Experimental study on mechanical properties of sandstone with single fracture under fully-mechanized top-coal caving mining stress path

In the process offully-mechanized top-coal caving, the stress path is different from the conventional triaxial compression. In order to study the mechanical properties of roof rock mass under fully-mechanized top-coal caving mining compared with conventional triaxial compression, for single fissure sandstone with specific length and angle, carry out the loading and unloading triaxial tests of constant unloading confining pressure and variable rate loading axial pressure to simulate the stress path in fully-mechanized top-coal caving mining process, analyze the failure form of single fissure sandstone, study the relationship between ultrasonic characteristics and stress-strain and the law of crack evolution by real-time ultrasonic testing. The results show that：①Under the conventional triaxial stress path, the failure mode of single fissure sandstone specimen with different lengths is basically tensile shear composite failure, and the failure mode of single fissure sandstone specimen with different angles includes pure shear failure, tensile failure, tensile shear composite failure and X-type conjugate shear failure. Under the loading and unloading triaxial stress path, with the increase of crack length, the failure mode is tension shear composite failure -“X” non-coplanar shear failure -“X” coplanar shear failure, and the tensile crack will be replaced by secondary coplanar crack; the fracture angle is the inducement of crack initiation, the failure modes of sandstone specimen with different angles include tensile shear composite failure, single shear failure and X-type non-coplanar shear failure. ②P wave velocity can effectively reflect the stress level and microstructure of the specimen. Compared with the results that the overall P-wave velocity first increases and then decreases with the increase of axial strain under the conventional triaxial stress path, under the loading and unloading triaxial stress path, the overall P-wave velocity increases with the increase of axial strain, while the growth rate slows down with the increase of stress level; when the specimen reaches the yield point, the specimen density reaches the maximum and the P-wave velocity reaches the maximum; when the specimen reaches the peak strength, the phenomenon of internal crack crossing and mutual combination intensifies, the P wave velocity fluctuates obviously; with the increase of fissure length, the P wave velocity stable section decreases gradually, with the increase of the fissure angle, the P wave velocity stable section increases first and then decreases. ③According to the change of crack width reflected by P wave velocity, the crack evolution stage under conventional triaxial stress path can be divided into four stages：crack compaction stage, crack development stage, crack stability stage and post failure instability stage. While under loading and unloading triaxial stress path, it can be divided into three stages：crack compaction stage, crack stability stage and crack propagation stage. The crack stability stage decreases with the increase of fissure length, and the fissure angle has almost no effect on the crack stability stage.