"Three hole" in-situ test and evaluation of fault impermeability of coal seam floor
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摘要: 针对“双孔”法现场压渗测试技术采用单一方向的压渗测试，未考虑岩层裂隙的方向和各向异性特点，导致结果与复杂岩层多向受压的实际情况有较大差异的问题，采用“三孔”法现场压渗试验方法，分别对城郊煤矿二水平煤层FN-6正断层进行了正向、反向压水试验。在典型巷道布置3个钻孔分别用于注水和监测水压，第1次压水试验为正向压水试验，第2次压水试验为反向压水试验。将水压监测孔水压和压渗流量明显随注水水压同步变化的点作为起始渗透特征点，对应的注水水压确定为起始导渗水压力，若注水水压按照设计值持续增大，测渗水压较注水水压小且保持稳定，注水流量相对稳定，则表明在原始状态下，现场岩层的抗渗能力极强，为隔水层，反之为导水裂隙。正向压水试验表明：在初始状态下FN-6正断层上段的导水能力极其微弱，属于隔水层；随着注水水压升高，压水孔和水压监测孔间的岩体密集发生劈裂产生了裂隙，但规模较小，导水能力较弱，仅以微小裂隙的渗流为主。在反向压水试验过程中，导水通道数量较正向压水试验时有所增加，导致压水孔和水压监测孔间的压差降低，在水压监测孔和压水孔间形成了优势导水通道，只有超过临界导渗水压（FN-6正断层的临界导渗水压为11 MPa）时才能形成实际的渗流状态。Abstract: 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 FN-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 FN-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 FN-6 normal fault is 11 MPa) is exceeded.
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