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By P. Tsang

The mechanisms and functions of yield pillar are analyzed by employing finite element method that takes time dependent and plastic failure of rock into consideration. The design criteria for different geological conditions are discussed. The results provide further evidences that yield pillar design will improve floor or roof condition, especially when the floor and/or roof strata are weaker than the coal. Based on this study, the yield pillar size is determined for one cod1 mine located in southern West Virginia.

Jan 1, 1989

By Andrew G. Martin

In 1991, Modular Mining Systems began marketing a Slope Monitoring System (SMS) now installed at twelve mines. The SMS uses automated wire line extensometers to monitor field conditions frequently. Every few minutes, the slope displacement values and current temperature are transmitted to a central computer via a packet radio telemetry system. The desktop workstation receiving the data continuously provides updated displays of active slope monitors and generates audible and visual alarms if movement exceeds user specified thresholds. The computer system also generates numerous graphs and reports for quick analysis of slope stability. This work describes the functionality of the SMS, details improvements to the SMS hardware and software developed for several clients, outlines different installation techniques needed for wall rock and spoil monitoring, and interprets slope movement data using illustrations of stable and unstable conditions.

Jan 1, 1996

By Thomas M. Barczak

The resultant load vector is the representation of the forces applied to a longwall roof support element by caving strata into a single, quantifiable measure of support resistance. The relatively complex kinematics of the shield support prohibit a determination of support resistance simply by summation of leg forces. A method is being investigated by the Bureau of Nines to determine the resultant load vector by instrumenting supports with pressure transducers and strain gages to measure leg, canopy capsule, and lemnisoate link forces. This concept has been laboratory tested in the Bureau's Nine Roof Simulator. Functional relationships among variables have been assessed, and confidence intervals have been established for prediction of the resultant load vector parameters.

Jan 1, 1984

By Dwayne C. Kicker

This paper introduces the concept of design theory and methodology as applied to rock mechanics for more innovative and efficient design. This is a "frontier" research area in rock mechanics because although design is a fundamental foundation to all engineering branches, very little attention has been paid to this aspect in mining. In discussing the theoretical premise of design, this paper identifies the principles which form the basis of design activity. Also, from a practical viewpoint, the findings from a series of interviews with mine design engineers are presented. These findings lead to the identification of needs arising from overlooked design methodology in practice.

Jan 1, 1989

By Mike Amick

As virgin coal reserves dwindle, more and more mines are being opened in areas with previous mining. As addressed in this conference before, there are solutions available for designing mines extracting coal from reserves with known underground workings above or below the target seam. Often times, however, mining begins with limited knowledge of the underground workings. Even with careful planning, the proposed design has limited success because the ?facts? as presented during planning turn out not to be as accurate as one would like. At National Coal?s Mine #14, mining the Walnut Mountain seam, after driving the main entries in some 34 crosscuts, it was found during routine mine examinations that the pillars had separated from the roof in a few areas of the mains. Immediate roof conditions proved to be sound but questions arose concerning the stability of the main roof and floor. Core tests were done to better determine the rock strengths and characteristics. The paper presents the ideology used to tackle the effects of mining over old works in an operating coal mine. This paper discusses the analysis of the symptoms observed and the steps taken to ensure that this pillar separation was caused by the underlying old workings and that the separation was expected and controllable.

Jan 1, 2009

By Floyd D. Varley

Outbursts, the sudden and violent out- pouring of material into mine openings, is a ground control problem increasingly encountered in the United States as deeper seams are developed. Outbursts, caused by several mechanisms, are the result of the loading of ground support systems be- yond their designed capacity to failure, manifested in a violent release of stored energy. There are several types of outbursts typical to deep underground coal mines. - Outbursts of the coal face due to the presence of pressure abutments in close proximity to the face. These outbursts are usually accompanied by liberation of large amounts of methane. - Instantaneous convergence of the ribs and floor due to overloading and failure of ribs which lengthens the beam of floor strata sometimes resulting in a chain reaction failure of the mine floor. - violent failure of the confined core of a pillar due to rapid loading beyond the yield point of the pillar.

Jan 1, 1986

By Hamid Maleki

Roof falls are one of the leading ground control problems that adversely influence productivity and worker safety in underground mines, construction sites, and nuclear waste repositories. To control roof stability problems, the operator needs to have practical methods for measuring changes in strata conditions and criteria for defining the timing of secondary support installation. The objective of this study was to develop measurement techniques and criteria for assessing roof conditions at the mining face. An integrated rock deformation and tomographic imaging method was developed and applied at two sites in a western U.S. trona mine. It was shown that rock damage occurred at a deformation of 30 to 60 mm (1.2 to 2.4 in) (2 to 4 pct strain) and was associated with a significant (16 pct) reduction in wave velocity. Thus, supplementary support may be installed when there is a measured change in wave velocity of at least 16 pct. Tomographic measurements can be obtained in rocks rapidly, thus providing a powerful tool for analyzing growth of failure and for installing supplementary support systems in a timely manner.

Jan 1, 1996

By R. Kanniganti

The primary problems of pillar design in seams subject to interaction are due not only to difficulties in predicting loading, but also to the often unsymmetrical nature of the stress fields. The effects of uneven stress fields on failure patterns within a pillar were studied wing a two dimensional finite element method. Lower seams, including ultra- close mining situations were evaluated considering conventional pillars. Pillar case study data for under mining were analyzed to determine if empirical trends in stable pillar sizing could be derived. Recommendations for multi-seam pillar design are made, based on anticipated loading conditions within a pressure bulb. Applications of the research results are demonstrated using case studies

Jan 1, 1995

By Dakota D. Faulkner

Cable bolts anchored with traditional two component polyester resin cartridges have successfully supported difficult roof conditions in underground mines. Resin cartridges provide a costeffective cable bolt anchoring system with consistent anchorage performance characteristics when properly installed. The most commonly used cable bolts in the U.S. mining industry are 270 ksi (1,862 MPa), 0.7 and 0.6 inch (18 and 15 mm), seven-strand smooth cable with resin mixing bulbs (also known as bird cages), and 40- and 30-ton nominal capacity, respectively. However, there are other types of cable bolt anchoring systems aside from bird cages. Recently, many questions have been raised regarding the anchorage characteristics of various types of cable bolts. Keystone Mining Services, LLC (KMS), the affiliated engineering company of Jennmar Corporation, conducted a variety of controlled tests developed to closely model actual underground installations. These tests compared the performance of three commonly used cable bolt varieties, including plain, bird caged and indented seven-strand cable. Details of the laboratory testing and comparative analysis of the results are provided.

Jan 1, 2013

By J. C. Johnson

High-grade ore found in a shaft pillar at the Homestake Mine prompted a request for a Bureau of Mines study to determine if selected areas of the shaft pillar could be mined without jeopardizing the shaft. The design approach was to perform a finite-element study of the shaft pillar. Plan view and vertical sections were developed using two-dimensional, elastic, plane strain assumptions. Input into the model consisted of in situ stress measurements geologic mapping rock mass properties, and previous mining history. Results from the study indicated that ore-beating sections of the shaft pillar could be removed within the displacement tolerances acceptable for the shaft. Homestake engineers, using the results of the finite-element study and considering other factors, have designed and are beginning to mine the shaft pillar.

Jan 1, 1989

By Ray Brandon

The recent introduction of polymer geogrids for ground control purposes in underground mines allows mine operators more flexibility in choosing improved products for several applications. This is evidenced most clearly in longwall recovery operations. Since November, 1994, many mine operators have chosen Fortrac geogrid assemblies for the purpose of securing longwall recovery areas prior to shield removal. Fortrac is a lightweight, strong, and flexible polyester geogrid available in many strengths. Utilizing the "diamond" seaming method, different strengths of Fortrac geogrids are easily fastened together so stronger geogrids may be placed in the most critical areas and lighter geogrids in less critical areas.

Jan 1, 1996

By Gerald L. Stolarczyk

Safe and efficient coal extraction requires advanced radar subsystems that can be mounted on cutting drums of coal cutting machines. A look-ahead radar (LAR) must solve the radio geophysics problem of transmitting an electromagnetic (EM) wave through at least 20 feet (6.1 meters) of coal to an air- or water-filled void. The reflected EM wave must be processed in the radar electronics to determine the physical distance to the void. Horizon sensing enables selective cutting near the undulating sedimentary rock boundaries of the coal bed to address coal quality and ground control issues. Both sensors require subsystems for dynamic electric power generation, real-time measurement of drum rotation angle, detection of reflected EM waves, and processing to determine distance through coal and RF modem transmission of measured data to the mining machine. This paper describes the development of the subsystems and field test results.

Jan 1, 2006

By P. Gaskell

The paper examines the development of rock movement between a longwall extraction horizon and the surface for a fully caved mining situation. Physical modelling has been used to examine the mechanics of ground movement and the propagation of fractures around the longwall working. The paper discusses dimensional analysis, model construction and measurement technique and data processing of the results into effective graphical forms for further study.

Jan 1, 1988

By M. M. Zarnan

This paper presents the results of s study involving characterization of ground subsidence of abandoned lead-zinc mines in northeastern Oklahoma. Two coordinated efforts were undertaken: (1) collection and analysis of available data such as driller's logs, geologic cross-sections, structure contour naps and stratigraphic sections, and (2) development of a numerical (finite element) technique for prediction of collapse and mine subsidence. An in-depth analysis of a major collapse in the region was conducted. The factors which contributed to this collapse were identified.

Jan 1, 1989

By John W. Fredland

Many accidents occur in the mining industry as a result of the instability of material during handling and processing operations. Accidents due to dump point instability at stockpiles, and at spoil or waste piles, for example, occur with alarming frequency. Miners must be trained to be better aware of these hazards. Information on safe working procedures at stockpiles and surge piles is provided. Mine operators must review their training and operating procedures regularly to ensure that hazardous conditions are avoided.

Jan 1, 1993

By Essie Esterhuizen, Michael M. Murphy, Chris Mark

"The response of the surrounding rock mass to the creation of mining excavations determines the ultimate load on a pillar support system. In conditions where the ground is relatively soft and weak, the full overburden weight can be transferred to the pillar system. However, in stiffer and stronger rocks, a greater portion of the overburden load is transferred to the unmined coal barriers or abutments, and the pillar stress is reduced. This paper makes use of numerical models to examine the interaction between typical pillar systems and the surrounding rock mass for weak and strong geological conditions at various spans and depths of cover. The concepts of structural failure and functional failure of pillars are used to assess pillar performance when pillars are deformed beyond their peak resistance. The results show that the span-to-depth ratio is an important factor in determining the pillar stress and the ultimate deformation of pillars. The ultimate pillar strain appears to be closely related to the functional success of pillar systems.INTRODUCTIONThe National Institute for Occupational Safety and Health (NIOSH) was requested by Congress to study the safety of retreat pillar mining following the Crandall Canyon Mine disaster that took place near Price, Utah in August, 2007. As a part of this study, an investigation was made to better understand how the rock mass responds to the creation of mining excavations and how the loads are distributed among the pillars and the surrounding abutments or barriers. The outcome of these investigations contributed to the development of a modified loading model for the ARMPS-2010 (Mark, 2010) method ofretreat mining pillar design.Structural and Functional Failure of PillarsPillar design is typically conducted by estimating the pillar strength and stress, and then sizing the pillars so that an adequate margin of safety exists between the expected strength and stress. The pillar strength can be defined as the maximum resistance of a pillar to axial compression (Brady and Brown, 1985). If a pillar is loaded beyond its peak strength, load shedding or yielding can occur, and the pillar is considered to have failed as a structure. Structural failure generally refers to a loss of loadcarrying capacity."

Jan 1, 2010

By M. K. Larson

The U.S. Bureau of Mines and Cyprus Shoshone Coal Corp. conducted a study of deformation mechanisms in strata around a longwall gate road system at two sites in an underground coal mine near Ham, WY. Of particular interest was timedependent (on the order of several months) roof deformation. Strata above and below the coal seam are very weak, carbonaceous mudstones that have cohesionless planes of weakness oriented along bedding, which dips approximately 8° to 16° at N 49° E. An extensive array of instrumented roof bolts, roof extensometers, and biaxial stressmeters were installed during development mining at these two sites under variable depth of cover [I83 to 335 m (600 to 1,100 ft)], seam dip (10° to 16°), and top coal thickness [estimated to be 0.30 to 0.61 m (1 to 2 ft) on the downdip side of the entry]. Bolt load and roof deformation histories during the entry development periods were compared. Bolt loads at site 2 were less than those at site 1, which is consistent with the amount of overburden. Deformation magnitudes at site 2 were similar to those at site 1, but deformation rates were approximately 44% greater and nearly constant over time, whereas the deformation rates at site 1 decreased exponentially over time. Also, most deformation occurred above the bolt horizon at site two, but extensometer measurements at site 1 showed that a significant amount of deformation occurred within the bolt horizon. The most significant factor that influenced bolt tension and timedependent strata deformation was strength and Young's modulus of the strata. Finite-difference models support this conclusion.

Jan 1, 1996

By P. M. Lin

A subsidence monitoring program over a longwall panel was established (1) to explore the impacts of dynamic subsidence on the ground surface and structures and (2) to correlate the movements between the structures and their corresponding points on the ground. The program consisted of a transverse line survey and a series of survey points around the structures on the ground surface and their corresponding survey points on the exterior walls of the structures. The vertical displacement of each point can be expressed as: where Si is subsidence at i point; Sif is the final subsidence at i point; F is face location in terms of distance from the i point and SD is seam depth. The development of subsidence can be divided into five stages. Damage to the structures starts at the beginning of the second stage, The maximum damage developed in the middle of the third stage. The relationship of the vertical displacement between ground surface and structures can be expressed as: G = aH + b where G is ground displacement, H is structures displacement, and a and b are constants. The relationship is linear. However, the strain is non-linear and more complicated because of the interaction of the ground surface and structure.

Jan 1, 1989

By G. J. Hasenfus

This paper presents the results of an extensive hydrological and geomechanical monitoring program which Gas conducted at a longwall coal mine in W. Va. The field program included monitoring of groundwater levels. overburden hydraulic conductivity, overburden movement, and surface subsidence relative to the passage of the longwall. Overburden geology and rock strength characteristics were evaluated prior to mining. A postmining corehole was drilled to evaluate main roof fracturing. An overburden response model is proposed based on the correlations between hydrological and geomechanical data.

Jan 1, 1988

By Mike Watts

Since the first longwall panel development started in the new mining, (Southern) area, a number of roof falls have occurred in L1 and L2 gate-entries. The longwall was delayed for a total of two months due to roof falls in the gate-entries. To prevent roof falls in future gate-entries, an extensive ground control analysis has been conducted which incorporated regional horizontal stress and panel orientation, tectonic stress, roof strata, surface stream valleys, etc. Results of the analysis indicate that a combination of these factors caused the roof falls. Therefore, a Roof Instability Rating (RIR) system was constructed to predict the roof condition and identify areas where supplemental support should be implemented. The L3 gate-entries, supported by the prediction using the RIR system, have not experienced any roof problems.

Jan 1, 2000