Theoretical and Experimental Investigation on Top Coal Drawing Mechanism Considering Overburden Pressures in Longwall Top-coal Caving

Longwall top coal caving (LTCC) mining technique is widely used in thick and extra-thick coal seams in China, India and Türkiye. Considering that the depth of coal seam varies widely (from 188 to 988 m in China), the different overburden pressure has great influence on the failure and drawing process of top coal in LTCC. In this paper, the top coal drawing mechanism considering overburden pressures was investigated through theoretical analysis, physical experiments and numerical calculation. The shape of the drawing body, coal–rock interface and recovery of top coal under different overburden pressure were analyzed deeply. The results show that the motion acceleration and fluidity of top coal particles increase first and then decrease with the increase of overburden pressure. When gangue-to-coal density ratio (γ) is larger than 1, the increasing of overburden pressure is beneficial to the recovery of top coal to some extent. With the increase of overburden pressure, the top coal boundary in the initial drawing stage gradually expands to the goaf, which increases the volume of the initial drawing body. The expansion of the initial drawing body’s width is attributed to the growth of the goaf-side width and is unrelated to the support-side width. With the increasing of the overburden pressure, the proportion of strong force chain gradually increases and the force arch is gradually formed between the goaf and the coal wall, which is conductive to the mixing of gangue form the goaf direction. This is the internal reason why the top coal recovery ratio decreases when the overburden pressure is too large. The “excessive coal drawing method” is proposed for deeply buried coal seams, which is conducive to recover more top coal and reduce the residual coal left in the goaf.