Thesis (M.Eng.) - Asian Institute of Technology

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Engineering and Technology

A major cutting for the spillway on the right abutment of Mae Song Dam exposed a structurally complex and layered sequence of shale, sandstone and slate belonging to the Rong· Kwang Formation within Phrae Group of rocks. During the construction of spillway in the first quarter of 1991, a modest excavation near the toe of the slope triggered a predominantly translational failure along pre-existing bedding shear, which eventually led to the cessation of further construction activities. The depth of slip surface was estimated by simpl e slip indicators in the field and also by the movement vectors; both of them gave similar results. The depth of slip surface as estimated was about 15 m. in average and the volume of landslip mass was approximately 375,000 M3 {u2022} Engineering geologic investigations and slope monitoring after failure established t he movement mechanism. Daclt analysis of the failed slope by Sarma's method showed the mobilized angle of internal friction for bedding shear as 17 to 22° whereas laboratory direct shear test on infill material showed 19.7° as the value of the internal angle of friction for bedding shear. Post failure monitoring of slope revealed a definite correlation of movement rate with rainfall, the movement being accelerated with increased precipitation. It was also found that movement at present is stopped but it may be accelerated during ensuing fall season. Continuing movement of the slope, therefore, had to be stopped, and alternative remedial measures including relocation, drainage and profile control were investigated. Such an analysis revealed that the reasonable stability could be achieved at comparatively lower cost by removing the mass from the crest area and filling near toe with spillway width reduced to 40 m. and dam crest height increase d by 2.25 m. Sensitivity analysis also showed the significant effect of slope drainage upon factor of safety, improving the stability from a factor of safety of 0.62 for fully saturated slope to 1.13 for fully dry slope thereby stressing the need for surface and subsurface drainage.