Under the Thames to Lewisham

From Railway Gazette International, January 1999

Skim the text to say if it contains information about:

a) the contractors of construction

b) the methods of construction

c) geological conditions

In contrast to the delays and cost overruns which have dogged London Underground’s Jubilee Line extension to Canary Wharf and Stratford, construction of DLR’s (DOCKLANDS RAIL) 4.2 km southern extension to Lewisham has gone smoothly.

Boring of the 1 080 m long twin tunnels under the River Thames from Island Gardens to Railtrack’s Greenwich station was finished in April 1998 - four months ahead of schedule - and civil works were substantially complete by the autumn of 1998. Project Manager Bill Shepherd expects test to start running in March. The extension is now expected to open for revenue service ahead of the January 2000 contract date, in time for the Millennium celebrations.

Work commenced in October 1996 after a 24.5 year concession tendered under the Private Finance Initiative was signed by City Greenwich Lewisham Rail Link plc. CGL Rail members – John Mowlem & Co, Hyder, London Electricity and Mitsui & Co Ltd – are committed to financing and building the Ј200m project, apart from certain grants, and then maintaining it during the life of the concession.

LRG Contractors in charge of construction is a joint venture of John Mowlem and Mitsui-Nishimatsu, the latter responsible for boring the tunnels. Electrical works is subcontracted to London Electricity Contracting Ltd, and train control Alcatel Canada Inc which installed the Seltrac automatic train operation system on the rest of DLR.

Under the river. The new line diverges from the present Island Gardens branch north of Mudchute and drops into a relocated station in cutting. It then runs in cut-and-cover under Millwall Park to a new underground station at Ialand Gardens. The viaducts at Mudchute and Island Gardens built as part of the original DLR are to be demolished, although the Grade II listed Great Eastern Railway brick viaduct across Millwall Park will remain.

The new Island Gardens station box includes the chamber from which Nishimatsu’s tunnel boring machine was launched for the first drive in May 1997. It will house ventilation plant installed primarily to control smoke in the event of a tunnel fire; matching fans are being installed south of the river crossing at Cutty Sark station.

A single TBM was used to drive each running tunnel in turn, starting twice from Millwall Park. Because it had to bore through water bearing sand and gravel under the tidal river, a full-face pressurized slurry TBM was supplied by Markham of Chesterfield. Excavated spoil was mixed with the slurry and piped to the surface, where the slurry was separated for re-use.

Finished diameter within the six reinforced concrete segments forming each 1.2m wide ring is 5.2m, larger than DLR’s tunnel to Bank because of a requirement by the Health & Safety Executive to provide a more generous evacuation walkway, and also low-level access from the opposite side for effecting repairs or jacking a derailed train. One cross-passage halfway between Cutty Sark and Island Gardens links the evacuation walkways in adjacent tunnels.

At Cutty Sark station, named after the famous sailing ship on display here, the rails are 19 m below ground barely 100 m from the river. Diaphragm walls 28 m deep were excavated using bentonite slurry to support the soft ground. On its first passage the TBM then cut through both end walls, using its cutter teeth to remove concrete which had been left unreinforced over the affected area, creating a 5.2 m diameter lined tunnel through the incomplete box.

The box was excavated using the top-down method. Once the running tunnel had been exposed, the rings were unbolted from inside and removed. By the time the TBM arrived on its second pass, the box had been fully excavated and the bottom slab cast so the 180 tone machine could be jacked through.

The bored tunnel runs into cut-and-cover beneath the ‘Up’ platform at Railtrack’s Greenwich station, avoiding the listed station building; once the terminus of the London & Greenwich Railway, it was dismantled and relocated when that line was extended.

The line rises until the tracks are at the same level as the London-Dartford line, where the DLR platforms are located to give convenient interchange with Connex South Eastern trains. The line then climbs on to a 786 m concrete viaduct of 20 spans which winds above the tidal Deptford Creek. Over the water the post-tensioned concrete deck was cast in-situ in sections by the balanced cantilever method because of the limited access from below.

On the viaduct the minimum curve radius of 100 m was needed to ‘wiggle’ the alignment between the buildings of Lewisham College to reach Deptford Bridge station which spans the A2 trunk road. The line then descends to ground level following the Ravensbourne river, which has been diverted in places. The contract includes the landscaping of parkland through which the river now runs in a natural bed rather than the previous ugly concrete-lined channel.

After one more stop, the line terminates in the Vee formed by two more Connex south Eeastern commuter routes. Jacking was used to slide three concrete boxes under both pairs of tracks and platforms, to form a running tunnel and two passenger interchange subways. Connex will move its booking office into the new station complex, which will also offer good interchange with over 30 bus routes that serve this focal point.

Noise prevention. Tracklaying is almost complete above ground, and is currently in progress through the tunnels. The 80a flat-bottom rail is secured by Pandrol clips to prestressed concrete sleepers on ballasted sections.

Where the track is on structures or in tunnel, baseplates rest on resilient pads up to 30 mm thick bolted to a cast in-situ concrete slab. Where the bored tunnels pass under the town of Greenwich, these slabs float on steel springs to reduce ground-borne noise and vibrations.

To avoid difficulties which have been experienced in replacing rubber pads under floating slabs, these springs are housed in steel cylinders cast in the track slab. A cover plate flush with the top surface of the slab is then secured in place. Removal of this plate allows the springs to be inspected easily, and they can readily be replaced using special tools without disturbing the slab.

The safety specification requires check rails or other measures to constrain derailed vehicles wherever the line is in tunnel, on an elevated structure or on retained earth structures. Check rails on independent baseplates will be laid throughout the tunnels, but above ground derailment constraint is provided by a heavily reinforced upstand, or curb, along the outside of each track slab.