Chen Huang

In this study, to address the overlying strata control issue for the isolated working face of a super-thick coal seam confined by surrounding goafs and open-pit boundaries, we focus on mining via upward slicing strip paste filling in a Ningxia coal mine. We adopt strip coal pillar stability theory for safety factor analysis and conduct FLAC3D three-dimensional numerical simulations to quantitatively reveal overlying strata displacement, stress redistribution, plastic zone evolution, and surface subsidence response. Our theoretical calculations show that, when the mining width is 5 m, the safety factors of retained coal pillars with widths of 5, 10, and 15 m are <1, 1.3, and 1.8, respectively. Our numerical results indicate that the overlying strata of the first slice are dominated by continuous bending subsidence, with a maximum vertical displacement of 21.4 cm, increasing to 55.5 cm after four slices without through damage. High stress is mainly controlled by mined-out area boundaries and inter-face coal pillars, with the maximum principal stress of the fourth slice reaching 18.9 MPa. The surface subsidence center stably corresponds to the underlying backfill goaf, with a maximum value of 11.4 cm. Our research demonstrates that slicing strip filling can suppress deformation and stress concentration risks by reconstructing load transfer and realizing synergistic bearing, as reflected by the limited surface subsidence of 11.4 cm, the controlled maximum principal stress of 18.9 MPa, and the improved coal pillar safety factor from <1 to 1.3–1.8 under wider retained pillars, providing a basis for optimizing strip pillar width and mining–filling parameters.

Backfill mining is a key technique for achieving mine stratum control and synergistic disposal of solid waste. Cemented backfill, as the core bearing structure of the mined area, plays an essential role in ensuring engineering safety and promoting green mining practices. This review systematically explains the multiscale mechanisms of strength development in backfill and establishes a three-tier analytical framework based on “Materials–Environment–Structure”. At the material level, previous research has revealed the microscopic mechanisms involving hydration products and the interfacial transition zone (ITZ); at the environmental level, the coupling of multiphysical fields during strength evolution has become a focal point of study. The core challenge lies in the fact that large-volume backfill under field conditions, which forms layered structures, is anisotropic and exhibits size effects during sedimentation and processing stages, significantly controlling the macroscopic mechanical behavior. This is difficult to fully reveal through laboratory research focused on homogeneous materials. The authors provide a comprehensive review of experimental, simulation, and theoretical results, with a focus on revealing the controlling mechanisms of factors such as layered weak interfaces, inclined interfaces, and structural scale for backfill strength and failure modes. Moreover, the proposed “Materials–Environment–Structure” framework offers predictive potential for linking multiscale strength formation mechanisms with field-scale structural performance. The goal is to advance the understanding of strength from material constitutive models to structural responses, providing theoretical support for the development of strength prediction methods and structural designs suitable for engineering practice.

In this paper, the effects of static stresses on the blast‐induced crack in polymethyl methacrylate specimens with an empty hole were investigated by caustic system. The propagation behavior and re‐initiation process of blast‐induced crack under initial static stress and high stress unloading were investigated. The results show that when the direction of the empty hole is perpendicular to the static stress, the static stress reduces the length of the main crack. However, when the static stress is very high, the length of main cracks does not decrease. In addition, the static stress reduces the crack velocity and dynamic stress intensity factor (DSIF) and suppresses the increase of DSIFs and crack velocity due to reflected wave. The crack arrested under high stress unloading propagates again in the later stage. Moreover, the duration of high stress unloading lasts approximately 1 ms.

In this study, the effects of preloaded uniaxial compressive stress and defect (prefabricated by laser cutting) curvature on blast-induced cracks and stress wave propagation were investigated in polymethyl methacrylate (PMMA) specimens using caustics theory. Based on the mathematical relationship between stress and optics, the fracture behavior (the propagation path, dynamic stress intensity factors (DSIFs), propagation velocity, initiation angle of the main crack, and damage degree between the defect and the blasthole) was calculated and analyzed quantitatively. The results show that the preloaded stress could mainly restrain the main crack propagation in the horizontal direction and reduce the initiation angle and damage degree at the defects. Meanwhile, the crack initiation position of the positive curvature defect was not at the end of the defect. In addition, the curvature of the defect significantly affected the blast-induced fracture compared to preloaded stresses. Comparing the main crack with other curvature defects, the DSIFs, velocity, crack arrest time, crack length, and horizontal offset distance of the main crack with negative curvature defect were the largest. The variation in the full stress field in the specimen under blasting and preloaded stress was simulated by a numerical model. The results show that the curvature of the defect has a significant effect on tensile waves at the end of defects, and the tensile stress from high to low was C_-25, C₀, and C₂₅, in that order.