Research On Energy Evolution And Fracture Mechanism Of Compressed Sandstone During Deformation

With the rapid exploitation of mineral resources in China,the shallow coal resources are gradually exhausted,while the deep coal resources are in urgent need of exploitation.Deep rock exists in the complex mechanical environment of "three high and one disturbance".Under the background of frequent rock dynamic disasters in the mining of deep coal resources,it is difficult to accurately predict and prevent disasters,which brings great challenges to the safety of engineering construction and personnel.It is found that the traditional method based on strength criterion and stress-strain relationship to analyse the mechanism of dynamic disaster in deep rock mass has some limitations,while the method based on energy theory seems to more suitable.Based on laboratory experiments,theoretical analysis and numerical simulation,the research on the energy evolution and fracture mechanism of compressed sandstone during deformation are carried out by four aspects,which mainly includes the experimental research on the evolution of the strain field and the energy evolution law of the compressed sandstone,the research on the damage constitutive model of the compressed sandstone,the theoretical research on the fracture mechanism of the compressed sandstone.The main achievements are as follows:(1)The evolution cloud map of the strain field of the compressed sandstone based on DIC method can reflect the expansion path of the strain localization zone,and thus predict the failure of the sandstone.As a brittle rock,sandstone has obvious effect of loading rate.(2)Under uniaxial compression,the crack starts from the tip of the prefabricated crack,and the stress is more concentrated in the tip of the prefabricated crack.At the same time,the crack tip is affected by tensile stress,and the closer to the tip,the more obvious the tension is.(3)Considering that the rock still has elastic modulus in the residual stress stage,the consumption coefficient of elastic modulus α is introduced to represent the consumption of elastic modulus in the residual stress stage.Based on the modified damage variables and the principle of energy conservation and energy dissipation,rock damage constitutive models under uniaxial and triaxial conditions were derived.(4)Based on the fracture mechanics analysis method,the stress of flat oval crack under compression is analysed.When subjected to the far field uniaxial compression,when the direction of pressure is parallel to the long axis of oval crack,the axial stress of crack in rock is tensile stress,and the maximum tensile stress is close to the crack tip.(5)Based on Maxwell model and Inglis formula,the internal stress field of rock under pressure is deduced theoretically.According to Maxwell equation,when reaching the uniaxial compressive strength,the induced tensile stress induced by cracks in rock is close to 4 times of its tensile strength,which is enough to cause tensile failure of rock samples perpendicular to the axis of rock,while the conclusion by applying Griffith criterion and Inglis formula is nearby 8times of the tensile strength,which is undoubtedly enough to cause the tensile fracture of the rock.(6)Based on the establishment of the rock column model,assuming that the failure of rock column is caused by the expansion of a large number of micro-cracks in rock,and the corresponding relationship between crack behavior and energy density is established,the instability failure condition of rock column caused by weak disturbance is deduced as follows:the instability failure occurs in the post-peak stage,and the post-peak modulus is required to be no less than the pre-peak modulus.(7)By using the finite element numerical simulation program to simulate the deformation and failure process of rock under compression.Compared the calculation results with the experimental results,it is found that the program developed in this paper has well applicability.It is further applied to the simulation study of tunnel excavation process,and the failure law and energy evolution law in tunnel excavation process are obtained.This paper has 96 figures,14 tables and 168 references.