The influence of bedrock geology and slip surface characteristics on failure mode and mobility: a comparative study of instability patterns in Nigeria

Abstract

The correlation among bedrock, regolith properties, groundwater table, and slip surface character and their combined effects on the mode and scale of landslides were evaluated. Standard penetration test, laboratory, and DEM-based analyses were integrated to study igneous, metamorphic, and sedimentary areas where instability was common. A clear pattern of the distribution of translational and rotational slides, and their pre- and post-failure mechanisms were established. It was found that factors such as climate, vegetation types and density, tectonic setting, and catchment area were generally similar. The key differences were in the dissimilar underlying bedrock types which weathered to yield soils with distinctive engineering property and in the nature of the sliding surfaces. Whereas the weathering of granitic and gneissic rocks (where groundwater table was <2 m) resulted in thinner regolith composed of unsorted rock fragments, sands, and fines, the weathering of schist produced thicker soils with a high percentage of finer fractions. Sediments in areas with deeper water table (>7 m) produced less compressible, fairly-sorted, porous materials. The hydraulic conductivity of the granitic and gneissic-derived soils decreased with depth. At saturation levels above a threshold, the soils derived from the igneous and metamorphic rocks contracted and generated positive pore pressure. Movement on discrete, planar, slicken-sided, fractured, and foliated slip surfaces transformed the slides to flow (translational failures). On the contrary, pronounced dilation and negative pore-water developed in the more porous, less crushable soils from the sedimentary area leading to the dominance of rotational slumps in this region of active erosion.