Study On Rock Penetration Characteristics And Mechanical Behaviour Under Borehole Conditions

The rock mechanical parameters are an important basis for the design and construction of rock mass engineering.As an important technique for in-borehole testing,the borehole in-situ penetration test has become an important means of testing rock mechanics parameters with its advantages of convenience and representativeness.And presents a good application prospect.In view of this,this paper focuses on a series of problems of borehole penetration,based on laboratory rock penetration test and numerical simulation.The method of obtaining the in-situ parameters of rock mass by the penetration was studied.The influence of rock penetration test under different confining pressure and its failure mechanism were analyzed.Through theoretical and numerical analysis,the stress state around the borehole was analyzed.And the penetration characteristics and influencing factors of the borehole wall were studied.The main research results of this paper are as follows:(1)Based on the laboratory penetration test,the relationship between the uniaxial compressive strength,elastic modulus and shear strength parameters of the rock was obtained by the penetration index.The influence of rock size and indenter shape on penetration characteristics were analyzed.The results show that there is a size effect of the penetration strength of the rock in a certain range,and the size and shape of the indenter will affect the rock penetration characteristics.(2)There is a tendency for the peak load and ultimate displacement to increase as the increasing confining stress level.And the minor confining pressure plays a dominant influence in the process.The fracture pattern was influenced by the different confining pressure of the specimen.Tensile failures dominated in the process.Based on PFC,size effect of rock penetration under different confining pressures was studied by numerical simulation.(3)Through theoretical and numerical analysis,the method for determining the magnitude of the principal stress of the borehole and its direction were obtained.The existence of stress around the borehole has a significant effect on the mechanical properties of the borehole wall penetration.And the confining stress perpendicular to the loading direction is the main factor influencing the penetration characteristics of the borehole wall.