Mix Shield TBM Support Pressure: Theoretical Calculation Approaches vs. Practical Experience for Crossrail C310 Thames Tunnel

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1 Mix Shield TBM Support Pressure: Theoretical Calculation Approaches vs. Practical Experience for Crossrail C310 Thames Tunnel Abstract In recent years there has been an increasing demand for tunnelling using Slurry and Earth Pressure Balance (EPB) Tunnel Boring Machines (TBMs) in urban environments with low overburden and sensitive structures to underpass. Therefore, there is an increasing need for reliable and realistic methods for the calculation of the necessary support pressure. For the Crossrail Contract C310 Thames Tunnel two methods for the determination of the support pressure were applied and investigated in detail. The primary calculations for the tunnel drive were conducted according to DIN 4085: ; as well as an approach based on the assumptions of Anagnostou and Kovári (1994), which was used to modify the limits, give guidance in exceptional situations and refine face pressure determination in Chalk for upcoming projects. This paper will give an insight of the experience with the calculated and applied support pressures during tunnelling in London Chalk and beneath the River Thames. About the author My name is Ester Sophia Elisabeth Karl and I was born in July I finished my engineering degree in June 2012 at the Technical University in Aachen (RWTH), Germany. In October 2011, I started a six month internship at the Crossrail Contract C310 Thames Tunnel with Hochtief Solutions and wrote my diploma thesis Determination of required limits of tunnel face support pressure for the project C310 at the Institute for Geotechnical Engineering. In my thesis I analysed the method for the determination of the support pressure from DIN 4085: and conducted comparison calculations based on Anagnostou and Kovári (1994). In addition, I conducted several sensibility analyses to investigate the limits and advantages of both approaches. As a fully integrated member of the HMJV C310 technical team, I developed a calculation tool to verify support pressures in exceptional situations that was successfully applied during problems, TBM interventions and planned standstills. In September 2012, I started working for Hochtief Consult Infrastructure. Since then I was involved in projects including Able Marine Park (UK), Ringsted Copenhagen (DK), Forth Replacement Crossing (UK), SAA GA (NL) and as Design Supervisor for the cross passage construction at Crossrail C310 Thames Tunnel (UK). According to the German tradition that every tunnel requires a female name, I took the honour to christen the first C310 TBM Drive from Plumstead Portal to Woolwich with the name Ester. It was also coincidence that the TBM was named Sophia, which is my middle name. In January 2014 I returned to the UK to work in the HMJV tender team for the Thames Tideway Tunnel and York Potash Mineral Transport System schemes as Design Coordinator. After the final breakthrough of the last C310 TBM in May 2014, I took the opportunity to analyse the support pressures and refine the calculation methods for deep tunnels in Chalk. This paper summarizes the calculations and the analyses I conducted for my thesis and my evaluation of the applied face support pressures during tunnelling. Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 1

2 1. Introduction In 1843 Marc Isambard Brunel completed the first tunnel underneath the River Thames, the Rotherhithe Tunnel, using a shield tunnelling concept. He was the first engineer to use this method. Today the shield tunnelling concept is the most widely used tunnelling method, especially in urban areas, difficult geotechnical conditions (geological and hydrological) and areas with low overburden. Crossrail is one of many projects to ease the pressure on London s infrastructure and prepare the city for the increasing population and future generations. Crossrail is currently Europe s largest construction project and will increase London s rail capacity by 10%. Planned completion is in 2018 and will provide London with a new East to West connection through the city centre (see Figure 1). The project includes the construction of 118km of new rail tracks, including 41.5km of tunnels and 40 train stations. In 2013/2014 a total of eight shield tunnelling machines bored their way through the subsoil of London to form the Crossrail tunnels. More than 80% of the tunnelling was completed by December Figure 1: Crossrail Overview [provided by Crossrail] Tunnelling in urban areas is challenging, expensive and can be very restricted in available space, Health and Safety (H&S) requirements and impact on residents. Sensitive or listed structures have to be protected and the impact of settlements minimised. The risk of settlements to overlaying structures and hazards such as blow-outs and collapses which could be caused by tunnelling, must be minimised and mitigated. The selection of the optimum tunnelling method plays an important role for the success of the tunnelling works, the cost efficiency and the programme reliability. Recent projects, for example Channel Tunnel Rail Link Contract 320, showed that slurry shield tunnelling is very reliable and safe particularly when tunnelling in the Chalk strata. On Crossrail Contract C310 Thames Tunnel (Tunnel Drive H), for the expected heterogeneous ground conditions, including Chalk with low overburden, high ground water level and overlying sensitive structures, Hochtief Murphy Joint Venture (HMJV) choose two Mix-Shield Tunnel Boring Machines (TBM) from Herrenknecht with slurry/compressed air support. Due to the tidal effects of the River Thames and the varying overburden and ground conditions, HMJV had to adjust the face pressure according to the strata and the current ground water level on a real-time basis. This paper gives an introduction for the theoretical and practical application of the face support pressure during tunnelling for the C310 Thames Tunnel. Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 2

3 2. Tunnelling in Urban Areas with low overburden underneath sensitive structures The greatest challenge of tunnelling in London is the high density of developments and the confined nature of the construction sites. Tunnelling underneath sensitive structures needs intensive preparation and precise execution. The impact on residents and third parties has to be kept to a practicable minimum and the construction works programme should therefore be optimised and reliable. Crossrail and its supply chain put much effort and work into third party and local community involvement to achieve a high public acceptance and a positive reception from the media. The effort taken in public relations will smooth the path for future tunnelling projects in London and other cities worldwide Crossrail C310 Thames Tunnel Crossrail Contract C310 Thames Tunnel (see Figure 2) is part of the Crossrail Eastern Tunnels section and is comprised of a twin-bored tunnel between Plumstead and North Woolwich with a length of 2.64km, including cross-passages and sumps. Figure 2: Crossrail C310 Thames Tunnel Geotechnical Longitudinal Section [provided by HMJV] It is an ambitious project, both in technical and geotechnical aspects, due to its location and difficult geological ground conditions. The two Mix-Shield TBMs Sophia and Mary with an outer diameter of 7.12m drove through variable ground conditions below the groundwater level and arrived on the 1 st February and 16 th May 2014 at North Woolwich (see Figure 3) with intermediate breakthroughs at the Woolwich Station Box on the 15 th May and 13 th August The strata encountered between Plumstead Portal and North Woolwich Portal were Made Ground, Alluvial Layers, River Terrace Deposits, Lambeth Group (Upnor Formation), Thanet Sand and the Chalk strata, which are typical for East London and southern England. Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 3

4 Figure 3: TBM Mary Reception at North Woolwich Portal in May 2014 [provided by HMJV] The tunnel drives commenced at Plumstead Portal which is located adjacent to the existing rail tracks of the North Kent Line. At the west end of the portal, the TBMs were launched on the 8 th January and 18 th May 2013 approximately 6.0m beneath ground level; additional ballast on the first meters of the drive was laid to increase the overburden and thus prevent blow-outs of the surface. After the launch, the TBMs descended beneath the operational rail tracks of the North Kent Line and encountered challenges such as a nearby railway bridge and in particular a Network Rail electrical substation; where the maximum allowed settlement was limited to only 5mm. The TBMs continued to Woolwich Station Box and passed underneath and in close proximity to several listed buildings as well as the operational subway tunnels for the Dockland Light Railway (DLR). After Woolwich Station Box the TBMs turned west and crossed below the River Thames, which is approximately 450m wide at this point. The tunnel drive underneath the River Thames was influenced by the continuous changes of hydrostatic pressure due to the tidal influences. The overburden beneath the River Thames varied from 12m to 16m. The ground conditions beneath the River Thames were characterized by fissured and weathered Chalk with layers of discontinued sediments. After crossing below the river, the TBMs ascended into North Woolwich Portal. The groundwater and the level of the River Thames were monitored constantly to ensure the application of the correct support pressure to minimize settlements and to prevent blow-outs. The C310 Thames Tunnel will be completed in 2015; the outstanding works such as construction of the tunnel invert and cross passages, finalization of the portals and Woolwich Box, will be completed to prepare the tunnels for their final fit out. HMJV has delivered two outstanding tunnels (both in quality and performance) and have received several awards from Crossrail and the construction industry C310 Tunnel Boring Machines Sophia and Mary HMJV used two Mix-Shield TBMs which could be deployed with slurry or compressed air support, to prevent the cave-in of the strata and/or the inflow of water. A Mix-Shield TBM consists of a steel cylinder that is about two to five centimetres larger than the outer diameter of the tunnel lining (see Figure 4). The pressure chamber is situated behind the cutter head, which is separated by a submerged wall. The slurry rises behind the submerged wall and a compressed air cushion applies the necessary pressure onto the slurry allowing compensation of pressure fluctuations in this part of the machine. The excavated soil is mixed with the slurry and is pumped out at the bottom of the excavation chamber for separation at the Slurry Treatment Plant (STP) located at ground level. The slurry has a high viscosity, which reduces the risk of uncontrolled leakage and has only a slightly higher density than water. Therefore, the excess fluid pressure in the crown is small and reduces the risk of ground heave. Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 4

5 HMJV selected Slurry TBMs due to their favourable behaviour in Chalk. The Chalk is predominantly a very pure micritic carbonate rock with flints and marl bands, consisting of calcium carbonate. The strength of the Chalk is significantly reduced on saturation due to the dissolution arising from flowing water and the weakening of the bonding between grains, which makes it favourable for pumping and separation in the STP. For the Slurry treatment, HMJV chose filterpresses for the STP to achieve a water content of less than 35% after processing to meet transportation requirements. In summary, Sophia and Mary constructed 3397 rings in 519 days and reached a peak advance rate of 156m/week. 3. Determination of the support pressure for safe and economic TBM operations For successful and safe Slurry/Mix Shield TBM tunnelling the necessary face support pressure has to be investigated and determined in detail beforehand. To increase reliability and safety, the face pressure calculation approaches are constantly refined. For accurate results, a regime of ground investigations has to be conducted to establish the basis for these calculations. Acting on the tunnel face are the horizontal rock mass pressure (p h), the water pressure (p w) and the influence of overlying structures (see Figure 5). Acting against these pressures is the support pressure (p S) from the slurry/compressed air which is induced by the pressure of the air cushion (p 0) behind the submerged wall. In a Mix-Shield TBM, the chamber is normally filled with slurry during the tunnel drive. For maintenance work the slurry level will be decreased partially (or fully) and substituted with compressed air. Figure 4: Concept of TBM with slurry face support [Wittke (2006)] Figure 5: Principle of the slurry face support [Wittke (2006)] To prevent ingress of water into the excavation face, the pressure of the slurry must exceed the pore water pressure (p w) in the soil; therefore the pressure difference (Δp) varies between a minimum at the tunnel crown to a maximum at the invert. The equilibrium at the tunnel face has to be evaluated taking into consideration safety factors, stress redistribution, minimisation of settlements, safety against vertical ground cracking and Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 5

6 blow-outs. For the ground it is assumed that the pressure-transmitting medium between the slurry or compressed air and the soil, is a filter cake or membrane, which is determined by the rheological characteristics of the slurry and the pore structure of the soil. This membrane does not possess any strength and is therefore not able to support any earth or water pressure. The stability against vertical ground cracking and blowing out has to be checked at the tunnel crown. This check is based on the model assumption that in front of the tunnel face, a coarse grained horizontal layer, without the possibility of forming a filter cake, is therefore exposed to slurry flowing through it. The result is that the supporting pressure pushes against the superimposed layers which may lift up or crack, especially when these layers are of small extent and lead to loss of the supporting slurry. The diverse effect results from unsufficient support pressure which can lead to cave-ins, collapses and/or settlements at the surface. In this case the earth and/or water pressure exceeds the pressure in the excavation chamber C310 calculation approach according to DIN 4085: The face support pressure calculations for C310 Thames Tunnel were conducted by Hochtief Consult Infrastructure according to DIN 4085: According to the underlying theory, the kinematically possible failure mechanism consists of a solid-state earth body for the two-dimensional case which can be adapted to fit a threedimensional earth body (see Figure 6). In cases of a relatively narrow diameter of the tunnel compared to the depth z, the active earth pressure can be reduced for the threedimensional case by introducing the spatial length. The spatial length is a Figure 6: Failure mechanism [Hettler (2008)] characteristic length, which is typically smaller than the actual diameter, and is used to consider the influence of the three- dimensional stress state and is directly dependant on the ratio of depth to diameter. This method typically leads to conservative results as DIN 4085: does not consider any stabilizing arch effect of the ground. Nevertheless this method is used on projects worldwide and was applied successfully by HMJV on the Crossrail C310 and CTRL320 projects Comparison calculation by Anagnostou and Kovári (1994) A comparison calculation based on Anagnostou and Kovári (1994) was conducted to verify the calculation according to DIN 4085: and to predict maximum and minimum pressure values. The approach according to Anagnostou and Kovári (1994) is based on the three-dimensional earth pressure model for tunnelling by Horn (1961) and the silo theory by Janssen (1895). The collapse mechanism consists of a right-angled prism from the surface to the tunnel crown which sits on a wedge. The soil on top of the tunnel face is limited by perpendicular sliding Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 6

7 surfaces and forms a weight on the tunnel crown (across the area shown as CFED in Figure 7) with an overburden. Figure 7: Sliding mechanism according to Horn [Anagnostou and Kovári (1994)] In case of collapse, the prism slides vertically downwards and pushes the wedge into the tunnel. The loosened soil in front of the tunnel face arches in two directions, parallel and perpendicular to the axis, and reduces therefore the earth pressure. The wedge interface in front of the tunnel face has friction and cohesion and therefore builds up a shear resistance. The shift of pressure from the slipping soil onto the resisting soil is mobilized by the arching effect. Therefore the influence of the weight of the prism is only applicable to a certain height. According to Terzaghi (1954) the arching effect can attain five times the width of the prism. For the calculation of the mean effective vertical stress σ V acting on the area CFED, a reduction factor λ is introduced. This coefficient of horizontal stress considers the deformation characteristics of the ground for the calculation of the shear stresses. Terzaghi (1954) proposes a value of λ = 1, whereas Anagnostou and Kovári (1994) use λ = 0.80 for the prism and λ = 0.40 for the wedge to compensate for uncertainties in the calculation of the shearing resistance. The calculations based on Anagnostou and Kovári (1994) lead to lower face pressures due to this arching effect. This gains special importance for tunnels in Chalk and with increased overburden Sensitivity analysis In addition to the comparison calculation, several sensitivity studies on a homogeneous soil were conducted to evaluate the limits, risks and application areas of both approaches. The sensitivity analysis showed that ground investigation and resulting geotechnical parameters can have a high influence on the calculated support pressure. For example, a variation of the friction angle can vary by ±2º before compromising design limits, whereas as a variation of the ratio of horizontal stress λ by ±0.1 can lead to unstable face conditions. Cohesion has the greatest but also the most contrary influence on both approaches. For the approach based on Anagnostou and Kovári (1994) the influence of cohesion decreases for high cohesion, which means the higher the cohesion the lower is the influence of variations; whereas the influence of cohesion on the approach based on DIN 4085: is much greater. However, the approach based on Anagnostou and Kovári (1994) tends to overestimate the holding forces due to cohesion as the internal stability is considered as a holding force. The influence of high overburden on the approach based on Anagnostou and Kovári (1994), is very small due to the height limitation of the arching effect as described above. In ground with high water pressure, for both approaches the water pressure becomes decisive and the resulting pressure of overlaying strata and structures are negligible. Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 7

8 In addition, a calculation tool to cater for exceptional situations was developed to verify the lowest possible support pressure without causing settlements. This allowed experienced technical tunnelling to have flexibility in these situations. 4. Comparison of calculated vs. actual applied values Examples for exceptional conditions Blow-outs, cave-ins or major settlements did not occurred during the C310 tunnelling works. In total, fourteen face interventions were carried out to inspect the cutter head, to change tools and to verify the quality of the Chalk at the locations of the cross passages. All interventions were conducted under compressed air up to a maximum of 2.45bar. Normally, the support pressure is never constant but varies around the optimum due to the Mix Shield technique using the double chamber system. The evaluation of the actual support pressure data from C310 showed the average variation from the optimum value between Plumstead Portal and Woolwich Station Box was 3% whereas between Woolwich Station Box and North Woolwich the variation was 8%. In general it was observed that the variation of the support pressure was to a level lower than the calculated optimum. The main reasons for the fluctuations are operational and include amongst others:- Loads from overlaying structures can differ from the estimated loads; Fluctuation of the slurry level after the completion of a ring or after standstill; Density of the slurry increases during excavation process; Slurry drawdown for interventions; Extensions of slurry pipes; Maintenance works; Exceptional situations. The face support pressure had to be constantly monitored to enable adjustment in cases of support pressure fluctuations Control of the support pressure Real time monitoring The TBMs for Drive H were equipped with the Tunnel Process Control (TPC) and Advanced Tunnel Drive Steering (ATDS) computer systems to facilitate the TBM steering process and the evaluation and storing of data. TPC is a software tool for collecting TBM data in real time that determines a detailed observation of different parameters and data. The information of the STP, Water Treatment Plant and Grout Mixing Unit were displayed and monitored by the software. For the support pressure TPC was used to compare and display the predetermined and applied values; predetermined threshold and limit values were monitored with an alarm system. The alarm system was based on three colours/trigger levels: green, amber and red (see Figure 8). In the case of the support pressure visualization the green limits were the optimal face pressure range for the operation of the TBM. The amber range was the second stage of the trigger values with a signal for the TBM driver to adjust the pressure. The red range which represents the design limits, was deliberately not displayed to the TBM operators who had to adhere to the amber limits. Triggering the red range did not lead to immediate face collapse or blow-outs due to the safety factors. ATDS collected all settlement information and was directly linked into TPC. Within the TPC software an appropriate Watchdog alarm function was used to inform key personnel in the Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 8

9 event of breaching any ground movement and support pressure trigger values. The maximum settlement trigger values were determined based on the volume loss calculations. For controlling the settlements, fifty three cross-sections along the tunnel drive were monitored; these were equipped with automated measurement devices such as Hydrostatic levelling cells or Shape Accelerator Arrays (SAA) and manual measurement devices such as Extensometer, Inclinometer and precise levelling points. In general the evaluation showed that the face support pressure stayed within the green limits 78 to 93% of the time for the drives between Plumstead Portal and Woolwich Station Box. For the two drives beneath the River Thames these values are less due to the tidal influences of the River Thames and the constant adjustment of the pressures. Figure 8 shows an example for the support pressure within the set limits. Figure 8: Example for regular fluctuation of support pressure and according trigger values Before the tunnelling commenced, HMJV prepared ring based tables with the calculated face pressure and trigger values in relation to the design water levels which were integrated into TPC. The Daily Plan of Advance of Bored Tunnel (DPABT) and the Daily Shift Review Group reviewed the tables on a daily basis. They decided which water levels were applicable and adjusted the tables where necessary to achieve a safe support pressure. In addition, the values based on Anagnostou and Kovári (1994) and the calculation tool were used to support additional investigations for any exceptional situations Exceptional fluctuations of the support pressure The main reason for the fluctuations of the support pressure is the operation and interaction of the TBM and STP. Exceptional short term and long term fluctuations occur during every tunnel drive; e.g. issues relating to the discharge pipe/ pump, tail skin and slurry loss which are described below in more detail: Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 9

10 Stop of the discharge pipe/pump The rapid rise of the slurry level in the excavation chamber (control system reacts slower than the slurry flow) results in a rapid rise of the face support pressure until the bypass slide valve is opened. Figure 9: Example of stop in discharge pump/pipe In the example in Figure 9, the face support pressure is increased in less than one minute from 1.65 to 2.37 bar, breaching the amber trigger value (and fell afterwards to 1.82 bar). The advance rate fell from 33 mm/min to 0 mm/min. This was due to a valve in the high pressure plant being defective and therefore closing. It was changed to continue with the regular advance. As the feed pipe was still working properly, the discharge pipe was not able to remove the slurry and a tailback occurred. Impact on the tail skin This can lead to a constant drop of the slurry level in the excavation chamber (control system reacts slower than the slurry flow) resulting in a reduction of the face support pressure until a counter-reaction is applied and the tail skin is fixed. Figure 10: Example for impact of tail skin The drop started during the construction of ring 1230 (see Figure 10) due to incoming annulus grout entering into the tail skin. This caused leakage through the tail skin-seals. At this Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 10

11 moment, the advance rate fell from 45 mm/min to 2 mm/min and the face pressure values fell below the lower amber limit. However, during low tide the values breached the upper amber trigger values. The tail skin had to be cleaned and to be pulled with the help of an installed power pack. Slurry Loss In the case of a leakage within the slurry circuit, the process of minimisation of the support pressure by the TBM Operator to reduce slurry loss is undertaken. This consequently leads to a reduction of the face support pressure, until the problem has been resolved. The face support pressure fell from 1.14 to 0.97 bar during the erection of ring 685 and increased after a period of 11 hours, when ring 691 was constructed (see Figure 11). The support pressure was minimized by the TBM operator within the limits of the lower amber trigger values, but the advance continued with the normal advance speed. After a loss of 65 m³/h of slurry, a fresh slurry pipe was connected and the slurry circuit was balanced to counter the loss. Figure 11: Example of bentonite loss During all of these events there was no occurrence of cave-ins into the excavation chamber, significant settlement at the ground surface or blow-outs, despite triggering the limits of the support pressure C310 project specifics for support pressure application For C310 the following measures which had an influence on the support pressures were applied (amongst others) to achieve H&S, programme and cost benefits: - Automatisation of the support pressure in combination with the real-time monitoring of tide levels of the River Thames provided by Port of London Authority (PLA); Substitution of the slurry with water during tunnelling in Chalk; Application of reduced face pressure during compressed air interventions. The groundwater variations due to the tidal River Thames led to the continuous adaptation of the support pressure. In combination with in real-time monitored groundwater and tide levels, Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 11

12 HMJV utilised TPC to provide the necessary information to automate the support pressure in the influence zone of the River Thames (see Figure 12.) Figure 12: Example of Automatisation of support pressure in the influence zone of the River Thames The automatisation was achieved with the air pressure regulation system SAMSOMATIC, which regulates the air cushion behind the submerged wall and the information from the TPC. The main advantage of the automatisation is the improved implementation of the support pressure within the green range and less breaches of trigger levels. At the beginning of the first tunnel drive underneath the River Thames, the support pressure was adjusted manually. For the second drive the automatisation was in place for the entire drive. This led to 10% less breaches of the green range limits for the second TBM drive compared to the first drive. The main part of the tunnelling, especially the deep section underneath the River Thames which was located completely in Chalk, was conducted using water instead of slurry as the face support fluid. Chalk has a diverse behaviour for long term and short term conditions. The long term behaviour is characterized by low to medium values for cohesion and friction angles whereas in short term conditions shows rock like behaviour (see Figure 13). HMJV took advantage of this short term behaviour (which is applicable for the face support pressure) to conduct part of the tunnel drives with water instead of slurry leading to benefits in the STP operations. In addition, the short term stability of the Chalk enabled compressed air works with reduced pressure. This offered significant for H&S benefits for the workers as well as providing programme savings and mitigation measure during periods when the TBM got stuck. Figure 13: Face Intervention in Chalk After rings 178 and 478 (see Figure 14) the crew carried out compressed air interventions to confirm the quality of the Chalk at the cross passage locations and to inspect the cutter head. Unfortunately after finalisation of the compressed air intervention and re-filling of the excavation chamber with support fluid the TBM could not be moved. To restart the TBM, several mitigation measures needed to be implemented. A hydraulic power pack on the tail skin articulation was installed to pull the tail skin centimetre by Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 12

13 centimetre. Afterwards the TBM was moved by means of the thrust rams restricted by the general maximum allowable thrust forces of 32000kN. At the cross passage rings (with conventional reinforcement) the trust force could be increased up to 38000kN. Figure 14: Reduced Face Pressure Furthermore, the support pressure was reduced to the minimum limit as per the alternative calculation approach with the calculation tool to reduce counter force to have more flexibility for the thrust force. For example, a reduction of 0.4 bar support pressure gives approx. 2500kN more thrust force. Alternative calculations were conducted with the calculation tool to determine the lowest possible value without a face collapse. The results after the investigation led to a possible reduction of the face pressure by 0.45 bar for the following advance. The face pressure was reduced, the mitigation measures were applied and the TBM could continue under normal face pressure and advance rate at ring 483. To ensure that the TBM did not stick after the next interventions, several additional provisions were implemented, such as bentonite lubrication, tail skin articulation and compressed air work with reduced pressures. 5. Summary The evaluation has shown that the applied support pressures did not lead to any face support failures or notable settlements during C310 tunnelling. The two step system with the back-up values for exceptional situations, proved safe and reliable. In addition, reduced face support pressures have shown that the arching effect in combination with the short term behaviour of the Chalk, suggest that for future tunnelling projects in Chalk reduced support pressures can be applied. In recent years the development of the calculation methods for support pressures has been pushed forward with more realistic approaches to the ground behaviour and other influences on the tunnel face. In the coming years more tunnels in Chalk will be constructed, for example the Thames Tideway Tunnel, Crossrail 2 and High Figure 15: C310 Tunnel Ester Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 13

14 Speed 2. The deep tunnels in Chalk will need intensive investigation for appropriate support pressure calculations for H&S, programme and cost benefits. It is the responsibility of the tunnelling team to select the most reliable/realistic model from the available calculation methods and to interpret the behaviour of Chalk to ensure safe but economic mechanised tunnelling. Acknowledgements The author would like to thank all parties involved in the face pressure analysis and application including Crossrail, HMJV and Hochtief Consult Infrastructure. Special thanks goes to Andreas Raedle for his mentoring during my diploma thesis. Also a warm thank you to the HMJV Tunnelling and Technical Teams for answering all my questions/providing all information and to Crossrail for their approval of this submission. Tunnel reception celebration of TBM Sophia in Woolwich on 22 nd May 2013: Left: Ester in front of the TBM; Middle: Ester with Crossrail Programme Director Andy Mitchell; Right: Ester with the TBM crew. References Anagnostou and Kovári (1994), The Face Stability of Slurry-shield-driven Tunnels. In: Tunnelling and Underground Space Technology, Vol. 9, No. 2, pp DIN 4085: , Baugrund - Berechnung des Erddrucks. DIN Norm 4085, Berlin: Beuth Verlag (in German) Horn (1961), Horizontaler Erddruck auf senkrechte Abschlussflächen von Tunnelröhren. Landeskonferenz der Tiefbauindustrie Budapest (Deutsche Überarbeitung STUVA Düsseldorf), pp.7-16 (in German) Hettler (2008), Erddruck. In: Grundbau-Taschenbuch, Teil 1: Geotechnische Grundlagen. 7. Auflage, Berlin: Ernst & Sohn Verlag, pp (in German) Janssen (1895), Versuche über Getreidedruck in Silozellen. In: Zeitschrift des Vereins deutscher Ingenieur, Band XXXXIX, No. 35, pp (in German) Terzaghi (1954), Die Brucherscheinungen in idealen Böden. In: Theoretische Bodenmechanik, Berlin: Springer Verlag, pp (in German) Wittke (2006), Stability Analysis and Design for mechanized Tunnelling. Translated from the German edition: Statik und Konstruktion maschineller Tunnelvortriebe. Geotechnik in Forschung und Praxis, WBI-PRINT 6, Essen: VGE- Verlag Glückauf Harding Prize 2015 Submission from Ester Sophia Elisabeth Karl 14