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By Kevin M. Connor, Malcolm J. McPhersqn

Examples are presented in which Time Domain Reflectometry (TDR) was employed to locate deformation of rock masses induced by mining. The first example involved motoring the propagation of overburden fractures above a longwall coal panel and the second example involved the we of a)R to monitor rock mass deformation in the vicinity of a strip mine highwall. Bock mass deformation, strata separation and large block movement generated strains and failure along coaxial cables which were installed in drill holes and the TDR reflections generated by these strains and failures were monitored. Analysis of field data and a preliminary laboratory test indicate that it may be possible not only to locate rock mass movements but also to quantify these movements with TDR data.

Jan 1, 1984

Continuing with the effort of providing new and more effective exploration methods to the mining industry, where direct methods (drilling, cross-cutting, etc.) are not applicable, either for logistical or environmental reasons or because they are costly and sometimes ineffective, we present here a new seismic technique based on full wave recording of 3-D channel waves generated within the coal seam across a longwall. In the past two years we reported the application on In-Seam High Resolution Seismic Reflection (ISHRSR) and In-Seam Seismic Tomography (ISST) to map non-coal inclusions that disrupt the coal seam continuity. In the present paper we present a 3-D In-Seam Seismic Tomography (3-DTSST) method using 3-D channel waves recorded by three component high resolution geophones. The method allows to distinguish longitudinal and transversal components of the channel waves, similar to the spheroidal and toroidal modes for the free oscillations of the earth. The recording of these waves allows a more complete analysis of the free modal vibrations of channel waves, permitting a more complete inversion of the structure of the coal seam through which they travel. The full wave analysis provides additional insights of the detailed coal structure for larger, distances than for the ones previously reported using the one dimensional method. The refinement of the inversion software is also a contribution towards the better definition of the coal seam geometry and structures disturbing them such as sandstone channels, sandstone rolls, partings, mine voids, faults, fractures, etc. The method is illustrated with three case studies where subsequent mining revealed the virtues and limitations. The first example deals with the problem of mapping mine works in one side of a longwall panel which geometry is well known, to verify its applicability to map old mine works. The second example deals with a sandstone body that partially scoured the coal seam withing a longwall panel. The third shows an application of the method to map roof conditions in a longwall panel where this problem critically affected production scheduling and costs

Jan 1, 1996

By Peter Cain

Twenty years of research into the design of permanent gateroad support (packs) in coal mines have resulted in two methods of pack design based on engineering principles. The first, the detached block concept, is based on static loading of the support system by an hypothetical detached block. The second, the roof beam tilt concept, is based on the dynamic loading of the support system as the roof beam tilts over the roadway about a pivot point located in the rib-side. The roof beam tilt concept has been used on a number of occasions to design packs in a variety of conditions. However, recent work in the Sydney Coalfield of Cape Breton Island, Nova Scotia, has shown that design criteria based on the roof beam tilt concept in its present state of development are far from universal in application. After a review of published pack performance data, including detailed studies conducted in the UK, France and Canada, it was found that consideration of the mechanical properties of the roof beam produced a design criterion that fitted the available data. Modifications to the design process using the roof beam tilt concept are suggested for use in pack design. The work also revealed that current research in other aspects of coal mine ground control, particularly the areas of powered support design and subsidence prediction, may provide valuable information leading to major advances in this field. Such developments could also extend the application of the roof beam tilt concept to the design of pillar systems.

Jan 1, 1993

By Miklos D. G. Salamon

If all stratifications are horizontal, the computation of surface deformation induced by coal mining involves the prediction of roof and floor convergence and then the transfer of the effects of this relative displacement to the surface through the use of the appropriate influence functions. A laminated model, in the form of a quasi- continuum, provides a simple means of computing the approximate convergence distribution and leads to the Gaussian distribution as the influence function. The ideas presented in this paper represent a subtle but fundamental generalization of the influence function Method. It is postulated that the influence of a small area of extraction is proportional to the roof and floor convergence and not to the thickness extracted. This difference in definition removes many conceptual difficulties associated with the influence function method. To demonstrate the utility of the model, the surface disturbances induced by the extraction of a parallel sided long panel are derived. Formulae giving subsidence, tilt, horizontal displacement and strain are given.

Jan 1, 1989

By Y. M. Jiang

A users friendly PC Computer Program has been developed to implement a systematic design of the powered support. The required input data are: (1) thickness, RQD, and compressive strength of the immediate roof; (2) mining height; and (3) type, thickness, and tensile strength of the main roof. Several important parameters for support selection under a given geological condition are generated through the implementation of the program, including type of support most favorable for the geologic condition, minimum inclination angle of the legs with respect to the horizontal axis, suitable setting load ratio of the front to rear leg for the 4-leg supports, optimum setting load, and support capacity.

Jan 1, 1989

By X. L. Yao

This paper analyses the effects of mining extraction geometry (width/depth ratio) and dip of seam on subsidence parameters based on a large Database, containing 120 longwall mining cases all within the UK coalfields. The subsidence parameters evaluated in this paper include the following: the maximum subsidence value with different goaf-treatments, the position of maximum subsidence, the position of half-subsidence, subsidence value over the rib-side, positions of maximum tensile strains, positions of inflection points and the magnitude of maximum tensile strains on both rise and dip sides. Additionally, the maximum compressive strain and two limit angles are also studied in this paper. Based on a comprehensive Database analysis, several equations are derived relating these parameters to the width/depth ratio and the dip of seam. The results should assist engineers to design new structures, or take measures to protect existing structures from strain effects. The findings of the research assist in extension of the SEH to inclined seams with greater certainty.

Jan 1, 1990

By Noor Mohammad

Comprehensive numerical modelling of the subsidence associated with longwall mining in UK Coal Measures strata has been conducted and validated against UK data. The Rock Mass Classification System (RMR) was used as the main basis to derive representative in-situ rock mass properties for stiffness and strength parameters for the numerical modelling input. A Finite Difference Method, Fast Lagrangian Analysis of Continua (FLAC) has been used to design a suitable non¬linear model and simulate the behaviour of the caved zone with a unique modification of the post failure properties within the zone of influence. The established model was validated for different configurations, varying depth, panel width and extraction thickness, in a sensible and realistic range. The Subsidence Engineers Handbook (SEH) and a stress abutment model were used as a validation tool in this respect. The approach developed will be modified for application to a number of different International Coalfields with different geological environments.

Jan 1, 1997

By Rebecca Hardy

The escalating demand for coal, in addition to the steady depletion of reserves, has produced increasingly difficult mining conditions for coal mining companies within the United States, resulting in mining above or below previously mined areas. The safe and productive extraction of coal in a multiple seam environment requires specific design technology for effective mine planning. Presently, one of the most useful design approaches for multiple-seam feasibility analysis is using numerical modeling techniques. Therefore, numerical modeling programs LaModel 2.1 and Lam2D were used to determine the feasibility of mining the Herrin (No. 6) coal seam over an existing room-and-pillar mine in the Springfield (No. 5) coal seam in the Illinois Basin.

Jan 1, 2006

By Axel Studeny

Geomechanical-numerical methods are an established tool for planning of underground excavations worldwide. By describing the interaction between heterogeneous layered strata and support elements it is possible to determine their suitability. The German hard coal mining industry has more than twenty years experience in investigation and using these parameters to carry out numerical calculations. Today, nearly all excavations (shafts, pit bottoms areas, bunkers, coal faces, development drivages, gate roads etc.) are calculated by geomechanical-numerical tools in combination with empirical planning methods. A high accuracy of planning results, which is a special requirement in the German coal industry with its deep-level longwalls, bedded and weak surrounding strata and multiple seam mining layouts, depends on the extensive calibration of the numerical models, which is achieved by: 1) Taking a large number of underground measurements with empirical evaluation. 2) Carrying out physical modeling. 3) Using the characteristic features of the installed support elements. This paper compares the planning approach used for multiple and single seam mining and presents the advantages of this planning tool. The main focus is on the feasibility of rockbolting as the sole means of roadway support for multiple seam mining operations, though the paper will also look at how the costs can be improved by using a low bolting density in single seam layouts.

Jan 1, 2007

By David Bigby

In the UK, it is becoming increasingly necessary to open new longwall mining areas above old, abandoned workings. In such cases, strategic and gate roadways must often be driven and maintained across highly stressed areas and pre-existing and sometimes only recently formed subsidence troughs. Ground control at the edges of the subsidence trough can be particularly problematic, with complex combinations of stress and rock conditions being encountered depending upon the precise position in relation to underlying pillar edges. For economic recovery of these reserves, it is essential that rockbolt support is used wherever possible. It is therefore very important to the industry to develop its understanding of ground conditions and support requirements in these areas. This paper describes; ? the historical background to these problems and a series of documented UK case studies from the last 20 years. ? the experiences at UK Coal?s Riccall colliery, where a number of longwall panels were worked at a depth of around 800m, some 65m above previously worked longwalls. Actual experience is compared with pre-mining expectations and model predictions. ? design considerations and experience to date at UK Coal?s Welbeck and Thoresby collieries, which are embarking upon a series of longwalls in the Deep Soft seam, 30-40m above previously worked longwalls. The paper outlines numerical modelling techniques used to optimise such mine layouts within the complex geological and geometric constraints of a mature coalfield. These involve combinations of 2D and 3D finite difference and 3D boundary element models. It also discuss new modelling approaches which will be investigated in forthcoming EU funded research.

Jan 1, 2007

By Murali M. Gadde

Perimeter mining is a special form of very productive partial extraction method practiced at some mines in the United States. In the Illinois Basin mines where all major coal seams have weak underclay in the immediate floor, experience shows that any long-term ground control issues will arise due to floor failures rather than instability of the coal pillar itself. While there is a significant amount of work done on floor stability associated with regular pillars, no work has been done so far on the stability of perimeter pillars. This paper reports the results of a comprehensive study that was conducted for the first time on floor stability during perimeter mining in the Illinois Basin mines. A special plate template, which mimicked a common perimeter pillar configuration was created to conduct in-situ bearing capacity tests at five mines in three states of the Basin. Additionally, because of the complex shape of pillars created in perimeter mining that defies any closed-form solution, floor bearing capacity was studied using three-dimensional nonlinear numerical modeling. Based on these in-situ and numerical studies, some guidelines are provided for perimeter pillar stability analysis.

Jan 1, 2009

By Khaled Morsy

Cementitious seals are made of foamed cements with strength ranging from 100 to 600 psi. These cementitious seals are usually used at locations where the seal will be subjected to some convergence, assuming that this material can absorb entry closure without compromising the structural integrity of the seal. Prior to 2006, Lake Lynn Experimental Mine (LLEM) conducted full-scale tests on various cementitious seals designed to meet or exceed the 20-psi horizontal static load requirements of the old 30 CFR1 75.335. Both hydrostatic and dynamic loading tests were conducted. The structural response (pressure and displacement readings) obtained from these tests were recorded. The stability of the cementitious seal is based on balancing the load applied to the face of the seal from the explosion against the shearing resistance at the seal/rock interface. Therefore, the properties of the seal/rock interface, such as stiffness, cohesion, friction, etc. are critical to determine the stability of a cementitious seal. These properties are usually assumed by the researchers in their models. This paper emphasizes the importance of having the correct properties of seal/rock interface in the numerical modeling. FLAC 3D models were developed to simulate Celuseal2 cementitious seals subjected to both hydrostatic and dynamic loadings. Using the hydrostatic and dynamic loading test data conducted at LLEM, the static and dynamic properties of the seal/rock interface were back calculated. The FLAC 3D models showed good agreement with the test results.

Jan 1, 2009

By R. D. Yancich

A case study of longwall mining surface subsidence was performed at a mine in Southwestern Pennsylvania. The mine operated in the Pittsburgh coal seam which averaged 70-72 inches with 6-8 inches draw slate in the longwall location. The average overburden depth ranged from 625 8eet to 660 feet. Three adjacent longwall panels were studied and classified as Panel 1, Panel 2, and Panel 3. The following is information on each and can be seen on the General Mine Map (Fig. 1).

Jan 1, 1984

By David Bigby

A remote reading dual height telltale system has been developed to provide mine management with early warning of impending roof failure. This system is a logical development from the visual dual height and triple height telltales which are increasingly being used worldwide for routine monitoring of mine roof. The paper describes the principles and history of telltales from their origins in France, their refinement and routine use in the UK and their subsequent spread around the world. They are now being used in various forms in USA, Canada, Germany, Spitzbergen (Norway), Poland, Ukraine, Russia, Australia, South Africa, Mexico, India, Egypt and China. The remote reading telltale system has coal mine intrinsic safety approval in Europe and the United States (MSHA) and the first commercial system was installed in a German colliery at the beginning of the year. Up to 100 dual height telltales can be connected, in a simple "daisy chain" configuration, to each of 4 underground interrogation units. These are connected through a twisted pair link to a surface computer which displays the readings, updates the database, generates warnings and provides the user interface. The paper describes the features and principles of operation of the system which differs considerably from conventional mine-wide monitoring systems. A European Community funded research programme is currently underway with the aim of developing improved alarm generating algorithms including evaluation of interfacing a neural network to the system. The paper concludes by exploring other potential applications of the system for ground control monitoring currently in development, including extensometry, stope closure monitoring and support load measurement.

Jan 1, 2001

By David Newman

The Southern Appalachian coalfield has a long history of coal bumps that are attributable to a unique combination of topography, geology, and multiple seam mining. The high pillar stresses that generate a coal bump result from: i. mountainous topography where the overburden commonly exceeds 1,500 feet beneath ridges, ii. multiple seam mining, iii. thin interburden between active and abandoned mines, iv. thick beds of competent sandstone and sandy shale strata throughout the stratigraphic section, v. strong coal seams, and vi. irregular pillar and mining geometry in older room-and¬pillar mines. Coal has been intensively mined for more than 100 years in Eastern Kentucky, Southwestern Virginia, and Southern West Virginia. As a result, it is commonplace to have abandoned mines above and/or below active operations. Therefore, those conditions that generate coal bumps will be more prevalent in the future, as underground mining continues in Southern Appalachia. The focus of this paper concerns two coal bumps that occurred in the Kellioka seam where both undermining and overmining had occurred. The purpose of the investigation is to determine the magnitude and distribution of vertical stress that initiated the bumps, to identify bump-prone areas in the unmined reserve, and to recommend pillar centers and mining layouts to avoid future bumps. The state of stress in three vertically adjacent seams was examined using the LAMODEL boundary element program. Large, detailed numerical models were constructed of both bump areas and of the remaining unmined reserve block. The results of the numerical analyses were compared with those obtained using analytical equations.

Jan 1, 2002

By Madan M. Singh

Currently longwall coal mining operations require at least 3 entries on both the headgate and tailgate ends because of ventilation and safety requirements. Driving of entries, however, is a slow and costly process. Often it is a bottleneck in longwall development. Single entry gateroads are permitted in Europe and commonly used there. This study entailed ascertaining the feasibility of using a single entry in the United States, without sacrificing safety. This design was conducted for an operating longwall mine in the Black Warrior coalfield in Alabama, An understanding of rock loads and consequent support requirements is prerequisite to the design and successful demonstration of the single entry system. Efforts in this regard were directed at determining center (packwall) support requirements for various single entry widths. Critical to the analysis is knowledge of the nature and extent of side abutment stresses. In collecting geological and geotechnical information relating to the stability of the single entry, Engineers International, Inc. engaged in the following: ? Examination and inspection of the single entry demonstration at the Sunnyside Mine in Utah. Severe ground conditions and unacceptable levels of entry closure were experienced there prior to the tailgate phase. Observations at the mine presented a clear perspective on possible ground control problems for future single entry designs. ? Reconnaissance inspection and classification of roof rock conditions. ? Detailed mapping of roof structure in the areas to be instrumented. ? Installation of convergence stations in the longwall headgate to monitor roof movement during mining. ? Measurement of side abutment stresses in a yield pillar in the area of the convergence measurements. ? Measurements of coal fracture distribution by the detail area method. ? Rock properties determinations of roof, coal, and floor rocks; samples obtained by core drilling. ? Determination of floor bearing capacity by in-situ testing.

Jan 1, 1982

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Thomas M. Barczak

With the development of the shield support, the primary requirement for successful ground control in longwall mining is to provide stable gate road and bleeder entries. Wood cribbing has been the dominant form of secondary support. However, the cost and limited availability of timber, along with the poor performance of softwood crib supports, has forced western U.S. mines to explore the utilization of other support systems. The recent success of cable bolts has engendered much interest from western operators. Several innovative freestanding support systems have been developed recently including: (1) "The Can"' support by Burrell Mining Products International, Inc., (2) Hercules and Link-N-Lock wood cribs and Propsetter by Strata Products (USA) Inc., (3) Variable Yielding Crib and Power Crib supports by Mountainland Support Systems, (4) the Confined Core Crib developed by Southern Utah Fuels Corporation; and (5) the MEGA prop by MBK Hydraulik. This paper assesses design considerations and compares the performance and application of these alternative secondary support systems.

Jan 1, 1996

By S. Yamashita

Recently in Japan, unexpected degradation (surface subsidence) at old underground quarry sites, such as 'Ohya-Ishi' (pumice tuff) pit field has often been occurred. At these quarry sites, the excavation was started 20 or 30 years ago with mechanical chain cutter and the Room and Pillar method, and stopped 7 or more years ago. Therefore, a reliable method has been required for an estimation of long term mechanical properties of the rock and for an observation of a stability of rock-pillars or structures. In this study, creep tests under uniaxial compressive load were carried out on the 'Ohya-Ishi', with the aim of estimating the long term stability of the rock. From these results, a creep mechanism of the rock was discussed using the failure mechanism hypothesis proposed by Bieniawski and by others. On this basis, critical time to failure was predicted by the relationship between the logarithm of life time and stress ratio of the sustained axial creep load. Additionally, outline of the monitoring system of AE/MS activity in the field, which have set by the Ohya Aria Consolidate Public Corporation and the others to predict or to prevent the degradation of those residual pillars, is to be reported. Key words : Ohya-ishi, Pumice tuff, Mine pillar, Long term stability, Creep, Critical time

Jan 1, 1996

By Gang Chen

Conventional methods designate a factor of safety, a deterministic number, to indicate the stability of a mine excavation. However, this deterministic description of excavation stability is frequently insufficient or inaccurate and, sometimes, misleading. Employing the probabilistic analysis to determine the likelihood of mine excavation failures will lead to a better understanding of ground control problems and result in more rigorous designs of the excavations. Based on the theory of probability, two approaches are taken in this study for the probabilistic analysis of excavation stability. The first one utilizes direct integrations for the estimation of failure probabilities of mine excavations. Due to the complexity of the integration, this approach can only be applied to solve relatively simple problems. In order to overcome the limitation of the direct integration method, the second approach employs the first-order second-moment approximation of the failure probabilities for more complex ground control problems. The probabilistic analysis offers a deeper insight into the uncertainty associated with the stability of mine excavations. The study indicates that the distributions of parameter values play a significant role in the mine excavation stability, which can not be disclosed by the conventional methods of stability analysis. Greater dispersiveness of the parameter values will lead to a different probability of failure, although the factor of safety estimated by conventional methods stays the same if the mean values of the parameters are constant. By taking consideration of the probabilistic distributions of the parameters, the probability model can provide more reliable designs of mine excavations.

Jan 1, 1996