Liquefaction flowslides in Rocky Mountain coal mine waste dumps

Over the past 25 years there have been a large number of long runout flowslides from Rocky Mountain coal mine waste dumps. The waste dumps are constructed as end-dumped fills with an angle of repose of 38 degrees. Dump heights range between 100 and 400 m. The dumps are normally founded on mountain slopes that are covered with a thin veneer of granular colluvial and dense stony till materials. Conventional stability analyses carried out for these dumps using friction angles equal to the angle of repose for the waste rock and typical values ranging from 30 to 32 degrees for the foundation materials indicate that many should be stable:. The flowslides occur rapidly and display surprisingly long runouts of up to 2 km in some cases. Detailed studies of three of these events indicate that static collapse of saturated or nearly saturated sandy gravel layers within the dumps may be responsible for the initial failure and the generation of high pore pressures which result in high runout mobility.