“[The U.K.] has plenty of qualified construction companies, says Otta. “This is probably the most tunnelled place in the world, and it has been tunnelled longer than anywhere else.” However, under pressure from the water-sector regulator to cut costs even further, Otta is planning a series of road shows to court bidders from East Asia and continental Europe.

Ahead of the bidding, approximately $17 million worth of geotechnical investigations have probed the uncharted soils along the tunnel route, 40 m to 70 m directly below the Thames. Two-thirds of tunnel will be in chalk or clay; trickier Thanet sands cover the remaining third. To deal with such conditions, Otta expects the project will need two slurry TBMs and three earth-pressure-balance machines. “We will learn a lot from construction of the Lee tunnel,” he adds.

Thames Water kicked off the scheme with the Lee interceptor tunnel partly because it would capture 40% of all Tideway's discharges for 20% of the cost, says Mark Sneesby, CH2M-Hill's project manager. The Lee project will intercept 16 million tonnes of discharges from a single CSO at the Abbey Mills pumping station. Its 7.2-m-dia tunnel and four shafts will provide 382,000 cu m of storage capacity during storms. The shafts will be up to 85 m deep with diameters ranging from 20 m to 38 m.

In January 2010, the $690-million design-build contract went to the MVB joint venture of Morgan Est PLC, Rugby; Vinci Construction Grands Projets S.A., Paris, and its affiliate Bachy Soletanche Ltd. Sitework started last June.

The joint venture is sinking one shaft at the Abbey Mills tunnel inlet and another at Beckton, about 6 km away. Sewage will flow by gravity from Abbey Mills to a 25-m-dia reception shaft at Beckton. The tunnel will continue for 850 m to a large overflow shaft with a culvert to the river. But the main flow will pass through a short suction tunnel to a 82.5-m-deep, 38-m-dia pumping shaft that will feed the treatment plant above.

The joint venture plans to launch the contract's sole tunnel-boring machine from the Beckton shaft, driving it uphill to ease drainage, says François Pogu, project director. The slurry TBM is being commissioned at Herrenknecht A.G.'s plant in Schwanau, Germany. Pogu plans to put the TBM to work by year's end.

The Lee tunnel will pass mainly through chalk containing fissures filled with water at about 8-bar pressure, says Pogu. The contractor plans to inject grout into these fissures from the TBM to stem water flows and minimize the need to work in compressed air.

But the machine also must drive through some 200 m of Thanet sands, which include mixed, abrasive and permeable materials. “Thanet sand is not treatable with injection [from the TBM],” says Pogu. The contractor will inject grout into this area from the surface. Even then, tunnelers may have to work in air pressures up to 3.5 bar to keep dry.

The contractor has completed more than 15% of the work at the Lee interceptor project, which is tardy due to various snags, says Pogu. “In 2010, we had a tight program and succeeded in getting most of it done,” he notes. With its 2014 completion deadline, the Lee tunnel is scheduled to be operational a year before work begins on the huge Thames Tunnel.

The Lee tunnel and its shafts are sited in brownfield and industrial areas, a construction advantage. But Thames tunnel builders will follow the historic river, negotiating modern utilities, World War II unexploded bombs and countless ancient artifacts, even passing below the central span of the iconic Tower Bridge.