Research on mining-induced fracture evolution characteristic of overlying strata in shallow-buried compound gob

Abstract

Abstract As the main mining shallow-buried and short-distance coal seams in Lijiahao Coal Mine, 2–2 medium coal and 3 − 1 coal are prone to spontaneous combustion. The overburden fracture of compound gob formed by large-scale and high-intensity coal mining are developed, and air leakage condition is complicated, which leads to prominent hidden danger of residual coal spontaneous combustion. Therefore, it is urgent to study the overburden fracture evolution characteristic of shallow-buried compound gob. Similar experiment was used to simulate the overburden movement characteristics of panel 1114 in 2–2 medium coal and panel 31114, and fractal geometry theory was utilized for quantifying the overburden fracture of compound gob. Based on the “key layer theory”, cause for evolution of the overburden fracture was revealed and corresponding countermeasures were proposed. The results showed that, when panel 31114 entered and left the overlying gob of panel 1114 in 2–2 medium coal, the overlying strata collapsed and the ground pressure appeared violently. The fractal dimension of mining-induced fracture network ranged from 1.32 ~ 1.58, with increase first large and then small, slight fluctuations in the middle and finally leveling off. The voidage distribution curve between neighboring monitoring lines on the overlying strata of compound gob presented a “W” shape, and the overburden fracture was developed near the setup entry and stop line. Meanwhile the overburden fracture zone presented a semi-enclosed “M” shape with the coal seam floor as the boundary, which was composed of irregular fracture zone Ⅰ in the collapsed zone, diagonal breakage fracture zone Ⅱ, Ⅲ and diagonal bed-separated fracture zone Ⅳ in the fractured zone. Also the overburden fracture ratio was 4.1%, and the fractal dimension was 1.4835. Among them, fracture ratio and fractal dimension of fracture zone Ⅰ was the largest, followed by fracture zone Ⅱ, Ⅳ and Ⅲ. The mechanical model on “three-hinged arch” bearing structure of key rock block was built to analyze the inducement of diagonal breakage fracture development and diagonal bed-separated fracture development near the setup entry and stop line. Then local reducing mining height, backfilling mining or accelerating advancing speed combined with regional surface backfilling and grout sealing methods were proposed, which could effectively block the overburden fracture channel, reduce surface air leakage and provided an important theoretical basis for the prevention and control of residual coal spontaneous combustion in compound gob.