Objective and Method Coals serve as a predominant energy source in China. However, coal mining has resulted in increasingly pronounced ecological issues such as ground subsidence and solid waste accumulation. Although the continuous mining and continuous backfilling (CMCB) technology allows for efficient resource recovery and ecological protection through backfilling with coal-based solid waste, the issue of insufficient roof-contact backfilling (termed underfilling) poses a serious threat to the overburden stability during mining. Given the unclear mechanisms behind the influence of coupled multiple factors of underfilling zones in current research, this study constructed three-dimensional numerical models using the 3DEC discrete element software based on the engineering background of a mine with backfill using aeolian sand paste-like filling materials in Yulin, Shanxi Province. Eight operating conditions were designed to compare the impacts of the underfilling zone size, roof conditions, and support conditions on overburden displacement, stress distribution, and surface deformation. Furthermore, a mechanical model was developed based on the elastic foundation beam theory to calculate the overburden displacement during the mining of a strip coal pillar under the influence of underfilling zones.

Results and Conclusions With the expansion of underfilling zones (volume ratio: ≥ 10%), the overburden displacement and the surface subsidence increased significantly. Most especially, the expansion produced a more significant impact on the overburden stability in the presence of a 20-m-width underfilling zone on each side of the mining face. The development of roof fractures further aggravated the overburden instability, leading to more pronounced overburden displacement and stress concentration. Notably, in the presence of a 20-m-width underfilling zone on each side of the mining face and the development of roof fractures, the maximum overburden displacement and peak stress post-mining increased by 40.3% and 44.5%, respectively, compared to those of the control group with no roof fracture. This seriously threatens mining safety. Although support failure also led to more pronounced overburden displacement and stress concentration, it exerted relatively limited influence. Under the operating condition with the presence of a 20-m-width underfilling zone on each side of the mining face and support failure, the maximum overburden displacement and peak stress post-mining were 2.1% and 5.6%, respectively, higher than those under intact support. In practical backfilling operations, the extent of underfilling zones can be effectively reduced by optimizing the roof-contact backfilling process. Meanwhile, enhanced control over fractured zones is required during mining to mitigate their adverse effects on the overburden stability. Additionally, sound support conditions can effectively reduce the overburden displacement and stress concentration in underfilling zones.