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英文原文Geological and geotechnical aspects of undergroundcoal mining methods within AustraliaB. Scott P. G. Ranjtih S. K. Choi Manoj KhandelwalAbstract : About one quarter of the coal produced in Australia is by underground mining methods. The most commonly used underground coal mining methods in Australia are longwall, and room and pillar. This paper provides a detailed review of the two methods, including their advantages and disadvantages, the major geotechnical and operational issues, and the factors that need to be considered regarding their choice, including the varying geological and geotechnical conditions suited to a particular method. Factors and issues such as capital cost, productivity, recovery, versatility and mine safety associated with the two methods are discussed and compared. The major advantages of the longwall mining method include its suitability for mining at greater depth, higher recovery, and higher production rate compared to room and pillar. The main disadvantages of the room and pillar method are the higher risks of roof and pillar collapse, higher capital costs incurred as well as lower recovery rate.Keywords : Longwall Room and pillar Geological GeotechnicalIntroduction :Mining in Australia is a significant primary industry and contributor to the economy of Australia and encouraged immigration to Australia. Many different ores and minerals are mined throughout the country. With the increase in coal demand and growing awareness towards sustainable development, the coal industry has drawn a consensus over the need for increased production from underground coalmines. Around the world, the majority of coal reserves are recoverable using underground mining techniques. At the moment, almost two-thirds of coal production comes from underground mines, however, in Australia this statistic is significantly lower (ACA 2006). Currently in Australia, the majority of underground coal mines are located in New South Wales and Central and Western Queensland, where thinner black coal seams suit the underground mining methods.There are a number of different types of access modes for underground mining. These include drift, incline/ decline and shaft, and can be used in conjunction with either of the three modes for underground mining within Australia. Drift is generally used when the coal deposit is inside of a hill, and mining is undertaken by entering directly into the hill (Ghose 1984). Incline/decline is created at the ground level of a valley, where an adit is constructed and slopes down to the coal. Shaft is used with an elevator, which stretches from the surface to the coal seam underground (Wilson 1983).The main aim of the paper is to identify and compare various techniques used for coal extractions and the selection process of those techniques for a particular site based on geological, geotechnical and other factors. The two major methods of underground mining within Australia and around the world are room and pillar and longwall mining, and these two methods will be discussed in details below.Current states of Australian underground and open-cut coal mining operationsIn order to gain an understanding of the current state of coal mining operations in Australia, a broad overview is given before method suitability for coal mines is discussed.In Australia, open cut mining produces the most amount of coal for both export and internal uses. In 2004 for example, 81.5 million tonnes of coal was mined using underground methods, whilst 296.3 million tonnes were obtained using methods within an open cut system (University of Wollongong 2006). This is of no surprise as nearly two-thirds of all operating mines within Australia are open cut, as can be seen from Table 1 and Fig. 1 below.Table1 Type of mines operating within Australia (GNSW 2006;GSA 2006; GWA 2006; GT 2006; GV 2006; GQ 2006)Mine types by stateStateUndergroundSurfaceTotal coal minesQueensland103040New South Wales272552Western Australia066Tasmania123Victoria178South Australia011Total3971110Fig.1 Map of Australian coal basins (DPMC 2006)Within Australia, brown coal is typically found in the southern part, with black coal found in the basins of New South Wales and Queensland. Before proceeding further, a quick overview of coal rank and classification is given. Typically, coal rank is classified into three distinct categories depending on the degree of metamorphism that the coal forming material has endured as it matures from peat to anthracite. These are lignite, sub-bituminous and bituminous, and the properties of these greatly influence the type of method used to exploit the coal.As mentioned, surface mining or opencast mining is the predominant method used in Australia. Opencast mining on a large scale first commenced in Australia in the 1960s, where imported draglines were the main means of stripping overburden. This method continued to be used over the next 2030 years, and still today, however, as the seams became deeper and the complexity of the coal seam increased, other equipments such as truck and shovel, and dozers, were introduced (Westcott 2004).Today, draglines and truck and shovel operations, or a combination of the two, are the predominant modes of equipment used in opencast mines, as seen in Fig. 2 below.Fig.2 Opencast Coal Mining Equipment used in Australia (Westcott 2004)Selection of excavation methodThe decision on whether to operate an above or underground mine is heavily influenced by a couple of important factors. The major factor in deciding on whether to go underground or open cut is the stripping ratio (Whittles et al. 2007). This is defined as the ratio of the volume of overburden (BCM) moved to the amount of coal produced (tonnes). As a general rule, anything past 20:1 is considered too large a ratio for above ground coal mining as large amounts of overburden are required to be moved in order to expose the coal seam, thus underground methods should be considered. Another factor in deciding on the technique to be employed is the type of coal to be mined. If the coal deposit consists of lignite, which is Tertiary in age and ranges from about 15 to 50 million years old, then above ground methods should be more closely considered. This is due to the fact that lignite is a much softer material than black coal, which increases the possibility of roof collapse or material collapsing from above during mining due to the younger, unconsolidated and softer material overlaying the brown coal. Careful consideration, however, would also need to be given, when mining brown coal above ground, as a strong base for the large, heavy equipment would be required to avoid bench collapse or other failure. Large, heavy coal haulage trucks may also find it difficult to operate on the softer lignite, especially during wet weather events, where the weight of the equipment results in large amounts of time lost due to trucks becoming bogged, wheel spin, etc.Other factors to be considered, which will be expanded upon shortly include: Life of mine. For example, is it feasible to outlay large capital for a small coal deposit? Required productivity. Do you need a high production machine or is it more feasible to mine constantly at a slower rate? Amount of capital available.If there is any plant currently available within the company, and can it be utilised?In the following sections on longwall and room and pillar coal mining, the typical geological and mining conditions for the techniques to be utilised will be discussed, including variations of the techniques, as well as expected productivity and costs experienced within the industry. The following is applicable to each proposed method (opencast or underground): No two mines are exactly the same. Geological profiles, weather, capital available, production, productivity requirements, recoverable reserves, etc. are all independent variables between different mines, and as such, it is impossible to prepare a standard document, which caters for every possible mine (Bise 1995). Large amounts of time are required by experienced engineers or specialist consultants during the preliminary and planning stages in order to design a mining method suited to a particular location. Geotechnical issues are independent at each location, and a detailed investigation at the very least should be undertaken in order to understand the groundconditions, geological profile, etc. (Wilson 1983). The geotechnical issues outlined within this paper include different modes of failure, soil properties, etc. however, it is not within the scope of this paper to detail every possible geotechnical issue with regards to coal mines. Major problems will be identified along with the conditions that cause such circumstances to occur. Costing of mine equipment and productivity is of a broad nature, and can be calculated using the following formula proposed by Noaks and Landz 1993. The cost in 1992 $A has been adjusted to 2006 value using the recommended formula:Cost Now = (Cost Then)(Cost Index Now)/(Cost Index Then)where, the cost price indexes (CPIs) for both 1992 (CPI, March 1992: 107.1, open cut; 108.1, underground) and 2006 (CPI, June 2006: 167.0, open cut; 152.0, underground) have been obtained from the Australian Bureau of Statistics (ABS 2006). All values and productivity are generated as a preliminary estimate for a pre-feasibility study level of accuracy (25%), and does not replace an engineered cost estimate or feasibility study. The accuracy of any estimate will be directly proportional to the quality and quantity of data available and to the time and effort put into its preparation and proper execution (Noaks and Landz 1993).The following sections outline the two underground coal mining alternatives (longwall, and room and pillar mining) specifying which geological conditions are better suited to each method, as well as geotechnical issues involved.Longwall miningLongwall mining is the most common of the underground coal mining methods used in Australia. It suits sites where coal seams are thicker, wide and have a consistent coal profile with gentle dip. In longwall mining, large rectangular sections of coal are identified and removed in one continuous operation (Trueman et al. 2009). Basically, a panel, or block, of coal is created by driving a set of headings into the section of coal (panel) for a certain distance (typically 1.53 km long). These sets of headings are generally spaced at a distance of approximately 100250 m (330820 ft) apart, and are joined together to allow the longwall mining machine to work along the longwall face. The mechanised shearer runs along the face, cutting and removing the coal as the mine advances along the length of the headings.As the coal is being cut and dropped onto a chain conveyor, temporary hydraulic-powered roof supports automatically follow the direction of the shearer to hold up the roof while the coal is being extracted. These supports provide a safe working environment, and as the mine advances, so do the support jacks, and the roof area behind the face is allowed to safely collapse, forming an area known as the goaf. In the main roadways within the mine, for use by mine personnel, transportation of maintenance equipment, etc. roof bolts are placed in the ceiling to avoid collapse. Once the shearer has completed extracting the coal from the panel, it is moved to a new location, and repeats the process.This method of mining is more efficient than the room and pillar method, with recovery rates averaging approximately 75%. However, the equipment is more expensive and cannot be used in all ground conditions. These issues, among others, will be discussed further below.As mentioned at the beginning of this section, longwall mining is the predominant underground method of extracting coal within Australia, with approximately 70% of underground coal mines utilising the technique, all of which are in either New South Wales or Queensland. The method accounts for 89% of Australias total underground coal production (University of Wollongong 2006). It is a relatively recent introduction in Australia, with the first longwall mine being developed in 1963, however, there are currently 27 in operation including the Beltana, Metropolitan and Newstan coal mines in New South Wales, and the Crinium, Kestrel, Oaky North and Newlands Southern mines within Queensland.Kelly (1999) identified the fact that shear failure, rather than tensile, is the major failure mechanism in a number of Australian longwall mines monitored by the CSIRO since 1994. The failure has occurred further ahead of the retreating face than traditional geo-mechanics theory predicts and is considerably affected by the geological conditions of the site. Other factors influencing failure include goafing mechanics of previous block and pore water pressure.Hebblewhite (2003) introduced the concept of core geotechnical risks associated with longwall mining, that is: any risk associated with a major hazard or potential hazard that is an inherent feature of a generic mining method. Almost by definition, core risks cannot be totally eliminated, and must therefore be controlled and managed during the life of the mining method or system of work. The paper identified some major core geotechnical risks associated with longwall mining, and can be seen in Table 2.Table2 Core geotechnical risks associated with longwall miningHazardConsequenceSurface subsidenceDisturbance/damage to surface features (natural and man-made), and to sub-surface, such as aquifers.Face instability/periodic weightingLoss of face/roof control; production disruption; equipment damage; operator safety threatenedCaving hang upWindblasts (range of consequent safety implications); excessive pillar and face loading; unpredictable subsidenceStructural geology disruption to panel blocksProduction disruption and potential sterilisation of reserves leading to major economic impact; adverse face ground conditionsAbutment stresses on developmentAdverse conditions/potential failure in gate roads and chain pillarsRisks identified within Table 2 such as surface subsidence are inevitable and will occur on almost all longwall mine sites, and must be managed effectively, whilst others such as caving hang up causing air/wind blasts are avoidable if appropriate planning is undertaken and precautions followed.Certainly, the geotechnical suitability, issues and risks mentioned herein are not in any way the sole elements to consider when planning, developing or operating longwall mines. It is simply an identification and description of major factors involved within the longwall mining process. Other issues and business considerations following, relevant to specific sites, should also be taken into account when considering coal mining methods.Other issues and considerationsThe previous section describes geotechnical considerations relevant to longwall mining. The following section will consider other issues which may arise during longwall operations including safety, production, productivity, equipment size, make, etc. and options for mine layout. As each site is independent of another, it is difficult to recommend certain templates for selecting the mining method, however, it is the aim of this section to discuss what options are available and under what conditions they are best suited for, as well as common issues arising during the application of a particular method.The importance of productivity in all mining cannot be overlooked, and the main increases over the years have generally come from advancements in technology. One of the key factors over the past 20 years in making longwall productivity gains has been the evolution of larger longwall panels, made possible by technology advancements, upwards of 3,350 m in length by 320 m wide as opposed to typical lengths and widths in the mid-1980s of 1,525 and 180 m, respectively (Kvitkovich and Weisdack 2005).It is expected that longwall technology will continue to improve for a number of years, thus providing greater options for longwall unit selection, which ultimately has a great influence on productivity (Peng and Chiang 1984). In1992 for instance, longwall equipment was very diverse in size and capacity, as it still is today, with shearer power ranging from 150 to 1,080 kW, and face conveyor capacity running between 940 and 2,600 tonnes/h. These statistics demonstrate the various options available when purchasing longwall equipment, which will ultimately have a large influence on the costs and expected productivity of a project.Tasman Asia Pacific (Anon 1998) conducted an analysis on longwall mining in 1998 by comparing best practice performing longwall mines in the USA with a number of longwall mines operating within Australia. This analysis concluded that the average productivity of the Australian mines was approximately 25% less than that of the USA mines. The analysis was limited to the core mining operations of longwalls (shearing, roof support, transportation of the coal, labour and maintenance) and therefore, because the operating characteristics of the mines were fairly similar, estimated productivity gaps likely indicated differences in management and work practices. The results from this report indicate areas which typically affect productivity in longwall mines.The report identified the major differences between the Australian and USA longwall mines as: higher non-production times during shift changeover within Australian longwall mines, lower utilisation of shearers within Australian longwall mines, and geological differences.These results indicate key areas which, if focused upon and managed appropriately, can influence productivity levels within longwall mines.As can be seen from Fig. 3, Australian mines had much more non-production, or joining and leaving, time than the USA mines. It was noted that the distance travelled by employees from surface access point to mine face was very similar between the average participating United States and Australian mines. So, large difference in joining and leaving time cannot be explained by travel time (Anon 1998). It was suggested that the main cause of this difference was either different work practice or employee transportation system. The difference in these non-production times resulted in 40 minutes extra productivity per shift for the USA mines, proving that what may seem a small or almost insignificant factor can have a large bearing on the overall successfulness of a longwall mine.Fig.3 Non-production time in shifts at longwall mines (% of total shift time) (Anon 1998)Another major difference (see Fig. 4) stated from the report was a lower utilisation of shearers in Australian longwall mines.Fig.4 Utilisation and availability of shearers (Anon 1998)This utilisation difference was mainly attributed to mine planning, and again demonstrates a major issue to consider in order to maximise the production of a longwall mine. These major factors mentioned, obviously along with some other minor factors, identify issues to consider when developing or operating longwall mines. This of course does not include individual geological conditions or equipment selection. Figure 5 shows what effect these issues can have on cost per tonne if they are not managed properly.Fig.5 Total factor productivity and cost per tonne for longwall mines(index, USA coal = 100, cost $A) (Anon 1998)When comparing longwall mining to the other major underground method, room and pillar, it is interesting to note that, at least in the USA, higher labour productivity levels are obtained utilising the longwall method (Darmstadter 1997). This is due to a number of factors. First, longwall mining is much less labour intensive than room and pillar mining, which is largely due to the fact that the longwall operation is of a highly mechanised nature, where there is a significant degree of computerisation and continuity in the extraction process. This minimal labour intensive factor also has a second added bonus, that is, superior safety performance is generally achieved due to less personnel at the cutting face. Secondly, longwall mining utilises a continuously hauling conveyor system, whereas the room and pillar method employs a somewhat more labour intensive shuttle car system in conjunction with continuous miners. This characteristic is an important aspect of underground mining techniques to consider, particularly when one looks at the outcome of a 1995 study in the USA comparing the ratio of longwall to room and pillar labour productivity levels throughout various states and regions, as shown in Table 3 below.Table3 Ratio of longwall to room and pillar labour productivity levels, selected states and regions 19831993 (Darmstadter 1997)19831993Alabama1.061.33East Kentucky0.851.35Pennsylvania0.911.62Virginia0.731.01West Virginia1.140.97Illinois1.251.12West Kentucky-0.75Colorado0.841.54Appalachia0.981.10Illinois Basin1.191.00West1.111.51US0.981.19Source: EIA 1995b, pp. 3940 (West Kentucky had no longwall production in 1983) As shown from this, an overall ratio of almost 1.2 in favour of longwall mining suggests this method will generally obtain higher labour productivity levels as opposed to room and pillar.Room and pillar miningConventional room and pillar mining was the traditional technique used during underground mining up until approximately 40 years ago, when continuous mining was introduced into Australia. This new form of room and pillar mining eliminated the need for drilling and blasting, and was a much more efficient process, and as such, practically replaced the conventional room and pillar method.Although continuous room and pillar mining is the most common type of underground mining in the world, it plays second fiddle to the more economic longwall mining in Australia, with approximately twice as many underground mines utilizing this method as opposed to room and pillar (ABS 2006).In continuous room and pillar mining, coal seams are mined by a continuous miner, which is an electric machine that breaks the coal mechanically. Its main component is a long rotating drum which has sharp picks attached around the outside, and allows the machine to cut and load the coal at the same time. The coal is then loaded onto shuttle cars and/or conveyor belts where it is transported to the surface.The continuous miner works in such a way that it cuts a series of rooms into the coal seam, and leaves pillars made up of coal (roof bolts are also added later) to support the roof of the mine as the machine advances, hence the name room and pillar. As the continuous miner moves deeper into the coal seam, each room requires a pillar of greater width to maintain the support of the mine. Careful planning is necessary during this stage of mining to ensure pillars of sufficient strength are designed to support the mine. In older mines, room dimensions generally ranged from 2.4 to 9.1 m (830 ft) wide, whilst a diverse range of pillar sizes are employed (DEP 2006), however, in more modern times the dimensions of the rooms and pillars are highly dependent on the type and size of the continuous miner and the haulage system in operation as well as the strength of immediate roof.Using this technique, as little as 35% of coal is recovered due to the use of coal as pillars to support the mine roof. However, if a process known as retreat mining is implemented, recovery of up to 70% of the target coal seam can be obtained (DEP 2006), quite a substantial increase in recovery rate. This process known as retreat mining involves the systematic removal of the coal pillars by pulling them towards the end of the mining section as the continuous miner advances through the coal seam.ConclusionsThis paper provides an overview of the various underground mining methods used in Australia. The major issues, the advantages and disadvantages of the various mining methods, and other mining and economic considerations are discussed.Room and pillar method is more versatile than longwall mining. It can cope with irregularities in mine geology and is less capital intensive. It is, however, only suitable for mining at shallow depth (up to 300 m) due to increased likelihood of roof and pillar collapse as well as lower recovery of the coal reserve. For mining at greater depth (300 m), longwall method has higher recovery, higher productivity and with enhanced safety and mine stability. However, there are other geological and geotechnical issues that also need to be considered.Roof, pillar and floor stability is the most important requirement in underground coal mines because it represents the leading cause of fatalities. During mine planning, geotechnical aspects may be compromised owing to unavailability of some field data. It is, therefore, very important to include a geotechnical investigation as part of the coal exploration programme. Data which should be collected basically include geological structures, rock strength and deformation properties, and the groundwater aspects. Very often the only source of such data is from exploration boreholes. Therefore, effort should be made to extract the maximum amount of information from the core and the borehole. Data collection methods can include the preparation of detailed geotechnical logs for the roof, coal and floor. Laboratory strength testing will provide estimates of the mechanical properties of the various lithologic types. Slaking and swelling tests provide an insight into the stability characteristics of the rock materials immediately enclosing the coal seam. The boreholes may be used to provide permeability and pressure data. The availability of a suitable geotechnical data base allows various stability prediction methods to be used. These include techniques for geotechnical appraisal of the roof and floor units, analysis of depositional environment, hazard analysis, and classification/rating systems for roof and floor. The basic geotechnical data and the stability prediction methods form a basis upon which preliminary rock mechanics designs may be developed. Of particular importance to future coal mining are those designs relating to longwall mining because most mining companies are thinking in terms of the greater safety and greater productivity offered by longwalls. Items which should be addressed when considering a longwall system include the panel layout in terms of a total mining plan, strata mechanics, roof/floor bearing capacities, face support requirements, face length, entry and pillar.中文译文澳大利亚地下开采方法的地质和岩土工程方面概述B.斯科特 P.G. Ranjtih S. K. Choi Manoj Khandelwal摘要：在澳大利亚，大约四分之一的煤炭产量都是采用井工开采的。在澳大利亚最广泛采用的井工开采方法是长壁采煤法、房柱式采煤法。本文将详细阐述这两种采煤方法，包括其各自的优缺点，主要的岩土工程和业务方面的问题，以及做出选择时必须考虑的因素。这包括鉴于各种地质条件所适宜采用的特定的采煤方法。对这些因素，诸如资金成本，生产力，恢复，通用性和两种方法相关的煤矿安全的问题进行了讨论和比较。长壁采煤法的主要优点包括其适用于更深的埋藏深度，更高的回收率，更高的生产效率。房柱式采煤法的主要缺点是顶板跨落和支柱倾倒的风险较高，较高的资本成本以及较低的回收率。关键词：长壁采煤法 房柱式采煤法 地质工程 岩土工程简介：在澳大利亚，采矿业是一个重要的支柱产业，在澳大利亚的经济中占有非常大的比重，并鼓励移民到澳大利亚。全国各地开采了许多不同的矿石和矿物。随着煤炭的需求和实现可持续发展的意识日益增加，煤炭行业已达成了需要扩大地下开采规模的共识。在世界各地，大部分煤炭储量是可以采用地下采矿技术的。目前，煤炭生产几乎有三分之二来自地下开采矿井，但是，在澳大利亚，这个统计数据显然更低（ACA 2006年）。目前，在澳大利亚，大多数矿井位于新南威尔士州和昆士兰州中、西部，那里较薄的煤层适合地下开采方法。地下开采有几种不同类型的开拓方式，其中包括平硐，斜井和立井。在澳大利亚境内，地下开采的三种模式都会被使用。平硐一般用于煤炭埋藏在一座小山时，采矿直接进入山体（戈塞1984年）。斜井，则掘进数条斜巷，一直延伸到煤层。立井，则使用罐笼，从地表延伸到煤层（威尔逊1983年）。本文的主要目的是确定和比较各种采矿技术，并根据地质，岩土工程和其他因素选择过程中使用的各种技术。在澳大利亚和世界范围内，两大地下开采方法分房柱式采煤法和长壁采煤法，下面将详细讨论这两种方法。目前澳大利亚矿井和露天矿的状况为了使读者理解澳大利亚煤炭开采业务的目前状况，在讨论合适的采煤方法之前，提供了广泛的概述。在澳大利亚，露天矿提供了主要的煤炭出口和供国内使用。例如，在2004年， 8150万吨的煤炭利用地下开采，而露天矿提供了29630万吨（伍伦贡大学2006年）。这一点并不令人吃惊，在澳大利亚近三分之二的生产煤矿都采用露天开采，如表1、图1所示。表1 澳大利亚生产煤矿类型（GNSW、GSA、GWA、GT、GV、GQ 2006年）煤矿类型州矿井露天矿合计昆士兰州103040新南威尔士州272552西澳大利亚州066塔斯马尼亚州123维多利亚州178南澳大利亚州011合计3971110图1 澳大利亚煤盆地的地图（DPMC 2006年）在澳大利亚，南部通常发现褐煤，在新南威尔士州和昆士兰州盆地发现烟煤。在进一步阐述之前，先给出煤炭排名和分类的快速概览。通常情况下，煤炭排名分为三个不同的类别，根据煤炭形成来源，由于它已经历从泥炭到无烟煤不同的变质程度。分别是褐煤，次烟煤，烟煤。这些分类的属性极大地影响了这些煤炭的使用方法。如上所述，露天开采是澳大利亚使用的主要方法。澳大利亚大规模采用露天矿开采起始于20世纪60年代。那时，进口挖掘机是剥离覆盖层的主要手段。在随后的20-30年里一直使用这种方法，至今。但是，随着埋深变得更深，煤层的复杂性增加，其他设备也开始引进，如卡车和铲子，推土机等（韦斯科特2004年）。如今，挖掘机、卡车和铲，或者是两者的结合，是露天矿的主要设备，如图2所示。图2 澳大利亚使用的露天矿采矿设备（韦斯科特2004年）开挖方法的选择采用露天矿还是地下矿井在很大程度上取决于几个重要因素。决定采用地下开采还是露天开采的主要因素是剥采比（怀特2007）。剥采比，是指每采出单位煤量（吨）所需剥离的覆岩量（BCM）。作为一般原则，只要剥采比大于20:1，露天矿开采的剥离量过大，因此应考虑井工开采。决定采用何种开采方法的另一个因素是可供开采的煤炭类型。如果煤炭矿藏包括褐煤，这是第三纪，约150至500亿年前形成，那么更多的将考虑露天开采。这是基于褐煤比烟煤更软这一事实，这增加了顶板垮落的可能性，由于形成年代较晚，褐煤上覆岩层较不稳定，岩性较软弱。但是，也需要仔细考虑，露天开采时，褐煤上作为一个大型、重型设备的坚实的基础，必须避免工作台倒塌或其他故障。在软媒上行驶，可能会发现大型、重型运煤卡车很难操作，尤其在天气潮湿时，大量的时间花费在处理卡车陷入煤泥，车轮打滑等。其他需要考虑的因素，这将在短期内扩大，包括：服务年限。例如，为价值不高的小煤矿投入大量资本可行吗？生产能力。你需要一套高产高效设备，还是低效率的开采，可行吗？可投入的资金。目前公司是否有闲置资金，它可以被利用？在下面的章节，将就基于典型的地质及采矿条件，是采用长壁采煤法还是房柱式采煤法展开讨论。涉及各层面的技术，以及行业内的经验预期的生产能力和成本。以下是适用于每个建议的方法（露天开采或地下开采）：没有两个矿是完全相同的。不同煤矿之间的地质剖面、天气、可用资本、生产能力、计生产能力、可采储量等所有独立变量的差别，因此，它是不可能准备一份标准文件来满足每一个可能的煤矿（碧丝1995年）。在初步规划阶段需要大量的时间，由经验丰富的工程师或专家顾问来设计采矿方法适用于一个特定的位置。岩土工程问题，在每个位置都是独立的，并应详细地调查，至少应采取措施以查明地貌状况，地质剖面等（威尔逊1983）。本文内所述的岩土工程问题，包括不同的故障模式，土壤性质等，然而，不是每一个关于煤矿的岩土工程问题的可能的细节都在本文的讨论范围内。重要问题确定的条件，将造成这种情况的发生。矿山设备和生产力成本是一个广泛的性质，而且可以使用1993年Noaks和Landz提出下列公式计算。资金$A在1992年的价值已经调整到2006年的价值，使用推荐的公式：现值 =（终值）（现值指数）/（终值指数）其中，1992年的成本价格指数（CPI指数）（1992年3月CPI，露天开采，107.1；地下开采，108.1）和2006年的成本价格指数（2006年6月CPI，露天开采，167.0；地下开采，152.0）来自澳大利亚统计局（ABS 2006年） 。所有的价值和生产能力产生于一个准确的预可行性研究的水平（25）的初步估计，并不会取代工程成本估算或可行性研究。任何估计的准确性将是提供数据的质量和数量的时间和精力投入及其制备方法和正确执行成正比（Noaks和Landz1993年）。以下各节概述了两个地下煤炭开采的替代品（长壁，房柱式开采）指定的地质条件更适合于每一种方法，以及所涉及的岩土工程问题。长壁开采长壁开采是在澳大利亚最常见的地下开采方法。适合厚煤层，缓倾斜煤层。在长壁开采中，煤炭以矩形形式用一个连续的操作进行开采（特鲁曼等2009）。基本上，划分采区，盘区，或带区，有一定的距离（一般为1.5-3公里长），开切眼。这些工作面一般间隔距离约100-250米（330-820英尺）外，并结合在一起，使综采工作面采煤机沿长壁工作。机械化采煤机沿工作面，破煤、装煤，沿巷道进尺。由于煤炭被破碎、装载到链式输送机上，临时液压支架自动跟随采煤机的方向向前移架，而煤炭也支撑顶板。这些支撑提供一个安全的工作环境，并作为煤矿的进展，允许安全地垮落，形成被称为采空区的面积和顶板面积背后的工作面。在矿井内的主要巷道，为井下人员，维修设备的运输使用等顶板支架放置在顶板下，以避免倒塌。一旦采煤机已完全从采区中采出煤炭，它被转移到一个新的位置，并重复这个过程。这种采矿方法的效率超过房柱法，回采率平均约75。然而，设备较昂贵，并不能在所有地质条件下使用，等等，这些问题将在下面进一步讨论。正如在本节开头提到的，澳大利亚矿井主要采用的就是长壁采煤法，约占70，无论是新南威尔士州或是昆士兰州。该方法的煤炭产量占澳大利亚总地下煤炭产量的89（伍伦贡大学2006年）。这是一个较为近期引进到澳大利亚的，在1963年掘进的第一个长壁工作面，然而，目前在Beltana有27个长壁工作面矿井在建。凯利（1999年）确认了事实，自1994年以来在CSIRO的监测中，发现是剪切破坏，而不是拉伸，是澳大利亚长壁工作面的主要破坏机制。破坏发生超前于回采工作面，比传统的地质力学理论预测的更超前，并认为受到地质条件大大影响。影响破坏的其他因素，包括早期的破裂面和孔隙水压力。赫本怀特（2003年）介绍了长壁采煤法的核心的岩土风险的概念：“一个重大危险源或潜在的危险，是一个通用的开采方法的固有特征有关的任何风险。几乎从定义，核心的风险不能被完全消除，在采矿方法或系统工作的生活，因此必须控制和管理”。该文确定了一些主要的核心与长壁开采相关的岩土工程风险，并如表2所示。表2与长壁开采相关的核心岩土工程风险危险后果地面沉降干扰/表面特征（自然和人为的），和亚表面损伤，如含水层。工作面不稳定/周期来压片帮/顶板控制；生产中断；设备损坏；操作人员的安全威胁顶板垮落风暴，随之而来的安全问题的范围；过多的支柱和工作面装载；不可预知的沉降顶板区域的地质构造的影响生产中断，并可能导致重大的经济影响；造成对地面的不良影响采动压力不利条件/煤