近距离煤层重复采动覆岩裂隙时空演化特征研究

Spatial and temporal characteristics of overburden fractures due to repeated mining in close distance coal seams

采用相似材料模型试验,在上行和下行开采方式下,对近距离煤层重复采动覆岩裂隙的时空演化特征进行了研究。采用数字图像处理技术对覆岩裂隙进行了量化分析,并基于分形理论与信息熵,提出了采用裂隙分形维数、裂隙熵和裂隙率3个特征参数来反映裂隙状态。研究结果表明:随着煤层开采的不断推进,上行开采重复采动覆岩裂隙的裂隙熵和裂隙率具有相同的变化趋势,下行开采重复采动覆岩裂隙的分形维数、裂隙率具有相同的递增趋势,但是裂隙熵在不同开采条件下的变化有较大的差异。在重复采动条件下,上、下行开采覆岩裂隙分形维数在沿开采方向上的变化分别呈“马鞍形”和“梯形”,裂隙率和裂隙熵呈相似的周期性振动变化,但裂隙率的振幅小于裂隙熵。相比于上行重复开采,下行重复开采的覆岩裂隙的产生与扩张作用较强。上行开采重复采动距离煤层底板越远,裂隙熵越小;下行开采重复采动距离煤层底板较远的地方裂隙熵较大,当夹矸岩层全部垮落后,上、下煤层的破坏带连通,导水裂隙带高度持续发育。

Spatial and temporal characteristics of overburden fractures under upward and downward repeated mining in close distance coal seams were studied with similar material model tests.The digital image processing technology was used to quantitatively analyze characteristics of overburden fractures.Based on fractal theory and information entropy,three characteristic parameters of fracture fractal dimension,fracture entropy,and fracture rate were proposed to reflect fracture state.Research results have indicated that along with coal mining,variation trend of fracture entropy and fracture rate of overburden fractures has the same varying tendency under upward repeated mining,and fractal dimension and fracture rate of overburden fractures has the same increasing trend.However,the variation of fracture entropy is quite different under different mining conditions.Fractal dimension of overburden fractures changes in the shape of"saddle"and"ladder"under upward and downward repeated mining along mining direction,respectively.Fracture rate and fracture entropy have similar periodic vibration changes,but amplitude of fracture rate is smaller than that of fracture entropy.The generation and expansion of overburden fractures caused by repeated downward mining are stronger than that under upward mining.The farther away from coal floor,the smaller the fracture entropy is under upward repeated mining.Results have shown that fracture entropy is larger in the place farther away from coal seam floor under downward repeated mining,and after all gangue strata cave,upper and lower coal seam failure zones are connected,and the height of water-conducting fractured zone is continuously developed.