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By Kenneth Rigsby

Black Mountain Resources' Highlands Mine No. I is a full extraction room and pillar operation located in Harlan County, Kentucky. Black Mountain extracts the 0.9 to 1.1 meter (36- to 42-inch) thick Owl Scam that is situated 60 to 70 meters (200 to 235 ft) above the longwall panels of the abandoned Arch No. 37 Mine, in the 3.4 meter (I I ft) thick Harlan Scam. In the areas where full extraction has taken place, the overburden above the Owl Seam ranges from 60 to 430 meters (200 to 1,400 ft). A short room and pillar panel was developed above an extracted longwall panel to test the selected pillar size, to evaluate support systems, and to verify the shape of the subsidence profile over the recovery end of the depleted longwall panel. It is over the recovery end that Highlands Mine No. I would access the Owl Seam reserves that exist above the No. 37 Mine. Black Mountain selected pillars sizes to be employed above the longwalls based on the ability to fully extract a pillar employing extended cuts. After a mechanical testing program was completed and mine behavior and top disturbance were observed in the test panel, John T. Boyd Company (BOYD) verified pillar stability and designed support systems. BOYD previously designed No. 37 Mine's gate pillars and studied subsidence and gob formation over its longwall panels; this effort aided the analysis of overburden movement and pillar integrity at Highlands Mine No. 1.

Jan 1, 2003

By Gregory Hendon

Jim Walter Resources, Inc. (JWR), has successfully longwall mined for many years at depths ranging from 1200'-2500'. However, full pillar extraction has proven difficult and generally economically unfeasible. Overburden and redistributed stresses at these depths require relatively large pillars, which become unmanageable with most pillar extraction practices. The economic benefits of taking gateroad pillars as part of a longwall face have been recognized for some time, including increased life of mine recovery, elimination of belt moves, reduced overcast requirements, and improved longwall to continuous miner ratios. Until recently, the associated risks of pulling pillars prevented any extensive use of this concept at JWR. Due to geological, economic, and longwall repeatability problems, JWR accepted the risks, and undertook pillar extractions with longwall faces in several different applications. The applications included taking a stable pillar (250' wide) as a longwall panel, taking yield pillars on the headgate and tailgate of panels, taking yield pillars and stable pillars in the middle of a panel, mining through chain pillars in the middle of a panel, and taking a support pillar on the tailgate end of a panel. This paper describes the background and experience of pillar extraction with longwalls at JWR.

Jan 1, 1998

By J. Marc Coolen

Marrowbone Development Company operates a large drift mining complex in the central Appalachian coal field. In 1997, five continuous miner supersections produced close to 9 million tons of raw plant feed. All production is from the Coalburg seam, a 5 to 10 ft thick coal seam with multiple coal benches and shale binders. The immediate roof lithologies consist of laminated shales, sandstones, or a mixture of sandstones and shales. The mine area is bisected by the regional Warfield anticline and associated Warfield fault. Mining conditions vary considerably due to frequent changes in seam and roof geology. The following geological categories can be distinguished: 1) presence of coal riders in the immediate shale roof, 2) excessive seam slopes around the Warfield fault, 3) splay faults associated with the Warfield fault, 4) zones of abundant slickensides, 5) abrupt roof lithology changes (sandstone - shale), 6) splitting and merging of different benches of the Coalburg seam, 7) sandstone washouts, 8) excessive roof dilution, 9) pillar sizing problems due to seam anomalies (soft fireclay binders). Several examples of these features are discussed in detail. Also, their relationship to roof fall occurrences is evaluated. Early recognition of potentially adverse geological conditions is of prime importance for the economic viability of the mining plans. Detailed core drilling in advance of the mining faces and coordination of the core data with underground mapping are the main forecasting tools.

Jan 1, 1999

By Yunqing Zhang

The 3-D numerical modeling using ABAQUS is performed to analyze the mechanisms of the tensioned bolts by taking into considerations excavation sequence, pre-tension, bedding planes and in-situ horizontal stresses. The effect of the pre-tension and the mechanisms of the tensioned bolts are analyzed based on the results from the modeling. Based on field observations, the failure modes for the tensioned bolted strata are classified by their causes and failure mechanisms. The support design consideration for each failure mode is given. Finally, a new design approach is proposed to do the tensioned bolting design using numerical method.

Jan 1, 2002

By Brian Clifford

A wide range of different cablebolt systems are available and in use in hard rock and coal mines around the world. The birdcaged cablebolt was initially used in UK coal mines in conjunction with rockbolts in the late 1980's. More recently other cablebolt configurations have been used. Innovations in cablebolt design in the UK have been driven by a demand to find a system as effective as the birdcaged cable, but which can be installed more rapidly and offer immediate support. A laboratory short encapsulation pull test (LSEPT) has been developed for both rockbolt and tendon systems which measures bond strength and stiffness in rock and assesses the full range of potential failure modes. This test is now used along with conventional 'gun barrel' and shear tests to provide full assessment of bolt and tendon performance. This test may be incorporated into a new performance based British Standard for rockbolts and long tendon systems. This paper describes the performance of some of these systems as being applied to UK and South African coal mines.

Jan 1, 2001

By Syd S. Peng

A total of 209 cases of subsidence data over longwall panels in 16 US coal seems have been collected and built into a subsidence database. The database is developed under MS Windows environment. It uses a very user-friendly menu driven system. The empirical formulae for a number of commonly used subsidence parameters have been derived from those collected subsidence data.

Jan 1, 1995

By D. J. Maconochie

This paper describes the investigations and results of monitoring of deformations of houses located within the area of influence of a pillar collapse over the Westfalen No. 3 coal mine near Ipswich, Queensland. The subsidence resulted in minor subsidence of at least 18 modern houses and serious damage to 4 others. The history of the events and the measures taken to deal with the physical problems arising are described. The investigations and results are described and related to the mining methods and geology. The stability of coal pillars up to 9.6 m high was considered and computer modelling undertaken to relate surface subsidence to estimates of the extent of pillar failure. Assessments were made of the likely extent of subsidence during the incident. Measurements of distortion and tilt were obtained as the ground slowly subsided and criteria established to determine when the structures were deemed to be unsafe. Detailed surveys of houses affected or anticipated to become affected were also undertaken. Four houses were condemned and subsequently demolished as a result of excessive tilt. The methods of assessment are described and the implications of the results related to the various house designs.

Jan 1, 1992

By Shiva P. B. Kolli

Surface mining of multiple scams by mountaintop mining methodology is complex in the Appalachian region of West Virginia. Excess spoil from the removal of overburden and interburden is disposed in the adjacent valleys. The valley fills so formed are some of the largest earth constructions, with a relief of 450 ft (137 m) to 650 ft (198 m) and a stretch of 500 ft (152 m) to 2000 ft (610 m). The stability of the valley fills has become an issue of public concern. This paper describes three cases of valley fill slopes. The analysis of each case involved the description of the geo-mechanical properties of different surface and sub-surface layers, and the surface configuration of the slope. Slope stability was carried out using deterministic as well as probabilistic approaches. Based on the analysis, it was found that valley till slopes were stable due to configuration, and the premise that valley fills with great stretch and high volume of spoil tend to be unstable does not hold.

Jan 1, 2001

By Nicholas P. Kripakov

The unpredictable massive collapse of roof in openings developed under apparently safe and stable roof conditions in coal mines of the United States has been of much concern for many years. One type of massive caving phenomenon is often attributed to cutter roof, a failure that initiates at one and/or both upper corners of an entry and propagates nearly vertically resulting in overall failure with little or no warning. This paper reviews current methods being used to control cutter roof and suggests new alternatives. The impact of basic parameters that include mine geometry, overburden geology, rock properties, and in situ stress conditions is discussed in the context of their relationship to this problem. The effects of various stress control measures are investigated with model analysis to gain insight into means of reducing the high level of stress known to exist at the entry corners of a West Virginia coal mine prone to cutter roof. The results are presented in the paper and the feasibility of their practical implementation is discussed.

Jan 1, 1982

By Wen H. Su

Multiple seam mining and its associated problems are very serious in Southern West Virginia where poor planning or lack of knowledge in seam interaction often results in complete loss of coal properties. Several measures have been proposed to alleviate the interaction prob¬lems under various multiple seam conditions, but few of them are spec¬ific in terms of mining plan. Two parallel approaches are adopted for this paper. One is field investigation which consists mainly of mine site visits and gathering of data. The other is the two-dimensional finite element simulation of mine workings using the data gathered as input. Two typical cases which represent the past and present multi¬ple seam mining practice in West Virginia were investigated. Much attention is focused on the interaction problems experienced and the important parameters controlling interaction. Several key parameters controlling interaction in multiple seam mining were analyzed. Proper mining plans, by which the interaction problems can be minimized, are proposed for the individual cases after a better understanding of the interaction mechanism was realized through finite element analyses.

Jan 1, 1984

By J. Nielen van der Merwe

During the process of pillar extraction at the Welgedacht Colliery, an unexpected pillar collapse resulted in the entrapment of equipment. The area was visited soon after the event to evaluate the stability of the area for the purposes of extracting the equipment. It was found that the area had stabilised sufficiently to allow work without undue risk to the safety of workers. The equipment was reclaimed successfully. A subsequent investigation into the reasons for the collapse was conducted. It entailed detailed underground and surface subsidence observations and numerical modeling. It was found that none of the perceived negative factors that were present at the time could result in pillar loads that were sufficient to cause the collapse. It was concluded that the local pillar strengths were less than the nationally used value and that they were possibly further reduced by aging.

Jan 1, 1999

By Chris Mark

Perhaps the most significant development in coal mine ground control during the last century was the introduction of roof bolting during the late 1940's and 1950's. From an engineering standpoint, roof bolts are inherently more effective than the wood timbers they replaced. Roof bolts promised to dramatically reduce the number of roof fall accidents, which then claimed hundreds of lives each year, and they were initially hailed as "one of the great social advances of our time." Roof bolting also emerged at a time of rapid technologic transformation of the coal industry, and greatly accelerated the transition to trackless, rubber-tired face haulage. The U.S. Bureau of Mines quickly became the new roof support's strongest advocate. Some state agencies and miners were skeptical at first, but nearly everyone was soon won over. Case histories were reported showing that roof falls could be largely eliminated while productivity increased dramatically. Little wonder that, in the words of one contemporary observer, "roof bolting has been adopted more rapidly than any other new technology in the history of coal mine mechanization." Yet by the end of the 1950's, it was clear that roof fall fatality incidence rates had actually increased. It would be another decade before the superior ground support provided by roof bolts would clearly save lives, The story of how roof bolting was implemented by the mining industry, but took so long to live up to its promise, is a fascinating example of the interaction between economics, technology, regulation, and science. It still has important lessons for today.

Jan 1, 2002

By Michael Karmis

During the last 25 years, technological advancement and subsidence research have resulted in more accurate and diverse prediction capabilities. The work presented in this paper focuses on the development of integrated prediction capabilities for dynamic deformation indices and for strains over sloping terrain. In order to assist subsidence engineers with accurate prediction methodologies, the research also addresses the development and validation of techniques that can enable model calibration using alternative measured subsidence parameters such as horizontal or ground strain instead of the traditional vertical subsidence values. This paper presents the basic concepts and validation of the above mentioned enhanced prediction and control methodologies, which are necessary for effective assessment and control of mining-induced subsidence, using examples and case studies. In addition, a risk assessment approach for evaluating landscape stability over mined areas is presented. The enhanced prediction methodologies have been incorporated into the Surface Deformation Prediction Software (SDPS) package.

Jan 1, 2008

By David A. Newman

Severe roof control problems have plagued a West Virginia underground mine since its initial development in the late 1970's. Adverse roof conditions in the Eastern portion of the reserve result from a combination of mining: - perpendicular to the major principal horizontal stress direction, - near lineament intersections, - in areas where the ratio of horizontal stress to vertical stress exceeds 2.00 to 4.00, and - where laminated strata are present in the immediate roof. The focus of this study is to identify the engineering (mine orientation, roof safety factor) and geological parameters (lineament and horizontal stress orientation, horizontal to vertical stress ratio, and immediate roof lithology) causing poor roof conditions and to quantify their influence on projected mining conditions in the Western portion of the reserve Rose diagrams were used to illustrate the correlation between the orientation of eight hundred thirty-six (836) roof falls with that of forty-four (44) lineaments and the principal horizontal stress Similarly a composite map was used to demonstrate the relationship between roof falls and the ratio of horizontal stress to vertical stress The immediate roof strata across the property were categorized as either laminated or non-laminated based upon a thorough examination of one hundred sixty-two core logs. The presence or absence of fireclay, a rider coal, and carbonaceous shale was also noted because of the historic correlation with poor roof conditions. Rock strength and physical property tests were conducted on selected immediate roof strata recovered from coreholes drilled along the mains projected to the Western portion of the reserve Potential roof control problems in the Western reserve were quantified based upon the correlations developed by analyzing engineering and geological data The main conclusion is that laminated roof strata, the rider seam, and its associated fireclay will have the greatest influence on mining conditions. Roof problems attributable to horizontal stresses will not be as severe in the West due to an increase in overburden depth and corresponding decrease in the stress ratio. Recommendations are provided for mining orientation, the layout of mains and submains, panel location, and roof support based on intended service life.

Jan 1, 1999

By Richard C. Repsher

The U.S. Bureau of Mines has developed a method and device designed to detect lose rock material in underground mines. The technology is designed to be an aid to mine workers in detecting hazardous roof conditions in underground mines which can complement or replace the traditional roof sounding techniques where the miner relies on experience to determine whether rock conditions are sound. The leading cause of accidents and fatalities in underground mines is falls of lose rock pieces or rock slabs from the mine roof. The device is an electronic roof sounding device that acoustically determines the integrity of mine rock. In previous research the Bureau of Mines found that loose rock, when impacted, vibrates at a much lower frequency than intact rock material. A major problem in determining rock stability using this technique has been the repeatability of the matt signal. This difficulty baa been greatly reduced in the current design by measuring the power spectra contained in two separate frequency bands of the signal produced by striking the rock in question. The ratio of the energy contained in each band is computed. This process minimizes any striking force differences, producing accurate, repeatable results for solid rock as well as loose, drummy material. The prototype has been successfully- tested in a variety of underground environments including coal, uranium, molybdenum, silver and salt. The technology has been investigated by the U.S. Mine Health and Safety Administration and the Department of Energy for use in detecting detached tunnel lining areas in nuclear repositories. The paper will discuss the technique, applicable results, and future applications.

Jan 1, 1990

By Herryal Z. Anwar

In recent years, many both mining and civil engineering projects are developed in the area which may be categorized unstable, such as fractured and jointed rocks or "water sensitive rocks". For example, in underground coal mining, in which the floor strata normally consist of weak shale, the problem that may arise in roadway is floor heaving. In this case cement injection in borehole appears to be an economically effective method in order to make roadway more stable. This method may be also applied in the projects in which the constructions are laid on fractured and jointed rock. Nevertheless, the successful cement injection is much dependent on the water-cement ratio and the injection technique. This paper discusses the application of cement grouting to improve the ground stability in longwall mining, the laboratory experimentation of cement grouting and the method of floor injection.

Jan 1, 2000

By Mikko Ahola

This paper describes recent innovative analyses conducted by the Bureau of Mines to demonstrate the application of the boundary-element method in evaluating the effectiveness of caving chambers or sacrificial entries to relieve the stress conditions initiating cutter roof failure in underground coal mines. The analysis assumes that cutter roof failure results when the shear strength of the roof rock at an entry corner is exceeded by the applied shear stress. The applied shear stress is due to large differences between the horizontal and vertical in situ stresses known to exist in many coal mining districts experiencing cutter roof failure. The results of boundary-element analysis suggest that driving an arched lead entry in advance of adjacent rectangular entries and allowing it to cave significantly reduces the difference between the principal stresses at the corner of the rectangular entry, thereby lowering the maximum shear stress. Slippage of the immediate roof along horizontal bedding planes into the caved entry is shown to further reduce the stress conditions in adjacent entries. Practical applications of this stress control technique to longwall and room-and-pillar mining are discussed.

Jan 1, 1987

By Jay Hilary Kelley

This paper proposes a modification of longwall roof support to meet new difficult mining conditions that are anticipated in the future. A gob canopy is a movable appendage attached on the rear of a longwall shield to counterbalance the troublesome forward downward force couple (FDFC). The gob canopy can be swung in both vertical and horizontal directions, powered by hydraulic cylinders. Several functions and applications are de- scribed

Jan 1, 1997

The objective of extensive underground experimentation on three longwall coal faces was to improve the stability of mechanised longwall faces through investigation of the relations between support resistance and convergence and resultant deformation of roof and face strata. In particular the influence of high setting and yield pressures on several strata and support parameters was studied in great depth. Increased setting pressures resulting in an increase in setting load densities from 0.15 MN/m2 to O.45 MN/m2 were shown to reduce convergence, face spalling roof flaking, lateral movement of roof and floor, extrusion of the face wall and progressive failure of the coal wall ahead of the face. This was shown to result mainly from the change in roof strata deformation from a wedge shaped compression zone with tension at the face edge to an even beam shaped compression zone over the face area. Debris thickness above the support canopy and below the base was observed to be the one of the most variable parameters which influenced the support characteristics particularly the rate of load acceptance by supports and the roof to floor convergence. A setting load density of 0.4 MN/m2 to 0.5 MN/m2 was found to be optimum under soft to moderately strong sandstone roofs. The authors demonstrated that use of guaranteed set valves with powered supports contribute to good strata control, reduce face down-time and increase face productivity.

Jan 1, 1984

By Wolfgang Schott

The propagation velocity of in-seam seismic (ISS ) waves depends on frequency which is also known as dispersion. This dispersion is not only determined by the thickness of the coal seam and its dirt hand content (i.e. thickness and position), but also by the rock type and its condition (soft or compact) above and below the coal. A coal panel completely surrounded by roadways was investigated using the in-seam seismic transmission survey technique. Along the roadways the immediate roof was either sandstone and shale- The seam thickness varied between 1.20 m (sandstone in the roof) and 1.40 m (shale in the roof) and a fault system (normal faults) crossed the panel with a total throw of about the seam thickness. Below this coal seam another coal scam with a thickness of about 80 cm exists. Its interval shown by exploratory drilling changes from about 60 cm to more than 6 m (scam splitting). Tomographic inversion was applied to the dispersion of the measured ISS waves and velocity distributions were calculated at frequencies found to he most sensitive to changes in the seam thickness and the distance to the coal seam below. By correlating the velocity distributions with known values along the roadways approximate distributions of the coal seam thickness and the seam splitting within the panel were produced. The mining of the coal panel had already been finished and comparison was made between the survey results and the actual situation as far as it was documented. Prediction by the seismic survey was partly in good agreement with the actual held conditions or showed at least the same trends.

Jan 1, 2003