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Backfill coal mining technology has drawn widespread attention due to its benefits of “controlling surface deformation and subsidence, reducing mining-induced disturbance in the stope, and recycling solid mine wastes”. However, the backfill coal mining technology is still progressing slowly in China. The geological environment of China’s mining areas is complex and highly diversified, and backfill coal mining is expected to fulfill different goals in a wide range of engineering scenarios. These facts explain the poor reproducibility of backfill coal mining projects. This study reviews the existing backfill coal mining systems in China. Based on findings from a survey of engineering cases, we summarize five types of new backfill coal mining methods classified by deployment style; namely, borehole grouting backfill, roadway backfill, borehole–roadway backfill, in situ backfill, and roadway-in-situ backfill. A total of 15 backfill coal mining methods falling into the above five categories are described. An engineering design workflow for backfill coal mining consisting of five steps is proposed; namely, identifying the targets of backfill, analyzing the feasibility of deploying the backfill system, comparing the engineering quantities of different engineering schemes, estimating the economic efficiency of backfill, and backfill performance tracking and monitoring. Real cases of backfill engineering design are analyzed to inform the fast and reasonable design of backfill strategy for specific working faces in certain coal mines.

During highwall backfill mining, the backfill substantially impacts the mechanical properties and stability of coal pillars. Analyzing the mechanical properties and failure characteristics of coal pillars under cemented paste backfill constraints is crucial for managing slope stability and enhancing resource recovery rates. The mechanical behavior of the backfill-coal pillar-backfill (BCB) was systematically examined under varying conditions of backfill ratios and strengths through a series of confined compression tests. The stress-strain curves, failure strength, and failure modes of BCBs were determined. Numerical simulations were used to evaluate the effectiveness of backfill in controlling slope deformation. The findings showed that the stress-strain curves of BCBs can be delineated into five stages: pore compaction, elastic bearing of the coal pillar, crack development and coalescence, failure of the coal pillar, and coal pillar bearing under backfill constraint. With low backfill ratios and strength, the failure strength of coal pillars was less than that without backfill. Increasing the backfill ratio and strength improved the failure strength of the coal pillars, changing the failure mode from localized shear failure in the upper unconstrained sections to a global shear-tensional composite failure. Once roof contact was made, the backfill provided active confinement and supported part of the load, greatly enhancing the coal pillars’ load-bearing capacity. Through numerical simulation, the efficacy of backfill control was quantified, further revealing a non-linear variation. The backfill transitioned from providing passive confinement to engaging in active, cooperative load-bearing when the backfill ratio increased from 90% to 100%, effectively eliminating the plastic zone on the slope surface and minimizing slope deformation.

The stability of backfill strips is a central focus in strip backfill mining, as it determines both the design rationale and the applicability range of this mining technique. However, analytical approaches for evaluating backfill strip stability remain underdeveloped. In this study, a novel aeolian sand-based paste backfill material was developed using aeolian sand as the aggregate and alkali-activated fly ash as the binder. The resulting material exhibits high fluidity and substantial strength, offering a cost-effective and high-performance backfill solution for coal mining in aeolian sand-rich regions. A numerical simulation model for strip backfill mining was established, showing that the plastic zone width of aeolian sand-based backfill strips increases with greater mining height and depth but decreases with a higher area filling ratio. Through ternary linear regression, an empirical equation was derived to relate plastic zone width to these three parameters. By comparing the mechanical behaviors of backfill and coal strips and integrating A.H. Wilson’s two-zone constraint theory with King’s effective area theory, a stability expression for backfill strips was formulated for the first time. Field verifications demonstrated that the proposed stability analysis method for aeolian sand-based paste backfill strips is both rational and accurate, providing a reliable theoretical basis for engineering applications.

Solid backfill technology, which can achieve precise control of surface subsidence, has become the primary method used to extract “under three” coal resources (under railways, buildings, and water bodies), especially under buildings. This paper proposes a probability integration model for surface subsidence prediction based on the equivalent mining height (EMH) theory and describes the basic control principle for surface subsidence, i.e., guaranteeing a maximum security standard for surface buildings, based on the maximum EMH, by controlling the backfill body’s compression ratio (BBCR). Based on this control principle, an engineering design process for solid backfill mining under buildings was established, and an engineering design method that employs the BBCR as the critical control indicator and a method for determining the key parameters in subsidence prediction are proposed. In applications at the Huayuan coal mine in China, the measured subsidence values were less than predicted; the measured BBCR was controlled at a level higher than 90 %, which was greater than in the theoretical design; the surface subsidence of buildings was controlled at mining level I. The results of application of the methods proposed in this paper show that the basic principles of controlling the BBCR and maximum EMH provide clear guidance for surface subsidence control in solid backfill mining engineering practice.