"Three hole" in-situ test and evaluation of fault impermeability of coal seam floor

The double-hole method in-situ pressure permeability test technology adopts single-direction pressure permeability test without considering the direction and anisotropy of rock fissures. The results are quite different from the actual situation of multi-directional compression of complex rock strata. The 'three-hole' in-situ pressure permeability test method is adopted to conduct forward water pressure test and reverse water pressure test on F_N-6 normal fault of the second horizontal coal seam in Chengjiao Coal Mine. Three holes are arranged in a typical roadway for water injection and water pressure monitoring respectively. The first water pressure test is a forward water pressure test, and the second water pressure test is a reverse water pressure test. The point where the water pressure and pressure permeability flow volume of the water pressure monitoring hole change synchronously with the water injection pressure is taken as the initial permeability characteristic point. And the corresponding water injection pressure is determined as the initial permeability pressure. If the water injection pressure continues to increase according to the design pressure, the permeability pressure is smaller than the water injection pressure and remains stable, and the water injection flow is relatively stable, it indicates that in the original state, the in-situ rock stratum has extremely strong permeability resistance and it is an aquifuge. On the contrary, it is a diversion fissure zone. The test results of the forward water pressure test shows that the water conductivity of the upper section of the F_N-6 normal fault is extremely weak in the initial state. This section is an aquifuge. With the increase of water injection pressure, the rock mass between the water pressure hole and the water pressure monitoring hole is densely split, resulting in fissures. However, the scale and water conductivity are relatively small, and only seepage from micro fissures is dominant. During the reverse water pressure test, the number of water diversion channels is increased compared with that in the forward water pressure test. This leads to the decrease of the pressure difference between the water pressure hole and the water pressure monitoring hole. And the dominant water diversion channel is formed between the water pressure monitoring hole and the water pressure hole. The actual permeability state can be formed only when the critical permeability pressure (the critical permeability pressure of F_N-6 normal fault is 11 MPa) is exceeded.