Rainfall-induced failures of volcanic slopes subjected to freezing and thawing

Rainfall- and earthquake-induced failures of slopes formed by volcanic soils occur frequently in Hokkaido, Japan. The aim of this study is to clarify the failure mechanisms of volcanic slopes caused by both rainfall and freeze–thaw action in cold regions such as Hokkaido. Using model slopes of different shapes formed by volcanic soils, a series of rainfall tests are conducted under field conditions in which a spray nozzle is used to simulate rainfall intensity. Test results show that the surface failure of volcanic slopes differs depending strongly on the angle and the initial moisture content of the slopes. Based on the results of the model testing, the effects of freezing and thawing on the failure mechanism are drawn upon to propose an evaluation method for slope stability. In consideration of the model test results, it is found that the formation of a frozen layer and the softening of the slope surface, due to the freeze–thaw action, are significant for the stability of volcanic slopes in cold regions, and that slope failure can be uniquely assessed by the changes in water content in zones subjected to rainfall and freeze–thaw action.