Study On Shear Behaviour Of Rock Joints Based On Discrete Element Simulation

The mechanical behaviors of natural rock mass are affected by the mechanical and geometric characteristics of joints.In the surface or shallow layer,the shear failures along the joint plane are the most common failure modes of jointed rock.Therefore,the researches on the shear mechanical characteristics of joints are of great significance for the design and construction of practical rock engineering.In the past few decades,researchers have made remarkable achievements in analyzing the shear behaviors of joints from macroscopic or microscopic perspectives through direct or indirect methods.However,there are still some problems in the study of rock joint shear.On the one hand,most of the research focuses on the shear behavior at low normal stress level,but there is still a lack of research on medium and high normal stress;On the other hand,as one of the important mechanical properties of rock joints,there is no clear and simple method to determine the transitional normal stress.In this paper,the numerical models are calibrated by the results of indoor experiments,and the direct shear simulations of regular sawtooth joints and irregular rough joints with different roughness are carried out by using the discrete element simulation software PFC2D,to study the shear mechanical properties and failure forms of rough joints.Based on the simulation results,the properties of transitional normal stress are discussed,and the calculation method of transitional normal stress is determined.The main results are as follows:(1)Based on the discrete element simulation software PFC2D to calibrate the parameters and generate the model,and the mechanical properties and failure characteristics of different joints are analyzed.The macro-responses through the simulation tests are close to the calibration parameters of the indoor experiment results,and the uniaxial simulation,compression simulation,and plane joint shear simulation are carried out respectively.The shear box genesis approach avoids abnormal simulation phenomena such as particle interlocking and particle overlap.Generated sawtooth joints with i0=5°?15°?25°?35°and standard JRC joints with JRC=0-2,4-6,10-12,14-16,18-20.The models are carried out direct shear simulations with normal stress ranging from 0.5 MPa to 27 MPa(uniaxial compressive strength of 27.4 MPa).The shear characteristics and crack were real-time monitored.(2)For rough joints,normal stress and joint roughness are the main factors affecting shear behavior.The increase of joint roughness and normal stress will promote the shear behavior.The increase of normal stress will change the failure mode from wear to gnawing,and suppress the dilatancy.With the increase of roughness,the gnawing failure is easier and the dilatancy is more obvious.Under high normal stress,vertical through cracks similar to compression failure will occur in joint shear failure,indicating that compression dominates the failure mode under high normal stress.The through crack generally occurs at the root of the first undulating body of the joint surface in the movement direction.The normal stress required to produce vertical through cracks is less than JCS and decreases with the increase of roughness.(3)The transitional normal stress ?T is related to the uniaxial compressive strength ?C of the rock and the joint roughness.Fit the dilation angle function to the shear simulation results of the jointed rock model,and determine the transitional normal stress corresponding to different functions.This paper proposes a new formula to estimate and calculate ?T of the rough joints.For the sawtooth joint,?T is connected with the sine value of the joint angle tan i0 and the uniaxial compressive strength?C.And for the irregular rough joints,the coefficient a is introduced,and ?T is connected with the joint roughness coefficient JRC.By comparing the calculation results of the formula with the results of numerical simulation and laboratory experiments,it is proved that the formula can better reflect the transitional normal stress of rock joints.