Construction and Application of Mathematical Model of Stable Fracture Development Zone of Roof

The fractured zone in the overlying strata of a coal mine goaf is vital for effective gas drainage. The development height of the fractured zone is lied on the “saddle type” theory and calculated by empirical formula, while the horizontal range is dependent on the caving theory of the “O” circle. However, the height of the fracture zone obtained using the above method is usually too large, and only the horizontal boundary of the fracture zone is defined. Besides, the fracture development degree in different regions is not described. To accurately determine the stable development area of the fractured zone and locate the gas migration channel as well as gas enrichment horizon, in this paper, a geometric and mathematical model for gas extraction in the fractured zone is established based on common high-level drilling. Then, a method of investigating the parameters of the directional high-level drilling in the roof is proposed, and a solving means is provided. Taking Wanfeng Coal Mine as an engineering background, the reasonable parameters of the common high-level drilling were investigated, and the effect was verified using the directional high-level drilling in the roof of an adjacent working face. The results show that the stable extraction volumes of the two directional drilling holes is 1~1.8 m3/min and 0.6~1 m3/min, respectively, which exceeds the optimal extraction volumes of the ordinary drilling holes. The new method of determining the stable development area of the fractured zone is significant for improving the gas drainage effect of the high-level drill holes in the roof.