深埋厚冲积层薄基岩煤层开采地表沉降特征与预测方法
Surface subsidence and its prediction method ofmining deep-buried seam with thick alluvial layer and thin bedrock
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摘要: 煤炭地下开采导致上覆岩层的破断和运动,岩层运动传递至地表引发地表沉陷,改变地表形态,破坏地表连续性和生态环境。为降低采矿活动的负外部性,以赵固二矿为工程背景,综合运用室内试验、理论分析和现场实测等方法研究深埋厚冲积层薄基岩煤层开采地表沉陷与覆岩运动的关系、地表沉陷演化特征和地表沉陷预测方法。结果表明:深埋薄基岩厚煤层开采覆岩运动存在关键层控制和厚冲积层主导2个阶段,第1阶段薄基岩呈现层状破断特征,关键层下缘离层现象明显,厚冲积层保持稳定;第2阶段采动裂隙萌生于高位厚冲积层,下行扩展导致薄基岩全厚破断,层间离层现象消失;得到了工作面推进过程中不同层位覆岩沉降曲线动态演化特征,发现厚冲积层对覆岩运动和地表沉陷具有强烈控制作用,冒落拱失稳导致覆岩沉降曲线呈现周期性突变现象;薄基岩全厚破断后整体下沉,厚冲积层冒落体下向压实,采动裂隙快速闭合,导致采空区上方垮落带和裂隙带岩层破坏后的碎胀系数小,地表下沉速度快,沉降系数高,最大下沉量达到开采厚度;将地表沉陷区划分为直接沉降和间接沉降2个区域,深埋厚冲积层薄基岩赋存条件下间接沉降区范围增大,构建了地表沉陷分区预测模型;对赵固二矿地表沉陷特征进行了预测,同时采用无人船搭载定位和声纳系统对赵固二矿地表沉陷盆地形态进行了实测和重构,预测结果和实测数据对比分析表明理论沉陷值的最大误差率为7.0%,平均误差率为2.4%,表明构建的地表沉陷分区预测模型适用于深埋厚冲积层薄基岩赋存条件,可为该类煤层开采地表沉陷治理提供借鉴。Abstract: Underground coal mining leads to a failure and movement of overburden strata, moreover resulting in surface subsidence. Surface subsidence changes the land topography, land continuity and even ecological environment of the land mineed. In order to reduce such adverse impacts caused by underground mining, the second coal mine of the Zhaogu mining area is investigated as an engineering case, where deep-buried coal seam with thick alluvial layer and thin bedrock is extracted. The relation between overburden movement and surface subsidence, dynamic evolution of surface subsidence and its prediction method are thoroughly studied by using lab experiment, theoretical analysis and field measurement. The results show that the overburden strata movement of longwall face with thick alluvial layer and thin bedrock is composed of a key stratum controlled stage and a thick alluvium controlled stage. In the first stage, thin bedrock presents lamellar failure mode. Bedding separation is obvious under the key stratum while thick alluvium remains stable. In the second stage, mining induced fractures initiate in the high alluvial layer. Downward propagation leads to the full-thick rupture of the thin bedrock. Correspondingly, bedding separation vanishes in this stage. Dynamic evolving process of subsidence curves at different level of overburden are achieved. It is revealed the thick alluvium exerts strong influence on strata movement and surface subsidence. The instability of the arch structure in the alluvium results in a periodic mutation in the subsidence curve. The thin bedrock moves downward quickly after full-thick rupture. The caving rock quickly compacts densely, leads to a quick closure of the caving fractures. As a result, the bulking factor of failure materials in the caving and fracturing zones is small. Correspondingly, surface subsides quickly, leading to a large subsidence coefficient. The largest subsidence nearly reaches mining height. The subsidence area is divided into direct and indirect regions. The range of the second region is enlarged by the special overburden composition. Based on such understanding, a regional prediction method is established, which is moreover used to evaluate surface subsidence. After that, the surface subsidence basin of the target mine is monitored and reconstructed with unmanned ship, which is equipped with GPS and Sonar systems. The comparison between predicting results and monitoring data reveals the largest and average errors are about 7.0% and 2.4%,respectively. That means the method is adaptable to deep mining with thick alluvium and thin bedrock, providing a guidance for addressing subsidence problems with similar conditions.