Many cut-slopes on the Izumi Belt in Shikoku suffered failure during the construction of Junkando expressway. Based on the orientation of rock bedding planes, these slopes are dealt in two categories: ukeban-rock planes with a reverse angle of dip and nagareban-rock planes with a dipping orientation. The number of failed slopes was greater in the nagareban structure than the ukeban, and most of the failed slopes consisted of medium hard rocks, whereas the slopes with soft rocks and ordinary soil were found stable. These observations interested the authors to investigate the causes of failure and the strength properties of the involved slope materials. As a result, it was found that the angle of friction for the medium hard rocks was comparatively small as they contain expansive clay minerals.