Radial Depth Damage Properties of Coal Tunnels Surrounded by Rock under Excavation and Unloading

In order to study the mechanism of radial gradient damage of surrounding rock caused by deep elevated stress coal roadway excavation, a triaxial loading unloading uniaxial reloading coal sample damage test was carried out. Based on the defined damage factor and fractal theory, the wave velocity damage characteristics, fracture characteristics, three-dimensional fractal characteristics and uniaxial reloading strength characteristics of coal samples under different loading unloading paths were studied. The results show that the shift of wave velocity can better reflect the damage of coal samples. Under the same confining pressure, the damage amount of coal samples increases with the axial compression ratio index, and the confining pressure can inhibit the damage of coal samples to a certain extent; the fracture characteristics of coal samples are closely related to the loading history. The surface cracks of coal samples tend to become complex with the increase of axial compression ratio, but eventually develop to the shear plane fracture surface; the surface cracks of damaged coal samples under the radial stress gradient of surrounding rock have excellent fractal characteristics. The fractal dimension has a linear function relationship with axial compression loading ratio and wave velocity damage; from the angle of wave velocity damage and strength loss, it is expounded that the increase of confining pressure can inhibit low damage to a certain extent, but can promote the occurrence of extreme damage.