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By John Hoelle

Coal mines in southern West Virginia, south-western Virginia and eastern Kentucky have experienced coal bumps at least since 1933. Most of the bumps have occurred due to high cover, strong roof and floor strata and stress concentrations due to the mining sequence. A longwall mine in eastern Kentucky first experienced coal bumps on the tailgate side of the longwall face in 1989. The bumps continued until 1996. The bumps were the result of: Thick overburden up to 2200 feet Strong roof and floor (strata strengths up to 25,600 psi UCS and elasticity modulus up to 4,800,000 psi Previous over-mining in places Sandstone channels Not all characteristics occurred simultaneously. The bumps produced seismic events recorded up to 4.3 (Richter scale magnitude), and damaged pillars that were up to 147 by 152 feet in size.

Jan 1, 2008

By Jan A. Nemcik

Steel mesh has been used successfully for many years to control friable roof conditions and prevent loose roof and rib material from caving into the roadway. Despite its extensive use, the installation of steel mesh is difficult to automate and many other products have been evaluated as an alternative for skin control. Ideally, the properties of these new products should be similar or better than those of the steel mesh. In particular, development of a strong and resistant shell that minimizes movement along the fractured rock and coal surfaces between the bolt anchors is recommended. A strong surface adhesion and the strength of a reinforced polymer skin can provide the necessary toughening mechanism required to enhance roadway surface support by forming a reinforced polymer/rock surface layer. Most of the Thin Spray-on Liners (TSL) evaluated in the mines are weak with slow curing times, and the plastic mesh currently used to support the coal ribs is relatively weak. Therefore, neither material can seriously compete with steel mesh. Currently, a new polymer product that cures in seconds and forms an instantaneous strata binder that surpasses the properties of steel mesh is under development at the University of Wollongong.

Jan 1, 2009

By Mingju Liu

China has been suffering from coal and gas outbursts since 1950s. Nowadays, the situation has become worse than ever with the need to mine deeper coal deposits at higher production rates in a more complex underground mining environment. In the year 2006 alone, 37 outbursts occurred in underground coal mines with a death toll of 232. This paper describes a case study of an outburst event, which occurred in July 2006 during the shaft development of the Mengjin Coal Mine, Henan Province, China at a depth of 768m below surface. Detailed analysis of the possible causes that led to the occurrence of the outburst is reported. These include the study of complex geological features, gas storage conditions, possible improper technical management practice as well as incomplete prevention and control measures. Based on the case study and the observation of outburst occurrence in China in the year 2007, suggestions have been put forward to help prevent and control future coal and gas outbursts.

Jan 1, 2007

By R. Balia

The upper part of the "Contatto Ovest" orebody, at the San Giovanni Pb-Zn sulphur mine (Sardinia, Italy), is mined by a sublevel caving method. With increasing mining depth, a progressive caving of the hanging wall and discontinuous subsidence occurs. The orebody, which is located very close to the contact between the Cambrian limestones and the Silurian shales, strikes NE-SW for more than 100 m and dips 75-80' NW. The limestone footwall is massive and strong while the hanging-wall consists of very weak shales. The mine considered is located near a town so that the observed subsidence effects must be taken into account for both stability of the nearby railway and houses and morphological and environmental impact. A limiting equilibrium analysis to predict the progress of hanging wall failure with increasing mining depth was used. A suitable computer program was developed for solving the model by a iterative procedure. The progressive hanging wall caving and its consequent superficial effects were predicted perfectly by means of the computer program used and later confirmed by a geophysical test (seismic tomography). Our study ends with a further analysis, carried out using the same procedure, which allows to have a different geometrical situation with the presence of a pillar to protect the mill area.

Jan 1, 1990

By T. N. Singh

An evaluation was made on the cavability of the roof formation of Dishergarh seam including caving sequence, caving span, and load on powered support.

Jan 1, 1994

By Ming Ju Liu

China has been suffering from the most serious coal and gas outburst accidents. The situation has become worse than ever with the mining of deeper coal seams at higher production rates in a more complex underground mining environment, in order to meet the huge national demand for coal-fuelled power plants. In the year 2007 alone, more than 30 outburst accidents occurred in underground coal mines with a death toll of 258, and in the year 2008, as of May 22, nine outbursts have already occurred in Henan, Hubei and Chongqing, killing 68 workers. At the same time, China also experienced new trends of coal and gas outburst accidents since its coal production boom in the year 2001. These trends are of great importance for government agencies, coal mine owners and operators to take initiative and effective measures to prevent and control the occurrence of coal and gas outbursts in China?s widely spread outburst prone mining areas. This paper presents outburst data of China coal mines since 2001 and describes new trends of outburst occurrence in recent years, which include outbursts coupled with shocks or rock bursts of competent roof, outbursts in mines under development, outbursts in new mines and mines that have never suffered outbursts before, outbursts in mines that are believed to be of low gas content as well as outbursts in more long-wall faces. Based on the observation of outburst occurrence in China in the period from 2001 to 2008 as well as mining and geological conditions, detailed analysis of the possible causes that led to those new trends of outburst accidents is conducted. The analysis helps shed light on possible future remedies and the necessary measures to alleviate or eliminate outburst accidents in underground coal mines. Suggestions are put forward on future studies that could be of interest to national and provincial government agencies regarding strategic policies, proper technical management practice, identification of outburst prone coal seams, online monitoring of impending outbursts as well as prevention and control measures.

Jan 1, 2008

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By Karl Zipf

In highwall mining, once a sufficient number of web pillars are developed, they can fail catastrophically in a domino-type failure, posing a safety hazard to highwall miners and sterilizing mineable reserves. Three recent case histories of catastrophic web pillar failure in U.S. and Australian mines are documented and analyzed with two design approaches. The conventional design approach uses only a strength criterion, while the postfailure design approach uses both strength and stability criteria to insure an acceptable failure mode should strength failure occur. Catastrophic highwall web pillar failures can cost money through equipment losses, production losses, coal reserve sterilization and lost opportunity costs. Coal recovery may be higher. but the benefits of higher recovery must balance the potential costs of failure. Application of the conventional or postfailure design approach will decrease coal recovery and revenues, but the risk of potentially costly failure is much less.

Jan 1, 1999

By Murali Gadde

The immediate floor bed in the Illinois Basin coal mines is often comprised of weak plastic underclay material. Operational and stability problems were encountered in the past due to the low strength characteristics of the underclay floor. Among several indicators of the nature of the floor, Atterberg limits are the most important ones that describe the plasticity behavior of these clayey materials. While several useful and interesting studies were conducted in the past on the Illinois Basin weak floor material, within the knowledge of the authors, a comprehensive database describing the plasticity characteristics of the floor material has never been published. In this paper, a large database comprising over 700 sets of Atterberg limits was put together from 39 inactive and active mines from all the three states covering the Basin. Analysis of this data shows several interesting trends related to the engineering behavior of the floor material. The paper also describes the practical implications of the trends shown by the Atterberg limits.

Jan 1, 2008

By A. G. Pasamehmetoglu

This paper describes the conver¬gence and load measurements carried out in inclined seams at five longwall faces, two of which were supported by hydraulic props and the other three by conventional timber supports in Karadon-Gelik mine. The minimum support capacities determined in this study are compared with the results obtained from two empirical approaches suggested by Wilson (1975) and Jacobi (1981). The characteristics of hydraulic props to be used and support patterns in the longwalls are found, conclu¬sions are drawn and suggestions are made. Design nomograms are prepared for field engineers, in order to be able to approach the problem more confidently and realistically.

Jan 1, 1992

By Tingkan Lu

To understand the reinforcing mechanisms of the fully grouted resin bolt and clarify its performance in weak roof strata, the field monitoring investigation using instrumented bolts has been conducted in a coal mine. The total time of monitoring was around ten months. In the first 8 month period, the rock bolt performance relates to the initial development of the gateroad and subsequent time dependent strata response. After 8 months, the rock bolt performance relates to the effect of the approaching longwall mining operation, up to the point where the measuring station was destroyed due to the mining operation.

Jan 1, 1998

By Erik Westman

Few studies have been conducted regarding the location, movement, and relative magnitude of the rear abutment of a longwall coal mine. The rear abutment, or the abutment pressure arch in the gob area, is controlled primarily by the quality of pack of the gob; the more compacted the gob is, the larger the stress would be theoretically. While there is no definitive location for the rear abutment, early studies showed that it could be located as far back in the gob as 300 m (1,000 ft). Like the location of the rear abutment, there has been little research conducted and very few answers as to the exact magnitude of the rear abutment load. Due to the fact that the rear abutment has been studied considerably less than the front or side abutment, the need for further studies is evident. From a ground control standpoint, the location and magnitude of a rear abutment could reveal information on the caving process associated with longwall mining, such as when the initial cave could be expected to occur in future panels. Besides the possible importance of the rear abutment from a ground control perspective, evidence related to the longwall rear abutment could be vital from a ventilation standpoint. The peak rear abutment location could serve as an area for harmful gases, such as methane, to accumulate because forcing air through repacked rock is more difficult than through loose rock. The objective of this study is to determine whether passive seismic tomography could image the location, movement, and relative magnitude of the rear abutment as the longwall face retreated, in addition to the forward abutment and gob. The results of a passive seismic imaging study at a western US longwall show evidence for the location, movement, and relative magnitude of a rear abutment. A clear zone of highlystressed strata is shown where the forward abutment is expected and a low-velocity, damaged zone is shown behind the face where the gob is located. These two zones retreat with the face over a three-week period. In addition, the images show a return to stress levels behind the gob that are slightly higher than overburden stress levels, which is presumably the rear abutment. This occurs at a distance behind the face that is approximately the same as the face width for this mine. We can conclude that passive seismic imaging can be used to identify the location, movement, and relative magnitude of the rear abutment of a longwall mine. This conclusion is dependent on the use of appropriate data collection and processing methods. The information provided can benefit the both ground control and ventilation engineers as they strive to improve the safety and efficiency of longwall mines. The knowledge associated with the rear abutment of a longwall could not only help increase production through proper initial planning and reduce down time, but it could also make positive strides in improving the overall safety surrounding a longwall mining operation.

Jan 1, 2012

By John Feddock

A recent increase in energy demand has resulted in the rapid growth of coal consumption in the People?s Republic of China (PRC), which is the largest coal producer and consumer in the world. Reportedly, the safety issues in Chinese coal mines have been improving, but the number of fatalities and serious injuries continues to be alarmingly high. The number of fatalities from catastrophic accidents have prompted researchers to review the Chinese mine safety situation and to look for potential solutions. Marshall Miller & Associates, Inc. (MM&A), along with various collaborators, has investigated these issues as part of its work on various projects in the PRC and has discussed the concerns with relevant Chinese officials. Preliminary investigations by MM&A of coal mine accidents during 2001?2006 in the PRC indicate that 65%-78% of the total fatalities were the result of gas explosions and roof falls in underground mines. Even though the drainage of methane has been practiced for decades, current practices do not always produce satisfactory results due to technical constraints and equipment limitations. Based on prior experience of in-seam degasification at the Daning Coal Mine located in Shanxi province, MM&A believes that in-seam directional drilling technology can be a viable solution to degas the low-permeability Chinese coal deposits. This paper 1) reviews current Chinese coal mine safety conditions, 2) analyzes the advantages/disadvantages of current degasification practices in China, and 3) presents the pre-mining degasification practices for the longwall panel at the Daning Coal Mine.

Jan 1, 2007

By Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The design of underground coal mines requires a clear understanding of the overburden response, the loading of pillars, the loading of the gob, the pillar failure process, and the ultimate load carried by partly or fully yielding pillars. Very few highquality stress measurements of yielding pillars and gob loading have been made in full extraction mining. Well-calibrated numerical models can assist in providing a better understanding of the load and failure processes, provided the coal, the overburden, and the gob are all modeled with sufficient realism. A program of numerical model calibration and validation was carried out using FLAC3D.1 The models were calibrated against observed and measured performance of coal pillars and the overburden in operating mines to provide a basic set of input parameters that can be used to provide a realistic first estimate of expected ground response and pillar loading. Input parameters for modeling coal pillar response were based on data from triaxial testing on coal samples, combined with both matching the depth of failure in the coal ribs to observations as well as matching the peak pillar resistance to an empirical equation. The models were calibrated against strong roof and floor case histories in which the pillar strength is governed by failure and yielding of the coal within the pillar and the surrounding strata only had a limited impact on pillar strength. Input parameters for the overburden were determined from a large database of laboratory tests and model calibration against maximum subsidence and subsidence curvature. Further overburden calibration was carried out by matching stresses in the mining horizon to field measurements. Three examples of the application of the calibrated dataset and modeling methodology to field measurements are presented. The results show that a reasopable estimate of the in-seam stress distribution and overburden response can be obtained for both strong and weak overburden scenarios at various depths of cover.INTRODUCTIONThe planning and design of coal mine excavations requires reliable estimates of the expected strength and loading of the mine structures to achieve global stability. Empirical methods are widely used to estimate the strength and loading of coal pillars and have been incorporated into pillar design procedures, such as Analysis of Longwall Pillar Stability (ALPS) (Mark, 1987) and Analysis of Retreat Mining Pillar Stability (ARMPS) (Mark and Chase, 1997), that are widely used in the United States. ~umerical models are finding increasing application as a tool for underground mine design because of their. versatility and the ever increasing computational power available to mine designers. A prerequisite for the application of numerical models is the calibration of the models against observed rock mass response (Hoek et al., 1990; Skiles and Stricklin, 2009). The calibration process may include comparison of model results to measured stress and deformation and modifying the input parameters in a systematic manner to achieve a satisfactory agreement between the model results and measurements. Once models have been calibrated, they can be applied to evaluating similar mining layouts in similar geological conditions."

Jan 1, 2010

By Ebrahim Ghasemi

Problem statement: One of the most challenging safety problems in room-and-pillar coalmines is controlling roof falls during retreat mining. Roof falls not only cause both fatal and non-fatal injuries to miners, stoppages in mining operations, and equipment breakdowns, but also results in the inefficient recovery of coal reserves. Approach: Sufficient awareness of effective parameters related to roof falls is an important factor in roof control. In this study, the major contributing parameters on roof falls during retreat mining are introduced based upon experience and judgment acquired from extensive retreat mining case histories and a review of relevant literature. Key findings: The main roof fall parameters in retreat mining can be divided into three categories: geological, design, and operational. Key conclusions: The design and operational parameters can be controlled by the engineer and/or operator. By the appropriate selection of these parameters in the design and implementation stages of retreat mining, the roof fall risk can be decreased to an acceptable extent.

Jan 1, 2012

By John Latilla

Exxaro Mining?s Matla Coal situated in Mpumalanga, South Africa operates two shortwall faces on the No 4 and No 2 seam while the No 5 seam has also been longwalled in the past. Severe strata loading events have lead to a number of serious roof falls on the wall face (facebreaks) resulting in safety threats as well as considerable production losses. In 2002 the No 4 seam shortwall suffered a facebreak restricted mainly to the tailgate side of the mining panel due to a thick, near surface dolerite sill not breaking properly. This was partly due to the premature suspension of longwalling in the overlying No 5 seam. The resultant overloading lead to severe pillar damage and subsequent bord collapse. Surface pre-split blasting was successfully used to relieve the pressure as well as to pre-condition the dolerite above the following two panels. The shortwall on the No 2 seam has experienced mid-face loading due to No 4 seam chain pillars lying over this area and creating a secondary cantilever effect. Pre-split blasting of the solid rock above the No 4 seam pillars has resulted in a significant reduction in the number of facebreaks. The panel layout of the No 2 seam was subsequently changed so that the chain pillars on both the No 4 and No 2 seam are superimposed whereas previously they were offset.

Jan 1, 2007

By Kot Unrug

Mine seals are necessary in nearly every mine to isolate mined-out areas from the ventilation network. There is a large number of seals in active mines and that number continually increases as the development spreads across the reserves. The recent accident caused by the failure of seals in the Sago mine resulted in new regulations. These regulations have been set based on a principle of eliminating the risk of failure, rather than the sound engineering and understanding of the conditions that may cause seal failure during an explosion of methane in sealed areas of the mine including the probability of such occurrences. This paper discusses the following subjects related to mine seals: The potential causes of methane explosions and the dynamic effects of pressure wave propagation. Seal materials and methods of construction in view of the interaction of the seal with the host rock environment as well as requirements for gathering geotechnical data. The design of seal dimensions is based on loading conditions, therefore the probable ranges of such loading is reviewed. There is also discussion of possible methods for shielding of seals from pressure pulses following an explosion. Particular seal?s types are examined in reference to the conditions at the seals location. A novel design is presented that will be a low- cost solution for the sealing of mined out areas. Recommendations are given for further research leading to a rational engineering system for design of new seals in coal mines.

Jan 1, 2008

By J. Brasfield

Jim Walter Resources, along with the Rock Mechanics Branch of British Coal and Strata Control Technology Pty. Ltd., is presently studying the yield-stable-yield pillar design in use at the Blue Creek No. 7 Mine located in Brookwood, Alabama. This paper describes the steps taken to determine stresses on the strata surrounding the pre-driven development entries. It also describes the installation and monitoring of stress cells and extensometers used to quantify the stress build-up and strata movement associated with the headgate pass of a longwall panel. The findings of this study will be used to design and predict the performance of a 2 entry yield pillar system in the Parkgate Seam at Welbeck Colliery in the United Kingdom. Stress measurements and strata movement data from No. 7 Mine and Welbeck will be used to develop a computer model that can predict the performance of such a pillar system in British mines. Jim Walter Resources will use this data to study the performance of the present yield-stable-yield pillar system in order to determine if conditions will allow the stable pillar width to be decreased while maintaining current standards of safety and production.

Jan 1, 1994

By Guo Junfeng

The strength of coal mass saturated with gas is an essential parameter in studying the mechanism of gas and coal outburst, and its prediction method. Up to now, there has not been a method for determining the strength of coal mass saturated with gas. This paper describes a new method whereby the strength of in-situ coal mass can be determined and stored continuously and automatically. The simulation results obtained in the laboratory indicate that the new method allows a fast, reliable, inexpensive and convenient determination of the strength of coal mass.

Jan 1, 1992

By Dao Hong Quang

This paper describes the numerical analysis of some geotechnical factors influencing the application of Longwall Top Coal Caving (LTCC) method. The study was aimed at comparing the differences in stress distribution in the chain pillars of the face ends and caving mechanism of the top coal in LTCC method between the flat and inclined thick coal seams. The outcomes from this analysis will benefit mine planners in designing the location of roadway development in the next longwall panel. In addition, results from the study will extend the understanding of caving mechanism in top coal section when applying the LTCC method (with the direction of the longwall face is down dip, retreating along strike) in inclined coal seams. The results demonstrate a clear effect of dipping factor on cavability of the top coal and on the stress distribution in the chain pillar of the face ends.

Jan 1, 2008