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Predicted and Observed Ground Movements around a Tunnel Boring
Predicted and Observed Ground Movements around a Tunnel Boring Machine at Heathrow Airport C. Pound, Y. S. Hsu and G R Walker Mott MacDonald Limited, London, United Kingdom Abstract The Airside Road Tunnel (ART) at Heathrow Airport comprises twin 8.1m diameter tunnels constructed by TBM in London Clay at shallow depth beneath taxiways and above an existing rail tunnel. To assess the effect of construction of the ART on the Heathrow Express (HEX) rail tunnel, a series of three-dimensional numerical analyses were carried out using the finite difference program FLAC3D. The analyses adopted a non-linear model to replicate the small strain stiffness of the over-consolidated soil stratum at tunnel horizon. Three construction methods were considered, comprising open face, compressed air and earth pressure balance tunnelling. The analyses demonstrated the significant contribution of closure of the ground around the tunnel shield on the overall ground movements caused by the tunnelling and the benefit of the use of ground support pressures in reducing the total magnitude of ground movements. The analyses predicted that the loads and deformations induced in the underlying HEX tunnel would be small whatever construction method was adopted and that they would be within operating tolerances for the railway. Instrumentation installed in the early part of the ART showed that heave was occurring at the ground surface ahead and over the tunnel face. The analyses were rerun to investigate this effect and the movements of the HEX tunnel were again predicted to be small. Monitoring of the HEX tunnel as the ART TBM passed above showed small movements and no damage or distress was caused to the tunnel or unacceptable movement of the rail tracks. The monitoring did indicate that the tunnel settled as the TBM passed and this could not be replicated in the numerical modelling. Introduction The Airside Road Tunnel (ART) at Heathrow Airport will provide an underground road link between the Central Terminal Area and aircraft stands to the west of the airport and at a later date with the new Terminal 5. The ART is 1265m long and comprises twin 8.1m internal diameter bored tunnels with cut and cover portals and access ramps formed in retained cut at the two ends. At two locations operational rail tunnels pass beneath the ART. The Piccadilly line passes beneath the west portal cut and cover structure with a separation of only about 5m between the tunnel crown and the invert of the portal base slab. Also, at approximately the mid-point of the ART bored tunnels, the Heathrow Express (HEX) tunnel passes beneath the ART with a clearance of only around 3m between the HEX tunnel crown and the ART tunnel invert. The tolerance of operating railway lines is such that small track movements can necessitate a temporary speed restriction to be imposed and a subsequent realignment of the track. This can have major implications for the rail operator. Consequently, there was significant concern that construction of the ART could cause unacceptable movement of the existing tunnels because of the limited tunnel separation. This paper discusses the design study carried out to confirm the acceptability of tunnelling with such limited cover over the top of the HEX tunnel and describes the observed behaviour as the first tunnel passed. Ground Conditions An extensive site investigation was carried out to determine the ground conditions along the alignment of the ART and this supplemented the existing database of information on the geological conditions at Heathrow Airport. Over much of the route, the investigation identified that Terrace Gravels overlie a considerable thickness of London Clay. However, the location where the HEX crosses the ART lies at the edge of a backfilled borrow pit where the Terrace Gravels have been extracted. The geological profile at the location of the crossing is as given in Table 1. Stratum Thickness (m) Borrow Pit Materials / 4.3 Terrace Gravels London Clay 55.4 Lambeth Group Undefined Table 1 Geological profile at HEX-ART crossing The Terrace Gravels comprise a dense to very dense, sandy medium to coarse gravel, becoming progressively more clayey towards the ground surface. The London Clay is a stiff to very stiff overconsolidated fissured silty clay with an average undrained shear strength measured in triaxial tests increasing from about 100 kPa at the London Clay surface to about 350 kPa at a depth of 30m below ground level. The surface of the London Clay is weathered, but along the route of the ART the depth of weathering is no more than 1m. The Borrow Pit materials typically comprise slightly sandy, slightly gravelly clay with occasional layers and/or pockets of silty, sandy gravel. The SPT N values within the deposit vary between 2 and 9. It is believed that the Borrow Pit materials are derived from a mixture of London Clay and Terrace Gravels. The water table is located at 2.2m below existing ground level and the water pressures are assumed to be hydrostatic through the Borrow Pit materials, Terrace Gravels and London Clay. Tunnel Geometry The HEX tunnel was constructed in the mid-1990s using a conventional open face shield. A 220mm thick, 1m wide, 6.115m external diameter, expanded pre-cast concrete segmental lining comprising nine segments and a key was installed immediately behind the shield. Review of surface settlements above the HEX tunnel showed that the volume loss was typically 0.7% during the tunnel drive between the Central Terminal Area and Terminal 4 (Pound et al2). The rails within the HEX tunnel are fixed directly to a continuous concrete track bed. The ART was constructed using a 9.18m diameter earth pressure balance tunnel boring machine. The machine had a 6.5m long main shield and a 3.7m long tail skin. The front of the shield had a 10mm bead and the shield and tailskin taper by 20mm on radius over their length. At the back of the tail skin the diameter was therefore 9.12m. The machine could be operated in open mode, compressed air mode or earth pressure balance mode depending on requirements. Around the shield bentonite could be injected to minimise ground loss prior to installation of the lining. A 350mm thick, 1.7m wide, 8.8m external diameter pre-cast concrete lining, comprising seven segments and a key, was installed within the tailskin. At the location where the ART crosses the HEX, the HEX tunnel axis is 23.0m below ground level. At the initial stage of the design the precise vertical alignment of the HEX tunnel was not fixed and could lie between 12.0m and 12.8m below ground level. The theoretical separation between the extrados of the two tunnel linings could therefore vary between 3.5m and 2.7m. It was considered prudent to assess the impact of ART construction on the HEX tunnel assuming the minimum clearance of 2.7m. The angle between the two tunnel axes is 77º and both tunnel alignments are on horizontal curves. Design Calculations Geotechnical Properties In view of the weaker nature of the Borrow Pit materials compared to the Terrace Gravels it was considered appropriate to assume in the design that the material overlying the London Clay was Borrow Pit materials, as this was anticipated to provide an upper bound to the ground movements. No sophisticated in situ or laboratory testing was carried out on the Borrow Pit deposits because of their natural heterogeneity and also because they were not considered to significantly affect the tunnelling conditions. Therefore the properties were derived from classification tests, SPT N values and triaxial compression tests. The parameters adopted in the design calculations are given in Table 2. Since no small-strain stiffness data exists for these deposits, it has been modelled as a linear elastic perfectly-plastic material with a Mohr Coulomb yield criterion. The basic properties of the London Clay are well understood from testing at Heathrow and elsewhere in London. The non-linear stiffness properties were derived from stress path triaxial tests complete with local strain measuring devices. The properties adopted for the London Clay are presented in Table 2. Soil Type γb Porosity E’ ν’ c’ φ’ (o) (kN/m3) (%) (MPa) (kPa) Borrow Pit Materials 19.5 30 10 0.15 0 21 London Clay (short term) 20 50 variable 20 25 London Clay (long term) 20 50 variable 8 23 Table 2 Soil parameters For excavation of both tunnels, the London Clay was modelled as an undrained material with peak effective stress shear strength properties back-calculated based on the undrained shear strength profile. A non-linear elastic perfectly-plastic constitutive model was implemented into FLAC3D based on the method outlined by Jardine et al1. The tangent stiffness expressions that describe the behaviour are as follows: - 3Gtan γ B.α .γ . X γ −1 = A + B. cos(α . X ) − . sin(α . X γ ) p' ln 10 εs X = log10 C K tan S .δ .µ .Y µ −1 = R + S . cos(δ .Y µ ) − . sin(δ .Y µ ) p' ln 10 εv Y = log10 T where Gtan and Ktan are the tangent shear and bulk moduli respectively, p’ is the current mean effective stress, and εs and εv are the shear and volumetric strains respectively. A, B, C, R, S, T, α, γ, δ, and µ are constants. The shear strain, εs, is equal to half the octahedral shear strain invariant. Up to a specified minimum strain, the stiffness varies only with p’, but thereafter the stiffness depends both on the current strain, ε, and the mean effective stress. The equations were implemented using a sub-routine within FLAC3D that continually updated the shear and bulk tangent moduli throughout the analysis. To prevent unrealistically low shear and bulk moduli developing during the analysis, a minimum value of p’ of 50kPa was used when calculating the moduli. The constants used in the analyses are given in Table 3 and were derived by curve-fitting of small-strain triaxial test data from the project site investigation and from a review of existing small-strain data for the London Clay. A B C α γ εsmin 800 700 3.0x10-6 0.96 1.0 3.1x10-6 R S T λ µ εvmin 150 100 4.9x10-5 2.0 1.0 5.0e-5 Table 3 Constants for London Clay stiffness model The ko profile for the London Clay was based on the results of pore suction measurements taken on high quality undisturbed samples tested immediately after extraction from the borehole. The ko profile reduced from 2.5 at the London Clay surface, to 1.5 at 20m below the London Clay surface and to 1.2 at 40m below the London Clay surface. The ko value adopted for the Borrow Pit materials was based on 1-sin φ’. FLAC3D model and modelling sequence The model developed for the analysis is shown in Figure 1. For greater numerical efficiency symmetry on a vertical plane through the ART axis has been taken. The model is 100m long in the direction of the tunnel axis, 50m wide and 60m deep such that the base of the model represents the top of the Lambeth group. The model width was equivalent to about 4 times the Figure 1 FLAC3D mesh depth of the tunnel axis which was considered sufficient to minimise boundary effects. The model contains a total of nearly 34000 brick elements. The base is fixed against movement in all directions whilst the vertical boundaries are fixed in the direction perpendicular to the boundaries only. For simplicity of mesh generation, the HEX tunnel axis was taken to be perpendicular to the ART axis. The analysis was initially run to ensure that the initial stresses were in equilibrium and the resulting small displacements were set to zero. The precise construction sequence for the HEX tunnel was not considered to be critical and therefore the tunnel was excavated as a single event. Elements representing the HEX tunnel were first removed and tractions applied to the HEX tunnel boundary to balance the in-situ stresses. These tractions were gradually decreased until a surface settlement corresponding to a volume of about 0.7% was achieved. The HEX tunnel lining was then installed with the properties given in Table 4. The lining was modelled using triangular shell elements and joints between segments were modelled by providing pin connections between these elements. Next the remaining tractions were removed and the model run to equilibrium. Finally, long-term pore pressures around the HEX tunnel were allowed to develop assuming an impermeable tunnel lining. The strains were next initialised to reset the small strain stiffness. Excavation of a 70m length of the ART was analysed, assuming undrained conditions for the London Clay. Excavation of the ART was modelled in 23 stages by progressively removing elements from within the tunnel profile. As the length of the TBM shield is about 10m, the ground 10m behind the tunnel face was allowed to deform with three different supporting conditions, namely open face, compressed air and earth pressure balance modes. The three different support modes were modelled as follows: • For the open face mode (OPFM), the 10m length behind the tunnel face was excavated assuming no ground support. • For the compressed air mode (CAM), a constant pressure of 140kPa was applied to the excavated surface for 10m length behind the tunnel face. This pressure is approximately equal to the groundwater pressure at tunnel axis. • For the earth pressure-balance mode (EPBM), a pressure varying linearly from 184kPa in the crown to 312kPa in the invert was applied to the excavated surface for the 10m length behind the tunnel face. This pressure variation was based on balancing the vertical stress at axis and assuming that the remoulded soil provides a variation of pressure given by a unit weight of 14 kN/m3. For all tunnelling modes the lining elements were then installed 10m behind the tunnel face. The TBM was advanced in stages of 1.7m representing the length of the segmental lining. The ART lining was also modelled using triangular shell elements with properties given in Table 4, which are considered to be realistic for the concrete lining. Lining type Thickness E (GPa) Poisson’s Unit (mm) ratio weight (kN/m3) HEX tunnel 220 25 0.15 23 ART lining 350 25 0.15 23 Table 4 Concrete Lining Properties Figure 2 shows the predicted surface settlement along the ART centreline relative to the ART face position for the three construction modes. As expected, due to the 10m unsupported length for the OPFM, the maximum ground settlement is 250mm, which occurs about 30m behind the tunnel face. For the CAM, the maximum ground settlement is 30mm occurring 30m behind the tunnel face. For the 50 EPBM, the ground settles slightly in 0 front of the tunnel face and heaves to Vertical displacement (mm) -50 a maximum of about 20mm 20m -100 behind the tunnel face. Based on these displacements, the estimated -150 volume loss of the open face -200 (OPFM), compressed air (CAM) and -250 Open face mode Compressed air mode earth pressure balancing (EPBM) Earth Pressure Balance mode -300 modes is 15.5%, 3.1% and 0.25% -40 -30 -20 -10 0 10 20 30 40 50 respectively. The deflections for the Distance from ART face (m) OPFM and CAM obviously Figure 2 Surface settlement on ART centre-line. overestimate the actual ground movements as the analysis ignores the presence of the shield which would limit the actual ground convergence to about 30mm around the tunnel profile. A uniform inward movement of the excavated surface of 30mm would represent a volume loss of about 1.3%. Figure 3a shows the vertical displacement of the HEX tunnel crown at the point below the centre-line of the ART relative to the ART face position. For the OPFM, the HEX crown initially settles as the ART tunnel face approaches the crossover point and then starts to heave when the ART face passes with a maximum heave of 21mm occurring 5m behind the ART tunnel face. For both the CAM and EPBM, there is only slight crown movement as the ART tunnel approaches but after the ART face has passed, crown heave occurs. For the CAM, a maximum HEX crown heave of 9mm occurs 7m behind the ART face. For the EPBM, a maximum HEX crown heave of 2mm occurs about 17m behind the ART face. It is interesting to note that although the OPFM and CAM analyses predicted very large surface settlements, the predicted ground movements below the tunnel were significantly smaller and these analyses were therefore useful in providing an upper bound assessment of the movement of the HEX tunnel. Figure 3b shows the HEX tunnel crown horizontal displacement beneath the ART centre-line plotted relative to the ART face position. Positive values indicate that the displacement is in the same direction as the ART tunnel drive. For all modes, the crown moves towards the ART face as the ART approaches and when the ART passes the centre-line of the HEX, the crown starts to move in the direction of the ART drive. A review of the predicted movements at track level, allowing for the effect of the tunnel shield, suggested that the track would undergo heave of between 2 and 5mm depending on the tunnelling method employed. 25 15 Horizontal displacement of HEX crown (mm).. Vertical displacement of HEX crown (mm).. Open face mode Open face mode Compressed Air mode Compressed Air mode 20 Earth Pressure Balance mode 10 Earth Pressure Balance mode 15 5 10 5 0 0 -5 -5 -10 -10 -15 -15 -25 -20 -15 -10 -5 0 5 10 15 20 25 -25 -20 -15 -10 -5 0 5 10 15 20 25 ART face position relative to the HEX-ART crossover point (m) ART face position relative to the HEX-ART crossover point (m) a) b) Figure 3 Vertical and horizontal movement of the HEX tunnel Instrumentation Layout and Monitoring System Instrumentation installed above and around ART The ART drive was monitored for surface settlement from the launch of the TBM. Surface monitoring typically comprised manual precise levelling in front of and over the TBM position as well as on the completed bore behind the TBM. Due to the presence of extensive airfield pavement areas, much of the surface monitoring utilised 1.2 m deep monitoring pins that allowed comparison between the pavement settlement and the movement of the underlying ground. Typically the monitoring points were set out either in perpendicular arrays spaced every 100 m, extending 25 m either side of the centreline or as points along the centreline spaced every 10 m to 25 m depending on the sensitivity of the locality to settlement. Monitoring intervals varied depending on accessibility of the TBM position, but in general was between 12 and 24 hours. Where considered appropriate the precise levelling has been augmented by the use of manual or real time monitoring techniques including a robotic total station, in-place electrolevel inclinometer and rod-extensometers. In the vicinity of the HEX Tunnel, surface monitoring comprised manual levelling and manually read extensometers and inclinometers. Instrumentation installed in the HEX Monitoring of the HEX tunnel was undertaken to provide Heathrow Express Ltd with assurance that the track and tunnel remained within operational tolerances permitting the safe transit of passenger carrying trains whilst the TBM continued to work above. To provide the robustness necessary for a safety critical system, primary and secondary real time methods were used. The remote data was confirmed by manual observations taken during Engineering Hours (hours during which the trains were not operating), which provided a tertiary monitoring system. The primary monitoring system used in the HEX tunnel comprised an optical technique using a high specification fully automatic total station instrument. A secondary real time system was designed using discretely placed electrolevel tiltmeters. This allowed a remote assessment to be made of changes in track cant and track gradient and thus, could be used to confirm the movements observed using the primary system. Manual observation of tunnel deformation formed the tertiary system and was carried out by precise levelling on levelling studs, gauge and cant surveys using a standard track gauge, together with alignment and versine surveys carried out using standard total station techniques. The manual data concentrated on the track-bed as the size of the HEX tunnel and the presence of the OHLE significantly increased the risk of using conventional steel tape extensometers or inverted levelling off the crown. Again the manual survey was used to confirm the data recorded by the robotic total station instruments, but was also the preferred method for assessing changes in rail cant and gauge as it was not considered possible to affix a target prism directly to the rails themselves. MP MP MP – Mini-Prism Target MP Lvl – Level Target TM – Tiltmeter RTS – Robotic Total Station OHLE Catenary RTS MP MP RTS MP MP TM Lvl Lvl MP Figure 4 HEX tunnel instrumentation MP The monitoring scheme required the installation of some 112 mini-prism targets, 21 electrolevel tiltmeters and 40 manual levelling points (as the ART comprised two separate tunnels drives, the initial deployment of instruments was designed to accommodate, as far as possible, both crossings; the second drive requiring only a small number of prisms to be relocated). HEX identified that the structural envelope was most critical at the edge of the emergency walkway and that movement of the overhead line electrification (OHLE) catenary supports would also be critical. Consequently, a mini-prism was located on the walkway edge on full arrays to give an assessment of the change in clearance of the structural envelope and a prism was bolted to the catenary support arm. A diagrammatic representation of a full array is shown in Figure 4; the robotic total stations are shown to indicate their position within the tunnel circumference. The arrays were spaced nominally at between 3 m and 5 m intervals along the HEX tunnel. Construction Optimisation of the tunnel alignment during the design period meant that the final design axis level of the ART at the point where it crossed the HEX was 12.0m below ground level resulting in a theoretical clearance of 3.5m between the two tunnels. Excavation of the ART tunnel commenced on 21st June 2002. The machine was generally operated with air pressure applied at the face and with bentonite injected around the shield. Prior to the TBM entering the zone of influence of the HEX, defined as a zone 40m either side of the HEX tunnel centre-line, the preventative maintenance of the TBM was carried out to ensure that tunnelling over the HEX tunnel could be carried out as a continuous operation. The ART crossed the centre-line of the HEX on 18 October 2002. Within the zone of influence the average rate of progress was 18.9m/day. Pressures were measured continuously in the TBM. The thrust applied by the TBM averaged between 12MN and 14MN during the period while the TBM was in the zone of influence. The face air pressure and bentonite pressure was maintained at 50 kPa and 100 kPa respectively during this same period. The grout pressure recorded at the tunnel crown varied between 180 kPa and 270 kPa, although as the HEX was crossed the crown grout pressure was maintained at about 200 kPa. The face pressure of 50 kPa if applied over the whole face area of 66.2 m2 represents a thrust of only 3.3 MN. Thus the TBM applied an additional force of about 10 MN directly to the ground through the cutterhead. The weight of the TBM in full operating mode is approximately 853 tonnes whereas the weight of ground of equivalent volume to the TBM is about 1300 tonnes and therefore the TBM is lighter than the ground excavated. Although the cutter head provided a significant proportion of the overall weight, the screw conveyor is cantilevered off the back of the machine and therefore it is considered that the weight of the TBM is relatively evenly distributed. Assuming that the TBM is in contact with the ground over the lower quarter of the tunnel circumference, the bearing pressure exerted by the TBM is only 130 kPa which is significantly lower than the original vertical total stress at tunnel invert level of about 330 kPa existing prior to tunnelling. Instrumentation Results Approximately 400m before the location of the HEX-ART crossing a full monitoring section was installed comprising surface settlement pins, inclinometers and magnetic extensometers. The settlement of a point above the centre-line of the tunnel is shown in Figure 5, with positive movements indicating heave. The point was monitored using an automated theodolite and therefore continuous readings were obtained as the TBM passed. The data shows that very little ground surface movement occurred until the tunnel was about 5m from the monitoring point. Thereafter the surface gradually heaved reaching a maximum of about 9mm approximately 8m behind the tunnel face. Then the direction of movement reversed and settlement occurred reaching about 7.5mm about 20m behind the face. Other manually surveyed surface settlement pins located above and to the side of the tunnel showed a similar pattern with the maximum surface heave of between 15 and 20mm. Pins to the side of the tunnel alignment showed a comparatively smaller initial heave and final settlement. Those pins showing the largest initial heave showed a net heave after the TBM had passed. The maximum horizontal movement of an inclinometer located 6m from the tunnel centre- line or approximately 1.5m from the tunnel sidewall is shown in Figure 6. Positive values indicate movement away from the tunnel. 12.5 The inclinometer shows very little 10 movement ahead of the TBM, but as the Vertical displacements (mm).. 7.5 TBM passes the ground moves away from 5 2.5 the tunnel by up to about 7mm and then 0 after the shield has passed, the ground -2.5 moves back towards the tunnel resulting in a -5 net inward movement of about 2.5mm. The -7.5 point of maximum lateral movement -10 -12.5 occurred approximately 2m above the tunnel -20 -10 0 10 20 30 40 50 axis. Monitoring Point Position relative to Tunnel Face (m) Figure 5 Vertical displacement above The movements recorded in these ART centre-line instruments suggested that the thrust against the tunnel face approximately balanced the 8 ground stresses so that very little ground 6 Horizontal movement (mm).. movement ahead of the face occurred. The 4 heave and horizontal movement away from the tunnel suggested that the bentonite 2 pressure around the tunnel exceeded the 0 vertical and horizontal ground stresses. The net settlement above the tunnel and the -2 inward movement observed in the -4 inclinometer after the shield had passed -2 0 2 4 6 8 10 12 Inclinometer position relative to ART face (m) suggested that the grouting allowed some ground relaxation around the segmental Figure 6 Horizontal movement of lining. inclinometer The vertical deflection of the HEX tunnel crown as the ART TBM passed above is shown in Figure 7a. Also shown in this figure is the position of the ART tunnel face relative to the HEX tunnel centre-line. The data indicates that there was an initial upward movement of the HEX crown when the TBM was just 3 m away from the HEX centreline. However the direction of movement was reversed when the TBM was 2 m past the HEX centreline and the maximum crown settlement of 4.7 mm occurred when the TBM was some 6.5 m beyond the centreline of the HEX. This settlement recovered rapidly and when the TBM face was 12m beyond the HEX centre-line the HEX crown settlement was less than 0.5mm. Movement of the HEX crown immediately after the crossing indicated a second phase of crown settlement of up to 0.7 mm. This is considered to be due to loading from the TBM back-up within the ART. As the ART progressed, this movement recovered rapidly. There has subsequently been a gradual long-term heave of the HEX tunnel crown of around 0.8mm. Horizontal displacement of the HEX crown also occurred during the crossing as shown in Figure 7b. The monitoring points 25 m north and south of the ART crossing data have been taken as base readings when plotting the data. Profiles are shown corresponding to ART face distances to or from the HEX centreline. First movements occurred when the ART face was around 1 m of the HEX centreline position and rapidly developed to a maximum value of 1.6 mm in the direction of tunnelling when the TBM was passed the HEX centreline by 2.3 m. Thereafter, the displacement reversed and at 9 m past the centreline position a maximum movement of 1 mm in the opposite direction had occurred. As the TBM progressed beyond the influence zone, the crown prisms recovered slowly and returned to within 0.2 mm of their original positions. Tunnel convergence confirmed the nature of the displacement experienced by the HEX tunnel, and indicated an initial phase of deformation that commenced when the ART TBM was still some distance from the HEX tunnel centreline. As the TBM approached there was a small amount of ovalisation in the HEX tunnel with about a 0.5 mm increase in vertical diameter and a 0.5 mm decrease in horizontal diameter. As the TBM passed directly over the HEX tunnel, the vertical diameter reduced rapidly by some 5 mm, at the same time the horizontal diameter increased some 3 mm. Both these responses recovered once the shield had cleared the HEX tunnel. The movement of the HEX track-bed during construction of the ART was considerably less than the crown movements and were considered to be very minor with maximum vertical displacements not exceeding 1.5 mm and horizontal displacements showing no discernable trends. The vertical displacements described above were confirmed by measurements taken using the electrolevel tiltmeters, both systems agreeing to within 0.5 mm. 3 40 30 Initial - 10 Oct 02 TBM Progress (m +/- HEX CL). Settlement data Vertical displacement (mm).. -4.0m 2 HEX centre-line 30 Offset from ART centre-line (m). 20 -0.8m TBM Progress +2.3m 1 20 +5.5m 10 +8.7m 0 10 +11.9m +15.1m -1 0 0 +24.6m -2 -10 -3 -20 -10 -4 -30 -20 -5 -40 17-Oct 18-Oct 19-Oct 20-Oct -30 -2 -1 0 1 2 3 Horizontal displacement (mm) a) b) Figure 7 Vertical and horizontal displacement of HEX tunnel crown Discussion of observed ground movements The small ground movements ahead of the tunnel suggest that the pressure applied to the tunnel face through a combination of air pressure and direct thrust through the cutter-head was approximately equal to the in situ horizontal stress. The ground surface heave over the TBM shield suggested that the bentonite pressure exceeded the vertical ground stress in the crown of the tunnel and the horizontal movement of the ground away from the tunnel at axis level again suggested that the bentonite pressure exceeded the horizontal ground stress. This would suggest that the horizontal stress cannot be significantly greater than the vertical stress. The ground surface heave indicated that the ground was not in contact with the top of the shield although the annulus would be filled with bentonite. The settlement of the HEX tunnel crown as the ART TBM passed above suggested both a high bentonite grout pressure combined with the direct load due to the weight of the TBM. Calculations indicate that weight of bentonite displaced by the TBM shield is slightly less than the weight of the TBM and therefore the TBM is probably sliding along the invert of the excavation. Lateral movement of the HEX tunnel crown in the direction of the ART tunnel drive is probably due to friction from the shield as it slides on the excavated invert. However, the magnitude of this movement is higher than expected based on the buoyant weight of the TBM. The monitoring data appears to demonstrate that despite backgrouting of the tunnel segments some ground relaxation occurs around the segmental lining. Back Analysis The settlement of the HEX tunnel as the ART TBM shield passed above had not been predicted in the design calculations. Based on the construction information, two further FLAC3D analyses were carried out taking into account the measured TBM thrust of 13MN, the weight of the TBM, the bentonite pressure around the shield and the grout pressure around the lining. The total TBM thrust of 13MN equates to a uniform face pressure of about 200kPa. The TBM shield is fully submerged in bentonite and therefore only the submerged weight of the TBM acts on the tunnel invert. The bentonite pressure was taken to vary from 100 kPa in the crown to 200 kPa in the invert based on a unit weight of 11 kN/m3. The submerged weight of the TBM is only 120 tonnes if the displaced volume of the shield and tailskin is considered, but increases to 360 tonnes if only the displaced volume of the shield is considered. This submerged weight of the TBM was applied as an additional pressure to that of the bentonite over a width of about 6.5m in the excavation invert. The grout pressure was taken to be applied over the first segment behind the shield and varied from 200 kPa in the crown to 350 kPa in the invert. Back analysis 1 assumed that the submerged TBM weight was applied only over the length of the shield and the additional pressure applied was 80 kPa. In back analysis 2 the submerged weight of the TBM was applied over the full length of the shield and tailskin as a uniform pressure of 17 kPa. In both cases the vertical stress applied to the invert of the tunnel beneath the TBM was less than original vertical stress of 330 kPa at invert level. Figure 8 shows the vertical and horizontal displacement of the HEX tunnel crown relative to the ART face position for the two back analyses and for the actual monitoring data. The pattern and magnitude of the displacement profiles are very similar, however, the observed HEX tunnel movement is of opposite direction to the movements predicted in the two back analyses. 6 2 Horizontal displacement of HEX Crown (mm).. Actual data Actual data Vertical displacement of HEX crown (mm).. 5 Back analysis 1 Back analysis 1 Back analysis 2 Back analysis 2 4 1 3 2 1 0 0 -1 -2 -1 -3 -4 -5 -2 -25 -20 -15 -10 -5 0 5 10 15 20 25 -25 -20 -15 -10 -5 0 5 10 15 20 25 ART face position relative to the HEX-ART crossover point (m) ART face position relative to the HEX-ART crossover point (m) a) b) Figure 8 Back analysed vertical and horizontal displacement of HEX tunnel crown The vertical displacement at the ground surface is similar in the two analyses with between 20 and 25mm of settlement occurring above the shield reducing to around 15mm over the completed tunnel. Field observations suggested that heave occurred above the shield with a reduced amount of heave or even settlement occurring over the completed tunnel. To achieve these ground movements in the analysis the bentonite pressure around the shield would need to be increased above that measured during tunnelling. However, although analyses indicated that the bentonite pressure could be increased sufficiently to cause heave, it was still not possible to cause the ground to settle beneath the TBM. Even when the bentonite pressure was increased in the analysis sufficiently to cause heave above the ART tunnel far in excess of that observed, the lateral pressure was not sufficient to cause outward movement of the ground at axis level and the downward pressure was insufficient to cause settlement beneath the TBM. The reason for the discrepancy between observed ground movements and the back analyses is not clear, but it is thought that the in situ horizontal stresses used in the analysis may be higher than those present in the ground. A lower in situ stress profile would result in smaller movement of the ground ahead of the face and the possibility of outward movement of the ground under a reasonable bentonite pressure. The settlement of the ground beneath the ART tunnel is even more surprising and would imply that the TBM is heavier than envisaged. Since the TBM is sitting on the invert it is possible that bentonite is absent in the invert. However, this would have no effect on the effective downward pressure on the tunnel invert as the increase in submerged weight of the TBM is exactly balanced by the reduction of the bentonite pressure acting on the invert. One possible explanation is that the shove rams force the TBM forwards, but may also force the rear of the shield downwards resulting in an increased vertical stress on the tunnel invert towards the rear of the shield. Further study of all the monitoring data is required to confirm whether this is indeed the case. Conclusions Initial design studies showed that the magnitude of the ground movements was sensitive to the type of TBM mode employed. Nevertheless the analysis showed that movements below the ART would be considerably smaller than the movements above the tunnel and that unacceptable movement of the HEX tunnel was very unlikely even adopting open-faced tunnelling techniques. To limit ground movements along the whole route a pressurised TBM was used and this was found to cause heave above the shield, outward movement beside the shield and settlement below the shield. Despite the unexpected direction of the ground movements, the movement of the trackwork within the HEX tunnel was of similar magnitude to that predicted and sufficiently small that rail operations were unaffected. Back analysis of the tunnelling process using measured tunnelling pressures could not reproduce the observed ground movements. It is concluded that the bentonite pressures around the TBM are higher than measured and the horizontal in situ ground stress is lower than assessed from the site investigation data. This case study indicates the difficulty of accurately predicting in advance the magnitude and direction of ground movements associated with tunnelling with a pressurised TBM. Acknowledgements The authors would like to thank their colleagues for stimulating discussions on the operation of the tunnel boring machine and how this might influence the ground movements around the tunnel. The authors would also like to thank BAA plc for permission to publish this paper. References 1. R.J. JARDINE, D.M. POTTS, A.B. FOURIE and J.B. BURLAND: “Studies of the influence of non-linear stress strain characteristics in soil-structure interaction”, Géotechnique, 1986, 36 (3), 377-396. 2. C. POUND and J.P. BEVERIDGE: “Recent experiences of the measurement of tunnelling induced ground movements”, In press. 2003.
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