A study of slope stability at Mae Moh lignite mine

Abstract

Landslides and slope failures which occurred at Mae Moh lignite mine were recorded .These failures were of three principal types; rock slides, topple rotational slumps and debris flows. Field and laboratory investigations were carried out to analyse the stability of the present pit slopes which divided into five subareas according to their homogeneity in geology. These subareas locates on the southwestern slope, the northwestern slope, and the southeastern ones respectively. The rocks on these slopes are of the Overburden Clay-stone which overlies the upper Lignite K Seam. The Overburden Claystone layer is composed mainly of claystones with thin lignite interlayers and subordinate red clays and residual soils, the latter are the topmost layer of the northwestern slope. The bedding plane of these rocks is striking 195◦ azimuth and dipping 25◦ to the west. Three sets of east-west, north-south, and northeast-southwest trending major joints pervade these rocks. Superimposed upon these structures are generally north-south trending moderately to high angle normal faults. These geologic structures were found to have an influence on the stability of both individual bench and overall slope and causing the small scale wedge failures which later gave way to a larger mass movement. Stereographical methods developed by Hendron (1971) and Markland (1972), Hoek and Bray stability charts method (1974), and Bishop’s simplified method of slices (1955) were used in the stability analysis. An additional back-analysis method for planar sliding area was also performed. Results of analyses indicated that the slope is generally unstable during the wet season except along the southwest and northeast wall of the pit. Overall-slope plays and important role on large-scale instability while the bench slope angle is too high for the value of friction angle and discontinuity orientation, thus the structural-controlled smaller-scale slope failures easily start in the limit of a bench especially during the wet season. In the working area where bedding dips into the pit, planar bedding-plane slides easily occur.