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By Frederick Carr

1. The application of retreat longwall mining to the North American coal mining conditions is be¬coming more advantageous as more experience is gained. The productivity results being achieved and potential possible illustrates that this system can be the keystone of future underground coal production. 2. In the American coal fields multi-entry is normally practiced as this allows profitable longwall development with existing equipment and systems. Difficulties are being experienced however in the longwall retreat phase by the col¬lapse of multi-entry roadways and excessive use of supplemental support in the following panel tailgates. In some instances the headgate of an operating face can also be in difficulty. 3. Jim Walter Resources Mining Division is developed on a mining plan that calls for three retreat longwalls at each of its four deep mines which have varying depths of cover and immediate roof and floor conditions. The #3 Mine has experienced longwall entry difficulty and there were differing viewpoints with respect to the best development systems to adopt particularly regarding the number of entries and pillar sizes. In an attempt to reconcile these different points of view J.W.R. asked Dr. Wilson to examine the problem and this paper outlines the new approach developed.

Jan 1, 1982

By Gabriel S. Esterhuizen

"Cable bolts are sometimes used in low-seam coal mines to provide support in difficult ground conditions. This paper describes cable bolting solutions at two low-seam coal mines in similar ground conditions. Both mines used support systems incorporating cable bolts as part of the primary support system. Two original cable bolt based support systems as well as two modified systems are evaluated to estimate their ability to prevent large roof falls. One of the support systems incorporated passive cable bolts, while the other used pre-tensioned cable bolts. The results and experience at the mines showed that the modified systems provided improved stability over the original support systems. The presence of the cable bolts is the most important contribution to stability against large roof falls, rather than the details of the support pattern. It was also found that a heavy steel channel can improve the safety of the system because of the ‘sling’ action it provides. Additionally, the analysis showed that fully-grouted rebar bolts load much earlier than the cable bolts, and pre-tensioning of the cable bolts can result in a more uniform distribution of loading in the roof.INTRODUCTIONCable bolting is sometimes used as primary support in coal mines experiencing difficult roof conditions. In low-seam mines the flexibility of the cable bolts allows greater length supports to be installed near the advancing face without the use of couplers. When used as primary support, the cables are typically installed in the same row as fully grouted bolts, replacing two or more of the bolts in each support row. A heavy steel channel may be used as a strap to spread the support load over a greater portion of the roof. Historically, the Mine Safety and Health Administration (MSHA) has not allowed widespread use of partially grouted un-tensioned bolts (e.g., passive cable bolts) for primary support; however, pretensioned cable bolts have been accepted.Various solutions using cable bolts as primary support were attempted at two low-seam coal mines in Western Pennsylvania that were experiencing difficult roof conditions. Both mines originally used fully grouted rebar bolts as primary support and cable bolts as supplementary support. It was found that when a large roof fall occurred, the cable bolts may be contained within the dome of fallen rock. As problematic roof conditions continued to exist, both mines decided to use cable bolts as part of the primary support system. The cable bolts were located near the ribs of the entry, to increase the likelihood that they would be anchored outside the dome of potentially unstable roof. The cable and rebar bolts were installed on a heavy steel channel that acts as a ‘sling’ to distribute the load across the width of the entry. At the first mine, Mine A, pre-tensioned cable bolts were used while at Mine B, un-tensioned cables were used. The two support systems consist of essentially the same support components installed in different patterns and with varying degrees of pre-tension."

Jan 1, 2015

By Francis S. Kendorski

High-extraction mining of coal and other stratified minerals has a predictable effect on the surface and subsurface. Impacts on ground waters in overlying strata and on surface waters, that are associated with overlying sequential zones of collapse, fracturing, aquiclude, constrained strata, and surface disturbances can be anticipated. Since first quantified around 1979 in Bureau of Mines sponsored contract research work and others (and often adopted by state regulators), with zone extents based upon multiples of mined seam height of 24 to over 60 for the fractured zone, much new data have become available. In examining the sources of data and the methods and intents of the researchers of over 65 case histories, it became apparent that the strata behaviors were being confused with overlapping vertical extents reported for the fractured zones and aquiclude zones depending on whether the researcher was interested in water intrusion into the mine or in water loss from surface or ground waters. These more recent data, and critical examination of existing data, have led to the realization that the former "Aquiclude Zone" defined for its ability to prevent or minimize the intrusion of ground or surface waters into mines has another important character in increasing storage of surface and shallow ground waters in response to mining with no permanent loss of waters. This zone is here named the "Dilated Zone." The existence of the dilated zone explains the ratios of up to 60 times the mined thickness. Surface and ground waters can drain into this zone, but seldom into the mine, and can eventually be recovered through closing of dilations by mine subsidence progression away from the area, or filling of the additional void space created, or both. A revised model has been developed which accommodates the available data, by modifying the zones as follows: ? collapse and disaggregation extending 6 to 10 times the mined thickness above the panel ? continuous fracturing extending approximately 24 times the mined thickness above the panel, allowing temporary drainage of intersected surface and ground waters ? development of a zone of dilated, increased storativity, and leaky strata with little enhanced vertical permeability from 24 to 60 times the mined thickness above the panel above the continuous fracturing zone, and below the constrained or surface effects zones, whose thickness or existence is dictated by the zones above and below ? maintenance of a constrained but leaky zone above the dilated zone and below the surface effects zone, whose thickness or existence is dictated by the zones above and below ? limited surface fracturing in areas of extension extending up to 50 ft or so beneath the ground surface

Jan 1, 1993

By Michael J. Castner

"INTRODUCTIONMultiple-seam mining is an ever growing concern for engineers and mine management in every portion of the mining industry and is especially true for coal mining operations. The goal of this report is to identify the location and severity of multiple-seam interactions, provide AMSS calculations performed on the worst of these locations, and follow up with a visual examination of the underground physical effects on the roof and ribs in the locations identified for analysis. Multiple-seam mining is increasingly being studied because:1. Multiple-seam mining can create a multitude of problems such as roof falls, pillar crushing, pillar failing, rib spalling, floor heave, and inundation of water, gas, or oxygen-deficient air that can disrupt mining operations and threaten the safety of the miners.2. Multiple-seam mining is becoming more prevalent due to the depletion of virgin coal reserves.Currently, coal mining in the United States has progressed to the point that a growing number of mining companies are operating in coal seams located above, below, and/or adjacent to previously mined out coal seams. As a result, the state and federal governments are enacting higher safety standards on mining operations. For example, in years past, pillars may have been properly designed to handle single seam interactions for a particular mine or mines; however, today, where mining is now being considered above or below a previously mined area, these loading conditions may not be adequate to handle the removal of additional seams above or below the previously mined reserve. Studies from 1981 estimated that two-thirds of the mineable coal reserves in the United States would be impacted by multiple-seam implications. Today, over three decades later, it is obvious that most of the remaining mineable coal reserve will be influenced by previously extracted seams. Thus, thefocus on multiple-seam mining interactions on current and future mining operations. In 2012, underground mining in the United States produced 342,287 thousand short tons of coal. Northern Appalachia accounted for over 30% (103,301 thousand short tons) of this total (U.S. Energy Information Administration, 2013). Rosebud Mining Company and Western Allegheny Energy (RMC and WAE) have a combined total of 23 actively producing underground mining operations and at least four new mines under or ready for development. All of these mines are located in coal seams (Upper and Lower Freeport; Upper, Middle, and Lower Kittanning) of northern Appalachia (Pennsylvania and Ohio). Currently, more than half of the RMC and WAE mines are dealing with multiple-seam interactions. Seven mines are undermining previously mined out coal seams, and five mines are overmining previously mined out coal seams. Also, between these companies, five mines have inner-seam sloping possibilities (2 completed, 3 planned), most of which will result in mining over or under previously mined areas."

Jan 1, 2015

By Eric Keller, Jamal Rostami, Wenpeng Liu

"Roof and rib instability is an important issue in underground mining. To optimize ground support design, enhance ground stability, and reduce the possibility of roof or rib failure with minimal use of artificial ground support, it is essential to have an accurate understanding of ground conditions. This includes the location of voids, cracks, and discontinuities, as well as information about the different strata in the immediate roof. This paper briefly introduces ongoing research on void detection by using the roof bolter feed and rotation pressure. The goal of this project is to improve the sensitivity of detection programs to locate smaller joints and reduce the number of false alarms. This paper has presented a brief review of the testing procedures, data analysis, logic, and algorithms used for void detection. In addition, this paper has discussed the results of preliminary laboratory tests and statistical analysis of the data from these two drilling parameters used for void detection.INTRODUCTIONRoof falls are one of the most serious and frequent accidents in underground mining. Each year, personnel are injured or killed because of roof falls. Equipment can also be damaged when this occurs. According to Mine Safety and Health Administration statistics collected from 1999 to 2008, ground falls were the largest cause of fatal accidents in underground coal mining, causing around 40% of all fatalities (NIOSH, 2015a). Out of 8 to 10 fatalities and over 800 injuries are recorded each year in underground mines, there are nearly 2,000 reportable non-injury ground falls every year (NIOSH, 2015b).Certain features in the ground, such as voids, cracks, and discontinuities, are significant factors that cause roof support failures and roof fall accidents. Currently, several manufacturers provide smart roof bolters with limited capabilities for void detection. While successful in many instances, in laboratory experiments, the existing void detection systems have limited success in detecting void openings that are less than 1/8 inch This limited success was observed in testing concrete blocks with gaps in between, simulating a rock medium with voids. Detection programs have missed some voids, while, in some cases, they have shown false detection. These algorithms need to be more accurate in sensing voids to improve the capabilities of these programs."

Jan 1, 2016

By Kun Li, Shuaishua Yue, Huigui Li, Yuan Ruifu, Yongxiang Xu, Huamin Li

"Coal seams in Tashan Mine of Datong Coal Group in China average 49.2 ft (15m) thick and have been mined by the top coal caving longwall mining method of large mining height. Mining height was 12.5 ft (3.8m) and the top coal caving height was 36.7 ft (11.2m). The gateroad pillar between panels was 124.6 ft (38m). During retreat mining, serious bumps occurred in the gateroads on both sides of the pillar affecting safety production. Therefore, pillarless mining was experimented.Using numerical modeling and comparative study of cases of similar mining condition, it was decided to employ a 19.7 ft (6m) wide pillar, rather than the previous 124.6 ft (38m) wide pillar. Support system for the gateroads was designed and implemented. During gateroad development, pillar failure conditions and entry deformation were monitored. Hydraulic fracturing method was employed to cut off the K3 sandstone along the entry rib so as to reduce the abutment pressure induced during retreat mining. Support reinforcement method combining grouting and advanced reinforcement methods was proposed to insure stable gateroad ahead of mining. Methane drainage and nitrogen injection were implemented to eliminate hazards associated with mine fire and spontaneous combustion.Since the development of gateroad has just completed, and retreat mining has not begun, the effectiveness of the proposed methods is unknown at this point. However, monitoring will continue until after mining. The results will be published in a separate paper.INTRODUCTIONGeneral descriptionTashan mine is located about 15 Km south of the city of Datong, Shanxi province, China (aure 1). The 2014 raw coal production was 25 million mt."

Jan 1, 2015

By John C. Stankus

The 3 Right pre-developed longwall recovery entry at Signal Peak Energy?s Bull Mountain No. 1 mine was designed to prevent the abnormal roof conditions encountered in its firsttwo recovery entries. Unique geological conditions, such as the low overburden of approximately 200 feet and prevalence of roof joints in the sandstone overburden, contribute significantly to abnormal conditions during longwall recovery. The 3 Right recovery entry was 42 ft wide and developed in two stages, 21 ft wide in each stage. The entry roof was reinforced with steel wire mesh/non-metallic recovery mesh, high-capacity primary bolts and cables. After all the support was installed, the entry was completely backfilled with specially designed, cuttable, low-density cement. The roof bolting and backfilldesign were based on current practices in this mine and successful practices in other mines in the U.S. and Australia. Extensive instrumentation was installed in the recovery entry for continuous monitoring of load transfer as the face approached and cut into the recovery entries. The support design was completely successful with a smooth cut-through on January 29 and 30, 2014, significantly reducing the shield recovery time and meanwhile protecting the safety of miners. Based on actual observance and practices of the 3 Right recovery entry, recommendations on improvement and modificationsof roof support for 4 Right recovery room are presented.

Jan 1, 2014

By Ihsan Berk Tulu, Keith A. Heasley, Chris Mark

"The Analysis of Retreat l\.fining Pillar Stability (ARJ\1PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover. To this end, the National Institute for Occupational Safety and Health (NIOSH) and the West Virginia University (WVU) l\.fining Engineering Department have been working together to improve both the ARJ\1PS and the Lal\.fodel programs.This paper compares and analyzes the overburden loads calculated by ARJ\1PS 2002, the new ARJ\1PS 2010, and the Lal\.fodel program with regard to the retreat pillar line, the gob, and the barrier pillars. The analysis shows a number of distinct differences between the ARJ\1PS and Lal\.fodel loading distributions. Ultimately, the programs, with their distinctive load distributions, are used to analyze a database of 52 deep cover pillar retreat case studies, and the ability of each program to discriminate between successful and unsuccessful retreat pillar plans is evaluated.INTRODUCTIONThe Analysis of Retreat l\.fining Pillar Stability (ARJ\.f PS) and the Lal\.fodel programs have been used successfully in the U.S. for designing safe pillar recovery operations for many years. However, the recent Crandall Canyon l\.fine collapse showed that further research is required to improve the pillar design for recovery under deep cover.Pillar recovery accounts for less than 10% of the coal produced from underground coal mines; however, in the period between 1989 and 1996, it accounted for more than 25% of all ground fatalities (l\.fark et al., 2003). In the past 15 years, the safety of retreat mining operations has greatly improved. l\.fark (2009) explains three steps promoted by l\1SHA and NIOSH for safer pillar recovery:1) Global stability through proper pillar design2) Local stability through proper roof support3) Worker safety through proper section managementThe ARJ\1PS and Lal\.fodel (Heasley, 1998) programs have been playing an important role in guiding mine operators in designing pillars to ensure the global stability of a retreat panel."

Jan 1, 2010

By David P. Conover, Jake Bain, Christopher M. Ross

"Highwall mining of thick (up to 100 ft) steeply dipping (20° or more) coal seams provides many challenges, both geotechnically and operationally because seam dips near or in excess of highwall mining machine capabilities are encountered. Maximizing coal recovery while maintaining highwall stability requires innovative techniques with regard to web and barrier pillar layout, depth of penetration, and choice of mining horizon within the seam. Stability of highwall mining slopes, openings, and pillars are typically analyzed using the ARMPS-HWM program, as well as LAMODEL, UDEC, and Slope-W modeling. Highwall stability can be maintained and highwall mining production optimized by applying design criteria in creative ways, including alternating miner penetration depths and initiating mining of thick seams toward the bottom of the seam. Highwall mining of thick, steeply dipping coal requires careful planning and execution, including close cooperation between geotechnical design engineers, the mining company, and the highwall mining contractor. This paper describes the application of creative design techniques to a specific pit arrangement at the Westmoreland Kemmerer Mine, Kemmerer, Wyoming. Highwall mining was accomplished by UGM ADDCAR Systems, LLC, on a contract basis. INTRODUCTION Highwall mining is a technique for attaining additional coal recovery after the economic strip limit is reached in surface mining. It involves remote deployment of a continuous miner in openings beneath the final highwall, with no personnel entry. Many candidate areas for highwall mining have thick and steeply dipping seams. Mining downdip presents challenges related to the maximum pulling capacity of the machine, traction of the cutting head, and material conveying, all of which limit penetration depth. Maximum penetration is greater for flatter slopes and decreases for slopes nearing the threshold of the maximum machine operating angle. Most highwall mining operations are relatively flat with slight undulations within the seam; therefore, the highwall mining pillar design criteria applies fairly equally to the entire mining area. However, in steeply dipping deposits, design criteria based on higher overburden loads at the far end of the penetration are excessively conservative for the shallower portions of the openings near the highwall."

Jan 1, 2017

By Peng Zhang

In previous research, the laminated overburden model from the LaModel program was effectively integrated with Analysis of Retreat Mining Pillar Stability (ARMPS) through ARMPS-like input to create a laboratory version of the new ?ARMPS-LAM? program. This program takes the basic ARMPS geometric input for defining the mining plan and loading condition, and then automatically develops, grids, runs, and analyzes a full LaModel analysis of the mining geometry to output the section stability factor (SF), all without further user input. The initial ARMPS-LAM results were encouraging with a case history classification accuracy of 55% to 71%; however, a few of the input variables that were nominally included in the SF calculation showed independent significance in the classification accuracy. Therefore, in order to further improve the accuracy of the ARMPS-LAM program, an investigation of the SFs calculated by the new ARMPS-LAM program and the ARMPS program is detailed in this paper. The initial results of a linear correlation between the ARMPS-LAM SF and the ARMPS SF showed a strong correlation (R2 = 0.88), with the ARMPS-LAM SF averaging about 8% higher. The difference in SFs between the programs were further investigated, and, ultimately, the results indicated that the laminated overburden model as implemented in ARMPS-LAM distributes relatively more load on the section pillars for depths less than about 1,000 ft and less load for depths more than 1,000 ft. The results of this research highlight the potential for improving the ARMPS-LAM program in the future by implementing a more accurate loading calculation on the section pillars.

Jan 1, 2014

By Christopher Mark

Coal bursts are typically associated with highly stressed coal. Most bursts occur during retreat mining (longwall mining or pillar recovery) in highly stressed locations like the tailgate corner of the longwall panel. Others are associated with multiple seam interactions. However, a small but significant percentage of coal bursts have occurred during development or in outby locations unaffected by active mining. Most development bursts have been relatively small, but some have been highly destructive. No theory of coal bursts can be complete if it does not account for this type of event. This paper focusses on the development mining coal burst experience in the US, putting it into the context of the entire US coal burst database. The first documented development coal burst occurred almost exactly 100 years ago during slope drivage at the Sunnyside Mine in Utah. Sunnyside subsequently had a long history of bursts, mainly during retreat mining but also during development. Several Colorado mines have also experienced multiple development bursts. Some, but by no means all, of the development bursts in these western US coalfields have been associated with faults. In the Central Appalachian coalfields, most development bursts have occurred in multiple seam situations. In some of these cases, however, there was no retreat mining in either seam. The paper closes with some lessons from this history, with implications for preventing such events in the future.

Jan 1, 2017

By Daniel S. Mazzei

This paper will be a general discussion of surface flooding considerations as they affect mining facilities site selection, based primarily on experience with several such problems with which MSHA's Bruceton Technical Support staff has been involved. The detailed analytical procedures used in arriving at engineering solutions of these problems will not he presented here; the emphasis is on drawing attention to this additional aspect of mine design which hitherto has been largely overlooked due to insufficient awareness, information and criteria. The paper's purpose is to stimulate thought and offer guidance concerning the selection of sites for mining facilities in light of the potential for flooding problems caused by surface waters. Flooding by surface waters, would generally not be a serious consideration in areas of relatively low topographical relief, i.e. flat lands, although there are exceptions to this rule, for example, in Western areas where flash flooding is a major concern. Therefore, this discussion will mainly apply to areas where the severity of surface flooding would be amplified by the topography-large, well defined watersheds of considerable relief, i.e. mountainous areas. In these regions, steep valley walls cause any rainfall to flow quickly into the relatively narrow valley bottoms. These valley bottoms provide little flow area during storm conditions, so deep and rapidly flowing streams result.

Jan 1, 1982

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994

By Rob G. Jeffrey, Ken W. Mills, Jesse W. Puller

"A coal mine in New South Wales is longwall mining 300m wide panels at a depth of 160-180m directly below a 16-20 m thick conglomerate strata. As part of a strategy to use hydraulic fracturing to manage potential windblast and periodic caving hazards associated with this conglomerate strata, the in situ stresses in the conglomerate were measured using ANZI strain cells and the overcoring method of stress relief. Changes in stress associated with abutment loading and placement of hydraulic fractures were also measured using ANZI strain cells installed from the surface and from underground. This paper presents the results of in situ stress measurements and stress change monitoring undertaken as part of the hydraulic fracturing program.Overcore stress measurements have indicated the vertical stress is the lowest principal stress so that hydraulic fractures placed ahead of mining form horizontally and so provide effective pre-conditioning to promote caving of the conglomerate strata. Monitoring of the full-scale hydraulic fractures has confirmed the hydraulic fractures grow horizontally.Stress changes monitored during hydraulic fracturing formed part of a broader program to investigate fracture geometry and growth rates to confirm the characteristics of the hydraulic fractures placed. This monitoring showed that vertical stress in the conglomerate strata was being increased by up to 1 MPa in close proximity to the hydraulic fracture and had the effect of tending to promote asymmetrical growth of subsequent fractures about the injection borehole.Monitoring of stress changes in the overburden strata during longwall retreat was undertaken at two different locations at the mine. The monitoring indicated stress changes were evident 150 m ahead of the longwall face and abutment loading reached a maximum increase of about 7.5 MPa. The stresses ahead of mining change gradually with distance to the approaching longwall and in a direction consistent with the horizontal in situ stresses. There was no evidence in the stress change monitoring results to indicate significant cyclical forward abutment loading ahead of the face consistent with the observations of face conditions underground adjacent to both measurement locations. The forward abutment load determined from the stress change monitoring is consistent with the weight of overburden strata overhanging the goaf indicated by subsidence monitoring."

Jan 1, 2015

By Michael Gauna, Christopher Mark

"Coal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. Despite decades of research, the sources and mechanics of these events are not well understood, and therefore they are difficult to predict and control. Experience has shown, however, that certain geologic and mining factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present, and then identifying appropriate control measures to mitigate the hazard. This paper summarizes the U.S. and international experience with coal bursts, and describes the known risk factors in detail. It includes a framework that can be used to guide the risk assessment process.BACKGROUNDCoal bursts involve the sudden, violent ejection of coal or rock into the mine workings. They are almost always accompanied by a loud noise, like an explosion, and ground vibration. Bursts are a particular hazard for miners because they typically occur without warning. During the years 2012-2014, serious coal bursts occurred at three different U.S. room and pillar mines. These events resulted in three fatalities and two permanently disabling injuries (MSHA, 2014; MSHA, 2013). In all three instances, the events occurred during pillar recovery at depths exceeding 1,000 feet (300 m). None of these three mines had previously reported a burst to MSHA. Coal bursts also occurred at three longwall mines during this same time period.Despite decades of research, the sources and mechanics of bursts are not well understood, and therefore these events are difficult to predict and control. Experience has shown, however, that certain risk factors are associated with an increased likelihood of a coal burst. A coal burst risk assessment consists of evaluating the degree to which these risk factors are present. In addition, some control techniques are effective in reducing the likelihood of an event or protecting miners from their effects."

Jan 1, 2015

By William J. Schloemer

INTRODUCTION The history of coal mining the Ostrava Region of the Czech Republic goes back nearly 250 years. Both coking coal for steelmaking and thermal coal for the production of energy are mined here; however, it was the coking capabilities that subsequently turned the region into a major steel-producing area in the early 1800?s, something that continues to this day. Ostrava lies in the northeast corner of the Czech Republic, near the southern border of Poland, and falls within the Upper Silesian Coal Basin. The region contains over 400 seams of coal of which about 150 have been mined over its history and continue to be mined. Eighty percent of the Upper Silesian coalfieldlies in Poland and is presently mined by about 35 coal mines; the remaining 20% falls in the Czech Republic, which presently has four mines operated by OKD, a.s., a subsidiary of Amsterdam-based New World Resources. Traditionally, all the mines in this region have employed the longwall method of mining, of which subsidence is an unavoidable consequence. In fact, due to the long history of longwall mining here, there are areas where the overall subsidence has exceeded 37 meters (121 feet). The region is fairly densely populated with residences and industry, and a considerable amount of coal lies under some of these areas where subsidence is not allowed or has to be minimized. It is this reason that has driven OKD to consider alternate methods of mining that will not create subsidence. The decision was made to consider the room and pillar method of mining in order to leave stable pillars of coal to support the surface. After considering several locations, OKD chose the shaft safety pillar at the CSM mine as a test mining location. To say the least, this is a very challenging location in which to conduct a test. As with the entire region, there are many faults from tectonic activity, the seams are pitching at 12 to 20 degrees, the area is affected by high horizontal stress, and the depth of 750 to 900 meters is perhaps the deepest room and pillar coal mining ever attempted.

Jan 1, 2014

By Dion Pastars

Kestrel Coal has undertaken a surface-to-seam consolidation program of faulted ground prior to longwall retreat in the 304 panel. Micro fine cement is used to improve the rock mass quality and therefore reduce the risk to personnel and equipment whilst increasing previously assumed sterilised reserves. The program is based on information attained from previously successful grouting campaigns in the Queensland coal area and follows the decision making process to ensure there is sufficient spatial permeation of cement across the faulted area. This data is used to assess the residual risk to the business in terms of health, safety and production

Jan 1, 2007

By Ding Xu Chu

There are many methods developed for measuring the crustal stress in China. Among them only two methods have been adopted in practice, i.e., overcoring method (for which the sensors include: Piezo-magnetic stress gauge, steel-ring strain cell, 3-dimensional hollow-inclusion stress ,gauge, and Leeman's-like stress gauge) and hydraulic fracturing method. For the former method, one of those sensors is installed in a borehole, 36 mm in diameter, then prestressed, and finally overcored with a drill bit of 130 mm in diameter. After that the deformation of the borehole wall is measured, from which the in-situ stresses are determined. In order to avoid the errors resulting from the difference in elastic modulus for various kinds of rocks as well as from the installment of the cells, we have developed a technique to calibrate the sensor on-spot using a confining pressure calibrator and produced a very good result. For the later method, the measurements are achieved by isolating a certain section of a borehole and then injecting water to pressurize it till a tensile break is induced. It is identical to the typical technology used internationally. For many years, through field observations and a large number of laboratory experiments, we believe that the most efficient way of stress measurement for engineering purposes is the overcoring method using Piezo-magnetic stress gauge. It is characterized by stable and reliable measurement values, high accuracy and high adaptability. Taking the measurements in the intact rock mass as an example, the measured stress magnitudes are within accuracies of 8% and the errors in orientation are about 5 degrees. It can even be used perfectly in deep water. Up to now, we have performed in-situ stress measurements in more than 70 sites and obtain more than one thousand sets of data. The deepest depth used in the overcoring method is 80 m in vertical borehole and 780 m in tunnels.

Jan 1, 1987

By Kot F. Unrug

The shale response to moisture changes and the major finding of the appropriate research concerning this subject are briefly reported in this paper. Also described are mine ambient atmospheric conditions which are responsible for moisture changes in mines. The need for a quantitative measure of the weathering potential of roof. rock is demonstrated in relation to its influence on safety and the potential for increasing the cost of mine operations. The weatherability test presented in this paper has been developed and used in many mines. It imitates the natural process in a mine by subjecting the rock sample to alternating wet and dry cycles. The natural changes are thus accelerated, but are similar to the natural conditions to which the surrounding excavation rock will be exposed. The measure of the weatherability index is the percentage of the initial sample mass remaining after the test as the largest fragment of the sample. Thus, the index changes, from 100% for rock affected by moisture changes which disintegrates completely, to 0% for rock totally unaffected. In this paper, the apparatus and testing procedures are described. Also presented are examples of test results. with a discussion of the design implication to he considered by mine planners.

Jan 1, 1997

By Keith Heasley

As part of the initial investigation and validation of a new boundary-clement formulation for stress modeling in coal mines. the underground stresses and displacements at two multiple-seam coal mines with unique stress problems are modeled and predicted. The new program, LAMODEL, calculates stresses and displacements at the seam level and at requested locations in the overburden or at the surface. Both linear elastic and non-linear seam materials can he used: and surface effects, multiple seams, and multiple mining steps can be simulated. In order to most efficiently use LAMODEL for accurate stress prediction, the program is first calibrated to the site-specific geo-mechanics based on previously observed stress conditions at the mine. For this calibration process. a previously mined area is "stress mapped" by quantifying the observed pillar and strata behavior using a numerical rating system. Then, the site-specific mechanical properties in the model are adjusted to provide the best correlation between the predicted stresses and the observed underground stress rating. Once calibrated, the model is then used to predict future stress problems abead of mining. At the two case study mines, the calibrated models showed good correlation with the observed stresses, and also accurately predicted upcoming high stress areas for preventive action by the mines.

Jan 1, 1998