Experimental Study On Shear Characteristics Of Rock Joints Under Cyclic Loading

The structural plane is the main controlling factor of discontinuity,heterogeneity and anisotropy of rock mass,and under the cyclic load and dynamic load caused by earthquake and blasting,the deformation and strength change associated with the closure or slip of structural plane are important reasons for the instability and failure of ground and underground engineering.Therefore,the study of shear properties of rock mass with structural plane under cyclic shear load can provide a reliable basis for the stability analysis and reinforcement design of engineering rock mass.For shear test of structural plane is destructive,this paper used similar materials to replicate split and shear structural surfaces in batches.Cyclic shear test and three-dimensional scanning of rock mass with structure plane under normal stress were carried out by using self-developed coal rock shear-flow coupling test device.Under the conditions of different normal stresses,different structural plane types,different filling materials and different filling thickness,the strength,deformation and three-dimensional（3D）morphology evolution of the rock mass with structure in cyclic shear were studied.The main results were as follows:（1）By analyzing the shape of shear stress-shear displacement curves in cyclic shear,it could be divided into three types:peak type,yield type and sliding type.It was found that the shape of curves was the least affected by normal stress,followed by the type of structural plane,which was greatly influenced by filling material and filling thickness.With the increase of cyclic shear times,the maximum shear stress under different normal stress and different structural plane type all decreased in deceleration rate,but the decreasing rates were different.The maximum shear stress of structure plane filled with yellow mud of 3mm and filled with plaster of 1mm decreased in deceleration rate,while filled with detritus of 3mm and filled with plaster of 2mm and3mm,the maximum shear stress decreased firstly and then increased in deceleration rate;filling materials weaken the shear strength of structure plane.the weakening degree of different filling conditions were different.（2）The shape of normal displacement-shear displacement curves in cyclic shear was analyzed,and average dilatancy angle was calculated to measure the shear expansion（shearing shrinkage）of structural plane in different shear cycles.With the increase of cyclic shear times,under different test conditions,the average dilatancy angle of structure plane all decreased in deceleration rate,the greater the normal stress,and the greater the filling thickness,the faster the average dilatancy angle decreased,and the smaller the average dilatancy angle was in the same cycle.Under the same normal stress,the average dilatancy angle of shear structure plane decreased faster than that of split structure plane,and the samller the average dilatancy angle was in the same cycle.Under the condition of different filling materials,the average dilatancy angle of structural palne filled with yellow mud decreased at the lowest rate,there was little difference in the rate of decline when filled with detritus and plaster.（3）Using the comparation the physical figures and three-dimensional scanning maps of structure planes before and after cyclic shear to observe the visual wear degree of the structure plane in cyclic shear.3D roughness parameters:the maximum height difference S_h,the ratio of contour area to area S_A,the root mean square of relative undulating height Z₂,and the coincidence coefficient JMC were calculated for quantitative analysis.It was found that the failure mode of structural plane was less affected by normal stress than by the type of structural plane,filling material and filling thickness.The greater the normal stress was,the more serious the wear of structural face was,under the same normal stress,the wear of shear plane was more serious than that of split plane.Under the condition of different filling materials,the wear of structural palne filled with yellow mud was the lightest,while the wear of structural plane filled with detritus and plaster had small difference,when filled with different thickness,the wear degree of structure plane were nearly the same.