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By Lewis A. Martin

Instrumented cable bolts developed at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health were used in conjunction with existing ground control systems to monitor rock mass loads at Tg Soda Ash's trona mine, Granger, WY. Axial and shear loads were determined to levels of strain gauge accuracy of ±5 N or ±5 microstrain. These gauges were embedded in a remanufactured king wire that replaced the conventional king wire. Cable bolt performance, quality of grout, and installation techniques were also assessed. By using instrumented cables, a mine operator can determine axial load along the cable at predefined gauge locations. These data provide necessary input for an operator to design a safer working environment for miners.

Jan 1, 2001

By Salah Badr

Longwall mine layouts with their entries, chain pillars, face support systems, advancing mining faces and compacting gobs represent a geomechanically complex mining operation. Geomechanical aspects are further complicated if mining occurs at depths exceeding 350 m, as stresses induced adjacent to the excavations exceed the unconfined strength of the coal and result in fracturing of the sidewalls of entries and chain pillars during development. Traditional methods of geomechanical analysis, whether empirical or numerical, are unable to represent such behavior satisfactorily. Achieving some meaningful results requires explicit representation of the three-dimensional boundary conditions and implementation of appropriate constitutive models for the rock mass. This paper describes a numerical modeling methodology for deep longwall mines operation using the FLAC3D finite difference de, which incorporates a strain softening constitutive law for modeling failure of coal. An important aspect of the modeling method includes accounting for stress relief provided to the pillars and entries due to the increasing load applied to the compacting gob.

Jan 1, 2003

By Olayemi Akinkugbe

As coal production in the United States continues to increase, the availability of unexploited or virgin fields continue to diminish. As a result, mine operators are forced to mine in unfavorable or multiple seam conditions. The safe, productive exploitation of coal in multiple-scam conditions requires specific design technology. At present, there are two general design approaches that the mining engineer can use to analyze the multiple-seam problem in question, either simple empirical relationships or complex numerical models. Often, the empirical approaches are too general for the specific problem and the numerical models require too much time and expertise for the practicing engineer. This paper introduces a new computer program, Lam2D, designed specifically to enhance multiple seam mining operation by quickly and easily calculating, stress, displacement, subsidence, horizontal strain and safety factors associated with multiple seam mining situations. The program implements a simplified, two-dimensional boundary-element method in order to model the complex multiple-seam stress and displacement interactions. Most importantly, the program is greatly simplified by incorporating automatic overburden, interburden, coal and gob property generation thereby reducing typical modeling and solution time to a few minutes.

Jan 1, 2004

By Chris Mark

Pillar recovery continues to be a significant ground control hazard. During the past decade, 10 miners were killed during pillar extraction operations in southern West Virginia. Studies conducted during the past decade have identified a number of "risk factors" that can be used to evaluate pillar extraction plans: Cut sequence Final stump Timber or Mobile Roof Supports Roof bolting Intersection span Depth of cover Roof quality Age of workings For each of these factors, rock mechanics science suggests which alternative would be expected to be more risky. For example, numerical models were used to evaluate different cut sequences, and indicated that less roof convergence occurred with the outside lift method than with the Christmas tree in the particular environment simulated. For many of the risk factors, accident statistics confirm the science. One finding was that currently almost 70% of the retreat coal in southern WV is being mined with MRS. In contrast, timber supports were used in 70% of the past decade's pillaring fatalities. This paper discusses each of the risk factors in turn, presents the relevant accident statistics, and shows how the risk factors can be combined to estimate the overall hazard. It also addresses the use of pillar design to minimize the risk of global stability hazards including squeezes. massive collapses, and bumps.

Jan 1, 2002

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992

The formation of tension cracks in slopes is well known. Analytical studies require the determination of a critical slip surface for any slope and, in doing so, it is necessary to consider an appropriate tension crack. A general yet simple approach is proposed for identifying the critical slope failure surfaces associated with critical tension cracks. The minimum factor of safety consequently reflects both safety against shear sliding and tensile failure. In general, the critical slope failure surface for a slope determined in this way differs from that which is obtained without optimizing the position and depth of the tension crack. In particular situations, the difference is significant.

Jan 1, 1989

By Y. Luo

An extensive subsidence research program conducted by the authors has greatly improved the accuracy and the capabilities of subsidence prediction under complicated mining and surface conditions. The techniques developed for assessing and mitigating subsidence influences on various surface structures have been applied in numerous cases with a very good re- cord of success.

Jan 1, 1997

By Hamid R. Nikraz

This paper reports the finding of a comprehensive investigation carried out to evaluate the compaction characteristics of saturated sandstone aquifers in the Collie Coal Basin under conditions anticipated during dewatering operations. A triaxial technique has been adapted to evaluate the compaction characteristics of the sandstone aquifer in the Collie Basin. This technique, which allows the strata stress regime to be reproduced by triaxial loading with zero lateral strain, also provides a precise evaluation of lateral stresses and consequently Poisson's ratio under in situ conditions. The paper contains a description of the equipment commissioned, test techniques, results, analysis and interpretation of the data obtained. The testing evaluation techniques are general in nature and can be applied to field situations in locations where similar weak sandstones occur.

Jan 1, 1993

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Christopher Mark

After the United States and Australia, South Africa has the largest. modern underground coal mining industry in the world. Historically, South Africa has been the cradle of many innovations in ground control, particularly in the area of pillar design. Today, South African mines are grappling with many of the same issues faced by their U.S. counterparts, including safety during pillar retreat operations, selecting the proper roof support, and maintaining long-term pillar stability. The author recently visited seven underground mines in the Mpumalanga, Free State, and KwaZulu-Natal provinces. Mining methods included longwall, room-and-pillar with continuous miners, and drill-and-blast. The mines represented a cross-section of geology and ground support practices. At each mine evaluations were made of the Coal Mine Roof Rating (CMRR), roof support practices, pillar design methods, and general ground control experience. The coal seams observed were generally thick (2-5 m), the depths of cover moderate (less than 200 m), and the roof rocks relatively strong (CMRR greater than 45). Unfortunately, the South African roof fall fatality rate is approximately three times greater than in U.S., perhaps in part because roof bolts seem to be more widely spaced in most South African mines. Roof bolting equipment was generally outdated, and the quality of bolt installation was a major concern. Pillar design, on the other hand, is quite advanced, and pillar failures are rare. Several novel partial pillaring techniques are employed, including "checkerboard" mining in which every other pillar is removed, 3-cut systems, and high-extraction mining where engineered final stumps are left by design. The paper also offers observations on ground control management techniques (including recently introduced Codes of Practice), the role of the mine inspectorate, and the status of the research community.

Jan 1, 1999

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By Robert C. Garson

The Mine Roof Simulator at the DOE'S Mining Equipment Test Facility is described. Its use in the research and development on the behavior and response of longwall roof supports to simulated mine environment is investigated. Advanced technology analysis methods such as finite element modeling, strain gage instrumentation, and photoelastic techniques are investigated for application in roof support design two case studies are presented of test data and results.

Jan 1, 1982

By Christopher Mark

Mines with exceptionally low-strength roof (UCS <3,500 psi and CMRR <40) are much more likely to struggle with roof falls than other mines. Weak-roof is a particular problem for many room and pillar mines in the Midwestern and Northern Appalachian coal basins. Traditional roof support techniques are often not up to the challenge at these operations. This paper focuses on two mines, one operating in the Upper Freeport seam and the other in the Herrin No. 6 seam. Together, the two mines had more than 300 roof falls during a recent 5 year period. Each has experimented with a variety of roof bolt types and lengths, and with different supplemental supports. Detailed statistical analysis was conducted to determine which support combinations have proven to be most effective. Geology, horizontal stress, and time are also important at both mines. Successful control techniques have included; ? Identifying particularly troublesome lithologic roof units; ? Installing longer and stronger roof bolts; ? Installing supplemental support in intersections, and; ? Limiting the duration that a panel remains open. The lessons learned from these mines, and others like them, could help improve ground control safety for the entire U.S. mining industry.

Jan 1, 2004

By Brad Seymour

In cooperation with Cyprus Twentymile Coal Co., researchers from the National Institute for Occupational Safety and Health (NIOSI-I), Spokane Research Laboratory, conducted a study at the Foidel Creek Mine, an underground coal mine near Oak Creek, CO, to evaluate the stability of underground working conditions as a longwall advanced through a series of backfilled cross-panel entries. Instruments installed in the cross-panel entry pillars, backfill, longwall panel, and adjacent headgate entry were continuously monitored by a computerized data acquisition system as the longwall approached and advanced through the backfilled section of the panel. Data collected from these instruments were analyzed to determine the magnitude and direction of the secondary principal stress changes ahead of the longwall face, deformation and closure of the headgate entry, and stability of the cross-panel entry pillars and backfill. A three-dimensional, boundary element model was calibrated to accurately predict mining-induced stress changes and the distance they occurred ahead of the longwall face. As the longwall advanced through the backfilled entries, most of the mining-induced load was supported by the cross-panel entry pillars. The backfill provided stability for the root and Moor of the in-panel entries and also confined the entry pillars significantly improving their load-carrying capacity. Comprehensive information derived from backfill material property tests, instrument data, underground observations, and numeric modeling results document the safety and stability of the 8-Right minethrough and should provide beneficial case study information fur other longwall backfilling applications in the future.

Jan 1, 1998

By S. S. Peng

This paper presents a convenient way for the preliminary design of the shield supports. The locus equations which are applicable for all kids of shield supports are derived. These equations facilitate the statics analysis of the shields. An indirect method in determining the coefficient of friction, the external toads and their locations are also developed.

Jan 1, 1982

By Dave Forrester

Mining subsidence over shallow abandoned coal mine workings is unfortunately not an uncommon experience in the coalfields of North America and there is a significant knowledge and literature base addressing the phenomenon and related issues. From time to time, however, a case history arises which is outside the more routine occurrence. This paper addresses one such case and presents an overview of a subsidence Event above abandoned coal workings, which badly damaged a two-story High School building. The subsidence at the Cape Breton & Victoria Regional School Board (CBVRSB)&apos;s MacDonald High School in Dominion, N.S., Canada on October 30, 2002 led to a detailed investigation. It was concluded that the ground movement and associated building damage had been caused by mining subsidence in underlying old underground coal mine workings of the 1890s, only 150 ft deep at the School. The Event was not isolated and a similar but smaller event happened nearby a few months later. The paper describes the events and summarizes the subsequent investigations and findings. The 17-year old High School building was eventually demolished.

Jan 1, 2004

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

By Lisa Burke

In underground coal mines, concrete block stoppings are widely used to control mine ventilation. Although stoppings are not intended as roof support, they are subjected to roof to floor convergence, and may fail as a result of this loading. Premature failure of stoppings can significantly affect mine ventilation, limiting the amount of fresh air reaching the working faces and increasing the risk for methane explosions. Softening materials are often used in stoppings to reduce the damaging effects of roof to floor convergence. However, to date, research has not focused on the behavior of stopping construction materials subjected to roof or floor loading, leaving the design process to trial and error. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The first series of physical models were constructed using standard CMU (concrete masonry unit) blocks and tested in a load frame. The behavior of these walls was simulated with a distinct element model. A key finding was that very small variations in block size and irregularities in block shape significantly affect wall performance. These variations, which are inherent to the type of blocks typically used underground, create stress concentrations that initiate the failure of stopping walls. A second series of models employed concrete stoppings that incorporated woos planks, a shredded wood material, and foam planks like those commonly used underground. Numerical models were again able to match the physical test data. Analysis of the softening layers used in stopping construction indicated that the strength and stiffness of the material relative to that of the wall is crucial in determining the amount of roof to floor convergence the wall can withstand prior to failure. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment. Testing was done in a laboratory setting with walls constructed on flat, parallel surfaces and subjected to uniform loading at a constant rate. Parametric studies with the model indicate that in this ideal environment it is possible to control the amount of vertical displacement the stopping can withstand by adjusting the number of softening layers built into the wall and the thickness of the layers. Additionally, under these conditions, the position of softening layers within the wall appears to have little effect on the amount of displacement walls can withstand prior to failure.

Jan 1, 2004

By Hamid Maleki

A novel ground control program has been developed at Plateau Mining Company through five years of geotechnical investigations. The study was initiated to determine the causes for roof stability problems and to develop alternative support and monitoring systems for increased safety and productivity. From systematic roof fall observations, extensive geologic mapping, roof drilling and monitoring ground movements along gate entries of four longwall panels, a sand- stone channel system was detected and shown to impact roof stability. In particular, underlying mudstones up to seven feet thick experienced instability due to differential compaction and load transfer from the gob. Roof bolter operators were trained to report roof lithology for mapping channel positions into the roof and for selective use of longer bolts. This boosted operator morale, and improved ground conditions. Monitoring rates of roof movement is used in these areas for detection of instability and for application of secondary support: This has significantly increased safety and productivity.

Jan 1, 1987