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By Laxminarayan Holla

There exist many formulae for designing coal pillars. However, when applied to a given set of mining parameters, they lead to different pillar sizes and factors of safety. From the subsidence point of view, the pillars designed as protection pillars not only have to be stable, but also have to limit subsidence over them to a level acceptable to the structure being protected. The subsidence over a pillar of given width to height ratio depends upon the mining depth. A pillar may be stable, but the subsidence over it may be quite large and unacceptable for the structure. In many cases even though pillars may yield, they support the undermined strata and reduce surface subsidence. In addition, the yielding pillars eliminate peaks and troughs in the surface subsidence profile and reduce ground strains. In a given situation, whether a pillar yields or not depends primarily upon its width in relation to mining depth.

Jan 1, 1992

By Peter Zhang, Todd Minoski, Mark Van Dyke, Daniel W. H. Su

"This paper presents the results of a 2017 study conducted by the National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Mining Research Division (PMRD), to evaluate the effects of longwall-induced subsurface deformations within a longwall abutment pillar under deep cover. The 2017 study was conducted in a southwestern Pennsylvania coal mine, which extracts 1,500-foot-wide longwall panels under 1,185 feet of cover. One 650-foot-deep, in-place inclinometer monitoring well was drilled and installed over a 150-foot by 275-foot center abutment pillar. In addition to the monitoring well, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 1,500-foot-wide longwall panel on the south side of the abutment pillar was being mined.Prior to the first longwall excavation, a number of simulations using FLAC3D™ (Itasca, 2017) were conducted to estimate surface subsidence, increases in underground coal pillar pressure, and subsurface horizontal displacements in the monitoring well. Comparisons of the pre-mining FLAC3D simulation results and the surface, subsurface, and underground instrumentation results show that the measured in-place inclinometer casing deformations are in reasonable agreement with those predicted by the 3D finite difference models. The measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models. Results from this 2017 research clearly indicate that, under deep cover, the measured horizontal displacements within the abutment pillar are approximately one order of magnitude smaller than those measured in a 2014 study under medium cover.INTRODUCTIONDue to a recent shale gas boom, approximately 1,400 unconventional shale gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, and Ohio over the past 15 years. These shale gas wells have penetrated many coal seams, such as the Sewickley, Pittsburgh, Upper Freeport, and Kittanning seams. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure. Strata deformations associated with longwall and other high extraction techniques could induce high stresses and deformations in the shale gas well casings. This could seriously compromise the mechanical integrity of the production, intermediate, and coal protection casings (Figure 1). Damaged well casings could potentially introduce high-pressure, explosive gas into underground mine workings, particularly during an extended well shut-in period, which could seriously jeopardize underground miners’ safety and health."

Jan 1, 2018

By Y. M. Jiang

A users friendly PC Computer Program has been developed to implement a systematic design of the powered support. The required input data are: (1) thickness, RQD, and compressive strength of the immediate roof; (2) mining height; and (3) type, thickness, and tensile strength of the main roof. Several important parameters for support selection under a given geological condition are generated through the implementation of the program, including type of support most favorable for the geologic condition, minimum inclination angle of the legs with respect to the horizontal axis, suitable setting load ratio of the front to rear leg for the 4-leg supports, optimum setting load, and support capacity.

Jan 1, 1989

By Rodger Fry

All coal deposits contain anomalous geologic conditions which have an impact on roof control, Run-Of--Mine (ROM) coal quality and production costs. To remain competitive in today's coal market and provide a safe working atmosphere, mine operators need to know in advance of mining the geologic conditions which prevail in the coal deposit. In order to identify geologic anomalies within a coal seam and surrounding roof rock in advance of mining, the Radio Imaging Method (RIM) survey procedure has been developed. This paper describes the RIM procedure and how it is used to predict various geologic conditions within a coal deposit. Figure 1 illustrates the RIM in-mine and surface surveys which can be used in an underground mine. [ ] RIM is an electromagnetic (EM} wave process which consists of in-mine and surface survey techniques. Both techniques are used in underground coal mines to detect faults, interbedding, sandstone channels, and thinning coal. Thesurface survey can also be used to determine seam correlation and continuity in strip and underground mining. In-mine and surface surveys work by propagating EM waves along straight paths through the coal. As the RIM signal travels along a ray path, it interacts with the seam material and surrounding rock. This interaction causes the signal to lose energy thereby decreasing its strength along the path. The amount of energy loss per unit path length depends upon the coal seam height and type of surrounding roof and floor rock. A reading of the signal level is taken at the end of the ray path at the receiver location. This reading is used to determine the loss (attenuation) rate of the signal along the path. The signal level data is compiled and analyzed through computerized algorithms which construct images of any anomaly present. This graphical depiction of the coal provides a basis for further geologic interpretation. By comparing the images reconstructed by the RIM analysis to in-mine geologic mappings made while mining through RIM study zones, it has been confirmed that RIM can help detect the following; Rapidly Thinning Coal Incompetent Roof Rock Poor Floor Lithology

Jan 1, 1984

By David C. Oyler

Few studies have specifically measured and documented the large-scale loading behavior and durability of ventilation stoppings to mining induced movements, particularly from longwalls. Ventilation stoppings are more commonly the concern of ventilation engineers, and ventilation, topping response to ground movement has only been documented in an incidental manner. This paper presents the investigations of underground measurements that have been conducted to determine the loading response of stoppings constructed from lightweight aggregate concrete masonry units (CMU). These investigations have produced some interesting results that may prove beneficial, not only for developing and assessing alternative stopping construction techniques, but also for designing and selecting standard construction methods for use in varying mining conditions. For instance, the interface friction that results from wedging a CMU stopping during construction plays a substantial role in its ability to resist both lateral and vertical loads. Although they are not intended for ground support, the study showed that block stoppings can resist vertical loads of at least 2,700 to 3,000 kN. Asymmetric loading of a stopping may result in localized failure of blocks within a stopping, which, depending on the severity, can be a precursor of impending stopping failure. In addition to measurement results and implications, the paper will also present details of field measurement methods used to assess stopping response. Stopping stiffness and material strength characteristics will also be presented,

Jan 1, 2001

By Kenneth L. Johnson

The effects of underground longwall coal mining on flow in perennial streams within the Pittsburgh seam basin were evaluated. Four streams above four different longwall mines were studied. Overburden depths at the study sites ranged from 500 to 600 feet. At all four locations, premining streamfow measurements were made to establish baseline flow conditions. Measurements were then repeated at each location after the longwall face passed beneath the stream. At one location, in which the longwall panel axis was roughly parallel to the stream, the streamflow was monitored as the longwall face advanced. Measurements at this location showed that temporary flow reduction occurred as the face advanced underneath; however, after the tensile portion of the traveling subsidence profile passed, this effect was lessened. At two of the sites in which the stream was roughly perpendicular to the panel axis, postmining streamfow increased across the panel and decreased over the development headings, such that the net lateral change in streamflow across the fully subsided panel and associated development headings was small compared to the measured flow.

Jan 1, 1997

By Larry Howe

Second, or retreat mhng with Mobile Roof Supports (MRS) has now been part of coal mining in the United States for a decade Their utilization has evolved into mining applications which vary in seam heights from 38 inches to 170 inches, and overburden depths from outcrop to beyond 1200 feet This paper will detail the pillaring plans, lift sequences, and mnchky utilized in these operations and the rational for the plans A comparison of the current productivities achieved will be outlined

Jan 1, 1998

By Thomas Barczak

Longwall mines typically use some form of standing support for secondary roof support in longwall tailgate entries. Although there have been several new support products developed for this application, there remains no universal design criteria to optimize the application of these support technologies. The requirement for optimization and proper support selection is to understand the degree of control that the support has on the ground behavior. The ground reaction curve and numerical modeling was used to evaluate the impact of standing support on ground behavior. LaModel was used to evaluate the impact of standing support on main roof and floor behavior and pillar yielding. The conclusion drawn from this study was that standing supports do not have sufficient capacity to control main roof or floor loading or prevent the resulting convergence of the tailgate entry. However, it is imperative that this "uncontrollable convergence" be considered in the support design to prevent premature failure of the support. A FLAC model was used to evaluate the near-seam roof and floor behavior in conjunction with the global vertical and horizontal stresses. The model suggests that standing roof supports can have some impact on the ground behavior as the elastic response of the rock is exceeded and rock structure deteriorates from the stress concentrations that develop around the tailgate opening. During this phase, the capacity and stiffhess of the standing support can be critical to the stability of the opening, as eventually the rock mass will be transformed into a partially detached structure whose weight must be supported by the standing support. This work is part of a ground control program of research at the National Institute for Occupational Safety and Health (NIOSH) aimed at improving mine safety by reducing roof fall injuries and fatalities.

Jan 1, 2005

By R. Kaund

"Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence, preventative measures can be taken to avoid severe hazards to mine safety.INTRODUCTIONWith the increasing demand of mineral resources and depletion of near-surface ores, the depths of underground mines have made the mining activities one of mankind’s most dangerous types of work (Ortlepp, 2005). The Mine Safety and Health Administration (MSHA) defines rockburst as “a sudden and violent failure of overstressed rock resulting in the instantaneous release of large amounts of accumulated energy.”The types of rockburst can be classified into three types: (1) strain burst, (2) pillar burst, and (3) fault slip (Müller, 1991). Strain burst is the most common type of unstable rock failure in underground openings, where the intensity and scale are usually smaller than pillar burst and fault slip (White and Whyatt, 1999; White, Williams and Whyatt, 2002; Ortlepp, 2005). The cause of strain burst can range from shattering of rock under high stress concentration to buckling of discontinuities parallel to underground openings. The occurrence of pillar burst also depends on the stress and discontinuity conditions in the rockmass, but pillar burst usually involves the sudden loss of strength in the core or foundation of a pillar (Whyatt and Blake, 2002; Kias and Ozbay, 2013; Khademian et al., 2016). Fault slip or slip burst can be either slip along pre-existing discontinuities or shear failure in the rockmass, when the shear stress is larger than shear strength in the fault (Rice, 1983; White and Whyatt, 1999; Sainoki and Mitri, 2014). The consequence of fault slip can vary from very small rockmass damage to consequential rockmass damage, but large seismic magnitude does not always result in large rockmass damage (White and Whyatt, 1999; Whyatt and Blake, 2002)."

Jan 1, 2018

By Francis S. Kendorski

Underground stone mines tier construction aggregates and industrial minerals are being planned in increasing numbers throughout the United States. At the middle of the last century (1950s) underground stone mines were much more common, and characteristically had no thorough site characterization other than proving reserves and trial-and-error planning. Ground control employed was minimal. Consequently, there was a tendency to conduct minimal pre-mining studies. However, the deposits that lend themselves to little site characterization and minimal ground control are few today, with the best being mined out. Failures of a number of underground stone or similar mines from that era are more common than realized, Underground stone mines almost always use some combinations of square or rectangular pillars, sized to support the overburden load, and spaced to allow the use of larger, more-efficient equipment. Heights are controlled by the available stone quality, interbeds of waste, or technological or safety considerations. Prior to committing to a specific mine plan with subsequent significant capital investment, and prior to having available the wealth of knowledge and experience gained once mining underground, a safe and workable mine layout must he achieved. Early attention to the several different rock units involved (roof, mine level. floor) and their strength as a rock mass with the inherent discontinuities, will allow first approximations of acceptable mine geometries and sequencing.

Jan 1, 2000

By S. J. Jung

A fundamental study of fragment size distribution and characteristics of fracture pattern is presented for the dynamic penetration of a conical bit into coal block. In this paper, damaged zones during crater formation and fragment size distribution were studied. Bit with a sharper tip angle and a smooth body configuration performed better penetration. Results suggest that in coal cutting the trend should be toward the face cleat direction using a sharper angled tip bits which consumes less energy for an optimum range of penetration depths, thus attaining maximum cutting efficiency and reducing fine fragments.

Jan 1, 1989

By John L. Hill

Cutter roof failure is a specific type of ground control problem which frequently results in massive roof failure. It is a cannon occurrence in coal mines of the Northern Appalachian Coal Sasin, causing delays in production and posing a safety hazard to mine personnel. The Bureau of Mines is conducting research on the causes of cutter roof failure togain a basis from which to prevent its occurrence and to support such roof when failure does occur. Research conducted in a coal mine of central Pennsylvania has revealed a correlation between the occurrence of clastic dikes and fonation of cutter roof failure. Inmine mapping of ground conditions showed an increase in roof failure in areas of high frequencies of clastic dikes. Rock pressure monitoring around clastic dires registered the greatest amount of roof loading near the inter- section of dikes with the rib. Load cells measurim horizontal pressure changes in the roof indicated that the greitest pressure changes were occurring perpendicular to entry headings when clastic dikes were present. Analysts of rock pressure nwnitiring shows that the roof behaved as two cantilever beams when severed by a clastic dike. Additional roof supports such as trusses and cribbing were found to effectfvelv suooort the roof in areas of clastic dikes and prevent cutter roof iailure. However, there lnethads were only successful when employed shortly after mining.

Jan 1, 1984

By Mao Bai

In predicting the integrity of under-mined structures, surface horizontal displacements and curvatures are frequently of greater importance than the more homogeneous vertical displacements. Accurate evaluation of mining induced subsidence and horizontal displacement further requires that the predictive model adequately represents the subsurface geology, mining geometry and extraction sequencing. The following documents an extension of the Subsidence Prediction and System Identification method (SPASID) to the evaluation of horizontal strains. Comparisons are made between predictions from SPASID and two other empirical methods against measured data from a case study. The system identification method is illustrated to predict horizontal displacements and strains most consistently with the in situ data.

Jan 1, 1989

By William Bookshar

The development of increasingly more sophisticated and powerful retreat longwall equipment has led to ever wider and longer panels. To maximize gateroad development efficiency, provide for effective face recovery, andlor reduce ventilation pressures, entries are cut across longwall panels in advance of longwall retreat mining. These entries must be supported to prevent falls of immediate roof in advance of the face. Recently, cut through entries and predriven recovery room entries have been supported with various combinations of roof bolts and passive support. The cost and relative success of these support systems has varied considerably. Eighty-Four Mining Company, Fosroc, and Sub-Technical have developed and implemented an efficient, yielding, multiple component, cement crib to support longwall panel cut through entries and pre-driven recovery rooms. The cutable crib consists of an inexpensive flyash cement pillar poured in a bag, within a temporary metal form. The form is moved to a subsequent cutable crib location after the initial hardening of the flyash pillar. The remaining entry height between the mine roof and the top of the flyash cement pillar is filled with a specially designed bag filled with Fosroc's Tekset cement formulation. The diameter of the pillar and thickness of the Tekset bag are designed to accommodate the anticipated front abutment loading and entry convergence.

Jan 1, 1998

By Lawrence R. Artler

The North American Coal Corporation, its subsidiaries and other coal companies, have been mining the Pittsburgh #8 seam in Eastern Ohio for several decades. Underground mining by the "room and pillar" method, and more recently by longwall methods, have been practiced. This presentation deals with ground control problems and subsequent solutions to those problems for both mining methods. But first, let me briefly discuss some pertinent points about the geologic conditions of the coal seam near the southwest edge of the Pittsburgh Coal Basin. The Eastern Ohio region of the Pittsburgh Coal Basin is situated along the western edge of the unglaciated Allegheny Plateau. The local terrain is quite steep, with topographic relief of 430-500 feet being common. The coal thickness in the region ranges from 36"to 120", with the average being 60" in the areas being currently mined. Regional dip of the seam is toward the southeast in an amount of 10 to 40 feet per mile. A cross-section of the mineable seam generally consists of the main bench (60"), a rider coal (6"-12"), and a claystone parting (6"-12") for a total excavated height of 6-7 feet. Greater excavated height occurs where the fireclay floor is so soft and friable that movement of mechanical equipment breaks up the first 4-8 inches. Generally, the main bench and rider coal thin and become more variable toward the southwestern portion of the mining area. The rider coal also grades laterally into a coaly carbonaceous shale. In those areas where the roof coal can no longer be counted on to help support the immediate top, some of the main seam (6") is left to protect the overlying 8'-12' of mudstone/claystone strata. These strata are virtually non-bedded, highly slickensided units and are "hung in suspension" by roof bolts from a more competent limestone stratum above. These mudstone/claystone units are very susceptible to moisture and thus to the weathering cycles. The rider coal or 6" of the main bench is left in place to "seal" the mudstone/claystone units in Mains and Submains development. For many years, it was thought that the strength of the limestone member was detrimental in attempts to fully extract the seam by conventional means; however, longwall methods that have been more recently employed have broken the "myth" that the Redstone Limestone prevents full extraction of pillars.

Jan 1, 1984

By Morgan M. Sears, lhsan B. Tulu, Ted M. Klemetti

"Why do some room and pillar retreat panels encounter abnormal conditions? What factors deserve the most consideration during the planning and execution phases of mining and what can be done to mitigate those abnormal conditions when they are encountered? To help answer these questions, and to determine some of the relevant factors influencing the conditions of room and pillar (R&P) retreat mining entries, four consecutive R&P retreat panels were evaluated. This evaluation was intended to reinforce the influence of topographic changes, depth of cover, multiple-seam interactions, geological conditions, and mining geometry.This paper details observations made in four consecutive R&P retreat panels and the data collected from an instrumentation site during retreat mining. The primary focus was on the differences observed among the four panels and within the panels themselves. The instrumentation study was initially planned to evaluate the interactions between primary and secondary support, but produced rather interesting results relating to the loading encountered under the current mining conditions. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the stress conditions. Both the LaModel 3.0 and Rocscience Phase2 programs were used to evaluate these four panels. The results of both models indicated a drastic reduction in the vertical stresses experienced in these panels due to the full extraction mining in overlying seams when compared to the full overburden load. Both models showed a higher level of stress associated with the outside entries of the panels. These results agree quite well with the observations and instrumentation studies performed at the mine.These efforts provided two overarching conclusions concerning R&P retreat mine planning and execution. The first was that there are four areas that should not be overlooked during R&P retreat mining: topographic relief, multiple-seam stress relief, stress concentrations near the gob edge, and geologic changes in the immediate roof. The second is that in order to successfully retreat an R&P panel, a three-phased approach to the design and analysis of the panel should be conducted: the planning phase, evaluation phase, and monitoring phase."

Jan 1, 2016

By Ben Mirabile

Over the past three decades, the U.S. coal mining industry has widely used cable supports to supplement traditional primary bolting systems, especially in difficult ground conditions. However, increased mining activity in adverse conditions has underscored the limited effectiveness of the non-tensioned, partially grouted cable supports used to supplement primary bolts. Roof failures in conditions such as corrosive environments, highly laminated strata, low overburden depth and water intrusion have demonstrated that the need for cable technology has reached beyond the traditional non-tensioned, partially-grouted cable tendon. Furthermore, declining market conditions and increased competition for coal customers has demanded that new cable bolt technology be costeffective and readily adaptable to the mining cycle. Jennmar Corporation has developed several cable bolt technologies that can be used as stand-alone solutions or in concert with one another to mitigate adverse conditions. These technologies include rapid tensioning of cable supports, structural surface control and polyurethane injection. The case studies presented in this paper use one or more of these technologies to address adverse mining conditions or special ground control problems.

Jan 1, 2013

By D. W. Evans

Mine subsidence problems due to coal extraction have occurred in a number of areas throughout the United States. Depending on the local geology, the depth of the mined seam, the type of mining method employed for extraction, and other related factors, subsidence can result in anywhere from slight displacements to a total loss of ground. These conditions can have serious consequences on the health and safety of the people living above the mined areas. A number of methods exist to provide support for the ground overlying mines. All of these methods employ some mechanism for providing additional support to either the roof of the mine or the sides of the pillars. Since failure usually occurs due to overburden stresses which are transmitted to the roof and pillars, these methods employ techniques to counteract stresses induced due to the coal extraction. The applicability of any particular method will depend on the availability of backfill material, the areal extent of the affected area, and of course, the cost. The primary use of coal ash for subsidence control has been as a backfill material for the stabilization of large areas. Large scale backfilling projects for subsidence control have been performed in Rock Springs, Wyoming, Wilkes-Barre and Scranton, Pennsylvania, and in Fairmont, West Virginia. Some of these projects have utilized sand, as well as, coal ash as backfill material. Much of the backfilling work performed in the United States has been done in an effort to control mine subsidence in abandoned mines. South Africa, on the other hand, has used backfilling extensively both for ash disposal and to increase extraction in active mines. The utilization of repetitive fill and mine techniques has resulted in extraction increases of as much as eight percent in shallow mines and as much as twelve percent in deep mines.(1) The coal ash injection system which can be used for backfilling depends largely upon whether the mine voids are dry or submerged. For dry mines, hydraulic or pneumatic conveyance systems can be used, while submerged mines are limited to backfilling by hydraulic methods. The use of coal ash as a backfill material has a number of advantages, such as: 1. Coal ash is available in areas throughout the United States and in many instances is conveniently located near areas requiring backfilling. 2. Coal ash is available in significant quantities since many coals when burned will result in about 15 to 30 percent ash by weight. 3. Coal ash as backfill is, in many instances, the most economical alternative fill material. Due to the costs associated with above ground disposal, power companies will often times give the material away to lower their overall operating expenses. 4. Coal ash is usually pozzolanic in nature, that is, this material results in cementitious bonding within the backfilled matrix of material.

Jan 1, 1982

By Venkata S. Vutukuri

Compressive strength of cylindrical specimens of a model material and sandstone with various diameter/height (d/h) ratios was determined in unconfined compression tests. It was observed that the compressive strength increases with increase in d/h ratio. Triaxial compression tests were also conducted on the specimens of the same materials with a d/h ratio of about 0.47. It was observed that the axial stress at failure increases with increase in confining pressure. A very interesting correlation was found between the effect of d/h ratio on compressive strength in unconfined compression tests and the effect of confining pressure on axial stress at failure in triaxial tests. The relationships between d/h ratio and confining pressure, s3 in MPa are:- For model material - s3 = -0.95 + 1.7957*(d/h) 0.7883. For sandstone - s- (-0.4554 + 0.9888* (d/h) 1.0355. The correlation coefficients for these equations are 0.9997 and 1. These equations are valid for d/h ratios of up to 10.

Jan 1, 1993

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013