Numerical Modeling Of The Elastoplastic－Damage Constitutive Relations For Sand

The theory of soil constitutive relations is the foundation of the design and simulation of geotechnical engineering. The mechnical characteristics of geological medium are very complex, that include the effect of confining pressure, dilatancy and the dependency of stress path. The traditional methods, based on plastic potential theory, are not able to exactly reflect these characteristics, in particular the influence of stress path on the constitutive relations. Therefore, the numerical method of modeling of constitutive relations for rock and soil is put forward by WANG Jing-tao. A series of the curves of elasto-plastic deformation are drawn through the triaxial compression tests for medium dense sand, under the stress path in which the mean normal stress P keeps a constant value. The stress-strain relation of sand is abstracted by using the numerical method of modeling and the elastoplastic-damage model has been built up from test data. The three-dimensional surfaces of stress-strain relations are drawn through visualization and corresponding yield locus are also given. In the elastoplastic-damage model, the coupling between elastoplastic deformation and damage is considered. In the entire process of elastoplastic deformation of the medium dense sand, the volume strain changes from compression to expansion before failure, called as dilatancy. Based on the recent results of CT tests on Shanghai clay, we think that the dilatancy is caused by the extension of internal micro-cracks and it is an expression of the damage of sand. In the triaxial compression tests, the values of moduli of elasticity and shear were measured before and after dilatancy. It is found that the values of moduli of elasticity and shear obviously decrease after dilatancy, which verifies that the dilatancy is a process of damage indeed. A variable of damage, based on the plastic volume strain, was introduced to describe the damage evolution. The research results show that the damage of sand after dilatancy gradually evolves with the increasing of plastic volume strain, which is dependent on the initial consolidation pressure P0. The greater the pressure P0 is, the smaller is the rate of damage evolution relative to plastic volume strain. In this paper, the elastoplastic constitutive model of sand is built up through the numerical method of modeling of the constitutive law for rock and soil. It reflects the effects of confining pressure, dilatancy and dependency of stress path. In addition, it is further proved that the dilatancy is essentially a process of damage. Through the measurement of the moduli of elasticity and shear before and after dilatancy, the process of damage is also described in the constitutive model.