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Design of precast concrete segmental tunnel linings
Design of precast concrete segmental t u n n e l h"m"n g s *
J P Copsey S R Doran -
Chief Engineer, CE & S Design, MRTC Senior Tunnel Design Engineer, MRTC
*This paper was read at the MRT Conference held in Singapore, March 1987 and is reproduced by courtesy of the authors and the Singapore Mass Mail Transit Corporation.
Abstract Two main items are developed to be borne in mind by designers of segmented tunnel linings - that the process of tunnelling involves a complex redistribution of stress around the excavation as it proceeds and second the inter-relationship between the loads and deflections to which the lining is subjected during construction and the final state stresses following consideration of the structure~ground interactiorL
Introduction The city area of Singapore, in particular the Central Business District, is studded with high rise building development and transversed by busy thoroughfares, typifying a modern day metropolis, in addition, Singapore is rightly proud of its garden-city image and its demands for cleanliness and order. When a decision was made to construct a rapid transit system, it was of no surprise that the requirement for all the central area trainway construction and s o m e areas beyond was for bored tunnelling rather than the more disruptive, more polluting construction by the cut and cover method. In total, a length of 11.2 km of twin tunnels were constructed in Phase 1 and 2 of the transit system with designs as a part of nine design-construct contracts in accordance with the table shown below. Bored tunnelling extended from Toa Payoh Station in the north to Collyer Quay in the south and from Tiong 13ahru Station in the west to Lavender Station in the east. W'Chin these lengths, the stations, the cross-over structures adjacent to Newton and Bugis Stations and the Singapore River crossing, which was built within cofferdams, were constructed by the cut and cover method. The heart of the MRT system therefore comprises the bored tunnel sections, the majority of which are lined with precast concrete segmental linings. This paper describes the design of these elements with particular reference to Singapore soil conditions.
Design criteria of segmental lining The Corporation specified that full consideration be given to structure/ground interaction and r e c o m m e n d e d that the design method given by Muir Wood I~l as modified by
134
Curtis (2) be used although a similar method would be considered. The r e c o m m e n d e d method was adopted by most designers. Maximum allowable deflections of the lining was given as 25 m m over the radius. All temporan] loading conditions were to be considered, ie. handling and jacking, and consideration was required to be given to inaccuracies in casting and erection. The watertightness requirements were stringent. A leakage rate of 5 ml/m2/hour was required overall with no l 0 m length greater than 10 ml/m2/hour. The specified tunnel diameter was 5.2 m minimum which provided the contractors with 100 m m tolerance on radius for driving, casting and building inaccuracies and long-term deformation. No contractor opted for the minimum radius; each selected an increased size ranging from 5230 m m to 5400 m m to provide more tolerance. The criteria required the contractors to provide a permanent reinforced concrete lining designed to British Standard Code of Practice CP 110 with a load factor on both ground and water loads of 1.4. Ground conditions
Three predominant material types were met during the tunnelling works. These are described as follows:-
Weathering products of the Bukit Tirnah granite (Material type G) The Bukit Timah granite was emplaced in early Mesozoic times (approximately 200 million years ago). The upper surface of this material was deeply weathered and whilst some hard rock was expected, the material at tunnel depth was generally indicated to be either a silt or clay of
CONSTRUCTION & BUILDING MATERIALS VoL 1 No. 3 SEPTEMBER 1987
Tunnel Designer (Subcontractor)
Type of lining
Tobishima-Takenaka Joint Venture
Mott Hay and Anderson Asia
Segmental & In situ
105
Bocotra Construction
Jenny Engineering Corporation
In situ
106
Campenon Bernard/Singapore Piling Civil Contractor
Campenon Bernard
Segmental
107
Kajima Keppel Joint Venture
Kajima Keppel Joint Venture
Segmental
107A
Taisei-Shimizu-Marubeni Joint Venture
Pacific Consultants
In situ
107B
Nishimatsu/Lum Chang Joint Venture
Mott Hay and Anderson Asia
Segmental & In situ
108
Taisei-Shimizu-Marubeni Joint Venture
Pacific Consultants
Segmental
109
Ohbayashi-Okumura Joint Venture
Ohbayashi-Okumura Joint Venture
Segmental
301"
Nishimatsu/Lum Chang Joint Venture
Nishimatsu Construction Co Ltd
Segmental
Contract No.
Contractor
104
* Engineer's design was for cut-and-cover tunnel Contractors' alternative of bored tunnel design/construct was accepted.
intermediate to high plasticity. As may be expected with the weathering product of rock, there was a clear trend of increasing shear strength as the degree of weathering decreased. As the degree of weathering with depth was highly variable, only very broad guidelines between shear strength and depth could be established. R e c o m m e n d e d lower limit values for the design of permanent structures in these materials were as follows:G2 Rock, properties dependent on weathering and jointing characteristics but typically for weak rocl~ Unconfined compressive strength q uc greater than 1 MN/m ~ Elastic modulus E greater than 100 MN/m ~
the now exposed granite. Five major facies were identified on the alignment of the North-South line with properties ranging from partially weathered, highly fractured, friable, fissile, slickensided siltstone and mudstone ($2), through conglomerate beds containing large (10 m 3 plus) sandstone boulders ($3), to poorly consolidated mudstones ($4 a/b) and mottled stiff clays ($4). R e c o m m e n d e d lower limit values for the design of permanent structures in these materials were as follows:-
$ 2 Weathered rock Undrained shear strength Elastic Modulus
Cu = 200 kN/m 2 Eu = 200 MN/m 2
$3 Boulder bed (Parameters controlled by clay matrix) Undrained shear strength Cu = 100 kN/m ~ G4 (Totally decomposed silts and clay) Undrained modulus Eu = 100 MN/m ~ Effective cohesion C' = 10 kN/m 2 Undrained shear strength Cu depth 5-20 m ~b' = 28 ° 55 kN/m ~ Effective angle of friction Undrained shear strength Cu depth 20 m plus 100 kN/m ~ S 4 a / b (Poorly consolidated mudstones) Undrained shear strength Cu = 150kN/m 2 Undrained modulus Eu = 150 Cu Remoulded strength 0.5 Cu Undrained modulus Eu = 50 MN/m ~ Effective cohesion C' = 0 kN/m ~ Effective cohesion C' = 0 Effective angle friction ~' = 30 ° Effective angle of friction ~' = 30 °
Weathering products of the Jurong siltstones and mudstones (Material type S) The Jurong formation was deposited in a sedimentary basin to the southwest of the Bukit Timah granite intrusion during middle Mesozoic times. Uplift of this basin outpaced that of the cooling granite pluton with the result that the partially lithified Jurong Formation beds were folded and faulted against the southwest margin of
$ 4 (Stiff clay) Undrained shear strength Undrained modulus Effective cohesion Effective angle of friction
Cu Eu C' ~'
= 150 kN/m 2 = 150 MN/m ~ = 0 kN/m ~ = 30 °
The Kallang formation Fluctuations in sea level during glacial advance and
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
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Des|gn method
As previously noted the recommended design method was an elastic idealisation of the deformations and pressure distributions on a continuous dng. Whilst being aware of the implications of these methods where the lining is introduced into the ground by some 'technical magic' without any disturbance of the existing ground stresses, it was considered that the assumptions were conservative and represented an upper bound solution to the loads that were likely to develop in practice. The Corporation was also influenced in its thinking by the work reported by Peck~4~which indicated that the rate of lining pressure build up and the rate of lining distortion was linear with the logarithm of time both for soft and stiff clays and that even in relatively stiff London clays, loading on the linings were continuing to increase after periods in excess of 10 years and were ultimately approaching full overburden.
~l Distortional loadings
~
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F/~! retreat led to the erosion of deep valleys in all the earlier formations. Subsequent sea level rises in recent periods were accompanied by the filling of these valleys with Alluvial and Marine members consisting of loose fluvial sands (F1) and soft to very soft, plastic, highly compressible clays and silts (material type E, F2 & M). These clays are normally consolidated. Recommended values for the design of permanent structures in these materials were as follows:F1 (Loose fluvial sands) Modulus depths 0-25 m Effective cohesion Effective angle of friction
E
=
C' ~'
= =
10MN/m ~ 0 30 °
1~! (Marine clay) Undrained shear strength Cu (depth 0-7 m) = 10 kN/m 2 increasing linearly with depth to 60 kN/m 2 at 40 m depth Undrained modulus Eu = 200 Cu Effective cohesion C' = 0 Effective angle of friction ~b' = 22 ° Sensitivity = 5 Alignment of the phase I tunnels, which traversed several of these infiiled river valleys, therefore presented highly variable ground conditions as the tunnels passed alternately through spurs of rock with variable weathering and valleys filled with surficial deposits. On the phase I! tunnels along Victoda Street, which lie substantially further east, deeper (up to 45 m) more uniform deposits of soft clays were encountered.
136
As virtually all the contracts were expected to encounter the soft clays of the Kailang formation, the manner in which the linings would perform in these materials was of particular concern. In specifying a maximum deflection criterion which limited the relative lengthening or shortening of any diameters, inclusive of building tolerances, to less than 1%, the Corporation was philosophically inclined towards a relatively dgid lining design. This accorded with the contractors' proposals who all elected to minimise, as far aS practical, the number of segments in the ring and to stiffen the joint between segments by staggering radial joint locations. In the elastic idealisation the deformations of the lining and hence the bending moments induced are controlled by three principal parameters. These are: 1) The distortional loading: taken by Muir Wood to be the excess of the pressure on the vertical axis over that on the horizontal axis. It is the authors' view that the distortional pressures applied to the lining are influenced by many factors, in particular time dependency, and that Ko, the at rest coefficient, should only serve as a guide to the designer in his judgement of a suitable horizontal load coefficient, termed K in this paper. This point is developed further below. 2) The elasticity of the lining: generally taken to be that of a continuous ring having the section properties of the uncracked concrete section. 3) The elasticity of the ground: generally taken to be the modulus dedved from the laboratory triaxial testing of clays factored to allow for the conversion between plane stress and plane strain conditions, and long term creep strains. In the real tunnel the deformations and the bending moments are controlled by the stress path to which the ground is subjected. This in turn is a function of the actual and effective stresses in the ground, the strength and plasticity of the material and the construction techniques involved in the tunnelling process. The following discussion is relevant. The process of tunnelling is such that the release of ground stresses occurs at or before the tunnel face. In soft
CONSTRUCTION& BUILDING MATERIALSVol. 1 No. 3 SEPTEMBER1987
clays considerable plastic movement in the form of 'face toothpasting' may occur if support such as face timbers, compressed air, or slurry pressure is not provided. A conventional shield is normally provided with a cutting bead at the leading edge in order to facilitate steering, even in firm clays the annular void created by this cutting bead is rapidly filled by radial squeezing of the clay. To be able to construct a segmental lining in the protection of a shield tail skin the external diameter of the shield must exceed the external diameter of the lining by the thickness of the tail skin itself plus a suitable construction margin. Whilst good practice is to attempt to grout this void as soon as the completed ring has cleared the tail skin, in soft ground it is not always successful and further radial squeezing of the clay into the annular void thus created almost inevitably occurs. Undrained squeezing of the clay is arrested after the ring is grouted and only at this stage does the lining begin to carry loads. It can be seen from the foregoing that in practice the deformations experienced by the clay prior to the lining being installed may be many times larger than that which occurs at~erthe relatively incompressible lining is in place. It is possible to envisage therefore that a relatively large volume of soil around the completed tunnel is in a plastic condition. Clough and Schmidt ~1 present a stress path analysis which enables at least a qualitative assessment of the extent of such plastic conditions around tunnels in clay to be made and examines the development of loading on the lining. The principal loading stages described in relation to a shallow tunnel in a normally consolidated soft clay are as follows: As the excavation takes place the total tangential stress around the opening increases to approximately 2(7v and radial stresses are reduced to the value of any internal air pressure. When the undrained triaxial strength of the material immediately adjacent to the tunnel is exceeded a plastic zone is formed around the tunnel which sheds loads to more distant regions, where with increasing confinement the soil is able to carry greater stresses. In soft clays, or if high overburdens are present, the sheared material in the plastic zone will squeeze against the body of the shield or the now installed lining which will provide increasing radial support; hence arresting the plastic zone development. In this plastic zone significant pore pressure reductions occur; the pore pressures may even become negative in a large region. In time these pore pressures dissipate as drained conditions are established and the boundary of the plastic zone moves closer to the tunnel. The water demand of the negative pore-water zone results in a volume increase near the tunnel. If this volume increase cannot occur because of the incompressibility of the lining, loading of the lining occurs and a new stress equilibrium results. Since arching of the soil around the tunnel occurs the total stresses in the final condition are quite different from the initial total stresses. The soil is therefore not subject to simple consolidation or swelling; a complex readjustment of both total and effective stresses must occur. A relatively shallow tunnel in soft clay will develop an undrained shear zone
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which is virtually certain to reach the ground surface. Hence the entire soil mass above the tunnel is affected and will influence the behaviour of the clay at tunnel depth. Ifthe clay is normally consolidated, or nearly so, disturbance and reconsolidation leads to a state of stress which closely approximates to the original normally consolidated state. In other words, the vertical load on the tunnel approaches the full overburden pressure and the horizontal pressure on the side of the tunnel reaches at least the normally consolidated "at rest" pressures. If the lining is flexible it must deflect to balance the ground loadings and an elastic analysis of this process is appropriate. The foregoing discussion serves to illustrate why the standard design assumption that a shallow tunnel in sot~ clays carries the full overburden is fair. It also serves to explain the apparent paradox of using an elastic idealisation to analyse a loading system that involves essentially plastic deformations and swelling of the soil mass.
Comparison of calculated and measured lining deflection Comparison of actual lining deflection and the calculated predictions are always of value and the following information abstracted from the site ring building and later survey records from Contract ] 04 provides some interesting results. Elastic theory had clearly predicted that the deflections of the lining would be highest in the soft clays which had low elastic moduli. Conversely deflection in G2 rock where moduli values were much greater were not critical.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
137
Ground conditions
K value Measured Calculated ' used in design deflections (1)i deflections (2)
M clay (Cu = 26kpa)
0.7
15 mm
G4 (Cu = 80kpa)
0.7
12 mm
G2 rock (E = 100Mpa)
0.5
4mm
Average 5-10 mm Max 15 mm Average 10-20 mm Max 25 mm Average 20-25 mm Max 35 mm
K value from back analysis
(1 -sine')
0.8
0.63
0.5
0.5
See Note 3
N/A
Notes 1) Calculated deflections are changes across vertical or horizontal diameters of tunnel under working loads. 2) K values calculated from back analysis are those that give the Upl~er limit of the average measure deflections 3) K = 0 gives a calculated deflection of 8 mm for this case, which is substantially below measured deflections.
The actual deflections measured on site indicate that the reverse trend is in fact the case. Two explanations are offered for this observation. Firstly, Ko pressures in the soft clays would be expected to be higher hence producing more uniform loading conditions. If we accept the classical theory that Ko for normally consolidated clays is equal to (1-sin~b') it can be seen that the distortional pressure acting on the lining in the final drained condition is equal to (or' sin~'). The higher density and angle of internal friction for G4 over Marine Clay therefore give rise to greater distortional forces. Secondly, as previously discussed, the rate and manner in which ground loadings are in reality applied to the lining is markedly different to the assumption implicit in the elastic idealisation. In the soft Marine Clays, undrained squeeze of the ground began to occur at all points around the ring as soon as the matedal was excavated. The excavated soil closed rapidly around the newly built dng and high lateral active pressures were obviously being developed. In the G4 material undrained squeeze of the ground was much less evident, especially when stabilised by compressed air. Whilst loading developed quickly in the crown of the tunnels the stand-up time for the material at the sides of the tunnel was greater and lateral loadings obviously developed at a slower rate. The lining is therefore subject to a pedod of distortional loadings under undrained conditions that act in addition to those that develop due to the process of pore-water pressure changes previously described. In the case where G2 rock was present at the axis of the tunnel with G4 in the crown, the material in the walls of the tunnel was stable and lateral loading conditions for clays based on undrained squeeze and long term swelling were obviously no longer relevant. As the primary action of the lining is to carny loads in ring compression, the observed deflection of the lining would indicate that squatting of the lining occurs until adequate passive pressures are developed against the rock at the axis of the tunnel. Under such mixed ground conditions, ie. clays overlying competent material, it appears possible that the whole of the overburden may act as a distortional load on the lining; ie. K = O. A moment's reflection on the manner in which the lining cardes the load will also indicate the importance and influence of adequate backfill grouting. For unless the
138
void between the liner and ground is completely filled with a low modulus material, the deflection of the lining required to mobilise the required passive pressures will be increased.
Design of adjacent tunnels in clays In many locations of the MRT alignment geometry restraints resulted in tunnels being required to be built in close proximity both horizontally and vertically. If two tunnels are excavated in close proximity they will mutually influence each other and due allowance should be made for these effects in the design. The following discussion is relevant:
Effect of the first tunnel on the second tunnel As noted in the previous section, the behaviour of the soil around a tunnel in soft clay is strongly influenced by the methods used to excavate and support the tunnel. Different excavation and support arrangements lead to differences in the stresses and displacements induced into the surrounding soils. In particular, ground lost through the face and radial inward squeezing into the grout void before grouting will result in shear failure of the soil and the development of a plastic zone around the tunnel: Clays will be subject to strength loss and reduction of stiffness as a result of such movements dependent on their sensitivity. If a second tunnel is excavated in a zone of ground disturbed by a previous tunnelling operation it would be normal practice to make a reduction in the elastic modulus of the soil surrounding the second tunnel to account for these factors. For the design of the second tunnel, if excavated in a zone of plastic ground movement around the first tunnel, the assumption of the elastic methods of virgin vertical and horizontal initial state of stress in the ground become invalidated. However, ground loadings onto the tunnel are related primarily to the shear strength of the clay, the overburden pressure and the manner in which pore pressure changes caused by the excavation are dissipated. As such, the strict principles of super-position, implicit in the elastic methods of analysis, do not necessarily apply. Rather the construction of the second tunnel will extend the magnitude of the plastic zone, and loads on both tunnels will be dedved from a consideration of the dissipation of the pore pressures between the various negative and positive pore pressure zones created by the
CONSTRUCTION& BUILDING MATERIALSVoI. 1 No. 3 SEPTEMBER1987
tunnel geometry, construction methods and sequences. Peck et al ~6~recommends for tunnels in soft ground that the full overburden pressure should be adopted as the load case for both tunnels.
Effect of the second tunnel on the first tunnel When the second tunnel is excavated there will be a certain loss of ground around the excavation. If the first tunnel is within the zone of influence of the second tunnel it will inevitably be subject to distortional loads associated with these ground movements. The magnitude of such ground movements are primarily a function of the shear strength of the soil, the overburden pressure and the construction techniques employed to stabilise the face and grouting of the annular void. In impermeable clays most of the displacement occurs as squeeze under undrained conditions. Subsequent swelling, while of significance for lining loads and distortions, usually has little effect on the surface movements and displacements within the soil mass. The recommended method for the design of the first tunnel was therefore to analyse the ground losses around the second tunnel in a similar manner to that for a settlement analysis and to allow for the correspondingly increased bending moments in the lining of the first tunnel induced by such displacements.
Detailing of the linings Whilst all contractors were faced with generally similar design ioadings in terms of axial thrust and bending moments, their particular solutions in terms of reinforcement detailing, bolting arrangements and waterproofing methods differed markedly. Isometric details and typical reinforcement details of the various designs are shown in Fig 3. The relative advantages and disadvantages of the vadous features, gained from the experience on the MRT are discussed in the following sections.
Circumferential joint bolting details Fabricated steel bolt pockets were employed by Contracts 104 and 108. Whilst this detail represents a relatively small reduction in the cross-sectional area of the segment body, the experience has been that, as predicted by tests carried out by the Singapore Institute of Standard & Industrial Research (SISIR), segments incorporating this feature were relatively weak in lateral bending and were susceptible to cracking under shield jacking loads. Steel bolt pockets to the circumferential joint, whilst offering excellent location of segments lacked compliances and the experience on Contract 104 was that circumferential bolts were sheared on several occasions whilst attempting to make alignment corrections to the shield. The cost of fabricating and gaivanising the steel bolt pockets was estimated to be greater than other details.
Concrete bolt pockets were employed by Contracts 106, 107 and 109. In both the 106 and 109 designs the pockets represented a reduction in the cross-sectional area of the lining in the order of 20%. In the 107 design, this reduction in area is avoided by the use of a steel sleeve cast into the body of the segment into which a threaded stud is withdrawn during segment erection. This detail does however preclude the replace-
ment of circumferential bolts should this become necessa~ dudng the life of the tunnels. The use of concrete bolt pockets does allow a stiff edge beam to be formed in the extreme fibre of the segment and the general performance of segments reinforced in this manner in resisting shield jacking forces was good. Companion papers submitted to this conference discuss the Corrosion of Underground Structures (CBM June '87). In the region of the bolt pocket a locally ve~ thin section of concrete panel exists which may be susceptible to accelerated corrosion due to percolation of aggressive groundwater. The cost of the bolt assembly is low and good location of segments during erection was achieved.
Curved bolts were adopted by Contracts 107B and 301. The detail requires only a small reduction in the crosssectional area of the segment, and allows excellent edge reinforcement to be achieved. The curved bolts can be replaced if required and no thin concrete panels that may be susceptible to corrosion are created. The cost of the curved bolt assembly is moderate and no particular difficulty of location of segments during building were reported with their use. Radial joint bolting detail Fabricated steel bolt pockets were employed on Contracts 104, 106, 108 and 301. The detail requires only a small loss of section in the critical joint zone and testing at SISIR indicated that the presence of the fabrication inhibited development of shear cracking in the joint zone and significantly contributed to the strength of the segment if any misalignment occurred. The detail provides excellent location of segments and good 'nip up' during building. MRTC Phase 11 Materials and Workmanship Specification required that exposed metalwork in the tunnels is protected by a duplex system comprising hot dip galvanising and coal tar epoxy paint. This requirement is anticipated to significantly increase the life of the exposed metalwork.
Concrete bolt pockets were employed by Contracts 107 and 109 and resulted in a relatively large loss of section in the critical joint zone associated with this detail. Radial joint packing. Having regard to the perceived limitations of bituminous packings as revealed by the SISIR tests, the use of sheet rubber packing to radial joint surfaces was incorporated into the Phase II Materials and Workmanship Specification.
Joint bursting reinforcement was shown in the SISIR test series to greatly increase the strength of the lining if subjected to construction misalignments. This detail was incorporated into Phase II designs.
Key design Three basic methods have been used to close the ring as shown in Fig 4.
Standard taper keys were used on Contracts 104 and
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
139
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CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
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107B. In this design a key block of arc length approximately twice the lining thickness (500 mm) was installed radially from within the plane of the ring. The relatively short length means that the angle of draw on the key can be shallow and in this manner shear stresses on the joint surface are minimised. Further, the area of the key subjected to a disturbing pressure when grouting is small and no problems of keys dropping dudng either grouting or backgrouting operations were reported on these contracts.
One bolt hole keys were used on Contracts 107, 108, 109 and 301. In this design the key comprised a one bolt segment, of arc length up to 1060 mm, which was installed radially from within the plane of the ring. The large arc length resulted in a higher angle of draw on the key being required with consequent increases in the shear stresses on the joint surface. Further the area of the key subjected to a disturbing pressure when grouting is large and problems of keys being displaced during both grouting and backgrouting operations were observed on
CONSTRUCTION 8, BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
141
several of the contracts that incorporated this detail in their lining design. Taper key blocks were used on Contract 106. In this design a key segment is erected slightly forward of the plane of the ring to allow sufficient construction tolerances and is then pushed back using the shield jacks. The advantage of this technique is that the key joint surfaces are truly radial, hence eliminating problems of shear stress and key displacement during grouting. However problems do exist in maintaining accuracy of build as the key is jacked back and some increase in segment damage local to the key was observed.
Waterproofing of tunnels In all cases designers elected to provide a sealing material located in grooves near the rear edge of both the circumferential and radial joints, as the main line of defence against water ingress. All contractors also provided a traditional caulking groove in the inner face of both the circumferential and radial joints. A variety of materials were used for sealing and caulking the linings and their relative performance is discussed.
Simple bitumastic sealing str/ps were employed on sections of Contracts 104 and 109. These materials proved to be of limited effectiveness as on both contracts water ingress, whilst reduced, was many times the specified rate. The experience has been that these materials behaved in a totally plastic manner and once compressed were unable to recover their original shape. Consequently leakage often developed as shield jacks were withdrawn and the sealant was unable to follow the relaxation of the segment. Similarly the effectivenss of the seal was impaired ifjoint packing was incorporated to correct alignment or achieve large radius curves, as the degree of compression was markedly reduced. Particular difficulties also arose around the key as the protruding face of the sealant was susceptible to damage or misalignment during dng building as the key was pushed home. On Contract 109 the contractor resorted to an extensive programme of back grouting involving the injection and reinjection of cement/water milk through both the grout holes and joints between the segments, in areas of high water ingress up to twelve passes were involved. On Contract 104 the contractor elected to supplement a back grouting exercise with the use of epoxy caulking materials. Water activated polyurethane foams were injected into the void between the defective sealing strips and the caulking, to prevent water flow in the channels thus created.
Composite neoprene and bitumastic sealing str/ps were employed on sections of Contracts 104, 107, 108 and 109. The experience has been that whilst the neoprene core of these materials maintains a sealing pressure once compressed, the effectiveness of the seal is markedly reduced if joint packing is required and that these materials are also subject to damage and misalignment around the key segment.
142
AS a consequence additional waterproofing measures have generally had to be carried out to linings built in water-bearing grounds.
Neoprene gaskets were employed on Contract 106. These items consist of an extruded section of proprietary design that is formed into a ring and fitted into a groove in all segments. The surface of the gasket stands proud of the surface of the segment by some 2-3 mm and pressure between the compressed mating surfaces effects a seal. The experience has been that the resilient nature of the neoprene ensures adequate pressure between mating surfaces and that due to the taper key detail employed in this lining design, no problems of gaskets becoming displaced have occurred. Numerous problems of achieving an effective seal at the corners of segments were encountered and the specified rate of water ingress has not generally been achieved in water-bearing grounds. Remedial measures have taken the form of chemical grouting and water activated polyurethane foam injection.
'Hydrotite' gaskets were used on Contracts 107B and 301. This material is formed by mixing non-water-soluble but water-expansive resins and synthetic rubber. When the gasket comes into contact with water it swells to ten times its odginal volume, filling the joint gap and sealing against water ingress under its own expansion pressure. As the matedal may be fitted flush within the groove few problems of damage or misalignment, even during key installation, have occurred. The experience has been that swelling of the sealing stzips occurs shortly after installation and that a virtually dry watertight tunnel has been achieved without additional waterproofing measures. Testing of segments by SISIR As part of the procedure for checking the contractors' lining designs, prototype segments were subjected to various full scale loading tests at SISIR's laboratory. Two tests proved particularly relevant to the ultimate performance of the segments.
Lateral bending test During construction minor misalignments between segments and errors in the plane of build inevitably give rise to imperfect mating of adjacent circumferential joint surfaces which are required to act in bearing to transmit shield jacking loads. Under such circumstances segments can be subjected to extensive 'break back' lateral bending stresses, as illustrated in Fig 5. The test was designed to simulate the loading condition on segment 'A' in this figure. The Corporation was obviously concerned to minimise segment damage due to shoving and several aspects of the contractors' designs indicated that they would be susceptible to such problems. • The general use of large segments (typically 5 or 6 segments per ring) would result in large lever arms if such misalignments did occur. • Many of the shields had been designed for blind operation in soft clays and were equipped with extremely powerful shove rams. For example Contract 104's shields were provided with 24 No. 100 tonne
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
JACK
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jacks. • Few contractors were proposing any form of compressible packing in the circumferential joints to cushion the segments during shoving. As was expected, segment designs which featured steel bolt pockets on the circumferential joints were at a considerable disadvantage in their ability to resist this type of loading as the steel bolt pockets not only restricted the
Segment type
104(G M) 104(M) 106 107 108 109(G) 109(M)
amount of steel that could be concentrated in the edge of the segment, but also acted as a point of stress concentration in the extreme fibre of the segment which initiated cracking. Whilst acknowledging the severity of the test, reinforcement changes and/or the use of compressible packing was required for those designs that the testing indicated would be susceptible to such damage.
Load to 0.1 mm crack width
Load to 0 . 3 mm crack width
Load to 0 . 6 mm crack width
12 17 20 19 21 24 32
17 18 30 25 27 38 60
20 24 36 31 32 60 not achieved
Results of lateral bending test Joint test Design calculation for all Phase I lining types assumed that radial joint surfaces were erected with mating surfaces in perfect contact. Only one design, that of Contract 106, considered the possibility of tensile bursting reinforcement in this zone to resist such forces. Considerable discussion occurred regarding the compatibility of the design assumption of perfectly mating radial joint surfaces with the relatively generous segment manufacturing tolerance on the joint surfaces themselves (+ l m m from the radial plane) and the build of the lining (_+ 25 mm deviation from a true circle). In adverse conditions where these tolerances may become cumulative, considerable 'birds-mouthing' of the radial joint will occur. The joint test was devised to study the performance of the various contractors' radial joint details, if they were subjected to such adverse conditions. The tests were carried out on two "L" shaped concrete
specimens each the full thickness of the segment but due I~ capadty limitations of me tes~g frames only 500 mm wide (1/~ segment width). The bolt pocket design, joint packing if any, and reinforcement details within 250 mm of the joint zone were exactly similar to the segment design being modelled. The specimens were erected in the testing frame with strips of plastigauge inserted in the joint and aligned square and plumb. A laser was fixed to the upper segment and the beam aligned parallel to its axis. The scale on the lower segment was zeroed. The upper segment was then given a small rotation equivalent to that which would occur in a misaligned joint and the new laser reading noted. The test procedure was to incrementally apply loading to the specimens via the main compression jacks P1 whilst simultaneously adjusting the force in the secondanj tension jack P2 to maintain the angle of misalignment (as measured by the laser) between the segments constant.
C O N S T R U C T I O N & B U I L D I N G MATERIALS Vol. 1 No. 3 S E P T E M B E R 1987
143
CIRCLE
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.
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t
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Demag gauge studs installed on either side of the joint enabled a measurement of the closure of the joint at each load increment. The test was continued to destruction or until the capacity of the loading f~ame was reached. On completion of the test the specimens were disassembled and the width of the plastigauge strips measured at various points across the thickness of the segment in order to estimate the nature of the bearing surfaces at failure. Simple resolution of forces and moments, combined with a knowledge of the geometry of the specimens enabled a calculation of eccentricity of line of action in the radial joint versus axial load to be plotted. The following observations were made: • It was noted that cracking in thejoint zone occurred at a lower threshold for the designs which were erected without joint packing, than for other designs which used various forms of bituminous packing materials. However, these materials were observed to b e c o m e fully plastic under moderate loads and were squeezed out of the contact zone. Concrete to concrete load bearing surfaces had generally been fully developed for all designs above applied loads of 60 tonnes. Use of bituminous packings were not considered to affect the ultimate load capacity of the joint. • Cracking in the joint zone generally occured initially as local spalling around the segment corners and bolt pockets. At about 70-80 tonnes load a shear crack developed from the highly stressed joint area running at a shallow angle to intersect the segment face. This cracking was generally associated with a load redistribution within the joint zone as load was shed from the
144
A
edges of the segment back towards the central core and the joint crushed down. Increased loadings developed further shear cracks originating from the core area at increasingly steep angles which intersected the original shear crack and led to the ultimate failure of the segment. The performance of the Contract 106 segment which had been provided with tensile bursting reinforcement in the end block of the segment was particularly good. This segment had developed only local spalling at the corners when the capacity of the loading frame was reached. Performance of the Contract 104 and 108 designs was largely governed by the presence of heavy fabricated steel bolt pockets in the radial joint. These items appeared to be capable of both carrying load away from the highly stressed joint zone and of controlling the development of secondary shear cracking at ultimate loads. The performance of designs without either of these features was poor and the segment was generally considered to be susceptible to damage due to construction misalignment. Conclusions In drafting this paper the authors have sought to develop two main themes that in their opinion should be borne in mind by the designer of segmental tunnel linings. Firstly, the actual process of tunnelling is a three dimensional process involving complex redistribution of stresses around the excavation as it proceeds. Real soils around the excavation undergo complex stress path
CONSTRUCTION & B U I L D I N G MATERIALS Vol. 1 No. 3 S E P T E M B E R 1 9 8 7
changes involving undrained plastic squeeze, changes in effective stress and pore-water pressures followed by subsequent swelling or shrinkage as these pore-water pressures are dissipated. The process of tunnelling in soft soils is therefore one that does not lend itself easily to mathema~cally modelling.,A rigorous analysis would require a three-dimension finite element analysis that would be able to model not only the non-linear responses of the clay under undrained conditions, but also the stresses induced by the volumetric changes of the clays. It is the authors' view that faced with the complexity of the problem, simple hand calculation methods that conservatively consider only the initial and final states of stress in the ground, are at least as relevant as other, more sophisticated methods, that have as their roots a twodimensional analysis based on the principle of elastic super-position. Secondly and perhaps more importantly, the authors have tried to highlight the inter-relationship, that is perhaps unique to tunnel design, between the loads and deflections to which the lining is subjected during construction and the final state stresses that follow from consideration of structure/ground interaction. The tunnel lining should not therefore be designed in a vacuum making ideal assumptions regarding its construction. Rather, the lining should be viewed as an integral part of the tunnelling process that must be fully compatible with all aspects of the construction. In particular, the designer must be aware that in the real world shields do stray off line and that in the process of correcting the alignment rings will be built with a less than perfect mating surface on the circumferential joint. Similarly in the real world segments are built that are not perfectly circular and that if plain radial joint surfaces are used, concentration of load at either the front or the back of the mating surfaces will occur.
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It is the acknowledgement of these realities and the detailing of the lining to accommodate suitable tolerances that ensures the practical constructability of the design. Similarly, attention to detail and material selection for waterproofing, caulking and protection of exposed metalwork are factors that have a major effect on the serviceability and long term maintenace requirements of the lining. The importance of these factors are all too often overlooked in the design process.
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CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
145
/
Re~erences
(1) Muir Wood, A M. The circular tunnel in elastic ground. Geotechnique 25 No 1, 1975. (2) Curtis, D J. Discussion on '1' above. Geotechn/que 26 No 1, 1976. (3) Dames and Moore. Detailed Geotechnical Study - Interpretative Report Internal MRTC document. (4) Peck, R B. Deep excavat/ons and tunnelling in soft ground, Mexico
146
1969, (5) Clough and Schmidt So/~ clay engineering. Published by F_Jsevier Scientific Publishing Company, Chapter 8. (6) Peck et al. Some design considerations in the selection of underground support systems. University of Illinois report to US Department of Transportation November 1969. (7) Doran, Robe~y, Ong and Robinson, The corrosion of buried sO'uctures. MRTC conference 1987.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
J P Copsey S R Doran -
Chief Engineer, CE & S Design, MRTC Senior Tunnel Design Engineer, MRTC
*This paper was read at the MRT Conference held in Singapore, March 1987 and is reproduced by courtesy of the authors and the Singapore Mass Mail Transit Corporation.
Abstract Two main items are developed to be borne in mind by designers of segmented tunnel linings - that the process of tunnelling involves a complex redistribution of stress around the excavation as it proceeds and second the inter-relationship between the loads and deflections to which the lining is subjected during construction and the final state stresses following consideration of the structure~ground interactiorL
Introduction The city area of Singapore, in particular the Central Business District, is studded with high rise building development and transversed by busy thoroughfares, typifying a modern day metropolis, in addition, Singapore is rightly proud of its garden-city image and its demands for cleanliness and order. When a decision was made to construct a rapid transit system, it was of no surprise that the requirement for all the central area trainway construction and s o m e areas beyond was for bored tunnelling rather than the more disruptive, more polluting construction by the cut and cover method. In total, a length of 11.2 km of twin tunnels were constructed in Phase 1 and 2 of the transit system with designs as a part of nine design-construct contracts in accordance with the table shown below. Bored tunnelling extended from Toa Payoh Station in the north to Collyer Quay in the south and from Tiong 13ahru Station in the west to Lavender Station in the east. W'Chin these lengths, the stations, the cross-over structures adjacent to Newton and Bugis Stations and the Singapore River crossing, which was built within cofferdams, were constructed by the cut and cover method. The heart of the MRT system therefore comprises the bored tunnel sections, the majority of which are lined with precast concrete segmental linings. This paper describes the design of these elements with particular reference to Singapore soil conditions.
Design criteria of segmental lining The Corporation specified that full consideration be given to structure/ground interaction and r e c o m m e n d e d that the design method given by Muir Wood I~l as modified by
134
Curtis (2) be used although a similar method would be considered. The r e c o m m e n d e d method was adopted by most designers. Maximum allowable deflections of the lining was given as 25 m m over the radius. All temporan] loading conditions were to be considered, ie. handling and jacking, and consideration was required to be given to inaccuracies in casting and erection. The watertightness requirements were stringent. A leakage rate of 5 ml/m2/hour was required overall with no l 0 m length greater than 10 ml/m2/hour. The specified tunnel diameter was 5.2 m minimum which provided the contractors with 100 m m tolerance on radius for driving, casting and building inaccuracies and long-term deformation. No contractor opted for the minimum radius; each selected an increased size ranging from 5230 m m to 5400 m m to provide more tolerance. The criteria required the contractors to provide a permanent reinforced concrete lining designed to British Standard Code of Practice CP 110 with a load factor on both ground and water loads of 1.4. Ground conditions
Three predominant material types were met during the tunnelling works. These are described as follows:-
Weathering products of the Bukit Tirnah granite (Material type G) The Bukit Timah granite was emplaced in early Mesozoic times (approximately 200 million years ago). The upper surface of this material was deeply weathered and whilst some hard rock was expected, the material at tunnel depth was generally indicated to be either a silt or clay of
CONSTRUCTION & BUILDING MATERIALS VoL 1 No. 3 SEPTEMBER 1987
Tunnel Designer (Subcontractor)
Type of lining
Tobishima-Takenaka Joint Venture
Mott Hay and Anderson Asia
Segmental & In situ
105
Bocotra Construction
Jenny Engineering Corporation
In situ
106
Campenon Bernard/Singapore Piling Civil Contractor
Campenon Bernard
Segmental
107
Kajima Keppel Joint Venture
Kajima Keppel Joint Venture
Segmental
107A
Taisei-Shimizu-Marubeni Joint Venture
Pacific Consultants
In situ
107B
Nishimatsu/Lum Chang Joint Venture
Mott Hay and Anderson Asia
Segmental & In situ
108
Taisei-Shimizu-Marubeni Joint Venture
Pacific Consultants
Segmental
109
Ohbayashi-Okumura Joint Venture
Ohbayashi-Okumura Joint Venture
Segmental
301"
Nishimatsu/Lum Chang Joint Venture
Nishimatsu Construction Co Ltd
Segmental
Contract No.
Contractor
104
* Engineer's design was for cut-and-cover tunnel Contractors' alternative of bored tunnel design/construct was accepted.
intermediate to high plasticity. As may be expected with the weathering product of rock, there was a clear trend of increasing shear strength as the degree of weathering decreased. As the degree of weathering with depth was highly variable, only very broad guidelines between shear strength and depth could be established. R e c o m m e n d e d lower limit values for the design of permanent structures in these materials were as follows:G2 Rock, properties dependent on weathering and jointing characteristics but typically for weak rocl~ Unconfined compressive strength q uc greater than 1 MN/m ~ Elastic modulus E greater than 100 MN/m ~
the now exposed granite. Five major facies were identified on the alignment of the North-South line with properties ranging from partially weathered, highly fractured, friable, fissile, slickensided siltstone and mudstone ($2), through conglomerate beds containing large (10 m 3 plus) sandstone boulders ($3), to poorly consolidated mudstones ($4 a/b) and mottled stiff clays ($4). R e c o m m e n d e d lower limit values for the design of permanent structures in these materials were as follows:-
$ 2 Weathered rock Undrained shear strength Elastic Modulus
Cu = 200 kN/m 2 Eu = 200 MN/m 2
$3 Boulder bed (Parameters controlled by clay matrix) Undrained shear strength Cu = 100 kN/m ~ G4 (Totally decomposed silts and clay) Undrained modulus Eu = 100 MN/m ~ Effective cohesion C' = 10 kN/m 2 Undrained shear strength Cu depth 5-20 m ~b' = 28 ° 55 kN/m ~ Effective angle of friction Undrained shear strength Cu depth 20 m plus 100 kN/m ~ S 4 a / b (Poorly consolidated mudstones) Undrained shear strength Cu = 150kN/m 2 Undrained modulus Eu = 150 Cu Remoulded strength 0.5 Cu Undrained modulus Eu = 50 MN/m ~ Effective cohesion C' = 0 kN/m ~ Effective cohesion C' = 0 Effective angle friction ~' = 30 ° Effective angle of friction ~' = 30 °
Weathering products of the Jurong siltstones and mudstones (Material type S) The Jurong formation was deposited in a sedimentary basin to the southwest of the Bukit Timah granite intrusion during middle Mesozoic times. Uplift of this basin outpaced that of the cooling granite pluton with the result that the partially lithified Jurong Formation beds were folded and faulted against the southwest margin of
$ 4 (Stiff clay) Undrained shear strength Undrained modulus Effective cohesion Effective angle of friction
Cu Eu C' ~'
= 150 kN/m 2 = 150 MN/m ~ = 0 kN/m ~ = 30 °
The Kallang formation Fluctuations in sea level during glacial advance and
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
135
Des|gn method
As previously noted the recommended design method was an elastic idealisation of the deformations and pressure distributions on a continuous dng. Whilst being aware of the implications of these methods where the lining is introduced into the ground by some 'technical magic' without any disturbance of the existing ground stresses, it was considered that the assumptions were conservative and represented an upper bound solution to the loads that were likely to develop in practice. The Corporation was also influenced in its thinking by the work reported by Peck~4~which indicated that the rate of lining pressure build up and the rate of lining distortion was linear with the logarithm of time both for soft and stiff clays and that even in relatively stiff London clays, loading on the linings were continuing to increase after periods in excess of 10 years and were ultimately approaching full overburden.
~l Distortional loadings
~
~
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F/~! retreat led to the erosion of deep valleys in all the earlier formations. Subsequent sea level rises in recent periods were accompanied by the filling of these valleys with Alluvial and Marine members consisting of loose fluvial sands (F1) and soft to very soft, plastic, highly compressible clays and silts (material type E, F2 & M). These clays are normally consolidated. Recommended values for the design of permanent structures in these materials were as follows:F1 (Loose fluvial sands) Modulus depths 0-25 m Effective cohesion Effective angle of friction
E
=
C' ~'
= =
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1~! (Marine clay) Undrained shear strength Cu (depth 0-7 m) = 10 kN/m 2 increasing linearly with depth to 60 kN/m 2 at 40 m depth Undrained modulus Eu = 200 Cu Effective cohesion C' = 0 Effective angle of friction ~b' = 22 ° Sensitivity = 5 Alignment of the phase I tunnels, which traversed several of these infiiled river valleys, therefore presented highly variable ground conditions as the tunnels passed alternately through spurs of rock with variable weathering and valleys filled with surficial deposits. On the phase I! tunnels along Victoda Street, which lie substantially further east, deeper (up to 45 m) more uniform deposits of soft clays were encountered.
136
As virtually all the contracts were expected to encounter the soft clays of the Kailang formation, the manner in which the linings would perform in these materials was of particular concern. In specifying a maximum deflection criterion which limited the relative lengthening or shortening of any diameters, inclusive of building tolerances, to less than 1%, the Corporation was philosophically inclined towards a relatively dgid lining design. This accorded with the contractors' proposals who all elected to minimise, as far aS practical, the number of segments in the ring and to stiffen the joint between segments by staggering radial joint locations. In the elastic idealisation the deformations of the lining and hence the bending moments induced are controlled by three principal parameters. These are: 1) The distortional loading: taken by Muir Wood to be the excess of the pressure on the vertical axis over that on the horizontal axis. It is the authors' view that the distortional pressures applied to the lining are influenced by many factors, in particular time dependency, and that Ko, the at rest coefficient, should only serve as a guide to the designer in his judgement of a suitable horizontal load coefficient, termed K in this paper. This point is developed further below. 2) The elasticity of the lining: generally taken to be that of a continuous ring having the section properties of the uncracked concrete section. 3) The elasticity of the ground: generally taken to be the modulus dedved from the laboratory triaxial testing of clays factored to allow for the conversion between plane stress and plane strain conditions, and long term creep strains. In the real tunnel the deformations and the bending moments are controlled by the stress path to which the ground is subjected. This in turn is a function of the actual and effective stresses in the ground, the strength and plasticity of the material and the construction techniques involved in the tunnelling process. The following discussion is relevant. The process of tunnelling is such that the release of ground stresses occurs at or before the tunnel face. In soft
CONSTRUCTION& BUILDING MATERIALSVol. 1 No. 3 SEPTEMBER1987
clays considerable plastic movement in the form of 'face toothpasting' may occur if support such as face timbers, compressed air, or slurry pressure is not provided. A conventional shield is normally provided with a cutting bead at the leading edge in order to facilitate steering, even in firm clays the annular void created by this cutting bead is rapidly filled by radial squeezing of the clay. To be able to construct a segmental lining in the protection of a shield tail skin the external diameter of the shield must exceed the external diameter of the lining by the thickness of the tail skin itself plus a suitable construction margin. Whilst good practice is to attempt to grout this void as soon as the completed ring has cleared the tail skin, in soft ground it is not always successful and further radial squeezing of the clay into the annular void thus created almost inevitably occurs. Undrained squeezing of the clay is arrested after the ring is grouted and only at this stage does the lining begin to carry loads. It can be seen from the foregoing that in practice the deformations experienced by the clay prior to the lining being installed may be many times larger than that which occurs at~erthe relatively incompressible lining is in place. It is possible to envisage therefore that a relatively large volume of soil around the completed tunnel is in a plastic condition. Clough and Schmidt ~1 present a stress path analysis which enables at least a qualitative assessment of the extent of such plastic conditions around tunnels in clay to be made and examines the development of loading on the lining. The principal loading stages described in relation to a shallow tunnel in a normally consolidated soft clay are as follows: As the excavation takes place the total tangential stress around the opening increases to approximately 2(7v and radial stresses are reduced to the value of any internal air pressure. When the undrained triaxial strength of the material immediately adjacent to the tunnel is exceeded a plastic zone is formed around the tunnel which sheds loads to more distant regions, where with increasing confinement the soil is able to carry greater stresses. In soft clays, or if high overburdens are present, the sheared material in the plastic zone will squeeze against the body of the shield or the now installed lining which will provide increasing radial support; hence arresting the plastic zone development. In this plastic zone significant pore pressure reductions occur; the pore pressures may even become negative in a large region. In time these pore pressures dissipate as drained conditions are established and the boundary of the plastic zone moves closer to the tunnel. The water demand of the negative pore-water zone results in a volume increase near the tunnel. If this volume increase cannot occur because of the incompressibility of the lining, loading of the lining occurs and a new stress equilibrium results. Since arching of the soil around the tunnel occurs the total stresses in the final condition are quite different from the initial total stresses. The soil is therefore not subject to simple consolidation or swelling; a complex readjustment of both total and effective stresses must occur. A relatively shallow tunnel in soft clay will develop an undrained shear zone
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which is virtually certain to reach the ground surface. Hence the entire soil mass above the tunnel is affected and will influence the behaviour of the clay at tunnel depth. Ifthe clay is normally consolidated, or nearly so, disturbance and reconsolidation leads to a state of stress which closely approximates to the original normally consolidated state. In other words, the vertical load on the tunnel approaches the full overburden pressure and the horizontal pressure on the side of the tunnel reaches at least the normally consolidated "at rest" pressures. If the lining is flexible it must deflect to balance the ground loadings and an elastic analysis of this process is appropriate. The foregoing discussion serves to illustrate why the standard design assumption that a shallow tunnel in sot~ clays carries the full overburden is fair. It also serves to explain the apparent paradox of using an elastic idealisation to analyse a loading system that involves essentially plastic deformations and swelling of the soil mass.
Comparison of calculated and measured lining deflection Comparison of actual lining deflection and the calculated predictions are always of value and the following information abstracted from the site ring building and later survey records from Contract ] 04 provides some interesting results. Elastic theory had clearly predicted that the deflections of the lining would be highest in the soft clays which had low elastic moduli. Conversely deflection in G2 rock where moduli values were much greater were not critical.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
137
Ground conditions
K value Measured Calculated ' used in design deflections (1)i deflections (2)
M clay (Cu = 26kpa)
0.7
15 mm
G4 (Cu = 80kpa)
0.7
12 mm
G2 rock (E = 100Mpa)
0.5
4mm
Average 5-10 mm Max 15 mm Average 10-20 mm Max 25 mm Average 20-25 mm Max 35 mm
K value from back analysis
(1 -sine')
0.8
0.63
0.5
0.5
See Note 3
N/A
Notes 1) Calculated deflections are changes across vertical or horizontal diameters of tunnel under working loads. 2) K values calculated from back analysis are those that give the Upl~er limit of the average measure deflections 3) K = 0 gives a calculated deflection of 8 mm for this case, which is substantially below measured deflections.
The actual deflections measured on site indicate that the reverse trend is in fact the case. Two explanations are offered for this observation. Firstly, Ko pressures in the soft clays would be expected to be higher hence producing more uniform loading conditions. If we accept the classical theory that Ko for normally consolidated clays is equal to (1-sin~b') it can be seen that the distortional pressure acting on the lining in the final drained condition is equal to (or' sin~'). The higher density and angle of internal friction for G4 over Marine Clay therefore give rise to greater distortional forces. Secondly, as previously discussed, the rate and manner in which ground loadings are in reality applied to the lining is markedly different to the assumption implicit in the elastic idealisation. In the soft Marine Clays, undrained squeeze of the ground began to occur at all points around the ring as soon as the matedal was excavated. The excavated soil closed rapidly around the newly built dng and high lateral active pressures were obviously being developed. In the G4 material undrained squeeze of the ground was much less evident, especially when stabilised by compressed air. Whilst loading developed quickly in the crown of the tunnels the stand-up time for the material at the sides of the tunnel was greater and lateral loadings obviously developed at a slower rate. The lining is therefore subject to a pedod of distortional loadings under undrained conditions that act in addition to those that develop due to the process of pore-water pressure changes previously described. In the case where G2 rock was present at the axis of the tunnel with G4 in the crown, the material in the walls of the tunnel was stable and lateral loading conditions for clays based on undrained squeeze and long term swelling were obviously no longer relevant. As the primary action of the lining is to carny loads in ring compression, the observed deflection of the lining would indicate that squatting of the lining occurs until adequate passive pressures are developed against the rock at the axis of the tunnel. Under such mixed ground conditions, ie. clays overlying competent material, it appears possible that the whole of the overburden may act as a distortional load on the lining; ie. K = O. A moment's reflection on the manner in which the lining cardes the load will also indicate the importance and influence of adequate backfill grouting. For unless the
138
void between the liner and ground is completely filled with a low modulus material, the deflection of the lining required to mobilise the required passive pressures will be increased.
Design of adjacent tunnels in clays In many locations of the MRT alignment geometry restraints resulted in tunnels being required to be built in close proximity both horizontally and vertically. If two tunnels are excavated in close proximity they will mutually influence each other and due allowance should be made for these effects in the design. The following discussion is relevant:
Effect of the first tunnel on the second tunnel As noted in the previous section, the behaviour of the soil around a tunnel in soft clay is strongly influenced by the methods used to excavate and support the tunnel. Different excavation and support arrangements lead to differences in the stresses and displacements induced into the surrounding soils. In particular, ground lost through the face and radial inward squeezing into the grout void before grouting will result in shear failure of the soil and the development of a plastic zone around the tunnel: Clays will be subject to strength loss and reduction of stiffness as a result of such movements dependent on their sensitivity. If a second tunnel is excavated in a zone of ground disturbed by a previous tunnelling operation it would be normal practice to make a reduction in the elastic modulus of the soil surrounding the second tunnel to account for these factors. For the design of the second tunnel, if excavated in a zone of plastic ground movement around the first tunnel, the assumption of the elastic methods of virgin vertical and horizontal initial state of stress in the ground become invalidated. However, ground loadings onto the tunnel are related primarily to the shear strength of the clay, the overburden pressure and the manner in which pore pressure changes caused by the excavation are dissipated. As such, the strict principles of super-position, implicit in the elastic methods of analysis, do not necessarily apply. Rather the construction of the second tunnel will extend the magnitude of the plastic zone, and loads on both tunnels will be dedved from a consideration of the dissipation of the pore pressures between the various negative and positive pore pressure zones created by the
CONSTRUCTION& BUILDING MATERIALSVoI. 1 No. 3 SEPTEMBER1987
tunnel geometry, construction methods and sequences. Peck et al ~6~recommends for tunnels in soft ground that the full overburden pressure should be adopted as the load case for both tunnels.
Effect of the second tunnel on the first tunnel When the second tunnel is excavated there will be a certain loss of ground around the excavation. If the first tunnel is within the zone of influence of the second tunnel it will inevitably be subject to distortional loads associated with these ground movements. The magnitude of such ground movements are primarily a function of the shear strength of the soil, the overburden pressure and the construction techniques employed to stabilise the face and grouting of the annular void. In impermeable clays most of the displacement occurs as squeeze under undrained conditions. Subsequent swelling, while of significance for lining loads and distortions, usually has little effect on the surface movements and displacements within the soil mass. The recommended method for the design of the first tunnel was therefore to analyse the ground losses around the second tunnel in a similar manner to that for a settlement analysis and to allow for the correspondingly increased bending moments in the lining of the first tunnel induced by such displacements.
Detailing of the linings Whilst all contractors were faced with generally similar design ioadings in terms of axial thrust and bending moments, their particular solutions in terms of reinforcement detailing, bolting arrangements and waterproofing methods differed markedly. Isometric details and typical reinforcement details of the various designs are shown in Fig 3. The relative advantages and disadvantages of the vadous features, gained from the experience on the MRT are discussed in the following sections.
Circumferential joint bolting details Fabricated steel bolt pockets were employed by Contracts 104 and 108. Whilst this detail represents a relatively small reduction in the cross-sectional area of the segment body, the experience has been that, as predicted by tests carried out by the Singapore Institute of Standard & Industrial Research (SISIR), segments incorporating this feature were relatively weak in lateral bending and were susceptible to cracking under shield jacking loads. Steel bolt pockets to the circumferential joint, whilst offering excellent location of segments lacked compliances and the experience on Contract 104 was that circumferential bolts were sheared on several occasions whilst attempting to make alignment corrections to the shield. The cost of fabricating and gaivanising the steel bolt pockets was estimated to be greater than other details.
Concrete bolt pockets were employed by Contracts 106, 107 and 109. In both the 106 and 109 designs the pockets represented a reduction in the cross-sectional area of the lining in the order of 20%. In the 107 design, this reduction in area is avoided by the use of a steel sleeve cast into the body of the segment into which a threaded stud is withdrawn during segment erection. This detail does however preclude the replace-
ment of circumferential bolts should this become necessa~ dudng the life of the tunnels. The use of concrete bolt pockets does allow a stiff edge beam to be formed in the extreme fibre of the segment and the general performance of segments reinforced in this manner in resisting shield jacking forces was good. Companion papers submitted to this conference discuss the Corrosion of Underground Structures (CBM June '87). In the region of the bolt pocket a locally ve~ thin section of concrete panel exists which may be susceptible to accelerated corrosion due to percolation of aggressive groundwater. The cost of the bolt assembly is low and good location of segments during erection was achieved.
Curved bolts were adopted by Contracts 107B and 301. The detail requires only a small reduction in the crosssectional area of the segment, and allows excellent edge reinforcement to be achieved. The curved bolts can be replaced if required and no thin concrete panels that may be susceptible to corrosion are created. The cost of the curved bolt assembly is moderate and no particular difficulty of location of segments during building were reported with their use. Radial joint bolting detail Fabricated steel bolt pockets were employed on Contracts 104, 106, 108 and 301. The detail requires only a small loss of section in the critical joint zone and testing at SISIR indicated that the presence of the fabrication inhibited development of shear cracking in the joint zone and significantly contributed to the strength of the segment if any misalignment occurred. The detail provides excellent location of segments and good 'nip up' during building. MRTC Phase 11 Materials and Workmanship Specification required that exposed metalwork in the tunnels is protected by a duplex system comprising hot dip galvanising and coal tar epoxy paint. This requirement is anticipated to significantly increase the life of the exposed metalwork.
Concrete bolt pockets were employed by Contracts 107 and 109 and resulted in a relatively large loss of section in the critical joint zone associated with this detail. Radial joint packing. Having regard to the perceived limitations of bituminous packings as revealed by the SISIR tests, the use of sheet rubber packing to radial joint surfaces was incorporated into the Phase II Materials and Workmanship Specification.
Joint bursting reinforcement was shown in the SISIR test series to greatly increase the strength of the lining if subjected to construction misalignments. This detail was incorporated into Phase II designs.
Key design Three basic methods have been used to close the ring as shown in Fig 4.
Standard taper keys were used on Contracts 104 and
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
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107B. In this design a key block of arc length approximately twice the lining thickness (500 mm) was installed radially from within the plane of the ring. The relatively short length means that the angle of draw on the key can be shallow and in this manner shear stresses on the joint surface are minimised. Further, the area of the key subjected to a disturbing pressure when grouting is small and no problems of keys dropping dudng either grouting or backgrouting operations were reported on these contracts.
One bolt hole keys were used on Contracts 107, 108, 109 and 301. In this design the key comprised a one bolt segment, of arc length up to 1060 mm, which was installed radially from within the plane of the ring. The large arc length resulted in a higher angle of draw on the key being required with consequent increases in the shear stresses on the joint surface. Further the area of the key subjected to a disturbing pressure when grouting is large and problems of keys being displaced during both grouting and backgrouting operations were observed on
CONSTRUCTION 8, BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
141
several of the contracts that incorporated this detail in their lining design. Taper key blocks were used on Contract 106. In this design a key segment is erected slightly forward of the plane of the ring to allow sufficient construction tolerances and is then pushed back using the shield jacks. The advantage of this technique is that the key joint surfaces are truly radial, hence eliminating problems of shear stress and key displacement during grouting. However problems do exist in maintaining accuracy of build as the key is jacked back and some increase in segment damage local to the key was observed.
Waterproofing of tunnels In all cases designers elected to provide a sealing material located in grooves near the rear edge of both the circumferential and radial joints, as the main line of defence against water ingress. All contractors also provided a traditional caulking groove in the inner face of both the circumferential and radial joints. A variety of materials were used for sealing and caulking the linings and their relative performance is discussed.
Simple bitumastic sealing str/ps were employed on sections of Contracts 104 and 109. These materials proved to be of limited effectiveness as on both contracts water ingress, whilst reduced, was many times the specified rate. The experience has been that these materials behaved in a totally plastic manner and once compressed were unable to recover their original shape. Consequently leakage often developed as shield jacks were withdrawn and the sealant was unable to follow the relaxation of the segment. Similarly the effectivenss of the seal was impaired ifjoint packing was incorporated to correct alignment or achieve large radius curves, as the degree of compression was markedly reduced. Particular difficulties also arose around the key as the protruding face of the sealant was susceptible to damage or misalignment during dng building as the key was pushed home. On Contract 109 the contractor resorted to an extensive programme of back grouting involving the injection and reinjection of cement/water milk through both the grout holes and joints between the segments, in areas of high water ingress up to twelve passes were involved. On Contract 104 the contractor elected to supplement a back grouting exercise with the use of epoxy caulking materials. Water activated polyurethane foams were injected into the void between the defective sealing strips and the caulking, to prevent water flow in the channels thus created.
Composite neoprene and bitumastic sealing str/ps were employed on sections of Contracts 104, 107, 108 and 109. The experience has been that whilst the neoprene core of these materials maintains a sealing pressure once compressed, the effectiveness of the seal is markedly reduced if joint packing is required and that these materials are also subject to damage and misalignment around the key segment.
142
AS a consequence additional waterproofing measures have generally had to be carried out to linings built in water-bearing grounds.
Neoprene gaskets were employed on Contract 106. These items consist of an extruded section of proprietary design that is formed into a ring and fitted into a groove in all segments. The surface of the gasket stands proud of the surface of the segment by some 2-3 mm and pressure between the compressed mating surfaces effects a seal. The experience has been that the resilient nature of the neoprene ensures adequate pressure between mating surfaces and that due to the taper key detail employed in this lining design, no problems of gaskets becoming displaced have occurred. Numerous problems of achieving an effective seal at the corners of segments were encountered and the specified rate of water ingress has not generally been achieved in water-bearing grounds. Remedial measures have taken the form of chemical grouting and water activated polyurethane foam injection.
'Hydrotite' gaskets were used on Contracts 107B and 301. This material is formed by mixing non-water-soluble but water-expansive resins and synthetic rubber. When the gasket comes into contact with water it swells to ten times its odginal volume, filling the joint gap and sealing against water ingress under its own expansion pressure. As the matedal may be fitted flush within the groove few problems of damage or misalignment, even during key installation, have occurred. The experience has been that swelling of the sealing stzips occurs shortly after installation and that a virtually dry watertight tunnel has been achieved without additional waterproofing measures. Testing of segments by SISIR As part of the procedure for checking the contractors' lining designs, prototype segments were subjected to various full scale loading tests at SISIR's laboratory. Two tests proved particularly relevant to the ultimate performance of the segments.
Lateral bending test During construction minor misalignments between segments and errors in the plane of build inevitably give rise to imperfect mating of adjacent circumferential joint surfaces which are required to act in bearing to transmit shield jacking loads. Under such circumstances segments can be subjected to extensive 'break back' lateral bending stresses, as illustrated in Fig 5. The test was designed to simulate the loading condition on segment 'A' in this figure. The Corporation was obviously concerned to minimise segment damage due to shoving and several aspects of the contractors' designs indicated that they would be susceptible to such problems. • The general use of large segments (typically 5 or 6 segments per ring) would result in large lever arms if such misalignments did occur. • Many of the shields had been designed for blind operation in soft clays and were equipped with extremely powerful shove rams. For example Contract 104's shields were provided with 24 No. 100 tonne
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
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jacks. • Few contractors were proposing any form of compressible packing in the circumferential joints to cushion the segments during shoving. As was expected, segment designs which featured steel bolt pockets on the circumferential joints were at a considerable disadvantage in their ability to resist this type of loading as the steel bolt pockets not only restricted the
Segment type
104(G M) 104(M) 106 107 108 109(G) 109(M)
amount of steel that could be concentrated in the edge of the segment, but also acted as a point of stress concentration in the extreme fibre of the segment which initiated cracking. Whilst acknowledging the severity of the test, reinforcement changes and/or the use of compressible packing was required for those designs that the testing indicated would be susceptible to such damage.
Load to 0.1 mm crack width
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Results of lateral bending test Joint test Design calculation for all Phase I lining types assumed that radial joint surfaces were erected with mating surfaces in perfect contact. Only one design, that of Contract 106, considered the possibility of tensile bursting reinforcement in this zone to resist such forces. Considerable discussion occurred regarding the compatibility of the design assumption of perfectly mating radial joint surfaces with the relatively generous segment manufacturing tolerance on the joint surfaces themselves (+ l m m from the radial plane) and the build of the lining (_+ 25 mm deviation from a true circle). In adverse conditions where these tolerances may become cumulative, considerable 'birds-mouthing' of the radial joint will occur. The joint test was devised to study the performance of the various contractors' radial joint details, if they were subjected to such adverse conditions. The tests were carried out on two "L" shaped concrete
specimens each the full thickness of the segment but due I~ capadty limitations of me tes~g frames only 500 mm wide (1/~ segment width). The bolt pocket design, joint packing if any, and reinforcement details within 250 mm of the joint zone were exactly similar to the segment design being modelled. The specimens were erected in the testing frame with strips of plastigauge inserted in the joint and aligned square and plumb. A laser was fixed to the upper segment and the beam aligned parallel to its axis. The scale on the lower segment was zeroed. The upper segment was then given a small rotation equivalent to that which would occur in a misaligned joint and the new laser reading noted. The test procedure was to incrementally apply loading to the specimens via the main compression jacks P1 whilst simultaneously adjusting the force in the secondanj tension jack P2 to maintain the angle of misalignment (as measured by the laser) between the segments constant.
C O N S T R U C T I O N & B U I L D I N G MATERIALS Vol. 1 No. 3 S E P T E M B E R 1987
143
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Demag gauge studs installed on either side of the joint enabled a measurement of the closure of the joint at each load increment. The test was continued to destruction or until the capacity of the loading f~ame was reached. On completion of the test the specimens were disassembled and the width of the plastigauge strips measured at various points across the thickness of the segment in order to estimate the nature of the bearing surfaces at failure. Simple resolution of forces and moments, combined with a knowledge of the geometry of the specimens enabled a calculation of eccentricity of line of action in the radial joint versus axial load to be plotted. The following observations were made: • It was noted that cracking in thejoint zone occurred at a lower threshold for the designs which were erected without joint packing, than for other designs which used various forms of bituminous packing materials. However, these materials were observed to b e c o m e fully plastic under moderate loads and were squeezed out of the contact zone. Concrete to concrete load bearing surfaces had generally been fully developed for all designs above applied loads of 60 tonnes. Use of bituminous packings were not considered to affect the ultimate load capacity of the joint. • Cracking in the joint zone generally occured initially as local spalling around the segment corners and bolt pockets. At about 70-80 tonnes load a shear crack developed from the highly stressed joint area running at a shallow angle to intersect the segment face. This cracking was generally associated with a load redistribution within the joint zone as load was shed from the
144
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edges of the segment back towards the central core and the joint crushed down. Increased loadings developed further shear cracks originating from the core area at increasingly steep angles which intersected the original shear crack and led to the ultimate failure of the segment. The performance of the Contract 106 segment which had been provided with tensile bursting reinforcement in the end block of the segment was particularly good. This segment had developed only local spalling at the corners when the capacity of the loading frame was reached. Performance of the Contract 104 and 108 designs was largely governed by the presence of heavy fabricated steel bolt pockets in the radial joint. These items appeared to be capable of both carrying load away from the highly stressed joint zone and of controlling the development of secondary shear cracking at ultimate loads. The performance of designs without either of these features was poor and the segment was generally considered to be susceptible to damage due to construction misalignment. Conclusions In drafting this paper the authors have sought to develop two main themes that in their opinion should be borne in mind by the designer of segmental tunnel linings. Firstly, the actual process of tunnelling is a three dimensional process involving complex redistribution of stresses around the excavation as it proceeds. Real soils around the excavation undergo complex stress path
CONSTRUCTION & B U I L D I N G MATERIALS Vol. 1 No. 3 S E P T E M B E R 1 9 8 7
changes involving undrained plastic squeeze, changes in effective stress and pore-water pressures followed by subsequent swelling or shrinkage as these pore-water pressures are dissipated. The process of tunnelling in soft soils is therefore one that does not lend itself easily to mathema~cally modelling.,A rigorous analysis would require a three-dimension finite element analysis that would be able to model not only the non-linear responses of the clay under undrained conditions, but also the stresses induced by the volumetric changes of the clays. It is the authors' view that faced with the complexity of the problem, simple hand calculation methods that conservatively consider only the initial and final states of stress in the ground, are at least as relevant as other, more sophisticated methods, that have as their roots a twodimensional analysis based on the principle of elastic super-position. Secondly and perhaps more importantly, the authors have tried to highlight the inter-relationship, that is perhaps unique to tunnel design, between the loads and deflections to which the lining is subjected during construction and the final state stresses that follow from consideration of structure/ground interaction. The tunnel lining should not therefore be designed in a vacuum making ideal assumptions regarding its construction. Rather, the lining should be viewed as an integral part of the tunnelling process that must be fully compatible with all aspects of the construction. In particular, the designer must be aware that in the real world shields do stray off line and that in the process of correcting the alignment rings will be built with a less than perfect mating surface on the circumferential joint. Similarly in the real world segments are built that are not perfectly circular and that if plain radial joint surfaces are used, concentration of load at either the front or the back of the mating surfaces will occur.
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It is the acknowledgement of these realities and the detailing of the lining to accommodate suitable tolerances that ensures the practical constructability of the design. Similarly, attention to detail and material selection for waterproofing, caulking and protection of exposed metalwork are factors that have a major effect on the serviceability and long term maintenace requirements of the lining. The importance of these factors are all too often overlooked in the design process.
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CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987
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Re~erences
(1) Muir Wood, A M. The circular tunnel in elastic ground. Geotechnique 25 No 1, 1975. (2) Curtis, D J. Discussion on '1' above. Geotechn/que 26 No 1, 1976. (3) Dames and Moore. Detailed Geotechnical Study - Interpretative Report Internal MRTC document. (4) Peck, R B. Deep excavat/ons and tunnelling in soft ground, Mexico
146
1969, (5) Clough and Schmidt So/~ clay engineering. Published by F_Jsevier Scientific Publishing Company, Chapter 8. (6) Peck et al. Some design considerations in the selection of underground support systems. University of Illinois report to US Department of Transportation November 1969. (7) Doran, Robe~y, Ong and Robinson, The corrosion of buried sO'uctures. MRTC conference 1987.
CONSTRUCTION & BUILDING MATERIALS Vol. 1 No. 3 SEPTEMBER 1987