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To realize the recycling of iron tailings powder (IP) and desulfurization ash (DA) and reduce the high preparation cost of mine filling cementitious materials (MCs), this article adopts sodium carbonate (SC) as an activator to prepare iron tailings powder-desulfurization ash mine filling cementitious materials (IDMC). The effects of IP content, DA content, SC content, and mirabilite content on the mechanical properties and setting time are experimentally investigated. The micromorphology and phase compositions of the hydration products of IDMC are analyzed and characterized by scanning electron microscopy and X-ray diffraction. The results show that the initial setting time of the IDMC is reduced by 0.87 and 21.83% when the mirabilite content is increased from 0 to 1% and 2%, respectively, and the compressive and flexural strengths of the IDMC are increased by 24.01 and 86.25% when the IP content is increased from 0 to 20%, respectively. The IP not only participates in the hydration reaction but also plays an aggregate filling effect, significantly improving the mechanical properties of the IDMC. The pozzolanic effect is gradually enhanced with the increase of the DA content, and the hydration degree of the IDMC increases. The SC as an activator can moderately reduce the shrinkage rate of the IDMC. Based on the multi-index optimization analysis, the optimal mix proportion of the IDMC is obtained, which provides an effective reference for the preparation of the novel MC.

Paste backfill prepared with ultrafine tailings often faces difficulties in meeting engineering requirements. To address the challenge of using ultrafine tailings, this work utilized a slag cemented ultrafine tailings paste backfill (SCUPB) composed of slag, Portland cement, and desulfurized gypsum as the cementitious materials, with a binder–tailing ratio of 1:4 and a solid mass concentration of 60%. The fluidity and compressive strength of the SCUPB were tested at 3 days, 7 days, and 28 days, while the microstructure and hydration process of the SCUPB were studied by scanning electron microscopy and isothermal calorimetry. The results reveal that the fluidity of the SCUPB decreases with the increase in the slag content and increases with the decrease in the c/g ratio. SCUPB attains the highest early compressive strength with a slag content of 70% and a c/g ratio of 3:2 (with the cement content at 18% and the desulfurized gypsum content at 12%). In this slag–gypsum–cement system, the combined activation of the slag by the cement and desulfurized gypsum exhibits a higher early compressive strength. This work utilized ultrafine tailings to prepare paste backfill that meets the engineering requirements, achieving the effective utilization of ultrafine tailings in mine backfilling projects. [ABSTRACT FROM AUTHOR]

The lateral response of combined pile-raft foundations (CPRFs) adjacent to tunnel excavation is a challenging problem owing to the complexity of the pile-raft connections. In current engineering practices, the impact of these connections on the lateral performance of CPRFs is frequently overlooked, despite their importance. To address this issue, this study conducted three-dimensional finite element analyses to evaluate the contribution of pile-raft connections to the tunnelling-induced lateral performance of CPRFs in saturated clay. In the analysis, both passive and active loading at the pile head could be considered by varying the tunnel depth. Several parameter studies, such as relative pile-raft modulus, pile embedded modulus, pile embedded depths, and pile shaft skin friction, were conducted to determine the optimal design parameters for CPRFs. The results indicate that pile-raft connections significantly affect the tunnellinginduced deflections and bending moments of pile groups. Inspired by the results, a simplified design method, the pile-raft connection coefficient Kc was proposed. Additionally, the pile-head restraint percentage was established to make a relationship with the pile-raft connection coefficient in order to assess the pile-raft connection and guide the pile-raft design. In this paper, the recommended range value of Kc is 10--200 and the range value of pile-head restraint percentage is 24%--42%. [ABSTRACT FROM AUTHOR]

In regard to goaf risk prediction, due to the low accuracy and single prediction method, this study proposes a method that combines the improved arithmetic optimization algorithm (IAOA) -- support vector machines (SVM) with GoCAD--FLAC3D numerical simulation. Thus, goaf risk is comprehensively predicted. From the perspectives of geological and engineering conditions, eight factors that affect goaf stability and 176 sets of sample data were determined. We utilized eight influencing factors such as rock mass structure, geological structure, and goaf burial depth as inputs, and the goaf risk level as the output. Moreover, an IAOA--SVM goaf risk prediction model was established. The 30 goaf areas of Yangla Copper Mine in Yunnan Province were selected as the research subject. First, the rationality of mechanical parameter values in the numerical model was verified using the parameter inversion method. Second, based on the GoCAD--FLAC3D numerical simulation method, the goaf risk analysis in Yangla Copper Mine was performed. Subsequently, using numerical simulation verification, the goaf filling effect was analyzed. Finally, the prediction results of the IAOA--SVM model were compared with that of other intelligent algorithms. The results indicate that the numerical simulation results of the GoCAD--FLAC3D model are consistent with those of IAOA--SVM and the actual results, which further verifies the effectiveness and superiority of the IAOA--SVM prediction model. Therefore, an innovative approach for goaf risk prediction is developed. [ABSTRACT FROM AUTHOR]

The interval time between successive filling processes in a stope is a critical parameter for metal mines. It is imperative to accurately determine this interval time. The relationship curves between curing time and heavy hammer settlement height under various types of cementing materials and slurry concentration conditions were obtained through the self-developed heavy hammer experiment. The mathematical model for the relationship between curing time and heavy hammer settlement height was fitted and regressed, with further analysis of the impact of slurry concentration and cementing material type on initial setting time. The results demonstrate an exponential relationship between curing time and heavy hammer settlement height, wherein the heavy hammer settlement height gradually decreases with increasing curing time. The initial setting time is significantly influenced by both the concentration of slurry and the type of cementing materials. Generally, with the same cementing material conditions, higher backfill concentrations result in shorter initial setting times. When the slurry concentration is 68 %, the initial setting time of A-type cementing materials occurs approximately 36.8 % earlier compared to that of C-type cementing materials, highlighting the crucial significance of selecting appropriate cementing materials for mine filling. The implementation of a consecutive filling strategy based on heavy hammer experimentation holds significant implications for enhancing the efficiency of mine filling, as exemplified by an iron mine in China.

Coal and electricity integration plays an important role in ensuring national energy security, but it still faces the challenge of carbon emission reduction. The development of in-situ CO2 sequestration and utilization technology for pithead power plants is an effective way to achieve low-carbon and efficient utilization of coal power. The preparation of foam concrete for mining using CO2 is a type of carbon capture, utilization, and storage technology featuring in-situ CO2 sequestration and utilization in pithead power plants and mine filling and sequestration. The purpose of this study is to evaluate the basic performance and carbon sequestration potential of Portland cement-based CO2 foam concrete (PC-CFC) as a mining material. In this study, PC-CFC was prepared through physical foaming and the carbonation pretreatment cement process. The influence of carbonation pretreatment time and CO2 foam content on density, strength, and carbon sequestration of PC-CFC was investigated. The experimental results showed that carbonation pretreatment could enhance the stability of CO2 foam in Portland cement and improve the CO2 foaming performance. With the extension of carbonation pretreatment time, the extend of dry density reduction decreases from 16.6% to 0.8%. A 60 min–90 min of carbonation pretreatment can achieve the best treatment performance. Carbonation pretreatment and CO2 foam can promote the degree of cement hydration, optimize the PC-CFC pore structure, and improve the compressive strength of PC-CFC. However, the PC-CFC material strength owing to the extended carbonation treatment time, which leads to well-developed vesicle distribution, has an overall decreasing trend. In addition, the 7 day (d) compressive strength of PC-CFC can reach more than 60% of the 28 d compressive strength, which has evident early strength characteristics. Extending the carbonation pretreatment time and CO2 foam content increased the PC-CFC carbon sequestration that ranged from 61.0 kg/ton to 85.7 kg/ton. The dry density of the PC-CFC material was significantly and positively correlated to the 28 d compressive strength and negatively correlated to the amount of carbon sequestration. In the case in which the best carbon sequestration effects is achieved, a single filling of the working face end can store ∼3929.31 kg of CO2; in the most economical condition, it can store ∼3642.79 kg of CO2. The conducted research provides new ideas for the low-carbon and green development of coal and electricity integration.

In order to solve the problem of formaldehyde release from the traditional phenolic foam (PF) in the process of coal mine filling and plugging. In this paper, the effect of different factors on the efficiency of formaldehyde trapping agents were studied by adding formaldehyde trapping agents containing amino compounds after PF synthesis. The effect of formaldehyde trapping agents on the performance of resin was investigated. The results showed that the addition of formaldehyde trapping agents could greatly reduce the residual formaldehyde content in phenolic resin. Fourier‐transform infrared spectroscopy (FTIR) data showed that the addition of formaldehyde trapping agent not only successfully prepared PF, but also produced a stable formaldehyde trapping agent product. Compared with the unmodified PF, the pore size and foaming rate of the modified PF decreased in varying degrees, and the foaming temperature also decreased in varying degrees. Among them, urea modified phenolic resin (PUF) had the highest formaldehyde removal efficiency. It had the t lowest heat release during combustion and the foam pore size was large and uniform. In addition, through the application of urea‐modified PF in the goaf return air corner fissure sealing application, it was found that the CO concentration in the goaf maintained at the lowest state of 0.002%. It has reached the national industry standard of polymer gas plugging materials and conforms to the green development concept in the field of coal mine fire prevention. [ABSTRACT FROM AUTHOR]

In order to build green mines, goaf is often filled, supported, and sealed with a high-water material to eliminate a series of environmental problems and safety hazards caused by goaf. In this study, ordinary Portland cement, sulphoaluminate cement, and alkali-activated cement were used as binders to prepare full-tailings high-water materials for filling, with various water-to-cement ratios. The compressive strength development of consolidated tungsten tailings specimens prepared with various curing binders was observed, and the influence of various water–cement ratios on the strength development was analyzed. The environmental impact of mine backfill materials was assessed according to the life cycle theory (LCA), and these mine backfill materials were prepared by using various binders. The results show that when the water-to-binder ratio is 3, the strength of alkali-activated cement can reach 3 MPa at 28 days; at that ratio, the microstructure of alkali-activated cement is more compact. Through LCA analysis, the environmental load of alkali-activated cement is shown to be significantly lower than that of either Portland cement or sulphoaluminate cement; the LCA results show that the primary energy consumption using alkali-activated cement is reduced from the Portland and sulphoaluminate cements by 1319.32 MJ and 945 kg, respectively. These unusual reduction percentages are achieved because the production of alkali-activated cement by LCA does not have any negative environmental impact—the production of alkali-activated cement, with its primary component being industrial byproduct slag, so that the use of alkali-activated cement in tailings' consolidation has a positive environmental impact. [ABSTRACT FROM AUTHOR]

Copyright of Mining & Metallurgy (10057854) is the property of Beijing Research Institute of Mining & Metallurgy Technology Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To ensure the stability of an overlying roadway in close proximity to a 1000-m deep coal seam, this work, through theoretical analysis and numerical simulation, studies the deformation and failure mechanism, failure types, and prevention methods of the overlying roadway while mining a lower coal seam. Based on the study of mining conditions and overburdened space structures formed through mining, the deformation and failure law of the roadway in the strike and dip directions is obtained. Moreover, the stress evolution law of roadway deformation and failure is obtained by studying the influence of different mining stages and filling rates of the lower face on the mining of the overlying roadway. Results show that there are two types of potential roadway deformation and failure: (1) roadway section reduction failure and (2) roadway strike step subsidence failure. The fundamental cause of the deformation and failure of the overlying roadway is the influence of the large buried depth, strong mining stress, and unequal distance between the coal seam and roadway. The direct causes are the sudden increase in the stress of the roadway surrounding the rock and the subsidence of the key strata. In the mining process, when the mining range of the working face is more than 400 m, the roadway section reduction failure and strike ladder subsidence failure superimpose each other, inducing further roadway failure. Therefore, a scheme of partial filling and strengthening of the roadway along the working face is designed. Field measurements revealed that the above scheme can meet the stability requirements of the overlying roadway and the high efficiency and yield of the underlying working face. The results provide some references for the safe mining of coal in the 1000-m deep coal seam.

The article presents an innovative technology of fire prevention in goaves, based on injection of nitrogen and carbon dioxide inert gases mixture from treatment of exhaust gases from a methane combusting gas engine. The developed innovative technology and the constructed prototype of the installation producing inert gases are the final result of the research project entitled "Innovative and effective technology of inerting the goaf active or dammed longwall in an underground mining plant, extracting hard coal, using mixtures of inert gases obtained from the purification of exhaust gases from a gas engine and preventing the formation of endogenous fires", co-financed by the National Center for Research and Development (NCBR). The prototype installation was demonstrated in real conditions at the "Borynia" part of "Borynia- Zofiówka" mine, where, at the end of the research project, the prototype and the developed fire prevention technology with the use of gas engine exhaust gases were optimized and validated. Until now, under normal conditions in a hard coal mine, the fumes generated as a result of methane combustion in gas engines were emitted directly to the atmosphere. Innovative inerting technology, processing and reusing the exhaust gases produced by methane-fueled gas engines, in ecological context will contribute to the reduction of pollutant emissions in the mining sector, and, with mixing at the same time the carbon dioxide and nitrogen in the proper ratio in the mixture, it will fully utilize the advantages of each of these gases, as known when used in separate form. As a result, considering the possibility of generating a much higher amount of inert gases per time unit compared to the available technologies, and combining the physicochemical properties of nitrogen and carbon dioxide in one mixture, the innovative technological solution significantly increases the effectiveness of fire prevention, thus reducing the possibility of an endogenous fire. [ABSTRACT FROM AUTHOR]

Intelligent mine filling is an important part of unmanned and intelligent mine construction. Aiming at the lack of a systematic understanding of how to construct intelligent mine filling and what functions the intelligent control system should have in the domestic, this paper analyzes the key points of mine filling control and designs the "cloud, edge and end" three layer intelligent control system for mine filling based on the industrial Internet platform. Combined with the engineering case of Wushan copper mine′s unclassified tailings paste filling system construction, the intelligent control system has been introduced in detail, which can provide reference for the realization of intelligent filling construction in domestic mines. [ABSTRACT FROM AUTHOR]

Copyright of China Plastics / Zhongguo Suliao is the property of Journal Office of CHINA PLASTICS and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Phosphorus slag (PS) is an industrial waste discharged during the refining of yellow phosphorus by a high-temperature electric furnace. The slag is generally deposited on the surface in situ, and this uses up considerable areas of land and causes environmental pollution problems. The glass phase content in electric furnace PS is over 90%, indicating that the PS has potential cementitious property. As a cement admixture, PS has a retarding effect on cement, thus, its application to cement has become less adopted. Based on the hydration characteristics of PS and the application conditions of mine filling, the feasibility of alkali-activated cement–PS used as cementitious material (CPCM) to cement sulfur tailings was studied. Quicklime, NaOH, and Na2SiO3 were used as the activators to investigate the CPCM curing performance in a laboratory, and the compatibility of CPCM with sulfur tailings was evaluated. The results show that when the PS-to-cement mole ratio is 1∶1, quicklime is 3% of PS, the final setting time of CPCM is 300 min, and the CPCM strength for 28 days is 40.6 MPa, indicating that the material can replace P.O 42.5 cement in the subsequent filling process. When the Na2SiO3 is 4% of PS, the final setting time of CPCM is 39.3% less than that of cement, and its strength for 7 days is 31.1% higher than that of cement. Compared with cement, CPCM shortens the setting time of backfill by 8 hours, and its strength does not deteriorate after curing for 28 days. Therefore, CPCM is suitable for cementing sulfur tailings. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) analyses show that the CPCM hydration products are mainly Ca(OH)2 and C−S−H. Moreover, the PS hydration resulted in a decrease in the Ca(OH)2 content, and a compact C−S−H gel with a low Ca/Si mole ratio is formed. This not only improves the later strength of CPCM but also prevents the strength deterioration of the backfill.

In view of the coal spontaneous combustion accident caused by air leakage in the mine, this paper studied the foaming cement solidified filling material used for mine filling and plugging with cement as the base material, foaming agent, gypsum and polypropylene fiber as additives. Through the single factor test method, the optimized ratio of the main components affecting the compressive strength and expansion ratio of the foamed cement solidified filling material was determined as the water-cement ratio 0.5, the foaming agent content 1%, and the gypsum content 6%, polypropylene fiber 0.4%. Taking Cuimu Mine as the experimental background, through the design of reasonable and effective filling technology scheme, the 21303 wind tunnel in the lower part of 22306 working face of Cuimu Mine was filled. The test results show that the foamed cement solidified material effectively enhances the airtightness of the wind tunnel and the roadway.

In order to build green mines, goaf is often filled, supported, and sealed with a high-water material to eliminate a series of environmental problems and safety hazards caused by goaf. In this study, ordinary Portland cement, sulphoaluminate cement, and alkali-activated cement were used as binders to prepare full-tailings high-water materials for filling, with various water-to-cement ratios. The compressive strength development of consolidated tungsten tailings specimens prepared with various curing binders was observed, and the influence of various water–cement ratios on the strength development was analyzed. The environmental impact of mine backfill materials was assessed according to the life cycle theory (LCA), and these mine backfill materials were prepared by using various binders. The results show that when the water-to-binder ratio is 3, the strength of alkali-activated cement can reach 3 MPa at 28 days; at that ratio, the microstructure of alkali-activated cement is more compact. Through LCA analysis, the environmental load of alkali-activated cement is shown to be significantly lower than that of either Portland cement or sulphoaluminate cement; the LCA results show that the primary energy consumption using alkali-activated cement is reduced from the Portland and sulphoaluminate cements by 1319.32 MJ and 945 kg, respectively. These unusual reduction percentages are achieved because the production of alkali-activated cement by LCA does not have any negative environmental impact—the production of alkali-activated cement, with its primary component being industrial byproduct slag, so that the use of alkali-activated cement in tailings’ consolidation has a positive environmental impact.

In order to study the law of spontaneous combustion in goaf under the condition of gas extraction, the method of beam tube monitoring and numerical analysis is adopted to obtain three dimensional distribution of oxygen concentration and contour map under the condition of different negative pressure. Based on the way of regression analysis, reasonable negative pressure are determined. The results show that with the increase of negative pressure, the spontaneous combustion "three-zone" in goaf: the heat dissipation zone and the spontaneous combustion zone become wider and the suffocation zone narrows. At the same time, the trend of spontaneous combustion zone presents to the deep part of the goaf; the amount of gas extraction increases first and then decreases with the increase of negative pressure. Combined with the trend of spontaneous combustion zone and the amount of gas extraction in goaf, and the reasonable negative pressure value is 32kpa and verified. [ABSTRACT FROM AUTHOR]

A large number of accidents caused by goafs have great influence on the ecological environment and public safety. In order to assess the risk degrees of goafs, a hybrid Tomada de Decisão Interativa Multicritério (TODIM) method is proposed in this paper. Firstly, on account of the geological conditions, shape parameters and environmental factors, an evaluation index system is established. This system includes seven quantitative sub-criteria and six qualitative sub-criteria. The combined weight of each index is determined on the basis of game theory by combining the subjective weight and objective weight. Afterward, a fuzzy-TODIM method is put forward to obtain the rank of risk degree and the specific risk level. Finally, the hybrid TODIM method is used to assess the risk degrees of goafs in Shuikoushan lead-zinc mine under uncertainty. Sensitivity analysis is demonstrated to reveal the robustness of this approach. Results show that the proposed hybrid TODIM method can be adopted to assess the risk degrees of goafs reliably and stably. [ABSTRACT FROM AUTHOR]

Highlights • Development of mathematical model and numerical solution to self-heating of coal. • Transient results of heating in longwall goaf. • Proactive inertisation in longwall goaf. Abstract Spontaneous combustion of coal has long been a thermal dynamic hazard during coal mining, storage, and transport, posing a great threat to coal mine safety. Especially coal spontaneous combustion in longwall goaf poses a great threat to underground working crew. To investigate such a mine safety issue with more insights and " what if " scenarios, this study employs CFD technique to develop a proactive inertisation plan in a longwall goaf. Based on real on-site conditions of the longwall goaf, a three-dimensional transient non-equilibrium thermal CFD model was developed to study heating evolution and proactive inertisation plans of the longwall goaf. The theoretical model incorporated a set of governing equations including low temperature kinetics of coal oxidation, energy and mass conservation, momentum balance, and continuity equation. After the base model (1000 m) was validated and calibrated with field gas monitoring data, another model (500 m model) was studied to obtain an optimum inertisation plan. Both steady state and transient simulations were conducted to study the flow dynamics of air velocity, oxygen ingress, dispersion of gaseous products and heating evolution in the longwall goaf. Based on the flow dynamics field, proactive inertisation plans using nitrogen to suppress the onset and development of goaf heatings were then developed and studied. [ABSTRACT FROM AUTHOR]

Bayueshan tunnel (BYS) is an important construction crossing over coal mine goaf. The underground mining subsidence has led the tunnel cracked seriously in three years after it was built. In order to evaluate the coal mine influence and future stability of the tunnels, probability integral method (PIM) was used to calculate the tunnel deformation. PIM is an experience function method based on random medium theory which is used widely in China. With the parameters analyzed, the tunnels' subsidence was calculated. The results show that it can interpret the tunnel damage well, and the maximum normal strain positions fit the damaged tunnel positions well. It proved that PIM can be used to evaluate the tunnel's radial deformation caused by underground coal excavation. In order to maintain tunnels to keep a long-term stability, the future deformation was calculated in case the coal excavation continues. It shows that the tunnel would be cracked again if the excavation continued. Other reasons such as the old goaf deformation and water and vehicle dynamic load are also important reasons for the tunnels' deformation. In order to keep traffic safety, it needs to reinforce the cracked foundation under the tunnel. Then, grouting injection was proposed to reduce the old goaf deformation under the tunnels. If the fracture zone under the tunnels disturbed by the dynamic traffic load, the old goaf will face a risk of sudden collapse. So, to ensure the grouting effect, the grouting depth should be deeper than the sum of traffic load influence depth and height of coal mine caved fissure zone. The grouting scope should keep all the crack rock area under the tunnel from being disturbed by the dynamic traffic load. This design can reduce the sudden collapse risk of the goaf and reduces the vehicles' load disturbance on the cracked rock. The researched technology provides an engineering guidance to tunnel subsidence calculation, stability evaluation, and maintenance in complex geological and engineering situations. [ABSTRACT FROM AUTHOR]

Interbedded strata and their collapse are vital to mining pressure control for extremely thick coal seam under goaf. To ensure the stability of the support and to avoid roof collapse, some traditional underground pressure theoretical models had been widely used in the control of surrounding rock and the selection of support. However, one of the challenges for extremely thick coal seam under goaf is that the abnormal disasters, such as support crushing and water inrush that were occurring frequently. To solve this problem, the movement characteristics of overburden rocks during the mining of extremely thick coal seam under the conditions of the interlayer thickness of 5 m and 40 m were studied by using the similar simulation experiments, while the numerical simulation experiments were carried out for the interval between coal seams of 15 m and 60 m, respectively. Finally, the structure and mechanical transfer mechanism of overburden in stope under different thickness interbedded strata were analyzed dynamically, and the condition of full-thickness connection between upper goaf and lower goaf and corresponding judgment criteria are obtained. These results can guide future research on the mechanical of extremely thick coal seam under goaf, which can provide a theoretical basis and engineering reference for similar projects. [ABSTRACT FROM AUTHOR]

In order to reveal mining pressure in the workface using a gangue backfilling method and a caving method in Pingdingshan No. 12 coal mine, this paper establishes a numerical analysis model for different advance lengths of workface based on geological conditions. By the analysis of numerical models and field measurements, the following conclusions about mining pressure behavior in the mixed workface were drawn. (1) As the mixing face advances, its goaf is continuously compacted. Therefore, the vertical stress in the roof of the mixed workface increases gradually. (2) Because the gangue has a certain supporting effect on the roof, the peak value of abutment stress in the gangue backfilling section is less than that in the caving section, while the roof vertical stress in the goaf of the backfilling section is greater than in that of the caving section. (3) Because roof break and movement are different in the gangue backfilling section and caving section of the mixed workface, a sudden increase of vertical stress in the transition section appears. (4) Through the analysis of the working resistance of the hydraulic support and the advance stress of the coal wall in the mixed workface, the supporting role of gangue for the goaf roof and the abrupt increase of vertical stress in the transition section are verified. (5) The mining pressure of the mixed workface shows that the mining management of the caving and transition sections should be strengthened. [ABSTRACT FROM AUTHOR]

To explore the feasibility of cemented paste backfill with phosphogypsum (PG), bleeding water and rheological tests (slump and on-site pipeline loop tests) were performed with PG backfill slurry (PGBS). In the bleeding water test, the PGBS concentration with minimal bleeding water was measured between 60.87 and 67.61%; in the rheological slump test, values of 61 to 68% were determined for the on-site pipeline loop test. The rheological pipeline loop test demonstrated that the resistance coefficient is lowest when the concentration is no higher than 65%. Through industrial experiments, PG slurry with a concentration of 64%–65% backfill was successfully applied to the goaf. The experimental results demonstrate that PGBS with characteristics of "less bleeding water" and "improved pumpability" is obtained when its concentration is between 61 and 65%. Paste-like PG slurry was proven to be optimal for cemented PG backfilling technology. [ABSTRACT FROM AUTHOR]

In view of the ground drilling of the N2206 working face in Shanxi Wangzhuang Coal Mine, the gas concentration is low and the extraction effect is not good. Fluent computational fluid dynamics software was used to simulate the ground extraction drilling position of the N2206 working face in the goaf (the distance from the top of the coal seam and the distance from the return to the wind). The numerical simulation results show that when the final hole of the ground extraction hole in the goaf is 16 m from the roof of the coal seam and the distance from the return air is 45 m, the extraction effect is optimal. The average extraction gas volume is 9.78 m3/min, and the average extraction gas concentration is 43.95%, the best extraction effect is obtained. After optimizing the ground drilling position in the goaf and combining with the site implementation, the maximum gas scouring amount of the extraction is 12.59 m3/min, which is 3.42 m3/min higher than the original. The maximum gas concentration of extraction was 63.54%, which was 28.82% higher than the original. After optimization, the gas concentration of the extraction is more than 30%, and the extraction effect is very good. Field application results further validate the reliability of theoretical analysis and numerical simulation results. [ABSTRACT FROM AUTHOR]

Coal combustion product (CCP) is generated after combustion of coal in thermal power plants. Generation of CCP depends on the combustion efficiency and properties of coal. Developing countries are producing large quantity of CCP due to the high ash content of coal. Utilization of CCP is a major challenge to the technical and scientific community. In recent years, researches are trying to solve this problem using of CCP in mine fill. Hydraulic transportation of CCP is a major concern for this type of utilization. Present paper tries to compile various types of CCP along with studies related to hydraulic transportation. [ABSTRACT FROM AUTHOR]

The roof deformation and damage of roadway driving next to goaf in the vertical and horizontal directions are serious, the process of horizontal squeezing-expansion is difficult to be controlled using the traditional anchor truss structure. To solve this problem, a new type of cable-truss structure was designed to effectively control the deformation and damage of the roof along the vertical and horizontal directions. Besides, it had the function of horizontal bidirectional constant resistance. The mechanism of the structure is explained first. Then, the basic mechanical properties obtained through laboratory tests are presented. Finally, field practice is discussed. The results show that the new cable-truss structure has a load bearing force acting on its combination beam according to the horizontal movement of roadway roof. It can also adjust its length to prevent or adapt to the horizontal squeezing-expansion movement of roof, which ensures that the combination beam will not be distorted, torn, or broken. Thus, this new cable-truss structure can effectively mitigate the roof-caving accidents. [ABSTRACT FROM AUTHOR]

The precise prediction of surface subsidence under mining influence is significant for ecological environment and personal safety. This paper presents a numerical approach for surface subsidence analysis based on a case study conducted at Gucheng mine, China. Considering the rock mechanical characteristics of fractured zone and caved zone, the tension-weakening model and the double-yield model with back-analyzed properties are implemented into FLAC3D to simulate the mining-induced fracture weakening and the goaf compaction process, respectively. The reasonable goaf material and the relationship between the intensity of fractures and the residual stiffness of rock mass are built in the numerical model. An analysis of tension-weakening and double-yield models is carried out and the simulated results are compared to strain-softening model. It is shown that the combination of tension-weakening and double-yield models provides a more realistic simulation of surface subsidence than strain-softening model. The numerical approach can be utilized for control and prediction of surface subsidence under similar geotechnical conditions. [ABSTRACT FROM AUTHOR]

The stability of the goaf support system is the key to safe production in gypsum mines. Therefore, this study constructed a pillar-beam support system which contained pillar plastic zones. In this support system, the beam and pillar were taken as energy releaser and energy dissipater, respectively. Through establishing a cusp catastrophe model based on energy theory, the new criterion for instability was obtained which is related with geometric stiffness and system energy dissipation. The results indicate the instability of the support system is caused by the incompatibility of energy release, dissipation, and geometric deformation. When K > 1, the energy released by the support system is compatible with geometric deformation. The support system experiences a quasistatic process from the static state in bottom page to the static state in top page along Path I. When K < 1, the energy released by the support system cannot be in tune with geometric deformation. The support system experiences a catastrophe process along Path II. The evolution from the static state in bottom page to the static state in top page is not progressive, but catastrophic. The redundant energy released in this process leads to mechanical instability of the support system. This study provided theoretical foundation for the mining and treatment of mines. Based on actual engineering examples, the sensitivity of the geometric parameters of the support system was analyzed as well. These parameters are ranked by their sensitivity from high to low, as is shown below: beam thickness, plastic zone width, room span, pillar width, and pillar height. Then, the goaf was classified according to the geometric parameters. Energy catastrophe theory was applied to analyze the stability of the support system in different classes of goaf. The analysis results showed that Class D goaf should be labeled as the unstable zone, which was consistent with the result of field research. To conclude, energy catastrophe theory can be used to demonstrate the nonlinear mechanical mechanism of support system instability in room-pillar mining goaf. [ABSTRACT FROM AUTHOR]

Highlights • Recycling of wastes as backfilling materials in mine goaf is an optional way. • CCS replacement of cement can elevate the slump value of DRS and ITS mixture. • Optimal proportion among DRS, ITS, OPC and CCS is 60:40:16:4 in mass in this case. • An updated expression to predict the strength is proposed just with two variables. Abstract Re-utilization of industrial wastes or by-products in construction, including dredged river sediments (DRS), iron tailing slag (ITS) and calcium carbide slag (CCS) etc., can reduce the construction cost, decrease the storage requirement and also be of great benefit to the environment. In this research, the aforementioned wastes (i.e. , DRS, ITS and CCS) were synthetically recycled and used as the backfilling materials in the mine goaf to solve the problem of subsidence. Systematic laboratory experiments were conducted to ensure that these recycled materials can fulfill the requirement on the slump value and unconfined compressive strength after 7-day's curing. The results show that despite that Ordinary Portland cement (OPC) suppresses the slump value, CCS elevates the flowability, and then the maximal flowability is achieved when the mass ratio of DRS to ITS was 70:30 with a cement content of 16.7%. ITS addition into DRS upgrades the strength of the mixtures of DRS and ITS stabilized by OPC and CCS for the skeleton effect and water content reduction. The optimal proportion between DRS, ITS, OPC and CCS in this case is 60:40:16:4 in mass, with a slump value of about 160, and unconfined compression strength after 7-day's curing about 2.8 MPa. At last, a simple expression to predict the strength of the backfilling materials was proposed referring the gel-space theory and the concept of volumetric solid content, where just two variables (i.e. , the binder content and the volumetric solid content) can characterize this complex matrix. [ABSTRACT FROM AUTHOR]

The effect of lime-sand ratio and slurry concentration on the mechanical properties of backfills is important. To achieve green and high-efficiency mining, accurately determining the optimum ratio of cemented tailings for certain tungsten tailings and ensuring the safety and stability of the mine stope structure are important. The cement-sand ratios used in this research were 1 : 6 and 1 : 8. The mechanical properties were evaluated by using 68%, 72%, and 78% of tailing cemented filling materials. The corresponding physical and mechanical parameters were obtained through uniaxial compression, splitting, and shearing mechanical experiments on the backfill specimens. FLAC3D was used to investigate the mechanical properties of cement-filled pillars and the stability of supporting surrounding rocks on the basis of the mine’s current room pillar structure size parameters and mining sequence. The key factors that affect the stability of the goaf were analyzed by evaluating the plastic zone area of the stope, maximum and minimum principal stresses, and displacement change. The structural characteristics of stope structures and changes of rock mass damage were obtained under different cement-tailing ratios and concentrations. A cemented backfill with a cement-tailing ratio of 1 : 8 and a concentration of 68% was selected as the best solution for the mine in terms of safety and economic considerations. [ABSTRACT FROM AUTHOR]

To study the evolution of porosity in the process of filling slurry hydration, samples of tailing filling with different mixing ratios and different curing ages were prepared, and the pore structure was tested by low-field nuclear magnetic resonance (LF-NMR). According to the T2 spectrum distribution curve and T2 spectrum area of the filling slurry hydration process, the porosity and the evolution of the pore distribution features of the filling samples were analysed. The results showed that the higher the mass concentration of the filling slurry, the slower the hydration reaction, the smaller the average size of the pores, the higher the proportion occupied by harmful pores, and the lower the proportion occupied by multiharmful pores. The smaller the cement-sand ratio of the slurry, the higher the proportion occupied by harmful pores and the larger the aperture of the pores. The porosity of the filling slurry with the cement-sand ratio of 1 : 8 is the highest, and the hydration velocity of the filling slurry with the cement-sand ratio of 1 : 3 is the fastest. [ABSTRACT FROM AUTHOR]

With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO3·SO4-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO3·SO4-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ18O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr2+ concentrations differed considerably, the ratios of Sr2+/Ca2+ and 87Sr/86Sr and the 34S content of SO42− were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas. [ABSTRACT FROM AUTHOR]

Soils of Coastal Tablelands are important for the economic and social development of the coastal region of Brazil. However, these soils appear to have a number of chemical and physical limitations. One of the most expressive physical limitations is the occurrence of cohesive character, whose genesis is still not fully understood. Thus, considering that the tensile strength is a direct measure of the cohesion of particles, this research was undertaken to discover how the size of soil particles influences the genesis of the cohesive character in Bt horizons of the Coastal Tablelands of Ceará, Brazil. The study was carried out in the municipality of Aquiraz-CE, where samples of two soils were collected (Argissolo Vermelho-Amarelo Distrófico típico – PVAd; Argissolo Amarelo Eutrocoeso abrúptico – PAex) in the horizons Bt1 (cohesive) and Bt2 (non-cohesive) in the top, middle and bottom portions. The results demonstrated that the size of the particles led to sandy clay loam texture in horizons of PVAd and sandy clay texture in the horizons of PAex. Regarding sand sorting, for all investigated horizons and in the three positions (top, middle and bottom), the medium sand fraction prevailed, followed successively by fine and very fine sands together, coarse and very coarse sands. Tensile strength values were higher in cohesive horizons, with reduction in the tensile strength of aggregates from top to bottom. It was concluded that tensile strength was a useful indicator for differentiating cohesive properties in the different soil horizons. There was significant variation of tensile strength in the soil horizon defined by the pedologist – which suggests that it is necessary to include as a valuable addition to the diagnostic tools available to the working pedologist, a tensile strength scale for the identification of cohesion to avoid subjectivity in the definition of the cohesive horizon. For the studied set of horizons, tensile strength decreased from top to bottom. Sand and clay fractions, despite not being the only determinant factor, influenced the genesis of the cohesive character in soils. For the sand fraction, poor degree of sand sorting led to a denser packing of particles, significantly contributing to the cohesion of aggregates. Our experiment strongly suggests that the packing of particles allows the increase of cohesion provided there is a cementing agent even in small quantity. [ABSTRACT FROM AUTHOR]

Cemented paste backfill (CPB) has been extensively used as a popular mine fill system for underground mining operations. Understanding the CPB’s in situ properties as an element of ground support is indeed really important for developing a well-organised mine design in terms of costs and security. In this paper, a comprehensive experimental work was conducted to better understand the effect of stope depth on in situ behaviour and performance of CPB having three different binding agents (ordinary Portland cement alone, slag-based cement and fly ash-based cement) over different curing times. A stope depth of 0 (it replicates the stope’s top), 5, 10, 15 and 20 m (it replicates the stope’s bottom) were virtually simulated by using an improved laboratory set-up, escorting with conventional plastic moulds. Results show that the highest compressive strength is obtained from CPB having slag-based cement. The strength of CPB increases with increasing stope depth mainly due to resulting improved geotechnical properties, such as reduced water content, degree of saturation and porosity as well as increased specific surface area retained at the bottom of a simulated stope. Geochemical testing shows that sulphate SO4−2 and calcium Ca contents increase with increasing curing time and reduce with increasing stope depth. As a critical point, a number of experimental relationships considered typically for different curing times, overburden pressure, arching stress and binder contents within CPB material were expressed and discussed. [ABSTRACT FROM AUTHOR]

A new type of incident in the form of a methane explosion caused by spontaneous combustion of coal is receiving more and more attention due to the high ground temperature and high methane content in coal mines. To investigate the formation process of this type of incidents and advance the research on methane explosion risk caused by spontaneous coal combustion, a self-designed experimental platform was used to determine the influence of spontaneous coal combustion on methane migration in the goaf. Additionally, the effect of different ventilation velocities at the mining face on methane migration in the goaf was studied. The results reveal that air leakage plays a key role in methane migration in the goaf. The bigger the ventilation velocity, the larger the area affected by air leakage. In addition, the superposition of the air leakage and the horizontal and vertical movement of hot gases creates the entrainment effect in the spontaneous combustion area. Eventually, the methane concentration at the location of spontaneous combustion is the lowest and it gradually increases outside the spontaneous combustion area because of the superposition of air leakage, 'fire pressure' and the entrainment effect. [ABSTRACT FROM AUTHOR]

Goaf fires are prevalent in underground coal mines and have drawn increasing public attention. Inhibition foam is recognized as an efficient means of fire extinguishment in a huge volume of porous material. A verisimilar goaf platform setup and a foam preparation system were developed. Experimental results show that increasing the gas/liquid ratio improves the foam stack effect. The optimum gas/liquid ratio and the ratio of infusion pipe length and cooling zone length are suggested for a practical application. The goaf interior pressure test indicates that the air leakage is controlled effectively by the inhibition foam. A perforated pipe will be superior to the standard one for a practical application due to its better diffusivity and lower infusion pressure. [ABSTRACT FROM AUTHOR]

When cemented backfill mixture (CBM) is transported through pipelines from a backfill plant to stopes, it experiences shearing forces over the transport time. In this paper, the effects of solids concentration, binder content, shear rate and curing time on the time-dependent rheological behaviour of CBM were studied. It was found that over long periods of shearing at a constant rate greater than 5 s−1, the shear stress decreased at first and then increased gradually with time. When the shear rate was less than 0.5 s−1, shear stress increased slightly firstly with shearing time, then it started to behave similar to the test with a higher shear rate whose shear stress decreased firstly and then increased. The samples that were sheared at a higher shear rate exhibited a lower apparent viscosity and the higher yield stress CBM samples displayed more pronounced shear-thinning properties. It was also found that for transient flow, increasing the solids concentration and the curing time lead to both a higher initial shear stress (τ0) and minimum shear stress (τmin). When the cement to tailings ratio increased, the τ0, τmin and final shear stress after shearing for 3600 s (τ3600) increased at first, and then subsequently decreased. Moreover, changes in the solids concentration profile and the cement hydration property were displayed during the rheological tests. [ABSTRACT FROM AUTHOR]

Abundant residual methane stored in abandoned mine goafs can be effectively extracted by surface vertical wells. However, abandoned mine goafs are in a closed state and are not influenced by underground ventilation systems, which differentiate them from production mine goafs. Thus, the studies on the optimization of surface vertical well location in production mines cannot be directly used to guide methane extraction in abandoned mine goafs. To determine a reasonable location for surface vertical well in abandoned mine goafs, this study, based on the mathematical model of methane seepage in an abandoned mine goaf, employed the COMSOL software to numerically explore methane seepage characteristics in the goaf under the conditions of four typical vertical wells. The well locations included central position of goaf, middle of the boundary in the trending direction of goaf, middle of the boundary in the inclination direction of goaf, and corner position of goaf. The methane seepage characteristics of the goaf in the 3D, 2D, and 1D flow fields were analyzed separately, and the high-speed flow zones in the goaf were divided. The optimal well location for extraction was determined based on the methane seepage characteristics and the range of high-speed flow zone. Results indicate that under the conditions of surface vertical well, methane in the goaf initially flows slowly into the high-speed flow zone and then rapidly to the well location under the negative pressure of the drainage. The corner position of goaf is the best well location. The maximum methane flow velocities in the upper and lower parts of the goaf under the conditions of the well location are more than 2.05 times of those under other well locations. The weak belt of the surface vertical well is in the range of 0-10 m from the bottom of the goaf. Methane flow velocities in this area are only 0.035-0.28 m/s under the four well conditions. The control effect of single-well on methane flow in the goaf is limited, and the goaf has blind areas for drainage under the single-well condition. The conclusions provide a theoretical reference for the layout and optimization of surface vertical well locations in abandoned mine goafs. [ABSTRACT FROM AUTHOR]

Historical global coal resource exploitation has resulted in goaf abandonment at most mines, which, during the life of the mine, exerts a strong influence on the ability to exploit new working faces adjacent to any abandoned goaf. Thus, the distribution law of the surface subsidence basin above the new working face differs from the general law. Given the lack of research on this distribution law, this study uses a similar material simulation method to examine the influencing mechanism(s) of abandoned goafs on the subsidence basin in the same coal seam. Synthetic aperture radar (SAR) monitoring results of the 9310 working face in the Nan Tun coal mine, Yanzhou, China, are used to verify analytical results, followed by numerical simulation to study the influence of the abandoned goaf on the subsidence basin. Finally, to accurately predict surface subsidence influenced by the abandoned goaf, inflection point deviations are selected and discussed in terms of the probability integral method. This allows for prediction of the distribution of the surface subsidence basin, and moreover, the predicted results are found to be consistent with the SAR monitoring results when the derivation of inflection points is revised. [ABSTRACT FROM AUTHOR]

In the paper there are presented results of influence of types of rocks forming the goaf with caving on the physical parameters of air stream flowing through these goaf and adjacent headings. It was assumed. that goaf with caving are porous medium. which permeability and porosity depends on the distance from the front longwall and from stratification resistance of rocks forming the heap. On the base of stand tests values of stratification resistance of rock littering at the roof of tested longwalls were determined. The main aim of the tests was to determine an impact of type of rock littering in mined seam on values and distribution of velocity and pressure of air stream in the gobs This aim was realized by a determination of value of stratification resistance of roof rocks on the tested parameters of air stream flowing through these gob. Analysis was performed in the ANSYS Fluent software. Modeling studies were performed for real exploitation longwall and additionally for two types of roof created with the most often presented rocks. For each of these systems distributions of velocities and static pressure along assumed measurement lines were determined. Analysis included determination of physical parameters of the air stream flowing through the gob for real exploitation conditions of this longwall and additionally for assumed two (the most often presented in reality) variants of rock presented in the roof of mining longwall. Developed research methodology and obtained results should be use during selection of ventilation parameters and preventive actions connected with reduce methane and fire hazard in underground mining. [ABSTRACT FROM AUTHOR]

It is crucial to locate underground goafs (i.e., mined-out areas) resulting from coal mining and define their spatial dimensions for effectively controlling the induced damages and geohazards. Traditional geophysical techniques for locating and defining underground goafs, however, are ground-based, labour-consuming and costly. This paper presents a novel space-based method for locating and defining the underground goaf caused by coal extraction using Interferometric Synthetic Aperture Radar (InSAR) techniques. As the coal mining-induced goaf is often a cuboid-shaped void and eight critical geometric parameters (i.e., length, width, height, inclined angle, azimuth angle, mining depth, and two central geodetic coordinates) are capable of locating and defining this underground space, the proposed method reduces to determine the eight geometric parameters from InSAR observations. Therefore, it first applies the Probability Integral Method (PIM), a widely used model for mining-induced deformation prediction, to construct a functional relationship between the eight geometric parameters and the InSAR-derived surface deformation. Next, the method estimates these geometric parameters from the InSAR-derived deformation observations using a hybrid simulated annealing and genetic algorithm. Finally, the proposed method was tested with both simulated and two real data sets. The results demonstrate that the estimated geometric parameters of the goafs are accurate and compatible overall, with averaged relative errors of approximately 2.1% and 8.1% being observed for the simulated and the real data experiments, respectively. Owing to the advantages of the InSAR observations, the proposed method provides a non-contact, convenient and practical method for economically locating and defining underground goafs in a large spatial area from space. [ABSTRACT FROM AUTHOR]

This paper researches specialized reverse-circulation drilling bit for multi-layer goaf detection based on the multilayer goaf detection project of iron ore mine in Yuan Village and designs a sliding-block reverse-circulation drilling bit with innovation, which is designed with large-through hole central channel, double-row inner orifice and sliding block. The structure of sliding block is used on reverse-circulation drilling bit for the first time. The performance of slidingblock reverse-circulation drilling bit is observed in practical drilling test, indicating good effect of reverse circulation, and high drilling efficiency, with the average drilling efficiency of 33.75m/h on extremely cracked land and 10.17m/h on hard land. The sliding-block reversecirculation drilling bit can effectively solve the problem of blocking central channel since the structure of sliding block can meet the requirement on drilling. The top plate of multilayer goaf can be drilled through successfully. C-ALS threedimensional laser scanner can be put down through the central channel to solve the difficulty of simultaneous scan and detection on multi-layer goaf. [ABSTRACT FROM AUTHOR]