Interaction Mechanism of the Upper and Lower Main Roofs with Different Properties in Close Coal Seams: A Case Study

Abstract

Close-distance coal seams are widely distributed in China, and the mining of overlying coal seams leads to floor damage. To grasp the properties and the fracture spans of the damaged main roof in the underlying coal seam, combining the calculation of the floor damage depth with rock damage theory and the formulas for calculating the first and periodic weighting intervals of the damaged main roof and the instability conditions of the damaged key blocks are obtained. Three interaction stability mechanics models are proposed for key blocks with different properties of the upper and lower main roof, and the instability conditions of the lower damaged key blocks are obtained when the fracture lines overlap. When combined with a specific example, the field monitoring verified the calculation results. The research results are as follows: (1) The first and periodic weighting intervals, horizontal thrust between blocks, and critical load of instability of the damaged main roof are significantly reduced. Still, there are differences in its reduction under different loads, rotation angles, and lumpiness. (2) When the fracture lines of the upper and lower main roofs overlap, the stability of the damaged key blocks is the lowest. There are three linkage stability regions in the critical load curves of the two key blocks. (3) In this case, the damage equivalent of the main roof is 0.397, which belongs to the local damage type. Its first and periodic weighting intervals are 40 m and 16 m, which is 22% and 24% less than when there is no damage. (4) A supporting load of 0.489 MPa is required to maintain the stability of the upper key block, and the lower damaged key block is prone to rotary and sliding instability during the first and periodic weighting, respectively. Thus, the supports need to bear a total of 0.988 MPa and 0.761 MPa to maintain the stability of the two key blocks simultaneously. The ground pressure data monitored on-site is in accord with the calculation results.

Coupling support technique for coal roadway under double gobs in close coal seams

“…On the basis of the difficulties in roadway support and support principles, 23‐29 in view of the large area roof fall disaster of the broken surrounding rock of No. 11103 fully mechanized caving roadway under the double gob, the U‐steel + anchor + prop coupling support structure is used to bear the broken surrounding rock, resist the top pressure, and prevent the U‐shaped steel from appearing weak section under the eccentric load conditions of coal pillar stress concentration and gob stress reduction, which reduces the bearing capacity 30‐37 . “U‐steel + anchor + prop” refers to U‐shaped steel shed, hollow grouting anchor cable and single prop.…”

“…11103 fully mechanized caving roadway under the double gob, the Usteel + anchor + prop coupling support structure is used to bear the broken surrounding rock, resist the top pressure, and prevent the U-shaped steel from appearing weak section under the eccentric load conditions of coal pillar stress concentration and gob stress reduction, which reduces the bearing capacity. [30][31][32][33][34][35][36][37] "U-steel + anchor + prop" refers to Ushaped steel shed, hollow grouting anchor cable and single prop. The coupling support of single prop, hollow grouting anchor cable and U-shaped steel frame is used to deal with…”

Failure analysis and prediction of roof instability in end face under repeated mining using early warning system

“…In view of the research on the theory of surrounding rock failure under repeated mining of the coal seams, Li et al 9 combined the two research methods of physical similarity simulation and numerical simulation, and clarified that the process of roof failure of the underlying coal seam in the repeated mining of the coal seams is a process of filling the goaf with the natural caving method of coal gangue after the mining of the overlying coal seam, the integrity of the main roof of the underlying coal seam is destroyed, and the rock strata between the coal seams are broken and collapsed. Xie et al 10 combined the theory of floor damage depth with the theory of rock damage, calculated the first weighting interval and periodic weighting interval of the damaged floor strata (i.e., the main roof of the underlying coal seam) after the overlying coal seam mining, and clarified the instability conditions of the internal key blocks after the rock damage. Experts and scholars in the mining industry have conducted a lot of explorations on the factors affecting the stability of end face roof.…”

Study on the Law of Repeated Mining Strata Movement and Surface Subsidence in Close Distance Coal Seam

“…Taking Shendong Coal Mine as the research background, the development law of hanging stress arch in roof strata is obtained through the analysis of shallow coal seam [13]. Through the study of the migration law of key strata in close distance coal seam mining, the calculation model of floor damage depth is constructed [14]. Based on the physical similarity simulation experiment, the stable roof structure formed by repeated mining of near coal seam and the evolution characteristics of roof and floor cracks under the influence of mining are discussed.…”