Study On The Strength Characteristics Of Jointed Rock Mass Based On The Numerical Manifold Method

Jointed rock mass is one of the most common construction objects in rock mass engineering.Its mechanical properties are factor that must be considered during support design and stability analysis.It is of great significance for the safety and stability of tunnel,slope and mining engineering to correctly understand and rationally evaluate the strength characteristics of jointed rock mass.Due to the long-term complex geological structure,there are a larch number of discontinuities with different size and direction in the rock mass.The existence of these discontinuities causes the process of rock mass contains continuous and discontinuous deformation,which brings great difficulties to numerical simulation.The traditional numerical methods based on continuum or non-continuum mechanics can not simulate the failure of rock mass well.Through the introduction of the finite cover technique,the numerical manifold method(NMM)can solve the continuous and discontinuous problems in a unified way.The two sets of independent grids,mathematical and physical grids,makes it avoids a large amount of mesh reconstruction when simulating the crack propagation using NMM.Because of the advantages above,NMM can simulate the failure process of rock mass effectively.It has been rapidly developed and widely used in the field of rock mass engineering.The research topic comes from the national natural science foundation project.Based on NMM,the strength characteristics of rock mass with persistent and non-persistent joints are studied in this paper.Aiming at the research objects in this paper,the solving ability of NMM is extended.The extended NMM can realize the elastoplastic calculation of rock mass with persistent joints,and can also simulate the crack propagation of the rock mass with non-persistent joints.By comparing with the classic examples,the reliability of the calculated results of the extended NMM is verified.Then the NMM is used to simulate the compression test of rock mass with a set of persistent joints,two sets of persistent joints and non-persistent joints.The focus of this paper is the influence of the joint spatial distribution on the strength of the rock mass.Through the above research,the following main achievements are obtained in this paper.1.The strength characteristics and failure mechanism of rock mass with a set of joints was revealed.And the corresponding strength prediction model was established.(1)The compression test of rock mass with a set of joints was simulated by the elastoplastic numerical manifold method.The result showed that the strength of rock mass a set of joints is significant anisotropic.The curve of rock mass strength-joint inclination was shaped like a spoon with asymmetrical sides.With the decrease of joint spacing,the strength of rock mass decreases gradually and tends to a certain value.The influence of joint space was also anisotropic.(2)Through the regression analysis of the numerical results,a new parameter of joint distribution was proposed.A strength prediction model for rock mass with a set of joints was established.The influence of the joint inclination,joint spacing and confining pressure on the strength of rock mass can be accurately reflected in this model.The fitting constants have clear physical meaning,which can be determined through conventional laboratory tests.2.The strength characteristics and failure mechanism of rock mass with two sets of joints was revealed.And the corresponding strength prediction model was established.(1)The compression test of rock mass with a set of joints was simulated by the elastoplastic NMM.The results showed that joint inclination,joint spacing,confining pressure and the angle between the joint sets were the important factors of the rock mass strength.According to the joint state,the 3 failure modes of rock mass with two sets of joints were summarized.Through qualitative analysis,it was concluded that there was interaction between the joint sets.Therefore,there were some drawbacks to extend the single weaken plane theory model to rock mass with multi-sets of joints according to the principle of superposition.(2)Through the regression analysis of the numerical results,the strength prediction model suitable for rock mass with two sets of cross-persistent joints was established.This model was based on the strength model of rock mass with a set of joints.Not only the influences of joint inclination,joint spacing and confining pressure on rock mass strength was reflected in this model,but also the interaction between joint sets in different failure mode was reflected by a quantized joint group influence coefficient.3.In view of the strength characteristics of rock mass with non-persistent joints,the NMM,which can simulate the crack propagation process,was established.The key term of the crack tip displacement field was introduced to the basis function of NMM as a reward function.Through this,the singularity of the stress and displacement field at the crack tip is considered.The maximum circumferential tensile stress criterion was used to determine the crack initiation and propagation direction of?-? mixed type.The J integral method was used to solve the stress intensity factor at crack tip.Multiple crack propagation and crack intersection were also consider in NMM.The simplex integration was still used as unit integral.By extending the finite covering system,unit matrix and simplex integral in the original NMM,the numerical manifold program was compiled through Visual C++.4.The strength characteristics and crack propagation characteristics of rock mass with non-persistent joints were revealed.In view of the strength characteristics of the rock mass with mixed cracked of type? and ?,the compression tests of rock mass with non-persistent joints were simulated by the extended NMM.The influence of joint inclination,joint spacing and joint connectivity on the strength of rock mass was analyzed.The joint inclination,joint spacing and joint connectivity have a significant influence on the strength characteristics of the intermittent jointed rock mass.With the changing of the joint inclination a,the crack propagation mode of the rock mass will change.With the decrease of joint spacing,the strength of rock mass decreases gradually.With the increase of joint connectivity,the strength of rock mass decreases gradually.