Permeability characteristics of fracture zone by high-pressure water injection tests

The permeability characteristics in different fracture zones of roadway floor are studied by the high-pressure water injection tests. A total of three boreholes are arranged and five water injection tests are carried out. During the tests, water is injected into one bore hole and the water pressure is monitored in the other bore holes. The step wise increase and decrease of the water injection flow rate is adopted. The flow rates of water injection and the pressures of water injection and monitoring bore holes are recorded. The results show that: (1) At the stage of the increasing water flow rate, the water injection pressure in the fracture zone exhibits the process of "rapid increase, sudden drop and small fluctuation", which conforms to the characteristics of typical hydraulic fracturing pressure curve of rock mass. (2) The difference of fracturing pressure in a single hole is small, the fracture in the fracture zone has strong healing capability, and the repeated water injection tests produce new cracks. (3) The water-injection fracturing pressure and the initial permeability coefficient in the fracture zone decrease and increase with the decrease of the distance between the test point and the roadway floor, respectively. (4) Under the same water injection flow rate, the fracturing width at the stage of the decreasing flow rate is significantly larger than that at the stage of the increasing flow rate, and the water injection after fracturing will cause the fracture zone to be washed gradually. The ratio of water-blocking intensity of the fracture zone to water pressure of the aquifer is defined as the safety factor of water-inrush prevention in the fracture zone, the risk evaluation method for water inrush in the fracture zone is established, and the prevention and control measures for the water inrush in the fracture zone are put forward.