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Pressure point


Starting at the east end of the Stratford box the two bored tunnels of Contract 240 will pass below the Central Line Tube tunnels with just 4m clearance.

The alignment then passes under hundreds of individual properties for 3.7km before tucking beneath the route of existing surface railway infrastructure.

Railways are then followed all the way to Barrington Road where a vent shaft excavation will be dug on the edge of playing fields beside the North Circular Road. Tunnel boring machines (TBMs) will be dismantled to leave chambers ready for arrival of Contract 250's TBMs driving west from Ripple Lane near Dagenham.

Immediate priorities on C240 are consents, regional dewatering to allow tunnelling through Thanet Sands, shaft sinking and TBM manufacture. Dewatering will be far more elaborate than on C220 (see box) to cope with the much higher water pressures.

Wirth is building the two TBMs for C240 to Rail Link Engineering (RLE) specification.

Again they will be earth pressure balance machines capable of working in open or closed mode.

Heads will be adaptable to be dressed in picks or roller cutters, all of which can be replaced from inside the tunnel. The sole opportunity to examine the outside of the machines once they are under way will be at the intermediate vent shaft on Woodgrange Road.

'To all intents and purposes all tunnelling is in Thanet Sand, ' says RLE contract manager Kevin McManus. 'There's a small part in Woowich & Reading and Upnor Formation then it's water bearing sand at up to 3 bar all the way to Barrington Road.'

Costain/Skanska/Bachy Soletanche JV project director Bob Ibell points out that although the machines are designed to deal with the ground, much dewatering skill will be needed.

'Having looked at the specification we were happy, went out to tender and got four sound proposals back, ' says Ibell. 'We are very keen to get up and running.

I am concious that there are six machines on order for CTRL London tunnels with three suppliers and this is putting pressure on bearing and hydraulics manufacturers.'

The TBM design allows for spoil to be removed from the face chamber by screw conveyor and transferred to a belt as with the other London tunnels machines. Negotiations are under way for the supply of 36km of belt conveyors for the three contracts.

These will eliminate potentially hazardous muck trains from the tunnels which will have protected walkways throughout for the miners and surveyors. A canteen cabin and toilets are being included in the equipment on the TBM tail skid. 'There's no reason not to have decent facilities at the workface, ' says Ibell.

Grout for filling the void left by the tailskin will be brought into the tunnel ready mixed in hoppers. Sophisticated settlement control techniques include an interlock between the shove rams and the tailskin grout pumps. The TMB cannot shove forwards unless the pumps are squirting grout into the void behind the tailskin.

Double articulation of the shield and separate articulation for the tailskin will enable the lining to be built very tightly inside the machine leaving mininum annulus to fill, even on curves, to reduce settlement.

'Certain buildings will be be identified as needing a detailed structural survey as opposed to a condition survey, ' says Ibell. But he does not anticipate extreme measures will be needed as on his London Bridge Jubilee Line Extension contract where substantial and sensitive structures were situated directly above some extremely large diameter contiguous tunnel bores.

Lining is a fibre reinforced tapered design and will be made at Stratford along with material for C220 (see box). Hybrid concrete and bolted cast iron rings will be installed at the eight cross passage sites.

Construction of the sump 10m below the tunnel invert will be challenging. The plan is to freeze the ground locally so it can be excavated, rather than trying to pull the water pressure down a further 1bar locally. Compact equipment using brine tubes could be set up in the tunnel, avoiding deep drilling from the surface.

Work began at the surface early in June on clearance of the former club buildings at the site of the vent shaft just off Woodgrange Road. The 28m by 12m rectangular shaft has to be squeezed in behind a parade of shops at the end of a narrow side street. Bachy-Soletanche will use grabs to excavate the upper levels of the diaphragm wall panels then switch to a hydrofraise rig to cut them down to their 35m maximum depth. It is a confined site in a busy residential area where noise and hours restrictions plus severe local traffic congestion will rule progress.

The Barrington Road site at the contract limit will be much easier, as it is in the corner of a field. But the CTRL up line is beneath the tracks of the District Line and the London Tilbury & Southend Railway. Hence the shaft is turned at right angles and a 16.5m by 21.65m pit has to be excavated in diaphragm walls beside the existing railway embankment. A tunnelled connection will be made to the up line from the base of the shaft.

The TBMs of Contract 240 are due to arrive there in October and November 2003.

Contract 240 London Tunnels Stratford to Barrington Road Costain/Skanska/Bachy Soletanche JV 4.68km Target Price £120M

Key Features: Twin running tunnels of 7.15m internal diameter descend east from Stratford box into saturated Thanet Sands.

Natural ground water pressures will be reduced by a regional dewatering system which is designed to improve ground conditions and the performance of the two Wirth earth pressure balance tunnelling machines. Most of the route lies below the North London Railway Line. Near half way, the big rectangular shaft at Woodgrange Road has to be excavated in a cul-de-sac behind a parade of shops.

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