相较于单一煤层开采，多煤层开采的空间性和复杂性十分突出。当多煤层开采时，上下分层采空区覆岩形成新的裂隙带，采动裂隙带空间将相互影响扩大，其分布范围并非简单的几何叠加，而是与采高、煤层埋深及层间距等因素密切相关。因此，研究采动裂隙场的演化及瓦斯运移规律对提升瓦斯抽采效率具有重要意义。本文采用理论分析、物理相似模拟、数值模拟和现场工程实践等研究方法，对陕西某矿下位厚煤层重复采动条件下覆岩裂隙演化及裂隙场内瓦斯运移规律进行了系统研究。具体研究成果如下：

(1) 根据陕西某矿下位厚煤层重复采动实际情况，开展二维物理相似模拟实验和3DEC数值模拟实验，研究了单次采动和下位厚煤层重复采动条件下覆岩变形破坏规律。得到单次采动和重复采动覆岩裂隙带发育高度和采场围岩裂隙的发育形态。

(2) 结合3DEC数值模拟软件获得的单次采动和下位厚煤层重复采动覆岩裂隙场分布特征，利用COMSOL软件进行模拟分析，探究了下位厚煤层在重复采动前后，卸压瓦斯在采空区及其上覆岩裂隙网络中的运移和富集过程，揭示了下位厚煤层重复采动裂隙场内部卸压瓦斯的运移富集规律。

(3) 综合考虑采高、煤层埋深及下伏煤层开采时引起两煤层间隔层顶部位置的二次下沉值，改进两煤层开采综合采高计算方法；结合修正后的裂隙带高度预计公式计算得到陕西某矿下位厚煤层重复采动覆岩裂隙带发育高度。

(4) 综合以上研究成果，结合现场裂隙带高度探查结果，现场施工高、中、低三个层位的定向长钻孔，现场工程应用效果表明，高位定向长钻孔瓦斯抽采量占绝对瓦斯涌出量的37%-63%，平均抽采占比53.5%，进一步验证论文研究结果及抽采系统布置优化的合理性，保证了下位厚煤层重复采动工作面的安全开采，为同类开采条件矿井卸压瓦斯抽采提供了参考依据，对于实现瓦斯精准抽采具有重要意义。

Compared to single coal seam mining, the spatial and complexity features of multi-seam mining are very prominent. When mining multiple coal seams, new fracture zones are formed in the overlying rock of the upper and lower mining voids. The spaces of the mining-induced fracture zones influence each other and expand, and their distribution is not a simple geometric superposition, but closely related to factors such as mining height, coal seam burial depth, and inter-seam spacing. Therefore, studying the evolution of the mining-induced fracture field and the laws of gas migration is of great importance for improving the efficiency of gas extraction. This paper adopts research methods such as theoretical analysis, physical similar simulation, numerical simulation, and field validation to systematically study the evolution of overburden fractures and the laws of gas migration within the fracture field under the conditions of repeated mining of a thick lower coal seam in a mine in Shaanxi. The specific research results are as follows:

(1) Based on the actual situation of repeated mining of the thick lower coal seam in the Shaanxi mine, two-dimensional physical similar experiments and 3DEC numerical simulation experiments were conducted to study the deformation and failure patterns of the overburden under the conditions of single mining and repeated mining of the thick lower coal seam. The development height of the fracture zone in the overburden and the formation pattern of the surrounding rock fractures in the mining area were obtained.

(2) Combined with the distribution characteristics of the overburden fracture field under single mining and repeated mining of the thick lower coal seam obtained using 3DEC numerical simulation software, simulation analysis using COMSOL software was conducted to explore the migration and enrichment process of depressurized gas in the goaf and its overlying rock fracture network before and after repeated mining of the thick lower coal seam, revealing the migration and enrichment laws of depressurized gas within the fracture field under the conditions of repeated mining of the thick lower coal seam.

(3) Taking into account the mining height, coal seam burial depth, and the secondary subsidence value of the top of the interlayer caused by the mining of the underlying coal seam, the comprehensive mining height calculation method for mining both coal seams was improved; Combined with the modified formula for predicting the development height of the fracture zone, the development height of the overburden fracture zone under repeated mining of the thick lower coal seam in the Shaanxi mine was calculated.

(4) Integrating the above research results, combined with the field exploration results of the fracture zone height, directional long drilling holes at high, medium, and low positions were conducted on-site. The field engineering application results show that the gas extraction volume of high-position directional long drilling holes accounts for 37%-63% of the total gas emission volume, with an average extraction proportion of 53.5%. This further verifies the research results of the thesis and the rationality of the optimized layout of the extraction system, ensuring the safe mining of the working face under repeated mining of the thick lower coal seam, providing a reference for depressurized gas extraction in mines under similar mining conditions, and is of great significance for achieving precise gas extraction.

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