Micromechanics-based Continuum Method And Constitutive Model For Jointed Rock Masses

Rock as natural geological materials, the presence of various defects in rock mass plays an important role on mechanical properties. The instability and destruction of many geotechnical projects are often occured due to the joints. This paper mainly studies the jointed rock masses, first start from a variety of discontinuities for the purpose of investigating the static mechanical characteristic of joints, and then together the micro-joint and macro-rock. As a result, a micromechanics constitutive model has been established by use of the elasto-plastic theory. The main contents and results include:(1) Base on the test results in the literature, the shear stress-shear displacement curve and the dilatancy curve of rock joints are extended from the previous linear elastic to the nonlinear elastic constitutive model by use of analog equivalent method. According to the characteristics of each curve, the curve relations are developed by the corresponding assumptions and constraints condition. Then derivative of displacement from curve function and stiffness coefficient can be obtained. Compared with the CSDS model, the new model is more likely derivation and calculation.(2) The micro and macro deformation mechanism of joints under shear loading was analyzed. A new constitutive model is presented, the essence of which is climbing phenomena and strengthen phenomena to be considered separately when asperity surface will undergo the wearing, grinding and crushing. From many direct shear tests it is concluded that jointed rock mass will be generate dilatancy and wear as the behavior of overriding and shearing for asperity surface under normal and shear loading. To a certain extent of damage accumulated will result in softening. A new concept named initial shear stress was introduced to reflect the above characteristic. On the other hand, as the motion of broken granule, the mechanical behavior of shear which is controlled by asperities will gradually transition to compact layer controlled, and it makes shear strength increase. This microscopic change effects are described by considering elasto-plastic kinematic hardening model. The constitutive equations are derived when combine kinematic hardening model and initial shear stress. These two phenomena together influence the deformation failure of jointed rock mass.(3) On the basis of previous findings of joint surface morphology, the behavior of two rough surfaces in contact with the non-coupling joints is change into the smooth undulate asperity and the rough plane behavior. The two basic assumptions were adopted of failure mechanism for rock joints under cyclic shear loading to reflect the damage and a constitutive model under shear cyclic loading is proposed. And the salient "shift" phenomena and "large" phenomena of residual shear strength after several cycles are investigated.(4) Infilled jointed are likely to be the weakest element of rock mass, the mechanical behaviour of infilled rock joints are to a great extent dependent upon the properties of the joint and of the infill material. Therefore, the failure mode and deformation mechanism are very different compared with joints without filling. Learn from the direct shear test results, and investigates the influence of the ratio of infill thickness to asperity height (t/a) on the mechanical parameters of joints. On the basis of analyzing several kinds of shear strength models of infilled rock joint, an increment stress-displacement of constitutive model is established with the classical theory of elastic-plastic, and the dilation and compression behaviour of joint were studied.(5) As rock masses contain a large number of discontinuities, based on the continuum micromechanics and two assumptions, the average response problem of the non-uniform medium material is replaced by the response of equivalent continuous medium material were studied. Main idea is take out of joint from rock masses, and then discuss the effects of sliding, closure as well as properties of the joints on the overall mechanical properties of rock masses. For the filled jointed rock masses, the stress concentration tensor is obtained and the corresponding overall stress-strain constitutive model is proposed. To examine the performance of the model, some uniaxial compression tests are carried out on qualitative analysis, and discussed the influence of relative thickness of filled jointed rock masses on the deformation behaviour.(6) Taking into account the the stress concentration tensor, the viscoplastic theory and the series multi-layer viscoplastic model are used to establish the finite element program for the equivalent constitutive model of jointed rock masses. They were applied to the numerical simulation of some typical engineering problems. It can be seen that the new model avoids the complex modeling and grid division of joints and can be applied in practical projects.