Radial seepage-axial stress characteristics of hollow rock sample and seepage mutation mechanism of roadway surrounding rock

To explore the seepage mutation mechanism of roadway surrounding rock under confined groundwater, a series of laboratory tests and model studies were conducted for the seepage-stress characteristics of roadway surrounding rock. A set of seepage-stress test system for hollow rock samples was developed, which broke through the limitation of traditional test conditions that only conduct axial seepage flow and achieved the simultaneous operation of axial stress and radial seepage. Based on the new test system, the radial seepage tests of several groups of hollow red sandstone are carried out by using bore diameter(5-20 mm) and hydraulic pressure(0.5-2.5 MPa) as variables. The permeability evolution law and radial seepage characteristics of hollow rock samples are obtained. The test results show that erosion damage of radial seepage causes the attenuation of mechanical parameters such as cohesion and internal friction angle of rock samples, with the increase of water pressure, the erosion damage of seepage becomes stronger, the formation time of fracture through channel in the hollow specimen is shortened, and the seepage mutation of seepage occurs in advance. The hollow specimen with large bore size is more sensitive to axial stress, the internal structure is more easily damaged by radial seepage erosion damage, and the time of seepage mutation is shortened. The permeability of the rock specimen experiences three states during the test:(1) the pore seepage state with pore structure as the main seepage medium,(2) the unsteady seepage state with the partial fracture structure as the main seepage medium, and(3) the seepage mutation state with fissure structure as the main seepage medium. When the permeability reaches the peak value, the seepage medium of the specimen is the through fracture structure caused by the influence of water pressure, the peak permeability increases linearly with the increase of water pressure, and has no obvious relationship with pore diameter. According to the change of fracture volume, the characterization model of seepage mutation of hollow rock sample is established, and the calculated results of the model are in good agreement with the experimental data. According to the critical points of yield stress/strain and peak stress/strain, the permeability evolution curve was divided into three phases:(1) permeability increase evolution phase,(2) permeability rapidly increase evolution stage, and(3) post-peak permeability evolution stage. Among them, the sudden increase in permeability evolution stage is the control stage of water inrush disaster occurrence, before that, measures such as grouting and blocking, water delivery and pressure reduction shall be taken to prevent disasters.