Study On Failure Law And Mechanism Of Cross-Fractured Rocks Under Biaxial Cyclic Loading And Unloading Conditions

The natural rock mass contains complex discontinuous weak surfaces such as faults,joints and bedding,thus showing heterogeneity and anisotropy.According to a large number of existing theoretical and practical results,this failure is mainly caused by the expansion and penetration of joints,fissures and faults under external loads.Therefore,the study on the mechanical properties and deformation and failure laws of fractured rock mass has certain practical guiding significance for understanding the mechanical properties of engineering rock mass under external load and predicting engineering stability.In this paper,the methods of laboratory test and numerical simulation are adopted: cement mortar rock materials are embedded with plastic sheets to prepare specimens with different combination forms,and after the specimens are cured for 28 days,biaxial compression and biaxial cyclic loading and unloading tests are carried out by means of YY-L600 biaxial compression test system to analyze the mechanical properties and crack penetration failure modes of specimens with cross cracks;At the same time,a numerical model similar to the laboratory experiment was established by using particle flow software PFC2 D,and the mechanical properties and fracture failure mechanism of fractured rock mass were further analyzed and verified by combining the experimental results.The main achievements of this paper are as follows:(1)Through biaxial compression and biaxial cyclic loading and unloading tests,the change law of peak strength of cross-fractured rocks under different combination forms is obtained.It is found that the strength of single fractured rock samples is generally higher than that of cross-fractured rock samples when the angle β between primary and secondary fractures is 0,but the strength of some cross-fractured rock samples is higher than that of single fractured rock samples,which shows that the occurrence of secondary fractures is helpful to provide the bearing capacity of the whole model.The strength of cross fracture sample changes in V shape with the dip angle of main fracture: when the dip angle of main fracture is 60,the rock strength is the lowest;Under the same dip angle,the rock strength is not affected by the change of the angle between the main fracture and the secondary fracture,but the influence of the main fracture on the rock strength is more obvious,and the failure mostly occurs at the end of the main fracture,which is the main controlling factor of rock fracture Compared with a single group of cross-fracture samples,multiple groups of crossfracture samples have earlier crack initiation time,earlier crack penetration time and lower peak strength;Cyclic loading and unloading has a weakening effect on rock samples.Under the same conditions,the strength of rock samples under cyclic loading and unloading is smaller than that under conventional loading There is no difference in failure modes between biaxial compression and biaxial cyclic loading and unloading.The crack propagation modes of specimens with cross cracks under biaxial compression and cyclic loading can be divided into three types: tensile penetration,shear penetration and composite penetration,and the corresponding failure models can be divided into three types: tensile failure model,shear failure model and composite failure model.(2)Based on the laboratory rock mechanics test,the PFC numerical model of rock samples with cross cracks is established,and the mechanical characteristics and evolution law of crack propagation and penetration of rock samples with cross cracks are comparatively analyzed.The numerical simulation results are in good agreement with the experimental results,which verifies the correctness of the conclusions obtained from indoor experiments At the same time,the stress-strain curve relationship,hysteresis loop characteristics and energy dissipation characteristics of the model with cross cracks under different cyclic loading and unloading times are analyzed by using the energy principle.The results show that the dissipation energy increases with the increase of cyclic loading and unloading times;On this basis,the influence of inclination angle and included angle of cross fracture on dissipation energy is analyzed.The results show that the dissipation energy of single fracture is the lowest when the included angle is 0,and the dissipation energy of main fracture is the highest when the inclined angle is 60.Different deformation stages of the specimen correspond to different energy evolution laws: the failure mode of the specimen is well compared with the concentrated release degree of energy after the peak.The more energy dissipated in front of the peak,the more cracks are produced in the rock,and the higher the fracture degree is.(3)PFC2D numerical simulation software is used to simulate and reproduce the failure process of surrounding rock after excavation of semi-circular arch tunnel with cross joints under cyclic loading and unloading conditions,and the influence of the position,length and distance from the opening on the mechanical properties and failure mode of surrounding rock is analyzed.The results show that the preset cross joints located at the vault and floor have little influence on the strength of the whole model,and the cross joints at the side wall of the opening are the most unfavorable to the stability of surrounding rock.The peak strength of the model decreases with the increase of cross joint length.The closer the joint end is to the cavern,the earlier the crack initiation occurs,and the smaller the crack initiation stress is.