Solid Backfill Mining Research Articles

Dynamic leaching behaviors of heavy metals from recycled coal gangue aggregate under loading conditions during solid backfill mining

In solid backfill coal mining, recycled coal gangue aggregate (RCGA) is subject to the combined effects of overlying strata stress and leaching by mine water in the goaf. This process causes heavy metals to be leached and released from RCGA, which can lead to groundwater contamination. In this study, the release patterns of heavy metals in RCGA under the coupled effects of stress, solution pH, and solution flow rate were investigated, and the interactions between RCGA and the surrounding environment were explored. The findings indicate that: (1) The combined action of effective stress and mine water promotes the leaching of heavy metals. Specifically, the leaching of Pb, Zn, and Mn of RCGA is primarily influenced by the pH value of the leaching solution, while the leaching of Cu and Cr of RCGA is more closely related to the stress level; (2) Acidic environments accelerate the release of carbonate-bound fraction (CAR) in elements, facilitating the transformation of Fe/Mn oxide-bound fraction (XXI) into soluble forms; (3) The leaching ratios (Lr) of heavy metals follow the order: LrZn > LrPb > LrMn > LrCu > LrCr. This research provides guidance for the safe application of RCGA in solid backfill coal mining.

Study on Deformation Response Law of Surrounding Rock in Solid Backfill Mining Stope

To fully control the roof and surface and reduce the ecological damage caused by mining, it is necessary to understand the deformation response law of the surrounding rock in solid backfill mining. The similarities and differences of overburden movement characteristics between natural caving method and solid backfill method are analyzed and compared by means of on-site investigation and monitoring, theoretical derivation and analysis, numerical simulation and effect verification. The control mechanism of backfill mining on the roof is clarified, and the relationship between stope roof displacement and support pressure of backfill body is established. Furthermore, the mechanical conditions of immediate roof in equilibrium state during backfill mining are established. The FLAC3D simulation indicates that the solid backfill mining face has a concentrated stress peak located 15 m ahead of the coal wall. Within the range from 0 to 5 m, the abutment pressure is reduced gradually. However, within the range from 5 to 15 m, the abutment pressure is increased evidently. The stress concentration is located 35 m behind the working face. With continuous backfilling of the goaf, the maximum displacement occurs in the backfill body near the working face. The total deformation of the surrounding rock in the goaf is reduced by 58%. The total deformation of coal wall is reduced by ∼37.5%. The subsidence coefficient is reduced from 0.59 to 0.013 by using the backfill mining technology. The research results provide theoretical guidance and reference for roof control and ecological protection of backfill mining.

Autonomous Process Execution Control Algorithms of Solid Intelligent Backfilling Technology: Development and Numerical Testing

This paper analyzes the typical technical problems arising from dumping and tamping collision interferences in the working faces of conventional mechanized solid backfilling mining (SBM). Additionally, the technical and consecutive characteristics of the solid intelligent backfilling (SIB) method, the execution device, and the corresponding process categories of the SIB process are analyzed. A design for an SIB process flow is presented. Critical algorithms, including automatic recognition and optimization planning based on the cost function and laying the algorithm foundation, are proposed to develop a backfilling process control system. A joint simulation test system is built on a MATLAB/Simulink simulation toolkit (MSST) to simulate and test the optimized algorithms. The results show that the optimized algorithm can realize the automatic optimization planning and automatic interference-recognition adjustment of the backfilling process under actual engineering conditions. In conclusion, this paper analyzes typical technical problems in the conventional backfilling process, designs the SIB process flow, and develops key algorithms to achieve the automatic control of the backfilling process.

Study on dynamic adsorption characteristics of gangue to heavy metals in goaf and its water purification mechanism under leaching condition

Arid and semi-arid mining areas in Western China are faced with the problems of water shortage, large water inflow of mines and high-salinity of mine water. When the broken gangue is backfilled to the goaf by the solid backfilling mining technology, a large number of voids between the blocks is generated, which can store groundwater and purify water quality. In this study, the real void structure of the gangue backfilling body in the goaf was obtained through CT scanning. By using the COMSOL numerical simulation software, a coupling model of Free and Porous Media Flow and Transport of Diluted Species in Porous Media was established. From the mesoscopic void scale, the effects of particle size distribution, inflow velocity and ion concentration on the flow field characteristics, ion migration and dynamic adsorption characteristics of gangue in the leaching column were explored, and the purification mechanism of broken gangue to heavy metals in mine water was revealed. The results show that: (1) The larger the gangue particle size gradation, the larger the void size, the higher the velocity in the void, and the more significant the average flow velocity difference between the void area and the gangue area. With the increase of inflow velocity, the average velocity in the gangue area and void area also increases, but the average flow velocity in the void area is 5.36 times that in the gangue area; (2) Both the larger particle size gradation of gangue, higher inflow velocity and inlet concentration can improve the velocity of the ion migration in gangue leaching column; (3) The greater the inlet concentration, the faster the flow velocity, the higher the total adsorption rate of gangue in the leaching column; (4) Both decreasing the particle size gradation of the gangue and increasing the inlet concentration can increase the total adsorption capacity of gangue in the leaching column; (5) The removal of heavy metals by gangue is the result of physical adsorption, ion exchange and other reactions. The research results can provide theoretical reference and data support for the treatment of high-salinity mine water in mining areas of Western China.

Control mechanism of the migration of heavy metal ions from gangue backfill bodies in mined-out areas

In the process of solid backfill mining, the leaching of heavy metal ions from the gangue backfill body in the mined-out area can pose potential risk of polluting water resources in the mine. Accordingly, based on the environment of the gangue backfill body, the migration model of heavy metal ions from the gangue backfill body was established to reveal the pollution mechanism of water resources by the gangue backfill body in the mined-out area. The main factors that affect the migration of heavy metal ions were analyzed, and prevention and control techniques for the leaching and migration of heavy metal ions from gangue backfill bodies were proposed. Research showed that the heavy metal ions in gangue backfill bodies were subjected to the coupled action of seepage, concentration, and stress and then driven by water head pressure and gravitational potential energy to migrate downward along the pore channels in the floor, during which mine water served as the carrier. The migration distance of heavy metal ions increased with time. According to the migration rate, the migration process can be subdivided into three phases: the rapid migration phase (0–50 years), the slow migration phase (50–125 years), and the stable phase (125–200 years). It was concluded that the leaching concentration of heavy metal ions, the particle size of gangue, the permeability of floor strata, and the burial depth of coal seams were the main influencing factors of the migration of heavy metal ions. From the two perspectives of heavy metal ion leaching and migration, prevention and control techniques for the leaching and migration of heavy metal ions from gangue backfill bodies were proposed to protect water resources in mining area. The present study is of great significance to realizing utilization of solid waste in mines and protecting the ecological environment.

A Design Method and Application of Backfilling Hydraulic Supports in Reforming Top-Coal Caving Hydraulic Supports

Solid backfilling is an environmentally friendly method of coal mining. Backfilling hydraulic supports are important devices in solid backfill mining. In order to speed up the design and manufacturing process of them, this paper proposes a method of hydraulic support reformation and evaluation. The mechanical models of top-coal caving hydraulic support and backfilling hydraulic support were established to analyze the stress characteristics of the main structure. ABAQUS was used to establish the 3D numerical model to analyze the load-bearing characteristics, as well as verify the strength requirement. In addition, the structural characteristics, compaction characteristics, geological adaptation characteristics, and interference characteristics of the structure were analyzed using motion simulation. After reforming, the ramming force is just above 2.0 MPa, and the utilization ratio of the old parts of the top-coal caving hydraulic support is more than 50%, which saved the costs of the new backfilling hydraulic support and reduced the waste of idle top-coal caving hydraulic support. The successful application of the backfilling hydraulic support in the Tangshan mine showed the rationality of the reform design.

A Theoretical Model of Roof Self-Stability in Solid Backfilling Mining and Its Engineering Verification

Roof self-stability in backfilling mining was proposed to explore its connotation and characteristics after a comparative analysis of roof structures under long-wall caving and backfilling mining. The mechanical analysis models of roof self-stability along strike and dip were established. After that, the mechanical equations for cooperative roof control were constructed to analyze the elastic foundation coefficients of the backfill, support peak load, unsupported-roof distance, and drilling effect of the working face along strike, the size of the working face, and the section pillar effect along dip. Research showed that the roof self-stability was greatly impacted by the elastic foundation coefficient of backfill, and it was less impacted by the support peak load along strike. The unsupported-roof distance had no obvious effect on roof self-stability. Roof self-stability was significantly affected by the working face and coal-pillar length along the dip. Therefore, the engineering control method of roof self-stability was proposed. The backfilling engineering practice in Xinjulong Coal Mine showed that the maximum roof subsidence was 438 mm, and the backfill ratio was 86.3% when the supporting intensity of backfilling hydraulic support was 0.94 MPa; the advanced distance of the working face was greater than 638 m; the foundation coefficient of backfilling material was 4.16 × 108 Nm−3. The roof formed the self-stability structure, which satisfied safe coal mining under buildings, water bodies, and railways.

Study on Macro-Meso Deformation Law and Acoustic Emission Characteristics of Granular Gangue under Different Loading Rates

Bulk gangue is a common backfill material in solid backfill mining. After backfilling into the goaf, bulk gangue serves as the main body to bear the load of overlying strata, and its deformation resistance is the key factor affecting the backfill quality. In this study, the laterally confined compression test of broken gangue was designed, the compaction deformation characteristics of gangue specimens under different loading rates were studied, the acoustic emission (AE) energy characteristics of gangue specimens under compression were analyzed, and the relationship model between macroscopic deformation of broken gangue under compression and AE energy was established. The particle flow numerical software PFC2D was used to stimulate the particle breakage in the gangue compaction process, and the coal gangue particle model was established through particle cluster units. The particle force chain distribution and fracture evolution characteristics of gangue specimens in the compression process were studied, and the macroscopic deformation mechanism was revealed from the mesoscopic perspective. The results showed that: the porosity variation of the gangue specimen increases with the increase of loading rate; the porosity increases with the decrease in the strain, the porosity decreases with the increase in the stress, and the relationship between porosity and stress is monotonously decreasing. With the increase of loading rate, the AE signals produced by particle breakage become stronger, while the influence of the loading rate on the maximum strain, fragmentation and AE signal of the specimen is gradually weakened. Under different loading rates, the “instability-optimization” of the skeleton force chain structure of the gangue model and the crushing-recombination of cracks are the main reasons for the compaction deformation of gangue specimens at the early stage of loading. The research results are of great significance to reveal the deformation mechanism of coal gangue as backfill materials under compression.

Evaluation of Operating Performance of Backfilling Hydraulic Support Using Six Hybrid Machine Learning Models

Previously conducted studies have established that surface subsidence is typically avoided by filling coal mined-out areas with solid waste. Backfilling hydraulic supports are critically important devices in solid backfill mining, whose operating performance can directly affect backfill mining efficiency. To accurately evaluate the operating performance, this paper proposes hybrid machine learning models for the operating states. An analysis of the factors that influence operating performance provides eight indices for evaluating backfilling hydraulic supports. Based on the data obtained from the Creo simulation model and field measurement, six hybrid models were constructed by combining swarm intelligent algorithms and support vector machines (SVM). Models of the SVM optimized by the modified sparrow search algorithm have shown improved convergence performance. The results show that the modified model has a prediction accuracy of 95.52%. The related evaluation results fit well with the actual support intervals of the backfilling hydraulic support.

Numerical Simulation and Field Measurement Analysis of Fracture Evolution and Seepage Response of Key Aquiclude Strata in Backfill Mining

Solid waste backfill mining can effectively deal with gangue and other mining wastes, as well as control the movement and damage of rock strata. In this paper, the RFPA2D rock failure process analysis software is used to study the fracture evolution and seepage response mechanism of the key aquiclude strata (KAS) under the conditions of different structural characteristics, interlayer rock thickness, and backfilling ratios in backfill mining. The simulation results show that, in backfill mining, soft rock plays a crucial role in the fracture repair of KAS with different structural characteristics. An increase in the KAS thickness from 15 to 35 m is shown to results in a continuously improved repair of KAS fractures. At the advancing distance of the working face of 50~100 m and the KAS thickness of 35 m, the minimum vertical seepage velocity of 0.06 ‐ 0.78 × 10 − 2 m / s is reached. An increase in the backfilling ratio from 45 to 80% improves the control effect on the overlying strata. A case study of backfill mining in the Wugou Coal Mine located in the Anhui Province of China was conducted. At the goaf backfilling ratio of 80%, the composite KAS’s good control effect was achieved, which minimized seepage and avoided the water in rush phenomena. The above engineering application ensured the safe backfill mining of coal resources.

On-Site Measurement on Compaction Characteristics of Coal Gangue and Surface Subsidence Disaster in Deep Backfilling Mining

Gangue is the main backfilling material in solid backfilling mining, and its compaction characteristics determine the overburden control effect and surface subsidence. Under the action of compaction force and overburden pressure, the gangue will be broken, rotated and occluded, resulting in different compaction characteristics of gangue in the field and laboratory. To this end, a laboratory gangue compaction test system was established to test the compaction characteristics of gangue in the laboratory, such as stress-strain, stress bulk density and stress deformation modulus. Based on actual geological conditions of backfilling mining in the Tangshan coal mine, the characteristics of stress deformation modulus of gangue under different inclination angles and mining heights were tested on-site. Through the Beidou satellite CORS system, the surface subsidence of working face F5001was monitored. The research results show that the stress deformation modulus of gangue measured in the field is slightly less than that measured in the laboratory, and it is maintained at about 27 MPa under the overburden pressure in the on-site measurement. Backfilling technology with an initial compaction force of 2.5MPa can be used to effectively control the surface subsidence, and the maximum subsidence value is only 61 mm.

Nonlinear Elastic Foundation Plate Model Study on Overlying Strata Movement in Working Face with Solid Backfilling Mining

Although solid backfilling materials are featured with obvious nonlinear stress-strain properties, for a long time, they have been usually simplified as linear elastic materials for approximate calculation in mechanical analysis, so it is difficult to accurately reflect their deformation process. Based on test results of solid backfilling materials’ compaction characteristics, this paper provides a solution method to generate their elastic foundation coefficient. One multiparameter elastic foundation has been used to reflect stress-strain characteristics of solid backfilling material. In addition, the paper establishes a thin plate on a nonlinear elastic foundation model by adopting semianalytical and seminumerical method and obtains the relational expression between roof deflection, roof stress, and backfilling material’s compressive deformation. In combination with geological conditions in a specific mine, the paper probes into what influence both backfilling material’s particle size and the initial compaction force that the backfilling material bears could exert on roof subsidence and stress. Finally, the proposed model has been verified with measured data from industrial tests.

Control of Water-Flowing Fracture Development with Solid Backfill Mining: Designing a Backfill Body Compression Ratio for Water Resources Protection

Control of Water-Flowing Fracture Development with Solid Backfill Mining: Designing a Backfill Body Compression Ratio for Water Resources Protection

Deformation response of roof in solid backfilling coal mining based on viscoelastic properties of waste gangue

Solid backfill mining (SBM) is a form of green mining, the core of which is to control and minimize the deformation and movement of strata above longwall coal mines. Establishing a mechanical model that can reliably describe roof deformation by considering the viscoelastic properties of waste gangue is important as it assists in improving mine designs and reducing the environmental impact on the surface. In this paper, the time-dependent deformation characteristics of gangue under different stress levels were obtained by using lateral confinement compression, that reliably represents the compaction of goaf. The viscoelastic foundation model for gangue mechanical response is different from the traditionally used elastic foundation model, as it considers the time factor and viscoelasticity. A mechanical model using a thin plate on a fractional viscoelastic foundation was established, and the roof deflection, bending moment, time-dependent, viscous and other characteristics of SBM were included and analyzed. Compared with the existing elastic foundation model, the proposed fractional order viscoelastic foundation model has higher accuracy with laboratory data. The plate deflection increases by 50.9% and the bending moment increases by 37.9% after 100 days, which the elastic model would not have been able to predict.

Constitutive Model of Solid Backfill Materials and Numerical Simulation of Overburden Movement and Deformation in Backfill Mining

Solid backfill mining is an efficient and environmental-friendly coal mining technology, which can effectively solve the problems of coal gangue pollution, water resource loss, and surface subsidence. Based on the mechanical behavior of backfill materials in the compaction process, volume strain was used to express the deformation modulus, and a constitutive model of backfill materials was proposed in this study. The ABAQUS UMAT was used to develop the numerical calculation subroutine of the model. Finally, the rationality of the model was verified that simulated stress-strain curves of the backfill materials during the compaction process agree well with experiments. Based on the proposed constitutive model, the influence of three factors (the initial compaction rate of the filling body, the mining height, and the mining depth) on the key strata and surface subsidence was analyzed systematically. The results show that the initial compaction rate and the height of coal seams have significant influences on surface subsidence. When the thickness of topsoil is only changed and the structural composition and lithology of overburden are not changed, the mining depth has little influence on surface subsidence, but a significant influence on surface subsidence range. The influence of mining height and mining depth on the deformation of key strata of overburden and surface subsidence is approximately linear, while the influence of the initial compaction rate is nonlinear.

Transparent characterization of spatial-temporal evolution of gangue solid wastes' void structures during compression based on CT scanning

In the solid backfill mining, gangue solid wastes (GSW) are used as the main filling materials. The void ratio and void structure of GSW filling materials under the loaded compression in the goaf are the key factors that affect the heavy metal ion migration with the mine water. In this paper, the transparency of the internal structure of GSW filling materials was realized by CT scanning experiment and 3D reconstruction technology, and then the spatial-temporal evolution characteristics of void ratio and void structure of GSW filling materials were studied under the loaded compression. The results show that: (a) With the progress of loading process, the connectivity between adjacent voids is getting worse, and closed voids are gradually formed; (b) The void's equivalent diameter is mainly concentrated in the range of 15 mm - 25 mm; (c) The change in the overall void ratio of GSW is mainly caused by the change of large voids, and the change of small voids has slight contribution to the change of the overall void ratio. However, the sharp increase in the number of small voids can reflect the poor connectivity and permeability between the voids. This research is of great significance to further understand the migration mechanism of heavy metal ions in the filled goaf, and protect the groundwater environment.

Compaction property prediction of mixed gangue backfill materials using hybrid intelligence models: A new approach

Solid backfill coal mining has become one of the main means of green mining in coal mines because of its ability to control surface subsidence and disposal of surface gangue. Mixed gangue backfill material (MGBM) is the key factor for stratum control in solid backfill mining, and its compact mechanical performance directly affects the efficiency of backfill mining. In order to better carry out backfill mining design and backfill effect evaluation, a new hybrid artificial intelligence model integrating support vector machines (SVM), differential evolution algorithm (DE) and gray wolf optimization algorithm (GWO) is proposed to predict the compaction property of MGBM. The cement, lime and fly ash materials are selected to be mixed with gangue backfill materials and a large number of compaction tests using a self-made circular cylinder barrel are carried out to provide the dataset for the DGWO-SVM hybrid model. The input variables of this model include cement content, lime content, fly ash content and overburden stress, and the output variable of the model is the compaction property of MGBM. The performance of the DGWO-SVM model is evaluated by R2, MAE and RMSE. The predictive results indicate that the DGWO-SVM hybrid model can accurately predict the compaction property of MGBM, and the R2 of the training set and the testing set are 0.9518, 0.9137. Meanwhile, the relative importance of each input variable is implemented using the MIV method, and the importance scores of cement content, lime content, fly ash content and overburden stress are 0.3266, 0.0738, 0.2448, 0.3548, respectively. The research results can provide guidance for the optimization design of solid backfill mining.

Formation mechanism and height calculation of the caved zone and water-conducting fracture zone in solid backfill mining

To study the heights of the caved zone and water-conducting fracture zone in backfill mining, the failure mechanism of strata during backfill mining was analyzed, and a method for determining the heights of the two zones was proposed based on key strata theory. The movement and failure regularity of the strata above the backfilling panel were revealed through numerical simulation. Considering the geologic conditions of the CT101 backfilling panel, the height of the fracture zone was determined using the proposed method along with empirical calculation, numerical simulation, and borehole detection. The results of the new calculation method were similar to in situ measurements. The traditional empirical formula, which is based on the equivalent mining height model, resulted in large errors during calculation. The findings indicate the reliability of the new method and demonstrate its significance for creating reference data for related studies.

Moving set‐up room theory of weakening the mining pressure caused by the backfilling body in solid backfilling mining

AbstractThe core of backfilling mining is to reduce the mining pressure in order to achieve precise control of strata position and status. Since disturbances and damages to the original rocks of the stope by set‐up room and backfilling working face are low, and due to the similar ground behavior mechanism, the “moving set‐up room” theory (short as MSUR [moving set‐up room] theory) is proposed to characterize the reduction of mining pressure caused by the backfilling body. The scientific connotation includes the qualitative characterization of the mining status, quantitative expression of the mining pressure weakening degree, and critical conditions while weakening the mining pressure caused by the backfilling body in the stope, as well as controlling the mining pressure in the stope based on the weakening effect. By combining with the analysis of the controlling features of backfilling mining pressure in the stope and the stress and displacement evolutions, the concept and characteristics of the MSUR theory of backfilling stopes were proposed. Simultaneously, the qualitative and quantitative scientific characterization methods of MSUR were investigated. Therefore, solutions to the critical conditions of the MSUR effect were proposed, and the control of mining pressure in the stope was designed based on the MSUR theory. This study provides references for precise control of strata position and status in backfilling mining.

Engineering design of selecting particle size of gangue under solid backfill mining for protecting water resources

In view of the ecological environment damages caused by the loss of water resources and the gangue accumulation during coal mining, this paper proposed a solid backfill mining method to recover the coal seam under the water body, which could fundamentally reduce the sinking space of the overlying strata, and better prevent water-flowing fractures expanding. Consequently, according to the deformation characteristics of the overlying strata of the solid backfill mining, a mechanical model was established for superimposed beams on elastic foundation with simu?lating the expansion water-flowing fractures under solid backfill mining. A method of calculating the height of the water-flowing fractured zone was provided, and the mechanical mechanism of the development of water-flowing fractures in the overlying strata under solid backfill mining was expressed. Meanwhile, the backfill rate of the experimental working face was designed as 80% to avoid the ecological environment damages caused by gangue accumulation. The stress-strain characteristics of gangue samples under different grading size schemes were further studied.