浅埋综放覆岩非连续大变形条件下顶煤破碎机理与冒放性控制研究

There are many ultra-thick coal seams in the west of China. Such seams have shallow cover depth, thin bedrock and thick surface layer. The overburden shows large discontinuous deformation and top-coal cavability is unfavorable when extracted with longwall top-coal caving mining method. Effective utilization of the large discontinuous deformation in optimizing top-coal cavability remains a critical problem in such longwall faces. In this project, top-coal caving mining of the western ultra-thick seams is taken as the engineering background. Physical modeling system for longwall top-coal mining in shallow ultra-thick seams is modified to study the influence provided by large discontinuous deformation of the overburden on mining-induced stress environment. Rotation trace of the principal stress in top coal is established and determination method of roof impact force is put forward. Meso-strength criterion for the coal is proposed and the coupled macro- and meso-mechanical model constructed before is improved, with which the coupled continuous-discontinuous simulation method is developed in FLAC3D-PFC3D platform. Co-analysis of mining-induced stress and fracture field is realized and the role played by stress rotation and roof impact in driving propagation of top-coal fractures is achieved. Three dimensional reconstruction of fractured top-coal make it possible to quantitatively characterize top-coal cavability. Layer-layer progressive relationship from large discontinuous deformation of the overburden to top-coal stress environment to top-coal fracture field and finally to top-coal cavability is obtained. On the basis of such relationship, optimizing principle for top-coal cavability is proposed. The project aims to reveal the influences provided by large discontinuous deformation of the overburden on top-coal cavability. Thus, the recovery rate can be maximized by reasonable design of mining parameters.

我国西部赋存大量特厚煤层，具有埋深浅、基岩薄、表土层厚等赋存特征，综放开采条件下覆岩呈现非连续大变形，顶煤冒放性不理想，如何利用覆岩大变形促进顶煤破碎成为亟需解决的关键难题。项目以西部特厚煤层综放开采为工程背景，改进浅埋综放物理模拟实验台，研究覆岩非连续大变形对采动应力环境的影响，构建顶煤主应力旋转轨迹，提出顶板动载冲击力确定方法；建立考虑主应力方向的煤体细观强度准则，改进煤体宏-细观本构模型，开发基于FLAC3D-PFC3D平台的连续-非连续耦合模拟技术，实现采动应力场、裂隙场的协同分析，揭示主应力旋转和顶板动载冲击驱动下顶煤裂隙扩展机制；提出破碎顶煤三维重构方法，实现顶煤冒放性定量表征，得到覆岩非连续大变形→顶煤应力环境→顶煤裂隙场→顶煤冒放性的递进影响关系，提出冒放性控制原理。项目旨在揭示浅埋综放覆岩非连续大变形现象对顶煤冒放性的影响机制，优化开采参数，最大限度提高资源采出率。

我国西部赋存大量15 m以上特厚煤层，大采高综放是开采该类煤层的技术首选。高强度开采条件下覆岩运动剧烈，但特厚顶煤冒放性不理想，为提高优质煤炭资源开采效率，项目研究了特厚煤层综放开采条件下顶煤主应力旋转特征和顶板动载冲击效应、特厚顶煤裂隙扩展机理、顶煤冒放性预测及控制原理。研究发现强采动影响下超前采动应力向采空区倾斜，走向影响范围达到200 m，两侧采动应力向巷道倾斜，倾向影响范围达到15 m；以平行于工作面推进方向的竖直平面为基准，采动应力旋转轨迹经历面外慢速偏离、面外快速靠近和面内协同旋转三个阶段。顶板动载由坚硬顶板破断引起的应变能释放效应引起，建立了坚硬顶板动力破断的折叠突变模型，提出了顶板动载冲击力计算方法。顶煤最大和最小主应力均存在超前峰值现象，最小主应力峰值至煤壁区间，顶煤应力路径同三轴卸围压实验煤样加载历史相似；煤壁后方，顶煤最小主应力进入反向拉伸阶段；采动后顶煤主应力方向发生旋转现象，促进顶煤破坏进程。超前非贯通阶段微裂隙沿最大主应力方向扩展，顶煤发生局部拉裂，随着最小主应力的降低，顶煤沿最大裂隙贯通率或最大剪应力方向发生剪切破坏，产生贯通裂隙，同三轴卸围压实验中煤样破坏模式相似；由于顶煤主应力旋转现象，贯通裂隙向工作面后方采空区倾斜，根据顶煤裂隙产生机理的不同，可将顶煤划分为原岩应力区→微裂隙加密区→压剪破坏区→拉剪破坏区→散体冒落区。将工作面前方顶煤超声波速分布划分为单阶段（I）、双阶段（I-II）和三阶段（I-III）三种类型，对于I型分布采场，顶煤不具备冒放性，I-II型分布采场，顶煤冒放性差，采出率介于50%~70%，I-III型分布采场，顶煤冒放性良好，采出率大于70%；采用累积塑性应变表征煤中裂隙发育程度，构建超声波速预测模型，实现顶煤冒放性的定量表征和预测。针对顶煤冒放性不理想问题提出了水力预裂和采动应力旋转轨迹控制方法，有效提高了顶煤冒放性，研究成果为西部特厚顶煤综放开采顶煤冒放性控制提供了借鉴。