Study On Damage Mechanism Of Rock Mass Based On Joint Nonlinear Mechanical Response

The strength,deformation and stability of jointed rock masses were closely related to its multi-scale damage mechanism.Not only the strength and stiffness of the rock mass,but also the cracking process of the rock matrix(the initiation,propagation and coalescence of new discontinuities),were affected by the nonlinear mechanical response of the joints(such as their opening,closing and sliding).In this thesis,through the combined method of laboratory experiment,theoretical analysis and numerical simulation,the mechanical response of rock mass with a single joint as well as with multiple joints and structure of rock mass were investigated,in which the influence of nonlinear mechanical response of the joints on the multi-scale damage mechanism was studied.The main research results were obtained as follow:(1)In the case of no joint interaction,the influence of the nonlinear mechanical response of the joint at different orientation on the macro-mechanical behavior of the jointed rock mass was investigated.four failure modes were found;the relationship between opening/closing and sliding of the joint,as well as cracking of the matrix and the AE characteristics was analyzed;the nonlinear mechanical response of the joint and the microscopic damage mechanical response of the rock matrix are explained.(2)In the case of joint interaction,the PFC analysis method was established for the binary phases model,and PFC modeling was carried out for the rock mass with multiple joints.The damage mechanism and especially the interaction mechanism of the joints and the rock bridges were studied.(1)A large amount of local measurement circles were placed along the pre-defined Measurement Lines(ML),and stresses and joint response parameters at different scales(the measurement circles,the MLs and the whole specimen)were defined and calculated.Based on discontinuous quantity defined in the measurement circle,loadcarrying capacity of rock bridges and joints along the MLs of the joint planes were deduced,and the PFC analysis method of the binary phases model was established,and the corresponding Fish program was written and compiled.(2)The damage mechanism was studied for the rock mass with the combined variation of geometrical parameters of the joint netwrok,i.e.variation of joint spacing(s)-joint inclination angle(β),and variation of the number of jointed segments(m)-joint inclination angle(β)-joint continuity factor(k).Because of the interaction of joints,the nonlinear mechanical response of joints was affected by the joint geometrical parameters and the interaction mechanism between the two phases,i.e.,the rock bridge phase and the joint phase,were revealed,it was found that the peak stresses of the two phases,i.e.,the rock bridge phase and the joint phase on the joint plane,may not be reached at the same time.On the central Measurement Line,the interaction between the two phases can be divided into three stages,i.e.,(I)Elastic deformation dominated stage(before the peak stress of the rock bridge phase),in which the rock bridge phase carries most of the load,(II)Inelastic deformation developing stage(between the peak stress of the rock bridge phase and the last peak stress of the joint phase),in which the two phases carry the load together,and(III)Residual deformation stage(after the last peak stress of the joint phase),in which the joint phase carries most of the load.(3)Physical model test with AE monitoring was carried out for a rock structure under uniaxial compression,i.e.,the jointed rock mass containing a circular hole.The macro-mechanical properties,AE characteristics and their relationships were studied.(1)The macro-mechanical properties of the rock mass structure(such as strength,deformation and failure mode)were analyzed.Totally,three types of stress-strain curves and four failure modes were found.(2)AE characteristics were analyzed and related to the different crack evolution process of the specimens.It was found that the joint opening/ closing and sliding,as well as the failure around the hole can be corresponded to the AE characteristics.By using digital image processing methods,cracks on the surface of the specimen was identified and the corresponding Oda damage variable F was calculated,and the correlation between the cumulative of AE hits(HPS)and F was investigated.(4)Based on the PFC analysis method of the binary phases model,PFC modeling was carried out for the rock structure models by placing a large amount of local measurement circles along the MLs and around the hole.The damage mechanism of rock structure and the interaction mechanism of the hole,joints and rock bridges was studied.(1)the failure characteristics of the rock structure models were analyzed by the relative displacement of particles in the PFC model,The six simplified crack types were found,including three types of tensile cracks and three types of shear cracks.For the rock structure model with medium inclination angles,the damage zone in the matrix is mainly composed of tensile cracks,and the damage zone distributed along the joints are composed of shear cracks.The damage zone of the matrix and joints of other rock structure models are mainly composed of tensile cracks.For all rock structure models,the damage zone on the left and right sides of the circular hole is shear cracks.(2)The nonlinear mechanical response of the joints around the circular hole were analyzed.For the rock structure models with a low or medium joint inclination angles,the joints adjacent to the circular hole in the middle ML were closed firstly to carry the load,and the joints adjacent to the circular hole in the upper and lower MLs were lastly closed to carry the load,which are completely opposite in for the rock structure models with high joint inclination angle.(3)The influence of joints on the tangential stress at the left side,right side,the top and bottom of the circular hole were analyzed.It was found that the maximum value of the tangential stress of the circular hole left and right sides increases or shows an overall increase with the joint inclination angle increases.At the top and bottom of the circular hole,three different types of the tangential stress evolution curve were presented with the change of joint inclination.The first type showed that the tangential stress of the circular hole was tensile stress in the early stage and mainly compressive stress in the later stage.The second type showed that the tangential stress of the circular hole was mainly compressive stress in the whole stage.And the third type showed that the tangential stress of the circular hole was mainly tensile stress in the whole stage.