Study On The Strength And Failure Modes Of Rock Mass Models With Non-persistent Joints

With the rapid development of the domestic economy, the higher requirements are needed for the infrastructure constructions with the massive funding into major water conservancy projects, energy, and municipal engineering. The geological complexity is higher and the scale of these projects is larger dramatically compared with the past and the problems faced is increasingly complex. Researching on the intermittent jointed rock masses is of great theoretical and practical significance, because the great life and economic loss will occur with the sudden damage of rock mass. In the paper, Realistic Failure Process Analysis system (RFPA) is used to study the strength and failure characteristics of rock by establishing some numerical models of non-intermittent jointed rock mass. The effect of geometric joints distribution and stress conditions on the failure modes and strength characteristics is summarized, and the results of numerical simulation fit in with the results of physical tests very nicely.The main contents of the dissertation are as follows:1. By reading a large number of relevant papers, the current achievements on the strength and failure characteristics of non-intermittent jointed rock are summarized in the aspects of theory, lab tests and numerical simulation.2. The finite element method of RFPA is applied to simulate the failure process of jointed rock mass with initiation, propagation and perforation of crack. The numerical models with different joints geometric distribution and stress conditions are established in order to study the effect of joint angle, rock bridge length, layer spacing, step angle and lateral pressure on the failure modes, strength and crack propagation. The following conclusions are summarized:(1) It shows that failure modes of non-persistent joints can be divided to four categories: Failure through a plane, rotation of new blocks, stepped failure, and mixed failure. Failure through a plane and stepped failure are associated with higher peak strength and strain than the rotation of new blocks.(2) With the joint inclination increasing, the peak strength and elastic modulus tend to increase but then drop.(3) Joint step angle y has a greater effect on rock mass for the failure mode changed from stepped failure to rotation of new blocks when β=90. Layer distance d has less effect on stepped failure but more effect on rotation of new blocks. Even though rock bridge length Lr does not affect the failure mode, it has a greater impact on peak strength and failure strain for plane and stepped failure.(4)With the lateral pressure increasing, the damage zone is transformed from the central region to the edges with the failure mode change. With the joint inclination increasing, the peak strength and elastic modulus tend to increase but then drop. As the confining stress increased, the failure areas expand from middle to both ends and it can increase rock strength dramatically, while the growth rate is inclined to reduce with the confining stress increasing.(5)In random jointed rock mass, the dominant failure surface is the position with weak joints distributed densely. The effect of angle on the failure process is larger. With jointed angle increasing, the dominant failure surface of the block change into dominant failure block.(6)With the model size increasing, the failure mode changes from brittle damage to the progressive deterioration. By loading of lateral pressure, jointed rock still has a significant size effect.3. The compression test of three-dimensional model with surface cracks is conducted to analyze the rules of crack initiation and propagation. The results show that:(1) Compared with two-dimensional model, the stress concentration at joint tips in the three-dimensional rock mass is not obvious and the crack cannot propagate along the direction of maximum principal stress with the same conclusion in lab tests.(2) Compared with two-dimensional conditions, the failure of simplified case of a crack is different in three-dimensional model. The stress concentration occurs at the elliptical crack edge in the thickness direction, and then there is no clear expansion direction with divergent trends.(3) The failure mode of blocks with different thickness is similar roughly and consistent with the surface destruction. With the depth increasing, the effect of surface tends to weak gradually, and the inner failure of rock depends on the crack initiation in the thickness direction.