Observations of coal stockpiles subject to unusual levels of instability
showed that the particular coals exhibited 'gap grading', a relative lack of
intermediate size particles. This led to a suggestion that fine coal
particles within the pile might have been induced to migrate under the
influence of water flow and to concentrate in relatively impervious layers,
resulting in local water saturation and reduction of shear strength. To investigate this suggestion, a large test column has been constructed
and washed coals with various levels of fines content have been subjected
to a downward flow of water. Subsequent examination of the bed of coal
within the column confirmed the percolation of fines and their
concentration into horizontal layers, and showed the development of
zones of saturation above those layers. Shear box tests showed differences in cohesion and internal friction
angle between coals with different percentages of fines, supporting the
hypothesis that fines percolation may lead to decreased stability.