A case study of the pore water pressure fluctuation on the slip surface using horizontal borehole works on drainage well

The infiltration of rainfall reduces shear strength on the slip surface by increasing pore water pressure, resulting in increased landslide movement. Although pore water pressure fluctuation is an important factor influencing landslide activity, it is difficult to observe its mechanism at various depths of the slip surface because of the complex hydrogeological structure. This study aims to examine the pore water pressure fluctuation in shallow and deep strata of the slip surface. The rainfall, pressure head and drainage rate were observed at the Tairasawa landslide before and after the drainage well work. The pressure heads were measured in the vertical observation borehole with a screened section at the depth of slip surface. The horizontal drain borings of drainage well were drilled for confining groundwater in a screened section at the same depth of slip surface. After drilling a horizontal drain boring, the pore water pressure on the slip surface was reduced and the decreasing volume in the deep stratum was larger than that in the shallow stratum. The effects of horizontal drain boring could be easily evaluated by comparing the decrease in pore water pressure before and after the works. Furthermore, the decrease in pore water pressure on the slip surface shows two significant characteristics. First, the variation in pore water pressure corresponds to drilling depth and seepage water in a horizontal drain boring. Second, the decreasing effects of pore water pressure can be clearly seen when the drilling direction of a horizontal drain boring is near the vertical observation borehole.