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Tunnelling on the latest extension to London's Docklands Light Railway, under the Thames to Woolwich, got under way last month. Max Soudain reports.

A quarter of a century after the last vessel was loaded and left, one of east London's once thriving Royal Docks will soon come alive again with the movement of cargo.

This spring alluvium, gravel, chalk and Thanet Sand will be loaded onto barges at King George V Dock for transport to Tilbury, further down the Thames Estuary.

The source of the payload will be two parallel bored tunnels being driven under the River Thames as part of a 2.5km long extension to the Docklands Light Railway (DLR).

From its current terminus at King George V Dock at North Woolwich, one stop on from London City Airport, the line is being extended to Woolwich Arsenal on the south bank.

The new link is being built by Woolwich Arsenal Rail Enterprises (Ware), a joint venture of contractor Amec and the Royal Bank of Scotland. The same team built the DLR London City Airport extension which opened in December.

The DLR chose Ware as preferred bidder in December 2004 and awarded it the 30-year concession in June 2005. Ware will finance, design, build and maintain the extension and will be paid by DLR for every day the line is open for service.

Construction began in June 2005 and the new line is due to open in February 2009. Four route options were originally considered, with the nal choice made at the beginning of 2002. From the northern end, the twin track extension will head east from King George V station and will run in 105m of open cut and then 85m of cut and cover along the dockside (including the TBM launch chamber). The tracks will deepen and enter the 1.8km long bored tunnels, passing beneath North Woolwich and Gallions Reach.

After crossing the river, the route turns south, passing under the redeveloped historic Royal Arsenal site and emerging in Woolwich in cut and cover alongside the mainline railway, ending at a new terminus next to Woolwich Arsenal mainline station.

Woolwich itself is a deprived area and it is hoped the new link to the Docklands and the City will help improve its fortunes.

The extension is also an essential part of London's Olympic transport strategy. It will provide connections to the Excel exhibition centre in Docklands, which will host boxing, table tennis, taekwondo, weightlifting, wrestling and judo, and to the main Olympic park.

Amec is building the route, including the tunnelling and associated geotechnical work such as diaphragm walling, piling and grouting. Designer for the scheme is Halcrow.

'The extension will cost £177M, with the civils element coming in at £133M, ' says Peter South, Amec tunnelling contracts director and project director for the extension.

Ground conditions are typical for the area: fill over alluvium (with organic silt and clay and peat bands), river terrace gravels, Thanet Sand and chalk.

Amec geologist Roger Margerison says: 'The chalk is between 14m and 18m below ground level and the water table is about 2m below ground level.'

The 500t, 6m diameter Lovat earth pressure balance TBM forming the 5.3m internal diameter tunnels can move at up to 3m an hour - equivalent to two tunnel lining rings. Each ring has eight 250mm thick e reinforced concrete segments, supplied by Buchan Concrete Solutions, which is casting them on site.

'We expect a peak rate of about 30m a shift but of course this will vary, ' South says. Earth pressure balance was chosen over slurry as fissures in the chalk on the south side would mean it would be difficult to control the slurry, he adds.

The TBM has cutters set back from the head to allow cut ground to move and mix with the conditioning material - a polymer and foam from Degussa. Etac accelerated grout is injected behind the shield through the tail seal and around the rings as the TBM moves forward.

The TBM began boring the upline tunnel from the 45m long diaphragm walled launch chamber at King George V dock last month.

The walls, up to 26m deep, were formed using a conventional grab in 1m wide, 7m long panels with water bars between.

Across the floor of the box, 28 tension piles were installed 21.5m below the base slab to counter uplift. There is groundwater in the gravels and also in the chalk, where it is at 3.5bar. Dewatering, by WJ Groundwater, was carried out during construction of the box.

'The box was finished just before Christmas, ' South says. He explains that the 10m high headwall includes a glass bre reinforced eye to allow the TBM to cut through and begin boring into the he alluvium and terrace gravels.

Early on, the TBM will pass 3m beneath the piles for a building over the route. Margerison explains that permeation grouting has been used in the gravels to control settlement.

'We have also installed arrays for compensation grouting which can be used if needed.'

Similar measures are in place where the route runs under buildings at the southern end.

'We are expecting average face loss of 0.5% but have designed for 1% and 2% under properties at risk, ' Margerison says. 'If we get more face loss, then compensation grouting can be used.'

Once tunnelling is under the river it will be entirely in chalk, to a maximum depth of 35m below the river level. 'To err on the sensible side, we have assumed it will be of reasonably good quality, ' Margerison says.

'The Seaford Chalk has some flint bands which has implications for wear of the TBM but ints should only be up to cobble size.

The ground conditions are fairly well understood but it is variable - which means wear, ' he explains.

South says: 'The chalk and ints can be nasty, but the TBM is fitted with ribbed Trimay plate that is welded onto the face, cutters and screw conveyor, reducing wear and protecting the TBM head.'

Once the tunnel begins to rise to the surface at its southern end, the next challenge is the chalk and Thanet Sand boundary. The sands can be dif cult to tunnel and can draw fluid in - a particular problem when using earth pressure balance.

A grout block, one of three intervention points along the drive, has been placed just before this boundary. 'The big unknown is what condition the TBM will be in when it finishes the first drive, ' says South. 'There is a wear indicator which will tell us when we need to go in and inspect.'

Another grout block has been installed on the north side of the river and an intervention shaft, which will form part of the nished tunnel, is being built on the southern bank.

The shaft position clashed with river wall anchors. These have been destressed and will be cut during excavation, with new anchors already installed to replace those lost. The shaft caisson is being jacked into the ground and has point attack cutters at its base to ensure it can get past any ints.

All three intervention points will create dry environments for the team to enter the tunnel to inspect the TBM cutting head for wear and carry out maintenance. 'We will have to go in and inspect, find a dry spot, muck out and then go in under an air lock, ' South says.

The TBM is expected to arrive at the reception chamber on the southern bank in September. It will then be taken apart and ferried back across the river for the second drive to build the downline tunnel in December.

Rather than simply turning the TBM around and heading back across the river, both drives are from the northern end. The main reason is so spoil can be loaded onto the barges at King George V Dock.

Otherwise it would mean using the completed tunnel for spoil transfer - holding up track laying and t out, South explains.

Both he and Margerison are upbeat about the project and seem ready to take on any geological challenges that may be thrown at them.

Docklands railway spreads its wings

London's Docklands Light Railway opened in 1987 at a cost of £77M, with two main arms running north-south and east-west, intersecting in Poplar, in London's East End.

The original 13km network had 15 stations and 11 driverless trains. It has grown in line with the extensive regeneration of London's dilapidated docklands area and is now 27km long, with 34 stations served by 94 trains. Annual passenger levels have grown from 8.2M in 1994 to 50M today and are expected to rise to 80M by 2009.

More than £1bn has been spent on upgrades and extensions since the DLR opened. The main extensions are those to Bank (July 1991), Beckton (March 1994), under the Thames to Greenwich and Lewisham (November 1999) and to London City Airport (December 2005).

Much of the DLR is on viaduct to enable it to cross the old docks, and its stations are also elevated.

There are plans to push DLR in virtually every direction. The Beckton line could be extended to Dagenham Dock from a new junction at Gallions Reach by 2015, serving new business and housing developments. The line may even go as far Rainham in Essex.

London's successful bid for the 2012 Olympic Games has also given the DLR the opportunity to take over the ailing North London Line from Network Rail. As well as passing through the Games village site, there will be a new junction with Stratford International Station on the Channel Tunnel Rail Link.

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