Overcoming Fault Zones at Bodio on Gotthard Base TunnelLot 452/554 between Bodio and Faido, in the south section of the Gotthard base tunnel, is built by consortium TAT, whose partners are Implenia, Alpine Mayreder Bau, CSC, Impregilo and Hochtief. This unique lot is numbered 452/554 as it combines lot 554 (Bodio tunnel) and lot 452 (Faido tunnel). All the Bodio section (east and west tunnels, and cross passages) forms part of lot 554. Visit www.tat-ti.ch, www.implenia.com, www.alpine.at, www.csc-sa.ch, www.impregilo.it and www.hochtief.comThe consortium of engineers Galleria di Base del San Gottardo Sud, responsible for the project design and work management, is formed by Lombardi, Amberg Engineering and Pöyry Infra. Visit www.lombardi.ch, www.amberg.ch and www.poyry.com or www.infra-environment.poyry.comThe east tunnel is 15,971 m long and the west tunnel has a length of 15,702 metres. The outer diameter without overbreak is 8.80 metres while the inner diameter is 7.90 metres.In the fault zones, TH29 steel ribs provided by Bochumer Eisenhütte were installed at intervals of one metre, as well as K283 welded mesh provided by Frigerio and 15-20 centimetres of shotcrete without fibres of Holcim and Sika. Visit www.be-heico.de, www.frigeriospa.com, www.holcim.com and www.sika.comExcavation of lot 452/554 is completed. The Herrenknecht TBM S-210 working on the east tunnel broke through in early September 2006. Breakthrough of the west tunnel by the twin TBM S-211 took place at the end of October 2006. Click here and here. To this day, approx. 85% of final lining works has been completed in the west tunnel and around 75% in the east tunnel. Click ch/20 & ch/21. Visit www.herrenknecht.comAccording to geological data, only some short fault zones perpendicular to the tunnel path were expected on the section between Bodio and Faido. The presence of long unstable zones, in particular zones of squeezing rock where the overburden reaches 1,000 metres, was unforeseen and could therefore not be considered as a risky situation for support tasks in the final project design which was used as a basis for the tender documents. Click here.Probe drilling and prepared proceduresDespite those sound predictions and to cope with possible surprises, the construction planners envisaged to implement in Bodio an exploration concept ahead of the tunnel face as tunnelling progressed. The exploration concept was adapted during work on the basis of the gained knowledge and experience. Initially, it was envisaged to use the seismic exploration method but this proved ineffective and was abandoned after approximately 1,000 metres of tunnel, after crews met the first important sub-horizontal fault in the east tunnel, that the method failed to anticipate. Exploration was then ensured by destructive radial and longitudinal probe drilling, complemented if necessary by core drilling.Concurrently with exploration during tunnelling and measures planned in case of exceptional events, a concept aiming to crossing difficult massifs had been elaborated preventively. Whether the unstable zones had been foreseen or not, a procedure describing the interventions step by step was established, not only for the advance of the TBMs but also for the cross-passages excavated by drill and blast.Fault at chainage 2,705After approximately 200 metres, the TBM in the eastern tunnel had to cope with a totally unexpected unstable sub-horizontal fault. A few metres thick, this fault is primarily composed of kakirite, cataclasite and highly fractured rocks, and follows the tunnel path on approximately 400 metres, mainly in the top heading. This fault was also detected in the western tunnel, over a length of approximately 100 metres. It also disrupted the excavation of two cross passages.The main risk situation at the time of crossing such a fault dealt with cohesionless materials falling down onto or right behind the cutter head. These falling materials led to the formation of many cavities in the top heading, which in extreme cases spread to all the tunnel cross-section reaching up to six metres in height.Support was installed immediately in all the tunnel face area. TH29 steel beams were placed at intervals of one metre, combined with welded mesh and an approx. 15 cm-thick shotcrete layer. The cavities were subsequently filled, generally with shotcrete, in some cases with cast-in-place concrete. The project envisaged the possibility of widening the current 8.80 m bored diameter by 30 centimetres at maximum to cross possible local faults. This measure was intended to create additional space, if necessary, to deal with important convergence or to install more resistant final support and lining. The maximum bored diameter actually achieved is only 8.89 metres, because its widening was impossible but by increasing the gauge of the peripheral tools. Consequently, the project had to be adapted quickly.In order to avoid interference with the future drainage system, complete TH29 steel beams of a special type, characterised by a reduced height at the walls, were designed and installed. In addition, the 30 cm layer envisaged in the tender documents for the cast-in-place concrete inner ring in the fault zones was reduced to 25 centimetres, such as for the current profile. For static reasons, the vaults of the tunnel and cross passages in the unstable zone were entirely reinforced. The reinforcement was dimensioned by combination of the bending moment and the normal stress. These stresses correspond to the most unfavourable load case, which takes in particular into account the dislocation load, the actual weight, the temperature variation and the load due to traffic. The final invert did not require to be reinforced.Finally, to meet the new fire protection requirements, the vault lining in this tunnel section was concreted with two kg/cu m monofilament polypropylene fibres. Taking into account the particularly difficult geological conditions in the unstable zone and the time to get the works running smoothly, the TBMs could only progress by approximately 2.5 metres per day.Unstable faults and squeezing groundThe two TBMs started from Bodio and crossed a new fault zone at chainage 13,460. This second zone first generated the formation of a cavity at the crown and benches in the eastern tunnel, approximately at chainages 13,450 and 14,340, then in the western tube at approximately chainages 13,745 and 14,425. These are two sub-vertical faults, almost parallel, which cut the eastern bore and continue towards the western tube with an angle of 5-10° compared with the tunnel path, and then plunge towards east with an angle of about 60°. Looking like a 3 to 5 m-thick plate, the faults are formed by cataclastic rocks, in which kakirite layers are intercalated. They are bordered by strongly fractured rocks in the contact zone.When boring through the ground adjacent to the faults, the TBM underwent strong pressures exerted by the rock on the short tail shield located just behind the cutting head. The TH29 closed steel arches installed in the advance zone crept, shotcrete plates fell down and wire mesh was heavily deformed as a result of the tunnel convergence. Such phenomena reveal the presence of squeezing rock. Excavation generates a rock mass overload around the cavity and induces plastic deformations, which tend to close the cavity even more.This phenomenon was observed more particularly in the western tunnel, between chainages 13,595 and 13,692, at a spot where the sub-vertical fault of unstable rock located between the two tunnels gets closer to the western tube. The pressures on the tail shield were such that the TBM did not manage any more to struggle against the friction forces and was forced to stand still on 3rd March, 2006 close to chainage 13,692. Tunnelling did not resume until ten days later, after a cavity was dug at the top heading.In the disrupted zone, the geological conditions - and consequently their effects on the advance process - varied considerably along the two tunnels. On a distance of at least one kilometre, delimiting the transition between the stable, unstable and squeezing rock zones was not obvious at all.The support deformations were recorded by transverse convergence measurements at spots spaced five to 15 metres apart. However, the measurements could only be taken once the support had been installed and the reference sections could only be measured at seven or eight metres behind the tunnel face. First, the results show that the transverse deformations are strongly asymmetrical and unequal along the tunnel path and, secondly, that they completely stabilise 80 to 100 metres behind the face. The maximum convergences reached ten centimetres, while convergences occurred ahead of the reference sections are estimated between four and seven centimetres.The invert could be concreted in accordance with the construction project, without direct consequence on the TBM progress. Then, the invert did not undergo major deformations.Not taking into account the west TBM blockage, progress of the TBMs through this second fault area ranged from seven to 15 daily metres, which is much more than when crossing the fault at chainage 2,705. This performance is mainly the result of the experience gained by the crews and the improvements to the equipment to install the steel arches.ModellingLike the fault at chainage 2,705, structural safety and serviceability of the final lining in this new fault zone are also based on a 25 cm reinforced concrete ring and unreinforced invert. To achieve a uniform reinforcement on the entire section, engineers considered either stresses of the same type than those at chainage 2,705 or stresses related to the squeezing rock mass. The stresses due to the squeezing rock mass were determined by a back analysis, using a finished element 2D model which considers the observed deformations and the type of support (TH29 steel arches at one metre intervals, welded mesh and 14-18 centimetres of shotcrete). The model takes into account the influence of the fault between the tunnels, the anisotropic properties of the rock due to its schistosity as well as the mutual influence of both tubes.At the time when the west TBM was unjammed, core drill samples were taken to define the mechanical properties of the rock (triaxial compression tests carried out at the Zurich Federal Polytechnic School). These tests underlined the influence, on the rock resistance, of the schistosity direction compared with that of the stress.The tests and the measured deformations made it possible to calibrate the model, in particular using a sensitivity analysis of the calculation parameters. Then, typical curves were determined for the rock mass above the top heading, benches and invert. These curves make it possible, according to the support strength, to estimate the actual pressures exerted by the rock mass on the support, then on the inner ring (convergence-confinement method). In view of the substantial deformations undergone by the support, estimating its actual strength was very difficult. Minimal and maximum values of this strength were estimated and various stress assumptions on the final lining were then assumed. Calculations confirmed in situ inspections, i.e. that the pressures due to the actual squeeze of the rock mass are asymmetrical, which is normally not very common in very deep tunnels where the pressure is supposed to be homogeneous. This asymmetry of the stress is the consequence of the coexistence of two particular situations: the presence of the inclined fault at 65° between the two tubes featuring mechanical properties very different from those of the rock mass, and the anisotropy of the rock mass in terms of resistance (horizontal schistosity).Reprofiling the gaugeThe substantial deformations that affected the support in the fault zones generated underbreaks of about 15 centimetres so that approximately 300 metres in each tunnel has to be reprofiled, since such deformations do not allow to construct the final lining. If reprofiling works are not rare during tunnelling, the Bodio case includes - in addition to geological risks - peculiarities that complicate the situation: invert already built including the water drainage ducts, limited cross-section and logistics problems (long distance between the portal and the place of the works).It was decided that reprofiling would be performed using a new installation in each tunnel. This installation is integrated to the crew installing the final lining. The length of the intervention stages is conditioned by the local geological situation. The works, including the demolition of the deformed support, reprofiling and new support, must guarantee labour safety during several months, until concreting of the final lining. The actual reprofiling thickness strongly varies according to the deformations, the existing support and the new support work that is required. According to situations, interventions range from light works, in sections where reshaping the profile remains local and is limited to some support tasks, to heavy interventions, in squeezing ground where further deformations and upheave of the invert are to be feared. In the latter case, what is at stake is recreating a new complete support as soon as possible to form a stiff ring. Read E-News Weekly 42/2006.PerspectivesThe experience gained in Bodio proves that careful and thorough geological investigations do not safeguard against unexpected situations, particularly for tunnelling at great depth. Cooperation from all project participants and determination of planned measures with a clear assignment of everybody's competences are basic conditions to keep unexpected events under control but preventive measures can however be taken, such as for example:- even in the event of favourable geological studies, scenarios related to exceptional events must be worked out and prepared;- the equipment required to undertake heavy support work - in particular to install steel arches - must be included right from the very start of tunnelling and the crews working at the face must be trained as early as possible;- probe drilling throughout tunnelling must be regularly adapted according to the geological conditions actually encountered. Core drilling during tunnelling - which can be carried out during the maintenance shift - must be sufficiently long and detailed, even if the geological studies are favourable;- it is recommended to plan enough room reservation in the normal profile to face substantial deformations of the support, especially in case of excavation by hard rock TBM, where the actual ability of the devices to increase the bored diameter is uncertain.In such complex projects, unforeseen circumstances during works unavoidably have significant impact on costs and time frames. This impact can however be limited by the immediate search for suitable technical solutions and by an adequate specific decision-making. Click here to read in French. Read E-News Weekly 5/2005, 35/2004, 29/2004 & 50/2003. Visit www.alptransit.ch and www.ticino-tunnel.chConclusionThe solutions brought about to manage the fault zones between Bodio and Faido during TBM excavation of the south section of the Gothard base tunnel and the invaluable experience gained by the involved participants will certainly be put at the service of the major alpine projects announced in the future, both the Maurienne-Ambin base tunnel on the Lyon-Turin high speed line and the Brenner base tunnel, to the greatest benefit of eveybody. 30/07. Thanks to Alessandro Ferrari of Lombardi, Project manager Bodio for consortium Galleria di Base del San Gottardo Sud