Cadia dam: a critical state view of residual soil behaviour

Critical state theory evolved through various contributions relating to both sands and clays, but all were based on remoulded or reconstituted samples. This evolution has produced a perception that the theory applies only to ‘structureless’ soils, with the corollary of no relevance to natural soils which have developed structure and/or bonds through geological processes; residual soils are a particular example. A structured residual soil at the Cadia Northern Tailings Storage Facility displayed brittle undrained behaviour comparable to static liquefaction and contributed to a slump of the dam. The effect of structure is conventionally taken into account by expanding a soil’s yield surface but doing so leads to an increased elastic response whereas residual soils normally show plastic strains near immediately. Here we add ‘structure’ to the theory using two additional soil properties, one a true cohesion attributable to structure and the second characterising the rate of decay of that structure with distortion. This extended theory replicates the measured behaviour of the residual soil considered. The extended theory can be used with existing geotechnical modelling software to simulate progressive failure provided that the software includes user-defined routines (‘scripts’) to allow updating of the critical friction ratio as analysis proceeds.