In the next year, they already forecasted a 5km long cut-off wall project for this 10m high embankment. But EDF decided to carry out a 160m long cut-off 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).

 

 

GROUNDWATER MONITORING

EDF is closely monitoring the groundwater inside the embankment, especially as the hydroelectric power station is located in an urban area. They required six additional piezometers to be installed downstream of the cut-off wall in order to monitor the impact of the works. Daily monitoring showed conclusive and conformable results. We can also note that the standpipes have not been grouted by the Geomix® treatment, contrary to the grouting works.

 

SUBSIDENCE AREA

Prior to Geomix® works, the subsidence area has been treated with cement slurry through three drilling bores.

After that, the linking to the existing sluice gates was closely monitored by the Geomix® crane operator. The cutting head were slightly driven to the masonry and the cement content of the slurry has been increased. Two injection bores were then drilled to grout both sides of the cut-off wall to the masonry.

 

GROUTING OF THE ALLUVION

In an area of the embankment, raw alluvion without matrix was found in contact with natural compacted gravels. These gravels provoked the caving of soil below the cutting head. The same technique as the subsidence area has been successfully used: grouting the area with cement. This solution made the Geomix® mixing easier.

 

COMPLEMENTARITY WITH GROUTING

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. The works carried out in Villeurbanne have showed how sensitive are the works in an embankment because of possible very permeable soil layers.

The speed and the quality of the Geomix® technique make this process an innovative solution that can be easily combinated with grouting.