Abstract
Hard roof fracturing characteristics and mining-induced stress behaviors can be studied to ensure production safety in top coal caving face. Field tests were conducted at top coal caving face No. 8309 in the Tongxin mine of the Datong Coal Mining Group in China. The monitoring results indicated two modes of periodic weighting: short weighting steps with low intensity and long weighting steps with high intensity. According to three periodic weighting phenomena, three models of hard roof fracturing were proposed based on the cantilever-voussoir beam compound structure. And the equations of the working resistance for long/high periodic weighting were derived. Furthermore, a difference in the pressure changes among the front leg, back leg and front beam of the supports was observed, and the back leg pressure significantly increased during periodic weighting. The change in the single prop pressure was divided into a slow growth stage and a rapid growth stage. And the front abutment pressure fluctuated with the periodic instability of the roof compound structure. Moreover, the fracturing of the roof above the stope resulted in roadway deformation. The instability of the cantilever-voussoir beam structure also intensified the roadway deformation. Finally, a support-surrounding rock mechanical model was established considering the behavior of an elastic cantilever beam. The required work resistance considering the bearing capacity of top coal were discussed, providing a basis for reasonable support selection. These research results can provide references for mining pressure and stratum movement control.
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Abbreviations
- F j :
-
Pressure on the support, [kN]
- \(\varphi\) :
-
Friction angle between rock blocks, [°]
- h :
-
Thickness of the block, [m]
- Q 0 :
-
Total weight of the fractured block itself and its controlled overlying strata, [kN/m]
- h m :
-
Thickness of the coal, [m]
- k p :
-
Fracturing coefficient of the top coal
- h z :
-
Thickness of the immediate roof, [m]
- Q z1 :
-
Weight of the top coal and the immediate roof in Model B, [kN]
- \(\gamma_{z}\) :
-
Bulk density of the immediate roof, [kN/m3]
- F B :
-
Working resistance of the hydraulic support in Model B, [kN]
- k :
-
Dynamic load coefficient of the fractured cantilever beam structure
- F C :
-
Working resistance of the hydraulic support in Model C, [kN]
- BD :
-
Distance between points B and D, [m]
- AC :
-
Distance between points A and C, [m]
- x′:
-
Another section of BD, [m]
- y′:
-
Another section of AC, [m]
- Q :
-
Roof pressure, [kN]
- \(\sigma_{x}\) :
-
Normal stress on the cross section of the elastic cantilever beam, [MPa]
- \(\sigma_{\tau }\) :
-
Uniaxial tensile strength of the top coal, [MPa]
- l :
-
Weighting step, [m]
- \(\theta\) :
-
Broken angle of rock blocks, [°]
- s :
-
Subsidence of the fractured block, [m]
- b :
-
Width of the support, [m]
- h d :
-
Thickness of the top coal, [m]
- \(\eta_{m}\) :
-
Recovery rate of the top coal
- k z :
-
Fracturing coefficient of immediate roof
- \(\gamma_{m}\) :
-
Bulk density of the coal, [kN/m3]
- l d :
-
Length of the canopy of hydraulic support, [m]
- Q z2 :
-
Load of the top coal and immediate roof in Model C, [kN]
- l z :
-
Length of the fractured immediate roof, [m]
- AB :
-
Distance between points A and B, [m]
- AD :
-
Distance between points A and D, [m]
- x :
-
A section of BD, [m]
- y :
-
A section of AC, [m]
- M :
-
Bending moment in elastic cantilever beam, [N·m]
- q :
-
Required working resistance, [Pa]
- \(\sigma_{x\hbox{max} }\) :
-
The maximum tensile stress in the cantilever beam, [MPa]
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Acknowledgements
This study was funded by National Natural Science Foundation of China (Grant No. 51822403, 51827901) and the Sichuan International Technological Innovation Cooperation Project (Grant No. 2018HH0159).
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He, Z., Xie, H., Gao, M. et al. The fracturing models of hard roofs and spatiotemporal law of mining-induced stress in a top coal caving face with an extra-thick coal seam. Geomech. Geophys. Geo-energ. Geo-resour. 7, 2 (2021). https://doi.org/10.1007/s40948-020-00202-9
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DOI: https://doi.org/10.1007/s40948-020-00202-9