Geotechnical and physico chemical properties of clays associated with landslides in volcanic and metamorphic terrains in Cameroon, Central Africa

Abstract

Comparative studies of geotechnical properties of soils from landslide scars within volcanic rocks in Alou Sub-Division located in the Bambouto caldera and from a major landslide scar within Precambrian metamorphic rocks (gneisses and granites) located at Kekem indicated a range of bulk densities of 0.93 to 1.12 g/ml and water content from 27 to 44% from the volcanic terrains while bulk densities and water content in the metamorphic rocks ranged from 1.14 to 1.52 g/ml and 7 to 35% respectively. Granulometric analyses based on the determination of Atterberg's limits indicated the predominance of silts of high plasticity in both areas

although the proportion of the fine fraction (clay, silt and fine sand) was generally higher in the volcanic rocks. Mineralogical properties obtained from X-ray diffraction (XRD) patterns indicated the predominance of feldspars, chiefly microcline and gibbsite, with appreciable proportions of clays (montmorillonite, kaolinite, goethite, chlorite and halloysite) within the volcanic rocks while the gneisses were dominated by quartz and muscovite with some proportions of montmorillonite, kaolinite and feldspars with the clay fractions being more variable in the volcanic rocks (13 to 44%) than in the metamorphic rocks (23 to 29%). The presence ofmontmorillonite, a type of clay which has the ability to absorb water and swell, is an indication of a major contributor to the susceptibility of both terrains to sliding when subjected to high rainfall. This was revealed by data on intense precipitation during the specific months of the landslide events which led to elevated groundwater levels with raised pore water pressure in the highly weathered and loose materials resulting in increased shear stress and reduced shear strength with the eventual failures of the slopes causing the landslide disasters witnessed in both geological terrains.

Keywords: Landslide scars, geotechnical properties, montmorillonite, Cameroon Volcanic Line, metamorphic rocks, high rainfall.