Experimental Study On Post-peak Mechanical Characteristics Of Rock Under The Condition Of Loading And Unloading Confining Pressure

In order to give full play of the bearing capacity of rock mass to maintain stability in underground engineering, to reveal the evolution of plastic zone in rock mass and to do optimum design for support of the rock engineering, it is necessary for scholars to study the post-peak residual strengths of rock. By conducting a series of confining pressure uninstall- recovery test at the phase of residual strengths of rock, we study the mechanical characteristics of rock in the residual phase. We simulate the post-peak sliding friction deformation of rock with single structure plane and the interlayer sliding friction deformation characteristics in the layered surrounding rock when tunneling. The main research contents and results of this paper are as follows:(1) The residual strength of rock are affected by the peak strength and the confining pressure, and the residual strength is more sensitive to the low confining pressure. The reason is there is little contact between fracture planes at low confining pressure, and the interaction force is small. With the increase of confining pressure, the contact area between fracture planes increase, the stress-strain relationship also change from brittle stress drop to plastic slip. It is mainly the process of cohesion losing from the peak strength to the residual strength for shale and the reduction of internal friction angle is very small.(2) In the loading and unloading confining pressure test at the residual stage of rock, the effect of unloading confining pressure on the mechanical characteristics at the residual stage for rock have the same rule in the three test plans(many times unloadingmany times loading confining pressure test, many times unloading- once loading confining pressure test and cyclic loading and unloading test): In the stage of initial residual we limit the axial displacement, when the confining pressure is unloaded with uniform speed, the corresponding axial stress decreased homogeneously; after the confining pressure reach stable, we applied axial displacement load and the axial stress keep essentially stable, The axial stress-strain relationship present plastic deformation features.(3) Loading confining pressure also has same affect rules in the three test plans: the axial stress-strain curve can be divided into three stages, the lateral elastic deformation stage, the Axial Elastic deformation stage and axial Elastic deformation stage.(4) The mechanism of residual stage can be simplified as the rock matrix with elastic deformation properties showing residual bearing capacity by friction sliding deformation between fracture planes.(5) According to the mechanical properties and deformation mechanism of the rock samples in the residual stage, we put forward a single structure plane post-peak sliding friction model for rock with reasonable simplification, and simulated the process of general tri-axial test, the results show that the model can descript the test process well.(6) Based on the sliding friction model, we simulate and analyze the process of tunnel excavation; the results show that the slip deformation along weak plane in the rock mass of post-peak stage is the main cause of deformation in the surrounding rock.