TBM CROSSING OF STATIONS Eng. Marco Barbanti Astaldi S.p.A. Eng. Tommaso Grosso Astaldi S.p.A. 18-11-2011
In the presence of groundwater the permeability of the ground is of great interest. It is important to know if: the permeability of the ground is low or high there is any relevant heterogeneity, and which is the distribution of individual formations with various permeability There are two main problems when the TBM is passing empty through the waterbed layer of an already excavated station Opening the barrier at the entrance to each station for the TBM Preventing any water from flowing through the station The pre-existing buildings
TBM CROSSING OF STATIONS A station or a shaft can be the starting point or the finishing end of a TBM and it is important all the problems concerning logistics sites for TBMs The intermediate stations are subjected to the passage of the TBM. It is important to pay attention to the aspects connected to the two different ways of passing the stations by TBM: void crossing, where the station has already been built, at least as far as the excavation and the main structures are concerned; full crossing, where the station volume has yet to excavated;
SOFT EYE WALL The concept of soft eye wall is to install fiber glass cage in the diaphpragm wall where the TBM will break in or break out. In this way: TBM avoid to stop the excavation The TBM can easily penetrate the wall Cutting tools can cut the soft eye bars without problems No problem of destroying and wearing of cutting tools The TBM anyway during this penetration has to proceed slowly and with care. The installation of soft eye bars is nowdays compulsory If you don t install soft eye bars???
TBM CROSSING FULL STATION If in the design has been expected to excavate the station not yet built, it means that the TBM will excavated inside the future station a tunnel, that it will be demolished in future when the excavation of the station will arrive to the depth of the tunnel. The excavation procedure has to consider the crossing of the perimetral walls entering in the station end going out from the station. The crossing of the station in this case is more safe because it is a functioning and operative TBM and faster than crossing the station empty.
TBM CROSSING OF EMPTY STATION Generally the most common method for TBM crossing of the stations is that of void crossing and it can usually be foreseen in the design stage of the underground line. In some instance, the construction of the stations can become more critical than the excavation of the tunnel, in fact it can happen that the tunnel construction is ahead of construction of the next station. In this case: if the design has foreseen void crossing of the station, slowing down is necessary; In the worst case the TBM advance has to be stopped in front of the station because it is not completely equipped for the TBM to pass through; this is an event that should be avoided, not only because the risk connected to consequent delays in schedule but moreover because of more general problems of making the tunnel safe ( a stopped TBM under buildings or under road is more dangerous than a functioning and operative TBM)
VOID CROSSING vs FULL CROSSING VOID station already built delay in launch of TBM (waiting station building) no possibilities to finish the station before to finish the tunnel >entries >communication > easy maintenance of TBM >safety < risk > ventilation passing by void station is slower brak in / out and launching procedure (pushing frame..) < segments to demolish FULL launching TBM in advance complete excavation and building of tunnel >risk > segments (rings) to demolish delay in finish station building because before it is necessary to finish the tunnel It is faster to cross the station the only slow activity is break in/out the walls
BREAK IN AND BREAK OUT SYSTEMS Soil improvement False tunnel Lean concrete Sealing system Break in and break out Break out break out Break in and break out Break in and break out
TBM CROSSING OF EMPTY STATIONS
SOIL IMPROVEMENT GROUTED BODY Behind the walls which we install the sealing system, it will be also executed a grout body of jet grouting piles or plastic piles, in order to make safer, avoiding seepage water inside the station, the phase of break out of the TBM from the station. These concrete blocks are jet-grouted behind the shaft/station wall eye for a length exceeding the shield length so that, when the last wall is crossed through, the TBM shield has already bored and one or two rings have already been erected within the block itself, thus ensuring water seal. The objectives of soil improvement are: to obtain a reduction of soil permeability through voids permeation and soil structure compaction; to increase soil strength, consolidating loose materials to avoids overbreaks when tunnelling with no face pressure applied.
JET GROUTING Jet Grouting is a ground modification technique used to create in situ cemented soil, inserting an injecting tube into the ground which has a nozzle disposed for directing a jet of fluids or binders at an angle inclined to the axial direction of the injecting tube. The jet of the fluids or binders is discharged through the nozzle generally at a velocity of 100-450 m/sec and at a pressure of 50-1000 kg/cm2.
LEAN CONCRETE FOR BREAK IN This system is a concrete blocks poured inside the station, attached to wall eye for a length exceeding the shield length so that, when the last wall is crossed through, the TBM shield is coming inside the body of the station avoiding possible water seepage from groundwater behind the walls.
FALSE TUNNEL FOR BREAK OUT The utility of the false tunnel is to have the TBM already inside a concrete structure, inside a station, before to start the excavation. In this way the TBM has already bored and one or two rings have already been erected within the block itself, thus ensuring water seal, entering in the natural soil.
SEALING SYSTEM A metal ring system provided with hydraulic seal, allowing to bore the shaft wall with groundwater behind it.
This is an alternative to tunnel-end grouting consolidation, and allows to carry out launch phase (as in this case) and the run-out phase from a Shaft or Station, through a system of seals waterproofing the area around the TBM's shield thus preventing possible water seepage from groundwater behind the walls.
The reception (or run-out) phase is much more complex than the start phase, mainly due to the following reasons: High-precision in TBM topographical guidance is required: it must be ensured that, when going through the metal ring, the shield's position has the lowest possible plano-altimetric deviation with respect to design. In fact, seal centering may be adjusted by means of hydraulic jacks the maximum extension of which does not exceed 10 cm along its radius. Pressure balance: after breaking through the wall, and up to going through the seal, the pressure of possible water flows which may occur around the shield because of the void between soil and the shaft chamber, must be counter-balanced.
Assembly of the equipment elements into the run-out shaft. Progress of TBM excavation under earth-pressure balance mode until the shield runs into the grouted zone. Progressive reduction in pressure supporting the excavation face during the last meters of excavation and until breaking through the last diaphragm Breakthrough and void crossing toward seals, while still keeping the cover closed. During this phase, the following supplementary activities have to be carried out: disassembly of peripheral tools, which might damage the seal; removal of muck accumulated in front of cutting head consequently to the breakthrough, removal of provisional protections from the seal, in the invert area, topographic verification of the actual position of the cutting head with respect to the metal ring. Cutting head goes through the seal and places pressure on it. Removal of the closing cover and subsequent translation of TBM shield up to reaching disassembly position.
The use of the equipment offers the following advantages. Control of water inrushes: there is an intermediate phase, between the breakthrough and pressurization of seal, during which it is necessary to balance possible water inrushes from the hollow space between the shield and the soil. Reduction in soil consolidation from surface: in the case of shafts or stations, to be crossed through as void volumes, in urban environments where the areas required for grouted zone consolidation may be considerably limited, the exploitation of such technology contributes to significantly reduce the dimensions of grouted zone. Reclaimable equipment: the equipment installed as above is entirely reusable in subsequent projects (obviously, apart from the shuttering pipe buried into the concrete template), which makes it possible to compare this solution with grouted body also from the economic point of view.
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