Landslides often occur due to the impoundment of a dam. Some physical tests, weathering tests and tria xial compression tests were carried out by using hearvy weathered materials from W1 layer in the landslide areas surrounding the Otaki Dam reservoir. From result of these tests, it became clear that the weathering rate of W1 material was low and that she aring strength of the materials was 0.2kg/cm2 and 30.6°in chohesion and internal friction, respectively. Fairly high factors of safty for

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... e landslide slopes are got by using the sheoring strength parameters. The reasonable control works at the impoundment of the dam should be done by considering a slide in W1 layer.