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By Danny J. Van Roosendaal

An investigation was undertaken to determine hydrogeological effects of subsidence over a longwall coal mine in the Illinois Basin. At this mine, approximately 200 ft of bedrock overburden is overlain by up to 150 ft of glacial till and localized water-bearing glacial sediments within a buried bedrock valley that overlies the longwall panel. A finite-element mechanical model of the subsided longwall panel shows sufficient strain to enhance permeability throughout the bedrock portion of the overburden. However, examination of the actual surface subsidence profile indicates that the overburden deformation is localized in narrow zones at the panel edges. The aquifer units at the base of the glacial sediments are confined by glacial clays (above) and weathered shales (below). Elevated water levels were observed in the glacial aquifers during subsidence, which indicates that the clays and shales maintained confinement as the overburden subsided and deformed. A three-dimensional ground-water flow model was used to simulate the effects of subsidence on the hydrogeologic system and to estimate inflows into the mine. Longwall retreat was simulated by several model runs, each representing a new longwall face position. Simulated heads and mine inflows were calibrated with recorded water levels and observed inflow conditions by modifying the hydraulic conductivity of particular model layers. Model calibration indicates that the permeability of certain lithologies, such as plastic underclays and weathered shales, is not significantly enhanced by subsidence. A worst-case scenario was simulated by inserting a deeply incised, sandfilled valley into the model, which had only minimal impact on the simulated mine inflows.

Jan 1, 1995

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

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 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 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 Christopher Mark

Mine planning for a new reserve is based on information obtained from exploratory coreholes. A critical component of an exploration program is the geotechnical evaluation. Poor assumptions about roof conditions greatly add to the risks of mining. Rock mechanics testing is central to a geotechnical exploration program. Typically, 3 to 5 uniaxial compressive strength (UCS) tests are made to characterize a particular roof unit at a given corehole. The average (mean) of these tests is taken as "The UCS" for that location. Isopach contour maps are then used to show spatial trends in roof strength. Two issues are raised by this traditional approach. The first is due to the large variability in UCS values that is typical even within a single unit from a single hole. The average UCS might be higher at Corehole A than it is at Corehole B, but the difference may not be statistically significant. The second issue is whether widely spaced coreholes can identify valid spatial trends in rock strength. The answer depends upon whether rock strength changes over distances that are longer or shorter than the corehole spacing. This is a classical geostatistical problem. While geostatics have been used to investigate many coal quality parameters, they have seldom been used to evaluate rock strength. This paper describes an extensive investigation of these issues conducted by the National Institute for Occupational Safety and Health (NIOSH) in collaboration with Peabody Energy. The study employed the Peabody Rock Mechanics Data Base which contains more than 10,000 individual test results. Data from four important roof units were subjected to statistical analysis: ? Brereton Limestone above the Herrin 6 seam (Illinois) ? Turner Mine Shale above the No. 9 Seam (Kentucky) ? Sandstone above the Eagle Coal (West Virginia) ? Shale above the Eagle Coal (West Virginia) The study did not find significant spatial trends in rock strength in any of the cases. Perhaps there are none, or perhaps the exploration coreholes were just too far apart to see them. These results have valuable implications for the design of geotechnical exploration programs.

Jan 1, 2004

By D. E. Winston

Roof falls are the major cause of fatalities in the, coal mining industry and the prevention of roof failure is a major concern of mine management. Many ground failures observed underground are associated with geological discontinuities such as fractures, faults, clay veins and sandstone channels inherent in the earth's crust (Rinkenberger, 1977). To properly design a mine requires an advance knowledge of such geologic discontinuities and this is not always available from exploration boreholes. The use of remote sensing for mine planning is a relatively new development, and is at present being used primarily by the Roof Control Support Branches of the Mane Safety and Health Administration (MSHA) in Denver and Pittsburgh. However, technical advances, coupled with public access to previously unavailable photographic products, make remote sensing a useful aid in the difficult process of mine planning (Jansky and McCabe, 1979). Remote sensing techniques are being used to predict the location of geological features in advance of mining which influence roof conditions. The features appear on photographic images as linears, which are plotted on a mine map and serve as a planning tool for mine management. Also, during development of entries toward a linear, mining personnel will be on alert for signs of unstable roof conditions which might normally go undetected. Preventive action can then be taken before a fall occurs. A confusing aspect in remote sensing, analyses is terminology such as linear, lineament, fracture trace and joint. In this report the term "linear" refers to a line-like appearance on an aerial photograph or satellite imagery which signifies a naturally occurring geologic feature no the earth's surface. The objective of this paper is to describe the use of remote sensing techniques as an aid in predicting geological discontinuities which might produce hazardous roof conditions to a mine operating in the Pocahontas No. 4 Seam in southern West Virginia. Mine projections and roof control plans will he developed based on this and other available information such as data from cored drill holes.

Jan 1, 1982

By Kirk W. McCabe

RokLok binder is a two-component polyurethane system consisting of a polymeric isocyanate (Component A) and a polyol resin (Component B). The two chemicals are mixed and injected into the mine rock under pressure. The initial low viscosity enables the RokLok material to permeate many of the smallest fractures. About 3-5 minutes after injection, the reacting material begins to expand in the fractures; the reaction is completed after 1-2 hours, and the material sets to a mechanically strong binder which adheres tightly to the rock. The consolidated rack provides resistance to any further f movement w thin the strata. The RokLok binder injection system is fast becoming an important engineering tool for coping with adverse underground geological conditions. Two case histories discussed in this paper, a longwall mining problem and an entry-roof problem, demonstrate the effectiveness of using the RokLok binder system to help solve complex ground-control problems in a cost-effective manner.

Jan 1, 1981

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 P. Forrest

Based on the results of mathematical analysis, photoelastic modelling, and case studies, the mechanisms of longwall under- mining interaction has been thoroughly investigated. The effects of using yield pillars in combination with rigid pillars in lower seam mine design have been found to provide significant improvement in ground conditions during undermining. Off- setting of solemnized longwall panel entries relative to upper-seam remnant structures has proven to be effective in alleviating adverse stress conditions.

Jan 1, 1988

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 Bruce K. Hebblewhite

Tower Colliery is a longwall mine operated by BHP Coal Illawarra Collieries, Southwest of Sydney, Australia It mines the Bulli Seam at a depth of approximately 450m. The surface topography overlying the mine consists of several steep-sided river gorges, up to 68m in depth, which run at oblique angles across a sloping terrain Apart from some light rural development, the surface land is essentially natural bushland, but is traversed by a major freeway. This crosses one of the gorges on twin, six-span, box-girder bridges, with bridge piers up to 55m in height. Consequently, a major surface subsidence monitoring program has been in place for several years now, including intensive conventional, GPS and E DM surveying, plus real-time monitoring of critical components of the bridge structure. Although the bridge and freeway are outside the conventional angle of draw' subsidence influence criteria, and have seen only negligible vertical deformation as a result of mining, there has been widespread evidence of regional horizontal deformation of the land surface, large distances away from the mining area The effect at the bridge has been well within acceptable tolerances, primarily because of the regional nature of the movements, with very little differential movement observed at the bridge piers Gorge closure and evidence of large headlands of land moving `en masse' have been observed. Horizontal movements of hundreds of millimetres have been recorded in some locations where gorge closure has been monitored. Possible explanations of these movements include one or a combination of mechanisms such as pre-mining stress relaxation. valley notch effects, valley bulging, regional joint patterns, movement toward active goaf areas, vertical gorge shearing' and shear failure of horizontal bedding planes below the surface This paper presents a case study of the regional horizontal ?subsidence displacements? measured during mining. and a discussion of the possible explanations for these phenomena

Jan 1, 2000

By George J. Karabin

The Roof Control Division of the Pittsburgh Safety and Health Technology Center, MS HA, is routinely involved in the evaluation of ground conditions in underground coal mines. Assessing the stability of mined areas and the compatibility of mining plans with existing conditions are essential elements in assuring a safe working environment at a given site. Since 1985, the Roof Control Division has successfully used the Boundary Element Method of Numerical Modeling as a tool to aid in the resolution of complex ground control problems. This paper will present an overview of the modeling methodology established and details of techniques currently used to generate coal seam, rock mass, and gob backfill input data. A summary of coal and rock properties used in numerous successful evaluations throughout the country will be included and a set of Deterioration Indices that can aid in the quantification of in-mine ground conditions and verification of model accuracy will be introduced. Finally, a case study will be detailed that typifies the complexity of mining situations analyzed and illustrates various techniques that can be used to evaluate prospective design alternatives.

Jan 1, 1994

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 Mostafa Sharifzadeh, Ebrahim Ghasemi, Kourosh Shahriar

"The Tabas Central Mine (TCM) is one of the underground coal mines located in the Tabas coal region, in the mid-eastern part of Iran. This mine is the first mechanized coal mine in Iran that uses the room and pillar method. Pillar design is one of the most important issues in underground coal mines, especially in room and pillar mining. In this paper, a new method for coal pillar design in the main panel of TCM is presented. This method is able to estimate the magnitude of the various loads (both development and abutment loads) that pillars might experience throughout the mining process. Also, the method reduces the pillar failure risk. Based on this method, proper pillar sizes in TCM are obtained (11.6 x 11.6 m (38.l x 38.1 ft)).INTRODUCTIONPillar design and stability are two of the most complicated and extensive mining problems related to rock mechanics and ground control subjects. Although these problems have been investigated for a long time, to date only a limited understanding of the subject has been gained. Prior to this, the dimensions of a pillar were largely determined by experience based on trial and error, intuition, or established rules of thumb; any research available tended to be isolated and sporadic. But nowadays, various pillar design formulas have been developed based upon laboratory testing, fullscale pillar testing, and back-analysis of failed and successful case histqries. In 1980, a number of ambitious field studies conducted by the U.S. Bureau of Mines developed the ""classic"" pillar design methodology. It consisted of three steps (Mark, 1999, 2006):1. Estimating the pillar load using tributary area theory2. Estimating the pillar strength using a pillar strength formula3. Calculating the pillar safety factorAlso, Bieniawski (1981) presented a step-by-step method to determine coal pillar dimensions in room and pillar mines. In this method, similar to the classic method, pillar load is estimated by using tributary area theory.Currently, in mqst of the room and pillar mines, remnant pillars in panels are recovered by retreat mining (pillar recovery). Hence, the above mentioned methods are not appropriate for pillar design, because in these methods pillar load is estimated by the tributary area theory, and the effects of abutment loads, which result from retreat mining and the creation of a mined out gob, are ignored. Abutment loads affect the pillars in the adjacent of pillar line and a load greater than what is estimated by tributary area theory is applied on the pillar. So, pillar design without considering the abutment loads leads to pillar failures during retreat mining. Pillar failures continue to be one of the greatest single hazards faced by underground coal miners. Pillar failures are responsible for unsatisfactory conditions, which include the following (Mark et al., 2003):"

Jan 1, 2010

By Russell C. Frith

This paper presents recent findings concerning longwall caving mechanisms relating to Australian mining conditions and their effect upon longwall support requirements and behaviour. The findings arc based on a program of intensive hydraulic support monitoring in a variety of Australian mining conditions coupled with geotechnical observations. Results from the study illustrate periodic weighting cycles of varying severity and these are related to the overlying geology in terms of the strength of individual strata units, their thickness and proximity to the longwall extraction. Several different caving mechanisms are identified which contribute to periodic weighting cycles. The importance of recognising weighting cycles in relation to the operation of the longwall face will be discussed in detail. Longwall support design methods are critically examined in relation to the case histories of the monitoring program. The applicability of the said design methods to Australian coal mining conditions is discussed and an outline of design considerations to facilitate productive longwalling is presented. Overall, the paper only "scratches the surface" of the work currently being undertaken by ACIRL in this area, and the findings of that work.

Jan 1, 1992

By Kevin Jinrong Ma

Underground mines often experience roof falls in entries, crosscuts, and intersections of active mining sections, main travel ways, and belt entries. Roof fall heights greater than 20 ft (6 m) make re-bolting of the newly exposed roof dangerous and impractical. To protect mine personnel, belts, moving vehicles, and other facilities, mine operators typically install steel structures such as square or arch sets in the roof fall areas. Wood lagging is usually installed between the steel-sets to enclose the area and protect the entry from recurring falls. Usually the void space above the steel-sets and laggings is backfilled. However, backfilling high roof fall voids is costly, which causes some operators to leave the voids open. In this case, durability of wood over time and the capability of the steel-set and wood lagging to resist falling rocks are unknown. During the last few years, Keystone Mining Services, LLC (Jennmar Corporation affiliate engineering company) designed, tested, and developed an impact-resistant lagging system to protect steel-sets1. Various steel-sets protected with the impact-resistant lagging were approved by MSHA and installed in different underground roof fall rehabilitation projects. This paper presents (1) design, development, and laboratory testing of the patent-pending impact-resistant lagging panel, (2) steel-set design methodology according to the American Institute of Steel Construction (AISC) national standard, and (3) impact-resistant steel-set design method based on Law of Conservation of Energy. Two underground cases are reviewed to demonstrate the successful application of the impact-resistant lagging system.

Jan 1, 2011

By Joseph C. Zelanko

Cable bolt support techniques. including hardware and anchorage systems, continue to evolve to meet U.S. mining requirements. For cable support systems to be successfully implemented into new ground control areas, the mechanics of this support and the potential range of performance need to be better understood. To contribute to this understanding, a series of 36 pull tests were performed on 10 ft long cable bolts using various combinations of hole diameters, resin formulations, anchor types, and with and without resin dams. These tests provided insight as to the influence of these four parameters on cable system performance. Performance was assessed in terms of support capacity (maximum load attained in a pull test), system stiffness (assessed from two intervals of load-deformation), and from the general load-deformation response. Three characteristic load-deformation responses were observed. An Analysis of Variance identified a number of main effects and interactions of significance to support capacity and stiffness. The factorial experiment performed in this study provides insight to the effects of several design parameters associated with resin-anchored cable bolts.

Jan 1, 1995

Design and selection of support systems for longwall faces call for in-depth knowledge of strata mechanics and in particular of strata-support interactions. The paper presents the results of field investigations and analysis of a difficult-to-cave roof using powered supports under a massive dolerite sill in Central India using a state-of-the-art datalogger system. The mechanised longwall face was equipped with 4/680 tons chockshields operated on the IFS mode using a 300 K W DERD shearer. The datalogger system was capable of storing data simultaneously in 32 channels. The paper describes the monitoring system and the monitoring programme. The system provided useful information on support behaviour, build-up of leg pressures and face convergence during an intense dynamic periodic weighting phenomenon which destroyed the integrity of almost half of the face supports. At this point, the face had advanced by about 197 metres from the barrier. The paper outlines a post-mortem analysis of this dynamic event and provides a critical evaluation of the data collected for understanding strata support interaction and the behaviour of the massive sill in the overburden.

Jan 1, 1992

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