;

tive humidity and shrinkage;

• deformation capacity parameters: - Young's modulus E(x, t),

b) Intersegment contact joint structural properties:

- Poisson's ratio ν.

• sectional area, inertia

4.1.2 - Parameters for analysing ring stresses

In surrounding rock, a reduced value of Young's modulus, compared with laboratory-measured values, should be considered to take into account the potential deformation capacity of discontinuities influencing the rock matrix.

These properties allow the capacity for load transfer between contact joint sectional areas to be deduced;

Reference may usefully be made to the summary table provided under § 4.3.5. 4.1.2.1 - Parameters associated with the surrounding ground

The main parameters concerning the surrounding ground, which can come into play in terms of analysing the behaviour of a precast concrete segmental tunnel lining, are recalled hereunder on the basis of the A.F.T.E.S. recommendations entitled "le choix des paramètres et essais géotechniques utiles ‡ la conception, au dimensionnement et ‡ l'exécution des ouvrages creusés en souterrain" (the choice of geotechnical parameters and tests of use in the design and constr uction of underground excavated structures). a) Parameter s associated with natural constraints The geological history of the soil-rock mass must be known; it may have been influenced by tectonics, consolidation or erosion. The basic parameters are: • intensity of principal stresses (in particular, evaluation of the term Ko = (

σ’ Ho ); σ’ Vo

• direction of stresses (effects of slope and dipping of ground layers, etc.). b) Physical parameters It is essential to have good knowledge of parameters such as: • the swelling potential; • the aggressivity of the surrounding environment; • the interfaces between ground layers, anomalies such as discontinuities, non-uniformities (voids, blocks, faults, etc.).

Good command of this parameter is of prime importance for the design of structures such as water conveyance internal pressure tunnels (danger of intersegment contact joints opening); • seismicity Evaluation of the surrounding ground dynamic characteristics may be necessary in high seismic risk areas. d) Hydrogeological parameters Knowledge of groundwater pressures within the soil-rock mass is essential in ever y underground structure project. In particular, it is necessary to determine the influence of tunnel driving on the behaviour of the groundwater table(s) (drainage, barrier effect, danger of a collapse in groundwater pressures, etc.). e) Ground characteristic curves Ground behaviour can be represented by shor t- and long-term convergence curves based on the geological, hydrogeological and geotechnical parameter s identified above. 4.1.2.2 - Parameters associated with TBM characteristics

The table below reveals the main functions of each TBM structural parameter and its potential impact on stress analysis. 4.1.2.3 - Lining structural properties

The tunnel lining is discontinuous and its structural proper ties depend on those of the segments and contact joints between lining parts. a) Segment structural properties:

• Poisson's ratio.

• ring composition The discontinuous nature of a ring (an assembly of elementary segments) leads to a reduction in its stiffness in bending, whilst its stiffness in compression is, in general, only slightly affected by the presence of intersegment contact joints. On the other hand, installation of adjacent rings incorporating combined radial contact joints, associated with rigid assembly systems between rings, such as plugs, enables this effect of reduced ring stiffness in bending to be limited. 4.1.2.4 - Soil - structure interaction

The main parameters influencing behaviour of the tunnel lining in contact with the ground are: • lining/back grouting material and ground/ back grouting material contact conditions. These can vary between total slippage and total adherence; however, they tend towards total adherence with the use of mortars grouted under good conditions. Under cer tain ground and groundwater pressure configurations, the idea of separation can complement these conditions (lack of contact between lining and surrounding ground). • environment - nearness of existing or planned structures underground or at the surface (buildings, utilities, existing tunnels or shafts, deep foundations, etc.), - superimposed loads (traffic, buildings foundations, etc.).

4.2 - Design assumptions

• sectional area, inertia

4.2.1 - Regulations and references

Two types of engineering parameter are usually characteristic of the soil-rock mass:

The inclusion of more or less large pockets needs to be taken into consideration in the evaluation of these parameters;

4.2.1.1 - Foreword

• strength parameters:

• modulus of deformation

c) Engineering parameters

- soil shear strength properties (Cu, φ', C') - direct compressive strength and tensile

This depends essentially on concrete strength and parameters such as creep, rela-

It should be recalled that limit states analysis allows checking of both the structure's factor of safety with respect to failure and its satisfactory behaviour with respect to serviceability.

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 227 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Main functions

Impact on stress analysis

- to ensure excavation face stability, - to limit ground deformations.

- increase in ground deconfinement at lining installation, → decrease in ground loads on lining.

- to reduce pressure on TBM shield tail and lining, especially when advancing in a curve,

- augmentation du déconfinement du terrain à la pose du revêtement

- to allow a degree of ground convergence over the length of TBM shield tail to reduce friction (especially in expansive ground)

- increase in ground deconfinement at lining installation → decrease in ground loads on lining.

- to reserve space for fitting different mechanical equipment from head to thrust mechanisms.

- when ground is not in continuous contact with the TBM shield tail extrados, ground loads on the lining are increasingly reduced as the shield tail length is increased, - conversely, ground convergence is limited to the annular gap between excavation and shield tail extrados.

Parameters

a) Confinement pressure

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b) Overcut

c) Shield tail conicality

d) TBM length

e) TBM thrust

f) Back grouting

→ diminution des charges de terrain sur le revêtement

- to ensure longitudinal advance, - to ensure a reaction to the confinement pressure, - to guarantee temporary stability of lining segments, - to ensure compression of impervious gaskets between rings prior to installing assembly system, - to ensure key segment can be driven in.

- depends on thrust loads implemented and thrust cylinder ram pad/segment contact conditions (eccentricities due to installation tolerances, curved alignment, buoyancy, localized loads) and ring/ring contact conditions (geometrical defect in segments, eccentricities due to installation tolerances, localized loads).

- to ensure blocking between lining and ground, - to limit ground deformations.

- local pumping thrust, grouting pressure, strength properties of back grouting material (modulus, Rc, etc.), - reduction in ground deconfinement, → increase in ground loads on lining.

Ultimate limit state

Serviceability limit state

Failure of a section due to crushing of concrete

Excessive opening of cracks (infiltration, corrosion)

Excessive deformation of steel Instability of shape (buckling, bulging)

Excessive compression of concrete causing microcracking

Loss of static equilibrium at ring erection

Excessive ring deformations

The main characteristics of the two limit states are recalled in the following table. 4.2.1.2 - Applicable regulations and recommendations

The following regulations or recommendations apply to limit state analyses of reinforced concrete lining segments: • "Fascicule n° 62 Titre I section 1 - Régles techniques de conception et de calcul des ouvrages et constructions en béton armé suivant la méthode des états limites (Régles BAEL 91)" (Engineering rules for limit state design and analysis of str uctural and constructional reinforced concrete based) (French reinforced concrete code of practice). • Eurocode 2, published and annotated by AFNOR (French Standards Institute) in December 1992; • "Instruction technique sur les Directives Communes relatives au calcul des constructions" (Engineering guide to Common Directives covering str uctural design) (French Govt. Circular n° 79-25 of 13th March 1979); • CEB-FIP International Recommendations, 1990; • NCF (French national railway company) "Livret 2.01" covering railway loadings and reinforced concrete design rules; • CPC Fascicule n° 61 Titre II covering road imposed loadings (French code of practice); • AFTES Recommendations currently in force; and, in some cases: • "CCTG Fascicule n° 62 Titre V - Règles techniques de conception et de calcul des fondations des ouvrages de génie civil" (Engineering rules for design and analysis of civil engineering structures) (French code of practice); • "Récommandations provisoires relatives à la modification des règles de prise en compte de la fissuration et à l'emploi des bétons à hautes performances" (Provisional recommendations for the amendment of rules for considering cracking and for the use of high-strength concrete), edited and circulated by SETRA (French national highway engineering agency) in June 1997; • "DTU n° 14.1 - Travaux de cuvelage" (French unified code of practice - Lining and tanking work), October 1987. The project specification must detail the regulator y documents applicable to the contract, as well as their priority of application.

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 228 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) These methods and their conditions of application are described in detail in "texte des réflexions sur les méthodes usuelles de calcul du revÍtement des souterrains" (recorded thoughts on the usual design methods for tunnel linings) presented by A.F.T.E.S.Working Group 7 in T.O.S. Issue 14, March-April 1976.

they must be considered when sizing the segments.

b) Loads applied to the ground surface

• the characteristic tensile strength Ftj;

Actions thus determined are then used to analyse lining stresses by the different methods reviewed in Section 4.3 below.

• longitudinal instantaneous and long-term moduli of deformation;

c) Loads induced by neighbouring structures

• stress-strain diagrams;

These loads are determined from drawings of neighbouring structures (foundation drawings, etc.) passed on by the Owner.

4.2.2 - Material properties 4.2.2.1 - Lining concrete

Reinforced concrete properties to be introduced in design analyses are those specified by the BAEL and Eurocode rules in force, i.e.:

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• the characteristic compressive strength Fcj;

• Poisson's ration, which is usually taken as 0.20. 4.2.2.2 - Steel for passive reinforcement

Steel properties to be introduced in design analyses are those specified by the BAEL and Eurocode rules in force, i.e.: • the guaranteed elastic limit Fe; • the modulus of longitudinal elasticity, which is taken as 200,000 MPa; • stress-strain diagrams.

d) Hydrostatic and hydraulic pressures With regard to the external hydrostatic pressure exer ted by the hydrogeological environment, this will be determined on the basis of maximum and minimum water levels. In this connection, the potential impact of buoyancy on the lining and its assembly mechanisms, especially during construction at ring break-away from the TBM, should be recalled.

4.2.3 - Nature of actions and loadings

In the special case of water conveyance tunnels, the contract must detail values representing internal fluid loads to be considered and how they should be taken into account.

4.2.3.1 - Permanent actions (G)

e) Annular gap back grouting pressures

Permanent actions include the following loads:

If back grouting pressures exer ted on the lining extrados are higher than the ground pressure, they should be considered when sizing the segments.

a) Dead weight of the structure and weight of fixed equipment, b) Surrounding ground loads.

4.2.3.2 - Variable actions (Q)

The convergence-confinement method evaluates ground actions on the lining. This method has already been described in several publications. Recommendations on the use of this method, presented by A.F.T.E.S. Wor king Group 7 in T.O.S. Issue 59 of September-October 1983, will be updated in the near future.

Variable actions include the following loads:

Moreover, in the case of shallow soils and structures, various authors have proposed semi-empirical formulae, derived from theor y or experience, for calculating the vertical load exerted by the ground on the lining in terms of the density, cohesion and angle of internal friction of the soils, the excavation radius and the depth of overburden above the crown (notably the TERZAGHI, PROTODIAKONOV and LAUFFER methods).

On the other hand, when these loads are applied to a road pavement or rail trackbed suppor ted by a slab bearing or built into the tunnel lining,

Most frequently, these are pedestrian or vehicle loads. c) Loads applied during construction • Loads induced from precasting to ring building The sizing of each tunnel segment should be checked in relation to the different construction stages: segment handling, possible turning, storage, transpor t, loading onto cars, unloading, pick-up by erector and installation. • Loads induced by TBM penetration TBM thrust cylinder loads represent longitudinal forces applied to the lining segments through the load distribution pads. Each thrust load distribution pad is acted upon by one or more cylinder s, whose resultant thrust is eccentrically exerted.This eccentricity, which is measured with respect to the centre of gravity of the ram pad bearing surface, comprises a known structural component and an additional random component. It should be recalled that when the TBM follows a curved alignment (horizontal or ver tical), the structural eccentricity of the resultant thrust from the cylinder(s) with respect to the ram pad bearing surface is often increased and must consequently be integrated in the analysis.

EXAMPLE OF ECCENTRIC PLANE TRANSVERSE CONTACT JOINT extrados

a) Operating loads inside the tunnel When these are wheelinduced rolling loads applied directly to the tunnel inver t slab, their influence is usually ver y modest and they can therefore be neglected in the design of the lining cross-section.

For example, in the case of water conveyance tunnels, the contract should specify, if necessary, the value representing the variation in internal hydraulic pressure to be considered and the manner in which it is to be taken into account.

peripheral groove extrados side

half lining wall thickness

width of transverse contact joint

Centre of gravity of thrust cylinder ram pad bearing surface half lining wall thickness

eccentricity of thrust

resultant load exerted by a thrust cylinder group

peripheral groove intrados side intrados

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 229 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) The load exerted by each group of thrust cylinders and its eccentricity are two parameters for which rated working and exceptional values must be specified.

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

In a basic combination, the values of these two parameters to be considered are the rated working values. In an accidental combination, only one of these parameters reaches its exceptional value, the other parameter maintains its rated value. It is acknowledged that both parameters cannot reach their exceptional value at the same time. Whilst it is rare for the rated thrust to be reached simultaneously by all cylinders, it is frequently so for each group of cylinders. Those data are used to prove the strength of the concrete and reinforcement, in particular under the localized loads exerted by each group of thrust cylinders. Fur thermore, segment sizing must also be checked under the loads exer ted by the passage of the TBM back-up equipment. • Loads induced by the grouting material when filling the annular gap between the lining and the surrounding ground These transient-type loads result from a localized increase in grouting pressure ("local pumping thrust") directly behind the segment grouting holes. In certain special cases, where total control of this phenomenon is essential, the impact of these actions on the lining should be checked. Data is then required on the grouting system incorporated in the TBM (number of grouting points, grouting process, etc.) and on the procedure for implementing these grouting operations retained by the contractor.

Note:

4.2.3.3 - Accidental actions (FA)

dead weight loads), it is very important to ensure the validity of both the ground engineering properties taken into account and the stress analysis method. The non-uniformity, anisotropy, jointing and fissuration of the soil-rock materials and the difficulty in forecasting their long-term behaviour must also be considered.

In the case of certain structures, for which accidental actions (e.g. earthquakes, explosions, vehicle impacts or "waterhammer" in water conveyance tunnels) must be considered, the contract shall detail values representative of the corresponding actions and how they should be taken into account.

The 1979 Common Directives are ver y clear on this subject: Section 4.1.3 states that "the maximum and minimum characteristic values of actions corresponding to ear th pressures shall be evaluated taking into account the uncer tainties resulting from their method of calculation".

4.2.4 - Combined actions

However, when detailed analysis is not required or when a calculation process does not allow the effects of the ground and water to be separated, the following combined actions shall be retained:

In the case of certain structures, for which temperature gradient-based effects must be considered, the contract shall detail values representative of the corresponding actions and how they should be taken into account.

4.2.4.1 - Design stresses with respect to strength ultimate limit states

a) Foreword In the case of models which introduce a non-elastic law in terms of ground behaviour (occurrence of zones in a plastic state), soil-structure analysis should be carried out, without uplifting the actions involved, and the ultimate limit state weighting coefficients detailed below should be applied to the resulting forces. b) Basic combined action put forward by BAEL 91 and Eurocode 2 1,35Gmax + Gmin + γQ1 Q1 + Σγ Qi ΨOi Qi Gmax : total unfavourable permanent actions (e.g. lining dead weight, ground pressure); Gmin : total favourable permanent actions; in some cases, water pressure can be favourable and should then be weighted by 1 ; in other cases, it should be weighted by 1.35;

• Uniform temperature variations

Q I : basic variable action; in the present case, such actions would be road, rail or water conveyance operating loads or loads applied during construction;

In general, uniform temperature variations do not require consideration.

γQ1 is equal to 1.5 in general, except for railway loads (1.45).

Note:

Σγ QiΨ OiQ i are the accompanying variable actions.

d) Climatic temperature-induced actions (t°)

In the case of certain structures (very deep tunnels, energy conveyance tunnels, etc.), uniform temperature variations must be considered; the contract shall detail values representative of the corresponding actions and how they should be taken into account.

The basic combined action can therefore be expressed as: 1,35Gground + 1,35(ou l)Gwater + 1,50Q1

• Temperature gradients (∆θ)

The largest contributor y factors in this expression are obviously the actions on the lining due to water and ground.

In general, temperature gradients do not require consideration.

Because the action due to the ground is only weighted by 1.35 (e.g. for permanent

1,35xS{Gground + Gwater + (1,50/1,35)xQ1} obtained by multiplying the infrequent combined action by 1.35: S{Gground + Gwater + (1,50/1,35)xQ1} For checking the lining under construction, the basic combined action is expressed as: 1.35Glining dead weight + 1.35QI QI : TBM thrust cylinder load, back grouting pressure or weight of back-up train. c) Basic combined action derived from 1979 Common Directives A combined action more directly derived from § 7.2.1 of the 1979 Common Directives and featuring in BAEL 91 Fascicule n° 62 - Titre V "Règles techniques de conception et de calcul des fondations" (engineering rules for the design and analysis of foundations) can also be put forward. Characteristic values of actions are increased by two coefficients γ F3 et γ F1 : γF3S{ γ F1GmaxGmax+ γ F1Gmini Gmini

+ γ F1Q1Q1 + ΣγF1Q1ΨOiQi }

- the coefficient γ F3 must enable one to take into account the uncer tainty of the stress calculations and the simplifications resulting from the models and diagrams; its value usually lies between 1.125 and 1.15; - the coefficient γF1 must allow one to take into account the risk of exceeding the characteristic value of the action; it can take a value very close to 1 for the action due to water if, for example, it represents a hydro-

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 230 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) static pressure corresponding to an accurately known water level and can never be exceeded; but, for the action due to the ground, it can attain values near to 1.2, and even higher if the ground investigation is limited and risky. The resulting combined action can be expressed as:

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1,35*Gground + 1,15Gwater + 1,50Q1 * this coefficient can be higher if the action due to the ground is difficult to estimate. This type of combined action must be laid down in the project specification. d) Accidental combined action put forward by BAEL 91 and Eurocode 2

accident-related situation corresponding to loss of lining permeability; the check will then be carried out using the following accidental combined action:

4.2.4.2 - Design stresses with respect to serviceability limit state

This involves proving the strength of the segment ring with respect to the ultimate states without neglecting so-called second order effects such as buckling, bulging, etc. This proof is only to be considered for very thin linings installed in ground with very low moduli of deformation.

Gmax + Gmin +Q1 +ΣΨoiQi

4.2.5.3 - Static equilibrium limit state

When the tunnel is in operation, this combined action is expressed as:

In some cases, non-buoyancy checking should be anticipated.

Gwithout water ground + Gtotal water pressure + 0.6QI

Gground + Gwater + QI or Gground + Gwater

Gmax + Gmin +FA +Ψ11Q1 + ΣΨ2iQi

4.2.5 - Sizing criteria

FA : nominal value of accidental action;

4.2.5.1 - Strength ultimate limit state

Qi : variable accompanying actions.

Design stresses derived from basic or accident-related combined actions must not exceed the ultimate limit capacities for reinforced concrete sections specified by § A.4.3 of the BAEL 91 rules or Eurocode 2 and resulting from the following limiting strains established for the relevant materials:

The maximum capacity of the TBM thrust cylinders is to be considered as an accidental action. In the case of an earthquake or explosion: Gground + water + Earthquake (or explosion) + 0.6QI Notes : Another type of stress, comparable to an accidental stress, can be provided in the project specifications. This is the total overburden load and is usually considered for shallow tunnels (overburden of 1 to 2 diameters) or deeper tunnels likely to be subjected to long-term effects which are difficult to predict in ground surveys (creep, swelling, etc.). This action is therefore considered with a coefficient of 1: Goverburden weight + 0.6QI In this combined action, accompanying actions are usually neglected and the action due to the ground is often assumed to be uniformly distributed over the lining. This combined action has no geotechnical significance but it has the merit of testing the factor of safety associated with the lining bearing capacity with respect to the weight of overburden. It does not apply to very deep tunnels in ground with a high modulus of deformation (e.g. Alpine tunnels). Finally, in the case of impervious jointless precast concrete lining installation, project specifications can suggest considering an

4.2.5.2 - Shape stability ultimate limit state

• 10 x 10-3 for the elongation of reinforcing bars; 10-3

for the shortening of partially • 3.5 x compressed sections comprising concrete with a strength fcj of less than 60 MPa; for high-strength concrete (fcj ≥ 60 MPa), the limit for shortening is given by the relation: (4.5 - 0.025fcj) x 10-3 ; 10-3

for the shortening of concrete in • 2x a fully compressed section. Material design stresses are obtained by applying the following coefficients to their characteristic strengths: γ s = 1.15 for steel under basic combined action; γs = 1 for steel under accidental combined action; γb = 1.5 for concrete under basic combined action; γ b = 1.15 for concrete under accidental combined action. Because segments are concreted in a precast plant, coefficient γb can be reduced (only in strength ultimate limit state analysis) to 1.3 under basic combined action as long as quality inspections comply with an ISO system; these special provisions must be laid down in the project specifications.

Nominal permanent downward ver tical loads associated with permanent actions must be at least 1.05 x the water-induced upward loads resulting from exceptional water level conditions. Possible time-dependent variations in structural overburden (e.g. sinking of a riverbed, etc.) must also be taken into account. 4.2.5.4 - Ser viceability limit states with respect to structural durability

a) Crack opening limit state Because concrete is highly alkaline (pH ≈12), reinforcing steel is normally protected by passivation (formation of Fe(OH) 2 at the surface of the reinforcing bar). However, care must be taken to ensure that: • cracks are not excessively open; • concrete cover is sufficient in relation to the environment; • the concrete mix is satisfactory. The r ules specify conditions governing concrete cover and cracking. In accordance with the new recommendations amending Section A.5.3. of BAEL 91, reinforcement tensile stresses are limited as follows: • for detrimental cracking (lining in the presence of moderately aggressive water):

σs=sup[240MPa,110√ηftj] η : cracking coefficient equal to 1.6 for hightensile deformed bars (φ > 6 mm), 1.3 for high-tensile deformed (high bond) bars (φ ≤ 6 mm) and 1 for plain (hot-rolled) reinforcing bars; • for highly detrimental cracking (lining in highly aggressive environment):

σs=sup[200MPa,90√ηftj] In addition to the above limiting tensile stresses, Section A.4.5. of BAEL 91 specifies the maximum diameters and spacings for reinforcing bars.

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) b) Concrete compression limit state In accordance with Section A.4.5.2. of BAEL 91, the concrete compressive stress at the serviceability ultimate limit state is limited to 0.6 fcj.

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4.2.5.5 - Fire resistance

Project specification requirements in relation to fire resistance must state the level of fire stability for the lining and this must be justified by the organization and emergency facilities in the event of fire in the tunnel(s).

4.3 - Determination of stresses in the tunnel lining

Moreover, this method of analysis remains equally applicable in the following situations:

to be determined. The normal force can be directly derived from this.

• tunnel located at shallow depth;

In this case, initial stresses are considered uniform and isotropic, but the surrounding ground can be considered as obeying a law of elastic-plastic behaviour.

• non-uniform surrounding ground (several different formations); • dissymmetrical external loads (dissymmetrical existing structures, load transfers due to excavation of nearby structures, etc.); • dissymmetrical lining (dissymmetrical distribution of radial joints). The main simplifying assumptions associated with this type of model are as follows: • ground behaves elastically; • "springs" modelling the surrounding ground are mutually independent.

4.3.1 - Introduction Given the circular shape of the tunnel lining, several methods of analysis are possible for determining the stresses due to interaction of the soil and the structure. The two main methods are:

Because this method is quick, it is used for the selection of critical sections and for impact studies of certain parameters, especially when the analytical resolution method cannot be applied. It is used at preliminary design, design or construction study stage. Naturally, this method does not allow possible surface settlements to be tackled.

• the hyperstatic reaction method • the composite solid method.

4.3.3 - Composite solid method

4.3.2 - Hyperstatic reaction method

This method enables the behaviour of the ground-structure system to be studied.

The hyperstatic reaction method studies the behaviour of the lining alone by likening the action of the ground to external loads. It therefore favours the role of the lining. As a result, it should preferably be applied to a rigid lining located at shallow depth and in enclosing ground comprising weak soils or very closely fractured rocks. The drawback of this method is that it does not take into account: • the behaviour of the ground after failure and with respect to time; • the deformation the surrounding ground has already reached when the lining is installed; • the different excavation stages. However, a number of specific assumptions made for modelling the structure and the external loads enable the principles of the composite solid method to be approached (e.g. maintaining of "springs" in tension) when conditions of application are satisfied. Stresses and strains are calculated using a numerical resolution method usually based on a 2-dimensional model made up of wireframe elements with straight or cur ved members.

It considers the surrounding soil-rock mass as a continuous medium (basic assumption often made). 4.3.3.1 - Analytical solutions

Based on soil and rock mechanics theories for continuous media, analytical solutions allow the lining forces (normal forces, shear forces, bending moments) and elastic line to be determined.

Characteristic of soil-structure interaction and contributing to the application of this method, the impor tant parameter is the deconfinement ratio CARSPECIAUX 108 \f "Symbol" . Among its current methods of determination, we find those of: • Panet; • Corbetta; • Bernaud; • Minh-Guo. The last two methods allow the stiffness of the support to be considered. Because of the simplifying assumptions referred to above, analytical solutions are not valid in the following situations: • tunnel located at shallow depth; • non-uniform surrounding ground (several different formations); • dissymmetrical external loads (dissymmetrical existing structures, load transfers due to excavation of nearby structures, etc.); • dissymmetrical lining (dissymmetrical distribution of radial joints). However, because these methods are very quick, they are often applied for the selection of critical sections and for impact studies of certain parameters. 4.3.3.2 - Numerical resolution

• loads are uniform isotropic or anisotropic (Ko ≠ 1); they can be derived using the convergence-confinement method or take into account the total stress exer ted (Erdmann formula);

Based on the use of finite element, or sometimes finite difference, numerical models, this method allows 2- and 3-dimensional problems to be tackled. It favours neither the role of the lining nor that of the surrounding ground. As a result, it applies to a lining of any stiffness located at any depth in uniform or non-uniform surrounding ground comprising several different formations and in the presence of symmetrical or dissymmetrical existing structures.

• the assumed single layer of ground behaves elastically;

This method of analysis has the advantage of taking into account:

• contact between the lining and the ground can either be considered as in total adherence or as in total slippage.

• the deformability of the ground and, in par ticular, its behaviour after failure and with respect to time;

Convergence-confinement method

• the redistribution of loads resulting from lining deformation;

Standard assumptions are as follows: • lining geometry is circular and uniform (joints not directly considered);

The convergence-confinement method allows, on the one hand, the loading and, on the other hand, the ring radial displacement

• the 3-dimensional nature of the excavation associated with the presence of a cut-

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) ting face, through the concept of a deconfinement ratio C ARSPECIAUX 108 \f "Symbol" introduced either in the 2-dimensional model or directly in a 3-dimensional model;

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• the excavation stages. In standard cases analysis is limited to 2dimensional modelling in which the influence of the cutting face is considered by applying the convergence-confinement method referred to in the above section. This numerical resolution method of analysis is also valid for non-uniform and anisotropic initial stresses, i.e. even when a dissymmetrical feature is present in the structure (dissymmetrical distribution of radial intersegment contact joints), in the surrounding ground (several different formations, etc.) or in the external loads (nearby existing structures, etc.). Several types of ground behaviour can be modelled: elastic, fully elastic-plastic, elasticbrittle with softening (uncommon), anisotropic with respect to deformation and/or strength, etc.. The simplifying assumptions remain as follows: • initial deformation after lining installation is neglected; • every segment is not usually considered individually;

estimation methods, which are easy to implement but for which the area of application is limited to situations considered through the feedback of experience on which they depend (cf. Recommandations relatives aux tassements liés au creusement des ouvrages en souterrain Recommendations concerning settlements associated with the excavation of underground str uctures - T.O.S. Issue 132, November-December 1995), numerical resolution is the only method of analysis valid for approaching surface settlements. 4.3.4 - Adaptation of analysis methods to a segments lining and to TBM-based excavation The specific nature of designing a tunnel lining to be installed behind a TBM arises, on the one hand, from the tunnelling method and, on the other hand, from the nature of the structure. When it allows confinement of the excavation face, the tunnelling method can be reflected in the analyses by: • either using a deconfinement cur ve, based on a (σ0 - ps) stress condition, combined with the application of a pressure p s representing the confinement pressure applied at the workface;

• concrete shrinkage is neglected.

• or adopting an extension in space of the deconfinement curve behind the excavation face; thus, the deconfinement ratio taken into account at a certain distance from the workface (i.e. at the last ring installed) is much lower than the value used in the conventional method (or when tunnelling using a TBM in open face conditions).

In general, this more elaborate method is restricted to the final design of a few critical sections. Apar t from empirical settlement

Moreover, confinement can result in additional excess porewater pressures in the surrounding ground.

• segment blocking tolerances are not taken into account; • the lining is installed behind the cutting face and becomes effective at a certain distance from it;

Impacts of specific structural characteristics on design assumptions are as follows: • installation of lining at a certain distance from the excavation face: lining sustains loading from par t of the ground deconfinement, from the back grouting pressure and from delayed effects; • the lining is not monolithic: reduced inertia at the contact joints is reflected in reduced lining flexural stiffness. This phenomenon can be taken into account either by modelling contact joints directly or by designing an equivalent ring with a smaller inertia (Muir-Wood formula); however, it should be noted that this behavioural assumption is no longer borne out in the case of adjacent rings incorporating combined radial contact joints in association with rigid assembly systems between rings (e.g. plugs or tenonand-mortises) nor in the presence of very soft ground. 4.3.5 - Parameters which can be integrated in the different methods of analysis Based on the design stage and the context of the project, the following table shows the possible status of taking into account parameter s for the different considered approaches to analysis. These modes of parametric consideration are identified by the following coding: 0

not necessary

1

desirable

2

necessary

I

indirect consideration

D

direct consideration

N

not considered

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) Project stage / context Parameters

Preliminary Design Construction Studies studies

Methods of analysis

Urban Hyper- Composite Composite / static solid solid Sensitive reaction method. method. method. Analytical Numerical solutions resolution

Comments

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- Definition of loading and/ or actions * Initial constraints - levels of ground layers - groundwater level ° minimum groundwater level ° maximum groundwater level - dry unit weight - Ko

1

2

2

2

I

I

D

0 2 2 2

1 2 2 2

2 2 2 2

2 2 2 2

I I I D

I I I D

D D D D

1

2

2

2

D

I

D

- superimposed load: ° uniform superimposed load ° linear superimposed load (Bc truck, load-bearing wall), surface superimposed load - direction of principal stresses

0

1

2

2

D

N

D

1

2

2

2

D

D

D

- continuous medium assumption: ° continuous medium ° fissured medium

2 0

2 1

2 2

2 2

D D

I N

D D

* Loading: - segment installation distance

2

2

2

2

I

I

I in 2-d. D in 3-D.

2 0

2 0

2 0

2 1

I D

I D

I D

0 0

1 0

2 0

2 2

D I

I I

D I

- ground deconfinement law: ° convergence-confinement method

2

2

2

2

I

I

° method of similarities (Corbetta’s law)

0

0

0

1

I

I

I in 2-d. D in 3-d. I in 2-d. D in 3-d.

- ground convergence curve: ° unsupported (elastic-plastic behaviour)

2

2

2

2

I

I

° supported (Bernaud’s law)

0

0

0

1

I

I

- delayed effects (behaviour law): ° E(x,t)

1

2

2

2

I

I

1 2

2 2

2 2

2 2

I D

N D

Iin 2-d. D in 3-d. D D

1

2

2

2

I

I

I

1

2

2

2

D

D

D

- confinement pressure

° considered as artificial support ° considered by means of equivalents stress condition - back grouting pressure - overcutting, shield tail conicality

° long-term shear parameters ° effective stress analysis (excluding porewater pressures) ° physical chemical swelling - change of groundwater pressure ° drainage

- Ko obeys Jaky's law for sands and normally consolidated clays; - Ko depends on tectonics, consolidation and erosion for rocks and overconsolidated soils.

- slope effect - dip effect (ground layers)

Shield tail effect only to be considered in very soft ground and in an urban/sensitive context - max. value for stress calculations; - min. value for settlement calculations.

Effect only to be considered in very soft ground and in an urban/sensitive context

I in 2-d. D in 3-d. I en bi-dim D in 3-d.

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 234 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM)

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Paramètres

Phase / Contexte

Méthodes de calcul

Commentaires

Etudes Prélim

Projet

Exécut

Urbain / Sensib

° restoration of hydrostatic pressure

2

2

2

2

D

D

D

Application of pressure: - to ground and to lining; - to lining only;

II - Structure disign - overall structural design (possible consideration of contact joints, etc.)

1

2

2

2

I

I

I

Ring inertia is reduced by applying the Muir-Wood formula, except in the case of combined contact joints in very soft ground or rigid assembly systems between rings

2 0

2 0

2 0

2 1

D D

D N

D D

1

2

2

2

N

N

D

1 0

1 0

1 1

1 1

D N

D N

D D

2 1 0 0

2 1 1 1

2 1 1 2

2 2 2 2

I I I I

D D N N

D D D D

1

2

2

2

I

I

D

1

1

2

2

I

N

I in 2-d. D in 3-d.

- section and inertia of segments - properties of each contact joint - moduli: ° short-term modulus >< long-term lmodulus ° use of average modulus - back grouting material (model) III - Soil-structure contact conditions - adhérence - slipping - friction (Coulomb’s law) - separation IV - Environnment - nearness of other underground structures - existence of transition structures

Méthode Méthode Méthode des du solide du solide réactions composite composite hyperst. Solutions Résolution analytiques numérique

4.4 - Proof of concrete and reinforcement

contact joints between units: bearing surface area, installation of waterproofing gasket, chamfers, etc.;

4.4.1 - Choice of segment wall thickness

• the minimum segment wall thickness must be compatible with the bearing surface area of TBM longitudinal thrust cylinders.

Segment wall thickness must satisfy several criteria: • the segment strength capacity with respect to combined circumferential bending must be sufficient when the percentage of longitudinal reinforcing bars contributing to this strength is less than 1 % and is generally close to the minimum percentage; if segment wall thickness is constant throughout the tunnel alignment, an economic study shall be conducted to examine whether it would be preferable to vary the concrete strength (new mix design, alteration of mixing plant parameter s, feasibility of increasing strength) or, on the other hand, the percentage of steel in order to satisfy the calculated stress variations along the tunnel; • the minimum segment wall thickness must satisfy the conditions imposed by the

4.4.2 - Circumferential reinforcement (hoops) The sectional area of circumferential reinforcing bars arranged behind the internal and external segment faces shall be derived from: • analysis of combined bending (normal load - bending moment interaction diagram) in relation to anticipated loadings in ultimate limit state and serviceability limit state combinations. If bending moments are low compared with the normal force, the sectional area can be justified in "simple compression" (BAEL 91, Section B.8); • analysis of simple bending during handling and storage of segments. It should also be noted that the loading case involving thrust of the TBM main cylinders on the lining seg-

ments can be dimensionally critical (bursting forces due to spreading of this thrust); • minimum percentage of reinforcing steel considerations: - for units in compression (BAEL 91, Section A.8.1,21), - for units in bending: especially under the action of TBM main cylinder thrust (BAEL 91, Section A.4.2.). Hoop diameter s and arrangement (concrete cover and spacing) must be derived in accordance with Section A.8.1,22 for units in compression, Section A.7.1. for concrete cover protection of reinforcing bars and Section A.4.5,3 for cracking. It should be noted that the use of highstrength concrete mixes of very high compactness allows the concrete covers prescribed by Section A.7.1. to be reduced. 4.4.3 - Longitudinal reinforcing bars (arranged parallel to the tunnel axis) Outside segment end zones subjected to localized loads requiring iron-banding reinforcement, these longitudinal bars must

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 235 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM)

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satisfy both the requirement of BAEL 91 Section A.5 concerning shear force if this is large in relation to the accompanying normal force (τ u >0,07f cj / γ b ) and Section A.8.1.3. concerning transverse reinforcing links for units in compression. It is perfectly acceptable not to incorporate transverse reinforcing links around circumferential bars with a diameter less than 20 mm, which are not in corners and not taken into account in the relevant strength calculations (i.e. only derived from minimum percentages of reinforcing steel). In some rather exceptional cases, such as fire resistance checking, stirrups must also be checked in relation to their potential role in preventing unsuppor ted thrust of cur ved bar s under tension (BAEL 91, Section A.7.4,2). Finally, correctly designed longitudinal reinforcing bars contribute to balancing the shear force resulting from the TBM cylinders thrust loads. At unit ends, the loads transferred from one segment to another through each localized reduced bearing zone are spread through the total thickness of the segment. Surface and bursting reinforcement must be provided in these bearing zones. These can be designed in accordance with the recommendations provided in BAEL 91, Annex E.8 (not taking into account the minimum sectional area of bursting reinforcement under TBM thrust cylinder temporar y loading). Reinforcement can comprise small diameter bars bent into coils or welded bars (§ 5.25 of Eurocode 2).

bridges and civil engineering structures; • French standards NF P 22-430 and NF P 22-431 for non-prestressed (plain) bolted assemblies; • French standards NF P 22-460 and NF P 22-469 for control-tightened bolted assemblies; • Eurocode 3 "Design of steel structures" adopted by the European Standardization Committee in 1992. 5.1.2 - Nature of actions and loadings 5.1.2.1 - Permanent actions (G)

These actions are associated with keeping the waterproofing gaskets compressed and are to be considered especially near stations. They will be determined from crushing force - deformation curves provided by the waterproofing gasket supplier. 5.1.2.2 - Variable actions (Q)

These are represented by loads applied during construction. In par ticular, those associated with: - crushing of waterproofing gaskets, - segments overhanging from the previously installed ring during erection, when a thrust ring is used for TBM penetration (rare), - action resulting from the erector arm (possible). 5.1.2.3 - Accidental actions (FA)

Possible accidental actions will be detailed in the project specifications.

5 - DESIGN OF ASSEMBLY SYSTEMS

It should be recalled that a commonly retained accidental loading case is that associated with overhanging of a segment especially following a hydraulic failure in the TBM thrust cylinders.

5.1 - Design assumptions for bolts and socket bolts 5.1.1 - Regulations The following regulations or standards apply to the design of steel bolts or anchor bolts: • Design rules for structural steelwork or CM 66 design rules for steel buildings; • Additional Clause 80, which takes into account the concepts of plasticity and limit states; • CPC Fascicule n° 61 Titre V "Conception et calcul de ponts et constructions métalliques en acier" (Design and analysis of bridges and str uctural steelwor k) for

This loading case must also be combined with possible action resulting from the waterproofing gaskets. 5.1.3 - Combined actions - Design stresses a) Basic combined action put forward by the CM 66 rules 4/3 Gmax + Gmin + 3/2 Q b) Basic combined action put forward by Eurocode 3 1.35 Gmax + Gmin + 1.5 Q

c) Accidental combined action put forward by the CM 66 rules and Eurocode 3 Gmax + Gmin + FA

5.2 - Proof of assembly and pick-up components using materials other than steel 5.2.1 - Introduction Amongst the assembly systems most often used can be mentioned sockets for bolts and pick-up bolts or plugs positioned between consecutive rings. Although the design and analysis of steel assembly systems is based on regulator y documents and standards, the use of different materials is not necessarily included in an engineering regulation framework covering the behaviour of the components concerned. The forces likely to be imposed by such components must therefore be surveyed, then factors of safety to be applied to the inherent strength of these elements as well as their behaviour under service conditions must be evaluated on the basis of suitable mechanical tests. It should be recalled that, because of the actions applied, these different segmentinser ted components transmit often high local stresses to the concrete. Additional reinforcement may be required to balance these stresses and ensure concrete integrity. 5.2.2 - Actions to be considered The same type of actions as for steel assembly systems (cf. § 5.1) are found in relation to bolt sockets and plugs. Attention should be drawn to the following specific characteristics associated with the use of plugs in relation to the actions already referred to: • high local pressures around interlock pockets when installing segments; • structural continuity of the lining between consecutive rings ensuring greater longitudinal rigidity (limited relative mutual displacement of components); • systematic permanent presence. Structural continuity may be the cause of stresses which are difficult to quantify: • Thus, when an inert or semi-inert grout is used for back grouting behind the segments at the rear end of the shield tail, overriding

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) buoyancy with respect to the lining weight can induce significant loads in assembly systems;

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• It should also be noted that in small radius cur ves, thrust cylinder action can induce large forces resulting from the transverse component of thrust. Most of these actions occur in the shor tterm. One should also mention long-term actions which can be associated with, for example, local defects or losses of contact between lining and surrounding ground due to par ticular geological conditions (dissolving of gypsum, presence of karsts, etc.) or resulting from the construction of later structures. 5.2.3 - Combined actions - Stresses The different combined actions must be quantified on the basis of project data. If no regulation applies, a factor of safety of one is applied to each action. Stresses applied to assemblies are then evaluated by considering models and simple behaviour. 5.2.4 - Behaviour of materials and assemblies - Tests

This file will focus specifically on:

• refuges for vehicles;

• a description of tests carried out;

• wastewater drainage;

• the statistically aspect of the results;

• collection and discharge of groundwater ;

• technical data-sheets for the products used.

• maintenance of the structures.

6 - TRANSITION AND ANCILLARY WORKS Design of underground works is not limited to designing just the running tunnel; civil works for an underground project usually comprise: • tunnels, which are continuous structures allowing circulation of trains, vehicles, fluids or transmission of energy; • transition works: - with the surface: specific examples are stations, ventilation shafts, emergency shafts, inspection shafts and galleries, etc., - between tunnels: specific examples are branches providing communication, piston relief, ventilation, rolling stock depot and turning galleries, etc.; • ancillary works: specific examples are - ventilated refuges or collection areas,

In practice, actual material and assembly behaviour can only be fully understood through static and dynamic testing (shear, tension, etc.). These types of test can go as far as testing the capacity of the whole assembly chain including the surrounding concrete (with its design strength), as well as the reinforcement measures adopted around the connector inserts.

- safety recesses,

In general, test stresses differ from actual stresses; consideration of factors of safety which are adequate with respect to test results and conditions particular to the project therefore appears necessary. 5.2.5 - Conclusions Tests on anchor and pick-up sockets, for example, fall within the scope of conventional testing. On the other hand, because the plug system is relatively recent, experiments should be pursued to define more accurately the sizing problems and advantage should be taken of experience feedback from different projects in order to extend knowledge in relation to the operation of this intersegment connection system. However manufacturers must supply a full engineering file covering whatever systems are adopted.

- fire recesses,

Internal sizing and spacing of in-line works is often codified under reference documents. Thus, in France, the following documents apply: • the Dossier Pilote des Tunnels (guidelines for tunnels) published by CETu and the latest circulars in force covering road and motorway tunnels (and, by extension, all roads); • Owner-published (SNCF, RATP, etc.) rules for rail tunnels; • the Instruction technique relative aux réseaux d'assainissement des agglomérations (Engineering directive covering wastewater systems for urban areas) (Circular n° 77.284 INT of 22 June 1977).

6.2 - Construction of transition and ancillary works Communication between transition ans ancillary works and the running tunnel invariable requires the construction of various size openings in the running tunnel lining.

- vehicle turning galleries,

For example, construction of a transverse opening from inside the running tunnel usually involves the following operations:

- sand traps,

• special spacing of the lining rings;

which are essential for the operation of the structure.

• temporary support of rings to be cut into by propping or bolting;

The present section is simply aimed at drawing attention to a number of design and construction aspects of these transition and ancillary works, which commonly represent unusual points in terms of precast concrete segmental lining design. Moreover, they represent a major burden on the project in terms of cost and time.

• possible treatment of ground around the future opening;

- plantrooms,

6.1 - Design of ancillary works

• in one or several stages, cutting out and removal of segments in front of the opening under construction; • in one or several stages, excavation and concreting of the final reinforced concrete lining of the strengthening structure around the opening; • removal of temporary support.

• transfer and safety of the public;

Construction of in-line works must be foreseen as early as possible; consequently, their geometrical proper ties and method of construction must be fully defined by the Engineer and Owner's Representative right from design stage.

• ventilation (with respect to its twin aspects of health-related ventilation and fire safety);

Their construction stage must have a minimum impact on progress of TBM-excavated construction of the main works.

Design of ancillary works is closely dependent on their functions. In particular, functions resulting in extensive geometry can involve:

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) 7 - INSTRUMENTATION

7.2 - Monitoring methods

7.1 - Aims

To achieve the above aims, the principal effects to be measured are as follows:

Precast concrete segments lining instrumentation and monitoring schemes must satisfy several aims:

• stresses and pressures:

• to check that lining actual behaviour complies with forecasts resulting from planned theoretical sizing and, in turn, to verify the level of safety of the structure in terms of forces and deformations; • to gain greater knowledge of: - the intensity and distribution of external actions impacting on the lining during the different stages construction progress (TBM thrust, annular gap back grouting, short- and long-term pressures exer ted by the surrounding ground),

- measurement of pressures exerted by the back grouting material and ground on the lining extrados, - measurement of pressures exerted at segment contact joints. These measurements are often disrupted by the effect of sensor interaction (total pressure cells) with the medium in which it has been installed. Results should always be subjected to careful analysis; • segment deformations: These are often measured using vibrating wire tensometers embedded in the segments and arranged:

- the intensity and distribution of segment internal loads, especially:

- in a longitudinal direction, with a view to analysing stresses induced by TBM main cylinder thrust,

• resulting from TBM thrust (presence or absence of defects in bearing between different elements, evaluation of TBM length influencing the lining),

- in a radial direction, with a view to analysing stresses induced by back grouting, the surrounding ground and possible disruptive effects associated with TBM thrust.

• resulting from successive actions involving back grouting and the surrounding ground under par tial cutter section confinement and showing, for example, possible load redistribution due to alternation of radial contact joints over a succession of rings.

A concrete control block fitted with sensors is often provided as a reference, although its behaviour is somewhat different of that of the lining segments.

In any case, Owners and contractors should be made aware that these instrumentation schemes do not originate from the research field but must be regarded as a driving factor in terms of fur thering the technical nature of this type of lining and allowing both acquisition of quantifiable reference data, concerning the quality of the completed tunnel, and advantage to be gained from analysing information fed back in order to refine the actual behaviour of this form of discontinuous lining.

• convergences: - Invar wire or optical convergence measurement, with a view to evaluating short- and long-term deformations withstood by lining rings. Temperature probes and relative humidity sensors must also be implemented at instrumented sections in order to complement effectively the data collected. Whilst tunnel lining monitoring has, until now, been carried out conventionally by a technician, this method is now being challenged by automatic data acquisition, which offers many advantages such as:

• rapid and virtually simultaneous measurements; data acquisition from a large number of sensors therefore provides an "instantaneous picture" compared with TBM penetration rates; • scheduled frequency of measurement suited to both the nature or variability of actions concerned and data sought during different construction stages; • data acquisition facilitated, in par ticular, when instrumented rings are within the area enclosing the TBM back-up equipment. Conversely, several drawbacks inherent in the use of automatic data acquisition should be mentioned: • danger of a systematic loss of data due to defective installation or malfunction of a component after fixing (closely supervised installation then periodic checking to be carried out by body in charge of measurement); • compatibility of recording box spatial requirements with respect to clearances to be provided temporarily for shor t-term (erector arm, segments being erected, temporary equipment, back-up equipment, etc.) and long-term (rolling stock, permanent equipment, etc.); • possible problem of energy supply and independence; depending on the quantity of data to be collected, various options can be envisaged: large-capacity batteries, sophisticated power plant controlled by energysaving electronics). In particular, all monitoring schedules must specify who is responsible for interpreting results, the timescale and the data transmission chain. The validity of the monitoring system must be questioned if this often neglected interpretation stage does not exist. In general, reference should be made to A.F.T.E.S. "Auscultation" (Monitoring) Working Group n° 19 recommendations on tunnel monitoring measurements.

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The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) REFERENCES •••••••• TERZAGHI K. - Rock defects and loads on tunnel support - Rock Tunnelling with Steel Supports, Commercial Shearing Co - Youngstown, Ohio, pp. 15-99, 1946. PROTODIAKONOV M.M. - Klassifikacija Gornych Porod - Tunnels et Ouvrages Souterrains I, pp. 31-34, 1974. LAUFFER H. - Gebirgsklassifizierung f¸r den Stollenbau - Geologie Bauwesen, 74, pp. 46-51, 1958.

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

DUDDECK H., ERDMANN J. - On structural Design Models for Tunnels in Soft Soil - Underground Space,Vol 9, pp. 246-259, 1985. CORBETTA F., BERNAUD D., NGUYEN-MINH D. - Contribution à la méthode convergence-confinement par le principe de la similitude. Revue Française de Géotechnique n° 54, pp. 5-12, 1991. BERNAUD D., ROUSSET G. - La nouvelle méthode implicite pour l'étude du dimensionnement des tunnels - Revue Française de Géotechnique n° 60, 3e trim., pp. 5-26, 1992. PANET M. - Le calcul des tunnels par la méthode convergence-confinement - Presses de l'Ecole Nationale des Ponts et Chaussées, 1995. NGUYEN-MINH D., GUO C. - Sur un principe d'interaction massif-soutènement des tunnels en avancement stationnaire - Eurok'93, Lisbon, Portugal, 1993. NGUYEN-MINH D., GUO C. - Tunnels creusés en milieu viscoplastique - Géotechnique et Environnement, Colloque Franco -Polonais, Nancy, 1993. MUIR WOOD A.M. - The circular tunnel in elastic ground - Geotechnique 25, n° 1, 1975. HOEK E., KAISER P.K., BAWDEN W.F. - Support of Underground Excavations in Hard Rock - A.A. Balkema, Rotterdam, Brookfield, 1995.

ANNEX :

impervious seals of the mould are then correctly positioned.

TUNNEL LINING CONSTRUCTION - PRECASTING AND INSTALLATION

Concrete contact surfaces are spray-lubricated with a release agent.

1 - GENERAL The aim of the present section is to describe the main tunnel lining construction stages in such a way that actions applied to the segments during constr uction are recorded. In general, these operations are covered by construction procedures describing general and specific requirements for production organization and which are consolidated under a single more general document: the project Quality Assurance Plan (Q.A.P.).

2 - DESCRIPTION OF SEGMENT PRECASTING

2.1.2 - Reassembly of mould elements The mould will be reclosed in accordance with the specified flank and end closing order. 2.1.3 - Self-inspection After closing the mould, the operator will ensure proper closure of the mould by keeping a watch on alignment of flank and end reference marks and by undertaking other checks detailed in the Q.A.P.. The operator will then examine visually each mould; these operations will be entered on the compliance inspection record specified in the Q.A.P..

2.2 - Placement of reinforcing cages

2.1 - Mould preparation 2.1.1 - Cleaning Careful brush-cleaning of the peripheral ends and flanks (especially their bearing faces) and mould bottom is under taken with the mould in an open position. The

With respect to the logical sequence of precasting operations, it is assumed that reinforcing cage production has followed its own fabrication Q.A.P., incorporating all joint contracting and subcontracting fabrication Q.A.Ps. as well as all the supply items entering into the composition of a cage.

2.2.1 - Cage preparation Each reinforcing cage will be fitted with spacers designed to ensure its accurate positioning in the mould and, thus, compliance with specified concrete cover to the reinforcing bars. 2.2.2 - Self-inspection and placing of the reinforcing cage Before placing the cage in the mould, the operator will ensure, in accordance with the reinforcing cage production Q.A.P., that this cage: • is not deformed in any way; • corresponds perfectly with the mould for which it is intended; • is fitted with all the designed spacers, both in number and in position. After placement of the reinforcing cage in the mould, the operator will ensure that the cage is correctly centred with respect to the mould and will then fill in the compliance inspection record.

2.3 - Mounting of connector inserts and accessories Whatever the systems selected, they will be positioned generally after installing the reinforcing cage in the mould.They usually …/… comprise:

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 239 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) • a pick-up device; • sockets and pins for implementing socket bolts or any other connection system;

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

• a system for possible grouting behind segments, etc..

2.4.4 - Heat treatment, preheating of concrete

- inspection of first reinforcing cages fabricated in accordance with mass production procedures.

Depending on the type of concrete thermal maturing selected, several methods can be envisaged:

• During production

As in other fabrication operations, self-inspection will be carried out by the operator, who will check the correct positioning of all these connector inser ts and accessories before filling the compliance inspection record.

• hot water mixing of concrete (maximum water temperature 80 °C);

2.4 - Concreting

2.5 - Mould stripping Handling - Pre-storage

The concrete Q.A.P. or CONCRETE FILE specifies the type of concrete, its properties, constituent materials and, in particular :

Unbolting and removal of connector insert and accessory supports;

• the concrete mix;

Release of lock-bolts for opening flanks and ends of mould;

• the physical chemical analysis of constituent materials: fine aggregate, coar se aggregate, cement, water ; • the active alkali balance; • admixture engineering data sheet(s); • test results for trial mixes produced for design and suitability purposes; • the Q.A.P. for the approved testing laboratory. 2.4.1 - Concrete production and preliminary checks At the star t of each concreting shift, the operator will check proper operation of: • the concrete plant PLC-controlled system for weighing and accurately recording the weights of constituent materials; • the water, admixture, plasticizer, accelerator, etc., regulation system; • the mixing time control system. 2.4.2 - Concrete placement Continuous uniform placement in the mould of the segment concrete volume will be ensured.

• heating of mould underside; • steam curing under controlled temperature and relative humidity.

Positioning of lifting beam fitted with suction pads, or other gripper- or sling-based handling system. After mould stripping, segments will be set down and stacked on suppor ts located in prepared sections of the pre-storage (curing) area inside the precasting shop or outside under suitable protection. Timber blocks will be placed between segments taking care that they are aligned with the supports. Curing time will be approximately 8 hours and in general imposed by the sequencing of mould stripping, storing, gluing, turning, packing and then loading-out operations. Self-inspection The operator will then carry out the necessary inspections and fill in the relevant compliance inspection report.

2.6 - Inspections Moulds for the same lining ring must be fabricated and inspected from every angle in relation to the ring. Similarly, in the case of several ring moulds, the type of mould must be identical irrespective of the ring it forms. The following inspections are usually carried out:

2.4.3 - Finishing of extrados or non-shuttered surfaces On completion of concreting, the free surface of the concrete will be floated as accurately as possible. After waiting for the initial set of the concrete (approximately 20 - 40 minutes), final trowelling of the top of the segment will be carried out to eliminate bug holes and unevenness.

• Prior to starting mass production of segments - inspection of mould fabrication, - inspection of ring geometry and assembly using reinforced concrete segments from initial precast shop casting, - inspection of moulds on delivery to precast plant,

Segment precasting involves mass production, therefore procedures specifying details of inspections to be carried out at regular intervals throughout the production period should be established to ensure that tolerances for moulds, segments and reinforcing cages, as well as concerning reinforcing cage assembly quality, always remain less than the initially established values. - inspection of moulds approximately every 50 casting operations, - same frequency inspection of corresponding segments at line output and correction of mould adjustment if deviations are observed. These inspections are essentially based on: • overall dimensions of the assembled ring; • lengths of segments generating ring taper ; • segment wall thickness; • flatness of ring/ring contact surfaces; • roughness; • geometry of designed pockets and their layout (impervious gasket grooves, etc.); • positioning of connector inserts (pick-up socket, connection system, etc.); • geometry of reinforcing bars, quality of reinforcing cages and their position in the moulds.

2.7 - Repairs A repair report, which will be attached to the relevant compliance inspection report, will be drawn up for every repair. The Q.A.P. will specify repair procedures and materials to be used for the different cases encountered: • honeycombing; • bubbles in waterproofing gasket grooves; • spalling at edges, etc. Self-inspection after repair • inspection of waterproofing gasket groove surface condition; • inspection of extrados surface condition; • inspection of bearing surfaces.

2.8 - Installation of waterproofing gasket In the case of a glued waterproofing gasket section, gluing will be carried out using an adhesive payer recommended by the sup-

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 240 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM) plier of the gasket and installed in accordance with his instructions and procedures.

2.9 - Packing and marking In general, segments of the same ring will be packed together on timber blocks.

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

Timber blocks between segments must be perfectly aligned. Each segment will be marked according to instructions given on the contract drawings (intrados bearing face), to enable it to be identified and cross-referenced with the relevant inspection record (traceability).

2.10 - Internal preshipment inspection The segment loading supervisor will indicate the transported ring number, as well as the precasting date of its component segments, on the preshipment inspection report. He will check that connector inser ts and accessories are clean and protected, that there is no concrete spalling, that waterproofing gaskets and possibly pads are properly glued. He will also check segment packing, alignment of timber blocks and the size of the pack.

Segments will be positioned at the precast plant storage yard to avoid a segment turning operation between yard storage and pick-up by the segment erector, whose suction pads or handling mechanisms pick up the segment from the intrados side. When the erector is supplied in the upper section of the TBM, segments are stored intrados lowermost. When the erector is supplied in the lower section of the TBM, segments are stored intrados uppermost and, in this case, a segment turning machine enables this operation to be carried out at the precast plant. In its storage position, the segment rests on two timber blocks of the same length as the segment itself and positioned directly in line with the longitudinal assembly systems. Segments are stored by stacking each set of segments comprising a ring. Segments must be stored after precasting and can only be installed if their strength exceeds or is equal to that required by the project specifications.

4 - SEGMENT COLLECTION, TRANSPORT AND ACCEPTANCE ON SITE

copy will be returned to the segment manufacturer.

5 - SEGMENT SUPPLY TO THE WORKFACE After segment acceptance, unloading and storage in an area near the tunnel access (shaft, adit, etc.), supply to the workface usually depends on the area available and it represents a minimum stock. Storage can be organized by: • segment; • by segment pair ; • by rings palletized on steel frames. Storage design is therefore based on the selected transport methods within the tunnel and to the TBM, where the segments are unloaded, then placed on a belt or roller feed conveyor which delivers them to the front where they are picked up by the segment erector.

6 - LINING RING BUILDING Section 3.5.5 "Segment assembly systems" of the present recommendations specifies and describes:

A copy of the inspection report will be sent to site along with the delivery note.

In principle, the precast plant loading supervisor is responsible for collecting segments from the plant storage yard and the haulage contractor is responsible for transport.

3 - STORAGE AT PRECAST PLANT YARD

Segments are picked up at the precast plant storage yard by lifting beam fitted with suction pads, grippers or slings and they are then loaded onto lorries or another means of transport.

The following table complements this information by considering an example of building a lining ring comprising rectangular(standard) and trapezoidal-shaped (key and counter) segments. It details:

On-site segment inspection for acceptance purposes will be conducted on the lorry or other means of transpor t. Prior to unloading, the site representative will sign and make any written remarks on the delivery note.

• the successive segment erection stages;

Special care is required in relation to storage and possible thermal protection conditions to prevent segment concrete microcracking at the precast plant storage yard. Handling will be undertaken using a lifting beam fitted with either suction pads or slings, which allows a pack of several segments to be picked up.

Detailed observations will be included on the pre-unloading inspection report and a

• the different assembly systems; • the aims sought under construction and service conditions.

• recommendations associated with the operating environment for each erection stage; A few remarks of a general nature complement the table.

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 241 •

The design, sizing and construction of precast concrete segments installed at the rear of a tunnel boring machine (TBM)

SCHEDULE OF OPERATIONS

RECOMMENDATIONS

REMARKS

1) Supply of first segment to erector.

Supply possible from: - upper level; - lower level.

2) First segment pick-up.

Pick-up possible using suction pads, grippers, bolts.

All reproduction, translation and adaptation of articles (partly or totally) are subject to copyrigth.

3) Retraction of thrust cylinders corresponding to placement of first segment. 4) Positioning of first segment by rotating erector.

Detailed analysis of loads in each pick-up system position and of indirect loads on segments.

Light ray guidance systems can facilitate approach and final positioning of segment.

5) Radial approach of first segment. 6) Final approach with rotational, longitudinal and transverse balance adjustment.

Control of approach speeds by selection of proportioning hydraulic controls.

7) Holding of first segment on ring.

Pads of other thrust cylinders remain under pressure in contact with other segments to safely ensure: - segments holding and assembly, - compression of waterproofing gaskets and prevention of their decompression, - stability of the machine under the confinement p ressure.

TBM main cylinder thrust on the other segments must prevent any forward displacement of the machine. At this time, the segment is simultaneously held by the erector and the thrust from the main cylinders.

8) Fixing of first segment

see § 3.5.5 "Segment assembly systems"

By ring/ring (longitudinal), segment/segment (transverse) connection.

9) Installation and fixing of standard segments.

Same recommendation as for the first segment. Provide alternate installation of segments in each ring to minimize tube roll effects.

ame remark as for the first segment.

10) Installation of counter segments.

Use of template to calibrate gap between counter segments.

Same remark as for first segment.

11) Key segment installation.

Use of template prevents: - tearing of waterproofing gaskets, - concrete chipping. - greasing of waterproofing gaskets.

It should be noted that on completion of erection, the ring is stabilized by the prestress between the erection jacks and the previously installed ring. The only contact between the shield tail and the segmental lining is the shield tail seal.

TUNNELS ET OUVRAGES SOUTERRAINS – HORS-SERIE N° 1 – 2005 • 242 •

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