Abstract
Steep rock slopes composed of pyroclastic rocks of middle Miocene to Pliocene age are formed around the coastal part of Shakotan peninsula, Hokkaido, Japan. Rock mass failure frequently occurs at these area which can be examplified by the rock failure accident at Toyohama tunnel, Feb. 10, 1996. The pyroclastic rocks comprising the slopes is classified into the soft, inhomogeneous type. They are characterized by presence of smectite filling the interspaces between the particles as well as the pores in the rock. It may be possible that the microscopic occurrence of the smectite can be an important factor which controls the change in physical properties of the rock such as durability and hardness under an aqeous weathering condition. In addition, mineralogical feature of the rock may be a promoting factor of crack propagation causing the rock mass failure at such steep slope area.
