EDF operates the hydroelectric power station of Cusset, located in Villeurbanne (Rhône, France). The station was built around 1895. A bypass canal, called Canal de Jonage, 8km long, was built at the same time to supply the water turbines of the dam. But since 1995 EDF noticed the formation of a subsidence on the right embankment where the right bank joins the sluice gate of the dam.
In the next year, EDF forecasted a 5km long cut-off wall project for this 10m high embankment. But they decided to carry out a 160m long cutoff wall nearby the station as a matter of emergency.
The embankment is made of 10m sand and gravel backfill and pebbles, sometimes without matrix, and then natural compact gravels. EDF determined a socket of 2m in this compact soil layer and 5m nearby the station. The depth of the cut-off wall is between 12 and 15m.
The technique Geomix® consists in mixing the soil, without excavation, with a cement slurry. The cement content of the slurry is adjusted to match a hydraulic conductivity criterion of 10-9m/s. The Geomix® process has been chosen to the detriment of injection treatment or slurry wall for the following three reasons:
- Less spoil, reduction of environment impact (waste, traffic),
- Quick setup of the wall (no repetitive grouting steps),
- No fluid under pressure inside the embankment (risk of instability).
When obstacles (concrete, boulder, wood), first recognized or not, have been punctually prevented the Geomix® cutting head to drill deeper, an additional grout treatment has been implemented below the obstacle.
The combination of Geomix® and grouting allow to reduce the impact of encountered obstacles to the bare minimum of resources, duration and costs.