Weak Plane Failure of Phyllitic Sandstone: Back Analysis for Slope Stabilization and the Use of Probabilistic Approach for Design Optimization

The main access road of Mae Mao dam hydropower project, Chiang Mai province, Thailand, has failed during a sunny day. The question has been raised of what might have triggered the failure. At the day of failure, the precipitation amount was very small and the accumulated rainfall amount in the past days was also not significant to trigger the failure. Detail geotechnical investigations were carried out including the geological survey, seismic refraction survey, bore holes drilling and various laboratory testing. The rock found at site was Phyllitic and Quartzitic sandstone. It appears that the engineering properties of Phyllitic sandstone are more susceptible for slope failure. Rock and soil slope stability analysis have been done, however none of them was found to be the cause of failure. After carefully reexamining the geologic structure, it was found that there is a weak plane layer that resulted in the failure. A suitable stabilization method has been selected as the economical remedy of the slope failure. In this case, flattening the back slope is the economic method along with the drainage system specially designed to reduce the pore water pressure of the weak plane layer. Rock bolts and rock anchors were found to be uneconomical and unsuitable. Probabilistic approach was used to optimize the engineering safety and construction cost.