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By John W. Fredland

Conducting mining operations under a body of Water can create hazardous conditions for miners. The potential for a sudden inrush must be considered and abnormal ground control conditions can be encountered. This paper synopsizes the factors which mine operators should take into account when evaluating whether an inrush may occur. It addresses the applicable Federal regulations and discusses some items which coal operators should consider in applying for a permit, Through collecting and analyzing the pertinent information, the likelihood of encountering problems can be judged. Through proper mine planning, the potential for creating hazardous conditions can be mitigated.

Jan 1, 1982

By Yuanwci Shi

This paper discusses the results of research on the interaction between roof strata and shield supports including the active and passive supporting functions of the shield supports which prevents roof strata separation; pressure distribution on the canopy and under the base plate as measured in the laboratory and underground; comparison between various shield structures regarding their effectiveness on roof and floor control; and determination of optimum support capacity under given geological and mining conditions including hard-tocave roof

Jan 1, 1997

By R. Karl Zipf

Stress analysis programs such as MULSIM/NL, LAMODEL, MinSim 2000, and EXAMINE TAB are used in the mining industry to analyze stresses and displacements in coal mines, platinum mines, gold reefs, and tabular-type deposits. These relatively simple numerical models can efficiently simulate yielding and failure of a rock mass near a mine opening and subsequent stress transfer. The main input parameters for these models are a family of nonlinear stress-strain curves for the in-seam material. For many applications of these models, such as in coal mining, extensive field measurements and observations exist from which to estimate these stress-strain curves and calibrate the numerical models; however, such measurements are lacking for other types of mines. A three-step method is presented to determine nonlinear stress¬strain curves for boundary-element (BE) programs used in many mining applications. The method requires a suite of laboratory¬scale strength tests at various confining pressures. Note that this analysis uses laboratory-measured strength and modulus properties of alluvium. The dependency of the mechanical properties of alluvium on scale has not been established, although more tests are being planned. However, it is believed that the relative comparisons performed are valuable regardless of scale effects. First, the FLAC2D computer program is used to model the laboratory tests and determine cohesion (c) and friction angle (0) for a Mohr-Coulomb material model. Second, another FLAC2D model uses the c and 0 values to calculate vertical stress and strain around a single opening in the rock mass. This model calculates the stress-strain path of points at various distances from the opening boundary. Finally, based on these stress-strain paths, the stress-strain curves needed for a BE analysis are derived. This three-step method was demonstrated in a large, flat-lying underground test facility in very weak rock. The BE analyses agreed well with observations of failure at the test facility and provided a basis for evaluating the behavior of alternative layouts.

Jan 1, 2003

By I. W. Farmer

Weightings and water inflows into longwall workings often occur together, giving rise to discussions on their relative genesis. Case histories are introduced which indicate that most water inflows are associated with heavy weightings. Resultant large releases of energy can lead to extensive vertical fractures, which can drain water from aquifers or other sources, such as separation accumulations, onto the face. Effectively control of water inflows in these cases depends on face support and weighting control.

Jan 1, 1996

By N. P. Kripakov

Results of a complex three-seam interaction analysis in a western U.S. coal mine are presented. The physical situation simulated by the models represents the development of a set of entries underneath two previously mined seams with all three seams in proximity to each other. The layout and the geometry of the remnant pillars in the upper seams were nonuniform and not always directly superimposed over each other, creating a very challenging modeling problem. The U.S. Bureau of Mines' MULSIM/PC computer code was upgraded and enhanced to perform the required three-seam interaction calculations. Analysis was limited to simple linear-elastic assumptions as only peak stresses were required. Results clearly indicated that stress concentrations in the lower seam increased approximately 100 percent when compared to cases simulating no extraction in the two upper seams. These stress concentrations generally occurred in zones directly below points of highest abutment pressures in the upper seams. Overall, the model results compared favorably with actual documented observed behavior where roof failures were known to have occurred.

Jan 1, 1996

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 D. F. Scott

Seismic tomography, as used in mining, is based on the principle that highly stressed rock will demonstrate relatively higher P-wave velocities than rock under less stress. A decrease or increase in stress over time can be verified by comparing successive tomograms. Personnel at the Spokane Research Center have been investigating the use of seismic tomography to identify stress in remnant ore pillars in deep (greater than 1220 m) underground mines. In this process, three-dimensional seismic surveys are conducted in a pillar between mine levels. A sledgehammer is used to generate P-waves, which are recorded by geophones connected to a stacking signal seismograph capable of collecting and storing the P-wave data. Travel times are input into a spreadsheet, and apparent velocities are then generated and merged into imaging software. Mine workings are superimposed over apparent P-wave velocity contours to generate a final tomographic image. Results of a seismic tomographic survey at the Sunshine Mine, Kellogg, ID, indicate that low- velocity areas (low stress) are associated with mine workings and high-velocity areas (higher stress) are associated with areas where no mining has taken place. A high stress gradient was identified in an area where ground failed. From this tomographic survey, as well as four earlier surveys at other deep underground mines, a method was developed to identify relative stress in remnant ore pillars. This information is useful in making decisions about miner safety when mining such ore pillars.

Jan 1, 1996

By H. N. Maleki

The results of five years of geotechnical investigations are presented to develop productive and stable longwall layouts for the Foidel Creek Mine. The program was initiated during the pre-mining stage, and has continuously provided the data required for mine design. From several stress determinations at the mine, the relationship between geologic structure and the stress field/depth was established. The results were used for orienting the entries for improved stability. Cave conditions and load transfer to panel boundaries were evaluated from closely monitored full extraction mining and computer analysis. Back-analyses of ground movements, as compared with the actual measurements, indicated that cave conditions were favorable in spite of the presence of competent, thick-bedded sandstones in the mine roof. Gate pillar design using a yield-rigid concept was developed through computer analyses and underground instrumentation, resulting in 35 and 75 ft wide pillars. The influence of a slickensided bedding plane at the roof contact on pillar yield zones, core confinement, and pillar behavior was evaluated.

Jan 1, 1988

By Jun Lu

This paper presents the evaluation, enhancement and comparison of two hydraulic fracturing techniques which were employed to mitigate the impact of a massive sandstone channel on two successive longwall faces. The hydraulic fracturing technique and Longwall Visual Analysis (LVA) software were initially employed to mitigate the impact of thick, massive sandstoneroof during mining of longwall Panel 21. Improved longwall face conditions and productivity were observed as Panel 21 mined through the sandstone channel. However, moderate sandstone fractures and falls at the face were still present in areas where hydraulic fracture influence was not present. Such adverse conditions caused two to three weeks of production delays due to large face cavities, jammed face conveyor and subsequent face gluing. Based on the mining experience in Panel 21, and site-specific roof geology in Panel 22, a modified and more effective hydraulic fracturing program was employed. Fewer well-placed holes were drilled, and larger volume of water was pumped into each hole in order to propagate the individual horizontal pancake fractures over a larger area, thereby enhancing caving and relieving face pressure caused by overhang of the massive sandstone. Longwall Visual Analysis (LVA) software was also employed at Panel 22 to predict the cavity risk index and guide the longwall face crew in double-pulling shields behind the shearer drum. This helped to reduce the tip-to-face distance at the critical spots along the longwall face, which helped mitigate the formation of the roof cavities and failure of massive sandstone onto the face conveyor. Underground observations, face advance rate and face rock delay analysis confirmed that longwall face conditions and advance rate in Panel 22were significantly improved compared to those of Panel 21. Consequently, safety and productivity were improved.

Jan 1, 2014

By Zhijie Zhu

In order to determine the overburden failure characteristics of extra-thick seam mining, EH4 electromagnetic image system and borehole televiewer survey were applied to panel 8100, Tongxin coal mine. EH4 electromagnetic imaging system was used to map the overburden strata before and after mining. Analysis of the distribution of the electrical conductivity showed that the fractured zone height was 492-558 ft (150-170m); Observation through the borehole televiewer showed that the fractured zone height of water conductivity (or water flowing fractured zone height) was 110m-142m. The results of the two methods showed that the fractured zone height of water conductivity was 492-558 ft (150- 170m) or 10-11.3 times the mining height.

Jan 1, 2014

By Andre Vervoort

The roof in coal room and pillar panels is normally stratified and can be thought of as beams or plates supported by the pillar sides in the roadways and by pillar corners in the intersections Underground measurements showed the effect of the face advance on the additional total movement of the roof skin and on the relative movement between various layers in the roof. The latter consists of the opening of bedding planes and of the lateral sliding of layers over one another. Both phenomena increase locally the axial load along full column anchors. Four models have been used to come to a better understanding of the roof behavior: a beam either clamped above the pillar side or laying on the pillar sides and a three-dimensional boundary clement model with and without bedding planes. The shape ref the curvature of the roof deflection is host approached by a clamped beam: the other models overestimate the movement at the sides. The largest roof deflection for a given Young's modulus is estimated by the model incorporating bedding planes. This model estimates that the bed separation occurs close to the face (within the first meter) and within 0.2 m of the sidewalk. In some simulations, openings are even recorded just ahead of the face. Further underground measurements should focus on these findings, as the effectiveness of the support is directly related to the amount of bed separation and sliding, which have occurred prior to the support installation and which still cart occur abler installation.

Jan 1, 2000

By Vladimir I. Klishin

Complex method of ruling of rock pressure is considered in conditions of rapid roof subsidence. The purpose of this method is elaboration of emergency devices into hydraulic props. Also method of unloading of solid roofs is worked out. An analysis of means of protecting hydraulic props of powered supports 6om dynamic loads made it possible to divide them into four classes according to principle of operation: emergency, operating hm an increase of pressure; emergency damping; emergency, operating according to the "plastic element" principle; inertial emergency, operating hm dynarmc loads. It was established that the overwhelming 'majority of designs of the first three classes of emergency devices (EDs) are based on !mditional approaches, which exhausted their possibiiities of high-speed response. Delay on their opening is caused mainly by inertia of the moving locking parts. In our opinion, the most prospective are EDs using the inertia of the moving locking parts for depressurizing the piston cavity of the hydraulic prop with a minimum delay (inertial EDs). The essence of the designs of EDs having an inertial principle of action is that into the hydraulic prop containing movable and immovable elements is additionally introduced a spring-operated contracting support, which is installed on the movable element. Under dynamic loading of such a hydraulic prop hm the roof, the support moves relative to the movable element, performing the role of an inertial mass, which leads to instantaneous depresswization of the piston cavity of the hydraulic prop. Technological measures for unloading of solid roofs consists in creating of long oriented cracks in rock massif. Description of method for creating cracks, demanding equipment and means for cone01 the progress of splits are included. Also technological schemes for realization of the method of saving underground mines and rock mass are included. The results of experimental realization of this method on the Kuzbass and Poland coal mines are also included.

Jan 1, 1997

By Klaus Opolony

The world's most popular method of longwall mining requires multiple entry systems for the panels. In contrast to this mine layout the roadways in German coal mining are used for advanced mining from the rock mechanic point of view or are even re-used by a second panel. This procedure of roadway use is presented within this paper using a 3D visualisation software tool. In the previous conferences in Morgantown several papas had addressed the basics of rock mechanic background, planning tools and variants. This paper gives a practical case study for reuse of a roadway in a German coal mine. The heading and the use of various alternatives are compared under the aspect of efficiency and costs. Within the comparison the specific requirements and benchmarks of German coal mines are taken into consideration. The costs for a multiple entry system and first of all the development rates that can be achieved with common road heading systems are compared with international layout alternatives. A prospect includes a discussion of pro and con for various development methods. Devices are given due to this prospect if and how multiple entry systems are applicable to German coal mines. The paper gives an initial point for international comparison and is a base for further discussion.

Jan 1, 2003

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 Thomas M. Barczak

The engineering of a support system is driven largely by economics. The goal is to provide effective support at minimal cost. For this reason, wood has been extensively used as a support material for centuries. The cost of timber for mining applications has risen by more than 50% in the past 5 years and is likely to continue to increase as the supply diminishes and further environmental restrictions are placed on timber harvesting. The increased cost and lack of high strength wood in the Western United States has provided incentives to improve the utilization of wood and to develop alternative support materials. This U.S. Bureau of Mines' (USBM) report discusses the basic engineering requirements for the application of various materials for supplemental roof support construction and provides an overview of support systems that use materials other than wood. A comparison of support performance and cost for various material constructions is made to assist the mine operator in support selection. The scope of materials discussed in the report include: (1) conventional mine timber loaded perpendicular to the grain, (2) parallel to the grain timber loading, (3) steel fiber reinforced and air entrained concrete, (4) steel, (5) granular fill materials, and (6) combinations of these as composite support structures. The range of support systems analyzed include wood and concrete cribs, concrete columns and props, steel beams, friction props, and several novel support concepts.

Jan 1, 1994

By Jinsheng Chen

To maintain the stability of longwall tailgate entries, they have to be supported by not only the primary roof support but also the secondary roof support due to the impact of the longwall mining¬induced abutment loads. Depending on the mining condition of a typical coal seam and its distance from the previous panel's gob, the longwall tailgate entry is often reinforced by the secondary roof support in the form of standing supports and/or cable bolts or trusses during longwall retreat, Even though lots of research work has been done to study the behavior of different standing supports in the laboratory. their function and requirements for a typical mining condition are still not fully understood due to lack of field data and correct assumptions about the behavior of the already-bolted entry roof under mining-induced abutment loads. Therefore, the application of the secondary roof support for tailgate entry roof control is still limited to trial and error, which often results in either higher cost for tailgate roof control if the design is too conservative or more production delays due to roof falls in the outby tailgate entry if the design is inadequate. To have a better tailgate roof control with reasonable roof control cost, RAG American Coal, NIOSH, and HeiTech have been conducting a joint research project to monitor actual roof loading, roof deformation, and entry convergence in the tailgate entry of 10 North panel at RAG Emerald mine. The actual roof loading data are used to calibrate and improve the roof control model for longwall tailgate entries, which has been developed by RAG American Coal, and to further reduce the roof control cost for longwall tailgate entries. The authors in this study attempt to discuss the Roof Control Model developed by RAG American Coal. in which some guidelines or important parameters for designing the secondary roof support for longwall tailgate entries are proposed.

Jan 1, 2003

By Madan M. Singh

Mining under bodies of water such as oceans, lakes, rivers, streams, ponds, and other impoundments, is not a new technique. However, in the United States it has been fairly restricted to date. In order to recover the maximum amount of the nation's reserves, in the future, it is important that further mining under such bodies of water be planned. However, it is imperative that this is not done at the expense of danger to the nation's miners or appreciable property damage. If all the bodies of water are considered large, and it is assumed that these constitute a hazard when mining in their vicinity irrespective of their size and geometry, a significant percentage of the coal reserves are likely to be sterilized. How- ever, by adopting special mining plans and procedures, it may be possible to release reserves for exploitation that would otherwise be unmineable since the surface of bodies of water below which they lie are relatively small in size. In 1977, the Bureau of Mines issued Information Circular (IC) 8741, entitled "Results of Research to Develop Guidelines for Mining Near Surface and Underground Bodies of Water". This document was based on the work done by Skelly and Loy (under Contract No. H0252083) and K. Wardell and Partners (under Contract No. H0252021). How ever, these guidelines have limited application, because these did not (1) define the bodies of water of concern, and (2) take into account the nature of the strata being encountered. These guidelines assumed that all bodies of water which would be undermined were large. For the purposes of this discussion, water bodies may be divided into three categories, depending upon their potential to cause damage to the underlying mining activity: ? catastrophic ? major ? limited Bodies of water which pose the threat of catastrophic destruction in the mine, should there be an inrush, include oceans, large lakes, big inland reservoirs, and rivers that could flood the mine complete- ly in a short period. Those with major potential for damage would include lakes and streams which might present a danger to both life and property if not considered in the mining plans. Finite bodies of water which have a volume considerably smaller than the mine volume offer a limited potential for damage in the case of an inrush. These include flowing streams which have a relatively small rate of replenishment and which can be dealt with by pumping or isolation in the event of a breakthrough into the mine. Such bodies of water may be inconvenient and cause a temporary disruption of production, and could even cause some damage to equipment, but only in rare instances would personal injuries occur. The scope of this paper is limited to the exploitation of stratified mineral deposits with overlying -face bodies of water. Vein and dessiminate ore deposits are not discussed, nor are breakthroughs into adjacent mines in the same horizon. Overlying, water-logged strata also pose a serious threat to mining, but are not treated in this discussion. Pending further investigation these strata may be considered as bodies of water with catastrophic potential for damage, with the base of the strata being considered the bottom of the water body. When the body of water is large, so that it may cause catastrophic damage to the mine in the event of an inrush, the guidelines suggested in IC 8741 are applicable. In order to develop criteria and procedures for mining under smaller bodies of water, however, it is necessary that the nature of the strata disturbance due to underground mining be examined.

Jan 1, 1981

By A. Chrzanowski

The integrated approach to monitoring surveys is based on a simultaneous utilization of geodetic, photogrammetric and geotechnical measurements of displacements, tilts, strains and any other geometrical parameters of deformation. New techniques for the integrated surveys are being developed at the University of New Brunswick in cooperation with the Canada Centre for Mineral and Energy Technology. A telemetry system for continuous monitoring of geotechnical measurements under difficult climatic and topographic conditions has been constructed and successfully tested in a coal mining area in western Canada. A generalized method for analysing the integrated deformation measurements has been developed and applied in the test area to yield three dimensional empirical modelling of surface movements from geodetic, photogrammetric and tilt surveys. The most appropriate model can be chosen from a statistical assessment of several modelling possibilities. The empirical model of the deformation can be employed in a "calibration" of the prediction (deterministic) model of the ground movements. A computer programme for two- and three-dimensional finite element modelling of elasto-plastic deformations has been developed at UNB as a part of the integrated approach. The :EM has been tested in predicting deformation in the test area in difficult geological conditions which were associated with an extraction of a steeply inclined coal seam. The results gave good agreement with the actual displacements obtained from geodetic measurements and confirmed the existence of a suspected fault.

Jan 1, 1984

By B. N. Whittaker

The paper briefly reviews the type of support and stability problems encountered in mining tunnels in U.K. Carboniferous rock conditions, particularly the Coal Measures where almost all such tunnels are located. Stability prediction of mining tunnels is discussed and a computer-based model which deals with stratified rock conditions is described. Various schemes for monitoring the stability of mining tunnels are examined and the application of the results discussed. Comparisons between predicted and measured Closure values are presented for a major Coal mining tunnelling project at Selby in the North Yorkshire Coalfield. Conclusions are drawn concerning the application of the present prediction technique together with the importance of underground monitoring. Examples of different types of mine tunnel stability problems encountered in the U.K. Coal Measures are discussed and comments made on the secondary effects of tunnel peripheral rock weathering and corrosion of the associated support systems.

Jan 1, 1987

By A. Mahyera

BETTER ROOF SUPPORT During the last decade, resin-grouted roof bolts have taken a substantial foothold in the coal mining industry - now accounting for about one-third of all bolts installed annually, due to the improved roof support. However, the high cost, toxicity and flammability of the resin cartridges appear to be a limiting factor in even greater use of such bolts. Terra Tek, Inc. has, over the past few years, been developing a novel roof bolting system to mix/inject a very fast-setting and high strength inorganic cement into bolt holes and install point-anchored or full-column grouted rebars - all automatically and at substantial cost savings. This grouting system utilizes a unique method to mix the grout ?in-the-hole?. It has been integrated with a conventional bolter, retrofitted with an indexing-head mechanism and an on-board pneumatic computer to sequence and Control the drilling, grouting and bolting. Cycle times are the same or better than for a resin system. Following laboratory testing/development, the prototype system has undergone field trials in a Utah coal mine. A description of the bolting system, results of pull-tests in the laboratory and field, and economics and productivity gains with this system in comparison to a conventional resin-cartridge bolting method are presented. The relationship of this technique to improved roof control is also discussed.

Jan 1, 1984