Roof stability evaluation of large section open-off cut in lower slice of slicing mining

In order to study the roof stability of large-section open-off cut in lower slice of slicing mining, the 14 boreholes in the top coal of the 203 open-off cut in the lower slice of 1⁻² coal seam in Huojitu Mine of Daliuta Mine is taken as the research background. The influence of the upper slice mining and the lower slice open-off cut driving on the plastic zone of the top coal of the open-off cut is analyzed by theoretical analysis, numerical simulation and field borehole peep technology. And the stability of the roof structure is evaluated by the rock mass integrity index. From the perspective of the top coal structure form, the top coal of the open-off cut is partially over-excavated or under-excavated due to the upper slicing mining. The maximum over-excavation of the open-off cut is 1.2 m, the maximum under-excavation is 0.8 m, and top-coal uneven rate is 27.7%. The result of theoretical analysis show that due to the influence of upper slice mining of the top coal of the open-off cut, the depth of plastic zone in the floor is 2.02 m. Due to the disturbance of the lower slice mining of the open-off cut, the depth of the plastic zone in the top coal is 1.50 m. Borehole peep results show that the plastic zone of the open-off cut top coal is divided into the theoretical plastic zone and the measured plastic zone according to the theoretical calculation and borehole peep respectively. Due to the influence of upper slice mining, the depth of the measured plastic zone in the floor is 1.06-2.04 m. Due to the disturbance of the lower slice mining of the open-off cut, the depth of the measured plastic zone in the top coal is 0.34-1.50 m. The measured plastic zone caused by the influence of upper slice mining on floor is 17.63% smaller than the theoretical plastic zone. The measured plastic zone of the top coal caused by the disturbance of lower slice mining of the open-off cut is 25.82% smaller than the theoretical plastic zone. The result of numerical simulation analysis shows that due to the influence of upper slice mining, the depth of plastic zone in the floor is 1-2 m. Due to the disturbance of the lower slice mining of the open-off cut, the depth of plastic zone in the top coal is 1 m. The results obtained by the above three methods are highly consistent. The stability evaluation results of the top coal in the open-off cut show that the top coal integrity index ranges from 42.9% to 87.9%. The top coal thickness is positively correlated with the integrity index and negatively correlated with the fracture development. The proportion of top coal integrity evaluation as good or above is more than 1/2, indicating that the overall structure of top coal is basically complete. The research results can provide reference for the design of top coal thickness retention standard and support scheme of large section open-off cut in lower slice of slicing mining under similar mining conditions.