Time Series Effect on Surface Deformation above Goaf Area with Multiple-Seam Mining

The surface subsidence caused by coal mining is a large area, and computer simulation is a fast and intuitive method, which can help us understand the macroscopic subsidence law. The mined-out area left over by coal mining is not disposed of appropriately for a long time. Thus, it can easily cause ground subsidence, collapse, or spot cracking, especially when mining multiple coal seams, which seriously restricts the construction and safety of the near-surface rock and soil layers. Based on the engineering background of five-layer coal mining in the Beibu Coal Mine of Laiwu City, a “Fast Lagrangian Analysis of Continua in 3D” numerical calculation model was established. The model was used to analyze the surface deformation indexes of four groups with different mining sequences in multiple coal seams, revealing the sequence effects of mining time on the surface deformation law in the goaf collapse areas, hence obtaining optimal mining sequences. The results showed that the four groups of mining sequences (including vertical settlement and horizontal deformation) have stable surface deformation centers, but the deformation ranges and amounts are quite different. The settlement deformation is the main difference. Mining sequence I has the largest deformation of 62.7 cm, followed by mining sequence III. Mining sequences II and IV are basically the same, at only 22% of the value of mining sequence 1. A multi-index analysis of the surface deformation curve including the inflection point, stagnation point, and slope showed that the larger the surface deformation, the more evident the change of the curve (concave or convex) and slope, the more uneven the foundation stress, the more severe the damage to the surface structures, and the less suitable the surface construction. Finally, upon analyzing the indicators of surface stability and adaptability, mining sequence IV was indicated as the optimal scheme. It is suggested that an optimal mining sequence should be appropriately selected before the mining of multiple coal seams. The research results can provide effective guidance for addressing surface deformations under similar geological conditions, and can provide scientific evaluations for the safety and stability of surface buildings and structures, leading to considerable economic and social benefits.