Characterization of landslides: a preliminary vertical electrical sounding approach

Groundwater flow and accumulation within a landslide mainly govern its movement along the slip surface. Thus, a comprehensive understanding of the subsurface flow regime would be imperative in mapping the landslide body. Geoelectrical resistivity can be used to detect the significant changes in the ground caused by water flow and accumulation. A systematic vertical electrical sounding (VES) at selected locations would provide preliminary information on the subsurface nature of the landslide that can guide future surveys including 2-D electrical resistivity traversing. A VES survey using Schlumberger electrode configuration was carried out at the creep-type landslide that occurs on the southwestern part of the Uva Wellassa University, Sri Lanka. Four survey points were established for the preliminary VES survey, one at the toe area of the landslide, the other on the south-west boundary, and two points outside the landslide body. Each survey line spanned 200m in length. The data revealed the presence of partially dry and quartz-rich overburden and a clay-rich, water-bearing layer with an approximate thickness varying between 24 m -32 m with significantly low apparent resistivity around 30- 60 Ωm sitting on the partially weathered bed rock at a depth of approximately 30 m – 40 m. This was found to be the prominent character at all locations. Preliminary VES data indicate that the thickness of the clay-rich water-bearing layer increases toward south (uphill) while the quartz-rich overburden wedges in the same direction from the toe across the boundary to the adjacent ridge and up and over the ridge. This preliminary finding leads to understanding the general anatomy of the landslide and the surrounding undisturbed mass in terms of the number of layers, type of layers, their thicknesses, the presence of water and the depth to the bed rock as a first approximation.