摘要: The ever increasing need of smoother flow of traffic in Japan is being met by constructing newer and wider wads as well as developing best possible road network. Many such roads need to pass across the hills and mountains that make nearly 75% of the country's land. When a tunnel is not feasible or possible, the option goes to cutting slopes to acquire the necessary space for the highways. Cutting natural slopes, however, results in a risk of slope failure. The instability of a cut slope might be caused by various factors but one prominent reason is stress release, which disturbs the balance of stresses along the potential lime of failure. This paper introduces some cases of cut slope failures in decomposed granite zones under the influence of clay veins, and discusses the causes of failures from mineralogical and geotechnical points of views. As a result of the laboratory tests, it was revealed that the clay veins consist of expansive clay minerals such as smectite, which up on stress release absorb more water and weaken the clay veins. This process then results in failure of the cut slopes along the veins.