Behaviour of Groundwater in the Slope Failure Area of the Gamahara Torrent

In the previous paper the background and mechanism of the Gamahara Torrent debris flow of 6 December 1996 were reported on the basis of the results of various geotechnical investigations. This report deals with results of the geological survey in two bore holes just behind of the slope failure scar and observation results of the behavior of the groundwater in the same bore holes due to the continuous measurement of the water table. As a result of the initial field survey it was estimated that the debris flow was triggered by the slope failure which occurred at the geological formation border at an elevation of about 1,300m and further the soil layer above the formation border probably contained very much water due to antecedent rainfall before the occurrence of the slope failure. Two bore holes for surveying are dug just behind of the slope failure scar in order to clarify the geological formation structure and to grasp the behavior of the groundwater regulated by the geological formation border. It was confirmed that the geological formation structure of the target area is generally corresponding with the structure estimated from the in situ field survey in the slope failure scar. The upper soil layer, the formation of the quaternary volcanics, is classified into two layers namely the upper pyroclastic flow deposit layer and the lower andesite lava layer according to the observation results of the boring core from the two bore holes. The observation results of the water table in one of the bore holes showed that the water table has significantly risen up by snow melt and quickly dropped down in the middle of April 1998