Three 40-m sections collapsed

A local resident observed subsidence and slippage over a 40-metre section of a flood protection dyke. This structure extends 4,500 m along the Saint-George Canal leading to the Gironde estuary. Upon arriving at the site, the operator observed the same phenomenon at two other locations on the dyke, over identical lengths. This major damage seriously jeopardised the structure’s ability to withstand flooding or a major maritime phenomenon.

Difficult repair work

Emergency work to reinforce the dyke was undertaken three days later. Using an excavator, a specialised company placed several dozen cubic meters of high-cohesion clayey earth. The soil was placed in successive layers with compaction at each stage to ensure a good cohesion. Five days after the work, the operator noted that the structure had collapsed once again, to the same extent as before the repair. He concluded that the excessive compaction operations may have further weakened the dyke’s foundation (due to a phenomenon known as liquefaction).

New interventions were decided upon, although using a different method. This time, the soil was simply reworked and very slightly compacted with the excavator’s bucket (not with the excavator itself). Following these operations, the structure still did not seem to be permanently stabilised. The operator commissioned a study to find a satisfactory solution: the solution called for the installation of sheet piling or wooden piles at the base of the dyke and possibly set the structure back in certain sectors.

Cause related to the proximity of the channel

The origin of the degradation was due to an internal erosion phenomenon as the dyke is located very close to the canal bed. This leads to permanent instability of the foundation and the structure as a whole. A very dry summer, coupled with high tidal coefficients, also accentuated this mechanism.