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By Axel Preusse

Underground coal mining can cause seismic events. There is currently no way to predict such phenomena reliably, in particular stronger events. That is why, according to the current state of the art, empirical methods are employed in this field. Some assessment criteria, which relate to the geological and mining situation and either stimulate or rather help to avoid seismic events, are presented in this paper. These criteria are examined for a practical mining situation in Germany, with a view to estimate possible seismic events.

Jan 1, 2009

By John Magyar

Special foamed cements were used to stabilize and secure caving ground in a newly-reamed 12.5-ft diameter underground ventilation raise. Two zones of very poor Rock Mass Rating (RMR) material were encountered during reaming of a ventilation raise at Placer Dome's Turquoise Ridge Mime in Northern Nevada These zones caved to large cavities, continued failing, and could not be safely secured using standard rock bolting, wire mesh, and shotcrete procedures. In stable sections of the raise, the ground control plan called for the contractor to install 8-ft and 124 length resin grouted No. 7 Dywidag bolts, chain-link fence mesh and shotcrete. This work was conducted on three decks installed on the raise bore reamer. The first of the caved areas was approximately 1,500 cubic yards in volume and extended out to 40 ft beyond the wall of the raise. Placer Dome considered several options for structural fill to repair the area including pumped shotcrete, concrete, and placing and grouting aggregate. A safe, cost effective, and expedient method of stabilizing the raise with foaming grout and bolted liner plate was selected and successfully executed. The special fast-setting, foaming cement grouts used were Tekfoam and Tekseal® manufactured by Minova. The raise was quickly placed in service. Factors considered in selecting the foaming grout are discussed.

Jan 1, 2005

By Frank E. Chase

During the past quarter century, coal mine geologists have evolved from providing a qualitative description of drill core and coal reserves into quantitative geotechnical specialists who bridge the gap between geology and engineering. In the late 1970?s and early 1980?s, hazard prediction mapping was in its infancy. At best, hazard maps identified rudimentary structures and conditions such as stream valley locations, sandstone/shale transition zones, and lineaments. A ?technical disconnect? had existed between geologists and mining engineers as each profession operated largely independent of each other. Engineers did not readily integrate geology and geologic conditions, with the exception of coal thickness, into mine planning. Geologists were relied upon for reserve estimates and did not participate in mine planning or design. The employment and market slump of the 1980?s and 1990?s combined with mining more challenging reserves mandated that this gap be bridged and forced geologists to learn to speak more ?numerically? to engineers so they could be understood. The purpose of this paper is to describe the procedures and thought processes used by coal mine geologists when determining various mineability and reserve extraction feasibility issues. The paper will also address underground geologic hazard mapping and prediction techniques. To achieve these objectives, the opinions and experiences of leading coal mine geologists, geotechnical and strata control specialists across the United States were solicited.

Jan 1, 2006

By Chaojun Fan, Du Wenzhang, Sheng Li, Hong-Hong Shi

"Coal and gas outburst is an extremely complex dynamic disaster in coal mine production process which will damage casualties, equipment facilities and disorder the ventilation system by suddenly ejecting a great amount of coal and gas into roadway or working face. This paper analyzed the interaction among the three essential elements of coal and gas outburst dynamic system. A stress-seepage-damage coupling model was established which can be used to simulate the evolution of the dynamical system, and then the size scale of coal and gas outburst dynamical system was investigated. Results show that the dynamical system is consisted of three essential elements, coal-gas medium (material basis), geology dynamic environment (internal motivation) and mining disturbance (external motivation). On the case of C13 coal seam in Panyi mine, the dynamical system exists in the range of 8-12 m in front of advancing face. The size scale will be larger where there are large geologic structures. This research plays an important guiding role for developing measures of coal and gas outburst prediction and prevention. INTRODUCTIONCoal and gas outbursts are a complicated mine gas dynamic phenomenon that pose a serious threat to coal mine safety production. Since the first recorded coal and gas outburst occurring in Isaac coal mine of the Lule coal field in 1843, more than 40,000 outbursts have occurred in the world. China ranks first in outburst frequency and intensity. In recent 20 years, China on average increases 37 pairs of coal mines and more than 280 times outburst accidents every year (Yan, 2013). The coal and gas outburst occurred on Oct 20, 2004 in Daping Coal Mine, Zhengzhou Coal Group, caused serious gas explosions and ejected 1,894 tons of coal into the face, causing 148 fatalities. In recent years, despite implementation of numerous control measures, coal and gas outburst still occur frequently ash mining depth and mining intensity increase. The outburst mechanisms can be summarized in the following four aspects: the gas leading role hypothesis, the geostress leading role hypothesis, the chemical effect hypothesis, and combination hypothesis. The last hypothesis has won the acceptance of most scholars who consider that an outburst is the result of combined effect of stress, gas and coal (Yu, 1992).Numerous experimental measurements and numerical simulations were conducted to investigate the permeability and damage development process in coal seam. Sobczyk, et al (2011) carried out a laboratory research of the influence of sorption processes on gas stresses leading to coal and gas outbursts. Wang, et al (2013, 2015) conducted laboratory experiments to investigate the rapid decompression and desorption induced energetic failure in coal using a shock tube apparatus. Xu, et al (2006) developed a coupled gas flow and solid deformation numerical model and applied to simulate the coal and gas outbursts in underground coal mines using RFPA2D-Gasflow. An, et al (2013) and Xue (2014) analyzed major parameters and the effect of gas desorption on outburst initiation and established a model of gas migration and mechanical processes during excavation."

Jan 1, 2016

By Rusty Mchenry, Craig Dickerson, Robin L. Oldham

"It has been proven that longwall faces can be moved safely and efficiently. However, abutment pressures and poor ground control conditions can halt operations and be hazardous to coal miners. Recently at a mine in Southwestern Pennsylvania, roof material collapsed above shields that created two large voids and caused major challenges for shield recovery. A unique, engineering solution was developed that utilized a modified concrete material to fill the voids, creating stability in the affected area. The many phases of this project included the construction phase, void pumping, cutting out, and bolting of the concrete material. This project eliminated the hazards associated with bolting the recovery face and removing shields in adverse conditions, making it possible for the mine operator to safely complete the longwall move.INTRODUCTIONFrequently in the mining process, especially during the recovery phase, voids above the longwall supports have occurred. These voids are simply roof cavities caused by falling rock strata above the longwall face. Most often, this condition slows down or halts mining operations and creates hazardous work conditions for coal miners. Numerous methods of supporting void areas on longwall faces have been attempted and utilized throughout the mining industry since the advent of the longwall mining system. These methods have utilized many different materials and processes and have included the installation of steel or wooden beams, crib blocks, and various types of roof bolts. However, these methods can be dangerous due to exposure of personnel to the mine voids, falling roof material, and the necessity to handle large amounts of heavy support materials.This paper describes a unique case study in Southwestern Pennsylvania where longwall recovery methods, through two void fills, were successfully completed, eliminating exposure of coal miners to the rock voids above. At the start of this endeavor, normal recovery operations were halted, and hazardous conditions became a concern when the voids were encountered. However, in the interest of safety, the decision was made to try a unique method to completely fill the void from the bottom of the coal seam to the top of the void with a cementitious material and then mine through it. This is in contrast to traditional methods where beams and support materials are used to stabilize the area above and in front of the mine shields, which can expose miners to the void above."

Jan 1, 2015

By Ashok Kumar, Rajendra Singh, Arun Kr. Singh

"A simple analysis of site conditions and capability of the mechanised depillaring (MD) operation found that this approach is suitable to address the strata control issues in coal pillar extraction under the weak and laminated roof strata of Pinoura mine. A continuous miner (CM) with a ram car and quad-bolter are used for single pass pillar extraction (first time in the country) to achieve a relatively faster rate of depillaring. A total of eleven panels are depillared under weak and laminated roof strata with a fair-rated rock mass rating (CMRI-RMR). It is observed that there is less influence of roof caving over the surrounding mining structures, especially those located out-bye the side of the extraction line. An attempt is made to quantify the goaf influence by field studies, which show that the maximum value of mining-induced vertical stress is 2.83 MPa, and the range of influence remains limited to around 15 m from the line of extraction. However, the problem of roof instability under the influenced zone was experienced due to the inherent nature of the weak roof strata. The chances of trapping of CM under weak and laminated roof strata of the panels is reduced by deriving a threshold value of the roof dilation, on the basis of a simple statistical analysis of the obtained field results. However, few incidences of roof collapses are observed over the CM due to the unpredictable behaviour of the geological structures below the freshly exposed strata, which also caused an obstruction in the speed of the extraction. This paper presents results of the field and laboratory studies, conducted for a successful application of the single-pass pillar extraction method, along with details of some CM trapping cases below the weak and laminated roof strata.INTRODUCTIONCoal is the major natural resource to meet the growing energy demand of India and this trend is likely to continue for the next 20 years in the country. A major part of the Indian coal belongs to Gondwana period, and only a small part belongs to Tertiary period. The estimated coal reserve in India per the exploration work done up to a maximum depth of 1200 m is 315.149 billion tonnes, including the proven coal reserve of 143.058 billion tonnes (CMPDIL, 2017). Even in presence of these huge resources of coal in India, there is a considerable gap between production and the users’ demand of coal. Most of the mines are operated by conventional semi-mechanised Bord and Pillar (B&P) methods; thus, the mining industry has gained a considerable amount of operational experience (Singh, Singh, and Dhar, 1996) and knowledge about strata behaviour (Singh and Singh, 1999; Singh et al., 2008) using this method. However, recently, the Indian coal mining industry has introduced some mechanised depillaring (MD) operations, where the level of production is relatively high. This approach of depillaring improves safety along with the production and productivity of a depillaring operation (Ram et al., 2017). The nature of the overlying strata of these depillaring operations vary greatly: from highly laminated, weak, and easily caveable to massive, strong, and difficult to cave."

Jan 1, 2018

By G. P. Neobutov

The near surface inclined gold ore deposit is mined out at freezing temperature. The replacement of ore pillars by artificial freezing rocks during the rock pressures control in this conditions presented no difficulties. At the present, the mining operations are maintained at a depth of more than 300 meters. The exploration of mining conditions showed partial warming of rock mass and unstable ground conditions. Analytical calculations using a mathematical model of heat exchange predicted undesirable duration of freezing of artificial pillars. Practical techniques and recommendations for mining with ice pillars as an artificial support system using a new freezing method are discussed.

Jan 1, 1996

By Martin Bolesta

The use of strata injection as a means of stabilizing brittle and heavily stressed ground as well as faulted zones is now state-of-the art technology. Users are obliged to select the right combination from a wide range of injection products and equipment in order to ensure that the operation will be a success. Laboratory tests and a fundamental understanding of the technical aspects of injection products not only help in drawing up the correct injection procedure but have in fact become indispensable for planning the most reliable and cost-effective solution.

Jan 1, 2001

By Ming-Gao Qian

How to monitor and control the support and surrounding strata system on a longwall face effectively is one of the major research problems in ground control. Based on observations on movements of the support and surrounding strata of face No. 2703 of Dongpang colliery, the relationship between the downtime related to support and surrounding strata and other influential factors is analysed systematically in this paper. Monitoring indices and control measures are then determined by taking corresponding downtime as an objective function. When the monitoring method, developed from the above findings, was used on face No. 2703, the downtime due to support and surrounding strata was remarkably reduced, and the face advanced normally.

Jan 1, 1992

By Thomas Barczak

Roof support systems are designed for roof control to prevent unplanned falls. It sounds logical and has been the conventional thinking in support design since supports were first installed. In order to determine which support system should be used or which would be the most effective, the degree of control provided by the support system must be known. This question embodies the concept of a ground response curve, which is a measure of support control by assessment of the convergence in the mine entry as a function of the support capacity. In this National Institute for Occupational Safety and Health (NIOSH) study to optimize standing roof support design, ground response curves were developed for longwall tailgate conditions from numerical models of Pittsburgh Coal Seam geology. The models were calibrated against tailgate convergence measurements that were made in two Pittsburgh Coal Seam mines as the depth of cover varied during the panel extraction. Ground response curves were developed for four loading conditions: (1) development, (2) side abutment, (3) front abutment near the longwall face, and (4) full extraction inby the face. In general, the tailgate convergence and required support capacity increase through each of these loading stages. It was concluded that prior to failure of the rock mass, the support system regardless of its capacity has relatively little impact on tailgate convergence. Recognizing that the support cannot prevent much of the (pre-failure) convergence that is occurring is an important part of support design. Supports that exhibit high loading stiffness but cannot sustain that loading through a displacement compatible with the ground response curve will not provide adequate roof control because they can fail prematurely. The required capacity depends on the loading plus yielding characteristics of the support and the amount of rock failure that has occurred. The loading plus yielding character establishes where the support loading intersects the ground response curve. Design criterion for support capacity based on the identifying the onset of strain-soften rock response leading to ?damaged roof? is proposed as a foundation for assessing support design requirements. The capacity without accounting for the loading and yielding character of one support should not be used to assess the capacity requirement of an alternative support design, particularly when the loading and yielding characteristics are significantly different. A sensitivity study was made to evaluate the impact of mining height and overburden depth. As expected, the results show that convergence increases with increase in mining height and increase in overburden depth. Some specific support examples are also analyzed. Although this is a first step and the two-dimensional numerical models have limitations, these initial studies provide valuable insight into the control provided by the roof support system and will ultimately lead to optimizing support design based on specific mine site conditions.

Jan 1, 2008

By Yi Luo

A longwall mining operation went under a tall, self-supporting and free-standing wireless communication transmission tower. In anticipation of that the ground subsidence process might affect the transmission tower, the mining company took a proactive approach to mitigate the expected subsidence influences. The efforts include prediction of surface subsidence process at the location of the tower, assessment of stability, structural integrity and functionality of the tower structures, and design and implementation of mitigation measures. The intension of this paper is to demonstrate the techniques that can be used to assess and mitigate the subsidence effects on tall self-supporting and free standing tower structures.

Jan 1, 2008

By Winton Gale

"The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report Cl3013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks. The study concluded that flow into mines is typically via an interconnected network of pre-existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia.INTRODUCTIONWater inflow into coal mines has been a design issue for many years. Guidelines as to the potential for water inflow have been developed in many countries based on local experience and the form of mining being undertaken. In most instances, the guidelines relate to inflows which would endanger underground personnel and operations.In more recent times, water inflow criteria for mines has been widened to include lesser inflows which may not impact on mine safety or operations, but have the potential to reduce water flow within streams and surface aquifers. For the purpose of this paper the larger inflows which are readily measured in a mine or may impact on mining safety are defined as mine inflow and the lesser inflow relating to aquifer water loss as environmental inflow.The water losses referred to are those which enter the mine from the surface or from near surface aquifers.The empirical relationships developed as mine design guidelines or regulations typically relate to high flow (mine inflow) situations and in this paper, these have been reviewed as to their application to environmental inflow. In many cases environmental inflows were unlikely to have been noted or recorded in many mining operations in the past."

Jan 1, 2010

By Yajie Wang

High horizontal stress and its adverse effects on longwall gate entries have been witnessed in many US coal mines for the past years. It can cause cutter roofs and leads to roof falls either in the headgate or tailgate entries depending on the relative orientation of the panel to the maximum principal horizontal stress. Underground observations have shown that if the maximum principal horizontal stress is perpendicular to the panel entries, the roof is in the worst condition and if the maximum principal horizontal stress is parallel to the entries, the entry is in the best condition. However, little has been done about the high horizontal stress effects on panel entries subject to different orientations of high horizontal stress. In order to design a longwall panel and control the roof effectively, the redistribution and concentration of high horizontal stress field must be fully understood. Using 3-D finite element analysis, this paper presents the high horizontal stress redistribution and concentration for a retreat longwall face with different orientations of high horizontal stress. Based on the results, possible roof failures for different orientations of high horizontal stress are analyzed. A panel design criterion is recommended.

Jan 1, 1996

By Hani Mitri

Understanding the interaction between rock bolts and underground rock movement is critical for safe and cost effective underground excavation design. Although early research on this subject involved a balance of theoretical analysis and field measurement, recent work has been heavily focused on analytical and numerical studies. This paper describes technologies that have the potential to redress the balance through instrumentation of rock bolts deployed under routine operating conditions in underground mines. Two new products are presented in this paper. Firstly the U-cell coupler is a load cell device that can be routinely coupled to the threaded end of a bolt prior to installation. It measures the ?series? load at the head of the bolt. Secondly, the d-REBAR involves an array of small-diameter, long-base length displacement sensors recessed into grooves along the entire length of the bolt. Both new instruments are interfaced with on-board digital signal conditioning and telemetry. Methodologies for the deployment of the new instrumentation and guidelines are presented for the interpretation of results based on data obtained from field trials. Successful field installations demonstrate the viability and practicality of these new technologies and, moreover, provide important insight in to rock bolt/rock mass interaction.

Jan 1, 2012

By Liang Dong, Jinxuan Liu, Biao Qiu, Jing Chai

"The Cenozoic unconsolidated strata in the east area of China are composed of clay, sandy clay, fine sand, hardpan, etc. Numerous shafts of the coal mines in these areas have been damaged in various degrees since 1987. The main reason for this disaster is the vertical additional stress on shaft caused by the subsidence of the thick unconsolidated strata. In order to monitor the subsidence deformation of the unconsolidated strata, a sensing network system with multiplexed Fiber Bragg Grating (FBG) for monitoring settlement deformation of the unconsolidated strata in deep borehole is proposed. The field monitoring results show that, soil layers settle or expand with decrease or increase in ground water levels. The thick hardpan generates the main compressive deformation. This FBG strain monitoring method can take place of traditional ground water measuring method. This system is stable and reliable, and the monitoring results can show the subsidence deformation of different layers, which has significance for the predictive maintenance of the shafts.INTRODUCTIONSince 1980s, a specific geologic hazard has occurred frequently on the coal mine shafts in eastern China: non-mining-induced shaft deformation, involving up to 100 shafts located in Jiangsu, Shandong, Anhui and Herran province, distributed in Xuzhou, Datun, Huainan, Huaibei, Yanzhou and Yongxia coal fields. This kind of geologic hazard can lead to shaft wall deformation, water infiltration, cage stuck, water-sand burst or shutting down of coal mines, which seriously risks the coal mines. Zhangshuanglou coal mine, Haizi coal mine and Linhuan coal mine shut down for 2-8 months due to this hazard (Chen, 1997; Cui, 1998). The main reason for this geologic hazard is the subsidence of the super thick unconsolidated strata (Liu et al., 2007; Xu et al., 2006).An innovative structure health ·monitoring and damage estimation method has been developed in recent years, i.e., optical fiber smart material and structure (Nobuo, 2002; Wang et al., 2000). A Fiber Bragg grating (FBG) sensor is a device that can transfer the change of the measured strain or temperature to the change of Bragg wavelength. One of the most important applications of FBG sensors is to monitor the internal stress and strain distribution of material and structure (Philipp et al., 2000; Ansari et al., 1998). Compared to traditional electrical sensors, FBG sensors have many advantages; they are adaptable to harsh environments, abrasion resistant, immune to electrical magnetism (EM) interference, safe and reliable. They also offer integrated sensing and transmission on a single fiber, have simple structure and are convenient to use, which satisfies the requirements of long distance, distributional and long term use in monitoring mining and geotechnical engineering structure. Due to such advantages, the FBG sensor has already been applied in petroleum engineering (Kersey, 2000), rock mechanics testing (Cornelia et al., 2003; Yang et al., 2007; Wei et al., 2008), physical simulation experiments (Chai et al., 2007; Chai et al., 2006; Zhu et al., 2008; Li et al., 2008; Chang et al., 2008), strain sensor (Li et al., 2007; Chai et al., 2008) and construction structure damage diagnosis (Dong et al., 2004; Kerrouche et al., 2008; Whitten et al., 2001)."

Jan 1, 2010

By Mark Colwell

This paper summarizes the results of a research project whose goal was to provide the Australian coal industry with a chain pillar design methodology readily usable by colliery staff. The project was primarily funded by the Australian Coal Association Research Program and further supported by several Australian longwall operations. The starting point or basis of the project was the Analysis of Longwall Pillar Stability (ALPS) methodology. ALPS was chosen because of its operational focus; it uses tailgate performance as the determining chain pillar design criterion rather than simply pillar stability. Furthermore, ALPS recognizes that several geotechnical and design factors, including (but not limited to) chain pillar stability, affect that performance. There are some geotechnical and mine layout differences between United States and Australian coalfields that required investigation and, therefore, calibration before the full benefits offered by the ALPS methodology could be realized in Australia. Ultimately, case history data were collected from 19 longwall mines representing approximately 60% of all Australian longwall operations. In addition, six monitoring sites incorporated an array of hydraulic stress cells to measure the change in vertical stress throughout the various phases of the longwall extraction cycle. The sites also incorporated extensometers to monitor roof and rib performance in response to the retreating longwall face. The study found strong relationships between the tailgate stability factor, the Coal Mine Roof Rating, and the installed level of primary support. The final outcome of the project is a chain pillar design methodology called Analysis of Longwall Tailgate Serviceability (ALTS). Guidelines for using ALTS are provided.

Jan 1, 1999

By Kevin Jinrong Ma

Underground mines often experience roof falls in entries, crosscuts, and intersections of active mining sections, main travel ways, and belt entries. Roof fall heights greater than 20 ft (6 m) make re-bolting of the newly exposed roof dangerous and impractical. To protect mine personnel, belts, moving vehicles, and other facilities, mine operators typically install steel structures such as square or arch sets in the roof fall areas. Wood lagging is usually installed between the steel-sets to enclose the area and protect the entry from recurring falls. Usually the void space above the steel-sets and laggings is backfilled. However, backfilling high roof fall voids is costly, which causes some operators to leave the voids open. In this case, durability of wood over time and the capability of the steel-set and wood lagging to resist falling rocks are unknown. During the last few years, Keystone Mining Services, LLC (Jennmar Corporation affiliate engineering company) designed, tested, and developed an impact-resistant lagging system to protect steel-sets1. Various steel-sets protected with the impact-resistant lagging were approved by MSHA and installed in different underground roof fall rehabilitation projects. This paper presents (1) design, development, and laboratory testing of the patent-pending impact-resistant lagging panel, (2) steel-set design methodology according to the American Institute of Steel Construction (AISC) national standard, and (3) impact-resistant steel-set design method based on Law of Conservation of Energy. Two underground cases are reviewed to demonstrate the successful application of the impact-resistant lagging system.

Jan 1, 2011

By G. M. Molinda

Ground Penetrating Radar (GPR) is being investigated for the potential to determine roof hazards in underground mines. GPR surveys were conducted at four field sites with accompanying ground truth in order to determine the value of GPR for roof hazard detection. The resolution of the current system allows detection of gross roof fractures (>63 cm (s1/4 in) zone) or rider beds in coal measure roof. Data quality is not yet sufficient to detect small bed separations or subtle lithologic changes in the roof. Differences in data quality are discussed, as well as suggestions for collecting improved data.

Jan 1, 1996

By D. A. Donato

It is often necessary for a mining company to extract coal from a seam underlying a previously mined coal seam. The impact of the overlying mine workings on the stress distribution within the underlying mine workings is very difficult to assess without the use of numerical modeling techniques. This paper demonstrates the usefulness of applying numerical modeling to evaluate alternate startup-room locations for a longwall panel considering the effects of multiple seam interaction. MULSIMIPC, a code adapted by the former U. S. Bureau of Mines, was used to compare stress patterns in two proposed startup-room locations. These model results were not only compared to each other, but also to results from a model with no overlying mine workings to provide baseline criteria for evaluation of the two proposed startup-room locations. The numerical modeling study was successful in providing useful information to the coal company and the regulatory agency (MSHA), resulting in a mine design that was both safe and efficient, and prevented the sterilization of a large volume of coal reserves.

Jan 1, 1996

By Rebecca Hardy

The escalating demand for coal, in addition to the steady depletion of reserves, has produced increasingly difficult mining conditions for coal mining companies within the United States, resulting in mining above or below previously mined areas. The safe and productive extraction of coal in a multiple seam environment requires specific design technology for effective mine planning. Presently, one of the most useful design approaches for multiple-seam feasibility analysis is using numerical modeling techniques. Therefore, numerical modeling programs LaModel 2.1 and Lam2D were used to determine the feasibility of mining the Herrin (No. 6) coal seam over an existing room-and-pillar mine in the Springfield (No. 5) coal seam in the Illinois Basin.

Jan 1, 2006