This paper deals with auther's research on details of actual sliding surfaces observed in Collector Wells at Hirayama, Okuooe, Nukedo landslide areas.
Observations on sliding surfaces lead to the recognition that the sliding surface is complex and not a geometrically single one. On the contrary, what we call sliding surface consists of a zonal structure having a thickness of a few centimetres. In this zonal structure there is a fine clay seam containing many cracks and slickensides which are produced by shearing and slipping.
Groundwater seapages are often observed at the position of several centimetres above the principal slip surface and the position of 2 or 6 metres above the sliding surface, whereat no seapages are observed in the fissured shearing zones. The presence of groundwater in the sheared zone, however, is obvious, because it is detectable by means of analyses of drilling records and groundwater logging. The sheared zone is usually detected as a leakage zone by the drilling record analysis and laminar inflow or upward flow in the process of groundwater logging in the borehole. These lead to an important hypothesis that groundwater pressure is transmitted through the fissured shearing zones and works as uplift on the sliding surfaces, though the amount of groundwater is not large.
