Izumi group is a mass of geological strata made basically of sandstone and shale. It is distributed near the southeastern part of Japan and mainly spreads all along the north side of the Median Tectonic Line in Shikoku Region. As expressway in this region is built nearly parallel to the tectonic line, a number of cut slopes in Izumi group suffered a failure despite a very gentle slope angle of about 34o. Observation of the failure pattern and the slope soil characteristics revealed that most of the failed slopes consist of medium hard rocks, whereas many slopes consisting of weathered soft rocks were stable. This typical failure pattern was considered center point of the study in this paper. Soil samples were collected from different cut slope sites of all the three states of soil, i.e., hard rocks, medium hard rocks, and weathered soft rocks. Strength and x-ray diffraction tests were carried out to elucidate the slope failure mechanism. Test results revealed that the shale rock has a small angle of external friction, far smaller than the angle of shearing resistance, and its bedding planes at failure sites make a positive acute angle with the horizontal plane.


Izumi group in Japan represents the geological strata distributed in southeastern part of the country. Owing to tectonic movements, most of the rocks along this line are in a fractured state that has caused a fragile geology in the area. This fragile geology has resulted in a number of landslides and slope failures, especially during and after the construction of Tokushima-Matsuyama Expressway that has been aligned along Izumi group and the MTL. Izumi group is supposed to have formed by an alteration of sandstone and shale some 64 to 100 million years ago during cretaceous period.

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