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The dam at Serre Poncon is one of Europe’s highest earth dams--122 m above the stream channel and resting on 110 m of alluvial fill. The main problem in construction was provision for an effective cut-off membrane in the alluvium to the bedrock. Throughout the investigation and construction, Electricite1 de France used a wide variation of modern exploration methods.

To protect against excessive ground-water leakage through the alluvial foundation and also to guard against piping, a vertical, water-tight grout curtain to bedrock was constructed. Selection of type of grout for consolidating alluvium and the most effective pattern of grout holes and application of grout mix are considered to be major advances in this field for construction of large earth dams. Extensive laboratory tests and large-scale field experiments facilitated the solution of these problems.

The completed vertical grout curtain, with a 4200-square-meter surface and 110 m maximum depth, is the first successful grout curtain emplaced in highly heterogeneous alluvial material to such depths. The coefficient of horizontal permeability was reduced from 8 × 10-4 m/sec to 2 × 10-7 m/sec by the grouting. A leakage of 25 liters (6.6 gallons) per second through the cut-off curtain is the maximum flow experienced. Although the first impression may be one of simplicity at this dam site, subsurface conditions are complex. The successful emplacement of the grout curtain and consolidation of alluvium in place was necessary at this site before construction of any dam was possible. Other dam designs were not feasible.

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