Geophysical and Geotechnical Evaluation of Landslide Slip Surface in a Residual Soil for Monitoring of Slope Instability

Abstract

Electrical Resistivity Tomography (ERT) technique has been widely used conventionally to monitor slope instability; however, a detailed study to validate ERT models with geotechnical properties for monitoring of slope instability is still rare. The objective of this work was to evaluate soil water conditions in gravelly sand soil that influences slope instability. The ERT investigation was conducted on a 60 m survey length with 1.5 m electrodes space to obtain subsurface geological structures and geotechnical laboratory tests, and experiments were carried out to obtain soil engineering properties for validation of the ERT model results. Our results revealed three geological zones namely: saturated, weathered granite, boulders and floaters, and resistivity values of 1–800 Ωm, 1,200–4,000 Ωm, and >4,000 Ωm, respectively. The saturated zones were characterized by subsidence during dry season and possible collapsed structures of boulders and floaters during prolonged heavy rainfall. The analyzed ERT models revealed that the slip surface of a potential landslide may occur at the boundary between the saturated and weathered granite zones within 3.0 m depth. The analyzed geotechnical properties also confirmed the ERT result because the potential landslide sliding surface was delineated within 3.0 m depth at the boundary between gravelly sand with a lowest angle of internal friction of 30° and a cohesion of 8 KPa, which was easier to destroy, and silty sand with a high angle of internal friction of 33.8° and a cohesion of 12.7 KPa, which was more difficult to destroy. These findings are very fundamental in shaping the earth's surface, especially in the formation of landscapes.