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The big dig


The Nishimatsu/Cementation/Skanska joint venture on Contract 220 may have the two longest tunnels to drive on the CTRL. But before digging can start the JV has to wait for Skanska on Contract 230 to dig the launch pit.

This needs to be ready for assembly of the first Kawasaki tunnelling spread next spring.

The tunnel boring machine (TBM) has to be under way by June and the second machine will start up almost immediately after the first. The plan is to drive the up line tunnel and then follow with the down line machine just 500m behind.

The first priorities, though, are getting the very deep Graham Road vent shaft excavation under way and sinking wells for dewatering to lower groundwater pressure in the Thanet Sands at the site of the deepest cross passages. Tunnel linings will be manufactured on site at Stratford jointly with Contract 240.

The structure of Graham Road vent shaft has to be in place by the time the two TBMs pass through after 3.2km of their journey from Stratford to the portal beside the East Coast Main Line from King's Cross.

Electrical and mechanical installations have to start as early as possible because the shaft will be relied on for ventilation during the system wide track laying in the western tunnels from Stratford. Similar constraints apply to the second shaft, at Corsica Street, but it is much shallower than Graham Road and over 2km further down the line so there is less urgency.

The 28m by 13m rectangular pit will be a landmark underground engineering project for London. Its diaphragm wall panels will reach 54m down from the surface, making them among the deepest ever constructed in Europe. Excavation within them for the base slab will be bottomed out 45.3m down - substantially more than West- minster Station, London's deepest basement dig to date.

Comparisons with Westminster should not be taken too far though. Graham Road is tight up against a live railway, as was Westminster, but the nearest buildings are an end of terrace house and a Scout hut, as opposed to the Big Ben clocktower. These buildings are no less important to the C220 environmental team, however.

Installation of the diaphragm walls will be on a target price subcontract by an unusual joint venture partnership between Cementation Skanska Foundations and Bachy-Soletanche.

'Bachy and Soletanche were the only companies in Europe building diaphragm walls so deep, ' says NCS JV project director Terry McDonald. 'The JV will do lots of work at Stratford as well and bring in equipment from all over the world.'

Panel sinking is scheduled to start by the end of August.

Access to the confined site and restrictions on working in a residential area near a live railway will govern work on the 1.2m thick panels which will need up to 500m 3of ready mixed concrete for each pour. The walls key deep into the Thanet Sands so dewatering will be needed early to secure the hole base.

Corsica Street shaft appears much less demanding. The RLE design is for a circular 18m diameter, bolted segmental concrete lined shaft which has to be sunk some 24m through London Clay to the Woolwich and Reading beds. It will encircle the line of the approaching down line tunnel. Adits will be driven south under the North London Line (NLL) to intersect the route of the up line drive and form the ventilation tunnels. This will create a reception chamber so the machine head can be serviced if necessary, ready for the final push to near King's Cross. Pressure relief tunnels must be dug at higher level linking across the top of the running bores.

The joint venture and RLE are currently developing an alternative design involving sprayed concrete lining for the shaft with an all enveloping cavern at its foot. This option will be fully mechanised to reduce hand mining substantially and limit the use of hand held power tools.

'There will be a demand for hand miners with Heathrow (Terminal Five) likely to start up, ' says McDonald.

Contracts 220, 230, 240 and 250 are all reliant on dewatering.

At their lowest point the C220 tunnel crowns dip just below the level of the lower aquifer in the Thanet Sands and temporary dewatering is needed to stabilise the face for planned interventions to the EPBM cutterheads and for cross passage construction. At Stratford box dewatering will be a permanent feature because of the upper aquifer (see opposite). Regional dewatering becomes a critical advance operation for the tunnels east of Stratford where the TBMs would otherwise meet water at up to 3.5bar pressure (see page 18).

When the C220 TBMs begin work at Stratford next June they will dive steeply down right to the Thanet Sands and then partly in to the Upper Chalk where bands of flint could be challenging for the cutterheads, as it is expected to be on the Thames Tunnel (see page 27). Spoil will be brought back by conveyor to Stratford where it will pass to Skanska for use in land raising and development. These conveyors will be 8km long by the time the TBM shields finish their job.

Co-operation between the contractors on 220, 230, 240 and 250 via an alliance known as Team 200 is so close that, at the moment, there seems little likelihood of a contractual argument about the condition of the spoil at this crucial handover point.

Settlement control is key throughout the drives. Some 900 dwellings are estimated to be in the zone of influence of the tunnels along with 4.5km of Railtrack infrastructure. Preliminary analysis by RLE assumed 2% volume loss. 'We expect to achieve 1% or less, ' says contract manager Gordon Battye.

Most tricky will be the shallow tunnelling work towards the western end of the contract.

There will be a relatively comfortable 9m clearance at Highbury and Islington station under the pairs of 12ft 6in Victoria Line and 16ft Northern City Line tunnels. But when the CTRL bores transit beneath Caledonian Road it will be 'quite challenging', says McDonald.

The 8.11m OD tunnels will be little more than half a diameter below the road surface. The down line is directly under the abutment and flank wall foundations of the NLL bridge over Caledonian Road. And Caledonian Road is home to the Fleet Sewer, some 1,650mm by 750mm, along with 750mm and 900mm diameter high pressure, cast iron water mains.

There is just 600mm clearance under the sewer and little more under the mains. An elaborate replacement and support operation will precede tunnelling so the services can be carried by properly engineered structures bridging the tunnel lines.

'We looked at all options of working below ground and above, including building a Bailey bridge to carry the traffic over the works, but there is limited clearance under the (NLL) bridge, ' says Battye. Trestles may also be needed to support the NLL rail bridge during tunnelling. A planning application has just gone in for the work.

Contract 220 London Tunnels - Stratford to Gifford Street Portal Nishimatsu/Cementation/Skanska Joint Venture 7.5km Target Price: £145M

Key Features Twin 7.15m diameter running tunnels will be driven 7.5km from Stratford Station box to emerge at the Gifford Street Portal about 1km north of King's Cross Station.

Strata to be tackled include Woolwich and Reading clay and silt beds, water bearing Thanet Sands, Harwich formation gravely silts and London Clay.

Two Kawasaki earth pressure balance tunnel boring machines are being designed in Japan. During excavation there will be two opportunities for full access to the cutter heads, at the Graham Road and Corsica Street vent shafts.

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