2019 Volume 7 Issue 2 Pages 556-564
Rainfall-induced failure is common after intensive rainfall events in mountainous areas, particularly in areas covered by residual soils. The infiltration of the surface water can have a significant influence on the stability of slopes. The increase of infiltration into the soil reduces the matric suction of the soil and hence results in a reduction of the shear strength of the soil, which increases the potential for slope instability. The mountainous road slopes in Timor Leste are located on scarp nearby the ocean. Slope failures repeatedly happened at several locations in Timor Leste after the intensive rainstorm events during the wet season. At some locations, the failure of the slopes occurred again even after the repair of the slopes. A geotechnical investigation, a geological study, and a surface seismic geophysical survey were conducted at the site to obtain soil/bedrock profiles and properties for the numerical modeling in this study. Seepage and slope stability analyses were conducted to evaluate the increase of the pore water pressures during the wet season and the resulting reduction of the slope stability. The results of the seepage analyses demonstrated that the pore water pressure within the slopes was significantly increased due to the increase in infiltration during the wet season. The results of the slope stability analyses showed that the as-built fill slopes at the dry season had only an initial factor of safety of about 1.3. The factor of safety of the as-built slopes was further reduced to reach unity or less during the wet season. The results of the analyses concluded that the triggering mechanisms for the failures are the combinations of geotechnical and hydrological properties in addition to the physical properties of the slopes. Conceptual repair plans including (1) regrading of the existing slopes, (2) installation of tieback anchors, and (3) installation of geogrids were considered in this study. Recommendations for the design of roadway in an area that is susceptible to a rainfall-induced slope failure potential were provided in the paper.
