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By Robert W. Bruhn

A 125 foot high steel lattice frame tower supporting a 500 kV EHV transmission line and located over the gateroads of a longwall mining operation was subjected to ground movements from the mining of adjacent longwall panels. Under an agreement with the owner of the powerline, the mine operator was required to minimize mining impacts on the transmission line. In view of this requirement, the mine operator prepared a forecast of the expected subsidence ground movements prior to the onset of mining. These forecasts, in conjunction with finite element analyses of the tower superstructure under the imposed ground movements and wind loadings. suggested that the structural capacity of the tower would probably be diminished little, if at all, by mining. Ground movements determined by survey during mining of the two longwall panels were found to be in close agreement with predictions. Strains measured in selected tower members during mining generally agreed with values calculated by the finite element analyses. The study confirmed a methodology to predict subsidence and to assess its effects on steel lattice transmission towers.

Jan 1, 1990

By Thomas P. Mucho

To document the performance of mine roof and roof support systems the U.S. Bureau of Mines (IJSBM) has been installing instrumentation at selected sites in U.S. coal mines. Much of this support and roof instrumentation has been installed in high stress conditions to maximize differences in performance. Summarized in this paper are the results of four such site investigations. The roof geology at the four sites is quite different and is quantified using the USBM's Coal Mine &f Rating (CMRR). The studies included detailed measurements of roof movement using multi-point extensometers, as well as measurements and monitoring of bolt loading. The investigations include developmental loading and abutment loading from longwall and room-and-pillar mining operations. Some of the issues examined are the effect of the horizontal stress field, the effect of installed tension (torque-tension versus resin bolts), the effect of reduced annulus bolt holes, and the differences between similar bolts supplied by different manufacturers.

Jan 1, 1995

By Daniel Su

Following several roof falls in the E3 longwall development of a Consol Pennsylvania Coal Company underground coal mine, an underground roof geology reconnaissance program consisting of twenty-nine 20-foot deep roof holes had identified the presence of thick sandstone layers with abundant shale and micaceous streaks. The base of the sandstone was 4 to 8 feet above the roof line within the fall areas. The presence of micaceous streaks created extremely pronounced planes of weakness in the sandstone. A structural irregularity, which extends into the E4 and E5 developments, exists between crosscuts I 1 and 30,where all roof falls occurred. Immediately to the east of this structural irregularity lies a huge syncline. The presence of thick sandstone units within the typically all shale environment and within such a structural irregularity helps to maximize stress concentration and creates closely spaced joints in the sandstone, which explains why many cracks are present in the macroscopically massive sandstone upon development. The approaching E2 longwall gob to the south and on the down-dip side, on the other hand, provided horizontal stress relief in the sandstone, which helps to explain the many roof falls in the E3 development that occurred during or immediately after the formation of the E2 gob. Finite element modeling confirmed the presence of such horizontal stress relief. Since the E3 longwall was a right-handed face, stress concentration was expected at the longwall headgate T -junctions, and further delamination of the sandstone was expected upon arrival of the E3 longwall face. Based on the results of the roof geology reconnaissance program and a roof stability analysis that took into account up to eleven parameters, supplemental headgate and track roof support consisting of 16-foot and 20-foot long cable bolts was successfully implemented within the zone of thick sandstone with micaceous streaks and structural irregularity and at other potential roof instability areas along the E3 development. The cable bolts were designed to be anchored at least 4 feet above the sandstone unit into the mostly competent sandy shale layers. No headgate roof control problem was reported within the sandstone and structural irregularity zone during E3 and the subsequent E4 longwall extractions.

Jan 1, 2002

By E. H. R. Schumann

The local stratigraphy and composition of the coal measures, including competent dolerite sill and massive sandstone lavers is shown to have a restricting influence on the development and magnitude of surface subsidence above total extraction panels in South Africa. The critical panel width necessary to induce nonviolent failure of a dolerite sill is quantified. Small subsidence due to the deflection of a 70 metre thick dolerite above subcritical panels is shown by example. Maximum surface subsidence tilt an; ground strain is compared to the NCB model. An exponential subsidence profile curve, SPC, describes a trough half-profile. A parabolic relationship between horizontal displacement and tilt, which has been discovered to exist can be substituted in the first derivative of the SPC in order to generate transverse displacement and strain profiles.

Jan 1, 1988

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

By Uros Bajzelj

When driving mine roadways in the vicinity of tectonic disturbances and fault zones, sudden outbursts of gas, along with ejections of fine coal, occur. Substantial dynamic changes occur around the roadways in the coal and around the support. The problem has several aspects. Studies have been carried out on the technical analyses of the phenomena which occurred in the Velenje Lignite Mine from 1957 to 1988, The second part of the investigation involves systematic laboratory studies of strength and deformability characteristics of lignite samples and "in-situ" measurements of stress changes during the driving of some roadways. Analyses of events (using the model of limit convergence and the finite element method) showed the possibility of gas outbursts, taking into account the deformability conditions when crossing over from compact lignite to the disturbed region. The rate of advance of roadways and the method of support strongly influence the occurrence of the phenomenon. The preventative measures used in driving roadways under such conditions include trial drilling and the use of blasting instead of machine cutting.

Jan 1, 1992

By Changshou Sun

Excessive deformation occurred in the underground openings of Jinchuan Nickel Mine, the largest nonferrous underground mine in China. Developed in weak rock formations at great depth, the underground openings are under great in-situ stresses with the horizontal stress being twice as high as the vertical one. Based on in-mine observations and in-situ testing results at the mine, time-dependent mite element simulations with coupled creep and plasticity constitutive models were conducted in this study to analyze the stability of underground openings In the study, the Burgers creep model and the Drucker-Prager plastic model were coupled to simulate underground drifts and cut-and-fill stopes. Two-dimensional finite element models were developed with ABAQUS - a comprehensive finite element simulation package The study investigated the behavior of underground drifts at great depth with high horizontal stress and the behavior of cut-and-fill stopes with various stope heights and fill conditions. The numerical simulations revealed important characteristics of opening deformations as a function of time after excavation and location relative to the wall of opening The results had good agreement with field monitoring data, They provided useful information to aid mine engineers in the design of mine openings, the selection of opening supports and the determination of support installation procedures.

Jan 1, 1999

By Michael A. Berdine

In early 1997, Twentymile Coal Company took a dramatic departure from traditional longwall recovery methods. Previously, Twentymile's standard longwall meshing procedure included the use of chain link fencing material. Beginning with the 9-Right to 8-Right longwall recovery operation, Twentymile incorporated a polyester geogrid mesh system supplied by DYWIDAG-Systems International. This paper discusses the subsequent utilization of the polyester geogrid mesh system at Twentymile.

Jan 1, 2001

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 Peter Hayes

Moonee Colliery operates a longwall extraction system in the Great Northern seam of the Newcastle Coal Measures, approximately 100km north of Sydney, NSW. In the initial mine area of longwall blocks1-4a, the Great Northern seam is overlain by a massive conglomerate approximately 35m in thickness. Unanticipated sudden, blocky caving of this material has produced windblasts with associated velocities of 100m/sec and over-pressures of 35kPa. Although these events have not resulted in fatality, the potential safety risk to face personnel was such that the mine was threatened with closure. The development of a Windblast Management Plan to minimise the exposure to windblasts, a Micro-seismic Management flan to predict caving events, and the use of hydraulic fracturing to generate caving on demand, have all contributed to the salvation of the mine. Prior to the introduction of hydraulic fracturing, falls were highly variable and ranged in goaf area from 2000m2 to 30,000m2 with an average of around 7.000m2. Hydraulic fracturing has introduced consistent and far less variable falls, planned as averaging around 5000m2 and all accompanied by microseismic warning, Moonee Colliery is believed to he the only operation in the world using real-time microseismic warning and using hydrofracturing as a day-to-day operational process. These processes have enabled it to return to financial viability in an environment of strict procedural control, whilst ongoing efforts are made to optimise and enhance these systems. Moonee's story is a case study of the successful application of research and development work to the economics of a mine.

Jan 1, 2000

By Winston Gale

This paper is presents a summary of recent investigations into fracture and caving about longwall panels. The results of these investigations indicate that rock failure initiates well ahead of the longwall face. Rock fracture typically forms in response failure through the material and bedding planes. Tensile fractures also form in massive units. These fracture patterns typically create a fracture network which determines the caving characteristics encountered at the faceline. The action of longwall face supports under such conditions is to maintain confinement to the fractured ground and develop a consistent caving line, The confinement developed above the canopy under these conditions can be variable on a shear by shear basis and the operational face support procedures play an important role in stability about the face area.

Jan 1, 2004

By Song Yong Jin

The Jurassic coals at Datong are characterised by their strong, massive sandstone and conglomerate roofs which fail suddenly over large areas, crushing pillars and creating destructive windblasts, collapsed areas have exceeded 180,000m2 and windblasts have extended over one km underground and 200m on the surface and have been accompanied by subsidence troughs on the surface. Scientific analysis has researched premonitory signs of caving and the nature of windblasts leading to considerations of alternative mining methods and ways of dissipating the energy of windblasts to reduce their damage and danger.

Jan 1, 1992

By Dave Forrester

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

Jan 1, 2004

By Wilfried Ufer

[1. Einführung in die Problematik 1.1 Die Bedeutung des Schildausbaus Seit fast 30 Jahren ist die Konkurrenzsituation auf dem Primärenergiemarkt der Bundesrepublik Deutschland durch einen von Erdöl and Erdgas geführten and insbesondere gegen die Steinkohle gerichteten Verdrängungswettbewerb gekennzeichnet, der den Steinkohlenbergbau zu tiefgreifenden RationalisierungsmaBnahmen zwang. In diesem Zusammenhang kam es Anfang der 70er Jahre zur Einrührung des Schildausbaus, der eine erhebliche Produktivitätssteigerung in den Strebbetrieben zur Folge hatte and damit die Uberlebenschance des Steinkohlenbergbaus in der Bundesrepublik Deutschland enorm verbesserte. Der Grund hierfdr lag darin, daB der Schildausbau im Vergleich zu dem vor ihm eingesetzten Rahmen- and Einzelstempelausbau eine wesentlich hdöhere Ausbauleistung erbrachte, weil er sich weit weniger empfindlich gegen Hangendausbruche and aus dem Alten Mann hereinbdschende Bruchberge erwies. So erklärt sich auch die stürmische Entwicklung and die schnelle Verbreitung des Schildausbaus. Bereits 1975 stammte rd. 50 % der Fdrderung aus Schildstreben, 1990 waren es fiber 75 % and heute sind es fiber 90 %. 1.2 Die Staubentstehung beim Schildausbau Die Erfolge des Schildausbaus sind unbestritten. Jedoch nahmen :nit seiner Einführung die Probleme der Staubentstehung and Staubbekämpfung im Vergleich zum Ein zelstempel- and Rahmenausbau erheblich zu. MaBgebend dafür sind hauptsächlich folgende Gründe: - Abbau von dozen nit gebrächen, weichen Hangendschichten, die erst durch Schildausbau abbaubar wurden, - Zerkleinerung des auf den breiten Kappen aufliegenden Bergematerials infolge der höheren Ausbaustiltzkrafte des Schildausbaus, Zermahlen der sick in die Spaltabdichtungen hineinarbeitenden Bergestückchen, - Erhöhung der Wettergeschwindigkeit infolge der durch die Baufarm des Schildausbaus bedingter. Verkleinerung des Strebquerschnitts verbunden mit der Aufwirbelung and Aussichtung von Staub, - Aufwirbelung von bereits abgelagertem Staub im Bruchraum beim Hereinbrechen der Dachschichten. Die genannten Gründe treten zumeist nicht in jedem Abbaubetrieb gleichzeitig auf. Sie rind vielmehr u.a. abhängig vom Ausbautyp, von den Gebirgsverhältnissen and von der Abbaugeschwindigkeit. Trotzdem ergeben sich beim Vorrücken des Ausbaus in der Regel hohe Staubkonzentrationen, die deswegen besonders kritisch sind, weil es sich um quarzhaltigen Feinstaub handelt, der besonders gesundheitsschädlich ist. Die Hauptzutrittsstelien]

Jan 1, 1984

By Stanislaw Prusek

The present state of Czech and Polish coal mining industry is presented at the beginning of the article. This paper will inform the reader of the general data concerning hard coal reserves? number of mines, number of miners, output, number of longwalls, productivity, depth of extraction, and extraction methods?in both countries. The main part of the paper describes the methods of gateroad development as well as gateroad support characteristics (support types, gateroad shape and dimensions, distance between support sets, types of reinforcements). The development prospects of gateroad support in the Czech and Polish coal mining industry are also presented.

Jan 1, 2011

By Larry Stolarczyk

The mining industry would benefit greatly by imaging geologic conditions well in advance of mining. In layered deposits such as coal, trona, quartz, and potash, natural waveguides form and enable electromagnetic seam waves to travel great distances. In the early 1980s, the Radio Imaging Method (RIM) was developed to capitalize on electromagnetic seam wave propagation in the assessments of seam conditions in longwall panels. The initial applications of RIM tomography and comparisons to in-mine mapping of geologic conditions proved that faults, paleochannels, and rapidly thinning coal could be detected and imaged with seam waves. This paper describes the application of electromagnetic seam waves in mapping the margins of a paleochannel crossing a longwall panel and the interesting possibility of adopting the newly developed Full Wave Inversion Code (FWIC) )Newman 1995) to significantly improve imaging resolution. When tomography images are combined with ground control science, a significant reduction in cost, roof fall potential, and waste rock in underground mining may be realized.

Jan 1, 1999

By S. P. Singh

Rockburst is a sudden manifestation of the release of elastic strain energy stored in the rock. Therefore the tendency of the rock to burst should depend upon its ability to accumulate elastic strain energy. The use of Burst Proneness Index as a simple and reliable indicator to recognize the burst prone situations in the mines has been advocated in this study. The Burst Proneness Index exhibited a strong dependence upon the brittleness, compressional and shear wave velocities and the point load strength index. It was also observed that the Burst Proneness Index increases with the increase in the strain energy per unit volume, c ressive strength and the modulus of rigidity of the rocks. In addition, important causes and the measures to alleviate rockbursts have been outlined in this paper.

Jan 1, 1986

By George J. Karabin

Since the inception of a qualitative pressure arch theory by Haak4 in 1911, much work has been done in an effort to apply this concept to underground mining. It has been used in both longwall and room and pillar mining to determine roadway and pillar stability (with varying degrees of success). This paper presents a summary of a case study where the pressure arch concept was extended in a totally qualitative manner to define the source of roof control problems in the mains of a Pennsylvania mine. It was concluded that, due to the method of mining employed, a pressure arch formed over a portion of the entries, and the high abutment stresses accompanying it resulted in extreme roof deterioration. The following discussion will include a general presentation of the arching mechanism, the physical response of the roof strata under mining-induced abutment stresses (both from the case study and in general), a computer simulation of ground stability under both "normal" and "abutment" stress conditions, and finally, potential means of alleviating such conditions.

Jan 1, 1982

Retreat longwall method with re-use of gateroad has been traditionally employed in Japan, mainly because of the advantage of higher recovery. But the Miike Colliery has newly introduced the interpanel-pillar system to maximize the production efficiency. The Department of Mining, Kyushu University, in cooperation with the Miike Colliery and the Coal Mining Research Center (CMRC), has conducted research into the optimum design of interpanel-pillar, because this system may be adopted in other coal mines as well. Field measurements of the deformation of the gateroads and of stress distribution in the pillar show that the designed layout of the panel and pillar and the supporting system are proper from the viewpoints of gateroad maintenance and spontaneous combustion in the pillar.

Jan 1, 1990

By Stephen P. Signer

This paper presents the results of a case study conducted in a two-entry gateroad in a coal mine where excessive roof deformation and bolt loading resulted in failure of many roof supports. The instruments consisted of 16 fully grouted, strain gaged resin bolts, load cells on both full and partially grouted cable bolts, and vertical deflection multipoint extensometers. These instruments were installed on-cycle to measure loading on bolts in an existing support pattern and to determine if cable bolts could be used to improve roof stability. Results showed significant amounts of movement within the bolted zone. In sonic cases, these movements were enough to load the supports past their ultimate strength. Geology appeared to be a significant factor in localized roof degradation; that is, a very weak rock layer in the immediate roof overlain by a very strong rock layer contributed to the development of shear planes. Only one instrumented bolt had a maximum strain of less than 2,000 microstrain, which was the yield point of the steel. The average maximum strain on all bolts was 20.000 microstrain. However, electrical continuity to many gages was lost, and a pattern was noted that would indicate possible bolt failure. Wire mesh and concrete cans installed as secondary support performed very effectively.

Jan 1, 1998