Immediate roof first fracture characteristics of suberect and extremely thick coal

The immediate roof fractures of suberect and extremely thick coal in Sub-Level Top-Coal Caving directly af- fects the stability of the workface and roadway. According to the stress condition and the fracture characteristics of im- mediate roof in Sub-Level Top-Coal Caving,a four-edges-clamped plate model was introduced to calculate the deflec- tion of immediate roof before the first fracture. To verify the normal stress distribution of the four thin plate models, FLAC3D software was used to perform the following tasks:simulating the mining process in the suberect and extremely thick coal seam mining face;undertaking an orthogonal numerical simulation experiment in three levels with different depths,coal seam angles,lateral pressure coefficients and orientations of maximum horizontal principal stress;transla- ting the immediate roof stress of the corresponding nine simulation experiments into the suberect immediate roof normal stress;and using the above results to determine the distribution regularity of normal stress along the dip direction of the roof under the circumstance of different advancing distances and different subsections. According to the simulation re- sults,the normal stress distribution of the immediate roof is uniform. The Galerkin method was used to calculate the so- lution of deflection equation under the effect of normal stress. The Hooke’s law and the arc length theorem were used to calculate tensile stress of immediate roof,and obtained the maximum tensile stress and corresponding position of im- mediate roof before first fracture. Taking the maximum tensile stress strength criterion as the suberect immediate roof fracture criterion,the first fracture span could be obtained.