Strategic sill pillar design for reduced hanging wall overbreak in longhole mining

Steeply dipping, vein and tabular orebodies are traditionally extracted with longitudinal retreat mining methods such as Eureka and Avoca in a bottom-up sequence with delayed backfill. To increase productivity, sill pillars in the orebody are used to separate mining zones thus allowing production to take place simultaneously in two or more zones. While such mining methods are productive, they may be accompanied with high volumes of hanging wall overbreak causing significant unplanned ore dilution. In this work, it is shown through a mine design case study of a narrow vein deposit that a sill pillar could also play a significant role in limiting hanging wall overbreak. To demonstrate the role of sill pillar, a novel numerical modelling scheme is proposed to account for progressive stope wall overbreak. A numerical modelling approach of element death and rebirth is developed to allow for the detected stope overbreak to be immediately removed and replaced with backfill material before upper-level stope extraction. It is further shown that the average overbreak volume could be reduced by as much as 33% when the sill pillar is strategically placed in the lower half of a mine plan.     

采动区建筑物下保护煤柱开采优化设计研究

采用岩层移动角进行留设保护煤柱的传统方法,增大了保护煤柱呆滞量,造成了煤炭资源的巨大浪费.为了探讨综放开采条件下保护煤柱留设的最优尺寸,基于潞安矿区王庄煤矿的地质采矿条件,提出了根据建筑物采动损害允许的临界变形值进行优化设计保护煤柱的新思路.按照概率积分法,预测计算开采工作面位于不同停采线位置时保护煤柱留设的合理尺寸,分析了不同开采方案下建筑物所受采动的影响面积、地表下沉程度及倾斜变形、拉伸变形等地表移动变形对建筑物采动损害的影响程度.通过11个方案的分析比较,结合经济与社会效益,给出了建筑物下保护煤柱留设的最优方案,为综放开采条件下建筑物压煤开采提供了科学依据.     

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.     

Subsidence over room and pillar retreat mining in a low coal seam

The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7 m × 7 m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.     

A roof model and its application in solid backfilling mining

Through changing the axial load on backfilling material compaction test to reflect different overlying strata pressure on backfilling material, the stress-strain relations in the compaction process of backfilling material under the geological condition can be obtained. Based on the characteristic of overlying strata movement in backfill mining, a model of roof thin plate is established. By introducing the stress-strain relation in compaction process into the model and using RIZT method to analyze the bending deformation of roof, the bending deflection and stress distribution can be obtained. The results show that the maximum roof subsidence and maximum tensile stress occurring at the center are 255 mm and5 MPa, respectively. Tensile fracture of roof under the geological condition of Dongping Mine did not occur. The dynamic measurement results of roof in Dongping Mine verify the theoretical result from the aforementioned model, thereby suggesting the roof mechanical model is reliable. The roof thin plate model based on the compaction characteristic of backfilling material in this study is of importance to research on backfill mining theories and application of backfilling material characteristics.     

Limitations and potential design risks when applying empirically derived coal pillar strength equations to real-life mine stability problems

The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database. This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength ‘‘index." These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of supercritical overburden behavior as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases. A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.     

Strata behavior investigation for high-intensity mining in the water-rich coal seam

This paper describes a specific case of mining in a water-rich coal seam in western China. Water inrushes, roof caving and other disasters induced by intensive mining operation could pose great threats to the safety of coal mines. The strata behavior during the high-intensity extraction in the water-rich coal seam is analyzed by employing the numerical simulation method and in situ monitoring. The results show that about 10 m ahead of the workface, the front abutment pressure peaks is at 34.13 MPa, while the peak of the side abutment pressure is located about 8 m away from the gateway with the value of 12.41 MPa; the height of the fracture zone, the first weighting step and the cycle weighting step are calculated to be 45, 50 and 20.8 m, respectively; pressure distribution in the workface is characterized by that the vertical pressure in the center occurs earlier and is stronger than those on both ends. Then, the results above are verified by in situ measurement, which may provide a basis for safe mining under similar conditions.     

Effect of protective coal seam mining and gas extraction on gas transport in a coal seam

A gas–solid coupling model involving coal seam deformation,gas diffusion and seepage,gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining.The research results indicate:(1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase,thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient,which will cause a decrease in gas pressure.(2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam,which can effectively reduce the coal and gas outburst risk.The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China.     

Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining

To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining,the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis.The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained.The results indicate that when the repeated mining face passes the residual pillars,the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face.A relatively stable ‘‘Voussoir beam" structure is formed after the advance fracturing of the main roof.When the repeated mining face passes the old goaf,as the large rock block revolves and touches gangue,the rock block will break secondarily under overburden rock loads.An example calculation was performed involving an integrated mine in Shanxi province,results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412 kN and 10,743 kN when repeated mining face passed through residual pillar and goaf.On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable.     

Catastrophe analysis on pillar instability considered mining effect

The instability of the pillar was discussed based on the potential energy principle and the cusp catastrophe theory, and a simplified mechanical model of the pillar was established considering the mining effect. The necessary-sufficient conditions, the jump value of displacement of pillar and the released energy expressions were deduced. The results show that the instability of the pillar is related to the properties of the rock, the external force and the relative stiffness of the elastic area to the plastic area. The instability of system is like to occur with the enlarging of the softening area or the decreasing of E/λ. The calculation done shows that the estimated results correspond to practical experience.     

Coupling control on pillar stress concentration and surface cracks in shallow multi-seam mining

In order to ensure safe mining and reduce surface damage in shallow multi-seam mining, the failure characteristics of interburden strata with different coal pillars offset distances between pillars in the upper and lower seams, the distribution characteristics of stress concentration in coal pillars, and the development characteristics of stratum cracks and subsidence were investigated by physical and UDEC2 D simulation. Meanwhile, the effect of different coal pillar offset distances on stress concentration of coal pillar and development of stratum cracks were studied. Based on those results, a formula for safe mining and reducing surface damage was established, which provided a theoretical basis for safe and environmentally friendly mining in shallow multi-seam. According to the results, the optimal coal pillar offset distance(the side to side horizontal distance of the upper and lower coal pillars) between the upper and lower coal seams was developed to reduce the stress concentration of coal pillars and surface damage.The results of this study have been applied in Ningtiaota coal mine and have achieved good results in safe and environmentally friendly mining.     

倾斜煤层区段煤柱变形破坏规律

煤层倾角是影响区段煤柱稳定性的关键因素之一.利用理论分析、相似模拟、数值模拟等方法研究了倾斜煤层开挖后倾向覆岩结构演化特征、煤柱变形及失稳破坏形式.研究结果表明,0~45°范围内随着煤层倾角增大,区段煤柱发生剪切失稳破坏的可能性增大;煤柱两侧覆岩结构呈现不对称分布,煤柱上侧砌体梁结构形成层位较低,煤柱下侧形成冒空区,砌体梁结构形成层位高于上侧;与水平煤层煤柱破坏以挤压变形为主不同,倾斜煤柱以沿着弱面剪切滑移破坏为主;不同倾角煤层煤柱围岩变形量呈不对称分布,煤柱下侧围岩变形量大于上侧,煤层倾角越大煤柱围岩变形量不对称分布趋势越明显.     

Numerical analysis for the prediction of bump prone conditions:A southern Appalachian pillar coal bump case study

Two miners were fatally injured when a pillar bump occurred during retreat mining in a southern West Virginia coal mine. The mine was operating in the Eagle seam with overmining in the No. 2 Gas and Powellton seams. A coal bump is defined as a sudden and violent failure of coal caused by the release of stored strain energy in the pillar. While significant strides have been made by academia, industry,and regulatory agencies to better understand bump conditions and mitigation techniques, coal bumps represent a long standing, highly site-specific engineering problem in which the exact failure mechanism is not clearly understood. In this case history, a cut-by-cut analysis of retreat mining operations was conducted on the 4 East Main leading up to the pillar bump event. Numerical input parameters were derived from site-specific geologic information and mine geometry for the analysis of pillar stress conditions and energy release using LaModel. An overview of stress conditions in the panel was presented including a precursor event that occurred two crosscuts inby the bump site. The methodology presented in the paper for the evaluation of the fatal bump event can be used for the identification of bump prone conditions prior to development and retreat of a mining area.     

Fold catastrophe model of dynamic pillar failure in asymmetric mining

A rock burst disaster not only destroys the pit facilities and results in economic loss but it also threatens the life of the miners. Pillar rock burst has a higher frequency of occurrence in the pit compared to other kinds of rock burst. Understanding the cause, magnitude and prevention of pillar rock burst is a significant undertaking. Equations describing the bending moment and displacement of the rock beam in asymmetric mining have been deduced for simplified asymmetric beam-pillar systems. Using the symbolic operation software MAPLE 9.5 a catastrophe model of the dynamic failure of an asymmetric rock-beam pillar system has been established. The differential form of the total potential function deduced from the law of conservation of energy was used for this deduction. The critical conditions and the initial and final positions of the pillar during failure have been given in analytical form. The amount of elastic energy released by the rock beam at the instant of failure is determined as well as. A diagrammatic form showing the pillar failure was plotted using MATLAB software. This plot contains a wealth of information and is important for understanding the behavior during each deformation phase of the rock-beam pillar system. The graphic also aids in distinguishing the equivalent stiffness of the rock beam in different directions.     

A study of mining fatalities and coal price variation

It has long been postulated that a relationship exists between commodity price cycles and fatalities in the mining industry. Previous studies have found only weak correlations in this area. This study analyses the fatalities recorded in coal mines over the period 1985–2016 in the State of Queensland as a function of thermal coal price variation. The study finds that the relationship between fatalities and coal prices is not linear. One to two fatalities occur in most years independent of the thermal coal price. When the price of coal falls below AUD 55/tonne(non-inflation adjusted), the likelihood of an incident involving multiple fatalities increases. The probability can be estimated at 2 in 18 events(equivalent to 11%). This paper postulates that in difficult economic times, mining companies react by downsizing direct employees. If not carefully managed, this can result in loss of knowledge around safety systems, and reduced effectiveness of safety supervision. Because of labour cost advantages, some jobs previously undertaken by direct employees will be replaced by contractors. Increased contractor numbers contribute to increased risk of fatalities occurring, as contractors are over-represented in accident categories involving vehicle accidents, tire handling and crushing incidents. Mine inspectorates, mining, and mining contractor companies need to be especially vigilant to enforce health and safety management systems during periods of low coal prices.     

Effects of overburden characteristics on dynamic failure in underground coal mining

Dynamic failures, or ‘‘bumps, remain an imperative safety concern in underground coal mining, despite significant advancements in engineering controls. The presence of spatially discrete, stiff roof units are one feature that has been linked to these events. However, an empirical stratigraphic review indicates that no significant difference exists in the relative commonality of discrete units between bumping and non-bumping deposits. Instead an apparent relationship exists between reportable bumping and the overall stiffness of the host rock. However, this initial study is too simplistic to be conclusive; to weight the relative impact of changes in a single variable, such as the thickness or location of sandstone members, it must be examined in isolation—i.e., in a setting where all other variables are held constant.Numerical modelling provides this setting, and the effects of variability in a stiff discrete member in a hypothetical longwall mining scenario are investigated within the context of three stratigraphic ‘‘types,Compliant, Intermediate and Stiff. A modelling experiment examines changes in rupture potential in stiff roof units for each stratigraphic type as discrete unit thickness and location are manipulated through a range of values. Results suggest that the stiff-to-compliant ratio of the host rock has an impact on the relative stress-inducing effects of discrete stiff members. In other words, it is necessary to consider both the thickness and the distance to the seam, within the context of the host rock, to accurately anticipate areas of elevated rupture-induced hazard; acknowledging the presence of a discrete unit within the overburden in general terms is an insufficient indicator of risk. This finding helps to refine our understanding of the role of individual stiff, strong roof members in bumping phenomena, and suggests that a holistic view of overburden lithology and site-specific numerical modelling may be necessary to improve miner safety.     

含水层下大采高综采面缩小防护煤柱留设研究

由五沟煤矿大量的钻探资料及现场试验数据,分析了含水层下大采高缩小防护煤柱综采工作面的工程地质特征.根据相关规程及相似模拟实验对综采工作面开采后导水裂缝带、冒落带高度的计算分析,得出五沟煤矿1012综采面缩小防水煤柱开采在技术上是可行的.     

综采面邻空煤柱漏风微循环现象研究

针对综采面"采场-邻空煤柱-采空区"间的漏风微循环现象,利用MATLAB中的PDE工具进行二维非线性渗流的数值模拟,同时分析研究漏风微循环现象形成的原因,以及漏风微循环对煤炭自燃的影响。研究表明,"采场-邻空煤柱-采空区"间的漏风微循环可以导致采空区内相对压力和气体成分的周期性变化,即"采空区喘息"现象。     

Vertical transportation system of solid material for backfilling coal mining technology

For transportation of solid backfill material such as waste and fly ash from the surface to the bottom of the shaft in a fully mechanized backfilling coal backfilling coal mining technology,we develope...