Characteristics of surface soil creep on a forest slope in Japan

•Intensive data which show relationships between soil creep and soil moisture change•Bending strain increased/decreased when soil were drying/wetting.•Not restored bending strain remained and formed net soil creep.•Miniature sliding sometimes occurred in the topsoil when rainfall exceeded the threshold.

Soil creep was monitored for 6 years using the strain probe method, and compared to soil moisture conditions in order to reveal details of the soil creep mechanism on a forest slope in a weathered granite area of Central Japan. The results revealed a relationship between soil creep and soil moisture changes. Bending strain of the topsoil was induced downslope by soil drying, and was reversed by soil wetting. However, an amount of bending strain remained, which accumulated annually to form net soil creep. On the upper slope, when rainfall exceeded a certain threshold, miniature sliding sometimes occurred, caused by soil destruction in areas where shear deformation became concentrated. On the middle slope, topsoil deformation was more complex. In order to monitor the soil creep phenomenon related to the concentration of shear deformation in a narrow zone, improvement of the strain probe method is needed.