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By Eugene D. Krupa

Mine 33 of Beth Energy has serious and complex roof cutter problems causing delay of the advance rate of both the entry development and longwall face retreat. The cost of maintaining these entries is very high due to the requirement of wry heavy artificial supports. It was suspected that this problem was caused by multiple factors including high in-situ horizontal stresses and weak roof in the problem areas. In order to alleviate the problem, proper design of panel entry and roof reinforcement were considered. This paper presents the results of extensive measurements in experimental panel entries of Mine 33 in order to assess the performance of the designed structure and roof support system during the entry development and longwall retreat. An experimental panel entry has been designed and implemented. The designed parameters were varied seeking for the most ideal parameters which provide stable roadways. Changing these parameters accompanied instrumentation of roof, floor, and pillars. Typical instrumentation, including stress meters, convergence measurement stations, and borehole borescope were utilued. Gnensive monitoring of the behavior of the designed structure and support system led to the development of a complete stress-deformation history of the roadway and an ideal roof reinforcement, support system, which reduced the cost of entry development by less than a third of the previous method.

Jan 1, 1990

Coal mass strength properties must be estimated to obtain realistic estimates of coal pillar strength. The limitations of existing techniques for approximating these properties are discussed. The coal mass is anisotropic. The Hoek and Brown approach to rock mass property estimation is modified to allow for this. This approach requires the Rock Mass Rating (RMR) for the strata. In the normal case where bedding is sub-horizontal, these major planes of weakness have little effect on the strength of pillars. As a result of this the RMR must be measured parallel to bedding. This means that a significantly higher rating than that usually obtained for coal is observed. The use of mass strength property estimates in extant empirical relationships is outlined. At high width to height ratios, an acceleration in the rate of increase in pillar strength with increasing width to height ratio has been observed. The empirical relationship developed by Salamon (the so called squat pillar formula) takes account of this. This relationship was found to have significant limitations with respect to its universal application. Modifications arc made to Bieniawski's empirical formula to extend its use to large width to height ratio (squat) pillars. This new squat pillar formula has none of the disadvantages associated with Salamon's relationship. Strain softening was incorporated into non-linear finite difference numerical modelling to estimate pillar strengths. The technique outlined above is used to obtain peak coal mass properties. Modelled pillar strength was found to be sensitive to both peak and post peak mass strength properties. Post peak properties were found to be difficult to obtain and required some back analysis. Modelling results were used to estimate both the strength of squat pillars and parameters for use in a now squat pillar empirical strength relationship.

Jan 1, 1992

By Erdal Unal

In underground mining today, safety and economical aspects demand a better understanding of the rock-mass conditions, particularly for design of underground mine openings excavated in weak and stratified rock mass. The rock mass classification systems, in essence, are empirical approaches utilized during preliminary design stage. These systems have been developed for specific purposes and rock mass types, therefore, direct utilization of the classification systems in their original form, for characterization of complex rock-mass conditions is not always possible. This is probably one of the main reasons why designers continue to originate new systems, or modify and extend the ones already existing. RMR and Q systems, for example, although widely used in mining and tunnelling, can not fully describe the specifications of the weak, stratified and flay bearing rock mass. Consequently, the engineering applications that would be carried out based on original RMR and Q ratings could be inadequate for making design decisions even during the preliminary design stage.

Jan 1, 1990

By Ulrich Langosch

In the 1990s the German mining industry introduced a new generation of shield supports. The new design of support has a maximum load capacity of 10,000 kN, making these units as strong as the shields used in Australia and in the USA. DMT took more than 3,100 underground observations in order to verify the roof fall frequency by statistical analysis. The results of this work have led to practical recommendations for roof control and the required shield support system on longwall faces. The underground observations have been correlated to Rock Mass Classification, to stress calculations and to the angle between the direction of the fissures and the direction of longwall mining_ The analysis work yielded the following two sets of results: 1. There is a critical distance between the canopy tip and the coal face (Tip to Face TF,n,). The TF,n, is predictable and relates to: the thickness of the first roof layer and its uniaxial compressive strength. The face support should have a TF that is less than the TFcrtt in every underground longwall situation. Exceeding the TF crit call immediately result in a roof fall. 2. Using the obtained regression equation DMT is able to calculate the probability of the roof fall frequency (FF), which describes the roof fall sensitivity. When TFcr;, is exceeded the predicted roof fall FF relates to: the measured support resistance (MSR) of the shield support the calculated vertical stress (p?) the fissure-direction index (DI = angle between main fissure direction and direction of mining) and the distance by which TFcrit is exceeded (ATF). Armed with these results DMT is now able to predict the critical distance between the tip of the canopy and the coal face (TFcrit), as well as the roof fall frequency, for all shield designs. By applying the new calculation method it is now possible to compare alternative longwall layouts and different shield support types under pre-set geological conditions. Mining engineers on site are therefore in a position to make the necessary roof control preparations required to run the longwall operation to maximum efficiency. The results provide a useful basis for making practical recommendations and for selecting the most effective design of shield support. The paper uses practical examples to demonstrate the R&D results and present the various methods now available for calculation and prediction in longwall roof control.

Jan 1, 2003

By K. Haramy

Coal mine bursts or bumps involve the violent, rapid failure of coal and rock in or around a mine excavation. Failure is normally associated with high stress and brittle or brittle-elastic materials; in coal mining, bursting may also be associated with desorbing gas, and this type of failure is termed an outburst. This paper discusses factors affecting coal mine bursts and several methods to predict bursts, including microgravity, microseismic, rheological, rebound, and drilling-yield methods. Methods to avoid burst conditions using volley firing, hydraulic fracturing, and auger drilling are also discussed. A case study is presented in which the redistribution of stresses and displacements was found to be associated with a burst at a deep longwall coal mine. The research showed that stresses are redistributed following the burst.

Jan 1, 1984

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 I. L. Follington

A breaker line support (BLS) monitoring system, BLSmon, has been designed and constructed by the CSIRO and installed and commissioned at Laleham No.1 Colliery, Queensland. This system was designed to record hydraulic leg pressure and canopy position measurements and relay these, in real time, to the surface. The system has shown itself to be capable of operating on the BLSs without any adverse affect on production, Analysis of the results of the monitoring exercise has shown that rate of change of leg pressures on the BLSs could be used as an aid in the prediction of adverse mining conditions. Under favourable mining conditions the supports were observed to initially unload from the set pressure, this unloading reduced with time and by the end of a typical lift cycle had either ceased or reversed slightly. This behaviour was considered to be due to one of four possible causes, these being; the compaction of floor debris, and/or compaction of the floor, and/or crushing of the roof and /or support creep. The actual cause can only be determined through detailed monitoring of both roof and support convergence. However observational data would suggest that compaction of debris and softened floor material beneath the supports was the most likely cause. The few positive loading rates recorded under these conditions were of short duration and occurred towards the end of individual lifts. Where mining conditions were unfavourable the negative rates of change of pressure, unloading of the support, were reduced throughout a lift, in both duration and magnitude, when compared with favourable mining conditions. Over the same period the positive rates of change were observed to be greater. The rate of change of loading and convergence can be utilised to identify the onset of instability in the lift area. However, before this facility can be utilised by the mine for the reliable prediction of instability more monitored data and sophisticated processing techniques are required. A mechanism has been postulated for the deformation behaviour of the roof and pillars in a mechanised pillar extraction operation from the continuously monitored data, observational data and results of previous studies. The 7 m wide fenders were observed to yield under abutment stresses before lifting commenced. The stability of these fenders throughout the lifting sequence was considered to be dependant upon the magnitude of their post peak load carrying capability. This is influenced by the post peak stiffness and the extent of roof lowering in the vicinity of the fender. Difficulties with fender extraction occurred where remnant coal left standing in the goaf caused uncaved goat to bridge to the mined fender. Difficulties were also experienced where sub-vertical jointing occurred in the roof of the split reducing it's ability to bridge effectively. Despite the relatively short monitoring period, the potential role and application of continuous monitoring in advancing understanding of the rock mass response to mechanised pillar extraction operations has been demonstrated. This understanding now requires enhancement and shows great potential for aiding the optimisation of mechanised pillar extraction operations.

Jan 1, 1992

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 Christopher Mark

Current longwall pillar design methods are based on many assumptions about pillar and entry response to longwall abutment loads. Knowledge of the magnitude and time-of-arrival of abutment loads is essential for Improving design procedures. The abutment loads are in turn related to the development of yield zones and horizontal confining stresses in pillars. Entry stability, which is the goal of any longwall pillar design, is affected by in situ stresses as well as the interaction between the roof, pillars, and floor. The paper describes a study performed in a longwall panel at a West Virginia coal mine. The behavior of two headgate pillars and the adjacent entries was- monitored as the longwall face approached and moved past. The results provided a detailed picture of the response of long- wall pillars and entries to abutment loadings.

Jan 1, 1986

By Frank E. Chase

Sacrificial entries, roof slotting, and other tactics have been used to combat high horizontal stresses during roadway development in U.S. coal mines. In Australia, the "pillar extraction on the advance" mining method has been successfully employed as a horizontal stress control technique. The concept of advancing and relieving mine workings through pillar extraction developed from observations that some mines experience difficult ground conditions when advancing and retreating the first panel in a new reserve. However, conditions dramatically improve in subsequent panels. Apparently, the creation of a gob area stress relieves the adjacent panel workings. After much debate and close scrutiny, the first U.S. roof control plan to advance and relieve was approved in 1998, for the Sargent Hollow Mine in Wise County, VA. The major concern was that this mining practice places section personnel inby of the gob. The tentative approval permitted barrier pillar slabbing and removal of the adjacent development pillar during panel development. The intent of the plan was to provide a "stress shadow" for the advancing faces and outby workings to reduce the excessive number of roof falls. Sargent Hollow successfully extracted their first panel by pillaring on advance and retreat mining using posts. Mobile roof supports were employed to extract pillars while advancing the second panel. This paper summarizes Sargent Hollow Mine's experiences with the advance and relieve mining method.

Jan 1, 1999

By Larry Stolarczyk

A clear vision of the future coal mining industry was developed into a hierarchy of technology needs by the National Mining Association (NMA) and the Chief Executive Officers of the mining industry. Although improvements in safety and environmental factors were the main drivers in the technology roadmapping workshop sponsored by the Department of Energy, the agile mining machine of the future must efficiently extract coal from deeper, thinner, and more geologically complex coal beds. One of the challenges will be to develop sensors and software that enable clean coal cutting in a changing geologic environment. The roadmap identifies imaging ahead of raining and horizon control within the top ten needed technologies. For these technologies to be adopted by the mining industry, they must be proven to be effective while driving down mining cost and risk. This paper describes how three-dimensional imaging ahead of mining and horizon sensors enable improved ground control and safety in the mining of underground coal.

Jan 1, 2003

By K. Y. Haramy

Rock burst and coal mine bump research using static and dynamic rock mechanics instrumentation has been conducted for several decades. Research efforts typically have been conducted using static instrumentation such as pressure cells, convergence measurements, multipoint borehole extensometers, and shield loading pressure measurements; dynamic rock mechanics instrumentation, for example, microseismic monitoring, has been used to help provide precursors to those violent failures. Unfortunately, these different styles of study have never been used in tandem. We report in this study the unique opportunity to have both static and dynamic instrumentation available for monitoring a longwall panel in a deep western coal mine and show that the two methods used in tandem provide information to understand the causes of a major bump. Several static rock mechanics instrumentation sites were carefully chosen along the longwall panel in the coal seam, packwalls, roof, and floor to provide the stress history during mining, including the stress redistribution that occurred before and after the major failures. Monitoring of the shield pressure on selected shields of the longwall face revealed that the cyclical loading anticipated during normal caving processes was not occurring. In addition, by locating each microseismic event (rock noise), the areas that were not adjusting to the changing stress field were clearly delineated by a lack of microseismic activity. The area of the longwall panel that did not adjust to the changing conditions ultimately failed. The combination of static and dynamic rock mechanics instrumentation data analysis provided the necessary information to determine the interaction of the structural members and the cause of the major failures at this mine.

Jan 1, 1988

By John Shepherd

Pillar extraction carried out using various methods (split and lift, Wongawilli and old Ben) involves the formation of narrow fenders (commonly 7 m of coal) formed by driving "splits" for lifting off with a continuous miner. The strata above a split and fender adjacent to the goaf consists of a cantilever beam (usually >3 m thick) that extends from the solid coal and bridges across the fender. Lifting off the fender from the split in safe conditions depends upon the bridging capability of the cantilever and the fender strength. The nature and competence of the immediate and near roof, the support capability of the coal fender and the presence of faults and or joints can adversely affect the bridging behaviour of the cantilever. Insufficient knowledge of these parameters can lead to extraction under dangerous conditions. Remnants of fenders ("stooks") also support the cantilever and their dimensions are critical to obtain regular caving and goaf falls. Stook area, goaf roof span and roof stand-up time after lifting can be related and only a relatively small increase in Btook area of 10-15 m2 can lead to a significantly increased stand-up time, especially under a massive roof. Such conditions strongly influence the redistribution of abutment stresses and can create increased outbye pillar loading. Based on detailed underground geotechnical investigations, it is concluded that strata caveability, roof cantilever behaviour, fender H:W ratio and remnant stook size are all critical parameters for successful and safe pillar extraction.

Jan 1, 1992

By Styles P.

To determine whether 130 felt earth tremors around Edwinstowe, Nottinghamshire U.K, which also experienced severe surface fissuring, were caused by coal extraction, a surface seismometer array was established around Thoresby Colliery. Over the next year, 785 microseismic events were detected. The spatio-temporal seismicity patterns are clearly associated with the commencement, continuing extraction and closure of faces. Of particular note are events which locate at the surface and appear to be related to the active fissuring. Events occur within days of commencement of production and cease when production finishes, with good correlation between face advance and hypocentral position. Naturally occurring microseismic events have also been detected up to 1 km ahead of active longwall faces in the Midlands using triaxial geophone packages grouted into the seam together with a surface seismometer in the top of the borehole. The quality of these data were very high and guided waves can clearly be seen with the dispersive characteristics associated with seam waves. In one experiment more than 2000 events were detected in only two days of monitoring even in a relatively noisy surface environment. This paper demonstrates how very accurate locations ( 5 10 metres) can be generated using three-component digital data from only one borehole. The surface seismometer and the borehole P-wave onset can give extra precision for the height of the event relative to the seam. The event distributions give a dynamic, three-dimensional image of the developing patterns of fracturing above, below and ahead of the longwall face with important implications for roof control, subsidence prediction and gas migration.

Jan 1, 1992

By Ken Mills

Dartbrook Mine (Australia) works the Wynn seam, which has a moderate propensity for spontaneous combustion. Prior to any mining a commitment was made to place a segregation barrier pillar between longwall blocks 4 and 5. This was to segregate goaves and improve management controls in the event of a spontaneous combustion event within previous longwall goaves. An alternative to the segregation barrier known as a Consolidated Barrier was developed; firstly as a concept, then computer modelled and finally field tested. It consists of a wider than normal chain pillar with concrete Mega-Seals placed in cut-throughs. Permeability and stress measurements were conducted in the surrounding coal of the seal. Remote monitoring took place as the longwall face approached and then passed by the seal. Results indicate that as the longwall retreats past the seal, horizontal stress increases. The additional stress leads to an increase in confinement and a subsequent reduction in permeability.

Jan 1, 1999

By S. J. Mitchell

An analysis of bolting reinforcement of several large [approximately 18 m (60 ft) cube] pillars was performed. The many overcoring stress profiles in pillars at the mine were used to produce generalized stress distributions at various stages in pre- and post-failure. A complete stress-deformation curve was estimated from this data. The effect of pillar bolting on the load-deformation behavior was extrapolated from the observed unbolted behavior. Bolting can increase pillar strength by up to 10 percent. The effect of bolting on general mine stability was examined with a shoW computer analysis. The computer model was calibrated against measurements performed at the mine, and produced good agreement between measured and modeled stresses. The analysis indicated that bolting increases the bolted pillar safety factors in the event of additional pillar failure by a small amount (approximately 5 percent). Progressive pillar failure is unlikely, because arching transfers most load from yielding pillars to large unmined abutments rather than to the smaller panel pillars.

Jan 1, 1984

By Claude A. Goode

Large quantities of high grade coal exist in thick seams in the United States. Many of these thick seams are too deep to be surface mined and do not lend themselves readily to underground extraction. To develop the new mining technology required to effectively mine thick seams, the Department of Energy has entered into a Cooperative Agreement with Mid- Continent Resources, Inc. to demonstrate the multilift longwall mining technique in a 28 foot-thick seam at their L.S. Wood No. 3 mine near Redatone, Colorado. The seam will be extracted in two lifts, with a 5-foot natural partition of coal left between the upper and lower lifts. Proper selection of face supports is critical to the success of this project. Specifications were defined for the roof supports, and it was concluded that the supports selected must provide full coverage, yield at the legs, and have a 60-inch push- out cantilever that can be set at any position under full load. A Hemscheidt two-leg shield was judged as the best qualified candidate and was selected for the multi-lift trial.

Jan 1, 1981

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

By D. W. H. Su

Following a roof fall in the B12 headgate of a CONSOL Pennsylvania Coal Company (CCPC) underground coal mine in February 1998, CONSOL R&D and Exploration initiated an underground roof geology reconnaissance program along the B13 and B14 developments to define potential roof instability areas and to recommend appropriate supplemental headgate support. This reconnaissance program revealed the presence of a thick, massive Pittsburgh sandstone channel overlying the B13 and B14 panels. Surface and underground core holes confirmed the close proximity and the very massive, nonlaminated nature of the sandstone. To mitigate anticipated adverse conditions at the B 13 and B 14 longwall faces due to delayed caving of the massive sandstone behind the shields, a task force consisting of Operations, Exploration, and R&D personnel was formed in May 1998. Several operational adjustments were made by mine management, including repairing damaged lemniscate links, maintaining immediate forward shield support, and implementing measures recommended by the task force intended to boost the shield set load density at the face. After reviewing available geotcchnical information, the task force also recommended fracing of the massive sandstone over the B13 and B14 panels to enhance caving characteristics and further reduce the bending-induced stresses in the sandstone near the face. Horizontal fracs were created at a horizon about 25 feet above the Pittsburgh seam main bench over the two panels in June and November 1998, respectively. To evaluate the effect of sandstone fracing and operational adjustments on the bending-induced stresses in the sandstone, three finite element models were constructed and analyzed. Results from the finite element analyses indicate that one horizontal frac, 25 feet above the Pittsburgh seam, would reduce the bending-induced stresses in the sandstone by approximately 50 percent, maintaining them at a level below the ultimate strength of the sandstone. Operational adjustments to increase the support set load density and support stiffness could reduce the bending-induced stresses in the sandstone by a maximum of an additional 10 percent. Shield pressure monitoring at the B14 midface confirmed the presence of a lower roof pressure zone inby and outby the 1314 frac holes. This lower face pressure significantly contributed to improving the B13 and B14 longwall productivity and safety. The total production downtime due to bad top at the B 12 face within the interval of channel influence was 30,600 minutes compared to about 3,500 minutes each for the B13 and B14 faces. The average longwall advance per machine shift for the B13 and B14 faces within the channel was about 70 percent higher than that for the B 12 face.

Jan 1, 2001

By Deno M. Pappas

The purpose of this U.S. Bureau of Mines study was to determine the material stiffness properties of the gob for use in numerical models of rock mass response to longwall mining. Photographs of actual mine gob were used to approximate particle size gradations of gob material. The gradation curve was shifted down to a laboratory scale, and 20 uniaxial compression tests were conducted on simulated gob material from 3 different rock types. The study found that the nonlinear stress-strain curves obtained from the three materials were similar, and could be represented in terms of moduli as follows: Es = 2.36s + 1,360 (1) Et = 0.00181s2 + 9.33s + 294 (2) where Es = secant modulus, psi Et = tangent modulus, psi and, s = stress level (due to overburden), psi. These curves correlated well with the theoretical solution curves developed by Salamon. Other effects on gob behavior were attributable to these studied factors including void ratio, thickness-to-width shape ratio, and rock compressive strength. The generated equations should provide numerical modelers with a practical means of estimating gob modulus as a function of stress level.

Jan 1, 1993