Abstract With the object of extending the powerhouse in the Cubatao Valley, 15 kilometers northwest of Santos, Brazil, an excavation on a 60 per cent slope, 40 meters high, was made in the side of a steep mountain slope. Just as the excavation was completed the ground began to move, cracks opened, and small springs issued in places. The site is the scene of pre-existing slides where soil and weathered material overlie weathered schists which dip steeply into the hill. Bore holes indicate that the boundary between slide material and weathered schist is sharp. The slide was controlled by back fill and drainage tunnels. Water level in the slide fell regularly after tunnels were drilled.