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By D. W. H. Su

A two-year study was conducted by CONSOL in a northern West Virginia coal mine to evaluate pillar design and support requirements to improve roof control in right-hand longwall headgates subject to high in situ horizontal stresses. Horizontal stress measurements conducted within undisturbed areas of the subject mine revealed a pre-existing regional horizontal stress field with a significant difference in magnitude between the east-west maximum and the north-south minimum stresses. Three¬dimensional stress cells installed to monitor horizontal stress concentration in the headgate roof during mining revealed that a narrow (60-foot center) headgate pillar reduces the east-west horizontal stress concentration in the roof ahead of the face such that the headgate is more stable and requires less support. This 60 foot x 140-foot pillar design, which has been employed in subsequent panels, still provides adequate stress attenuation to protect future tailgates under a maximum cover depth of 1000 feet. Roof geology derived from underground roof reconnaissance and underground bolt load and roof displacement measurements confirmed the effectiveness of 12-foot long cable bolts for controlling the headgate roof subject to high horizontal stresses. With the new headgate pillar design and the supplemental cable bolts, headgate roof conditions in the subsequent panels improved significantly, longwall production delay due to adverse headgate roof condition was totally eliminated, and record coal production was achieved by the subject mine in the subsequent years.

Jan 1, 2003

By Thomas Rymer

Mathematical modelling of mine subsidence measured at 21 locations in Pennsylvania and northern West Virginia allows for the refinement of current methods of predicting maximum land subsidence associated with longwall coal mining. This new method (RYBAD model) is an extension of that provided by Tandanand and Powell (1984; TP Model). The RYBAD model accomplishes more accurate subsidence predictions (error typically less than 10 percent) for a greater range of mining and geological parameters through the recognition of the following: 1) subsidence increases at a decreasing rate (curvilinear relationship) with greater overburden thickness up to a critical value beyond which subsidence decreases; 2) subsidence is strongly influenced by the stratigraphic proximity of strong rocks to the coal (size of potential caving zone); and 3) subsidence index is related to overburden thickness by a family of curves, each for incremental variations in the effective percentage of strong rock in the overburden.

Jan 1, 1988

By Archie Richardson

Periodic weighting of longwall supports occurs during retreat ofthe face under certain geologic conditions. Strong stratain the immediate and main roof tend to cantilever over the gob, weighting the supports periodically. The distance between weighting peaks, as well as the intensity ofperiodic weighting, is determined by the suength and thickness of roof members, their location relative to the seam, the frequency of joinling, and characteristics of the gob. In this paper the effects of varying several of these variables ate investigated using block modeling. The models are calibrated against a published case history of periodic weighting in the Pittsburgh Seam. It is concluded that block modeling is a useful tool for correlating measured support pressure histories with expected modes of strata behavior in the roof, with potential for predicting shield behavior in new geologic enviromnents.

Jan 1, 1998

By Jesse Puller, Ken Mills

"This paper describes the Australia, New Zealand Inflatable (ANZI) strain cell, its operation, and recent development for overcoring in exploration boreholes. The ANZI strain cell is an instrument system that uses the overcoring method of stress relief to determine the three-dimensional, in-situ stresses in rock. The instrument has been used successfully for over three decades in numerous underground mining and civil projects, but technical advances over the last decade or so have allowed the system to be deployed in surface exploration boreholes to greater depths than was previously possible. Recent development of a downhole electronic data logger, a wireline-enabled drilling system, and an instrument deployment system has simplified the process of obtaining three-dimensional overcore measurements at depths approaching 1km to a single shift operation.The capability to deploy ANZI strain cells in surface exploration boreholes represents a significant breakthrough for the design of underground mines and underground excavations generally. High-confidence characterisation of the in-situ stresses at the design stage provides the opportunity to design key infrastructure and mining systems to take advantage of the in-situ stress field from the outset before mining begins. Understanding the three-dimensional, in-situ stress field not only provides a measure of the magnitude and direction of loads acting within the rock mass, it also provides insight into the mechanics of all the various processes driving ground deformations, including which geological fault structures are at limiting equilibrium.INTRODUCTIONThe overcoring method of stress relief is one of the most direct methods for determining three-dimensional, in-situ stresses in rock. The Australia, New Zealand Inflatable (ANZI) strain cell combines overcoring with several other techniques to provide determinations of the three-dimensional, in-situ stress field and the properties of the rock and, importantly, provides an indication of how much confidence can be placed in the measurement. This paper describes the advances that have enabled overcoring of the ANZI strain cell in deep surface exploration boreholes and the development of various complementary techniques that build additional confidence in the determination of the in-situ stress."

Jan 1, 2018

By L. Mahony

This paper describes the design, construction and commissioning testing program for a laboratory facility at the School of Mining Engineering, UNSW, Australia, aimed at evaluating reinforcement tendon performance when subjected to shear loading. The project has been financially supported through the coal industry ACARP research initiative. The single failure plane design adopted in the test rig has been successful in allowing shear loading to be directly applied to fully installed (and where relevant, pre-tensioned) rock bolts. Initial results are extremely encouraging, and illustrate a significant axial component of load development as shearing takes place - particular in lower strength, softer material types. The paper also outlines future plans for use of this facility.

Jan 1, 2005

By Brian G. D. Smart

A scheme is presented whereby the complexities of design and operation of a hydraulically powered coalmine support are modelled using the finite element method. This enables predictions to be made of the interaction of the support with its environment in a variety of operating conditions. The model is a two dimensional representation. The modelling scheme was applied in two case studies. In the first, the performance of a four leg support was uprated by increasing the front leg force. The finite element model enabled changes in overall support performance to be verified. In the second case study, the change in lateral force developed by a two leg support was assessed with changes in support operating height and floor foundation stiffness. The modelling scheme is capable of further development with the possibility of extending the model into three dimensions and the addition of control elements that would enable an actual mining sequence to be represented.

Jan 1, 1992

By Quanxi Wang

This paper presents a new stability mapping system which tightly integrates structural and geologic mapping with geo- mechanical stress analysis for use in support design and mine plan- ning. The structural and geologic input can include such local characteristics as: rock strength, layer thickness, faults, stream val- ley effects, etc. The geo-mechanical stress inputs can include overburden stress, multiple-seam stresses, abutment stresses, etc. The mapping system utilizes the popular AutoCAD/SurvCADD platform as a foundation for inputting the geologic characteristics, and then integrates the boundary-element program LaModel for determining the geo-mechanical stress influences. Additional func- tionality in the system allows the user to transform each individual geologic or stress factor into an individual rating and then the user can apply various weighting functions to the individual factors to generate an overall stability index map for the mine property. A sample case study is included in the paper to demonstrate the basic concepts, procedures and utility of the stability mapping system.

Jan 1, 2005

By Mingju Liu

In this research, we measured fractal characteristics of over ten typical outburst areas on different scales and related them to their outburst proneness, in attempts to provide new methods for outburst prediction. It was shown that both faults and folds follow the power law on coal basin scale, mine scale or mining district scale, whichever it is. Fractal dimensions are appropriate indices to describe geological structures quantitatively. Preliminary relationships have been established between outburst proneness and fractal dimension. Comparisons have been made among fractal characteristics and degrees of outburst proneness on all three scales. The results indicate that multi-scale assessment of outburst proneness can be made based on fractal measurements independently or supplemented by other assessment methods.

Jan 1, 2000

By Kudret Tutuk, Serkan Saydam, Sungsoon Mo

"This paper presents an overview of the floor heave management at the Glencore Bulga Underground Operations and investigates the contributing factors to the behaviour of the floor. The mine experienced a number of major floor heave events in gateroads on development. As the longwall face approached the roadways, the magnitude of floor heave frequently increased, while new floor heave also developed. Furthermore, severe floor heave events took place along the longwall face. The most observed failure mode was buckling. While regular floor measurements were conducted to better understand the nature of the phenomenon, and various measures were considered to control the deformation of floor, the mining height was increased for the predicted floor heave domains, which facilitated effective management of the floor issues. The experience in the mine indicates that mainly high horizontal stresses with greater depths of cover and certain types of floor lithology configuration are likely to contribute to the failures of floor strata.INTRODUCTIONThe Glencore Bulga Underground is located in the Southwest of Singleton in the Hunter Valley of New South Wales, Australia. Blakefield South, part of Bulga Underground, commenced longwall mining in 2010, extracting coal from the Blakefield seam, while the overlying longwall panels were previously mined from the Whybrow seam. The coal seam ranges from 4.5 m to 8.0 m thick with the mining heights varying from 2.9 m to 3.5 m. The longwall panels are 320 m and 400 m wide and up to 3.5 km long.The mine experienced a wide range of floor issues around longwall panels LW7 and LW8, both 400 m wide. For instance, the concrete floor in the travel road failed and lifted as the longwall retreated in close proximity as can be seen in Figure 1. Figure 2 shows the beam stage loader (BSL) buried into the lifted floor, and Figure 3 shows a timber prop distorted due to the movement of the floor. In addition, severe deformation of the floor along the longwall face impeded the longwall operation, as shown in Figure 4."

Jan 1, 2018

By William Kendall

INTRODUCTION Stillwater Mining Company, with its two underground, hard rock mines in South Central Montana, is the largest primary producer of palladium and platinum in the Western Hemisphere. Platinum Group Metals (PGMs) are mined from a specific zone within the Stillwater Complex, a layered mafic/ultramafic igneous intrusion, named as the J-M Reef. Over much of the J-M Reef, the economically minable, mineralized horizon is contained at typical horizontal widths of 6.5 ft to 8 ft wide. This feature of narrow mining widths has made it challenging to safely and effectively mine these areas of the J-M Reef using mechanized cut and fill and sublevel sill development mining methods. Mechanization of most or all of the steps of the mining process is important to Stillwater in the continuous pursuit of high standards in safety and health. Mechanized bolting within the J-M Reef ore production headings is an important piece of reaching these objectives. In general, metal/non-metal mining requires varied mining techniques, as compared to more traditional bedded mineral/resource deposits. Narrow vein mining is a selective mining technique that generates a minimal amount of waste material while obtaining the more valuable ore deposits. Drilling and bolting in narrow vein mining has traditionally been performed with air-powered, hand-held ?jackleg? equipment because mechanized equipment is, typically, too large for the selective narrow vein techniques. In addition, traditional hand-held pneumatic equipment provides significant flexibility for small-opening, narrow-vein developments, reducing waste rock handling and improving ore recovery management. The hand-held equipment is also relatively inexpensive to purchase. Due to its simplicity, it is also relatively easy to train operators to use hand-held equipment. Hand-held equipment has been routinely used to safely install ground support and to develop production faces in hard rock mines for many years. Although it is effective, the hand-held equipment requires operator exposure to certain inherent hazardous conditions in the active face/development mine areas. These include falling rock, tripping hazards, bending/twisting/lifting hazards, high noise levels, and chemical fumes from the hammer exhaust. Based on health and safety statistics from the Stillwater mines, operation of the rotary-percussive hand-held equipment adversely affects the long-term health of the operator?continual jackleg operation frequently results in long term damage to hearing, shoulders, arm and wrist joints, and back injuries. This reduces the quality of life for the operator and also results in higher personnel turnover rates for the mining company.

Jan 1, 2014

By J. R. M. Hill

The U.S. Bureau of Mines is involved in research to improve geological sensing (or geo-sensing) techniques. This paper presents results from field trials of two research projects concerned with new technologies for geosensing. The goal of the first project was to develop a "smart" roof bolter drill so that it could probe into roof rock to evaluate rock conditions. The goal of the second project was to modify an existing mine-wide monitoring system in a continuous mining operation so that the system could interface with geotechnical instruments to provide near real-time geosensing. Integration of this mine-wide monitoring system with the smart roof drill to collect, reduce, and analyze geotechnical data and the potential applications of the resulting technology in mining, are described.

Jan 1, 1992

By Kevin J. Ma, John Stankus, Dakota Faulkner

"Expandable rock bolts are widely used in hard rock mines as an efficient ground control product. However, capacity and service life can be significantly reduced if the metallic body is subjected to corrosion. In some hard rock mines in the U.S., highly corrosive ground conditions exist, and it has been reported that inflatable rock bolts have corroded within a few months after installation. To provide mining industry a cost-effective inflatable bolt and combat the corrosion issues, Jennmar Corporation, Inc., and its subsidiary Keystone Mining Services, LLC (KMS), analyzed corroded bolt samples, identified root causes, evaluated properties of various coating materials, and developed a new inflatable rock bolt, Python M3TM, that is protected with an innovative PyFlexU2TM coating. The new generation Python M3 features improved steel chemistry for reliable performance, modified profile for better inflation, and surface preparation and coating application. The PyFlexU2 is impervious to liquid and air, durable, and UV resistant. With a flexible adhesive, and highly corrosion-resistant undercoating, and a very hard sacrificial surface coating, the PyFlexU2 coating system provides the Python M3 superior protection against chemical corrosion and physical scratch damage. The under-coating has exceptional flexibility and adhesion to prevent coating micro-cracks or fractures after bolt inflation and possesses excellent corrosion resistance to acids (pH <3), alkalis (pH >11), fuels, and salt solvents. The corrosion and scratch resistant PyFlexU2 coating offers very effective bolt protection for extra longevity in highly corrosive environments. The Python M3 coated with PyFlexU2 has been tested in the most challenging conditions, including laboratory corrosion tests in extreme acidic and basic solvents, rock slurry, and borehole scratch insertion tests. With demonstrated corrosion and scratch resistance, the product has been greatly welcomed by hard rock mines in the West and is currently installed in large scale. This paper identifies the root causes of the bolt corrosion, discusses the analysis process, and details laboratory and underground tests carried out on the Python M3 coated with PyFlexU2. The Python M3TM and PyFlexU2TM are subjects covered by pending U.S. Patent Applications assigned to FCI Holdings Delaware, LLC."

Jan 1, 2017

By John Stankus

Accurate evaluation of stress and geological conditions is critical to ground control design in underground openings. For a mine slope entry, the problem becomes more complicated because a slope transverses through many different types of strata. Mine slope ground control has traditionally been practiced based on trial and error and prior experience. There is currently no well-accepted slope and support design methodology available for an economical and technically sound ground control plan. Over the last few years, Keystone Mining Services, LLC (KMS), the engineering affiliate company of Jennmar Cooperation, has developed a ground support design methodology called the ?Stress, Geologic, and Support design system? (SGSSM). This patent pending method enables accurate analysis of actual geo-technical conditions and stress; identifies strong, fair, and weak sections along the slope; formulates primary and supplemental roof bolting systems, designs optimal long-term steel structure support; and verifies the adequacy of the developed support. The method has been successfully utilized on various new slope projects and has been well received by state and federal regulatory agencies during the approval process. This paper details the concepts of the methodology using an actual case study of a slope project in the Midwest.

Jan 1, 2011

By Dale S. Preece

In 1975 Congress passed the Energy Conservation Act to establish a U. S. Strategic Petroleum Reserve (SPR) with a capacity of 750 million barrels of crude oil. The most economic storage medium was determined to be salt caverns leached in salt domes in Louisiana and Texas. Salt caverns existed at several sites when the reserve was created. These were obtained by the U. S. Department of Energy (DOE) and used to initiate SPR oil storage. In order to meet the storage capacity approved by Congress, new caverns also had to be leached. To support the resulting design effort, finite element computer program have been used to determine the creep closure and structural stability of salt caverns. Using site specific material properties including creep models, elastic moduli and fracture data, the finite element analyses have replaced earlier empirical approaches to cavern design. This report presents results of such finite element analyses to determine the best cavern roof shape and the minium pillar to dimeter ratio. , P/D. These numerical predictions indicate that the current cavern design is safe.

Jan 1, 1984

By Gabriel S. Esterhuizen

"Cable bolts are sometimes used in low-seam coal mines to provide support in difficult ground conditions. This paper describes cable bolting solutions at two low-seam coal mines in similar ground conditions. Both mines used support systems incorporating cable bolts as part of the primary support system. Two original cable bolt based support systems as well as two modified systems are evaluated to estimate their ability to prevent large roof falls. One of the support systems incorporated passive cable bolts, while the other used pre-tensioned cable bolts. The results and experience at the mines showed that the modified systems provided improved stability over the original support systems. The presence of the cable bolts is the most important contribution to stability against large roof falls, rather than the details of the support pattern. It was also found that a heavy steel channel can improve the safety of the system because of the ‘sling’ action it provides. Additionally, the analysis showed that fully-grouted rebar bolts load much earlier than the cable bolts, and pre-tensioning of the cable bolts can result in a more uniform distribution of loading in the roof.INTRODUCTIONCable bolting is sometimes used as primary support in coal mines experiencing difficult roof conditions. In low-seam mines the flexibility of the cable bolts allows greater length supports to be installed near the advancing face without the use of couplers. When used as primary support, the cables are typically installed in the same row as fully grouted bolts, replacing two or more of the bolts in each support row. A heavy steel channel may be used as a strap to spread the support load over a greater portion of the roof. Historically, the Mine Safety and Health Administration (MSHA) has not allowed widespread use of partially grouted un-tensioned bolts (e.g., passive cable bolts) for primary support; however, pretensioned cable bolts have been accepted.Various solutions using cable bolts as primary support were attempted at two low-seam coal mines in Western Pennsylvania that were experiencing difficult roof conditions. Both mines originally used fully grouted rebar bolts as primary support and cable bolts as supplementary support. It was found that when a large roof fall occurred, the cable bolts may be contained within the dome of fallen rock. As problematic roof conditions continued to exist, both mines decided to use cable bolts as part of the primary support system. The cable bolts were located near the ribs of the entry, to increase the likelihood that they would be anchored outside the dome of potentially unstable roof. The cable and rebar bolts were installed on a heavy steel channel that acts as a ‘sling’ to distribute the load across the width of the entry. At the first mine, Mine A, pre-tensioned cable bolts were used while at Mine B, un-tensioned cables were used. The two support systems consist of essentially the same support components installed in different patterns and with varying degrees of pre-tension."

Jan 1, 2015

By Michael Jenkins

This paper reviews the history of rock burst research in the Coeur d&apos;Alene Mining District of northern Idaho over the last 30 years. Rock bursting is a problem because of a combination of geological factors in the region--complex structure, high horizontal stresses, and strong, brittle rock that readily stores strain energy. The Bureau of Mines has cooperated with industry to study rock burst phenomenon for many years. Early work focused on methods to mitigate bursting problems within the context of existing mining methods. In the 1970&apos;s, researchers began investigating methods for altering or controlling conditions that lead to rock bursts. These control measures become part of the stope development that precedes mining or are changes in the mining process itself. Although, researchers continue to develop microseismic techniques, the major research effort focuses on developing a new mining method to reduce the number and severity of rock bursts in order to improve safety and increase productivity in the Coeur d&apos;Alene Mining District.

Jan 1, 1987

By Khaled Morsy

In this paper a new method to evaluate the stability of yield pillars was introduced. The evaluation of yield pillar stability was conducted in three steps: (1) Estimation of pillar loading - numerical modeling is a good tool to estimate the pillar loading for most of the geological, geometrical and mining conditions. In this study. finite element technique was used to estimate the state of stress, strain, elastic strain energy and plastic energy, etc. at every point within the pillar. (2) Defining the yield pillar loading zones¬. The proposed method considered the non-uniform stress distribution of yield pillars by dividing the area of the yield pillar into three loading zones, namely core, transition and rib zones. (3) Estimation of stability indicators - an appropriate stability measure was proposed for each loading zone. such as; Core Stability Factor. CSF, Pillar Bump Index, PBI and Rib Instability Index. RIF. A case study of successful and unsuccessful yield pillar application was used to assess the applicability of the proposed method.

Jan 1, 2003

By Richard N. Campbell

The world?s ever-expanding demand for energy and coking coals has resulted in the underground mining industry looking at taking on increasingly marginal and geotechnically challenging deposits. The drivers for this change include a reduced availability of open cut reserves and a lack of remaining ?good coal leases?. The lack of plum deposits is forcing some companies wanting to enter the coal supply market to purchase and operate in higher risk, lower margin areas. As geotechnical professionals, we are expected to develop strategies to facilitate safe and productive mining in ground that once would have been avoided. To be successful in these reserves, we are required to use every tool available, from exploration data to longwall automation, and invent or reinvent some new tools along the way. This paper presents geotechnical case studies of challenges in longwall mining and some of the methods developed and used to allow longwall mining to succeed where others said we would fail.

Jan 1, 2014

By J. D. Cole

In order to reduce the number of longwall face moves and to increase the rate of gateroad construction, super longwall panels, 1000 feet wide and 12000 to 14000 feet long, are planned for Wolf Creek Collieries No. 4 mine. The development of the longwall panels at the mine has evolved from 5-entry, to 4-entry, to 3-entry systems. A three-entry system including pillar size and entry supporting method for the super longwall panels was designed for long-term stability. The design procedures included field observation and studies, laboratory properties tests, in-situ stress measurement, and computer analysis. Three pillar design methods consisting of PILLAR, ALPS, and the Wilson Method, and one entry supporting method, ROOFBOLT, were adopted in this study. The procedures, methods, and results are discussed in detail.

Jan 1, 1990

By Behdeen Oraee-Mirzamani

Since the advent of New Austrian Tunneling Method (NATM), shotcrete as a primary means of support in tunnels has been widely applied. It?s most important features are durability, speed of application and cost effectiveness. This paper introduces some tables that provide guidelines for the thickness of shotcrete required in some common situations of mine roadways. In order to devise such tables, two different arch sections, together with three different overburden types, were considered. Geotechnical parameters such as apparent cohesion and angle of internal friction of surrounding rocks were chosen, based on the five-category classification of Bieniawski. Two K0 factors (the ratio of horizontal stress to vertical stress) and an average rock density were utilized. Using numerical methods, 60 models were then devised in this way. By applying interaction diagrams of axial force and the bending moment for different thicknesses of shotcrete, appropriate shotcrete thickness for these models were calculated. The results of this research, as well as the methodology applied, can be used in mining roadway support design and all types of civil engineering tunnels.

Jan 1, 2011