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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

By E. Bane Kroeger

For many years, researchers around the world have been investigating coal pillar stability. Many have focused on trying to optimize the size of the pillars by examining stable and failed pillars in underground coal mines. For mines that are already in operation or companies that are opening a new mine adjacent to an existing mine where the pillar dimensions and stability can easily be determined, this is the preferred technique. However, when a new mine is planned for an area where there has been relatively little mining, then samples of the coal need to be drilled and tested for strength. Because of the fractured nature of coal, this lab testing is often problematic and may often yield poor results because of the limited size and number of samples that can be obtained from the core. To remedy this problem, research was conducted in the labs at SIUC to determine if improvements could be made when optimizing the size of pillars for underground coal mining in Illinois. Through a greater understanding of the relationship between dimensions and strengths of coal samples tested in the lab, it was hoped the in situ strength of coal pillars could be determined with greater accuracy. This could allow the extraction ratio to be increased and more of the coal resources in Illinois recovered without impacting the safety of the mine workers. Many different agencies around the world have published recommended minimum numbers of samples for strength determination based on the size of the coal samples. For two inch cubes, uniaxial testing of at least seven samples is recommended and this minimum number decreases to four when testing four inch cubes. However, the laboratory testing of coal from two seams thus far has suggested this number should be almost doubled to accurately determine the coal strength. Testing also showed there is a pronounced decrease in the variation in the strength of the samples tested as the dimensions of the sample were increased. From the testing results thus far, it is recommended that a sample size versus strength curve be constructed for every coal seam for Illinois. Creating such a curve will better determine both the minimum number of samples and minimum sample dimensions for that seam or region of a particular scam. A second set of tests were conducted in the laboratory to determine the increase in strength with reduction in height for samples with the same width and length. In Illinois, typical pillar dimensions are 40 to 80 feet (12.2 to 24.4 m) wide and long, with coal thickness varying from about 4 to 9 feet (1.22 to 2.74 m). This provides width/height ratios ranging from 4.5 to 17.5. Coal samples having width/height ratios ranging from 0.5 to 13 have been tested thus far. As with most pillar design formulae, the strength of the Murphysboro coal versus width/height ratio was linear and closely matched shape factors published by other researchers. The laboratory testing thus far has identified three factors that affect the strength of laboratory samples, the number of discontinuities, the angles of critical discontinuities, and the shape of the samples. The factor that probably had the most pronounced effect on the strength of the coal samples was the angle of critical discontinuities. This testing has confirmed that there is still a gap that needs to be bridged between the strength obtained from laboratory testing and the in situ coal strength predicted.

Jan 1, 2004

By Yi You

A research has been conducted to study the long-term subsidence and its possible effects on surface structures In this research, the surface movements in areas when potential for developing long-term subsidence is high have been monitored over an extended period of time Long-term subsidence data have also been collected from other sources The avail- able data from this on-going research have been analyzed and the results is presented in this paper.

Jan 1, 1997

By John W. Fredland

Conducting mining operations under a body of Water can create hazardous conditions for miners. The potential for a sudden inrush must be considered and abnormal ground control conditions can be encountered. This paper synopsizes the factors which mine operators should take into account when evaluating whether an inrush may occur. It addresses the applicable Federal regulations and discusses some items which coal operators should consider in applying for a permit, Through collecting and analyzing the pertinent information, the likelihood of encountering problems can be judged. Through proper mine planning, the potential for creating hazardous conditions can be mitigated.

Jan 1, 1982

By Yong-Ming Jiang

In cooperation with Cyprus Twentymile Coal Co., researchers from the National Institute for Occupational Safety and Health (NIOSI-I), Spokane Research Laboratory, conducted a study at the Foidel Creek Mine, an underground coal mine near Oak Creek, CO, to evaluate the stability of underground working conditions as a longwall advanced through a series of backfilled cross-panel entries. Instruments installed in the cross-panel entry pillars, backfill, longwall panel, and adjacent headgate entry were continuously monitored by a computerized data acquisition system as the longwall approached and advanced through the backfilled section of the panel. Data collected from these instruments were analyzed to determine the magnitude and direction of the secondary principal stress changes ahead of the longwall face, deformation and closure of the headgate entry, and stability of the cross-panel entry pillars and backfill. A three-dimensional, boundary element model was calibrated to accurately predict mining-induced stress changes and the distance they occurred ahead of the longwall face. As the longwall advanced through the hackfillcd entries, most of the mining-induced load was supported by the cross-panel entry pillars. The backlill provided stability for the root and Moor of the in-panel entries and also confined the entry pillars significantly improving their load-carrying capacity. Comprehensive information derived from backfill material property tests, instrument data, underground observations, and numeric modeling results document the safety and stability of the 8-Right minethrough and should provide beneficial case study information fur other longwall backfilling applications in the future.

Jan 1, 1998

By Daniel W. H. Su

A floor-bearing capacity test apparatus was designed and constructed to determine the true bearing capacity of soft floor materials beneath coal pillars or longwall face supports. The apparatus has been employed in entries up to eleven feet in height. Results of in-mine floor-bearing capacity tests suggested that the ratio of the soft fireclay thickness to the bearing plate width and the moisture content in the fireclay were the most critical parameters affecting the ultimate bearing capacity. Foundation engineering principles were applied to interpret the field test results to estimate the floor-bearing capacity beneath longwall face supports.

Jan 1, 1993

By Paul Hagan

A diverse range of rockbolt designs and resin anchors are available for use in underground mines. Despite their wide spread use, the factors that affect the performance a rockbolt is not fully understood. The laboratory-scale rockbolt test facility at the UNSW Mining Research Centre is being used to gain a better understanding of the behaviour of rockbolts leading to an improvement in ground support. A test program investigated the nature of load transfer between a fully encapsulated rockbolt and the surrounding rock mass. It was found that with all other factors held constant, there were differences in the behaviour of load transfer depending on the method of loading. The typical pull-out arrangement as used in most field test work resulted in a different load transfer function to that observed when the load on the rockbolt was induced through bed separation.

Jan 1, 2004

By André Vervoort

To learn more about the fundamental roof behaviour in South African coal mines, accurate underground measurements of roof deflection were conducted in bord and pillar panels during development and pillar extraction. To conduct these measurements a survey levelling technique was adapted. The overall system reading accuracy was established as being within 0,4 mm on a 15 m sighting distance. The underground sites were modelled and, for a roadway development, similar symmetrical curvatures were calculated by a linear elastic model, as were measured underground. In a pillar extraction panel, a significant amount of bed separation was measured and the roof underwent an a-symmetric movement due to the close presence of the mined out area at one side of the roadway.

Jan 1, 1992

By D. M. Shu

A two-dimensional finite element modelling technique is applied to predict the subsidence movements on a sloping ground surface above a completely mined panel. The modelling is carried out for the surface both directly and using a combination of an equivalent horizontal surface and a rays projection method and the result compared. The rays projection method provides the subsidence components on a sloping surface from the corresponding ones on an assumed equivalent horizontal surface through the point of mean elevation of the relevant part of the sloping surface. This method is used next to compare the subsidences, horizontal displacements and horizontal strains from the mining of a given panel at the surface for different inclinations. The results show that the magnitudes of the subsidence components are greater on the dip side of the slope from the point of the mean elevation, than the corresponding ones on the rise side and the asymmetry increases with the inclination of the surface. The findings are also compared with those of other research workers.

Jan 1, 1992

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By Duk-Won Park

The excavation of a longwall panel causes a continuous stress redistribution in the surrounding strata for every face advancement. The understanding of the behavior of rock to high level of stresses is instrumental to the structural design of underground mines. In order to achieve a realistic analysis of the behavior of strata, the three-dimensional finite element method is frequently used. Also the model developed for the analysis has to be designed large enough to represent the influenced strata in the study area and a sufficient number of elements has to be used to provide detailed information on the stresses and displacements. Due to the size of the model and its numerical solution the CRAY X-MP/24 supercomputer has been used for this study. The mine site selected for the analysis is a coal mine located in the Black Warrior Basin in Alabama. The mine has two longwall sections at a depth of 2,000 feet with various dimensions of chain pillars, including yield pillars. Various research programs at this mine have been carried out by Park et. al. ( 1984 ). Therefore data were already available on the geology, and physical properties of rock and coal. In addition a rock testing program has been carried out to obtain rock mass classification parameters. The progressive failure technique has been utilized to simulate stress redistributions. The failure criterion after Hoek and Brown ( 1980 ) which is based on rock mass behavior was used for the iterative technique. Simulated stress redistributions are therefore very realistic. In order to automate the process involved in the application of progressive failure technique, a computer program has been developed. The combination of a large scale three-dimensional finite element model and the automation of progressive failure simulation provided a tool that can generate a large amount of information related to the behavior of rock and coal as a response to a longwall mining operation.

Jan 1, 1989

By A. W. Khair

This paper presents an analysis of ground movements recorded from longwall operations in the Northern Appalachian region in West Virginia. The site chosen for the investigation was selected due to varying heights of overburden and topography and was instrumented for both surface and subsurface measurements. The instrumentation areas consisted of two panels covering various sections with unique geologic and topographic features. Instrumentation included surveying monuments and shallow boreholes using an inclinometer for measuring horizontal strata displacements. Vertical strata deformation was monitored utilizing a unique design of the mechanical grouting method. Full length borehole profile extension was measured using Sondex displacement rings in the inclinometer installation. Subsurface displacements, i. e. caving zone, fractured zone, and highly deformed non-fractured zone were identified. The nature of surface disturbances with respect to varying geologic settings were compared, and an attempt was made to simulate final strata caving and bed separation using finite element analysis.

Jan 1, 1987

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Stephen C. Tadolini

A new innovation in coal mine roof support has been developed by the Bureau of Mines at the Denver Research Center. The technique, called "Thrust Bolting,"1 converts a traditional passive roof support system, a full or partial column rebar bolt, into an active support system by following a specific installation process. The thrust bolting system provides two advantages from a ground control standpoint. First, because the system is active, no roof movement is necessary to put the bolt into tension. This eliminates the possibility of strata separations in sedimentary roots, minimizing progressive-type roof failures. Secondly, the tension placed on the system is independent of mechanical devices or torques. This eliminates the highly variable frictional losses that occur between the bearing plate and the bolt head and in the threaded portions of traditional active support systems. These two improvements create a safer working environment for mining personnel. An additional advantage of this improved support system is the reduced cost of installation and maintenance. The initial support cost is reduced by a minimum of 30 percent per bolt when compared to similar conventional support systems. The labor intensive and potentially hazardous torque¬tension test, required for most tensionable active support systems, is not required for thrust bolts. The potential for improved support performance and cost reduction of thrust bolting has generated a considerable amount of interest from both industry and MSHA. This paper will describe, in detail, the theory and application of thrust bolting and present the results of two underground coal mine case studies.

Jan 1, 1990

By Thomas L. Vandergrift

Multiple-scam mining is becoming more common in the United States, especially in the East, where primary reserves arc being depleted, forcing mining companies to develop secondary seams, Although mine engineers and planners are aware of the basic effects of seam interaction and generally follow guidelines for columnizing entries and avoiding load abutments, mine design earn greatly benefit bum more quantitative estimates of scam interaction effects. This paper presents case studies from two eastern mines subject to seam interaction effects where pre-mining stresses were quantified: the Mingo Logan Coal Company's Mountaineer Alma A Mine, and Peabody Group's Lightfoot No. 2 Mine. In each of these mines. mining is being carried out within 60 to 90 ft of overlying longwall and pillar panels under maximum cover of about 1,100 ft As part of overall mine planning and design efforts al these mines. NSA Engineering performed numerical modeling analyses using the non-linear boundary-element code MULSIM/NL Input parameters were calibrated by modeling areas where ground conditions resulting from seam interaction were well documented. By incorporating actual cover loading based on topography and seam dip, and the integrity of remnant, gate road, arid harrier pillars in the overlying mines, the pre-mining vertical stress in the underlying scants was estimated based on the pre-mining stress, plans for mains, gate road, bleeder, and pillar panels were developed. Although mining experience in these areas has beers somewhat limited to date, she designs developed as a result of these analyses have so far proven successful.

Jan 1, 2000

By Hamid R. Nikraz

This paper reports the finding of a comprehensive investigation carried out to evaluate the compaction characteristics of saturated sandstone aquifers in the Collie Coal Basin under conditions anticipated during dewatering operations. A triaxial technique has been adapted to evaluate the compaction characteristics of the sandstone aquifer in the Collie Basin. This technique, which allows the strata stress regime to be reproduced by triaxial loading with zero lateral strain, also provides a precise evaluation of lateral stresses and consequently Poisson's ratio under in situ conditions. The paper contains a description of the equipment commissioned, test techniques, results, analysis and interpretation of the data obtained. The testing evaluation techniques are general in nature and can be applied to field situations in locations where similar weak sandstones occur.

Jan 1, 1993

By Zuoliang Xiu

Experience indicates that rockbolting is the most efficient and cost effective means of supporting gateroads of retreating longwalls. However, gateroads supported solely by rockboltiug currently accounts for less than 16% of the 4500km of annual gatemad drivage in China According to a stated target of the Ministry of Coal Industry, this percentage should be at least 50% by the year 2000. Since the learning stage of the technology which is indicated by low drivage rates and frequent roof falls in rockbolted gateroads must be overcome , strategies for the application of rockbolting to gateroads have become the focus of discussions on this topic. This pap presents the recent development on longwall gateroad rockbolting technology in China The development work has concentrated on understanding the mechanism of rockbolting; establishing new design procedures of bolting parameters fix gateroad support upgrading the consumable; optimizing bolt installation procedures; adopting better roofbolting machines; and developing adequate secondary reinforcement technique. The authors believe that this development work together with the on-going wok in other aspects of gaterosd rockbolting, as discussed in this paper 9 represent the rational strategies for the application of rockbolting technology to a wider range of longwall gateroads in Chinese d mines .

Jan 1, 1997

By Thomas E. Marshall

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) examined and characterized conditions at a majority of the underground stone mines in the United States. Observations at these mines revealed a limited degree of roof monitoring beyond visual inspection and sounding. When monitors are used, they typically require the miner to measure movement at the roof. If conditions are unstable, the miner may be in a hazardous situation while recording data. Based on this scenario, researchers surmised that a simple, inexpensive monitor with the capability of recording data at a distance from the mine roof would be a safer way to gain the information. Additionally, more widespread use of monitors could potentially lead to better understanding of roof movement in general. A monitor to meet this need was designed, tested, and subsequently improved as experience was gained in its' use at a number of underground stone mines. The Roof Monitoring Safety System (RMSS) can provide an initial indication of movement in roof beams. By understanding and measuring roof movement in underground mines, the potential for injuries and fatalities to mine workers From falls of ground can be reduced. Also, officials at a mine with a history of data are better prepared to make a decision on remedial actions in the event of ground falls. This paper will outline the evolution of the RMSS and how it can be used in a comprehensive pro-active ground control safety program. Also included is a cast history describing how the RMSS was used in an evaluation of the effectiveness of a mechanical impact scaling machine at an operating limestone mine.

Jan 1, 2000

By W. G. Pariseau

A new approach to the direct inclusion of the effects of joints and geologic structure in rock mechanics analyses is applied to an elastic analysis of stability of a Lucky Friday undercut and 4111 longwall stope (WL) using the finite element method (FW). The approach is base+ on the concept of equivalent properties of a heterogeneous test volume, but does not depend on the assumption that a test volume is a representative volume element. Comparison with conventional FW results show greater variability in fields of stress, strain, displacement, safety factor and energy densities. Stability requires a knowledge of rock mass strengths as well as elastic moduli.

Jan 1, 1989

By David Newman

Landslides and soil creep often occur with varying degrees of severity on steep slopes within Southern Appalachia. Ground movement may take place over years with subtle changes in topography and vegetation or it can occur rapidly in response to water inflow or disturbance at the toe of the landslide. Although the ground movement typically is restricted within the soil and colluvial layers, failure can extend into the weathered rock zone. Certain areas of Appalachia are described as "slide prone" because of historical precedent. To develop a profile of "slide prone" areas, the strength and physical properties of the soil and colluvium, topography, groundwater and surface water runoff, present and historical land use, and any surface disturbance are examined in detail. These attributes are useful in determining whether an area is potentially unstable before surface development including construction, logging, or mining. Recommendations are provided to avoid initiating ground movement concurrent with mining operations.

Jan 1, 1998