The slope stability problem at Mae Moh Lignite Mine, Lampang Province, Northern Thailand

Landslides and slope failures have been recorded at the Mae Moh Lignite mine, Lampang Province, northern Thailand. These failures were found to be of three principle types, namely rock slides, topple rotational slumps and debris flows. Field and laboratory investigations were carried out to analyse the stability of the present pit slope which comprises mainly of weathered claystones with thin lignite interlayers. The slope stability analyses were performed using the stereographical techniques of Hendron et al. (19711, Markland (1972), Hoek and Bray's (1974) stability chart method, and Bishop's (1955) simplified method of slices for the northwestern slope, and the back-analysis method for the southeastern slope. Results of analyses indicate that slope instability is controlled by some or all of the following factors; (I) the bedding plane, the preexisting fractures and their concurrent orientation to that of the pit slopes, (2) the geometry of the overall slope and the inclination of the individual benches, (3) the water seepage in the rainy seasons, (4) the degree ofweathering and other engineering properties of the rocks. INTRODUCTION Lignite is a very important electric energy source in Thailand and is used to generate about 10 percent of the electricity required each year. The only electricity generating enterprise, the Electricity Generating Authority ofThailand (EGAT) owns several lignite deposits for such purpose. The most important and largest deposit is at the Mae Moh lignite mine which is located in Ban Mae Moh, Changwat Lampang, in northern Thailand. The lignite and associated claystones which belong to the Mae Moh Basin here dip gently to moderately to the north-west from the mining area to the central part of the basin. As excavation progressed down-dip, slope failures occurred in the claystone units overlying the lignite seams. These were wedge failures on the northwestern pit slope and down-dip block slides on the southeastern one. The wedge failures were controlled mostly by preexisting fractures in the clastic rocks while the block slides occurred along the bedding planes when the supporting toe of the slope was removed. GEOLOGY OF MAE MOH BASIN Gardner (1967) and Piyasin (1972) who studied the regional geology concluded that the lignite seams in this area belong to the Mae Moh Group, a typical non-marine