Effect of overburden bending deformation and alluvium mechanical parameters on surface subsidence due to longwall mining

Longwall extraction of coal seam disturbs the whole overburden strata, from the coal seam to the surface, and forms a subsidence basin. This paper investigated the mechanism of the overburden strata bending deformation, the alluvium mechanical parameters, and their effect on surface subsidence by the theoretical analysis and numerical simulation analysis. The dynamic processes of overburden failure and surface subsidence due to longwall mining was analyzed by a novel two-dimensional trapezoidal-area method. A “π-shaped” model was first developed to reveal that surface subsidence consists of the overburden strata bending deformation and the subsidence of the alluvium. The process and mechanism of the overburden strata bending transfer were analyzed, and two mechanical models of the strata bending deformation were established, i.e., the strata suspended bending model and the strata overhanging bending model. The maximum subsidence equation of the overburden strata bending basin was derived based on the theoretical analysis. Also, the effect of the alluvium mechanical parameters on the surface subsidence was investigated by numerical simulation analysis. The rationalities of the numerical simulation were validated by the field measurements. The results show that surface subsidence increases logarithmically during increased alluvium thickness and decreases exponentially and logarithmically during increased internal friction angles and increased cohesions, respectively.