Search Documents

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 Mictck Rataj

Methods of rock reinforcement in Australian coal mines are being advanced progressively through the better understanding of rock failure mechanisms around roadways and the-development of improved bolting hardware and installation techniques. It has been recognised that to improve the performance of rock reinforcement systems, the stiffness of the system as a whole has to be improved. New style bolts and accessories together with high pre-tensioned roof and cable bolts are being developed as a result. A number of the new techniques used to stabilise the roof, especially in weak roof/high horizontal stress areas have been site proven and are now currently used on a routine basis as a means of improving development rates and gate road stability during longwall retreat.

Jan 1, 1997

Numerical simulation for microseismicity due to mining at one of the collieries in Australia was carried out and the results were compared to the observed microscismicity. 3-D Stress analysis showed stress concentration on the edges of the mined areas and on the coal pillars- The stress concentration ahead of the coal face was biased to the tailgate. This is similar to the observed distribution of microseismic events. Stress reduction above and below the mined area was seen. Tensile stress was observed mainly above the mined area. It was realized that compressive failure due to nearly vertical maximum principal stress and nearly horizontal minimum principal stress in retreat direction (NS) might occur ahead of the face. Tensile failure due to nearly vertical minimum principal stress above the mined area was expected. Compressive failure due to nearly horizontal maximum principal stress in EW direction and nearly vertical minimum principal stress above and below the mined area were also expected.

Jan 1, 2001

By Gift Makusha

Surface subsidence associated with coal mining activities in the Mpumalanga Province of South Africa changes the natural environment in several ways, and current challenges for mining companies include rehabilitation of the natural environment and the prevention of further degradation. To monitor the spatial and temporal evolution of surface subsidence, traditional fieldbased monitoring approaches such as GPS and spirit leveling are employed at a number of locations. However, the resulting measurements are point-based, and frequent revisitations are necessary to map the evolution of surface subsidence basins over time. To address these limitations, differential interferograms derived from repeat-pass satellite-borne synthetic aperture radar (SAR) systems were tested for their ability to measure and monitor surface deformation. The SAR data was captured by the European ERS-1 and ERS-2 satellite and covered a timeframe between 2008- 09-15 and 2008-09-15. The resulting interferograms revealed several features indicative of surface subsidence. Ground truth data confirmed the presence of a subsidence basin, detected using differential interferometry techniques during the 35-day period between April 12, 2008 and May 17, 2008; a maximum vertical deformation of 3.2 cm (1.3 in) was recorded. Interferometric monitoring revealed an eastward migration of the subsidence basin between June 2, 2008 and September 15, 2008 with an additional 4.7 cm (1.9 in) of subsidence observed. This migration coincides with the advance of the working face of the mine during this period. Finite element modeling will be performed in order to model the rates and evolution of surface subsidence haloes as underground mining advances. These presented results demonstrate the ability of interferometric synthetic aperture radar techniques to measure surface subsidence as well as the monitoring of the evolution of subsidence basins over time. This implies that the technique could be included, together with traditional field-based surveying techniques, in an operational monitoring system. With knowledge on deformation rates and subsidence basin evolution, informed decisions on current and future infrastructure development can be made and remedial actions and prevention strategies can be formulated for the problems associated with environmental degradation.

Jan 1, 2011

By Reha Ozel

The stability in longwall faces depends on the interaction beman the roof strata, face supports and floor strata. The load distribution on the other hand, depends upon relative stiffness of the three supporting elements namely, the coal face, support and gob. Therefore, in defining support requirements, it is particularly important to analyze the strata loading, the face convergence and the controlling mechanism provided by supports. The controlling mechanism however, depends on the type and characteristics of supports used in long- wall faces. The design guidelines presented in this paper have bean developed based on a number of strata control oriented projects carried out in Turkey, load and convergence measurements carried out in 10 longwall faces, in Zonguldak Coal Region, and procedures given in literature. Evaluation of load and convergence measurements, and analyses of face supports were carried out by means of a computer program, called L0NGWAI.L-MEW, developed by the authors. The output of the design analyses include the following: (1) estimation of support capacity for roof supports namely; individual hydraulic props, chock-type power supports and various types of shield supports, and (2) analysis of loading conditions and response of shield support components in terms of stability and structural integrity.

Jan 1, 1990

By K. Biswas

So far, all pillar design methods or approaches consider the coal seam as a uniform structure From some mine visits in northern Appalachian Region. it is found that some coal seams have binder or parting material within the coal scan with varying thicknesses and numbers- And it has also been found that these kinds of hinder materials tire prone to weathering when exposed to ambient mine moisture In this paper an attempt has been made to study the weathering susceptibility of parting materials and coal when exposed to moisture and a two dimensional finite element analysis has been carried out to study the effect of the presence of partings on the structural integrity of the coal pillar in terms of short-term and long-term stability.

Jan 1, 1995

By William Gray

It has been ten years since the Mine Safety and Health Administration (MSHA) promulgated a safety standard requiring the use of Automated Temporary Roof Support (ATRS) systems in underground coal mines. The use of ATRS systems with roof bolting machines and continuous miners equipped with integral bolters has all but eliminated fatal roof fall accidents associated with manually setting temporary roof support. The last ten years have seen an evolution of the ATRS systems themselves and of MSHA's policy regarding ATRS systems. These changes are a response to operational and safety issues regarding ATRS systems often brought about by the ever-changing coal mining environment. As a result, MSHA's Technical Support personnel receive numerous inquiries from all segments of the coal mining industry on various ATRS topics. These requests for technical assistance range from an interpretation of a facet of the ATRS standard to an in-depth evaluation of a prototype ATRS system. This paper will address the aforementioned evolution of ATRS systems and will also summarize the factors MSHA Technical Support uses to evaluate these systems in the field. In addition, some of the more common misconceptions regarding the design, use, and regulatory requirements regarding ATRS systems will be covered.

Jan 1, 1998

By David Conover

Knowledge of the magnitude and direction of the horizontal secondary principal stresses is a critical factor in designing the layout and mining sequence of underground openings. Typically, horizontal stress measurement has been accomplished using overcoring or hydraulic fracturing. Hydraulic fracturing can be accomplished from surface boreholes prior to development, but the technique relies on numerous assumptions regarding the rock mass and the stress field, and interpretation of results is somewhat subjective. Traditional overcoring provides a more precise measurement, but requires underground access. To allow for precise measurement of the horizontal stress field from surface boreholes, the In-situ Stress Tool (1ST) was developed by SIGRA Pty of Brisbane, Australia. Used in conjunction with HQ wireline coring, the 1ST contains a battery-powered data logger that records tool orientation and monitors six diametral deformations as the tool is overcored. The IST has been used successfully for several years in Australia, but has only recently been introduced in the United States This paper discusses how the IST was used to measure the pre¬mining horizontal stresses at a western U.S. coal mine. Seven successful measurements were obtained in two holes at depths ranging from 250 to 800 ft. Results were consistent, showing moderately biaxial stresses greater than those expected from gravity loading, but relatively low compared to rock strength. In addition to describing the overcoring equipment and procedures, various problems encountered during testing and the accuracy of the results will be discussed.

Jan 1, 2004

By Y. P. Chugh

About 60-70% of the underground mined coal in Illinois is extracted using a room-and-pillar mining method that permits extraction of about 50% of the coal. The remaining coal is left behind in the form of support pillars to control surface and subsurface movements. Illinois coal seams are typically associated with water sensitive weak floor strata whose thickness varies 0.31 - 1.3 m. The current mining geometry mostly uses pillars of constant size within a mining area. This traditional practice suffers from the disadvantage that pillars within the center region are subject to much higher loads than the pillars located on the outer edges of the panel. This results in a higher incidence of rib spalling, and roof falls, creating a potentially hazardous work environment. In addition, this concentration of loading at the center of the panel provides initiation points for floor failure to occur. The primary goal of this study was to reduce ground control problems and production costs in Illinois mines by considering alternate mining geometries. The development and subsequent field demonstration of an alternate geometry is presented in this paper.

Jan 1, 2004

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 Hamid Maleki

The U.S. Bureau of Mines implemented a monitoring program in a western U.S. coal mine for the evaluation of both conventional and alternative support systems. The conventional support consisted of 2.5-m-long vertical bolts installed in an 5.5-m-wide entry. The alternative system consisted of 2.5- and 3-m vertical and inclined bolts installed at a higher density during widening of a cut from 3.6 to 5.5 m. Roof deformation, roof stress, rib deformation, and bolt loads were monitored during the 2-month test period. In addition, roof failure patterns and lithology were characterized within the six-entry main test section. Results were analyzed using roof deformation and failure profiles, roof stress profiles. bolt loading and bending moment diagrams, and numerical models. The mechanics of roof deformation, failure, and support reaction in three zones in the mine roof were characterized. Significant improvements were shown in roof stability in the section using the alternative system. Results are encouraging and indicate the effectiveness of alternative support systems and excavation techniques for limiting inelastic deformation of roof in highly stressed underground mines.

Jan 1, 1994

The purpose and current practice of cable trusses in underground coal mines is briefly outlined and details of various cable truss designs are presented. The main part of the paper describes the in-situ performance of cable trusses installed as secondary supports in gateroads and recovery roads of longwall coalfaces. The performance analysis is based on the results of in-situ monitoring carried out at a number of sites at two underground coal mines within Australia. Truss performance has been assessed as load developed in relation to roof convergence, face position and time. Additionally, the influence of strata conditions, installation pattern/density, roadway layout and geometry have been taken into account. An analytical model has been developed to predict truss loads. The principal variables are listed and the basis of the model is described. Based upon behaviour observed in the field the model has been progressively upgraded and validated against measured performance. Guidelines on truss performance and factors which influence it are presented and the implications of these to the use of cable trusses is discussed. Possible methods for improving truss performance in certain strata conditions are outlined and some comments are made on the value of cable trussing compared to cable bolting in these situations,

Jan 1, 1992

By H. Yavuz

A Numerical modelling study using the two dimensional finite difference code "FLAC" was performed for investigating the ability of numerical modelling to predict the performance of yielding pillars. The model was verified by comparing the stress and deformation data obtained from a yield pillar trial application at Welbeck Colliery, a UK coal mine. Two entries with-a 10 m intervening pillar were modelled. Both for development and for side abutment loading stages, the vertical stress measurement data over the pillar and side abutment and deformation measurement data in the roof and floor of the trial entry were compared with the results obtained from the model. Two difficulties were encountered, unavailability of data for caved rock and for the yielding pillar material. These were overcome by back analysis to estimate the unknown field properties of the pillar material and caved rock from known properties of these materials. Results from this model are given and the predictive capability of a finite difference model for two entry yield pillar applications is discussed.

Jan 1, 2001

By John K. Wood

The development of an expanded cement product represents a major advance in the technology of roof control systems. Such a product is gaining recognition as a valuable tool in today's cost- conscious coal mines. The National Coal Board's Bretby Research Lab, in conjunction with Polement Ltd., developed the first lightweight cement for use in the mines of the United Kingdom. The product, Aqualight, is a low-density (about 12.5 PCF) cement with a typical compressive strength of about 19 p.s.i. The final product increases in volume by approximately 15 times its dry powder volume. Aqualight gels in about 2 minutes, becomes self-supporting in 15 minutes, and fully cur& in 24 hours. When subjected to a load, the "cells" within its bubble structure collapse so that the product gains in density and compressive strength while retaining its integrity. Traditional methods to contain and support roof fall cavities through cribbing and blocking are both dangerous and labor-intensive. Aqualight represents a significant advantage over such traditional methods. It is non-toxic, non-cmbustible, and very easy to install. Aqualight can be pumped to the worksite from re- mote locations (as much as 600 feet), thereby reducing danger to men and equipment. Addition- ally, the strength of the cured product can be varied by adjusting the expansion ratio and resulting density. A compressive strength of 150 p.s.i. can be obtained in this manner. Although primarily designed to fill cavities resulting from roof falls. Aqualight has found a wide range of applications in mining industries around the world. In addition to cavity- filling, Aqualight has been used for ventilation stoppings, fire stoppings and emergency fire control, backfilling between arches and the roof and ribs, and backfilling of old workings. Since its recent introduction into the United States. Aqualight has been used by MICON SER- VICES, INC. to fill several roof fall cavities along headgate entries. In all cases, the cavities were contained and the longwall moved through without problems. Roof Pall cavities along main haulage entries and at shaft/slope bottoms have been successfully filled. MICON has also installed Aqualight as backfill material around arches. A current major project is the installation of trial ventilation seals using Aqualight as an alternative to traditional block and mortar construction. Pending MSHA approval, MICON anticipates the use of Aqualight in the construction of both ventilation seals and stoppings. In addition to new construction, Aqualight can be employed to repair stoppings damaged by roof convergence and floor heave. In labor savings alone, this is very attractive for remote stopping repair. Among the many other uses of Aqualight is the backfilling of old auger holes, which will support the highwall and allow access to coal re- serves which are presently sterilized.

Jan 1, 1986

By J. Riefenberg

U.S. Bureau of Mines (USBM) researchers are developing a personal computer-based hazard mapping system for use in underground coal mines. Hazard mapping is rapidly gaining interest as delineating areas of possible instability is becoming more complex. The development of a hazard map is based on the consensus that combined effects of local and regional stresses, roof, floor, and rib quality, geology, faulting, presence of ground water, etc., all contribute to stability of the ground. In order to demonstrate the concept, a hazard map was developed that combined stresses anticipated to occur in a multiseam setting using numerical modeling, and the newly developed Coal Mine Roof Rating (CMRR) where roof quality ratings in untested areas were modeled using geostatistical methods. Ten borescope readings from an underground mine were used to compute CMRR's for the geostatistical analysis. CMRR's for the mine ranged from 57 to 77, and are considered in the moderate to strong range. A hazard map was developed by combining the stress effects of interseam mining, scaled to weigh equally with CMRR's, and CMRR geostatistical analysis results. This hazard map demonstrates a synergistic effect between the mining-induced stress fields and the roof quality ratings, indicating, as expected, a greater hazard index in areas of high stress and lesser roof quality.

Jan 1, 1994

By Christopher Mark

Mine planning for a new reserve is based on information obtained from exploratory coreholes. A critical component of an exploration program is the geotechnical evaluation. Poor assumptions about roof conditions greatly add to the risks of mining. Rock mechanics testing is central to a geotechnical exploration program. Typically, 3 to 5 uniaxial compressive strength (UCS) tests are made to characterize a particular roof unit at a given corehole. The average (mean) of these tests is taken as "The UCS" for that location. Isopach contour maps are then used to show spatial trends in roof strength. Two issues are raised by this traditional approach. The first is due to the large variability in UCS values that is typical even within a single unit from a single hole. The average UCS might be higher at Corehole A than it is at Corehole B, but the difference may not be statistically significant. The second issue is whether widely spaced coreholes can identify valid spatial trends in rock strength. The answer depends upon whether rock strength changes over distances that are longer or shorter than the corehole spacing. This is a classical geostatistical problem. While geostatics have been used to investigate many coal quality parameters, they have seldom been used to evaluate rock strength. This paper describes an extensive investigation of these issues conducted by the National Institute for Occupational Safety and Health (NIOSH) in collaboration with Peabody Energy. The study employed the Peabody Rock Mechanics Data Base which contains more than 10,000 individual test results. Data from four important roof units were subjected to statistical analysis: ? Brereton Limestone above the Herrin 6 seam (Illinois) ? Turner Mine Shale above the No. 9 Seam (Kentucky) ? Sandstone above the Eagle Coal (West Virginia) ? Shale above the Eagle Coal (West Virginia) The study did not find significant spatial trends in rock strength in any of the cases. Perhaps there are none, or perhaps the exploration coreholes were just too far apart to see them. These results have valuable implications for the design of geotechnical exploration programs.

Jan 1, 2004

By Simon H. Prassetyo

Violent failures of coal pillars, known in practice as coal mine bumps, have long been a subject of investigation. Many field investigations have considered geological conditions that create high stress in the pillar to be the main causative factor leading to bumps. In recent years, constraint at interfaces has been observed to be associated with the occurrence of coal bursts. This paper shows how interface friction and width-to-height (W/H) ratio affect the degree of violence of coal specimen failure. A series of UCS tests on 178 coal samples from bump-prone (Utah, called UT coal) and non-bump-prone (West Virginia, called WV coal) mines were conducted. Three violent failure parameters? peak sound pressure level, core zone failure, and ultimate stress? were used to assess the violence of failure. The degree of violence were investigated at varying interface frictions (high: µ = 0.40, medium: µ = 0.22, and low: µ = 0.13) and W/H ratios (W/H = 2, 3, 4, 5, 6, 7, 8, 10, and 12). A simulation series of modeled coal specimen under uniaxial loading was also conducted using the finite element method to observe the equivalent plastic strain distribution experienced by the modeled coal specimens. It was found that the violence of coal specimen failure depends on W/H ratio and interface friction. Interface friction has more significant influence on the violence of failure at high W/H ratios than it does at low W/H ratios.

Jan 1, 2011

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 Thomas R. Scotese

A rock mechanics instrumentation and monitoring program was implemented during pillar extraction at Gulf Mineral Resources' Mt. Taylor Mine, the deepest uranium mine in the U.S. Three types of monitoring were employed: (1) drift convergence around stopes (using a portable tube extensometer), (2) stress changes in pillars (using vibrating wire stressmeters in horizontal boreholes), and (3) load changes in haulage ways under stopes (using vibrating wire load cells in jack stands). Results include convergence-time graphs, convergence contour maps, stress-time graphs, a stress increase contour map, load-time graphs, and abutment load limit maps. Major factors influencing results include (1) size of pillars around the extraction area, and (2) presence and orientation of a major fault adjacent to one extraction area. These factors, combined with pillar extraction, produced evidence of load transfer over 120 m from a stope measuring only 55 m x 45 m. Vertical stress increases of 7 to 17 MPa (1000 to 2500 psi) within 60 m of the stope were obtained using the stress- meter manufacturer's method. The fault apparently prevented load transfer in the down-dip direction but increased load transfer in the up-dip direction. Convergence rates of up to 1.3 cm/day (0.5 in/day) were measured at two stations. Instruments were generally reliable despite adverse underground conditions. The instrumentation and monitoring program at Mt. Taylor provided a warning system against ground control problems and a characterization of ground behavior for future development and extraction of pillars. Recommendations are made for future mining and instrumentation practices.

Jan 1, 1984

By Edward McHugh

While less than 1% of reported accidents are associated with slope stability problems. slope failures were responsible for about 15% of all U.S. surface mine fatalities between 1995 and 2001. Small rockfalls, which may involve hand-sized rocks weighing only a few kilograms, can cause fatal injuries to workers away from the protection of large machinery. Massive highwall failures containing a million cubic meters of material or more can be fatal even for heavy equipment operators. As part of an ongoing study at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, several remote-sensing technol¬ogies are being evaluated as tools to monitor slopes for hazards and to assess slope stability. Field tests of a hyperspectral imager were conducted to assess its value for improving geologic maps of poten¬tially unstable alteration zones on mine slopes. An interferometric radar device capable of detecting very small displacements on slopes awaits final assembly and field testing. Computerized moni¬toring methods using images from digital and video cameras are being assessed for application to mine slope surveillance. These and other techniques will eventually provide new tools to augment current methods for monitoring ground control hazards in mines.

Jan 1, 2002