It is very important to study the mechanism of the behavior of a slope in order to assess its stability, to predict its future behavior, and to design countermeasure works. There is an unstable steep slope along a national road in Yamaguchi Prefecture, Japan, where heavy rain has sometimes caused large displacements. Local collapses have occurred in some areas of the slope. The GPS displacement monitoring system was installed in 2012 to continuously monitor the slope displacements. The groundwater level of the slope has also been monitored at a few well points. The slope is composed of mainly rhyolite and granite. The surface is covered with colluvial deposits that accumulated due to past collapses.
In this paper, a numerical analysis is conducted to understand the mechanism of the slope behavior considering both the measured displacements and the groundwater height. The Universal Distinct Element Code (UDEC) is employed to better understand the mechanism of the displacement behavior brought about by heavy rain. The numerical analysis reveals that when the groundwater rises and reaches the height of a discontinuity (the boundary between the rhyolite and the granite), the water pressure in the discontinuity increases, the stress reaches the failure criteria, and the displacements increase largely. As a countermeasure, several horizontal drains were drilled toward the locations of the discontinuities, and the displacements became stable.
Knowledge of the mechanism of the displacement behavior of a slope is essential for assessing the stability of the slope, for predicting its future behavior, and for designing countermeasures. Monitoring the real behavior is important to investigating such a mechanism. There is an unstable steep slope along a national road in Yamaguchi Prefecture, Japan, where heavy rain has sometimes caused large displacements. Local collapses have occurred in some areas of the slope. A continuous displacement monitoring system, using the Global Positioning System (GPS), has been applied to confirm the behavior of the slope and to assess its stability (Nakashima et al., 2018).