The height of the development of a caved zone (CZ) and a water-conducting fractured zone (WCFZ), hereinafter “CZ and WCFZ”, in mining of soft coal with a soft mine floor, hard mine roof and different thicknesses of the coal seam, hereinafter “two soft and one hard” unstable coal seam, is of interest in the long wall coal mining. An observation borehole was drilled above a coal seam with an average thickness of 4.0 m. By using borehole imagery technique and comparing the borehole images at different distances from the working face, the height of “CZ and WCFZ” at the working face is calculated under the condition of insufficient mining. Based on field observations, a 3D numerical model is developed under the condition of full mining, the height of CZ is calculated to be 14.4 m and the height of WCFZ calculated to be 67.0 m. The numerical model also shows changes in the vertical stresses in the overburden strata above the roof of the coal seams and it is related to the development of the “CZ and WCFZ”. At the same time, based on the key strata theory and multiple linear regression, the height of WCFZ in the working face of this case study is predicted to be 67.0 m and 64.8 m respectively, which is close to the numerical simulation results. At the same time, 30 groups of measured data are collected to analyze the main factors affecting the WCFZ, and the influence of different lithology characteristics of overburden strata on the WCFZ is mainly discussed. The results show that the height of WCFZ is hard–hard > hard–soft > soft–hard. The results provide important practical guidelines for the prevention and control of roof water hazards in coal mine thus improving the safety of mining.

中文翻译：

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“两软一硬”不稳定煤层塌陷导水裂缝区的确定

在开采具有软矿层，硬矿顶和不同厚度煤层的软煤的开采中，陷落区（CZ）和导水裂隙区（WCFZ）（以下简称“ CZ和WCFZ”）的发育高度煤层，以下简称“两软一硬”不稳定煤层，在长壁煤层开采中引起人们的兴趣。在煤层上方钻了一个平均厚度为4.0 m的观察井。通过利用井眼成像技术，比较距工作面不同距离处的井眼图像，在开采不充分的情况下，计算出工作面上“ CZ和WCFZ”的高度。根据现场观察，在充分开采的条件下建立了一个3D数值模型，计算出CZ的高度为14.4 m，WCFZ的高度为67.0 m。数值模型还显示了煤层顶板覆盖层垂直应力的变化，这与“ CZ和WCFZ”的发展有关。同时，基于关键层理论和多元线性回归，该案例研究工作面的WCFZ高度预计分别为67.0 m和64.8 m，与数值模拟结果相近。同时，收集了30组实测数据，分析了影响WCFZ的主要因素，重点探讨了上覆岩层不同岩性特征对WCFZ的影响。结果表明，WCFZ的高度为硬-硬>硬-软>软-硬。