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By P. Tsang

In order to deal with ground control problems such as roof falls, floor heaves and pillar failures in underground coal mines, a new pillar design method based on the "yield pillar" concept is proposed. Using the finite element model simulation, the functions and mechanisms of the "yield pillar" are studied. The important variables that affect the stability of longwall entry-pillar system are identified. To simplify the geological and mining conditions, a new pillar design method classifies the in-situ roof and floor conditions into four categories: 1) strong roof and strong floor, 2) strong roof and weak floor, 3) weak roof and strong floor, and 4) weak roof and weak floor conditions. The design formulae are derived based on the finite element model simulation, stability study and nonlinear regression analysis. The finite element models used consider the plastic failure properties and time-dependent behaviors of rock. To better organize the finite element model simulation, the orthogonal experiment design is used to arrange the finite element models and proved to be very effective. An application example is used to illustrate the basic design procedures involved in the new pillar design method.

Jan 1, 1993

By Robert L. Schmidt

The support of underground openings by water is a natural phenomenon. Surface sinkholes, such as those that occur with some frequency in Florida, are attributed to a lowering of the water table resulting in the draining of existing limestone cavities. This withdraw the natural support and causes the surface subsidence. The role of water as a ground support medium was demonstrated in a mining situation during Bureau of Mines research on borehole mining of phosphate in St. Johns County, Florida. The borehole mining tool (BHT) incorporates a water jet nozzle and slurry eduotor in a single down- hole tool. The tool is lowered through a bore- hole into the ore zone, and an underground cavity is eroded by the motion of the water jet with the slurry simultaneously pumped to the surface. In addition to phosphate, the system has been successfully used in coal, oil sands, and uranium. The Florida phosphate ore is located at a depth of approximately 250 ft in strata that produce high water inflows. These factors preclude extraction by conventional methods, either surface or underground, but they present an ideal setting for borehole mining. Previous borehole mining research had led the researchers to believe that the system would only be effective in a void cavity, that is, a cavity where the water level is maintained below the water jet nozzle, so that the water does not impede the action of the let. It was soon discovered, however, that the void cavity presented ground control problems. The overlying strata started to cave soon after the support provided by the water was removed. This was an unacceptable situation as it adulterated the ore and threatened surface subsidence. Further tests wherein the size of the cavity -as reduced by cutting only a 30° arc also resulted in caving. A new concept, air shielding, was then introduced and tested. With this system, the water jet cutter was surrounded with a shroud of compressed air, thereby gaining the advantages of cutting in air while retaining the roof support afforded by a water-filled cavity. This arrangement proved to be highly successful. The air-shrouded jet was able to cut a cavity radius of about 18 ft, and the system production was 35 tph, which compared favorably to the production previously attained in void cavities. The borehole phosphate experience proves the efficacy of remote mining in a water filled opening. This system neutralises the hazards of heavy ground, and may, as in the case of the phosphate deposits, be the only viable alternative. It opens up the possibility of application in a wide range of deposits not only those amenable to hydraulic mining. Remote mechanical System might be developed using present robotics technology and then operated in water-filled openings.

Jan 1, 1986

Tailgate pillar and tailgate-face corner humps have been a major threat to longwall coal mines where hump-prone conditions exist. The yield pillar design concept, in which the longwall chain pillars are designed in such way that they can yield in a nonviolent or controlled manner during longwall retreat, has been used in many longwall mines to prevent tailgate pillar humps. However, this practice often shifts the side abutment load onto the tailgate-face corner especially when a massive and strong sandstone member is present within the caving zone or in the vicinity of the coal seam. Therefore, the abutment pillar design concept has also been used in some hump-prone coal mines to minimize the stress concentration on the tailgate panel rib for the control of tailgate-race corner humps. The disadvantage of using the abutment pillar design concept is that it is more costly and puts additional pressure on mine operations for gate entry development and significantly reduces the recovery of coal reserve. In addition to pillar size, panel layouts. such as the panel width and panel orientation also play a critical role in pillar or face humps and should he taken into account. Based Upon the Cyprus mine design and operational experiences at Willow Creek and case studies at other bump-prone longwall nines in the U.S., the authors attempt to identify the critical factors causing coal mine bumps and to introduce some guidelines and criteria for the design of both yield and abutment pillar systems and panel layouts.

Jan 1, 1999

By Stephen C. Tadolini

Cable supports offer several advantages over traditional secondary support methods. Cable supports enhance stress redistribution to pillars and gob areas, minimize or eliminate timbers and cribs which reduce ventilation capabilities, eradicate material handling injuries related to the placement of crib supports, and provide a cost-effective alternative to secondary support. The U.S. Bureau of Mines has designed and installed cable supports to improve the stability of tailgate entries in an underground western coal mine. Two entries closest to the Iongwall panel were solely supported with high-strength cables installed with cement grout. Basically, with modifications in the intersections, 360 ft of roadway nearest the longwall panel was supported with two rows of 22-ft-long cables installed on a 7-ft spacing. Approximately 250 ft of a second bleeder entry was supported with two rows of 16-ft¬long cables installed on a 7-ft spacing. Individual cable loads were monitored with hydraulic U-cells and pressure pads. The roof behavior was simultaneously monitored with differential magnetic sag stations and closure meters on 25-ft spacing during longwall panel retreat to evaluate the response of the immediate and main roofs. Cable supported gateroads appear to be a viable alternative in secondary support when compared to wood timbers and cribs. This paper describes the theory, application, and advantages of cable supports and presents the results of the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1993

By Robert P. Kudlawiec

The use of Mobile Roof Supports (MRS) in pillar retreat mining has been a fairly recent occurrence in coal mining operations in the United States (1988). Determining the pillar block configuration, the crosscut and entry centers, the number of entries developed, the method of ventilation, and the type and mixture of equipment used arc all factors in establishing a successful pillar retreat operation. This paper will evaluate and report on the history of one coal company's relationship with MRS usage for pillar retreat mining and focus on the company's use of horizontal pillars (inverted-blocks) for the panel configuration.

Jan 1, 2000

By M. Zaki Rashad

In this paper, if three slant distances are measured from three given points of known coordinates to an unknown point, the position of this point (X, Y, and Z coordinates) can be determined. A computer program was designed for observing the movement of a point in the roof in Abu-Tartour mine (western desert, Egypt) to measure the convergence. This method, for general using, is very fast and can be applied without any restrictions.

Jan 1, 1990

By Khaled Morsy

Most of the available numerical codes simulating the reinforcing effect of fully grouted bolt are accomplished by a series of bar elements which are connected along their length to the host rock by shear springs and sliders. The spring-slider system represents the shear bonding effect between the bolt and the host rock. This paper describes a three dimensional finite element modeling technique for the 3D simulation of fully grouted bolt reinforcement. The proposed technique introduced a physical simulation for the fully grouted bolts. The bolt simulation is composed of a series of beam elements (representing the steel rebar) embedded in brick elements (representing the grout). Because of the complexity of the grout/rock interface, ABAQUS explicit code was used. A modification of the ABAQUS default friction model was conducted to consider the cohesion of the grout/rock interface. The interaction between the grout and the host rock is simulated by the modified friction model. To verify the proposed technique, a simulation of the pull out test we conducted. The results from the simulation were compared with former studies

Jan 1, 2004

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By Alan A. Campoli

Fosroc continues as a leader in the development of cementitious technology for the mining industry. The considerable expertise accumulated in the development of the Fm cement line has been employed in the development of two pumpable, supplemental roof support systems. The Tekprop system is a unique combination of steel and concrete, specifically designed as a viable alternative to conventional wooden supplementd support. The second system is the Fosroc mineable mi which is a cost competitive variable yield system designed to support longwall ventilation and predriven recovery room entries. Two, separate, ongoing, full scale applications of these systems will be discussed during the inference presentation

Jan 1, 1997

By T. L. Dobecki

As a means of demonstrating the effectiveness of reflection seismology in determining continuity of coal seams, three U. S. field projects are reviewed. The three projects involve coals of varied thickness (2-14m) and age (Pennsylvanian to Eocene) from three coal areas of the U. S. (Pennsylvania, Wyoming, and Washington). Each project also employed its own particular seismic technique, recording system, and seismic energy source although all are considered state-of-the-art, high resolution, digital seismic surveys. Project 1 (thin, Pennsylvania coal) sought detection of sand channels using dynamite and standard in-line (2-D) seismic technique. Project 2 (thick, Wyoming underground coal gasification) involved a gas-explosion (Dinoseis) source with areal (3-D) acquisition methods. Project 3 (thick, Washington underground coal gasification) employed a shotgun-type source and standard in- line methods. Data processing was handled by different contractors for each project. Each project was successful in accomplishing its own particular objective; however, data quality and interpretation seem to be more a function of thickness of the target seam, complexity of the overburden, and processing contractor than a seismic source, acquisition scheme (2-D versus 3-D), or recording instrumentation.

Jan 1, 1981

By Dave Forrester

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

Jan 1, 2004

By Joshua Hescock, Zach Agioutantis

"The Surface Deformation Prediction System (SDPS) has been developed as an engineering tool for the prediction of subsidence deformation indices through the implementation of an influence function. SDPS provides a reliable and fast method for the prediction of mining-induced displacements, strains, tilt, and so forth at any elevation between the seam and the horizontal or varying surface topography. One of the key aspects in obtaining reliable ground deformation prediction results is the determination of the edge effect offset. The value assigned to the edge effect corresponds to a virtual offsetting of boundary lines delineating the extracted panel to allow for roof cantilevering over the mined out area. The objective of this paper is to describe the methods implemented in updating the edge effect offset code within SDPS. Using known geometric equations, the newly developed code provides a more robust calculation of the offset boundary line of the extracted panel for simplistic, as well as more complex, mining geometries. Assuming that an extracted panel is represented by a closed polyline, the new edge offset algorithm calculates a polyline offset into the extracted panel by the user-defined edge effect offset distance. Surface deformations are then calculated using this adjusted panel geometry. The MATLAB® program was used for development and testing of the new edge effect offset feature. In completing rigorous testing regimes, the new offset features will be integrated into SDPS, further increasing the speed and reliability of the programs resulting in a retrospective increase in capability and flexibility. INTRODUCTION Underground operations have been known to cause mining-induced subsidence inside and outside of the mine permit area (Karmis, Agioutantis, and Andrews, 2008). Additionally, subsidence can impact surface water bodies, such as rivers, streams, lakes, and wetlands (Karmis and Agioutantis, 2015; Newman et al., 2016), as well as infrastructure, such as roads, railroads, pipelines, and buildings. With an increase in regulatory focus on mining-induced impacts to local structures, the calculation of mining-induced subsidence indices has become necessary in determining potential damage and mitigation efforts. The accurate prediction and evaluation of mining-induced ground deformations is required to obtain underground mining permits due to potential impacts to nearby structures, as well as hydrogeological sources. The prediction of ground deformation indices is often a complex task due to the number of input parameters required, such as mining geometries, overburden lithology, surface topography, the rate of mining (Karmis, Agioutantis, and Andrews, 2008)."

Jan 1, 2017

By Chris Mark

The Coal Mine Root Rating (CMRR) was first presented at this Conference nine years ago. Since Its Introduction, the CMRR has been incorporated into many aspects of mine planning. including longwall pillar design, rout support selection, feasibility studies. and extended cut evaluation. It has also become truly international, with involvement in mine designs and funded research projects iii South Africa. Canada. and Australia. The purpose of this paper is to bring the minim community Up to date with recent improvements and applications of the CMRR. The must important new development is a streamlined process for determining the CMRR from exploratory drill core. Just three types of information are Jaw required: Fracture spacing (or RQD) Uniaxial compressive strength for axial Point Load Testing) Diametral Point Load Testing The moisture sensitivity adjustment has also been changed, and new research has related the immersion test to slake durability. Most recently, the CMRR has been implemented in a computer program. which can be obtained from NIOSH free of charge. The program facilitates calculation of the CMRR firm either underground or drill core data. Values from ninny locations can he saved in a single file. and an interface with Autocad allows CMRR contour plots to he integrated 11110 mine planning.

Jan 1, 2002

By Brian E. Travis

The recent development of polymer grids for the underground coal mining industry has created new alternatives for supplemental ground control practices. The grids utilize strong, lightweight polymers - usually special grades of polypropylene. High tensile strengths and resulting load support characteristics are achieved by molecular orientation of these polymers in the manufacturing process. One of the most important applications of polymer grids as supplemental ground control is in longwall shield recovery. When shields are moved from one face to another, the determining factor in the success of the recovery is the ground control provided by roof support structures along the old face. Whereas primary support is usually provided by roof bolts and cables which run the full width of the panel, supplemental support is often provided by metallic meshes of welded wire or chain-link fence. lightweight, high-strength polymer grids replace these heavy, cumbersome metallic meshes, giving the operation increased productivity by decreasing installation time and reducing injury downtime.

Jan 1, 1990

By A. K. Bhattacharyya

The excessive displacement of the seam floor in headings (i.e. roadways) called 'floor-heave' is often experienced in the underground mines of the Southern Coalfield of New South Wales. Apart from interfering with the mining operations, the phenomenon could be detrimental to the stability of the pillars left for support leading to an unpredictable increase in the subsidence of the overlying strata. The latter could pose a serious problem, particularly in the design of partial extraction layouts of mining for areas where subsidence is to be controlled. The susceptibility of the floor of different rock materials and thicknesses in a given 'panel and pillar' partial extraction geometry was examined by two-dimensional Finite Element Modelling of the displacements, stresses and failure zones around the excavations. The modelling, assuming elastic behaviour of the materials, indicated that the maximum vertical displacements of the floor increased substantially as the Young's Modulus of the material decreased to less than 7GPa and the thickness of the layer increased. The results also discount the possibility of a properly designed panel and pillar geometry in the Southern Coalfield of New South Wales being unable to adequately control surface subsidence through the pillar, or its core independently of the yielded ribs, penetrating the floor.

Jan 1, 1992

By H. N. Maleki

Tension-torque relationships were investigated *n a coal mine for bath standard 5/8 in. (16 mm) bolts and a vibrating wire instrumented bolt, using conventional and controlled installation techniques. The instrumented bolts differed from the standard bolts by their head size and hole preparation procedures. The controlled installation technique developed by the U. S. Bureau of Nines (USBN) involved using hardened washers and lubrication in the bolt assembly, and maintaining a minimum thrust level during bolt installations. The object of this study was to determine the tension to torque ratio (lb/ft-lbs; for standard bolts and instrumented bolts for both conventional and controlled installation techniques, and to modify the bolt assembly so that the ratios would be equal for the instrumented and standard bolts for both installation techniques. It was shown that the instrumented bolts installed in faced holes had a similar tension torque ratio td that of standard 5/8 in. (16 mm) bolts installed in unfaced holes for each installation technique, and thus they were representative of other uninstrumentea bolts. This was achieved by evaluating the influence of several factors oh the relationship, including: facing the hale surface, introducing washers and lubrication in the bolt assembly, and applying different levels of thrust at installation due to bolter operator practice. The tension to torque ratio fur conventional bolts was only 21 (lb/ft-lb) for the coal mine, resulting in an installation tension of 4000 lbs (17.8 kg) with the typical installation torque of 150 to 400 ft-lb. This tension level was approximately 50 percent of the required bolt tension, and thus, it was concluded that torque standards are not sufficient to assure adequate bolt tension. .he low tension levels were due to the particular wait hardware in use, operator practice, and bolt fencing.

Jan 1, 1984

By Ben Mirabile, Alan Campoli, Rodney Poland

"The majority of U.S. underground coal mines use some form of cable bolt as supplemental support to a primary roof bolting system. Many coal operators employ the high load capacity of non-tensioned cable bolts, cable roof trusses, or solid bar trusses in an attempt to suspend the primary bolted horizon to overlying strata. However, in cases where many discontinuities exist in the roof strata or where pre-existing fractures are present, these passive supplemental support systems often cannot adequately control roof deflection. This paper examines the application of tensioned cable systems to minimize roof deflection in very laminated and fractured strata. Three underground case studies are presented to demonstrate the application of tensioned cable bolts. The first case study involves the rehabilitation of a longwall head gate in the Eagle Seam with highly laminated and fractured sandstone. The second case study examines the recovery of a longwall face in the Alma Seam. The final case study addresses rehabilitation of a longwall head gate in the Pittsburgh No. 8 Seam that was developed in a sandstone channel margin area and subjected to horizontal stress concentration.INTRODUCTIONDuring the previous decade, Jennmar Corporation has recognized coal operators' increasing interest in tensioned cable bolts. The majority of these tensioned cable bolt systems have been applied in adverse roof conditions including broken ground, high horizontal stress areas, and highly laminated roof strata. In most cases, qualitative data on roof conditions after application of tensioned cable bolt support indicate successful implementation of these systems. The qualitative evidence suggests that the combination of high load' capacity and applied tension provides superior supplemental roof support in adverse roof conditions. This research aims to quantify the effects of applied tension to cable bolts. This paper will review the types and installation of tensioned cable bolt systems, present initial laboratory test data, discuss successful case studies, and propose an instrumentation system for quantification of tensioned cable bolt performance."

Jan 1, 2010

By Christopher Mark

Despite more than half a century of experience with roof bolting, no design method has received wide acceptance. To begin to improve this situation, National Institute for Occupational Safety and Health (NIOSH) conducted a statistical study of roof bolt performance at a number of mines throughout the U.S. Case histories were collected from 37 mines with a variety of roof bolt types and patterns in a wide range of geologic environments. Performance was measured in terms of the number of roof falls that occurred per 10,000 ft of drivage. The study found that roof falls are rare when the roof is strong and the stress is low, even with light roof bolting patterns. The focus of this paper is on the more difficult conditions, where the roof is weaker and/or the stress is higher. Analysis of the results led to guidelines that can be used to make preliminary estimates of the required bolt length, capacity, and pattern. The guidelines are based on the depth of cover (which correlates with stress) and the roof quality (measured by the Coal Mine Roof Rating (CMRR), and the intersection span. Another contribution is a formula for estimating the horizontal stress level in the eastern U.S. coalfields as a function of the depth of cover. The design guidelines are currently being implemented into a computer program called Analysis of Roof Bolt Systems (ARBS).

Jan 1, 2001

By T. P. Mucho

Mine roof failure due to excessive horizontal stress has been recognized as a major cause of hazardous roof conditions in some mines. Stress measurements gathered by the U.S. Bureau of Mines (USBM) at 24 different eastern mines show horizontal stress to be typically 2 to 3 times as great as vertical stress. The focus of current Bureau work is to develop detailed design parameters to assess and control the effects of horizontal stress. To facilitate the recognition of horizontal stress effects and to easily determine principal stress direction without resorting to cumbersome, expensive, and time consuming field measurements, the Bureau has developed a stress mapping methodology. Stress recognition and direction determination are required to successfully employ a number of control strategies such as mine layout, mining sequences, and support optimization to control the effects of horizontal stress, thereby increasing the safety of U.S. coal mines. This approach has been used in other major coal producing countries, most notably Australia and the UK, and has greatly enhanced the safety of their mines (1). This paper discusses horizontal stress, directional based control techniques, and provides guidelines for the use of the stress mapping technique. A case study of a mine where the technique has been used is also presented.

Jan 1, 1994

By Nikolaos Polysos

For the planning and driving of gateroads supported by roof bolts a practical concept has been developed in order to determine the geological and rock mechanical parameters derived from exploration drillholes. Based on this, an assessment scheme for the design of roof bolting has been established. The new methods of investigation comprise of geotechnical logging of drill cores for the determination of structural-geological rock characteristics and complementary geophysical well-logging which enables the evaluation of petro-physical and rock mechanical properties. The tools and their measurements will be briefly presented. The relationship between the geophysical log readings and the geotechnical parameters will be illustrated. The aim of the data acquisition is to improve roof bolting design which is based on the assessment scheme. This will be adjusted to the prevailing rock mechanical conditions and stands up to the required stability of the gate roads. In order to ensure consistent data acquisition, a method of core logging has been standardized for the respective demands. To achieve this it was necessary to introduce specific parameters and new methods. For the recording of linear and planar structural elements at the drill core, a special system for the documentation of the angles in relation to each other has been set up. The rock mechanical parameters and their acquisition will be explained. In order to carry out an objective analysis as fast as possible, software has been developed for the acquisition, management and presentation of the data. These programs and their functions will also be presented. The software is characterized by being user-friendly and the flexibility of its interfaces, Different options for the combined presentation of input and evaluated data give a more objective basis for the planning of roof bolt design. Examples will illustrate the operational implementation of the results during the planning and driving phases.

Jan 1, 2001