Numerical Simulation Study On Shear Failure Mechanism Of Fractured Sandstone Under Normal Unloading Condition

Unloading failure of slope engineering and underground engineering is mainly caused by natural disturbance and artificial disturbance,such as valley cutting,physical-chemical erosion,surface landslide and engineering excavation.At present,scholars at home and abroad mainly focus on unloading failure in three-axis constant axial pressure unloading confining pressure,axial pressure unloading confining pressure,equal proportion unloading,etc.Besides that,most of them use complete samples.There are few researches on the shear failure of fractured rock mass under normal unloading,and the research on this aspect is basically based on the analysis of mechanical properties and failure patterns by means of tests,while the research on the crack growth process and failure mechanism is rarely involved.In addition,through the collection of data,it is found that under the stress path of direct shear,triaxial and so on,the crack growth process and failure mechanism of rock mass are mostly studied by numerical simulation,and at present,PFC particle flow program is widely used.Based on this,this paper elaborates that PFC2 D particle flow program is used to analyze and explain the crack growth and evolution process and failure mechanism of single and double crack models under normal unloading condition on the basis of shear failure test of single and double cracks under normal unloading condition.The main research contents and achievements of this paper are as follows.(1)The meso parameters was calibrated and then obtained a set of meso parameters suitable for fracture red sandstone model on the basis of laboratory test.In the following chapters 3.1 and 3.2,the method of combining test and numerical simulation was used to check the specific applicability of the meso parameters.(2)The final failure diagram of single fracture model and double fracture rock bridge were analyzed under different geometry and loading conditions by using the method of controlling single variable,and then elaborated the change rules of failure mode and crack shape.(3)The failure patterns of single and double fracture test specimens and numerical models were compared with different fracture obliquity,and the differences between the test and numerical results in failure mode and crack shape are expounded.(4)The crack propagation mechanism and crack initiation,penetration and other crack propagation characteristics were described by means of the combination of macro micro crack change and micro force chain change on foundation that the analysis of the failure process diagram of single crack model and double crack rock bridge with cohesive force field under different geometry and load conditions.(5)The relationship between the total number of microcracks and the normal strain in unloading stage and each factor was expounded by analyzing the curve of the total number of microcracks in single fracture model and double fracture rock bridge under different geometry and load conditions.In addition,the relationship between failure mode and different geometry and loading conditions could be revealed more accurately by analyzing the ratio of pull-down and shear microcracks.(6)The mechanism of microcrack generation and model failure is revealed on foundation that the micro displacement field of three basic failure modes in single fracture model and double fracture rock bridge.