Stability analysis and simulated hydrologic response of some vulnerable slopes in Nigeria implications for rainfall-induced landslides

Abstract

Hillslope geometry, material properties, and hydraulic heterogeneities complicate slope stability models. To reduce uncertainties in the determination of Factor of Safety, parameters obtained at in-situ stress levels using ASTM standards were used in two slope stability models to identify and classify some vulnerable slopes in northern and southern parts of Nigeria.Steady-state and peak strength parameters were applied separately in an infinite slope analysis simulating the variation of slope attributes with degree of saturation. The application yielded FS that were consistent with instability, and accurately predicted the characteristics of slopes in which failure was likely. While rainfall was a common trigger, the probability of failure was higher on slopes > 38 o in the northern part of the country underlain by igneous and metamorphic rocks. Contrastingly, slopes with angles > 25 o were predicted to be at risk in the southern part underlain by semi-consolidated sandstones. These predictions are in good agreement with field and reported cases of mass movements in the two regions. Using another stability method based on Bishop Model to correlate and validate the findings, the research observed that the slopes were sensitive to moisture with considerable drop in FS as saturation gradually increased. The study discovered that about 80 % saturation was enough to induce instability and that beyond this threshold failure occurred when the slopes became marginally stable (FS ≤ 1). This threshold value and the decline in FS with rise in saturation have important implications for rainfall-induced landslides on the hilly areas of Nigeria.