The mechanisms and geotechnical characteristics of slope failures at a mining district, southeast Nigeria

A procedure encompassing field, laboratory and numerical analyses was employed to re-analyze the mechanism of slope failures at a mining district in south eastern Nigeria. The slopes have been re-evaluated on the basis of new evidence. The study identified 43 landslide events which were mainly shallow, short run-out slides triggered during rainfall. The sand grains were analysed by scanning electron microscope for evidence of microstructures, surface texture and grain crushing. The analysis revealed that the soil particles associated with landslide are somewhat rounded, fairly sorted, and absence of any grain crushing. Shearing tests were carried out at various normal stress and relative density conditions. The specimens with relative density of about 32 % responded to shearing in a strain-softening pattern; and as normal stress and over-consolidation ratio increased, there was no transition from contractive to dilative behaviour. On the contrary, under a constant normal stress (196 kPa) and increasing relative density (from 45.9 % to 75.5 %), the soils exhibited dilative behaviours. It is shown that increase in relative density, normal stress or overconsolidation ratio did not affect the brittleness index significantly. A numerical method based on strength reduction technique simulated a landslide which structure, run-out distance and distribution area were consistent with field observations. It is deduced that the slopes are unstable under intense rainfall conditions due to the effects of soil characteristics, excess pore water generation and the alternating swelling and shrinkage of the claystone layers interbedded with the sand bodies.