tbm crossing of stations

Course in Tunnelling and Tunnel Boring Machine

Kurs w zakresie drążenia tuneli oraz maszyny drążącej

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 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



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

TBM CROSSING OF STATIONS

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 WALLThe 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•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.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: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

•station already built•delay in launch of TBM (waiting station building)•no possibilities to finish the station before to finish the tunnel• >entries• >communication

•launching TBM in advance •complete excavation and building of tunnel•>risk•> segments (rings) to demolish• delay in finish station building

VOID FULL

• >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

• 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

� Break in and break out

False tunnel

�Break out break out

Lean concrete

�Break in and break out

Sealing system

�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 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 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.

JET GROUTING



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 tohas 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: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•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.



THANK YOU FOR YOUR ATTENTION!!

tbm crossing of stations

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