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By Joe Zelanko

In order to prevent roof fall accidents and injuries, U.S. underground coal mine operators must develop and implement effective roof control strategies. Various tools are available to assist engineers in developing designs. For example, criteria to address global stability issues (e.g., pillar stability) are well established in the U.S.; effective tools (e.g., empirical and numerical) are available to diagnose potential problems and avoid catastrophic failures. However, most miner injuries occur as a result of local instability (e.g., a roof fall in a single heading or a rib roll from a pillar corner). Hazard recognition and the availability of support plans suitable to those conditions are key to maintaining stability and safety on the working section.

Jan 1, 2011

By P. M. Lin

A subsidence monitoring program over a longwall panel was established (1) to explore the impacts of dynamic subsidence on the ground surface and structures and (2) to correlate the movements between the structures and their corresponding points on the ground. The program consisted of a transverse line survey and a series of survey points around the structures on the ground surface and their corresponding survey points on the exterior walls of the structures. The vertical displacement of each point can be expressed as: where Si is subsidence at i point; Sif is the final subsidence at i point; F is face location in terms of distance from the i point and SD is seam depth. The development of subsidence can be divided into five stages. Damage to the structures starts at the beginning of the second stage, The maximum damage developed in the middle of the third stage. The relationship of the vertical displacement between ground surface and structures can be expressed as: G = aH + b where G is ground displacement, H is structures displacement, and a and b are constants. The relationship is linear. However, the strain is non-linear and more complicated because of the interaction of the ground surface and structure.

Jan 1, 1989

By Huamin Li

Mine # 8 of Pindingshan Coal Group, Henan Province, China has an annual production 2 million metric tons. But it encounters many problems, e.g. mine layout is complicated, roadway maintenance is difficult, coal and gas outbursts are severe, production cost is high, etc. The major reason was due to the additive effects of abutment pressure interaction of previous mining in D, E, and F seams. At the request of mine management, an in-depth systematic analysis including underground observation, modeling with simulated materials, and photoelastic experimentation was performed to obtain site-specific results which include the dynamic and static influence zones of mining D, E, and F seams, propagation characteristics of abutment pressure in the floor strata, interburden movement caused by undermining, etc.

Jan 1, 2000

By K. Y. Haramy

This report describes the development of an equation for predicting the crushing strength of 2-inch (5.08-cm) cubes of coal from point load tests made on irregular lumps. The equation was developed from point load and cube crushing strength tests performed on coal samples from six Western U.S. coal seams. For the six coals tested, the means of the point loads [(P)] in kg at failure were approximately proportional to the deans it the distances [(V)], in cm between the platen points for lumps in the 2 to 10-cm size ramie. This finding is at variance with results of tests on other rock types reported in the literature, which tend to show that P is proportional to D2 , as is normally assumed (in theoretical grounds. [The average crushing strength of a 2-inch (5.08 cu) cane for each type of coal was plotted against the log 10 of the slope of the P vs D plots, AP/AID . A straight line was fitted to these plots. The equation of the fitted line is cr = 1.72 x 10 4 10gK -2.10 x 10 4, where Cr is re estimated 2-inch (5. 08-cu,) cube crushing strength of the coal in psi and K is the scope of the straight line fitted to a plat of P v D for the same coal.]

Jan 1, 1982

By Axel Preusse

For the last 150 years, subsidence damage to the surface has been an unavoidable phenomenon associated with underground coal mining. In particular, the mechanization of mining methods in multiple seam mining and the concentration of mining activities in densely populated European metropolitan areas (e.g., Ruhr, Upper Silesian industrial region) have led to mining having a very strong influence on the earth?s surface. In both these regions, surface subsidence of more than 20m has been reported up to now in comparison to the original state. Underground coal mining in these regions strongly impairs all kinds of surface use, such as urban and industrial development, infrastructure (roads, waterways, and railways) and pipelines?without restricting the functionality of those objects to a greater extent. Demanding and, in some cases, globally unique engineering solutions have been developed and implemented for this purpose. The previously mentioned mining and industrial regions are still very strongly characterized by the (increasingly declining) coal industry and will remain dependent to a large extent and for an indefinite period on artificial drainage measures because subsidence that has already occurred is irreversible. This paper will primarily discuss the prediction of subsidence that is caused by intensive multiple seam mining and its influence on surface objects (e.g., underground pipelines). However, an approach for predicting ground uplifts due to rising mine water as a consequence of mine closures will also be described.

Jan 1, 2012

By Axel Preusse

In the Ruhr district, pipelines of public and private infrastructure are made of cast iron, steel, plastics or asbestos cement. Legal framework, technical regulations and normative standards define which pipeline materials may be used for the transmission of which kind of product Thus ground water endangering substances as well as inflammable or explosive products are only permitted to be transported in steel-made pipelines. Ground movements, due to underground mining, are transmitted via the soil to the pipeline and cause extra strain within the steel pipeline (Figure I). In extreme cases durable deformations may occur, characterized by an oval profile or a fold in the pipeline. To avoid or minimize damages, balancing elements are installed into the concerning pipeline, reducing the ground movements which might otherwise cause damages. [ ] The distance between the balancing elements is not arranged by random, but by a calculation method developed in a doctoral thesis [Spielberg, 1999] which represents the actual available standards in science and technology. The calculation method considers all relevant influencing variables with impacts on the movement behaviour of steel pipelines in mining areas, such as technical data of pipeline design (e.g. steel grade, wall thickness, weld seam quality), soil conditions (e.g. friction), laying depths of the pipelines, the operational impacts (working pressure, corrosion) and the mining impacts. As of today, mining has caused no damages to steel pipelines protected by this calculation method.

Jan 1, 2004

In recent years, improvements made to the longwall system have accelerated at a faster pace than those made to the gate entry development system. As a consequence, unless planned years ahead of time, extending the panel length is nearly impossible without some innovative approach. At Cyprus Amax Coal's Emerald mine the G panel length was extended by 3,500 ft- by developing three entries across it to facilitate development of its headgate and tailgate entries simultaneously when the adjoining F panel was in operation. After evaluating alternative supporting methods and associated risks, determination was made to backfill these entries and crosscuts using a low strength flyash cement mixture prior to mining thru. Instrumentation was also used to monitor the stress change or front abutment stress in both the coal and the backfilled material ahead of the face. Longwall mining thru these three entries took about four eight-hour shifts and proved to be very successful. The designed flyash cement mixture provided adequate roof stability and caused no difficulty in the shearer's cutting while eliminating an in-panel longwall face move and minimizing the risk of roof or floor failure in the in-panel openings, or in the worst scenario loss of the face during the longwall cutting thru. This approach provided the intended result of extending G panel and recovered about one million tons of coal. This paper summarizes the engineering design of the cutting-thru project and flyash cement mixture, front abutment stress changes within the longwall coal block, backfilled material, and coal pillars ahead of the longwall face, and our underground observations

Jan 1, 1997

By Axel Preusse

Due to a political decision, German hard coal mining will come to an end in the year 2018. By then, there will have been more than 150 years of industrial grade mining activities. Located in the western part of Germany is one of the most densely populated areas of Europe. Multiple coal seams have embossed the earth?s surface up to 20 meters (21.87 yards), as mining has subsided. The aftercare of hard coal mining will consist of three tasks: groundwater management, polder management, and groundwater treatment. The Ewigkeitslasten (cost in perpetuity) at the current groundwater level has been determined in the past. The actual cash value is calculated to be about 12.4 billion euros (approximately 16.5 billion dollars). The cost of groundwater management is directly related to the groundwater level. Rising groundwater levels will reduce the cost of pumping water. Known effects of rising groundwater are uplift of the surface, emission of methane gas at the surface, higher risk of cave-ins, and contamination of drinking water horizons. Each of those processes produces cost in order to prevent negative consequences (e.g., uncontrolled methane emissions) or to handle reparations (e.g., damages caused by surface uplifts). In the long run, it is the goal to minimize the Ewigkeitslast. As a first step, all cost-generating processes are specified, and a cost type based characterization is performed. The objective of this case study is to identify key parameters to determine the quality of a cost-generating process (proportional cost, fixed amount cost). Afterwards, it is intended to specify the quantitative impacts of each parameter and to set up an optimization model to minimize the overall cost. The quantitatively specified parameters will form the base out of which the optimization model is created.

Jan 1, 2013

By J. K. Owens

As part of a larger research effort focused on ground control, the U.S. Bureau of Mines is currently evaluating the effectiveness of using stereological analysis for characterizing mine roof strata. Input for a stereological analysis of a rock mass is best obtained by scanning (imaging) the walls of clean drill holes using a video-imaging borescope or other video device. Two classes of features are being investigated: (1) Lithologic types that comprise the overall rock mass and (2) structural discontinuities (e.g., joints, foliation, bedding planes) within a given lithologic unit. In the former, the volume fraction of a given Lithologic unit in relation to the entire rock mass can be estimated. In the latter, the mean intercept length between adjacent structures and surface area density of the geologic structures can be estimated. Examples of sampling procedures and subsequent computations are presented for several situations using information collected from drill holes in roofs of underground mines. One such example is based on the trace of a cycloid curve used as a stereological sampling path on the cylindrical wall of a drill hole. Stereological measures, such as intercept length, volume fraction, and surface area density, are explained in the context of rock mass characterization.

Jan 1, 1994

By Jin He

Based on the simple-beam theory, a stress analysis model was established for undermining in multiple seam strip-pillar mining when the strip pillars were not columnized; the effects of various mining factors (e.g., mining depth, inter-burden thickness, recovery ratio, etc.) on inter-burden stability were analyzed; the changes in inter-burden stress distribution were investigated; and the important role that the inter-burden spacing and the mining width of the lower seams on inter-burden stability was studied. Finally, the actual practice used in Feng Feng Mine area was used as an example to demonstrate the rational mining width for a fixed mining depth.

Jan 1, 2006

By D. L. Price

This paper presents a method of measuring field power consumption of shearers and deriving specific energy consumption versus shearer haulage speed performance curves. A description of a computer program which simulates the cutting action of a shearer drum is presented. A comparison between the field specific energy curves and the computer simulation results is made. The simulation results compare well with the field data where all the coal seam is being cut by the shearer. The computer simulation results give hack-calculated field pick force cutting characteristics, which allow the performance of the shearer to be predicted for other operating conditions, e.g. effect of changes to pick lacing, effect of changing the drum rotational speed, allows the calculation of power requirements for a particular production rate.

Jan 1, 1992

By Russell Frith

For many years, underground rock mechanics and in particular, roadway/tunnel roof stability has been underpinned by the often unchallenged assumption that roof strength (as defined by the UCS) and stiffness (E) are key stability controls. This has logically led to the proliferation of laboratory testing of rock specimens and the development of indirect geophysical methods to gain estimates of these two rock parameters. Furthermore, many design methods are significantly focussed on replicating rock mass behaviour through either intact or failed constitutive models. Demonstrably the strength and stiffness of the host rock material is commonly used as one of the key indicators of excavation roof stability and it finds either direct or indirect use in just about every rock mass rating system in use today. In more recent times there has been a common move to consider and apply (even if only conceptually at the current time) structural engineering type principles (e.g. buckling) to coal mine roadway roof (and rib) stability. Similarly our knowledge of the in situ stress environment and its likely origins has improved significantly, largely through stress measurements and subsequent analysis. This paper combines knowledge in both of these fundamental areas through a deterministic model for roadway roof stability and in combination with field examples, reaches the almost certainly controversial conclusion that UCS and E are commonly irrelevant, albeit that the former may provide an indication of other relevant geotechnical parameters (e.g. bedding cohesion). As with all hypotheses or rules, there are naturally exceptions and in this case, the most obvious is the tailgate of the longwall panel (with adjacent goaf). Due to the significant change in the strata loading environment of a longwall tailgate as compared to first workings for example, the stability equation materially changes so that UCS and E become critical controls. The point of the paper is to present a different perspective on a traditional mining problem and to challenge geotechnical professionals to keep thinking ?outside of the square? in the never-ending endeavour to improve our understanding of the engineering problems we regularly face. Such an understanding impacts upon such issues as geotechnical data collection from borecore, support hardware requirements and design capabilities. Therefore making the assumption that our understanding is always fundamentally correct could in fact be limiting the development of new and improved engineering in the future.

Jan 1, 2006

By John H. Perry

Strata III is a full-spectrum roof control system which provides the mine operator the ability to upgrade his roof control in place. The procedure is easily adaptable with current roof bolting techniques. To use the system, the operator starts by installing a special hollow core mechanically anchored roof belt during the roof bolting cycle. This bolt can be upgraded to a fully grouted tensioned rebar bolt by injecting StrataGee G-300, a special polyurethane grout, around the bolt through an injection port in the roof plate. This same bolt can then be used to inject RokLok binders for roof consolidation by pumping the binders through the core of the bolt. This concept in roof control was developed to provide the mine operator a flexible way to maintain roof control in areas which deteriorate over time, or must be kept open for long periods of time. Possible locations of use would be: longwall set-up rooms, recovery rooms, headgates, intersections, and belt entries.

Jan 1, 1987

By Peter Fischer

Between 45,000 and 50,000 claims for subsidence are submitted each year to the German hard coal industry. In order to improve the service to claimants a quality management was established in 2005. This paper describes the process now used for handling these claims. A quality check has also been carried out by way of a customer survey, which was performed by an independent research agency. The results of the survey are presented in this paper. In Germany, research into mining subsidence mainly involves the prediction of ground movements caused by deep mining operations. Special areas of research are subsidence reducing aspects of the extraction panel planning, especially from the point of view of mining dynamics, as well as securing and restoration measures. This work seeks to prevent or minimize mining-induced damages close to the surface, for instance, buildings or infrastructure installations (such as pipelines), or underground mine openings (e.g. shafts). Roof collapses and surface fractures caused by shallow mine workings of an earlier era currently constitute a special area of investigation.

Jan 1, 2006

By Frank Schmidt

Due to changing geology, rib bolting is an ever-changing problem in ground control. With the economic status of the coal market, the need for safely increasing productivity in rib bolting is at an all-time high. The current solutions used in the market often require a long installation time (e.g. if collapsed holes have to be redrilled) or result in poor quality installations that can lead to insufficient ground control. A new bolting product based on the simple screw principle has been developed to bolt ribs faster and with a more repeatable, higher level of installation quality. The paper covers the product invention and development, the fastening principle and its advantages over current systems and first experiences in the market. The first underground trials have shown that the screw anchors can be installed safely and efficiently, often faster than traditional methods. The screw anchor does not suffer from conventional bolting issues such as collapsed holes due to bad ribs. The screw anchors are designed to obtain steel failure and load testing underground shows consistent fastening in all types of conditions.

Jan 1, 2013

By Karsten Zimmermann

Surface deformation monitoring is required all over the mining industry. For example oil and gas companies need this information to support exploration and production activities and to detect impacts from surface subsidence of existing hydrocarbon fields. Also opencast mines require large area information of subsidence due to dewatering of the overburden. Because of this, a lot of surface deformations measurements with GPS or traditional geodetic measurement techniques have been conduced during the last decades. This paper presents the actual capabilities to analyze GPS data using the state of the art Bernese GPS Software for GPS post processing. A new way of monitoring subsidence is a combination of GPS and satellite InSAR data. Considering this it is hoped that a combined strategy will permit receiving overall more information by integrating high point density of InSAR and accuracy of GPS. DMT is one of the leading actors on GPS services especially for monitoring surface movements from underground activities e.g. mining, water pumping or oil and gas exploitation. The paper will discuss preferences and problems of this approach on results of a current project of DMT.

Jan 1, 2008

By F. Ziaie

Line electrical resistivity method using physically infinite distance between current line electrodes is proposed to determine the location of mine workings. In this method at least three line electrodes are used. One of the current electrodes is used as a sinkhole electrode while the other two are used for different activiations of line electrodes when the survey area is between one of the line electrodes and outside the other one. The resistivity profiles perpendicular to the current line electrodes for different activation of electrodes is plotted versus the distance of the midpoint of the measuring potential electrodes. The anomaly due to presence of any locally lateral resistivity variation (e.g. mining cavity) will represent itself as peak or trough depending on resistivity contrast between the source of the anomaly and surrounding medium. Physical simulation of the proposed model was conducted by using a tank model in the laboratory. The results of experiments appear to be consistent with the theoretical model. Experiments for different depths and different spacial locations of the simulated cavities appeared to be successful for locating the depth and size of the cavity.

Jan 1, 1989

By Anton Sroka, Antoni Tajdus, Axel Preusse, Krzysztof Tajdus

"In Europe, the magnitude and range of mining deformation predicted on the surface have been, in most cases, determined using methods based on Gauss' influence function (e.g., Knothe's theory, Ruhrkohle method, Stochastic method). The increase in the application of numerical methods (finite element method FEM, finite difference method FDM, etc.) provides an opportunity to offer a different approach in describing mining deformations. However, this phenomenon is very complex, and due to numerical limitations, a two dimensional analysis was made (in plane strain) in most cases. Such 2D models could not accurately describe the mining geometry and geological conditions. The authors of this paper carried out the calculations in 3D models which describe a mining situation and the character of the strata in one Upper Silesian Polish mine. Furthermore, the authors include some tips for three-dimensional modeling.INTRODUCTIONThrough the years, many numerical models have been used to describe a phenomenon of underground mining exploitation influence on the ground surface. However, such rock masses are highly disturbed (cracks, fractures, changes in water conditions, etc.), and it is very difficult to suitably describe, in a mathematical manner, such materials. Therefore, many of the numerical models created have been corrected in various degrees.At first, models widely used to describe the behavior of strata in the region of exploitation appeared as elastic models, where scientests assumed that a whole strata behaved in a linearly elastic manner, according to Hook's law. Those models have been modifed to received suitable VALUES (of settlement and range of subsidence troughs. Scientests created this amongst others: elastic models with contact between the rocks layers (Shippam, 1975; Salamon, Chugh and Yang, 1993), elastic models with no tension (Chrzanowska-Szostak, 1988), and elastic models with a contact on the subsidence limit plane (Tajdus K., Tajdus A., 2005). However, in most cases, those models did not bring the expected results and the differences between shapes of measured and the calculated subsidence trough were substantial."

Jan 1, 2010

By Anastasia M. Suchowerska

The design of effective ground support requires a sound understanding of the pre-mining state of stress in the stratum to be mined. Irregularities in the stress field encountered during multiseam mining are governed primarily by the previous mining in the surrounding strata. The aim of this study is to quantify the effects of rock mass transverse isotropy on the vertical stress redistribution in the rock strata beneath supercritical longwall panels. Wilson?s equations for determining the stress distribution induced in and beyond a single longwall panel after it has been mined have been adopted, and a plane strain finite element analysis of an elastic medium, together with the stress changes predicted by Wilson, was used to calculate the final pre-mining stresses in the underlying strata. Values of the independent shear modulus (G¢) of the rock strata, ranging from 0.1 to 1 times the isotropic shear modulus (Giso) in the plane of the strata, and a Poisson?s ratio (?) of 0.25 have been investigated. The key finding of this study is that the predicted magnitude of vertical stress in the transversely isotropic strata beneath a mined supercritical longwall panel is larger and changes more quickly with horizontal distance than is the case for an isotropic rock mass. This finding is of significance for the design of ground support measures in multi-seam mining operations.

Jan 1, 2012

By Lightfoot. Nicholas, Na Liu

"Mining in multi-seam workings has been undertaken for many years in the VK Remnant pillars above and/or below current working areas have an important influence on retreat gateroad conditions at five of the remaining six deep coal mines. These interaction effects need lo be predicted ahead of mining so that longwall panels can be positioned to minimize roadway damage and to identify areas where additional support should be installed early into competent strata when and where it is most effective.Currently, three-dimensional elastic stress analysis using MAP3D is undertaken to predict local vertical and horizontal stress magnitudes during gateroad development and in front of the face on retreat. The predicted stress fields are often used as input to FLAC for roadway modelling.Using Maltby Colliery in South Yorkshire as a case study, this paper illustrates the MAP3D approach, together with its current limitations and uncertainties. Possible future improvements in the analysis method are proposed and the authors would welcome comments on these, or suggestions for alternative approaches.MODELLING OF MULTl-SEAM LAYOUT & MULTI-SEAM INTERACTIONThe remaining large coal mines in the UK arc operating at depths of 800-1, 150 m (2,625-3,773 ft). Production panels are now mined on retreat after the accompanying single entry access roadways have been completed. These gateroads are typically supported with a system of steel rock bolts and cable in the roof and steel bolts and glass re in forced polymer (G RP) dowels in the sidewalls.In addition to new panels being mined in the vicinity of old goafed districts, a significant amount of extraction has usually already taken place in coal seams above and/or below the current working areas. This means that there is also interaction between the current workings and the remnants of the old workings in other seams. As a consequence of the mining methods employed, old workings can have a complex geometry compri ing collapsed and partially collapsed excavations intersper ed with small and large pillars of unmined coal. The unmined coal pillar act as stress concentrators that carry the overburden load while the mined out excavations create stress shadows of varying degrees depending on how much collap e and reconsolidat ion has occurred. This is illustrated by a simple elastic model as shown in Figure I."

Jan 1, 2010