松散层下覆岩裂隙采后闭合效应及重复开采覆岩破坏预测

为研究开采覆岩裂隙发育的时间效应和为残煤复采评价提供依据。采用彩色钻孔电视、冲洗液漏失量观测、岩芯RQD指标等对厚松散含水层下煤层开采15 a后的覆岩裂隙闭合效应进行了研究。结果表明：距煤层顶板较远的岩体裂隙张开度、裂隙率均较小且易闭合，距煤层顶板较近的岩体裂隙张开度大、裂隙率大且未能闭合，开采15 a后覆岩导水裂隙高度比煤层开采结束时的最大导水裂隙带高度降低了近40%，垮落带高度稍有降低。对比分析发现，开采后的覆岩岩块平均强度比开采前降低了约20%，采用Hoek-Brown经验公式换算得到破碎岩体单轴抗压强度仅为0.237 MPa，破碎岩体抗压强度已小于自重应力值，处于“潜塑”状态。因此，残留煤层的重复开采覆岩破坏高度计算时，覆岩应按软弱岩类型考虑。计算表明，杨庄煤矿3号煤层复采厚度2和3 m时，分别留设15.7和18.2 m的防砂安全煤柱即可起到防溃砂作用。

Closure effect of mining induced fractures under sand aquifers and prediction of overburden failure due to re-mining

This paper presented an investigation of closure effect of mining induced fractures under thick sand aquifers 15 years after coal mining by using color borehole television，mud slurry leakage and RQD(Rock Quality Designation) of cores，for the purpose of understanding the time effect on mining induced fracture development and providing a basis for the evaluation of abandoned coal seam remining.The results show that the joint openness and fracture porosity in the rockmass far away from the roof of the coal seam are relatively small and easily compressed.The fractures with larger opening in the rock mass near the roof of the coal seam cannot be compressed to close.The height of water flowing fractured zones reduced by nearly 40% after 15 years，while the height of caving zone just reduced slightly.The uniaxial compressive strengths of overburden after mining reduced by about 20% compared with those before mining；the uniaxial compressive strength of broken rock mass with potential plastic state was only 0.237 MPa calculated with Hoek Brown empirical rock failure criterion.Therefore，the formulae for soft rock failure should be used to calculate the overburden failure due to re mining of the residual coal seam.The research also indicates that the height of safety pillars of more than 15.7 or 18.2 m is required to prevent panels with a re mining cutting height of 2 or 3 m of seam No.3 in the Yangzhuang Coal Mine，Shandong Province from quicksand.