Lee Tunnel: end of the D-Walls on the overflow shaft
Bachy Soletanche Ltd in a joint venture with Morgan Sindall and VINCI Construction Grande Projets (MVB) poured the last diaphragm wall panel of the first shaft on 10 February 2011. This diaphragm wall is 1500mm thick, 89m deep and reinforced top to bottom. Over 10,700m3 of concrete has been poured and as much excavated soil discharged directly into barges located on the Thames via a conveyor belt.
The verticality tolerance is 0.3%, which equals about 27cm deviation at 89m.
To meet this constraint the cablette system was adopted to control verticality during excavation. The hydrofraise was also equipped with a gyroscope to act as a secondary system and once the bentonite in the panel had been completely replaced, a Koden reading was taken.
The Koden was newly purchased for the project and was fitted with a data port. For the first time we were able to produce a 3D representation of the diaphragm wall through an automatic processing of the digital data from the Koden and then compared the results between the two cablettes, the gyroscope/cablette and the Koden reading.
Three other shafts remain to be done and one to a depth of 100m and 1800mm thick.
Now that the hydrofraise has moved out of the way at the first shaft, the fissure grouting can proceed unhindered. A grout curtain is being installed for 11m below the shaft with the TBM break-out zone being grouted at the same time.
Work began late September 2010 and the completion of the last shaft is planned for the end of 2011. Two hydrofraise machines have been mobilised to carry out these works.
Bachy’s geotechnical involvement in the project includes secant piling to two outfall shafts and our staff are spread throughout the project team to retain our involvement through to the end of the project in 2015.