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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.

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.

Underhand cut-and-fill mining has allowed for the safe extraction of ore in many mines operating in weak rock or highly stressed, rockburst-prone ground conditions. However, the design of safe backfill undercuts is typically based on historical experience at mine operations and on the strength requirements derived from analytical beam equations. In situ measurements in backfill are not commonplace, largely due to challenges associated with instrumenting harsh mining environments. In deep, narrow-vein mines, large deformations and induced stresses fracture the cemented fill, often damaging the instruments and preventing long-term measurements. Hecla Mining Company and the Spokane Mining Research Division of the National Institute for Occupational Safety and Health (NIOSH) have worked collaboratively for several years to better quantify the geomechanics of cemented paste backfill (CPB), thereby improving safety in underhand stopes. A significant focus of this work has been an extensive in situ backfill instrumentation program to monitor long-term stope closure and induced backfill stress. Rugged and durable custom-designed closure meters were developed, allowing measurements to be taken for up to five successive undercuts and measuring closures of more than 50 cm and horizontal fill pressures up to 5.5 MPa. These large stope closures require the stress–strain response of the fill to be considered in design, rather than to rely solely on traditional methods of backfill span design based on intact fill strength. Furthermore, long-term instrument response shows a change in behavior after 13–14% strain, indicating a transition from shear yielding of the intact, cemented material to compaction of the porosity between sand grains, typical of uncemented sand fills. This strain-hardening behavior is important for mine design purposes, particularly for the use of numerical models to simulate regional rock support and stress redistribution. These quantitative measurements help justify long-standing assumptions regarding the role of backfill in ground support and will be useful for other mines operating under similar conditions.

This paper reviews the design and application of paste backfill in underground hard rock mines used as ground support for pillars and walls, to help prevent caving and roof falls, and to enhance pillar recovery for improved productivity. Arching after stope filling reduces vertical stress and increases horizontal stress distribution within the fill mass. It is therefore important to determine horizontal stress on stope sidewalls using various predictive models in the design of paste backfill. Required uniaxial compressive strength (UCS) for paste backfill depends on the intended function, such as vertical roof support, development opening within the backfill, pillar recovery, ground or pillar support, and working platform. UCS design models for these functions are given. Laboratory and backfill plant scale designs for paste backfill mix design and optimization are presented, with emphasis on initial tailings density control to prevent under-proportioning of binder content. Once prepared, paste backfill is transported (or pumped) and placed underground by pipeline reticulation. The governing elements of paste backfill transport are rheological factors such as shear yield stress, viscosity, and slump height (consistency). Different models (analytical, semi-empirical, and empirical) are given to predict the rheological factors of paste backfill (shear yield stress and viscosity). Following backfill placement underground, self-weight consolidation settlement, internal pressure build-up, the arching effect, shrinkage, stope volume, and wall convergence against backfill affect mechanical integrity.

Intensive and massive coal mining causes a series of geological hazards and environmental problems, especially surface subsidence. In recent years, backfill-strip mining has been applied to control mining subsidence in order to realize sustainable development of the mining environment. To accurately predict the surface subsidence of backfill-strip mining, a prediction method of subsidence superposition of backfill-strip mining is proposed on the basis of the traditional probability integral method prediction model. In analyzing the distribution of the actual subsidence space, the surface subsidence problem of backfill-strip mining can be regarded as the superposition of surface subsidence caused by backfill mining and strip mining. Then, the appropriate prediction parameters will be chosen, and the surface subsidence caused by the backfill mining and strip mining will be predicted separately. The surface subsidence values of the backfill-strip mining are equal to the superposition subsidence values predicted by the backfill mining and strip mining prediction method at the same surface location. A similar material model and a numerical simulation model have been built to verify the feasibility and accuracy of the superposition prediction method. The comparison results of the surface subsidence values show that the superposition surface subsidence prediction method is reasonable. The average relative error of this superposition prediction method is less than 6.7%, and its accuracy is 3.9%~11.4% higher than that of the conventional prediction method. The superposition prediction method can satisfy the precision requirement of engineering applications. This study provides a scientific technical reference for safe mining engineering design and surface disaster protection for backfill-strip mining.

A backfill system in underground mines supports the walls and roofs of mined-out areas and improves the structural integrity of mines. However, there has been a significant gap in the visualization and monitoring of the backfill progress. To better observe the process of the paste backfill material filling the tunnels, a LiDAR-based backfill monitoring system is proposed. As long as the rising top surface of the backfill material enters the LiDAR range, the proposed system can compute the plane coefficient of this surface. The intersection boundary of the tunnel and the backfill material can be obtained by substituting the plane coefficient into the space where the initial tunnel is located. A surface point generation and slurry point determination algorithm are proposed to obtain the point cloud of the backfill body based on the intersection boundary. After Poisson surface reconstruction and volume computation, the point cloud model is reconstructed into a 3D mesh, and the backfill progress is digitized as the ratio of the backfill body volume to the initial tunnel volume. The volumes of the meshes are compared with the results computed by two other algorithms; the error is less than 1%. The time to compute a set of data increases with the amount of data, ranging from 8 to 20 s, which is sufficient to update a set of data with a tiny increase in progress. As the digitized results update, the visualization progress is transmitted to the mining control center, allowing unexpected problems inside the tunnel to be monitored and addressed based on the messages provided by the proposed system.

The ordinary cemented tailings backfill (CTB) is a cement-based composite prepared from tailings, cementitious materials, and water. In this study, a series of laboratory tests, including uniaxial compression, digital image correlation measurement, and scanning electron microscope characteristics of fiber-reinforced CTB (FRCTB), was conducted to obtain the uniaxial compressive strength (UCS), failure evolution, and microstructural characteristics of FRCTB specimens. The results show that adding fibers could increase the UCS values of the CTB by 6.90% to 32.76%. The UCS value of the FRCTB increased with the increase in the polypropylene (PP) fiber content. Moreover, the reinforcement effect of PP fiber on the CTB was better than that of glass fiber. The addition of fiber could increase the peak strain of the FRCTB by 0.39% to 1.45%. The peak strain of the FRCTB increased with the increase in glass fiber content. The failure pattern of the FRCTB was coupled with tensile and shear failure. The addition of fiber effectively inhibited the propagation of cracks, and the bridging effect of cracks by the fiber effectively improved the mechanical properties of the FRCTB. The findings in this study can provide a basis for the backfilling design and optimization of mine backfilling methods.

To test the impact of different mixture ratios on backfilling strength in Fankou lead–zinc mine, various mixture ratio designs have been conducted. Meanwhile, to improve the strength of ultra-fine tailings-based cement paste backfill (CPB), two kinds of fibers were utilized in this study, namely polypropylene (PP) fibers and straw fibers. To achieve these, a total of 144 CPB backfilling scenarios with different combinations of influenced factors were tested by uniaxial compressive tests. The test results indicated that polypropylene fibers improve the strength of CPB, while in some scenarios the addition of straw fibers decreases the strength of CPB. In this research, the support vector machine (SVM) technique coupled with three heuristic algorithms, namely genetic algorithms, particle swarm optimization and salp swarm algorithm (SSA), was developed to predict the strength of fiber-reinforced CPB. Also, the optimal performance of metaheuristic algorithms was compared with one fundamental search method, i.e., grid search (GS). The overall performance of four optimal algorithms was calculated by the ranking system. It can be found that these four approaches all presented satisfactory predictive capability. But the metaheuristic algorithms can capture better hyper-parameters for SVM prediction models compared with GS-SVM method. The robustness and generalization of SSA-SVM methods were the most prominent with the R2 values of 0.9245 and 0.9475 for training sets and testing sets. Therefore, SSA-SVM will be recommended to model the complexity of interactions for fiber-reinforced CPB and predict fiber-reinforced CPB strength.

In consideration of the high filling costs and backfill material shortage at present, backfill–strip mining, which combines the advantages of strip mining and backfill mining, has gradually been adopted to control surface subsidence. In this study, similar material modeling is established to simulate strata movement characteristics of backfill–strip mining. The displacement and deformation values of this similar material modeling are precisely acquired through close-range photogrammetry and optical image methods, respectively. On this basis, structural and movement characteristics of the overlying strata are investigated in different stages to reveal the strata subsidence control mechanism of backfill–strip mining. The dynamic deformation characteristics of the overlying strata in mining are also explored. This study provides a scientific technical reference for safe mining engineering design and surface disaster protection for backfill–strip mining.

In engineering practices such as backfill mining, the existence and characteristics of the rock-backfill interface significantly influence the propagation of explosive stress waves and the dynamic response of the rock mass. This study aims to reveal how, among these characteristics, different backfill media and their wave impedance differences with the rock specifically affect these processes. To this end, physical experiments were combined with numerical simulations, utilizing Digital Image Correlation (DIC) technology to capture strain field evolution under blasting, and LS-DYNA was employed for numerical analysis. The research indicates that the wave impedance difference between the rock and the backfill medium is key to controlling stress wave propagation and energy distribution. When the wave impedances are similar, stress wave transmission is dominant, leading to higher peak strain values at various points and a more uniform action of the stress wave on the rock mass, resulting in relatively uniform final failure. When the wave impedance difference is large, interface reflection is enhanced, forming a significant “blocking effect”; in this case, although the initial peak strain may be lower, the continuous action of the interface can lead to greater final strain and plastic deformation in the rock mass. Numerical simulations further show that the greater the wave impedance difference, the more pronounced the “guiding” and “blocking” effects of the backfill on crack propagation. This leads to intensified damage and crack accumulation in the rock mass adjacent to the backfill due to energy concentration, which also results in a higher fractal dimension of the cracks. Therefore, the type of backfill medium directly determines the strain response characteristics and final damage patterns of the rock mass by modulating the reflection and transmission behavior of waves at the interface. These findings have practical guiding significance for optimizing backfill material selection and blasting parameter design in backfill mining.