Role of vegetation in improving the stability of a tropical hill slope in Guam

Abstract

This paper aims at investigating and highlighting the hydrological benefit of vegetation by way of root water uptake (RWU) in preventing or at least delaying rainfall-induced slope failures in the island of Guam. In particular, the results of field instrumentation and monitoring by way of moisture and suction sensors, laboratory and field testing of geotechnical and hydrological soil properties and rainfall-induced numerical simulations of transient seepage coupled with slope stability analysis are presented. Two case studies, one with a 45° slope and another with a 60° slope, were studied for three different case scenarios. Only one set of vegetation and root reinforcement data was idealised, and the results confirm that matric suction was induced within shallow depths due to RWU helping in providing a buffer, which prevents slope failure during lower-precipitation events. However, this stabilisation effect ceases after saturation during long-duration precipitation. On the other hand, complementary analysis considering the mechanical root reinforcement effect showed that it is a dominating factor in improving the factor of safety even after the slope becomes saturated. In general, native hill slopes with a 45° angle were stable, but the one with a 60° angle failed when a real case rainfall event with climate–vegetation data was applied in this study.