煤层底板陷落柱突水模拟及机理分析

SIMULATION AND MECHANISM ANALYSIS OF WATER INRUSH FROM KARSTIC COLLAPSE COLUMNS IN COAL FLOOR

岩溶陷落柱是中国北方型石炭二迭系煤田的一种特殊塌陷,广泛分布于20个煤田45个煤矿区,其导致的突水具有隐蔽性、突发性且与岩溶水有天然联系等特点,对煤矿安全生产危害极大。为研究煤层底板陷落柱破坏特征及突水机理,采用FLAC3D模拟分析了陷落柱影响下采面推进过程的不同阶段。数值模拟及实验显示,由于陷落柱的面积有限,矿压和水压力联合作用下使其发生弯曲并形成拉张破坏的可能性较小,一般产生剪切破坏,虽然升、降错动产生的剪应力、上凸弯曲产生的拉应力和压应力的三重作用导致煤层底板岩层失稳破坏,但以剪应力作用为主;同时,陷落柱的存在改变了煤层底板的地质环境和岩体结构,底板有效隔水层厚度减小,岩体强度降低,应力-应变分布不均,局部应力集中系数增大,使关键层的最小主应力进一步降低,一旦承压水压力大于关键层的最小主应力,承压水的渗水软化和压裂扩容即起作用,使底板岩层破坏裂隙沿最薄弱方向进一步扩展,导致裂隙贯通,最终形成管涌,发生突水。特别是,陷落柱的边壁、工作面底板压缩区与膨胀区的分界线重合在一条线上时,是底板岩层发生剪切破坏的最佳状态,最容易发生底臌突水。

Karstic collapse column is a kind of vertical structures typically found in Carboniferous-Permian coalfields in north China, and is widely distributed in 45 coal mine areas of 20 coalfields. Because of its hidden characteristics of the outburst and natural relationship with karstic groundwater, the water inrush caused by karstic collapse columns is more harmful to mining safety. In order to study the failure characteristics of surrounding rock masses and mechanism of water inrush caused by karstic collapse columns in coal seam floor, several situations of movements of mining work face are simulated with FLAC3D when a collapse column exists in the coal floor. Numerical simulations and experiments show that the rock failure mechanism under hydraulic pressure is of a shear-tension-pressure complex with shear as the control factor. Because of the karstic collapse column, the geological environment and rock structure in the coal floor will change, and the effective thickness of the protective layers as well as the strength of rock mass will be reduced. In addition, the stress and strain distributions are asymmetrical, and local stress concentration coefficients become larger, so that minor principal stresses of key stratum decrease further. When hydraulic pressure is larger than that of the minor principal stress of the key stratum, mining pressure and seepage as well as dilation will cause fractures to expand along the weaker directions. Fractures eventually transfix and water inrush takes place. When the border cliff of the collapse column and division line of compressive and expansive section of coal floor are coincident on the same line, the shear yield and water inrush from coal floor are most likely to take place.