Failure mechanisms in weathered meta-sedimentary rocks

The weathering profile of the extended cut slope within the quartz mica schist along Pos Selim Highway in Perak State, Malaysia shows three broad morphological zones; bedrock zone, intermediate zone, and residual soil zone. Further thinner horizons within these broad morphological zones were differentiated based on differences particularly in textures and structures of original bedrock and degree of preservation of the constituent minerals. Both large and small scale discontinuities observed in the schist provide slip surfaces for failures, and field investigation showed evidence of failures in this extended cut slope. The implications of the micro-structures revealed through petrography showed shearing components along planes of weakness. From the determined index properties of the weathered profile, the quartz mica schist is not suitable for structural support. This is also evidenced by the high values of sand over silt and clay deduced from the sieve analysis. In the bedrock zone, weathering manifests itself by opening up of discontinuities within the rock mass rather than weakening of the material itself. From the kinematic analyses carried out on the rock cuts, there are possibilities of wedge, planar and toppling failures across this extended cut slope.