A new concept of backfill design—Application of wick drains in backfilled stopes

Backfilling represents an environmentally friendly mining waste disposal technique. It is increasingly used in underground mines all over the world. However, its primary purpose remains to improve ground stability and to reduce ore dilution. Previous investigations have shown that fill drainage plays a key role in backfill and barricade design. With a poor drainage system in the backfilled stope, the required dimension of barricade, which is constructed at the base of the stope near the drift entrance, has to be increased. A poor backfill drainage system can also lead to a significant increase in drainage waiting time and further reduction in mining productivity. In this paper, the drainage of conventional backfill design in backfilled stopes is briefly reviewed. For the first time, the application of the wick drain is introduced in the backfill within mine stopes. The drainage improvement from the introduction of the wick drain is illustrated using numerical modeling.     

Numerical investigation into the effect of backfilling on coal pillar strength in highwall mining

This paper attempts to quantify the effect of backfilling on pillar strength in highwall mining using numerical modelling. Calibration against the new empirical strength formula for highwall mining was conducted to obtain the material parameters used in the numerical modelling. With the obtained coal strength parameters, three sets of backfill properties were investigated. The results reveal that the behavior of pillars varies with the type and amount of backfill as well as the pillar width to mining height ratio(w/h). In case of cohesive backfill, generally 75% backfill shows a significant increase in peak strength, and the increase in peak strength is more pronounced for the pillars having lower w/h ratios. In case of noncohesive backfill, the changes in both the peak and residual strengths with up to 92% backfill are negligible while the residual strength constantly increases after reaching the peak strength only when 100%backfill is placed. Based on the modelling results, different backfilling strategies should be considered on a case by case basis depending on the type of backfill available and desired pillar dimension.     

3-D modeling of rock burst in pillar No. 19 of Fetr6 chromite mine

Fetr6 is an underground mine in which chromite is extracted using stope and pillar mining method. Despite of all improving works such as roof supporting and replacing of ore pillars with concrete pillars, pillar No. 19 failed and other pillars failed progressively as a domino effect and 4000 m² of mine collapsed within a few minutes, consequently. For detail investigation, two 3-D numerical models were developed by 3Dec. The first, a base model, was used for estimation of stress on pillars just before failure and the other for investigation of rock burst in pillar No. 19. The results show that discontinuity parameters such as friction angle and shear stiffness is critical parameters in this pillar failure. In addition, it indicates that W/H ratio equal 0.3, the lack of ore extraction strategy and inadequate roof support are the major reasons for this failure. In this paper, the procedure of study was described.     

Characteristics of rockburst and its mining technology in mines

In order to resolve how to mine under the condition of high stress with rockburst, this paper analyzes the law of rockburst, and considers that most of the rockbursts occur in the high stress area of stope. A method of rockburst forecast and its steps are given, and three different stages of rockburst are identified. Furthermore, this paper considers that blasting energy can effectively control rockburst by changing the characteristics of rock mass, which decreases the probability of rockburst happening. According to characteristics of rockburst, two feasible mining techniques under rockburst conditions are put forward, that is stoping-and-filling method, and sublevel caving method of mining without any sills in hanging wall.     

An improved method to assess the required strength of cemented backfill in underground stopes with an open face

Backfill is increasingly used in underground mines to reduce the surface impact from the wastes produced by the mining operations. But the main objectives of backfilling are to improve ground stability and reduce ore dilution. To this end, the backfill in a stope must possess a minimum strength to remain self-standing during mining of an adjacent stope. This required strength is often estimated using a solution proposed by Mitchell and co-workers, which was based on a limit equilibrium analysis of a wedge exposed by the open face. In this paper, three dimensional numerical simulations have been performed to assess the behavior of the wedge model. A new limit equilibrium solution is proposed, based on the backfill displacements obtained from the simulations. Comparisons are made between the proposed solution and experimental and numerical modeling results. Compared with the previous solution, a better agreement is obtained between the new solution and experimental results for the required cohesion and factor of safety. For large scale (field) conditions, the results also show that the required strength obtained from the proposed solution corresponds quite well to the simulated backfill response.     

Remediation and monitoring of abandoned mines

paper describes a recent study on using fly ash for backfilling abandoned room and pillar mines.Detailed investigations on fly ash properties such as the strength and stiffness of settled fly ash, flowability of fly ash grout, as well as chemistry and environmental aspects of fly ash backfill have been undertaken in the laboratory. Numerical modelling was also conducted to quantify the effects of fly ash backfill on the stability of underground pillars. The laboratory tests showed that with a solid concentration of approximate 50%, fly ash grout has an excellent flowability and very low viscosity. It is capable of penetrating and filling almost any voids underground if designed properly and settling as a reasonably stiff solid to provide support to the pillars. Several different types of strength tests proved that a consolidated fly ash should exhibit a friction angle above 42°. 3D numerical modelling on interaction between fly ash backfill and underground pillars has shown that fly ash backfill to 90% roadway height can raise the factor of safety(Fo S) of a marginally stable area to above 1.6, which is the number often used in rock engineering design for long term stability. Chemistry and leachate analysis of representative fly ash samples from a local power station showed that the elemental concentrations in the fly ash solid sample are lower than the allowed contaminant threshold and specific contaminant concentration levels. Geotechnical monitoring in the high risk areas of an abandoned mine has been carried out as part of the risk management and control for potential subsidence. The monitoring has been very helpful in understanding the ground behaviour around the abandoned mine which can provide timely information to the parties concerned in order to make correct decisions to control the subsidence risk.     

Analysis of coal pillar stability(ACPS): A new generation of pillar design software

Thirty years ago, the analysis of longwall pillar stability(ALPS) inaugurated a new era in coal pillar design.ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of longwall mining case histories.ALPS was followed by the analysis of retreat mining stability(ARMPS) and the analysis of multiple seam stability(AMSS). These methods incorporated other innovations, including the coal mine roof rating(CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods.Today, these empirical methods are used in nearly every underground coal mine in the US. However,the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example,in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice. The analysis of coal pillar stability(ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery.ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.     

Design of crown pillar thickness using finite element method and multivariate regression analysis

Minerals are now being extracted from deep mines due to drying up of resource in shallow ground. The need for suitable supports and ground control mechanisms for safe mining necessitates proper pillar design with filling technology. In addition, high horizontal stress may cause collapse of hanging wall and footwall rocks, hence designing of suitable crown pillars is absolutely necessary for imposing overall safety of the stopes. This paper provides a methodology for the evaluation of the required thickness of crown pillars for safe operation at depth ranging from 600 m to 1000 m. Analyses are conducted with the results of 108 non-linear numerical models considering Drucker-Prager material model in plane strain condition. Material properties of ore body rock and thickness of crown pillars are varied and safety factors of pillars estimated. Then, a generalized statistical relationship between the safety factors of crown pillars with the various input parameters is developed. The developed multivariate regression model is utilized for generating design/stability charts of pillars for different geo-mining conditions.These design charts can be used for the design of crown pillar thickness with the depth of the working,taking into account the changes of the rock mass conditions in underground metal mine.     

Analysis of factors and countermeasures of mining subsidence in Kaiyang Phosphorus Mine

1 INTRODUCTION After useful mineral body has been mined and dragged out, mining goaf is formed and country rocks would probably lose original equivalence. When span of the goaf is long enough, even if the roof is hard and intact, the rock around the goaf would move towards the mining openings, and this movement would pass on to the adjoining rock mass until it reaches the surface and ground displacement can be observed. In other words, underground mining may cause ground cracks and surface…     

小铁山矿分段分条充填法采场结构参数的三维有限元分析

采用三维有限元数值分析方法,结合小铁山矿分段充填法实际,对不同有场结构参数条件下矿体应力分布、能量释放和采场体积闭合进行了计算,为合理、安全垂采矿体提供了必要的切合实际的结构参数。     

An analytical solution to estimate the settlement of tailings or backfill slurry by considering the sedimentation and consolidation

A common and important task in mining industry is to estimate the settlement or final volume of the tailings or backfill associated with sedimentation and self-weight consolidation. Up to now however, most existing analytical solutions were developed by only considering the settlement induced by consolidation. In this paper, the process of shrinkage tests has been compared with those of sedimentation and consolidation. It was shown that the pore water and particles movement in the sedimentation are very similar to that in the normal shrinkage. The void ratio at the end of sedimentation is thus for the first time considered to be equal to the void ratio at desaturation onset of shrinkage tests. An analytical solution was then proposed to estimate the settlement of tailings or backfill by considering the sedimentation and consolidation. To validate the proposed analytical solution, tailing deposition tests were conducted in two molds to simulate the tailings impoundment and underground mine stope. The required parameters were obtained through shrinkage and consolidation tests. Good agreements were obtained between the measured settlements and those calculated by the proposed solution. The proposed solution can thus be considered as validated and used to evaluate the settlement of tailings or backfill slurry.     

露天煤矿端帮残煤开采及边坡暴露时间分析

为了提高露天开采的煤炭资源回收率，掌控露天煤矿端帮残煤开采工程安排，提出了水平和近水平大型露天煤矿汽车运输内排条件下，露天煤矿端帮露井联采、端帮陡帮开采、减少露天煤矿端帮边坡暴露时间的措施．结果表明：采掘工作帮帮坡角越大、内排土场工作帮帮坡角越大、工作线推进强度越大、内排土场下部水平排土台阶高度越大、采场坑底宽度越小露天煤矿端帮边坡的暴露时间越短；当露天煤矿端帮实行陡帮开采时，汽车运输内排通路可设在采场中部回填搭建。     

Mechanics criterion and factors affecting overburden stability in solid dense filling mining

The effect of controlling strata movement in solid filling mining depends on the filling rate of the goaf.However, the mechanical property of the overburden in the backfill stope and the designed size of the backfill mining workface should also be considered. In this study, we established a main roof strata model with loads in accordance with the theory of key strata to investigate the stability of the overburden in solid dense filling mining. We analyzed the stress distribution law of the main roof strata based on elastic thin plate theory. The results show that the position of the long side midpoint of the main roof strata failed more easily because of tensile yield, indicating that this position is the area where failure is likely to occur more easily. We also deduced the stability mechanics criterion of the main roof strata based on tensile yield criterion. The factors affecting the stability of the overburden in solid dense filling mining were also analyzed, including the thickness and elasticity modulus of the main roof strata, overlying strata loads, advanced distance and length of workface, and elastic foundation coefficient of backfill body.The research achievements can provide an important theoretical basis for determining the designed size of the solid dense filling mining workface.     

基于能量理论的深埋矿山矿柱尺寸设计方法

随着资源开发步入到深部开采阶段,深埋矿山开采过程中的动力灾害问题日益突出,传统静力稳定分析方法在应对这些问题时表现出一定的局限性,研究适用于深部矿山的矿柱尺寸设计方法意义重大。本文基于面积承载理论和金属矿山矿柱强度估算方法,以一种动态的视角从功能转换角度重点探讨了瞬间回采方式下的矿柱应变能积聚和转化特征,对地下矿体阶段性分层回采这一过程进行简化,提出了一种基于可释放能理论的适应不同分层钻爆开挖方式下的矿柱尺寸参数设计方法,通过依托某工程实例进行了能量法的应用并与传统强度理论方法进行了比较分析,据此建议了5组矿柱尺寸参数方案,进一步利用有限差分软件（FLAC3D）对其优化从而最终确定了合理矿柱尺寸设计值。研究成果表明：矿柱体受动态开挖作用将产生一定程度的动力响应,诱发产生的动应力和动态变形一度可达静力分析时的两倍,同时也会伴随大量能量的积聚;与传统强度计算理论相比,应用能量法可有效地避免快速回采情况下矿柱动力失稳的可能,设计得到的矿柱尺寸参数更偏保守,随着开挖分层数的增多,能量校核法逐渐趋于和强度静力法等价。经数值模拟优化比选,最终建议该工程矿体分三层回采,矿柱合理宽度22m,高度70m,基本上可以满足回采过程中安全稳定和经济效益的需求,与工程实际契合较好。     

深部条带开采下沉系数与采厚关系的数值模拟

以峰峰万年矿的地质采矿条件为原型,利用FLAC3D数值模拟软件,建立了深部大采宽条带开采数值模拟模型,对开采厚度与下沉系数的关系进行了研究.结果表明：在采深和采出率等条件相同的情况下,地表下沉值和水平移动值均随采厚的增加而增大,而下沉系数随采厚的增加以非线性关系逐渐减少.下沉系数与开采厚度关系的确立,对深部大采宽条带开采地表移动变形预计及优化设计具有指导作用.     

深部条带开采下沉系数与采厚关系的数值模拟

以峰峰万年矿的地质采矿条件为原型,利用FLAC3D数值模拟软件,建立了深部大采宽条带开采数值模拟模型,对开采厚度与下沉系数的关系进行了研究.结果表明:在采深和采出率等条件相同的情况下,地表下沉值和水平移动值均随采厚的增加而增大,而下沉系数随采厚的增加以非线性关系逐渐减少.下沉系数与开采厚度关系的确立,对深部大采宽条带开采地表移动变形预计及优化设计具有指导作用.     

锚杆加固条带煤柱的离散元模拟分析

结合煤巷锚杆支护技术与条带开采技术，首次提出了用锚杆加固条带煤柱的新方法。离散元数值模拟结果表明，采用这一方法，不仅可以提高煤柱的稳定性和承载能力，减轻采动损害，还可将煤柱缩小一定尺寸，提高回采率，在“三下”采煤中有着广阔的应用前景。     

Feasibility study of highwall mining in north surface mine of Yima Coal Corporation, China

Yima Coal Corporation is considering to adopt highwall mining method with auger machine to recover coal from north surface pit that has reached final highwall position. The major geomechanical issues associated with auger mining are highwall and pillar stability. Based on the field investigation and laboratory test results of mechanical parameters, numerical modeling is carried out to assess the stability of highwall and pillar. Field measurements of highwall deformation have been used to validate and ensure the confidence for the development of realistic models. The results of numerical modeling show that the mining method is feasible for mining the seam of 10 m thickness in north surface coal mine.     

Mechanisms of support failure and prevention measures under double-layer room mining gobs – A case study: Shigetai coal mine

In the practice of mining shallow buried ultra-close seams, support failure tends to occur during the process of longwall undermining beneath two layers of room mining goaf (TLRMG). In this paper, the factors causing support failure are summarized into geology and mining technology. Combining column lithology and composite beam theory, the key stratum of the rock strata is determined. A finite element numerical simulation is used to analyze the overlying load distribution rule of the main roof for different plane positions of the upper and lower room mining pillars. The tributary area theory (TAT) is adopted to analyze the vertical load distribution of each pillar, and dynamic models of coal pillar instability and main roof fracture are established. Through key block instability analysis, two critical moments are established, of which critical moment A has the greater dynamic load strength. Great economic losses and safety hazards are created by the dynamic load of the fracturing of the main roof. To reduce these negative effects, a method of pulling out supports is developed and two alternative measures for support failure prevention are proposed: reinforcing stope supports in conjunction with reducing mining height, or drilling ground holes to pre-split the main roof. Based on a comprehensive consideration of economic factors and the two categories of support failure causes, the method of reinforcing stope supports while reducing mining height was selected for use on the mining site.     

Numerical analysis and geotechnical assessment of mine scale model

Various numerical methods are available to model, simulate, analyse and interpret the results; however a major task is to select a reliable and intended tool to perform a realistic assessment of any problem. For a model to be a representative of the realistic mining scenario, a verified tool must be chosen to perform an assessment of mine roof support requirement and address the geotechnical risks associated with longwall mining. The dependable tools provide a safe working environment, increased production, efficient management of resources and reduce environmental impacts of mining. Although various methods, for example, analytical, experimental and empirical are being adopted in mining, in recent days numerical tools are becoming popular due to the advancement in computer hardware and numerical methods. Empirical rules based on past experiences do provide a general guide, however due to the heterogeneous nature of mine geology (i.e., none of the mine sites are identical), numerical simulations of mine site specific conditions would lend better insights into some underlying issues. The paper highlights the use of a continuum mechanics based tool in coal mining with a mine scale model. The continuum modelling can provide close to accurate stress fields and deformation. The paper describes the use of existing mine data to calibrate and validate the model parameters, which then are used to assess geotechnical issues related with installing a new high capacity longwall mine at the mine site. A variety of parameters, for example, chock convergences, caveability of overlying sandstones, abutment and vertical stresses have been estimated.