Characteristic state plasticity for granular materials Part II: Model calibration and results

A non-associated plasticity theory for granular materials has been developed in Part 1 based on the concept of a characteristic stress state of vanishing incremental dilation. The model is fully three-dimensional and is defined by six material parameters: two for elastic stiffness, one for plastic stiffness, two for the shapes of yield and plastic potential surfaces and one for the dilation at failure. In this paper a calibration procedure is developed using test data only from a standard triaxial test. It is found that the shape parameter for the yield surface can be estimated from the plastic how parameters, thus reducing the number of free parameters to five. Calibration examples are shown, as well as predictions made, for different confining stress levels and constant volume tests on sand. The model is found to represent stress-strain behaviour and development of volumetric strain in standard triaxial tests well. The model provides good predictions of constant volume behaviour of dense as well as loose sand on the basis of calibration by standard triaxial test data. A simple explicit formula is derived for the failure asymptote in constant volume testing, enabling explicit adjustment of the parameters, if incompressible-test data is available.