Fragmentary clay — a difficult waste material

The clayey cover of the coal seam in Northwestern Bohemia is excavated in the form of a fragmentary and blocky clay, and then deposited into large landfills >100 m high. Due to overburden pressure and increased moisture content, the clay changes from a granular to a saturated cohesive material. This transition is modelled by laboratory experiments considering the effects of stress, water and time. They provide characteristic values of the compression index, shear parameters and the coefficient of secondary compression and their variation. Due to high porosity, the source of a metastable structure, the material is collapsible. Bifurcation collapse and breakdown are differentiated. The latter acquired the form of a diffusion process. The prospects of the mathematical modelling of the collapsible behaviour are outlined.