Joint Propagation Modes And Fracture Mechanisms Of Jointed Rock Slope

The jointed rock slope,as a kind of common rock slope,its mechanism of deformation and failure and the slide path are uncertain due to the absence of obvious control structural surface.In this thesis,the slope of the Maji hydropower station in Nujiang River is taken as the analysis case,and the mechanical and geometrical effect of the joint expansion is taken as the core.The fracture mechanism and the fracture path of the jointed rock mass are deeply analyzed and the corresponding relation between the joint expansion mode and macroscopic rock mass fracture path is established.Finally,the rock mass structure and stress condition are used as the link to the slope engineering.The results of the research in this thesis can not only improve the theory of rock mechanics,but also guide the practical engineering,which is of vital significance in both theory research and engineering application.The main results are as follows:(1)Based on the theoretical analysis,the mechanics mechanism of joint extension is studied.The main work includes the extension criterion of joint,the calculation of plastic zone and the analysis of extended path,and a series of achievements are obtained.Based on the state of stress distribution at joint ends,the extension criterion of joint under composite stress conditions is given,and the corresponding relationship among the extension type of joints,the stress states and geometric conditions is established.It is considered that under the condition of compression-compression load conditions,the joint angle within the range of 20 ° <β <40 °,the shear failure is restrained and the tensile failure is dominant while outside of this range,the tensile failure is restrained and shear failure is dominant.Under the combination of tension-compression load,the shear failure is restrained,and most of the inclination angle range is in the area of tensile damage.From the point of view of stress distribution of the joint end,the calculation theory and method of the existing plastic zone are modified,and the calculation method of the plastic zone concerning the stress relaxation is proposed,and the calculation method of the plastic zone of the joint end about the different types of failure modes is considered.Finally,the joint expansion path is analyzed from the perspective of the extension criterion and the plastic distribution.The analytical calculation method of the joint extension path is put forward from the view of stress and plastic zone respectively.(2)Based on the stress zoning of the joint surface,several common combinations of joint sections are analyzed,and the mechanical significance is analyzed from the angle of stress shielding area and enhancement area.According to the stress zoning of joint,it is considered that under the load,the upper and lower areas of the joint will form a certain range of stress shielding area.At the same time,the end of the joint area will form a stress enhancement area(the concentrated area),which is also the most easily-expanded area of the joint.The combination problem of joint is transformed into two stress regional combinations of joint and several kinds of joint combination models considering both geometric and mechanical significance are summarized.(3)Based on the combination of stress zoning,the physical simulation of the joint expansion model is performed and the simulation experiment of the corresponding expansion mode is carried out.From the point of view of fracture site observation and the analysis of internal mechanics mechanism,the expansion mode and influencing factors of joint was discussed,and the general pattern of joint expansion is summarized.For the single crack,the expansion modes under different stress conditions is proposed: under uniaxial compression,the expansion of the wing crack is sufficient due to the unconfined stress,and the stress of the joint end increaseswing crack like flying geese after the wing crack is extended.Ultimately,the late formation of the shear crack is connected with the wing crack to formthe macro-fracture of penetrating;Under the condition of biaxial compression,the potential slip tendency of the joint is suppressed due to the existence of lateral stress restraint,especially for lager inclination angle,and the end of the joint is difficult to form the concentration area of tensile stress,resulting in the limitation of the expansion of the wing crack.Therefore,under the condition of biaxial compression,the wing crack first forms the geese-like arrangement at the end of the joint,and finally the geese-like crack passes through,forming macro-rupture.For the double joints,the lateral pressure has a certain limiting effect on the expansion of joints.In general,the greater the lateral pressure coefficient is,the more difficult the joint expansion is,and the higher the joint starting,expansion and penetration stress is,especially for the starting stress of the joint,the lateral pressure coefficient is the most sensitive.The sensitivity of the expansion and penetration stresses to the lateral pressure coefficient is slightly lower.The joints spacing has some influence on the starting,expansion and penetration stress of joint,and from the macroscopic stress(expansion and penetration),the greater the joint spacing is,the greater the expansion and the penetrating stress is.However,when different geometric parameters are small,the law is not the same for the starting stress of joints.There is a certain relation between the expansion and the penetration stress of joint and the expansion mode of joint under different geometric conditions.Specifically,when the expansion of joint is mainly the wing crack,the expansion and the penetrating stress of the joint are lower and when the expansion of joint is mainly shear crack,the expansion and the penetrating stress are higher.According to comprehensive experiment and theoretical analysis,when the inclination of the rock bridge is 45° and 90°,the expansion of the rock bridge is mainly based on the wing crack so its expansion and penetration stress is higher than that of 0° and 135°.(4)In the aspect of the structural effect of joint expansion,the model tests of large-scale jointed rock mass expansion are carried out,and the relationship among joint expansion mode,rock mass structure and macroscopic fracture of rock mass is initially established.The main results are as follows:The jointed rock mass will form one or several crack development belts during the process of penetrating.In general,the smaller the joint spacing is,the greater the possibility of the development of multiple cracks will be.Finally,the rock mass will follow one of the macroscopic fracture surface.The smaller the joint spacing is,the greater the connectivity rate is and the penetration of the rock mass is mainly through the expansion of the wing crack.The larger the joint spacing is,the smaller possibility the connectivity is and the shear crack rock mass is dominant.(5)Based on the analysis of the fracture path of jointed rock mass,a method of analyzing the expansion path of jointed rock mass based on flow field theory is proposed.According to the principle of similarity,the process of stress transfer of rock mass under load is equivalent to the condition of fluid flow in a planar potential flow.And the flow field equation of the fluid flowing around the plate in the plane potential flow is given.Finally,the fracture path of jointed rock mass is calculated,and the calculated results are in good agreement with the experimental results.(6)The rock mass evaluation system based on joint failure mode is established,which is a reasonable way to reflect the relation between joint expansion and macro rock mass destruction.Finally,the actual slope is used to verify the above research method,and the results obtained are consistent with those obtained by numerical calculation.Based on the typical structural evaluation system,the failure modes of the typical joint rock slope at home and abroad and the structural parameters of the rock mass are summarized and analyzed.The corresponding structure types of different failure modes are given,and the existing evaluation system of rock mass is modified.The failure mode of rock mass is incorporated into its rating system,and the rock mass grading system based on failure mode is put forward.The system can effectively connect the expansion mode of the joint and the macro-damage of the slope.Based on the modified rock mass evaluation system and the numerical calculation method,the fracture pattern of the joint rock slope is analyzed.It is found that the joint model and the stress field in the rock mass jointly determine the failure mode of the slope.If the joint density is large and the lateral pressure is small,the jointed rock mass is mainly formed by the tensile failure mode and the macroscopic rupture.Otherwise,it is easy to take the shear failure mode as the main method.Finally,the potential fracture surface of the slope is calculated and analyzed by the flow field theory.The results show that there are many potential rupture surfaces in the slope,and it is consistent with the particle flow calculation and macroscopic analysis of the rock mass structure.