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By Christopher Phillips

In light of recent events in the mining industry, there has been an increasing demand for technology to detect underground voids and abandoned mine workings. This paper summarizes the use of geophysical methods for this application. Geophysical methods have been successfully employed in the detection of underground voids in several disciplines, including foundation analysis, road and infrastructure analysis, and darn analysis, but is not yet being widely employed by the mining industry. The principal reasons are that the combination of the complex geological setting of a mine, mine design, surface conditions, and improper choice of geophysical method often makes the task of locating abandoned mine workings with geophysics a frustrating, time consuming, and often unsuccessful venture. While there are several geophysical methods that have been demonstrated for the successful detection of abandoned mine workings, there is still no 'silver bullet' to solve the problem in every case. The choice of geophysical method to detect abandoned mine workings should be made on a site specific, and sometimes target specific, level. This paper reviews current methods, and introduces recently developed methods, for the detection of underground voids and abandoned mine workings.

Jan 1, 2003

By I. Geldmacher

In-Seam Seismic (ISS) methods have been used extensively worldwide for the past 10 years to assist mine planning, espe¬cially longwall mining in coal seams. The technique is only starting to gain wide acceptance in North America where it is being applied in a much wider variety of applications than was initially considered. Surveys elsewhere have concentrated on the detection of faults (structures) in coal mines prior to longwall mining. The majority of surveys in North America has been the location of abandoned workings which were not adequately surveyed prior to closure. Modeling data based upon the lithological logs, geophysical logs and interpreted sections show whether the method will be successful prior to carrying out the survey. The surveys have been performed to locate abandoned wor¬kings from the surface and underground using both seismic reflection and transmission techniques. Data from coal seams in both Illinois and West Virginia will be presented. A second method where ISS surveys are being applied to detect old workings will be presented. The ISS tomography produces an image of the P-wave and Channel Wave velocities which is representative of the material through which the signal traveled thus identifying the location of the old workings. Data from Texas will serve as an example.

Jan 1, 1990

By Robert R. Thompson

Retreat pillar mining is highly productive but dangerous. The primary danger during pillar removal is premature caving of the roof. Because the safety of the miners is dependent on successfully controlling the roof, the roof must cave in a predictable and dependable manner. Temporary support during retreat is usually obtained by setting posts. They are set manually by a miner working in a dangerous area. A solution to this problem would be a system which could set and retrieve roof supports remotely and safely. The Bureau of nines has developed a remotely operated Mobile Roof Support (PIPS) machine which will place and retrieve temporary roof support. Two PIPS machines were fabricated under a cost-sharing agreement with Southern Utah Fuel Company and mining equipment manufacturer. Each machine carries four 50-ton jacks and is remotely- Controlled by radio. The machines are presently being used by Southern Utah Fuel Company under a long-team cooperative agreement. Not only have there been material and manpower cost savings in not having to set posts, but also the time saved bas resulted in increased productivity. The projected costs of mass-produced machines with tethered remote control rather than radio remote control are estimated at $150,000 each. The radio remote control is more expensive and is considered an optional extra.

Jan 1, 1986

By Jinsheng Chen

The major functions of longwall headgate entries are to provide access ways to the outby submains or mains for coal transportation and to the longwall face for fresh air and for transporting men and material during longwall operations. Any roof control problems such as cutter roofs or roof falls in the outby headgate entries could result in significant production delays to the longwall face and may be a threat to miners' safety Under a thinly laminated immediate roof and a high horizontal stress field, even a longwall headgate entry that has an angle of less than 45 degree to the maximum horizontal stress may experience roof deterioration in the outby headgate if the longwall panel mining sequence and retreat direction are not properly considered. This paper attempts to analyze the causes of the roof control problems experienced in the longwall gate entries at our Cumberland and Emerald mines in Pennsylvania and to determine the optimal longwall panel mining sequence and retreat direction for the future mining area at Cumberland

Jan 1, 1998

By M. Guana

Island Creek Corporation is predominately an underground mine operator deploying room-and-pillar and retreat longwall mining methods for extracting coal. Wide variations are encountered in overburden depth, stratigraphy, and insitu stress fields. This requires major adjustments in mine design among mines and occasionally within a mine. Simple rock mechanics principles are utilized in mine design to optimize safety and productivity. Physical observations and micro deformation monitoring of roadways are the primary techniques employed. Pillars and artificial support systems are the primary design elements modified. The specific case histories reviewed in this paper include narrow panels with high extraction for conventionally mined rooms, pillar sizing for longwall gate entries at great depths, and bolt type selection for difficult ground conditions.

Jan 1, 1988

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 Alexander Kattner

Surface fractures in urbanized areas show the risk potential of near-surface mining. This kind of mining subsidence damage occurs mainly in areas where hard coal was mined near the surface with very little overlying rock strata. In North Rhine-Westphalia (NRW), the mining authorities assess areas that are at risk for caveins by means of envelopes constructed according to Hollmann and Nürenberg (1972). Drafting abandoned underground structures depends on the assessment of historical documents such as mine plans, mine titles, and operation plans of prior mines in this area. A research and development project establishing a comprehensive Geo Information System (GIS) for hazard prevention was undertaken in cooperation with the mining authorities of NRW. It aims to display the areas endangered by abandoned, shallow underground mines for the purpose of hazard prevention. A tool was programmed using ArcGIS as a result of this project. It is used to digitize underground workings from preserved mine plans, calculate outcrop lines of coal seams and display hazardous areas. In the future, a database of shallow mine workings will be established. The tool will be used for a systematical mapping of shallow workings and assessment of endangered areas.

Jan 1, 2011

By D. J. Reddish

This paper illustrates how numerical modelling has been utilised to predict water and gas pathways around active longwall panels using two case studies. This information has proved invaluable m the design of methane drainage schemes and for risk assessments undertaken at the mining planning stage. The modelling approach described has now been applied successfully to several situations in UK coalmines in order to optimise the position, length and orientation of methane drainage boreholes. The first case study describes the numerical modelling of the stress and shear plane development mund a longwall panel. The simulation was used to predict potential gas pathways through regions of low stress confinrment which are conducive to fluid flow. The conclusions from the modelling were utilised to allow an increase in the volumes of gas drainage and allow the efficient mining of the panel. A detailed discussion is provided on the model construction, determination of input paramters and optimisation of the borehole positions Details are provided on how the models allow a prediction of the fracture patterns, stress distributions and zones of reduced permeability around an active longwall panel. The second case study describes the numerical modelling of a fault adjacent to a face line drivage of a panel that had just started to be retreated. An inrush of water had occurred and a concern was whether further mining would lead to more extensive inrushes or whether the water inrush was associated with more local, limited capacity water bearing features. The role of a fault that was positioned close to the locality of the mine workings was examined. Fracture patterns stresses, strata permeability and water flows were predicted using mechanical and hydro-mechanical coupled models.

Jan 1, 2003

By Zhanjing Yu

In this paper, an equation is developed to characterize the relationship between the UBC and plate size using a boundary integral approach. The equation was validated by the results from mo& than seventy (70) plate loading tests of different sizes conducted in various underground coal mines throughout the Illinois Coal Basin. The equation indicates that the UBC decreases as the plate size increases. The fundamental solution for this boundary integral problems is the Boussinesq equation. The load exerted by the plate on the floor surface is assumed to be uniformly distributed. Both normal and tangential loads are considered. It is also assumed that the floor fails in shear in a plate loading test. The equation may provide the mine operators with a useful tool to obtain from a plate load test of any plate size the UBC value required by the regulations.

Jan 1, 1993

By S. K. Sharma

The present study attempts to critically review the pillar-design as required by Coal Mines Regulations of India vis-a-vis latest rockmechanics formulae, Equivalent material modelling was the approach adopted for testing the safety and stability of the pillar sizes predicted by various formulae. Two parameters, considered here for calculating pillar sizes was the stresses acting over the pillars and the strength of the pillars. Stresses acting on the pillars were calculated by the tributary area method. Based on the studies of Coates and Hustrulid, the stress obtained was reduced by 40 percent. For calculating the strength of the pillars, three formulae, namely Holland and Gaddy, Salamon and Munro and Central Mining Research Station, India, were used. The pillar widths were then determined by equating the product of recommended factor of safety and pillar stress with the pillar strength.

Jan 1, 1992

By Frank Kendorski

Several startup or longtime underground stone mines have experienced unanticipated roof control difficulties due to the variable geologic character of the roof strata. Careful mapping of roof conditions, and identification of controlling geologic and mine geometric factors using sedimentological descriptors, lead to an understanding of the mechanisms causing roof problems. Types of bedding plane features can be categorized into geometric, internal structures, bedding-plane markings, deformed bedding, solution structures, and accretionary structures. Factors such as encountering open water-filled master joints; roof strata delamination due to internal water pressures; local karst features with mud, sand, and water hazards; and sudden loss of limestone roof due to paleo-channel erosion are some recent problems described.

Jan 1, 2002

By Erik C. Westman

The objective of this Bureau of Mines study is to determine the structural integrity and stress state of coal structure using seismic techniques. Bureau personnel conducted seismic transmission and refraction surveys on yield pillars and seismic tomography on rigid pillars. The pillars studied were in the headgate at increasing distances from the working face of a longwall coal mine, effectively evaluating pillars at lowering stress states and increasing levels of structural integrity. By examining characteristics of the seismic wave, including velocity and frequency, the survey results are used to infer yield zone thickness, core mss state, and stress distribution. An accurate definition of these parameters can not only provide mine engineers a useful tool for evaluating mine design, but also give valuable insight into the inherent safety of the mine structure.

Jan 1, 1993

By Yi Luo

The potential influences of mining subsidence on structural stability, integrity and functionality of guyed tower structures have not been systematically studied before. This paper presents a ease study where a guyed steel tower of a telecommunication company was mined under by a longwall operation. Prior to mining, every aspect of the subsidence influences to this structure was studied. The pre-mining assessment indicated that the structure could experience vibration problem in the second half of the dynamic subsidence process. Bawd on the study, a simple mitigation measure was recommended and implemented and the structure was successfully protected.

Jan 1, 2003

By Karl Brandt

Longwall panel 580 has recently been retreated in the Zollverein 1/2 seam at a depth of approximately 1000m at Auguste Victoria mine with rectangular rockbolted roadways used for both the maingate and tailgate. Although Auguste Victoria mine has been increasingly using rockbolted support since the mid 1990's. this has previously been restricted to tailgates for single use. This panel represents the first use of a rectangular rockbolted roadway as a maingate at Auguste Victoria mine. As such, it represents a significant change in support strategy. As part of the strategy numerical modelling was used to examine expected behaviour of rockbolted roadways at the site, stress changes during face retreat, the feasibility of adopting rockbolted support for the maingate and the reinforcement pattern required. Monitoring instrumentation was installed in both gates to confirm that the support systems were performing satisfactorily as the gates were driven and as the face was retreated. Although the panel is isolated from others in the Zollverein 1/2 seam, markedly different roadway conditions have been experienced in the two gate roads. Favourable conditions were obtained in the maingate, but particularly difficult conditions were encountered in the tailgate. Previous work for the mine had examined interaction with old workings in other seams and shown a vertical stress of 28MPa for the maingate and up to 45MPa for the tailgate. The values from the earlier work were used in the modelling work described, which showed contrasting results given the different stresses. The difference between the gates can be attributed to the interaction. The paper describes the numerical modelling work, the reinforcement systems adopted, the monitoring results, the interaction effects and the comparison between expectations from the numerical modelling results and the conditions actually experienced.

Jan 1, 2002

A computerised influence function approach to subsidence and horizontal displacement prediction is described, with particular emphasis being placed on the applicability of the method to irregular- shaped mine workings. Based on previous work [1], this approach has been put for- ward for inclined seam situations. Examples of graphical output presenting surface subsidence contours, horizontal displacement vectors and principal strain vectors are given and their significance discussed in relation to current problems.

Jan 1, 1988

By Russell Frith

The paper defines the various operational advantages associated with the use of cribless tailgates during longwall extraction and considers the associated ground control issues that need evaluating to ensure a successful outcome. Reference is made to longwall mines in Australia currently using cribless tailgates and the geotechnical and ground support factors that allow them to do so This includes the nature and behaviour of the roadway roof the stability of the chain pillar and also the type and timing of installed support A specific case history from a colliery in NSW will he referred to whereby heavy standing support was replaced with lower density tensioned cables as routine TV secondary support, which resulted in both improved TG roof conditions and a significant cost reduction. The conclusion will be drawn that cribless tailgate roadways can potentially he used in many environments either due to the existence of highly competent immediate roof conditions or through the appropriate application of active reinforcing support measures in less competent conditions. Use will he made of Strata Engineering?s roof behaviour and stability model in demonstrating the various concepts involved.

Jan 1, 2000

By Y. P. Chugh

The paper discusses laboratory and field studies to analyze time-dependent deformation behavior of immediate weak floor strata in an Illinois coal mine. In the laboratory, core samples of the immediate floor strata were subjected to incremental creep tests under unconfined compressive stress. Both axial and lateral creep deformations were recorded. Time-dependent behavior was reasonably characterized by a Burger model with one Kelvin-Voigt unit. The elastic and viscous parameter8 of the model were found to be stress-dependent, but in the range of 40% to 80% of the failure stress, the effect of stress was relatively small. Therefore, a linear visco-elastic model was developed to represent the laboratory time-dependent behavior of immediate floor strata. Such a model may be acceptable for the design of partial extraction mining systems having extraction ratios of 40% to 60%. Field observations of roof-floor convergence (floor heave) at the mine also indicated a time-dependent behavior similar to that observed in the laboratory. Field time-dependent behavior was also characterized in terms of a Burger model with one Kelvin-Voigt unit to obtain scaling factors between the field and laboratory parameters. This approach should help in predicting likely floor heave in the mine based on laboratory creep tests. It was concluded that additional studies in the field are required to arrive at more reliable correlations between the two sets of parameters.

Jan 1, 1987

By D. Newman

An underground limestone quarry, developed in the 1940's, transitioned from a surface quarry to an underground room-and-pillar operation. The quarry expanded to a second underground level when the reserves on the first level reached the property boundaries. The historical basis for the sill pillar thickness between the first and second level, pillar dimensions, and mining height was lost during the course of several changes in ownership. When reserves on the second level reached a five-year life, and plans were initiated for a third level, quarry management embarked upon a thorough evaluation of i. current ground control practices for the support of the immediate roof, ii. roof support in the declines between the surface, Level 1, and Level II, iii. pillar centers and pillar safety factors, iv. pillar alignment between levels, v. mining height within a level, and vi. the locations of declines to access future reserves on level three. The evaluation began with detailed mapping of the location and orientation of calcite veins, joints, fractures, and ground conditions in the immediate roof of Level II. The orientation and location of calcite veins, joints, and fractures on Level II were compared with similar mapping on Level 1, completed in the 1970's to establish whether these features were continuous and would be expected to affect conditions on Level III. Rock strength, physical properties, and fracture spacing were obtained from two (2) diamond core holes drilled from Level I to Level II and two (2) holes drilled from Level II to define the vertical extent of future reserves on Level III. The effort to characterize geological conditions was expanded to a rock mechanics investigation of the: i. performance of the current roof support, ii. optimization of roof and rib scaling, iii. calculation of safety factors for 416 pillars on Level II, iv. evaluation of future pillar centers and mining height within a level based upon pillar strength and pillar geometry, v. development of beam safety factors for the immediate roof on Level II, vi. identification of the most suitable strata for the roof in the future level three, and vii. use of LAMODEL to develop a large numerical model of overburden, inter-level, and total stress on the pillars in levels one and Level II with a stress "footprint" on the proposed level three. The characterization of engineering parameters and geological conditions resulted in changes to the: ? active operations in Level II regarding scaling ? equipment and scaling practices, ? pillar centers, ? mining height, ? raising the immediate roof horizon to avoid ? laminated strata in the current roof, and ? a change in rock bolts from a friction based system ? to active tensioned rebar. The planning and design of level three is ongoing and relies heavily upon the geological and engineering investigations conducted during the past year and one-half.

Jan 1, 2001

By B. T. Wells

This work concerns the stress fields encountered around mining tunnels. A critical review is undertaken of the various analytical assessments of the support requirements for mining tunnels. Comments are made regarding degree and type of support together with method and timing of installation. The results have been used to indicate the type of support required in different mining and geological conditions.

Jan 1, 1981

By Hanjie Chen

A mine panel collapse may occur when pillar sizes are too small or the surrounding rock strata (roof or floor) yields. After a small pillar fails, its loading is rapidly transferred to adjacent pillars that in turn fail. If the surrounding rock strata yield, the loading on pillars will redistribute. which may result in some pillars under high stress. This paper analyzes the causes of two cases of mine panels failures in two room and pillar mines. One case of massive pillar failure occurred in a phosphate mine and the other, mine panels failure, is in a coal mine. In the phosphate mine, both the roof and floor strata were a thick competent shale stratum. The finite element analysis shows that the major reason for the massive pillar failure is that the pillar sizes were too small in a panel. In addition, the high cliff topography has some influence on the failure. In the coal mine, the failed area was about 1,000 ft long by 760 ft wide (covering more than 3 panels) involving 390 pillars. The harrier pillars between the panels were punched on retreat into small chain pillars and the extraction ratio in this failed area was more than 80%. The roof remained intact, and pillars were crushed and punched into the floor. Based on the stress analysis, it is found that multiple panel collapse occurred because the barrier pillars were punched into small pillars and the loading on panel edges increased due to floor failure.

Jan 1, 1999