ABSTRACT
The occurrence of sinkholes in the WAC Bennett Dam in British Columbia in 1996 led to investigations of the condition of the dam which support a ‘working hypothesis’ that fine material has in places been washed out of the till-like core into the downstream transition zone. This is the converse of the crushing of sands that occurs at high pressures and which leads to an addition of fine material. In both cases the material is changing during its lifetime – with crushing the change is irreversible – and this cannot be ignored in attempts to model its mechanical behaviour. It is suggested that a ‘grading state index’ can provide an indication of the relationship of the current grading to two extremes: a perfectly single sized material with particle size equal to the maximum particle size of the actual material; and a perfectly packed material in which every void is filled with progressively smaller particles. Modelling the behaviour of such an evolving material requires that properties such as critical state conditions should be dependent on this changing grading state index.
