Theoretical Analysis And Numerical Simulation On Creep Damage And Fracture Of Open Discontinuous Jointed Rock Mass

Jointed rock mass is widely found in nature,and its mechanical properties are complex,which increase the difficulty of engineering design and uncertainty of engineering construction.Jointed rock mass will show obvious creep behavior under long-term load,which will have a great influence on the strength and deformation characteristics of rock mass.Meanwhile,under the influence of creep effect,the state of cracks inside rock mass will change with time,cracks expand and penetrate,eventually lead to the failure of the engineering rock mass.Therefore,it is of great significance to study the joint closure and crack propagation under creep condition for further understanding the mechanical properties of jointed rock mass.In this paper,firstly,based on the theory of damage mechanics and fracture mechanics,the creep deformation expressions of rock mass in the stage of joint closure and crack propagation are derived by using the theoretical derivation method;secondly,the uniaxial compression test of jointed rock mass is simulated by using the discrete element software PFC,and the instantaneous and creep mechanical properties of rock mass are studied;finally,the deformation and failure modes of surrounding rock mass after tunnel excavation are analyzed.The main research results are as follows:1.The progressive failure process of open jointed rock mass under compression is divided into two stages:joint closure stage and crack propagation stage.The theoretical model of two stages is established,and the creep expression of jointed rock mass with time under uniaxial compression is obtained.The accuracy of the theoretical model is verified by comparing the theoretical results with the experimental results.The theoretical model can be used to predict the creep failure time and deformation law of jointed rock mass.2.PFC software is used to simulate uniaxial compression test of jointed rock mass.The contact model between particles is defined as linear parallel bond model.Parameters in the model are calibrated by "trial and error method".The influence of joint length,joint angle,rock bridge length and joint spacing on peak strength and failure mode of rock is discussed.The failure modes in numerical simulation are divided into three types:tensile failure through joint surface,shear failure along joint surface and tensile failure along joint surface.The numerical simulation results show that the peak strength of rock mass with joint angle of 60°is the lowest;with the increase of joint length,the peak strength of rock mass will decrease gradually,and the failure mode of the specimen will be changed from tensile failure to shear failure;the influence of joint spacing on the failure mode of rock mass is small,and the peak strength of rock will gradually increase with the increase of joint spacing.3.The numerical simulation of creep test is carried out by using stress corrosion model.The creep behavior of jointed rock mass under graded loading is studied,including the relationship between strain and time,failure mode,the change of crack number with time,the change of creep peak strength and crack initiation stress with joint distribution.The curves obtained by numerical simulation are basically consistent with the experimental curves.The crack initiation stress and creep peak strength of rock mass with different crack distribution are different.When the load exceeds the crack initiation stress,the number of cracks increases gradually with time.When the load reaches the last level,the number of cracks and creep deformation increase rapidly,and the rock mass is destroyed as a whole.The maximum value of creep peak strength and crack initiation stress of rock mass appears when the crack angle is 90° and the minimum value appears when the crack angle is 60°.With the growth of rock bridge length and joint spacing,some cracks will be close to the left and right edges of rock mass,and the rock mass is prone to local failure,which leads to the decrease of creep peak strength.4.After the tunnel excavation,the cracks around the tunnel will increase with time.Under the influence of different lateral pressure coefficient,the number and location of cracks are different,and the scope of cracks is also different.At the same time,the tunnel surrounding rock will produce larger creep deformation,and the deformation is related to the stress level.Under the same conditions,the creep deformation of the tunnel surrounding rock on the side with larger stress is greater than that on the side with smaller stress.