Excavation of the working shaft for the deepest tunnel ever beneath the urban fabric of London in the UK has bottomed out and the base slab is cast. The 20m diameter x 75m deep shaft at Beckton in the East End of London is now being prepared as the working shaft for the Herrenknecht Mixshield that will excavate the 8.88m x 6.9km long Lee CSO interceptor tunnel as part of the massive Thames Tideway clean-up project by water company for the capital Thames Water.
MVB, a joint venture of Morgan Sindall, VINCI Construction and Bachy Soletanche was awarded the £422 million contract in January 2010 and mobilised to site to sink three deep shafts at Beckton and a fourth at the Abbey Mills pumping station to facilitate Thames Water's Lee Tunnel project.
The €635 million tunnel will help prevent 16 million tonne of sewage entering the River Lee each year during heavy rainfall. The storm water overflows occur as a result of London's Victorian sewers not being big enough to cope with a 21st century city that has trebled in size and continues to grow, and has areas of natural drainage concreted over.
Deepest shaft in London at 75m deep
The 8.88m o.d. tunnel, lined to 7m i.d. with primary precast concrete segments plus an in-situ concrete internal lining, will capture discharges from London's largest combined stormwater overflow at Abbey Mills Pumping Station in Stratford. The tunnel will transfer the flows to the Beckton sewage works that are being expanded by 60% to deal with the increased volumes. The tunnel will reach depths of up to 75m.
Table 1. Lee Tunnel shaft facts and figures
Internal diameter
Depth to slab level
Diaphragm wall depth
Diaphragm wall thickness
Abbey Mills Shaft
25m
68m
83m
1.2m
Beckton Connection Shaft
25m
78.5m
92m
1.5m
Beckton Pumping Shaft
38m
86.5m
98m
1.8m
Beckton Overflow Shaft
20m
74.5m
90m
1.5m
The completed shaft, at 75m deep, has had the 4.5m thick domed base slab installed and is now being prepared for the construction of the slip formed liner. The diaphragm walls for the shaft are 1.5m thick, and 90m deep.
Elsewhere on site, the 98m deep x 1.8m thick diaphragm walls for the 38m diameter pumping shaft are also complete as are the 1.5m thick x 92m deep diaphragm walls for the 25m diameter connection shaft. Work is now under way on the final 25m diameter x 68m deep shaft at Abbey Mills, constructed with 1.2m thick x 83m deep diaphragm walls.
Jet grouting will also be used at the Beckton Culvert and fissure grouting has been completed for the TBM breakouts and for an 11m curtain below the overflow diaphragm walls. The project also involves continuous flight auger and rotary piling for the inlet bridge, tower cranes, power supply complex and culverts. The Herrenknecht Mixshield TBM, named Busy Lizzie, will be launched at Beckton at the start of next year.
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