Ruofan Wang

Coal gangue (CG) backfill, utilizing magnesium slag as a cost-effective alternative to ordinary Portland cement, represents a critical technology for underground waste management in China. Crucially, the confined compressibility of this matrix governs its long-term deformation resistance. However, since CG is enriched with hydrophilic clay minerals that trigger intense water uptake, the compressibility of this weakly cemented backfill is highly susceptible to deterioration under the pervasive high-humidity conditions of underground coal mines, which remains poorly understood. To address this gap, this study investigated the multi-scale confined compression behavior of magnesium slag-based (MPB) and conventional cement-based (CPB) fine CG aggregate backfill cured under a high-humidity environment. One-dimensional confined compression tests using a triple-lever oedometer and scanning electron microscopy (SEM) observations were integrated across varied curing ages. The results indicate that the macroscopic confined compression response is governed by the coupling effects of the moisture-induced softening of fine CG aggregates and the progressive evolution of hydration minerals. With prolonged curing, increased moisture absorption elevates the void ratio variation and compression index (C_c), thereby increasing matrix compressibility. The post-yield C_c after the bond breaks increases with the higher binder content because abundant hydration products encapsulate aggregates, which undergo catastrophic shearing after the cement bond breaks. Conversely, the consolidation coefficient continuously decreases under advancing loads as matrices densify. Although CPB consistently maintains lower compressibility than MPB, both backfills exhibit a time-dependent decline in the yield stress of the cement bond under high-humidity curing. These macro-mechanical degradations are supported by SEM observations; the experimental accuracy was validated through benchmark tests on standard quartz sand.

Mechanical performance of the intervening pillar critically influences extraction efficiency and operational safety in multiple stope-and-fill mining operations. However, the time-dependent interactions between backfill and creeping rocks and the resulting impact on pillar behavior remain poorly quantified, posing significant challenges in scheduling optimization, designs of pillar and backfill. This study employed FLAC3D to investigate the time-dependent geomechanical responses of pillar during sequential stope-and-fill operations, explicitly incorporating volumetric hardening behavior of consolidated backfill and creep behavior of rock mass. Results indicate that a short stope sequencing interval enables the consolidated backfill in the first stope to provide immediate lateral confinement to the pillar upon excavation of the second stope, improving stability. Conversely, extended intervals allow greater rock creep and enhance backfill densification and swelling pressure that eventually causes a beam-like bending of the pillar after adjacent excavation. It further leads to an asymmetric stress state characterized by tensile minimum principal stress σ ₃ in the outer region (adjacent to the new stope), and high differential stress and elevated brittle shear ratio BSR in the inner part (against the consolidated backfill), marking the most critical state of pillar instability. An explicit dual-indicator framework based on σ ₃ and BSR were proposed to capture this asymmetric mechanical response. Substantial parametric analyses were conducted to analyze the effects of stope geometry, pillar width, mine depth, backfill compression and swelling indexes, and rock viscosity on the results. Notably, pillar stability significantly recovered once the second stope was backfilled. These insights provide a mechanistic basis for optimizing the extraction strategy and the design of backfill and pillar in squeezing ground to minimize dilution and enhance operational safety and efficiency.

Real-time evaluation of the damage location and level of rock mass is essential for preventing underground engineering disasters. However, the heterogeneity of rock mass, which results from the presence of layered rock media, faults, and pores, makes it difficult to characterize the damage evolution accurately in real time. To address this issue, an improved method for rock damage characterization is proposed. This method optimizes the solution of the global shortest acoustic wave propagation path in the medium and verifies it with layered and defective media models. Based on this, the relationship between the inversion results of the wave velocity field and the distribution of rock damage is established, thereby achieving quantitative characterization of rock damage distribution and degree. Thus, the improved method is more suitable for heterogeneous rock media. Finally, the proposed method was used to characterize the damage distribution evolution process of rock media during uniaxial compression experiments. The obtained results were compared and analyzed with digital speckle patterns, and the influencing factors during the use of the proposed method are discussed.

The compressibility of mining backfill governs its resistance to the closure of surrounding rock mass, which should be well reflected in numerical modeling. In most numerical simulations of backfill, the Mohr–Coulomb elasto-plastic model is used, but is constantly criticized for its poor representativeness to the mechanical response of geomaterials. Finding an appropriate constitutive model to better represent the compressibility of mining backfill is critical and necessary. In this paper, Mohr–Coulomb elasto-plastic model, double-yield model, and Soft Soil model are briefly recalled. Their applicability to describing the backfill compressibility is then assessed by comparing numerical and experimental results of one-dimensional consolidation and consolidated drained triaxial compression tests made on lowly cemented backfills available in the literature. The comparisons show that the Soft Soil model can be used to properly describe the experimental results while the application of the Mohr–Coulomb model and double-yield model shows poor description on the compressibility of the backfill submitted to large and cycle loading. A further application of the Soft Soil model to the case of a backfilled stope overlying a sill mat shows stress distributions close to those obtained by applying the Mohr–Coulomb model when rock wall closure is absent. After excavating the underlying stope, rock wall closure is generated and exercises compression on the overlying backfill. Compared to the results obtained by applying the Soft Soil model, an application of the Mohr–Coulomb model tends to overestimate the stresses in the backfill when the mine depth is small and underestimate the stresses when the mine depth is large due to the poor description of fill compressibility. The Soft Soil model is recommended to describe the compressibility of uncemented or lightly cemented backfill with small cohesions under external compressions associated with rock wall closure.