Abstract
During upward backfill mining of the close-distance coal seam group, the reasonable design of the backfill body compaction ratio (BBCR) of lower coal seam goaf plays a key role in controlling overburden strata movement and deformation. In this paper, the feasibility discrimination method of upward caving mining was adopted; it was found that when the coal seam spacing was small, the recovery was not possible due to the instability of the upper coal strata equilibrium structure caused by the mining. Hence, the upward backfill mining method was proposed; combined with the migration control mechanism of backfill mining overburden strata, the dynamic evolution of crack propagation height of overburden strata during upward backfill mining was analyzed. Based on that, a layer spacing-BBCR-mining height (L-R-H) prediction model based on multiple linear regression analysis was established and the BBCR of close-distance coal seam group safe upward mining was determined combined with the engineering background. The results demonstrate that the main controlling factors of crack propagation in overburden strata during upward backfill mining were BBCR and mining height; the crack propagation height in overburden strata caused by mining decreased exponentially with the increase of BBCR and increased with the increase of mining height linearly. The lower coal seam engineering safety BBCR of upward backfill mining in the Dafosi Coal Mine was 87.2%, and the close-distance coal seam group safe upward mining was realized.
Highlights
Owing to exploitation of coal resources, coal resources with good occurrence conditions were increasingly scarce [1] and the mining of various complex coal seams has attracted great attention
For the close-distance coal seam group, downward mining has some problems, such as difficult to form a hinged structure of the roof, instability of the roof structure, and air leakage; during upward caving mining, overburden strata movement and deformation caused by the mining of lower coal seams may influence the normal recovery of upper coal seams [3–5]
Ma et al [12] considered the relationship between the overburden strata condition of upper coal seams and upward mining; the results revealed that the key of upward mining was to analyze and control the longitudinal step dislocation of the coal layer
Summary
Owing to exploitation of coal resources, coal resources with good occurrence conditions were increasingly scarce [1] and the mining of various complex coal seams has attracted great attention. The geological conditions of the Dafosi Coal Mine were regarded as the research background; based on the analysis of the migration law and failure mechanisms of overburden strata during upward backfill mining of the close-distance coal seam group, the main controlling factors affecting overburden strata were determined. The results are of great significance to the supplement of the BBCR design method of the upward backfill mining of the close-distance coal seam group, the full exploitation and utilization of coal resources, and the construction of green and safe mines
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