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By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By Ken W. Mills

The ground movements associated with underground coal mining and, in particular, longwall mining, are recognised to include horizontal subsidence movements, but the mechanics of the processes that cause these horizontal movements are not well understood. Over the last two decades, three-dimensional subsidence monitoring has become routine in Australia and has provided a wealth of measurements of horizontal movements caused by mining subsidence. These measurements and other sub-surface observations allow the processes that cause mining-induced horizontal movements to be inferred and, subsequently, verified. In this paper, the mechanics of the processes that cause horizontal movements, particularly those in sloping topography, are described and discussed on the basis of field observations. There are several processes recognised to generate horizontal subsidence movements. In flat terrain, systematic horizontal movements cause the surface to move initially toward the newly created goaf and, subsequently, in the direction of mining. Tectonic energy stored as horizontal stress is released by mining, and, when the horizontal stresses are high, the magnitude of this horizontal stress relief movement is large enough to be perceptible for some kilometres from the panel. In sloping terrain, there is an additional component of horizontal movement that occurs in a down slope direction. This movement, sometimes referred to as valley closure movement, has a magnitude that is typically much greater than systematic or stress relief movements.

Jan 1, 2014

By Arthur Miller, Andrew Weakley, Heather E. Lawson

"IntroductionDynamic failure events in an underground coal mine, or “bumps,” are defined as “the sudden, violent bursts of coal from a pillar or pillars or a block of coal, resulting in a section, the whole pillars, or the solid block of coal being thrown into an open entry” (Peng, 2008). Reports of disastrous and often fatal dynamic failure events date back over one hundred years in the United States. Mining practices and technologies have significantly evolved over the course of the last century, yet these events continue to occur. The events at Crandall Canyon, Utah (Gates et al., 2007) and Brody No. 1 Mine in West Virginia (Barker and McNeely, 2014) are two recent failure events that resulted in a total of eleven fatalities. These events testify to the fact that dynamic failure remains an imperative safety concern. Furthermore, their continued occurrences indicate that engineering controls have proven inadequate at wholly mitigating the problem.Multiple conditions have been associated with the occurrence of dynamic failure phenomena, including• thick, competent strata that can create a bridging effect, resulting in high abutment stresses (Rice, 1935; Holland and Thomas, 1954; Iannacchione and Zelanko, 1995; Agapito and Goodrich, 2000; Peng, 2008; Whyatt, 2008; Whyatt and Varley, 2010)• overburden thicknesses greater than 500–700 ft. (Rice, 1935; Peng, 2008)• a strong coal that is resistant to crushing (Rice, 1935; Peng,2008) or that is “uncleated or poorly cleated, strong…sustains high stress and tends to fail suddenly” (Agapito and Goodrich, 2000)• the presence of sandstone channels or rolls that can serve to concentrate stresses (Iannachione and Zelanko, 1995; Agapito and Goodrich, 2000)• fracturing of strong units above or below the coal seam (Whyatt and Varley, 2010)• slip along pre-existing discontinuities (Peperakis, 1958; Whyatt and Varley, 2010)• multiple seam mining interactions (Campoli, Kertis, and Goode, 1987; Iannachione and Zelanko, 1995; Newman, 2002; Peng, 2008)• mining sequences that can cause anomalously high stress concentrations (Campoli, Kertis, and Goode, 1987; Iannachione and Zelanko, 1995)"

Jan 1, 2015

By Danny J. Van Roosendaal

An investigation was undertaken to determine hydrogeological effects of subsidence over a longwall coal mine in the Illinois Basin. At this mine, approximately 200 ft of bedrock overburden is overlain by up to 150 ft of glacial till and localized water-bearing glacial sediments within a buried bedrock valley that overlies the longwall panel. A finite-element mechanical model of the subsided longwall panel shows sufficient strain to enhance permeability throughout the bedrock portion of the overburden. However, examination of the actual surface subsidence profile indicates that the overburden deformation is localized in narrow zones at the panel edges. The aquifer units at the base of the glacial sediments are confined by glacial clays (above) and weathered shales (below). Elevated water levels were observed in the glacial aquifers during subsidence, which indicates that the clays and shales maintained confinement as the overburden subsided and deformed. A three-dimensional ground-water flow model was used to simulate the effects of subsidence on the hydrogeologic system and to estimate inflows into the mine. Longwall retreat was simulated by several model runs, each representing a new longwall face position. Simulated heads and mine inflows were calibrated with recorded water levels and observed inflow conditions by modifying the hydraulic conductivity of particular model layers. Model calibration indicates that the permeability of certain lithologies, such as plastic underclays and weathered shales, is not significantly enhanced by subsidence. A worst-case scenario was simulated by inserting a deeply incised, sandfilled valley into the model, which had only minimal impact on the simulated mine inflows.

Jan 1, 1995

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 R. Quentin Eades, Kyle Perry

"Surface mines continue to implement highwalls for several reasons, such as increasing recovery, improving margins, and justifying higher stripping ratios. Highwall stability is a complex issue that is dependent upon a variety of mining and geologic factors, and a safe design is necessary for a successful surface operation. To improve highwall stability, it is important to understand the connection between local geology and blasting.Explosives are employed throughout the mining industry for primary rock breakage. There are a number of controlled blasting techniques that can be implemented to improve highwall stability. These include line drilling, smooth wall blasting, trim blasting, buffer blasting, air decking, and presplitting. Each of these techniques have associated advantages and disadvantages. Understanding local geology is necessary for selecting the appropriate controlled blasting technique. Furthermore, understanding the limitations and conditions for successful implementation of each technique is necessary. A discussion of the impact of geologic conditions on highwall stability is provided. Additionally, discussion is provided for the successful incorporation of the controlled blasting techniques listed above, and the associated mining and geologic factors that influence the selection and design of controlled blasting plans. Finally, a new methodology is proposed.INTRODUCTIONExplosives are used throughout the mining industry as the standard for primary rock breakage, a critical part of the mining cycle. In 2015, the U.S.. consumed 2.2 million tons of explosives (Apodaca, 2015). The coal mining industry accounted for the majority of explosive consumption, accounting for approximately 63% of total explosives. The quarrying and nonmetal mining industries accounted for 12% of explosive consumption, and the metal mining industry accounted for 9% (Apodaca, 2015). This constitutes 84% of the explosives used in the U.S. However, the energy released during the detonation process is often in excess of that required to adequately fragment the surrounding rock (Jhanwar and Jethwa, 2000). This excessive energy, along with over confinement and poor blast geometry, will cause damage to the undisturbed rock mass beyond the intended boundary of the blast. This event is known as overbreak or back-break (ISEE, 2011)."

Jan 1, 2018

By Ross W. Seedsman

The logical framework recognises that a coal mine roof can be intrinsically stable or may collapse in one of four ways. Suspension of the immediate roof is one of fundamental ground control strategies available to a coal mine engineer and is recommended for three of the four collapse modes. Correctly implemented suspension offers the greatest improvement in roof stability and greatest reduction in longwall risk. For the control of maingates ahead of retreating longwall faces the ideal suspension support (if required) consists of angled, partially debonded, medium-length tendons installed as far behind the development face as practical. For situations where the roof may be subjected to tensile horizontal stress, immediate support by equally spaced short vertical tendons is required. The step away from fully-grouted tendons improves their survivability during the onset of compressive failure in the roof, minimises the risk of isolated loading, and allows a more robust TARP for roof movement. All suspension systems require a strap, sling or truss between the suspension elements.

Jan 1, 2014

By John C. Stankus

From research started six years ago, a new design criteria has been developed for mine roof bolt systems This design criteria precisely and quantitatively determines parameters such as bolt length, installed tension, and spacing that will produce the Optimum Beaming Effect (OBE) OBE, a new roof control term, is defined as the roof beam that has no separation above and within the bolted range with the shortest bolt passible Utilization of this design criteria has shown that in many cases shorter roof bolts with higher installed tension can produce a much stronger beaming effect (Stankus, 1995) This paper will first present a brief deception of how the design criteria was formulated From there, actual case studies where the design criteria was utilized will be discussed In conclusion, current and future research efforts will be presented

Jan 1, 1997

By Jun Lu

The Pittsburgh seam is the most extensively mined coal bed in the Appalachian Basin. Unlike most other prominent coal seams, the Pittsburgh seam typically includes a sequence of immediate roof coals and intervening rock binders (or ?draw slate?), most of which are within mineable horizons. Furthermore, unlike the main bench, the thickness of the overlying roof coals and their intervening rock binders can vary significantly. For instance, in southwestern Pennsylvania, the Pittsburgh seam main bench typically ranges from about 5.0 to 6.0 feet thick with minimal variation over short distances. On the other hand, the Pittsburgh seam roof coals and immediate roof binder typically range from a few inches to several feet, and in certain geologic settings can vary significantly over short distances. Additionally, in some locations, very thick draw slate can totally replace the roof coals. Since the draw slate, which typically consists of claystone or clayladen shale, is relatively weak, the presence of thick and variable draw slate in the roof and rib can significantly affect mine safety and productivity. This paper presents the adverse effect of thick draw slate on a 1500-foot-wide longwall face in a southwestern Pennsylvania coal mine. Mapping of the thick draw slate, numerical models which illustrate stress distribution and failure associated with thick draw slate, and operational adjustments instituted to mitigate the negative impact of thick draw slate will be discussed in detail.

Jan 1, 2013

By S. F. Smith

Following a roof beam collapse on a major production district at the British Steel Corporation&apos;s Dragonby Iron Ore Mine, Scunthorpe, South Humberside, UK, rock mechanics investigations were undertaken in this room and pillar operation to assess the stability of workings throughout all the underground mine area. Over a four year period room convergence and roof sag were monitored in different mining areas and various junction configurations in order to assess and compare the relative movement of the strata resulting from mining activities, where varying rock bolt lengths and patterns have been utilised. The paper, after shortly describing geological aspects and mining methods, presents the research procedure adopted together with the instrumentation employed and discusses the results obtained during these investigations.

Jan 1, 1995

By Taghi Sherizadeh, Pinnaduwa H. S. W. Kulatilake

"This paper examines the effect of different geological and mining factors on roof stability in underground coal mines by combining the field observations, laboratory testing, and numerical modeling. Observations at an underground coal mine that uses the room-and-pillar mining method showed several failures of the immediate roof, and is used as a case study. Three-dimensional distinct element analyses were performed to study the intrinsic roof failure mechanisms and to evaluate specifically the effect of in-situ stress magnitudes and directions, discontinuity mechanical properties, and variation of pillar to room size on roof stability.INTRODUCTIONThe lithology of the immediate roof with respect to the underground openings plays an important role in stability of the roof in underground excavations. Laminated, or thinly bedded shale, is one of the most frequently seen overlaying strata above the coal deposits. The bedding planes and interfaces have very low or close to zero tensile strength in the direction perpendicular to the bedding planes, and their shear strength is much lower than of the rock layers. As a result, the bedding planes are much weaker than the rock layers. Therefore, due to the stress concentrations caused by excavations, slippage and separation can easily happen along the bedding planes before initiation of failure within the rock layers.In the past, several attempts have been made to understand the mechanism behind the roof failure in underground coal mines. Most of these earlier attempts were based on empirical techniques and continuum based numerical methods that have limited capability in simulating the response of discontinuum rock mass behavior to excavation. Different factors such as the magnitude and direction of the horizontal in-situ stresses, type and mechanical properties of roof rock, surface topography, geological anomalies, gas pressure, slippage and separation of bedding planes at the roof and floor, entry width and excavation sequences are identified as key factors that could significantly impact the stability of the roof. Among all the aforementioned factors, in spite of its importance, the impact of slip and separation of bedding planes on roof stability has not received enough attention from researchers. In a few available numerical modeling studies, the bedding planes are modeled using the continuum based numerical codes such as ABAQUS (Chen, 1999) and FLAC3D (Ray, 2009). However, since the bedding planes among the roof layers become discontinuous during the numerical analysis, this behavior is most likely not be accurately captured by the continuum based numerical models. Chen (1999) conducted finite element analysis to study the impacts of the roof and coal interface slip and the bedding planes slip in the roof on the stress distribution in the roof strata by using the finite element analysis software named ABAQUS. To avoid the complications associated with the finite sliding between two deformable bodies, he limited his study to the two-dimensional finite element analysis (plane strain). He concluded that the slip and separation of interfaces and bedding planes have significant impact on the behavior of the roof strata and without considering the interfaces, the roof bolt functions cannot be simulated properly by using the numerical analysis methods. Gadde and Peng (2005) performed three-dimensional finite difference analysis using FLAC3D software with strain softening material behavior to simulate the cutter roof in an underground coal mine. Their numerical model was fully continuum and no bedding plane was included in the model. They suggested that the strain softening material model can help to simulate the cutter roof failure up to a satisfactory level (Gadde and Peng, 2005))."

Jan 1, 2015

By Changshou Sun

The geotechnical properties of mining zones at Leeville Underground mine (or simply called Leeville mine through this paper) are different from one zone to another. The typical ground condition is poor to very poor. Therefore, an appropriate ground control practice is the key for the successful and smooth ore production at Leeville mine. Based on the available geotechnical data, the empirical and numerical methods are used to design the right ground support systems. The main ground control practice at Leeville mine includes geotechnical data collection, evaluation of the stiffness of the support system, selection of rock bolt types, design bolt length and bolting patterns, development of the ground support standards, and performance of quality controls on bolt installation to ensure proper rock anchorage, and strength of shotcrete and backfill. Through all these activities, an appropriate ground support system has been established and applied at Leeville mine for the past four years. So far, a good result related to successful ground control and smooth ore production has been achieved, although the system is still being improved. The authors of this paper will summarize the ground control practice conducted in the last four years and will discuss the procedures and standards of establishing the appropriate ground support systems for Leeville mine.

Jan 1, 2013

By Sean C. Farrell

In underground mining and tunneling, machine design is predominantly dictated by mine conditions and individual customer desires. In partnership with Australian companies WDS Limited and Cram Fluid Power, the engineering department of J.H. Fletcher & Company was tasked to design and manufacture roof bolting modules to be mounted off both sides of a roadheader, providing in-cycle bolting. The objective was to produce a boom and bolt module that would allow an operator to bolt from a remote location, eliminating the need for miner to work in unsupported ground. The bolt module needed to operate in front of the cutting boom, but retract to a parked position out of the way during the cutting cycle. Due to Australian electrical approval requirements, it was desirable to design the module with minimal use of electrical controls. The need for multiple, complex, drill steel and bolt handling steps led to the need for a custom hydraulics package to automate several of the movements. The result of the design is dual drill and bolt modules mounted to a multistage boom with over 24 feet (7.31 m) of extension. Each unit rotates and tilts to meet the bolt pattern requirements in conjunction with the stow and load requirements. The bolt module builds on previous designs incorporating a modular assembly with each sub-component designed to be replaceable and adjustable independently from other machine components. Each major component is painted a different color to help the operator make the connection visually from the module to the operator?s control valves. Hydraulic package valves were designed to automatically control several of the sub-components during the drill cycle, reducing the number of steps a drill operator would, otherwise, have to do manually. This design removes the operator from working in unsupported roof and away from other inherent hazards, putting them at a safe distance on the operator?s platform. The cutting and bolting equipment has been combined into one machine, which is advantageous in single entry drifts because there is no need to place change equipment. The hydraulic package valves eliminate the need for any electronics, thus avoiding any electrical regulations with respect to the controls. The addition of these modules to the roadheading machine increases miner safety and streamlines production while meeting the ground control support requirements.

Jan 1, 2014

Virtually all mineable Appalachian coal seams exist in a multi-seam environment. This makes it inevitable that most seams will eventually experience interaction induced ground control and mining problems. Indeed, it has been estimated that every working face will experience interaction effects from previous or current workings at least once during its lifetime (1), and that these problems are considerably exaggerated when seams are in close proximity -- a distance of less than 110 feet. The continued increase in multi-seam problems has resulted in considerable research activities over many years (2,3,4,1). Theoretical analysis, numerical and physical model studies, statistical and empirical analysis of numerous interaction related field data have been applied to multi-seam mining situations and have identified many useful trends and design criteria (4,5,6,7). To fully utilize research results and transfer multi-seam mining technology to field engineers, a design model -- USEAM was constructed (8). This model, designed for close seam under-mining situations, was computerized to facilitate utilization. In the current research, a new computer program "MSEAM" was developed to produce a model for both under- and overmining close seam situations. It is hoped that both of these software packages will aid field engineers in using important interactive design criteria to gain a better understanding of interaction mechanisms. Through advance planning with these programs, detrimental interaction may be avoided and minimized, and occasionally a design can be selected which will allow interaction effects to be utilized to the benefit of mining operations.

Jan 1, 1987

By Anton Sroka

The German hard coal production predominantly derives from underground multiseam mining activities. Beside common surface damages, ?inner mine damages? on underground infrastructures (i.e. main roads, cross cuts and shafts) due to mining excavations occur to an increasing extent. As these "inner mine damages" dramatically increased because of the development towards high performance longwall mining technologies, this matter raised to be a severe problem concerning mining economics. To protect durable and long lasting underground infrastructures, precautionary measures such as suitable mine layouts, adapted planning of mining operations and moderate face advances have to be considered. Some examples of "inner mine damages" recorded by means of mine surveying methods are given in this paper. An empirical-mathematical model based on the principles of mining subsidence engineering methods will be introduced. This model allows to minimize ?inner mine damages? and helps to support and optimize mining processes.

Jan 1, 1999

By Thomas L. Hutchinson

The mechanical design of a roof support is basically a matter of a statics and dynamics problem, assuming of course, that the imposed loads and mining conditions are known. Here in the Appalachian coal fields the general conditions are known well enough to allow the majority of the proposed new faces of longwall to be furnished with standard commercially available supports equipped with suitable variants and options. While not trying to negate the primary importance of thoroughly investigating a potential face and the selection of the correct equipment for that face, it still becomes clear that an equally important aspect of the design of an adequate ground control program has in the past too of ten been overlooked, having been overshadowed by the ?endless? selection process of the roof supports proper. The equally important aspect which this paper discusses is the overall design of a mining system, which design incorporates not only the size and capacity of the equipment but the following points as well, namely: Equipment adaptability to mining scheme, Equipment va. human constraints, Operation at the designed levels, and Coordination of operation, maintenance and support design. Unless the coordination of these factors is considered in the design and selection of the equipment, a lopsided and inefficient operation is possible. How can we afford to put an 8-10 million dollar investment up to chance as to whether it will yield optimum results or not?

Jan 1, 1981

By Kot F. Unrug

The shale response to moisture changes and the major finding of the appropriate research concerning this subject are briefly reported in this paper. Also described are mine ambient atmospheric conditions which are responsible for moisture changes in mines. The need for a quantitative measure of the weathering potential of roof. rock is demonstrated in relation to its influence on safety and the potential for increasing the cost of mine operations. The weatherability test presented in this paper has been developed and used in many mines. It imitates the natural process in a mine by subjecting the rock sample to alternating wet and dry cycles. The natural changes are thus accelerated, but are similar to the natural conditions to which the surrounding excavation rock will be exposed. The measure of the weatherability index is the percentage of the initial sample mass remaining after the test as the largest fragment of the sample. Thus, the index changes, from 100% for rock affected by moisture changes which disintegrates completely, to 0% for rock totally unaffected. In this paper, the apparatus and testing procedures are described. Also presented are examples of test results. with a discussion of the design implication to he considered by mine planners.

Jan 1, 1997

By Stephen P. Signer

A method to assist in the evaluation and selection of roof bolts using in situ measurements of roof bolt loading has been developed by researchers of the Spokane Research Center, National Institute of Occupational Safety and Health. Both axial and bending forces are measured by strain gauges at many locations along the length of fully grouted bolts during various stages of mining. This information is then used to ( I) predict bolt loading for variations in bolt spacing, grade, and diameter, (2) calculate the design changes necessary in zones where bending loads are high, and (3) determine if bolt length is adequate. Results from several case studies of full-column bolt loading in coal mine gate roads are used to illustrate the design method. This knowledge will give design engineers a tool for the selection of roof support systems that will improve underground safety by reducing roof bolt failures.

Jan 1, 1997

By Al Campoli

The majority of the 100 million roof bolts installed each year in the United States are 5/8 inch deformed rebar fully grouted into a one inch borehole. Commonly referred to as B-Series or No. 5 bar, this system is proven effective and cost efficient in coal mines with average to above average roof conditions. One of the critical variables affecting resin bolt performance is the size of the resin annulus, calculated as the difference between the hole and bolt diameters. The optimum annulus has been proven to be &apos;/4 inch, which provides for effective mixing of the two polyester resin components and incorporation of the plastic film packaging within the cured resin. Glove fingering occurs when the plastic film is forced against the borehole wall potentially interfering with the mechanical interlock of the resin and the rock mass. Glove fingering is not a problem with the optimum &apos;/n inch annulus but has been know to occur with the B-Series 3/8 inch annulus. Laboratory testing correlated the degree of Glove fingering with resin formulation and the bolt head to shaft axes alignment. Glove fingering occurred over 45 % of the grouted length of standard B¬Series bolts. The newly formulated Eclipse B-Series resin when combined with a 1/8 inch offset between the bolt head and shaft axes reduced the glove fingering 70%. The Eclipse system allows the 3/8 inch B-Series annulus to achieve the positive mixing characteristics associated with the / inch optimal annulus systems.

Jan 1, 2002

By Rudrajit Mitra, Tien Dung Le, Chengguo Zhang, Bruce Hebblewhite, Joung Oh

"Geomechanical and geotechnical characteristics of coal seam are important parameters that directly affect the cavability of top coal in the longwall top-coal caving (LTCC) method. The impact of these parameters such as coal strength, coal deformation modulus, coal discontinuities and top-coal thickness on cavability has not been fully understood in previous numerical analyses. The limitation and selection of the numerical modelling codes were two of the main reasons. In this paper, detailed caving simulations are performed using a 2D distinct element method (DEM) code, UDEC, to study the effect of various critical parameters on top-coal cavability. The numerical analysis evidently shows that the coal deformation modulus, coal discontinuities, and top-coal thickness have considerable influences on cavability. Coal strength is also a factor but to a lesser extent. The results presented in this paper contribute to the understanding of top-coal cavability in different mining environments and facilitate the development of an advanced cavability evaluation criterion. INTRODUCTION The longwall top-coal caving (LTCC) method was originally developed in the 1950s in the former Soviet Union and France (Tien, 1998). The method was nearly abandoned in Europe after the mid-1980s; however, it has been significantly improved and widely applied in China since the late 1980s. A conceptual model of the LTCC method is shown in Figure 1 in which a thick coal seam is divided into two sections including the lower or cutting section, and the upper or top coal section. The lower section of coal seam is mined by conventional longwall method. Meanwhile, the upper section is recovered by caving through the face support. The LTCC method is considered to be the most viable option for mining general thick seams. Compared to the multi-slice longwall (MSL) method, LTCC has the major advantages (Zhongming, 2001; cited by Vakili, 2009):"

Jan 1, 2017