Study On Shear Creep Characteristics Of Rock-like Materials With Discontinuous Joints Under Loading And Unloading Conditions

Joints,which are widely distributed in rock mass engineering,control the mechanical properties and mechanical effects of rock mass,and are the main factors affecting the safety and stability of rock mass engineering.The shear creep properties of joints,as one of the important influencing factors of the mechanical properties of joints,are an important cause of deformation and instability of rock mass engineering.The current experimental studies on the shear creep properties of jointed rock mass mainly adopts the method of unloading confining pressure under constant axial pressure,while tests for unloading normal stress conditions and loading tangential stress conditions are less frequent.On the other hand,the actual joints are often undulating,so the effect of roughness needs to be considered.In view of this,discontinuous joints specimens with different roughness JRC were prepared with similar materials,the normal stress unloading shear creep tests and the tangential stress loading shear creep tests were carried out respectively.According to the test results,the influence of roughness on the shear deformation characteristics,strength characteristics and failure modes of discontinuous jointed rock mass was revealed.Then,the similarities and differences of mechanical properties of jointed rock mass under different stress paths were compared.Finally,based on the traditional element model and damage mechanics theory,the creep damage models under loading and unloading conditions were established.The main contents and conclusions of this paper are as follows:(1)The normal stress unloading shear creep tests were carried out for five discontinuous jointed rock masses with different joint roughness though the HYZW-500 L direct shear creep testing machine.Based on the results,the effects of different normal stress and different roughness on creep characteristics were analyzed,the variation of long-term strength with roughness was revealed,the types of failure modes of discontinuous jointed rock mass were defined based on the macroscopic section failure phenomena and microscopic SEM electron microscopy scans.By comparing with the normal stress unloading direct shear test,the similarities and differences in mechanical properties and failure modes due to different time effects were analyzed.(2)On the basis of normal stress unloading shear creep tests,tangential stress loading shear creep tests were carried out.Based on the results,patterns of deformation characteristics,strength characteristics and failure modes of jointed rock masses with roughness were analyzed.Moreover,the similarities and differences of creep properties under two different stress paths were analyzed by comparing with the normal stress unloading shear creep tests.(3)Based on the creep curve characteristics under loading and unloading conditions,the creep curves without the accelerated creep stage can be described by the traditional element model.For the creep curves with accelerated creep stage,the creep damage was considered as a function of time and stress level,and the damage creep models of the jointed rock mass were established by combining the traditional element creep model and the damage mechanics theory.The least squares method was used to identify the model parameters,and the reasonableness of the models was verified by comparing with the test results.