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英文原文A new coal pillars design method in order to enhance safety of the retreat mining in room and pillars minesE.Ghasemi*,K.ShahriarDepartment of Mining of and Metallurgical Engineering,Amirkabir University of Technology,Tehran,IranAbstract:Most of the proposed methods of coal pillar design determine pillar dimensions using pillar estimation only through the tributary area theory.Designing pillar based on these methods is not appropriate in room and pillar mines with pillar recovery because retreat mining and gob creation generate abutment loads.Neglecting abutment loads in design stage may lead to pillar failure and destructive effects during retreat mining.Thus proper pillar design has a remarkable effect on mining effect on mining safety.In this paper,a step-bu-step method is presented to design pillars with square shape in room and pillar mines with regard to existing pillars in the active mining zone(AMZ) and estimating abutment loads according to experiment-method. This method has been applied to determine optimum pillar dimensions in the main panel of Tabas Cental Mine(TCM),located in the mid-eastern part of Iran.Obtained results show the abutment loads account for 27%of the total loads applied on pillars in AMZ in this panel.Pillar width,based on this method, is also obtained 11.6m.Key words: Pillar design;Room and pillar;Retreat mining; Active mining zone;Abutment loads1. IntroductionIn underground coal mining ,room and pillar is the method of working preferable for flat,tabular deposits in thin seams,where rooms of entries are driven in the solid coal to form pillars in the development panels(Hustrulid,1982;Hartman,1978).Pillars o f coal are left behind to support the roof and prevent its collapse,thereby allowing miners to extract coal between them and to travel safely. In some cases, the pillars are removed partly or fully in a later operation, known as retreat mining(also known as secondary mining or pillar recovery operation). Coal mine pillar design has been the subject of sustained and intensive research in the major coal producing countries in the word. Pillar design and stability are two of the most complicated and extensive problems in mining related to rock mechanics and ground control subjects. Although these problems have been investigated for a long time, to date only a limited understanding of the subject has been gained. The subject of pillar design in the US goes back nearly a centry. Prior to this the dimensions of pillar were largely determined rules of thumb such research as there was tended to be isolated and sporadic. But nowadays, various pillar design formulas are developed, based upon laboratory testing, full-scale pillar testing, and back-analysis of failed and successful case histories. In 1980, field studies conducted by the US Bureau of Mines has developed the classic pillar design methodology. It consisted of three steps(Mark,2006): 1.Estimating the pillar load; 2.Estimating the pillar strength ,and 3.Calculating the pillar safety factor.Th average pillar, in regular layouts of pillars can be estimated by tributary-area theory, each individual pillar is assumed to carry the weight of the overburden immediately above it. In the other words, a pillar uniformly supports the weight of rock overlying the pillar and one-half the width of rooms and entries on each side of the pillar(Peng,1978).Pillar strength can be defined as the maximum resistance of a pillar to axial compression(Brady and Brown,1993). Empirical evidence suggests that pillar strength is related to both its volume and its shape(Salamon and Munro,1967;Brady and Brown,1993).Numerous formulas have been developed that can be used to estimate the strength of pillars in coal mines, which Table 1 shows the most applicable of them. Each of these formulas estimates the to pillar strength in terms of two various; width to height ration and in situ coal strength.Bieniawski(1981) represented very good classic approach to pillar design. He at fist described the issues involved in pillar design, and advantages and shortcomings of the available methods and then represented a logical, step-by-step approach to determine the coal pillars dimensions in room and pillar mines. Table 1Most applicable of empirical strength formula for coal pillars.Pillar strength formulas(MPa)ReferencePillar cross-sectionRemarksSalamon and Munro (1967)SquareBieniawski (1968)SquareMadden(1991)SquareMark and Chase (1997)SquareNowadays in most of the room and pillar mines in order to increase recovery and productivity,remanent pillars in panels are recovered by retreat mining. Since, the above mentioned methods are not appropriate for pillar design because these methods neglect the abutment loads due to retreat mining and creation of a mined out gob. Abutment loads affect on the pillar in the adjacent of pillar line and a load more than the one estimated bu tributary area theory applied on pillar(Mark and Chase,1997;Peng,1978).Studies conducted by van der Merwe(19990) confirm the increase of load on the pillars in the adjacent of the pillar line. He calculated the actual load applied on the pillar during pillar recovery using a two dimensional boundary element model and estimated the pillar safety factor for this condition.Pillar design without the abutment loads to failure of pillar during retreat mining. Pillar failures continue to be one the greatest single hazards faced by underground coal miner, Pillar failure responsible for unsatisfactory conditions includes(Mark et al.,2003):1.Pillar squeeze,2.Massive pillar collapse, and 3.Coal pillar bumps.The occurrence of pillar failure in underground mines entails detrimental effects on miners in the form of injury,disability or fatality as well as mining company due to downtimes, interruptions in the mining operations, equipment breakdowns,etc.For example in 1992,air blasts due to pillar failure at a southern West Virginia mine led to destroying of 103 ventilation stopping(Mark et al.,1997). On August 6th,2007,violent coal bump occurred in Crandall Canyon Mine in Utah, and caused the entrapping of six others(Heasley,2009a).So, proper pillar design is the key to prevent of pillar failure and the Analysis of Retreat Mining Pillar Stability(ARMPS) programs are used successfully for designing safe retreat mining(Tulu et al.,2010).LaModel is a PC-based program for calculating the stresses and displacement in coal mines or other thin seam or vein type deposits(Heasley and Barton,1999;Heasley,2009b).It is primarily designed to be utilized by mining engineers for investigating and optimizing pillar dimensions and layouts in relation to overburden, abutment and multiple seam stresses. The program was developed based on displacement-discontinuity variation of the boundary element method. Mark and Chase(1997) developed the ARMPS program based on empirical equations. ARMPS considering the active mining zone(AMZ) calculates stability factor(ARMPS SF) based on estimates of the loads applied to, and the load-bearing capacities of, pillars during retreat mining. More than150 cases of retreat mining were collected in US to verify the program(ARMPS help,2008).Analyses of all these cases show that pillar squeeze is the most frequent type of failure and occurs in about two thirds of cases. 14 cases of pillar sudden collapses were observed, which in every case occurred when the ARMPS SF was less than 1.5 and where the pillar width to height ration is less than 3. All but 3 of the 17bumps occurred when the depth of cover exceeded 400m.Mark and Chase(1997) understood that almost no considerable massive pillar collapses occurs when the pillar width to height ration more than 4 is selected. They also observed when the depth of cover is less than 200m; the minimum required stability factor to prevent massive pillar collapses is 1.5.One of the keys to miners safty and efficient recovery of the reserves is to design sufficiently sized pillars that will prevent pillar squeezes, excessive pillar spalling severe floor heave, roof falls,and pillar bumps.Regarding the above mentioned comments, a new method to design coal pillars with square shape in room and pillar mines is presented in the following sections. The proposed method is suitable in determining optimum pillar dimensions in room and pillar mines where remanent,pillars are supposed to be extracted after preliminary mining completion. This method, in addition to considering abutment loads,lowers pillar failure risk. The goal of this method is to help ensure that the pillars developed for future extraction are of adequate size for all anticipated loading conditions.2. Methodology Similar to the ARMPS program, the proposed method in this paper considers the pillars in the active mining zone(AMZ) because these pillars are exposed to maximum load throughout mining process therefore the pillar dimensions obtained by this method is more satisfactory. Before describing the design method, a describing on the AMZ is necessary. As shown in Fig.1, AMZ includes all of the pillars on the extraction front(or pillar line), and extends out by the pillar line a distance of 5 times the square root of the depth of cover. This width of AMZ was selected because measurements of abutment load falls within its boundaries(Mark and Chase,1997).The proposed method is based in five principles(Ghasemi et al.,2010a):1. Calculating the maximum load applied on the pillars in AMZ(including development load, abutment loads),2. Calculating the overall load-bearing capacity of pillars in AMZ,3. Selecting an appropriate safety factor,4. Calculating the pillar width, and 5. Correcting the pillar width to find the optimum pillar width.The method is made up of twelve steps which are described below. Fig,2 also illustrates different steps of this method in a flow-chart plot. The symbols used here are provided in Table 2.2.1. Step 1:Gathering essential dataEssential data to determine the optimum pillar dimensions in this method are as following:Fig.1. Schematic show of the AMZ (Mark and Chase,1997)1. Depth of cover:average overburden thickness over the pillar system.2. Pillar height(Mining height):note that the value of pillar height is not necessarily equal to the seam thickness.3. Entry width:entry width is usually determined base on roof rock quality, production rate and operational width of equipments. In this method, crosscuts are assumed to have the same width as the entries.4. In situ coal strength.5. Mean unit weight of the overburden.6. Abutment angle:the abutment angle determines how much load is carried by gob. Measurement of longwall abutment loads indicated that an abutment angle 21 is appropriate for normal caving conditions. For example, if no caving has occurred abutment angle is 90namely zero load transfer to the gob(Mark and Chase,1997).7. Panel width: panel width is usually determined base on geotechnical conditions, stress state in the region, economic criteria, and environmental conditions. Panel width affects on stress distribution loading conditions and caving mechanism. An increase in panel width results in an increase of the abutment loads applied on the pillars adjacent to the gob area. The tension zone height developed in the roof of the gob area also increase as the panel width increase and may lead to a large failure in overburden(Bieniawski,1987). Based on width to depth ration(P/H), panel are divided into categories:Sub-critical panels(P/H2tan)，and Super-critical panels(P/H2tan).8. Coal Mine Roof Rating(CMRR):this index is used to evaluate roof rock quality. In 1994 the CMRR was developed to fill the gap between geologic characterization and engineering design(Mark and Molinda,2005). This classification system considers geotechnical factors such as roof rock strength, bedding and other discontinuities, moisture sensitivity of the roof rock, groundwater, etc. CMRR varies between zero and 100. Based on this index, roof rocks in coal mines are put in three categories(Chase et al.,2002):Weak(CMRR45),Intermediate (45CMRR65).2.2. Step 2: Calculating AMZ dimensions AMZ length and width are determined from Eqs.(1) and (2) respectively:(1)(2)2.3. Step 3: Calculating development loadDevelopment load are resulted from the overburden weight over active mining zone. Based on tributary area theory, development loads are obtained from the following equation:(3)2.4. Step4: Calculating the maximum front abutment loadRetreat mining starts with the extraction of the panel pillars. When enough of pillars have been extracted, the overburden strata above the extracted pillars start to cave. As a result of this roof caving, the active gob is carried by the gob, but a considerable amount of the original overburden load over the gob is transferred to the pillars in AMZ and barrier pillars as a front abutment load(see Fig.1). Front abutment load is calculated based on abutment angle concept(Mark, 1992;Tulu et al.,2010) and its distribution is different in sub-critical and super-critical panels(see Fig.3). Depending on whether the panel is sub-critical or super-critical , the maximum front abutment load is given bu Eqs.(4) and (5) respectively(Ghasemi et al.,2010a):(4)(5)2.5. Step 5: Calculating side abutment loadThe gob area beside the mining panel is the source of side abutment load. Two gob areas may exist beside each mining panel. The side abutment load is shared between the barrier pillar and the AMZ. This load the same as front abutment is calculated by abutment angle concept. Gob area width and barrier width are required to calculate side abutment load applied on AMZ. Depending load is given by Eqs.(6) and (7) respectively(ARMPS help,2008):(6)(7)In both of them, regarding Eqs.(8),R is:(8)Factor R is transfer rate that shows the percentage of total side abutment load that is applied to AMZ.2.6. Step 6: Calculating the maximum load on AMZThe maximum load applied on the pillars in AMZ is calculated by summation of development load, maximum front abutment load, and side abutment load according to the following equation: (9)Fig.2. Flowchart for proposed coal pillars design method2.7. Step 7: Determining number of entriesThe number of existing entries is usually determined based on panel width,rock mechanics,operation equipments, and production rate. At least four entries are needed; one for accommodating the conveyor, one for fresh air, and wo others in two sides of panel to take the aie out (Stefanko, 1983). Economically and operationally, this number of entries is not adequate in continuous (mechanics) mining method and at least five entries should be planned which this number increases up to seven entries in mines with high production rate(Hartman,1987).2.8. Step 8：Calculating the load-bearing capacity of AMZThe load-bearing capacity of the pillars in AMZ is calculated by summing the load-bearing capacities of all of the pillars within its boundaries. The load-bearing capacity of each pillar is determined bu multiplying their strength by their load-bearing area(Mark and Chase,1997). In this method, pillar strength is estimated using the Bieniawskis strength formula. The number of existing pillars in AMZ is calculated according to the following equation:(10)Hence, the overall load-bearing capacity of pillars in AMZ is given by the following equation:(11)2.9. Step 9: Selecting an appropriate safety factorThe selection of an appropriate safety factor can be based on a subjective assessment of pillar performance or statistcal analysis of failed and stable cases(Salamon and Munro,1967;Mark,1992). According to the studies by Chase et al.(2002), Table 3 provides suggested safety factors for stability of the pillars in AMZ. These values are obtained from 250 analyses of panel design in US and as it is seen from table, safety factor depends on Coal Mine Roof Rating(CMRR) as well as depth.2.10. Step 10:Calculating pillar width In this step, putting the safety factor in Eqs.(12) and solving it, pillar width is obtained:(12)2.11. Step 11:Correcting pillar width to decrease the pillar failure riskAs it is pointed out before, one if the ways to decrease pillar failure risk ,especially large pillar collapse, is to choose a pillar width to height ration large than 4. In this step, if the ration of the obtained width from the previous step to pillar height is smaller than 4,pillar width is increased so a pillar width to height ration larger than 4 is reached. Of course, in order to control and avoid excessive increase of pillar width, the recovery rate is taken into consider. According to experiments and considering economic purposes in preliminary mining stage, the most suitable recovery rate varies from 40% to 60%. It should be notice 0.5m is added to the pillar width each time in this step.Table 2Used symbol in proposed coal pillars design methodSymbolDescription(unit)AMZActive mining zoneHDepth of cover (m)Renewal table 2pPanel width (m)hPillar height (m)BEntry width (m)Mean unit weight of the overburden (KN/m)Abutment angle()AMZ length (m)AMZ width (m)Development load (KN)Maximum front abutment load (KN)Side abutment load（KN）Side gob width (m)Barrier pillar width (m)Transfer rate (%)Maximum load applied on AMZ (KN)Pillar strength (MPa)Number of entriesNumber of pillars in AMZOverall load-bearing capacity of AMZ (KN)Safety factorPillar width (m)Width differenceOptimum pillar width (m)Fig.3. Abutment angle concept in sub-critical and super-critical panels (Mark,1992)Table 3Suggested safety factor for stability of the pillars in AMZDepth of cover (m)Weak and intermediate roof()Strong roof()2.12. Step 12: Determining the optimum pillar widthIn this step, the width obtained from previous step is corrected so that the optimum pillar width is determined based on the number od pillars in each row and the panel width. In order to at first should be calculated using Eqs.(13). If is less than or equal to the sum of pillar width and entry width, the optimum pillar width is obtained from Eqs.(14). Otherwise, the number of entries is added depending on value and calculating are repeated from step 8:(13)(14)In the following section optimum pillar dimensions in the main panel of the Tabas Central Coal Mine ,located in mid-eastern part of Iran, is determined in order to validate the proposed method and results are interpreted. This mine is the first mechanized one in Iran designed as a room and pillar mine. The pillars are left behind in this mine are supposed to be extracted as retreat mining in future after the preliminary mining finishes. Therefore a proper pillar design can has a remarkable influence on higher safety and efficiency of the reserve recovery in this mine.3. Tabas Central Coal MineTabas Central Coal Mine(TCM) is the case studies here, located in Tabas coal region approximately 85km south of Tabas town in Yazd province in mid-eastern part of Iran (Fig.4). The mine is working seam C1 by room and pillar method using continuous miner and LHD. The C1 seam gradient is 1 in 5(11) and seam thickness is about 2m. The immediate roof above the seam typically is weak(CMRR=37) and consist of 0.10.2m thick mudstone, siltstone/sandstone interfaces and sandstone channels in some areas within 3m which have potential to be water-bearing. The immediate floor is about 11.3m of weak seatearth/mudstone underline by stronger mudstones, siltstones/sandstones. The minable reserve accounts for 6 million tones of coking coal(Central Mine Design Report,2005). The in situ strength of coal, based on results from uniaxial compressive tests and Gsddy equations(Bieniawski,1987), is 6 MPa. As can be seen in Fig.4 the suggested layout for this mine includes two access drifts, a main panel, and eastern and western panels in both sides on the main panel. The main panel is initially developed in 2004 with five entries and pillars with 2020m distance between centers. This panel is developed with a continuous haulage system with 4.5m wide entries and crosscuts. So, pillars left behind this panel are square shaped and width is 15.5m. Because of weak floor, the pillar height is not equal to the seam thickness and is 2.6m. The current recovery rate is 40%. According to negotiations with the technical office and the caving behavior observed in the mine No.1(near the TCM), abutment angle in TCM is 25, so the main panel is super-critical (Ghasemi et al.,2010b).Because of extracting the pillars left behind in the eastern and western panels prior to beginning retreat mining in the main panel, gob is created in both sides of this panel. Therefore side abutment load should be considered in calculation of the maximum load applied on pillars in AMZ. The gob areas are super-critical and the barrier pillar width in both sides is 30m. According to the descriptions given in this section, the parameters required in pillar design in the main panel of the TCM are summarized in Table4.Fig.4. Location and suggested layout of the TCM4. ResultsThe results of proposed method for main panel of TCM are summarized in Table 5. Based on TCM conditions (i.e.,depth of cover=85m and CRMM=37) the minimum suitable safety factor for pillars stability is 1.5 (see Table 3). As can be seen in Table 5 the number of entries equal to six and optimum pillar width equal to 11.6m were obtained. In comparison with original mine layout, one unit was added to entries which can increase the production rate if efficient management is applied. Furthermore pillar width was decreased remarkably (about 4 m) which causes 8% increase in recovery rate that shows the proposed layout is smore economic. Also, it can be seen that the development load, front abutment load and side abutment load constitutes 73%,19% and 8% of the total load applied on pillars in AMZ respectively. The negligible side abutment load can be attributed to the great width of barrier and pillars in AMZ so as the barrier pillars width increase, the more load will applied to them.To confirm the results of proposed method, optimum pillar and other related parameters of the TCM were entered as input data to ARMPS program and acceptable stability factors were obtained, which indicates the mining operation, in both the preliminary and secondary mining stage, is safe.Table 4Essential data for pillar design in the TCMParameterValueParameterValueH85m25KN/m3h2.6m25oB4.5mP85mS16MPaCMRR37Table 5Summary for results for TCMParameterValueParameterValueLAMZ80.5mML10.87106KNWAMZ46.1mNE6DL7.89106KNwp11.6FL2.05106KNRr248%SL0.93106KN5. Discussion and conclusionsPillar design, especially in room and pillar mines, is one of the most important topics in the field of coal mine ground control. Various methods have been suggested in recent years most of which like classic method and Bieniawski method neglect abutment loads and determine pillar dimensions only based on development load (estimated by tributary area theory). Abutment loads increase the overall loads applied on the pillars adjacent to the gob area. When overall loads increase, pillar efficiency will decrease and eventually leads to pillar failure. So, properly sized pillars that are designed considering abutment loads can result in safety for miners and more efficient recovery of reserves. In this paper, by estimation overall loads (including development load and abutment loads) which may be applied on pillars during room and pillar mining, a simple method is provided to design coal pillars. One of the most important advantage of this method is decrease of pillar failure risk, especially massive pillar collapses. The result of this this method can be taken as the optimum pillar width which causes stability of pillars during preliminary and secondary mining.Although this method is similar to ARMPS program in structure, the main difference of proposed method with ARMPS program is that optimum pillar width in room and pillar mining is calculated using this method in order to decrease the pillar failure risk during retreat mining but in ARMPS program by inputting parameters such as pillar in preliminary and secondary stages are evaluated. It means that in proposed method the pillar width is unknown parameter (i.e. Pillar width is main output in proposed method) whereas in ARMPS program the pillar width is one of inputs so there is no need for calculation of pillar width.AcknowledgmentsThe authors would like to thank Dr.M.Sharifzadeh, Dr.H.Hashemolhosseini and Dr.F.Samimi for their kind helps and constructive suggestions during the preparation of manuscript.中文译文一种可以提高采用房柱式开采的煤矿在煤柱回采过程中安全性的新的煤柱设计方法摘要：大多数已有的煤柱设计方法仅仅通过分支领域的理论来估计煤柱的载荷从而确定煤柱的尺寸。在采用房柱式开采并且要回收煤柱的矿井中，由于随着煤柱的回收和采空区的形成将产生承载负荷，因此，居于这些方法设计的煤柱并不合适。在设计阶段对支撑载荷的忽视将在煤柱回收过程中造成煤柱失效，甚至破坏性的后果。因此，恰当的煤柱设计在安全性上将有显著地效果。在采用房柱式开采的矿井里，就采动影响区煤柱的留设和根据经验性公式估计支撑压力而言，本文将提出一种一步步设计煤柱的方法。这种方法最有意义的优点在于可以减少煤柱回收过程中煤柱的失效率。在位于伊朗中西部的Tabas Central煤矿，这种方法已经运用于确定最佳的煤柱尺寸。获得得结果表明，这个采区里，支撑压力是作用在采动影响区煤柱上总的载荷的27%。基于这种方法，煤柱也达到了11.6 m。关键词：煤柱设计；房柱式开采；煤柱回采；采动影响区；支撑压力1.1 前言在煤矿的地下开采领域，房柱式开采比较实用于那些开掘煤房或平巷形成煤柱的准备采区的水平或板状岩层的薄煤层。煤柱是留设下来支撑顶板并且防止其冒落，从来工人可以在煤柱之间安全的采煤和行走。有些时候，在后期开采中，要部分或全部回收煤柱，也就是大家熟悉的后退式开采（也称为二次开采或煤柱回收）。在世界上的主要产煤国家，煤柱的设计已经称为一个持续和深入的研究对象。煤柱的设计及其稳定性是涉及到岩石力学和地面控制学科的最为普遍和复杂的问题。尽管这些问题已经研究了很长时间，但到目前为止，也仅仅取得了有限的理解。在美国，对煤柱留设这个问题的研究可以追溯到差不多一个世纪之前，在这之前，煤柱的尺寸普遍通过反复试验获得的经验、直觉或者确定了的经验，这些研究往往是独立和零星的。但今天，通过实验室测试，一比一的煤柱模拟试验和过去失败和成功实例的反分析，已经发展起各种各样的煤柱留设原则。1980年，由美国国家煤炭部主导的一系列的现场研究已经确立了经典的煤柱留设方法。该方法主要有三步：1.估算煤柱载荷；2.估算煤柱的强度；3.计算煤柱的安全系数。按照常规的煤柱布设，煤柱载荷的平均值可以通过分支领域的理论估算出来。这种理论，假定每个单独的煤柱都要承载它正上方的地层表土。也就是说，每个煤柱一律都要承载它上方和两侧煤房或平巷一半宽度上方的岩石重量。煤柱强度可以定义为煤柱轴向压缩强度。实验证明煤柱的强度与它的体积大小和形状都有关。已经有可以用来估算煤矿煤柱强度的很多公式，表1列出了较为实用的几种。每个公式估算煤柱强度都根据两个变量：宽高比和原地煤的强度。Bieniawski提出了一种经典的方法来留设煤柱。首先，他描述了煤柱留设遇到的症结，和各种可用的方法的优点和缺陷，然后，提出了一种在采用房柱式开采的矿井中合乎逻辑的一步步来确定煤柱尺寸的方法。表1主要的煤柱计算的适用经验性强度公式煤柱强度公式(MPa)提出者煤柱断面备注Salamon and Munro (1967)方形Bieniawski(1968)方形Madden(1991)方形Mark and Chase(1997)方形现在，采用房柱式开采的大多数矿井为了提高回采率和生产率，也通过后退式开采回收采区里剩余的煤柱。然而，上面提到的这些方法由于忽略了因回采和产生的采空区带来的支撑压力而设计不出合理的煤柱留设方案。支撑载荷作用在煤柱线附近的煤柱之上和一个比利用分支学科理论估算的载荷大的压力也作用在煤柱上。一个有van der Merwe 主导的研究证实临近煤柱线的煤柱所受的载荷要大一些。他通过建立一个二维空间边界元素的模型准确计算出了煤柱回收过程中作用在煤柱上的载荷，并且估算除了这种情况下煤柱的安全系数。忽略了支撑压力设计的煤柱在回采过程中将导致煤柱失效。煤柱失效接着就是井下工人面临的巨大的一个危险灾害信号。煤柱失效导致的不如人意的状况包括：1.煤柱被压塌变薄；2.大规模的煤柱坍塌；3.煤柱受压移动。地下开采煤柱的失效将造成工人受伤、残疾甚至死亡的不良影响，同时，煤矿也将停工，中断采煤，设备被破坏等等。比如，在1992年West Virginia煤矿由于煤柱失效引起瓦斯爆炸，造成103工作面通风中断。在2007年8月6号，位于Utah的Crandall Canyon煤矿发生猛烈地煤爆，造成6名矿工死亡，又在10天后的全力冒险救援中，另一起煤爆又造成3名救援人员死亡和6名救援人员受伤。因此，防止煤柱失效和减少在回采过程中的事故的关键就是恰当的煤柱设计。在美国，LaModel和回采煤柱稳定性分析程序（ARMPS）成功的用于设计安全的回采方案。LaModel是用来计算煤矿或者其他薄岩层还有脉状岩层应力和位移的个人计算机程序。采矿工程师主要运用它来研究和优化有关上覆岩层的煤柱尺寸和布局、支撑载荷和多岩层压力。该程序是基于边界元素模型的移动和非连续变量开发出来的。Mark和Chase是在经验性方程的基础上研发ARMPS程序的。考虑了采动影响的ARMPS程序通过回收煤柱所受载荷的估计及其负荷能力计算出煤柱的稳定性系数。选择了250多个煤柱回收实例来验证这个程序。通过分析这些实例发现煤柱失效最频繁的形式是煤柱受压变薄，大约有2/3。观测到14例煤柱突然崩塌，并且这些都发生在ARMPS计算出的安全性系数小于1.5和宽高比小于3的情况下。17例中除了3例爆炸都是发生在井深超过400米的条件下。Mark和Chase认为当煤柱的宽高比大于4时，基本上不会发生大规模的煤柱崩塌。他们同时也观察到井深在200 m以内，防止大量煤柱崩塌的安全性系数最小为1.5.保证矿工安全和有效的回收剩下的煤的关键是设计足够尺寸的煤柱，可以防止煤柱受压变薄和大量的剥落，底鼓冒顶还有煤柱发生爆炸。综合以上提到的观点，下面将讲述一种设计方形煤柱的新方法。这种方法适用于房柱式开采煤田在初步开采结束后回收煤柱时最恰当的煤柱尺寸的确定。另外这种方法考虑到支撑压力和更低的煤柱失效率。这种方法的目的是帮助确定能承载后期开采所有的预期载荷的煤柱。2. 方法与ARMPS程序相似，这种方法考虑到了采动影响区的煤柱，因为留设的这些煤柱在采掘的始终都将承载最大的载荷，因此通过这种方法计算出来的煤柱更加安全。在阐述这种方法前，有必要先简述一下采动影响区。如图1所示，采动影响区包括掘进面之前的所有煤柱和沿煤柱线延伸5倍的埋深。之所以选择这个范围，是因为通过对支撑压力分布的观测显示90%的超前支撑压力作用在其边界上。这种方法基于一下5个原则：1、计算采动影响区内煤柱的最大压力；2、计算采动影响区内所有煤柱总的负荷能力；3、选择合适的安全性系数；4、计算煤柱宽度；5、修正并确定最合适的煤柱宽度。H-采深 FL-超前支撑压力 LAMZ-采动影响区长度 DL-支撑压力Wamz-采动影响区宽度 -支撑压力角图1 采动影响区示意图2.1、步骤1：收集必要的参数这种方法来确定煤柱最佳尺寸的参数有以下几项：1、埋深：煤柱上方的平均上覆岩层厚度；2、煤柱高度：需要说明的是煤柱的煤柱的高度不一定等于采高；3、平巷宽度：平巷宽度通常取决于顶板性质、生产能力和设备宽度，在这里，假定联络巷的宽度与平巷相同；4、在原地煤的强度；5、覆岩的平均密度；6、支撑压力角：支撑压力角决定采空区承载多少，通过对长壁式支撑压力的观测表明，在通常的开采条件下，支撑压力角为21是比较合适；7、采区宽度：采区宽度通常取决于土工技术条件，地压，经济性还有环境条件。采区宽度将影响压力分布和采掘设备。增大采区宽度将使作用在采空区附近煤柱的载荷增加，随着采区宽度的增加，将使采空区上方裂隙带的高低增加，甚至可能导致上覆岩层的大面积跨落。根据采区宽度和埋深之比（P/H），可以将采区分为两种类型：亚临界区超临界区8、矿井顶板等级（CMRR）：这个指标用来评估顶板性质。在1994年，顶板等级的划分将地质特征与工程设计联系在一起。这种分级系统考虑了诸如顶板强度，层理和其他不连续断面，湿度，地下水等地质因素。顶板等级指标在0到100之间，根据这个指标煤矿顶板可以分为3种：软（CMRR45)中等（45CMRR65）2.2、步骤2：计算采动影响区范围采动影响区长度和宽度分为由（1）和（2）确定：（1）（2）2.3、步骤3：计算附加载荷附加载荷是由采动影响区上方的覆岩重量产生的。根据分支领域的理论，附加载荷可以通过以下的方程计算：（3）2.4、步骤4：计算超前支撑压力的最大值煤柱的回收就标志着回采的开始。当开采出足够的煤柱，煤柱上方的岩层就开始下沉。上覆岩层的下沉就形成了采空区，采空区上方一部分的覆岩重量由采空区承载，但大部分覆岩重量作为支撑载荷转移到采动影响区内的煤柱上和安全煤柱上（见图1）。超前支撑压力的计算是支撑压力角，它的分布在亚临界采区和超临界采区是不同的。不管是亚临界采区或超临界采区，超前支撑压力的最大值可以分别由（4）式和（5）式给出：（4）（5）2.5、步骤5：计算侧向支撑压力两侧的支撑压力来源于开采水平两侧的采空区。每个开采水平两侧可能存在采空区。安全煤柱和采动影响区共同支撑侧压
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