Jenner stormwater tank, France

The diaphragm wall was excavated using a Hydrofraise® Evolution 5 to dig through the gaize (sandstone 8m thick and 30m deep), while maintaining a verticality of 0.3%.

The nature of the site also made the use of the Hydrofraise® necessary, as the diaphragm wall was constructed in the middle of a roundabout in the centre of Le Havre. This meant truck comings and goings had to be kept toa minimum, which was made possible by pumping the spoil directly from the digging tool to the treatment plant.

The size of the site also meant the bentonite plant had to be moved 70m, on the other side of the roundabout, with the piping routed via a pipe-rack.

With this organisation, the diaphragm wall was completed on schedule in 12 weeks. Four test micropiles were installed with a depth of 65m to test the characteristics of the strata at depth.

When tank excavation was completed and the diaphragm wall built, 520 micropiles were installed to anchor the raft in the sand. The micropiles were designed by the Soletanche Bachy design office to transfer compression stress (when the tank is full) and traction stress (uplift from the groundwater under the raft). The micropiles used Gewi® bars of diameter 63 or 75mm. The grid is about 2m by 2m.