Itacaja dam is a small concrete gravity structure built to supply electrical energy to the city of Itacaja located in northern Brazil. The dam is supported on soft rocks comprised mainly of verti-cally open jointed weathered sandstones. Seepage along the joints, during reservoir filling, initiated an erosion process that led to the complete removal of the dam's foundations central portion and the struc-ture in consequence becoming supported only on its abutments. Numerical nonlinear tridimensional flow analyses have shown that the water velocities in the vertical open joints were higher than the maximum allowable velocity for weathered sandstones and that failure would have occurred for joint openings of the order of 2 mm to 3 mm depending on the joints relative roughness. The lack of a site investigation program, the non implementation of an adequate foundation treatment and the absence of a proper sub-surface drainage system are the main causes behind the failure.

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