Abstract
Shallow soil slips are a main geomorphic process occurring on the hillslopes made of loosely consolidated sandstone for Iwasaka formation in central Boso Hills, Japan, where two alternate processes are repeated; (1) the denudation by soil slips and (2) the recovery of soil layer by transported soil and weathering. The purpose of the present study is to research soil structure, to clarify the process of the recovery of soil layer and to estimate the return period of shallow soil slips on hillslope. Three shallow soil-slip scars with different ages of failure were selected for detailed investigations. Field investigations such as Doken-type penetration test, vane shear tests, permeable capability tests and sampling of sand in soil layer were carried out. Physical properties and 137Cs concentration of soil sand layer were also measured to make clear the formative processes and dating of soils. Test results enable us to estimate that thickness of transported soil layer reaches 100cm in 100-200 years after land-slipping. Above discussion suggests that the return period of shallow soil slips on hillslopes of the present study area is 100-200 years.
