Investigation of a large-scale waste dump failure at the Mae Moh mine in Thailand

This research investigates the potential mechanisms and possible causes of a large-scale waste dump failure in the Mae Moh Lignite Mine, which is Thailand's largest open-pit coal mine. The large-scale failure of the waste dumpsite covered an area of approximately 1.56 km2 when the dump materials reached a height of 135 m from the original ground. The failure event involved the mobilization of a 70-Mm3 mass of the dump material with 1.2 km in length and 1.3 km in width. The causes of this failure were investigated in this research based on a scientific investigation approach for the remedial action. The geophysical and geotechnical investigation was first performed to characterize the waste dump materials and the perched water condition. The electrical resistivity tomography analysis was also performed within the failure zone and its results were calibrated with the boreholes and interpreted to depict the overview of the geotechnical conditions of waste dump materials. Finally, the stability of the waste dumpsite was investigated using the finite element method (Plaxis 2D program). Based on the critical analysis of the studied data, the causes of waste dump slope failure could be attributed to two main factors, which are the presence of a perched water table associated with the high porewater pressure and the presence of a weak basal layer at the interface between the hard foundation and the waste dump materials. This weak basal layer was the claystone deposited in the swamp area above the hard foundation and deteriorated over time with the increasing amount of waste dumped. The dumping of waste materials over the natural river and pond and the rainwater infiltration over time caused the presence of a perched zone in the waste dumpsite. Failure of the dump slope can be classified as due to a wedge mode of failure, which causes the kinematics with the horizontal translation of a passive wedge and vertical subsidence of an active wedge.