EXPERIMENTAL STUDY ON MECHANICAL PROPERTIES AND FAILURE CHARACTERISTICS OF COAL ROCK WITH PREFABRICATED FRACTURE UNDER STATIC LOADING

Coal rock with prefabricated fractures has different mechanical properties and failure characteristics from intact coal rock under external loading. The three-dimensional macro fracture prefabrications of intact coal rock samples with different dip angles(30°, 45°, and 60°)were carried out by diamond wire cutting method. Through uniaxial and triaxial compression tests and combined with acoustic emission monitoring techniques, the strength, deformation, failure mode, fracture propagation law and acoustic emission characteristics of the fractured coal rocks under different loading conditions were studied. The law of instability and failure of fractured coal rock with impact tendency were explored. The results show the follows. The strength and elastic modulus of intact coal rock have a good linear relationship with confining pressure. The strength increases by 400% as the confining pressure increases from 0 MPa to 16 MPa. The failure mode is shear failure and the shear angle increases. The existence of prefabricated fractures reduces the strength of coal rock. The strength of 30°, 45°and 60°fractured coal rocks decreases by 13%, 24% and 37%, respectively. When the failure occurs, the new wing fractures expand from the fracture tip along the direction of maximum principal stress and join the prefabricated fracture, resulting in instability failure of the coal rock. When the confining pressure of fractured coal rock increases to a certain level, the "reverse confining pressure effect" occurs, that is, the greater confining pressure is, the more obvious strength deterioration is. When the confining pressure reaches 8 MPa, the strength of three kinds of fractured coal rocks decreases by 200%. The acoustic emission intensity of the intact coal rock reaches the extreme value in the failure stage. The acoustic emission signal surge point of fractured coal rock is generated in each stage due to the fracture expansion and development, according to which the failure degree and stress state of fractured coal rock can be judged.