Experimental Study On Mechanical Properties And Fracture Evolution Process Of Hard Brittle Rock With Open Cracks

Rock mass failure in deep engineering is mostly the result of joint action of stress and crack,crack as a weak plane to a large extent influence and control the rock mass failure form and severity.The excavation of surrounding rock will destroy the balance of the original in-situ stress level and lead to rapid unloading in one or two directions.Finally,it can be simplified into unidirectional or bidirectional stress state.In addition,large-scale excavation and unloading of rock mass will inevitably cause strong disturbance of engineering rock mass,leading to gradual relaxation and crack of closed crack into open type crack,and the stability of engineering rock mass will be affected by subsequent excavation and unloading.Therefore,it is urgent to study the influence and control effect of cracks on mechanical properties of deep hard brittle rock and the evolution process of rock fracture.Based on Baihetanhydropower station and Jinping underground laboratory,the research work and achievement in this paper are summarized as follows:(1)Through the investigation and statistical analysis of a large number of field cases,the influence and control effect of rock fracture distribution on various failure modes of surrounding rock are obtained.The stress-structure controlled collapse and the structural plane opening failure are affected by both stress and fracture.The stress concentration leads to the generation of new fissure in a specific direction,which gradually extends and finally connects with the primary fissure to form the failure of potential unstable block.(2)Laboratory uniaxial compression tests and Brazil splitting tests were carried out on rock blocks with prefabricated open cracks to explore the influence and control of prefabricated open cracks on the mechanical properties and fracture evolution of hard brittle rock blocks.The results show that the dip Angle,number and location of cracks will affect the mechanical properties of hard brittle rock blocks.The peak strength,initiation strength,damage strength and elastic modulus of rock samples increase with the increase of crack dip Angle.The failure time of hard brittle rock is short,the energy is released rapidly,and the failure is extremely violent.(3)Through the laboratory uniaxial compression test on the rock block with prefabricated open fracture,digital image related technology is used to monitor the whole process of the rock block fracture evolution.By analyzing the surface displacement field and strain evolution law of the hard brittle rock mass failure process,the influence and control of the prefabricated fracture on the hard brittle rock mass fracture evolution process is explored.The results show that: The inclination,quantity and location of precast fissure will affect the fracture evolution process of hard brittle rock.The fracture of hard brittle rock mass with prefabricated cracks usually starts from the tip of prefabricated cracks.The types of newly formed cracks mainly include airfoil cracks and secondary cracks,and the types and numbers of newly formed cracks change with the dip Angle,number and location of prefabricated cracks.During the fracture process of hard brittle rock mass with precast fissure,the dip Angle,number and location of precast fissure will affect the displacement and strain evolution law.(4)Through the establishment of a two-dimensional discrete element numerical model and the uniaxial compression numerical simulation test,the results show that the numerical simulation results have a good agreement with the results of the laboratory mechanical test,and the reliability of the laboratory test results is effectively verified.Furthermore,the numerical simulation is used to verify that the crack opening degree and length affect the mechanical properties and fracture evolution process of hard brittle rock.