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By André Vervoort

In coal room and pillar, and pillar extraction panels, the sidewall conditions were investigated based on underground measurements and numerical simulations. Three-dimensional linear elastic models provided a useful and quick prediction of the amount of sidewall spalling. However, more complex constitutive models were required to simulate more accurately the extent of fracturing and its effect on the stress re-distribution. The input parameters of a Mohr-Coulomb model were determined by back-analysis of underground measurements, and the effect of the depth of the workings and the time dependent occcurrence of coal pillar fracturing were analyzed.

Jan 1, 1992

By W. A. Cincilla

The use of resin-grouted rock anchors continues to gain popularity in underground wall mines in the United States. It is estimated that during 1985, more than 30% and as many as 35% of all roof bolts used employed sane from of resin anchor. The so-called 'combination' bolting systems, which generally utilize a resin pint-anchor, have also found increased application. This increased application is particularly true in poor ground, where more 'traditional' support measures have performed poorly. Virtually all resin bolts in use today in U.S. mines employ cartridge-type delivery systems, which in general, have produced good results under a variety of conditions. However, a host of inconsistent and sometimes uncontrollable variables may significantly alter the optimum bolt performance. These variables have been identified and are well documented in the literature (Snyder, et al., 1979; Kwitoweki and Wade, 1980; Serbcusek and Signer, 1985). The beau of Mines, Denver Research Center (DRC) conducted a major research project aimed at investigating the effects of selected aspects of installation procedures on resin-bolt performance. One objective of the project was to establish anchorage criteria for full column resin bolts installed in actual mining conditions. This piper describes the results of a particular study which focused on quantifying, through field tests, possible reductions in bolt anchorage due to installation of less-than optimum resin columns. These columns might result from losses of resin to surrounding strata, or non-compliance with recommended installation procedures. Over 1000 resin bolts having grout columns ranging from 12 inches up to 48 inches (full column) were installed and pull-tested at 13 underground test sites located in a total of 11 mines (Fig. 1). Four basic roof-rock types were included: shale, top coal, sandstone, and limestone. The majority of the bolts employed 3/4-in-diameter, 48-in-long, type 40 rebar having a minimum '~ated' yield point of 17,600 lb (8.8 tons). Actual yield strength may vary by manufacturer.

Jan 1, 1986

By Fangting Dong

The broken rock zone (BRZ) and the tunnel support theory are investigated. The relationship between the BRZ and the related factors, the interaction between the BRZ and supports, and the function of supports are analyzed in detail based on the authors' underground observations and model taste. A summary of the application of the BRZ theory in the past is also presented.

Jan 1, 1988

By Yunqing Zhang

Weak shales refer to those with lower strength, thinly- laminated structure, sensitivity to moisture and weathering as well as significant time dependent behavior. It has been said that many future coal reserves with good quality have weak immediate roofs. Therefore their geo-mechanical properties and behavior must be known before an appropriate support plan can be designed. In this study, four types of shales collected from underground coal mines were tested in the laboratory. Their modes of failures in underground were observed and correlated with their lab behavior. The failure mechanisms of thinly-laminated weak shales were analyzed by the means of numerical modeling.

Jan 1, 2004

By Robert Trueman

This paper details the results of an assessment aimed at understanding the shield loading mechanisms associated with strata-related issues on a longwall face. Shield load cycle analysis theories developed by the authors were used to quantify the interaction between the shields and strata. Shield pressure data was back-analyzed using a version of the Longwall Visual Analysis (LVA) software that has been extended to enable efficient load cycle analysis to be undertaken. The software outputs enabled the shield strata interactions to be characterized. The results of the analyses indicated that the major cause of the roof falls was high-level periodic weighting, resulting in periodic shield overload. Exploration borehole data was analysed to identify and characterize the overburden unit that was the likely cause of the shield overloading.

Jan 1, 2011

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 Ross Seedsman

Discussion on the design of roof support in tailgates has often been conducted without a clear statement of the stress and failure conditions acting. There is general agreement that in the tailgate the vertical stresses above the chain pillar increase. Within the stress 'bulb' associated with this vertical stress increase there will be an increase in horizontal stress. This does not mean that the horizontal stresses acting across the roof line increase. In retreat longwalling, the proximity of the tailgate to the adjacent goof results in a substantial reduction in the horizontal stress field acting in the immediate roof. Furthermore, at the face/tailgate corner the onset of significant compression of the chain pillar. if it is designed to yield, will result in a further reduction in the horizontal stresses in the immediate roof. It is suggested that this latter mechanism can cause the horizontal stresses in the immediate roof to go to zero. Whatever, the horizontal stress acting across the roof in the face/tailgate corner will be significantly lower than in the face/maingate corner. The implication of the model is that support design in tailgates should be based on the assumption of zero horizontal (in fact tensile) stresses. How the roof behaves in a tensile stress environment is a function of joint spacing and orientation compared to excavation width and direction. It is speculated that the empirical relationship between support density and roof rating results from the relationship between joint spacing and bedding spacing. The onset of stress reductions has major implications to the design of cable and bolt anchorages in the tailgate environment.

Jan 1, 2001

By Bob Coutts, Matt Martin, Dan Lynch, Dan Payne

"The Broadmeadow punch longwall coal mine in Central Queensland Australia has experienced significant highwall movement associated with the effect of longwall subsidence when the longwalls approach their final position close to the open-cut highwall. In response to this movement, Broadmeadow employed two types of broadscale highwall monitoring (radar and laser scanners) to provide full coverage measurement throughout three consecutive longwalls approaching the highwall. This was to attain a better understanding of the mechanism causing the movement and potentially enable prediction of instability. Results from the monitoring found the highwall is displaced to magnitudes unlike those typically measured in open-cut mining and in direct contrast with typical longwall subsidence behaviour.This paper discusses the ground movements measured, monitoring methods used, safety measures established, as well as theorising the failure mechanism. Recommendations are made for mine and pit designs for future punch longwall layouts. The paper shows how the movements measured are more aligned to some measurements made during stream valley closure studies previously presented at the ICGCM and challenge the mechanisms suggested by previous literature.BACKGROUNDThe mining of longwalls under or adjacent to large voids (e.g., stream valleys, escarpments, or cliffs) is commonly associated with heavily vegetated or steep surface areas. In areas of extreme topographic variance, access to traditional survey pegs and stations or even new radar or laser technologies is limited. In addition, seldom have the longwall layouts aligned themselves parallel or perpendicular to the surface feature, making interpretation of any available surface movement data more complicated.The punch longwall layout (Figure 1) is also quite uncommon (only undertaken at a small number of longwall mines in Australia) but creates the perfect configuration to enable a somewhat controlled study of the effect of longwall subsidence on a steeply dipping surface feature (an un-vegetated, evenly excavated open-cut highwall). The relatively recently developed radar and laser scanning technology has also enabled near continuous, real-time, sub-millimetre monitoring of a full 500m wide x 100m high highwall. Because the punch longwall enables access and a clear view, the technology could be easily deployed as compared to the highly vegetated and variable topography of stream valleys."

Jan 1, 2018

By R. W. Hill

Multiseam mining has been carried out at many collieries in South Africa. It the seams are In close proximity difficult and dangerous mining conditions may arise. In the Witbank area multiseam bord and pillar mining has been carried out, while in Natal where the seams are thinner and of a higher quality, total extraction methods are common. Several combinations of multiseam mining have been carried out. The paper highlights a number of case studies and the strata control problems that can arise.

Jan 1, 1995

By C. Thomas Blevins

This paper is intended to be a hands on experience account about ground control in a Southern Illinois coal mine. Its aim is to show how a combination of real world mining practices and constraints along with practical engineering theory mesh together to formulate a ground control program in adverse conditions. These conditions - your typical localized slips, splits, rolls, clay intrusions, water, changing types and thicknesses of materials, etc. - being amplified by a very strong horizontal stress field. A review will be made about the formation of the overall ground control plan and to what degree it has been successful. A presentation of the various roof control systems and their component parts, how these systems were developed, and the theory behind them will be made. The pointing out of some of the flaws that we found in the components, installations, and/or theory will be part of this presentation. A brief look at some of the efforts in the following items will be made: quality control of the roof control products; using the computer to help keep track of roof control information and soliciting help from various universities, governmental agencies and consultants to give different perspectives to the problem.

Jan 1, 1984

By Yi Luo

The severity of disturbances caused by longwall mining subsidence to various residential structures can be re¬duced. The success of such mitigation efforts depends on accurate subsidence prediction, correct assessments of the subsidence influences, and careful design and implementa¬tion of the proper mitigation measures. This paper presents the systematic approach involved in the subsidence mitiga¬tion efforts with a case demonstration.

Jan 1, 2003

By Shosei Serata

Serious floor heave of up to 2.4 m in a 2.4-m high mine entry was eliminated by applying the stress control method of mining, as a last resort, at the No. 5 coal mine of Jim Walter Resources, Inc., in the Black Warrior coal basin near Birmingham. Alabama. Underground observation of the first 3-room entry created using the stress control method is discussed here. The behavior of the test entry, which eliminated the heave problem, is analyzed in relation to similar studies conducted in a salt mine under similar ground conditions by utilizing finite element analysis.

Jan 1, 1984

By M Nombe

Based on the analysis of a case study at POWELTON No.2 MINE, it was determined that mine stability problems had occurred due to floor softening. A fireclay (underclay) layer in the immediate floor was softening after coal extraction and failing under pillars load. The thickness of soft floor was considered as the governing factor in the determination of the magnitude of floor displacement. The effect of floor thickness on floor displacement was evaluated by creating seven models that were input into the LAMODEL program. The properties of the soft floor were determined from a bearing capacity test performed in the Mains close to the area where mine stability problems had occurred. Moisture increase was considered in the reduction of floor bearing strength. Various strain-softening properties were allocated to the materials representing the soft floor. The LOW GAP POWELTON No.2 MINE was modeled and LAMODEL program was used to determine stress and convergence on both the coal seam and the fireclay seam. The results of critical stress and convergence obtained from modeling were compared with the actual floor failure area in the mine.

Jan 1, 2002

By Michael J. Friedel

The fundamental problems associated with ground control reflect the inability of mining personnel to "see" the development of anomalous conditions. Toward this end, the U. S. Bureau of Mines is investigating the application of seismic refraction tomography for imaging the process of pillar yielding, at the Foidel Creek coal mine, near Steamboat Springs, CO. The inherent three-dimensional problem of imaging coal pillar stress is reduced to a single plane by considering only the stress transmitted from the pillar to the adjacent floor rock. This approach exploits the fact that refracted seismic energy is essentially confined to the adjacent higher velocity sandstone strata. The tomographic analysis using ray-tracing based on wavefront migration resulted in a robust P-wave tomogram from which regions of high velocity indicate zones of high stress concentration, whereas low velocity zones indicate regions of stress relief. The interpreted region of high stress concentration occurs at the pillar end nearest to the working face of the adjacent longwall panel. This finding is corroborated by vertical pressure cell measurements recorded in the pillar. The stress diminishes toward the pillar perimeter with local minima correlating to the locations of observed pillar spalling. Time-sequence monitoring, as the longwall face advances, is recommended for evaluating changing stress conditions.

Jan 1, 1993

By David Bigby

A remote reading dual height telltale system has been developed to provide mine management with early warning of impending roof failure. This system is a logical development from the visual dual height and triple height telltales which are increasingly being used worldwide for routine monitoring of mine roof. The paper describes the principles and history of telltales from their origins in France, their refinement and routine use in the UK and their subsequent spread around the world. They are now being used in various forms in USA, Canada, Germany, Spitzbergen (Norway), Poland, Ukraine, Russia, Australia, South Africa, Mexico, India, Egypt and China. The remote reading telltale system has coal mine intrinsic safety approval in Europe and the United States (MSHA) and the first commercial system was installed in a German colliery at the beginning of the year. Up to 100 dual height telltales can be connected, in a simple "daisy chain" configuration, to each of 4 underground interrogation units. These are connected through a twisted pair link to a surface computer which displays the readings, updates the database, generates warnings and provides the user interface. The paper describes the features and principles of operation of the system which differs considerably from conventional mine-wide monitoring systems. A European Community funded research programme is currently underway with the aim of developing improved alarm generating algorithms including evaluation of interfacing a neural network to the system. The paper concludes by exploring other potential applications of the system for ground control monitoring currently in development, including extensometry, stope closure monitoring and support load measurement.

Jan 1, 2001

By Khaled Morsy

Most of the available numerical codes simulating the reinforcing effect of fully grouted bolt are accomplished by a series of bar elements which are connected along their length to the host rock by shear springs and sliders. The spring-slider system represents the shear bonding effect between the bolt and the host rock. This paper describes a three dimensional finite element modeling technique for the 3D simulation of fully grouted bolt reinforcement. The proposed technique introduced a physical simulation for the fully grouted bolts. The bolt simulation is composed of a series of beam elements (representing the steel rebar) embedded in brick elements (representing the grout). Because of the complexity of the grout/rock interface, ABAQUS explicit code was used. A modification of the ABAQUS default friction model was conducted to consider the cohesion of the grout/rock interface. The interaction between the grout and the host rock is simulated by the modified friction model. To verify the proposed technique, a simulation of the pull out test we conducted. The results from the simulation were compared with former studies

Jan 1, 2004

By Peter Zhang

As simpler and easier reserves are exploited to depletion, underground coal mining continues in more complex geologic settings and conditions. Roof bolting for long-term entry stability is always a challenge. For long-term entries of a mine such as belt, track entries and return airways, roof could be under-supported if one uses the same bolting plan as in short-term entries or even seem over-supported with increasing use of longer, stronger bolts along with mesh, pans, straps or trusses. In most underground coal mines, roof deteriorates over time with cutters developing deeper in entry corners; roof sagging and popping out between bolts; entry width increasing due to rib sloughage; and separations developing deeper up into the roof. In deteriorated long-term entries, roof falls could occur at certain locations and re-bolting could be costly or sometimes unfeasible. Therefore, it is important to choose an appropriate bolting plan during development based on mining and geologic conditions if the entries will be used for the long-term. In both short-term and long-term entries, primary roof bolting is critical in building a strong and effective beam. But in long-term entries, supplementary bolting is necessary in certain cases to ensure roof stability can still be maintained even if a roof failure develops beyond the primary bolted horizon. This paper deals with roof bolting for long-term entries based on studying the progressive roof failure and roof falls under different geologic conditions. The paper presents bolting considerations in terms of bolt type, length, diameter and spacing as well as bolting strategy to minimize the risk of roof falls in long-term entries.

Jan 1, 2011

By Hui Chen

The paper describes the use of a physical model technique to investigate the failure modes of mine tunnels with reference to the in situ stress field. The characteristics of stratification commonly encountered within UK coal mining conditions have been taken into consideration. The investigation has indicated that the weak floor strata in coal mining tunnels, under the action of high rock stresses, will be subject to the floor lift problem in both predominantly horizontal and vertical stress fields. However, the failure mode and mechanism varies between the two stress field patterns. In the predominantly horizontal stress field, the tunnel floor tends to fail by gradual bending of the laminations, whilst in the predominantly vertical stress field, the floor tends to fail by a shearing action with the failed material lifting in the form of a block. The failure mechanism is discussed with reference to mechanics theory.

Jan 1, 1993

By William Monaghan

Over the period 2000 to 2003, roof falls have accounted for 4 to 14% of the fatalities in underground mining operations. The National Institute for Occupational Safety and Health (NIOSH) is conducting research to reduce the frequency, exposure, and risk of these events through an ongoing program of field and laboratory studies. One area of research involves the evaluation of polyurethane grouting technology that is commonly used to stabilize fractured mine roof strata. Concurrently, NIOSH is conducting work to evaluate the application of ground penetrating radar for advanced delineation of problematic mining areas. In this study, NIOSH partnered with Sub-Technical, Inc. who injected a polyurethane grout (also known as glue) into a roof area of NIOSH's Safety Research Coal Mine. The mine roof area was scanned using ground penetrating radar technology before and after grout injection in an attempt to determine the extent of grout infiltration. A comparison of the pre-and post-grouting radar records showed a significant change at a mine roof depth of about four to five feet. The interpreted radar records were then compared with drill core information, borescope evaluation of roof-bolt holes in the study area, and underground observations. At this site, the interpretations of the radar records correlated with data obtained from the core holes, borescope evaluations and underground observations. This study showed that ground penetrating radar technology can be a useful tool for detecting changes in mine roof due to the injection of the grout.

Jan 1, 2004

By J. G. Stewart

Polyurethane injection for the consolidation and control of broken strata is a common practice in the deep mines in Europe. The technology has gained wide acceptance in the coal mines of America since the introduction of RokLok polyurethane binder by Mobay Chemical Corporation in 1977. A popular and growing use for polyurethane consolidation grouting is to control the roof and prevent gob flushing during recovery of longwall face equipment. The technique provides many time and performance-related benefits compared to more conventional methods using wire mesh. Kitt Energy Corporation installed the nation's first 1000-feat longwall face in April and May of 1984. The face completed the first panel in October, 1984, and recovery was begun. Roof Strata consolidation for the recovery was accomplished by a RokLok polyurethane binder infection program designed and supervised by Halbach & Braun Industries. The application provided the most rigorous and demanding test to date of this method's performance and cost-effectiveness relative to bolts and wire screen.

Jan 1, 1984