综放工作面临空开采顶煤主应力旋转特征

Principal stress rotation in longwall top-coal caving face adjacent to the gob

综放工作面采动应力场分区特征决定顶煤裂隙场发育特征,为获得临空开采条件下顶煤裂隙扩展的应力驱动机制,本研究采用数值模拟、理论分析和现场实测手段对该类综放工作面顶煤主应力分布特征进行分析。结果表明:顶煤渐进破碎过程受主应力大小和方向的双重影响,最大主应力升高和最小主应力降低促进顶煤破碎,后者碎煤效率为前者的3.0~4.5倍;煤层为含有大量原生裂隙的复杂地质体,裂隙煤体具有各向异性力学特征,主应力方向旋转必然改变顶煤力学行为,进而影响顶煤破碎难度和破坏裂隙扩展特征;非临空开采条件下,顶煤主应力分布呈现对称分布特征,主应力旋转轨迹在赤平投影图中集中为一簇,旋转幅度小;临空开采条件下,顶煤主应力分布呈现非对称特征,临空侧顶煤主应力集中程度明显高于实体煤侧,主应力旋转轨迹在赤平投影图中分为两簇,旋转幅度大;采动影响下,顶煤最大主应力向竖直方向旋转,最小主应力向水平方向旋转,根据主应力同平行于推进方向竖直平面之间的空间位置关系,将顶煤主应力方向旋转轨迹划分为4个阶段:面外旋转阶段、面外反向回旋阶段、面内反向回旋阶段和非协调旋转阶段;最后采用临空开采工作面两侧巷道围岩的非对称变形现象验证了顶煤主应力方向旋转轨迹的非对称分布特征。

Mining induced fracture development is closely related to the stress redistribution in longwall top coal caving(LTCC) face. In order to provide the stress field for investigating the fracture propagation mechanisms in top coal, the distribution of the principal stresses in the LTCC face adjacent to the gob is analyzed by using numerical simulation, theoretical analysis and field investigation. The results indicate the progressive failure process of top coal is greatly influenced by the magnitude and orientation of the principal stresses in the LTCC face. The increase of the major principal stress and the decrease of the minor principal stress cause the failure of top coal. The efficiency of the latter in breaking top coal is 3.0 to 4.5 times of that from the former. The coal seam belongs to complex geological materials with many pre-existing fractures, which means the coal presents anisotropic mechanical behavior. Thus, the mechanical behaviors of top coal evolve with the rotation of the principal direction. Moreover, the failure initiation and propagation orientation of mining-induced fractures are influenced by such stress rotation. In the LTCC uninfluenced by adjacent gob, the mining induced stress presents a symmetrical distribution. There is one group of rotation trajectory on the stereonet, which has small rotation degrees. Under the influence of the adjacent gob, the principal stress presents an unsymmetrical distribution. The concentration degree at the gob side is significantly larger than that at the unmined coal seam side. Besides, the rotation trajectory of the principal direction is categorized into two groups on the stereonet, which have large rotation degrees. Under the influence of coal seam extraction, the major principal stress rotates to a vertical direction while the minor principal stress rotates to a horizontal direction. According to the spatial relation between the principal stress and vertical plane parallel to face advance direction, the rotation trajectory is divided into four stages, including external rotation stage, external reverse rotation stage, internal reverse rotation stage and inconsistent rotation stage. At last, the unsymmetrical deformation phenomenon existing between the gates on two sides of the LTCC face adjacent to the gob is reasonably explained by mining induced rotation in the principal stress.