Abstract
Pore water pressure fluctuation is proved to be an important cause of landslide movement in landslide areas, but it is difficult to observe the mechanism of pore water pressure fluctuation at the various depth of slip surface. The purpose of this paper is to examine the pore water pressure fluctuation in shallow and deep stratum at the slip surface. The data used in this study was collected at Tairasawa landslide area during 1983-1985. Two observational methods are considered.(1) The pressure heads were measured in the vertical observation borehole with a screened section at the designated depth of slip surface.(2) The horizontal borehole of drainage wells were drilled for confined groundwater in a screened section at the same depth of slip surface. The rainfall, pressure head and drainage discharge were observed before and after the drainage well works.
The results indicated that after drilling a horizontal borehole, the pore water pressure was low in shallow and deep stratum at the slip surface. It is possible to judge the effects of drainage well works by comparing the drop of pore water pressure before and after the works. Furthermore, the drop of pore water pressure at the slip surface have two significant characteristics. First, the dropping variation of pore water pressure is corresponding to drilling depth and seepage water in a horizontal borehole. Secondly, the dropping effects of pore water pressure can be clearly prove when the drilling direction of a horizontal borehole near the vertical observation borehole.