Study On Brazilian Splitting Failure Modes Of Rock Mass With Through Weak Cement Joint

Tensile failure is one of the main failure forms of tunnels,stope and mined-out area wall rocks in underground mines.This type of failure is generally controlled by the structure planes of surrounding rock,so it is of great significance to study the tensile failure characteristics of rock masses with structural planes.Based on the research project of key technology for disaster prevention and control of surrounding rock instability in a phosphate mine in Hubei province,this thesis studied the effect of joint angle on specimen failure strength and failure mode through Brazilian split test,and applied theoretical analysis method to explore the failure mechanism and the law of crack propagation of specimen.Moreover,the Mesoscopic finite element numerical simulation method was also used to analyze the influence of the distribution characteristics of joints on the process of specimen splitting failure.The main contents and results are as follows:(1)The peak strength of specimens with single joints and two parallel joints have similar trends.When the joint angle is 0°?60°,the peak strength of both decreases with the increases of angle ?,but when ? is 75° and 90°,the peak strength of the latter will suddenly increase;The peak strength of specimens with positive X-shaped joints shows an “S-type” increasing trend with the increase of the angle ? between the two joints;The peak strength of specimens with oblique X-shaped joints is mainly affected by joints with larger angles,and relatively less affected by joints with smaller angles.(2)The failure modes of specimens with single joints can be divided into 5 types:tensile failure of the matrix and joint occurs when ? is 0° and 15°;tensile failure of the matrix,shear failure and tensile failure of the joint occur when ? is 30°;shear failure of joints,tensile failure of matrix and joint occur when ? is 45° and 60°;shear failure of joint occur when ? is 75°;tensile failure of joint occur when ? is 90 °.The position of fracture initiation point of specimen with two parallel joints will become the middle position of each joint,and the process of crack propagation will become complicated.The failure mode of specimens with positive X-shaped joints can be regarded as the failure superposition of two single joint rock specimens;the failure mode of specimens with oblique X-shaped joint is mainly controlled by the joint with larger angle,but as the angle of joint with smaller angle increases,its influence on failure mode will also increase.(3)Under the action of tensile load,the mode of crack propagation of specimens with single joints is a combination of mode I and mode II fractures: when 0°???15°,the mode ? fractures of matrix and joint occur successively;when ?=30°,the mode I fracture of matrix,the mode II and the mode I fractures of joint occur successively;when 45°???60°,the mode II fracture of joint,the mode I fractures of matrix and joint occur successively;when ?=90°,the mode I fracture of joint occur.(4)The inclination angles of structural planes have a significant effect on the damage of the surrounding rock of the tunnel.When the inclination angle is 0°?30°,the roof and side walls of the tunnel are all damaged,and with the increase of the inclination angle,the shear failure of the structural planes will gradually increase;when the inclination angle is 45°?60°,the tunnel mainly occurs shear failure of the structural planes and tensile failure of the rock matrix in the side wall of surrounding rock,but the roof is rarely damaged;when the inclination angle is 75°?90°,the tunnel mainly occurs the damage of the structural planes in the side wall,while the roof is hardly damaged.