Moisture Movement Analyses for Coal Stockpiles

Flowslides and stability issues have occurred periodically within stockpiles of coking (metallurgical) coal at coal processing plants and export terminals in Queensland’s Bowen Basin, and to a lesser degree in New South Wales, since the early 1970s. A description of the issue and summary of research at James Cook University from 1973 to 2000 was published in ACARP Report C4057. Despite this work, coal stockpile flowslides remain a significant risk at mine and port stockpiles due to their initiation without warning and dramatic consequences. To be able to adequately model the redistribution of moisture that leads to collapse of a stockpile and then conduct realistic stability analyses for design of preventative measures remains an elusive prospect. This paper therefore updates the previous work with results from SEEP/W transient seepage modelling within a 12m high 14,700 tonne coal stockpile constructed at Hay Point in late 1991 for which initial moisture content, pore pressures at the stockpile base, outflows from subsoil drains and final density and moisture profiles were measured. The model was based on results of laboratory permeability and column drainage tests on specimens taken from a composite bulk sample obtained at the time of stockpile construction. The coking coal product was from an operation with a known history of stockpile instability. Results were found to correspond well with pore pressures measured at the stockpile base and the stockpile’s final moisture profile provided account was taken of a thin higher permeability zone just above the subgrade. The approach adopted and parameters developed provide a significant advance in modelling of moisture movements within production coal stockpiles, with a view to subsequent slope stability analyses.