A positive double cutoff wall was used to control the foundation seepage and to minimize the danger of piping, for a 107 m high earthfill dam founded on 126 m of previous and highly compressible alluvial deposits. At the end of the first year of operation (Dec. , 1976), the cutoff walls and deflected 285 mm downstream and had been vertically compressed by 140 mm. These deformations were caused by the combined action of the weight of the dam and the reservoir load and were monitored by a comprehensive instrumentation. The reading of the instruments indicates that 85% of the vertical deformation in the walls is mainly due to the load transmitted by the negative skin friction between the walls and the surrounding compressible alluvium. The presence of an arching effect in the foundation due to the particular shape of the canyon has been confirmed by the instrument readings structural analysis of the behaviour of the cutoff walls confirms the necessity of the twin walls, because the stresses induced in the single wall (negative friction on both sides) would have been dangerously close to the ultimate strength of the concrete.
