Dilatant Failure States for Drained Triaxial Compression of Some Geomaterials

The dilatant failure state in the stress ratio-plastic dilatancy relationship is crucial in the frictional state concept. This article presents a methodology for determining the dilatant failure state from the results of drained triaxial compression tests. For geomaterials undergoing dilative behavior during shearing, the dilatant failure state corresponds to the state of minimum plastic dilatancy. For contractive behavior, the proposed calculation procedure can be used to determine the dilatant failure state. In general, the dilatant failure state and the failure state are different. The points representing dilatant failure states in the stress ratio-plastic dilatancy plane can be approximated by a straight line (dilatant failure state line). Grain crushing and debonding during shearing significantly increase the slope of this line. The intersection of this line with the vertical axis determines the critical frictional state angle for granular materials. The dilatant failure state, with previously defined natural state parameter, allows for the determination of the critical state angle void ratio without physically reaching the critical frictional state. In general, given the fact that the dilatant failure state proceeds the failure state, the stresses and strains in the shearing specimen can be determined more accurately than in the ultimate, critical state. The frictional state concept may be viewed as an extension of the critical state concept developed over fifty years ago.