Author:

Abstract

The key to understanding the stability of slopes in residual soils lies in recognizing the behaviour of weak soil layers, underlying relict geological structural discontinuities inherited from the parent bed rock and highly permeable zones existing in the underlying bed rock. A clear understanding of the geological formation of such weak zones in the subsurface would help to assess stability of slopes with much more confidence and predict the depth of the potential slip surface materials. Detailed engineering geological and geophysical studies of the sub-surface of a slow-moving slope failure at Kahagolla, along Colombo-Badulla highway, Sri Lanka indicate that the sub surface of the slope mass had developed due to breakdown of underlying layers of garnet biotite sillimanite gneiss and a layer of impure crystalline limestone existing towards the head scarp. The test pits had shown that the slip surface had predominantly developed through the completely weathered garnet sillimanite biotite gneiss into sandy clayey silt layer. The rotary core drilling exploration reveals that the existence of a rock layer of impure crystalline limestone towards the head scarp which contain solution cavities supporting development of high pore water pressure below the slip surface material. The electrical resistivity studies indicated that the sub surface consists of loose upper layer and the lower layer of residual soil derived from weathered bed rock. Development and distribution of tensional cracks in the overlying beds control by the existence of two main joint discontinuity sets in the completely weathered rock and residual soil horizons. The study highlights the role of weak soils in the development of the slip surface, influence of solution cavities in the underlying marble bed rock on development of high pore water pressure and the soil weakening character of the relict discontinuities of the underlying rocks.

Keywords: Discontinuity, Pore water pressure, Scarp