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By J. F. T. Agapito

Overcore measurements at the North Fork Valley (NFV) coal mines in western Colorado have shown that horizontal stresses are highly anisotropic. Measurements have been made in four mines at various depths. In many measurements, the maximum horizontal stress is three to four times higher than the minor horizontal stress. At the West Elk Mine, maximum and minimum horizontal stresses of 24 MPa and 6 MPa, respectively, have been measured at depths of 640 meters (m). Under highly anisotropic stress conditions, ground control problems associated with both high and low horizontal stresses can develop. While high horizontal stresses can produce cutter failures and floor heave, low horizontal stresses can allow block fallouts. This paper summarizes the horizontal stress measurements in NFV mines and, in particular, in the West Elk Mine where their role in roof fall and floor heave failures is discussed. This experience has led to an improved use of ground support and safer mining operations at depths of 640 to 700 m.

Jan 1, 2005

By Tom Wynne

In order to shorten the time required and improve safety for a longwall face move, a 36-ft wide longwall recovery room was predriven. The room was supported with specialty bolts, trusses and concrete cribs. Rock mechanics instrumentation was implemented to evaluate the adequacy of the whole recovery system. This included instrumented roof bolts, entry convergence, load cells, and abutment stress in the surrounding pillars. This paper describes the methodology of support and instrumentation designs, and monitoring and evaluations. The results indicated that this method offers the safest and yet faster longwall recovery so far. But the rock mechanics monitoring also indicated some improvements are desirable in the truss system and concrete crib layout.

Jan 1, 1993

The applicability of four empirical strength criteria for rock masses, namely, Hoek and Brown, Bieniawski - Yudhbir et al., Rarnamurthy - Arora and Johnston - Sheorey et al. has been tested against the triaxial test data for jointed plaster of Paris of Arora. The following modified Bieniawski criterion shows best agreement with the triaxial test data for jointed plaster of Paris:- where st = axial compressive stress at failure, MPa; s3 = confining pressure, MPa; scm = uniaxial compressive strength of jointed plaster of Paris, MPa; and B = - 3.988 + 9.674 (scm )-0? 185 The modified Bieniawski strength criterion can be used to estimate the strength of a coal pillar, based on a knowledge of stress distribution in it.

Jan 1, 1992

The challenge of planning to minimize interaction effects in multi-seam mining is compounded by the difficulties inherent in extrapolating essential geological and spatial data between sampling points. To predict the effects of ultra-close mining on lower seam entry stability, simulated mine openings were studied using finite element analysis and body loaded photoelastic models. Both symmetrical and unsymmetrical loading was used to represent stress fields generated under multi-seam conditions. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Stability predictions require precise definitions of geologic and spatial data, which could not be accurately defined by interpolation between sampling points. To overcome this problem an expanded roof stability model, SESAME2, was developed to incorporate reliability concepts based on Monte Carlo sampling. The model was demonstrated in terms of variations of innerburden thickness using a case study.

Jan 1, 1994

By Ed Mchugh

Recent studies by researchers from the National Institute for Occupational Safety and Health indicate that shear loading contributes significantly to failure of bolts used for rock reinforcement in coal mine roofs. Laboratory tests on 17 bolts were conducted to study the behavior of roof bolts subjected to shear loading over a range of axial bolt loads. Fourteen strain gauges were attached to each bolt to measure axial and bending loads at seven locations along its length. The instrumented bolts were grouted through two high-strength concrete blocks, and axial tension was applied to as much as 75% of yield strength. With the block interface acting as a failure plane, shear loads were applied to the blocks at a constant rate of displacement to the ultimate strength of the bolts. The tests characterized the relationship of axial bolt loads to shear forces across a rock bedding plane and measured the distribution of axial and bending strain along the length of the bolts. These results will improve the selection of roof reinforcement in mine areas where high shear stresses are present, thus improving the safety of miners working in these areas.

Jan 1, 1999

By Deren Zhu

The principle of roof control in longwall mining is to control main roof fractures so as to reduce roof weighting, plus reasonable and effective design of supports to reduce the convergence and premature caving of the immediate roof. This paper introduces a comprehensive test for better roof control based on the geological conditions of coal seam No. 16 in Zaozhuan Colliery, China.

Jan 1, 1990

By Kenny D. Basnett

An analysis of the Roof Falls Data Base (RFDB) for underground coal mines in Illinois for the period 2004?2008 indicated that about 80% of the roof falls occur at intersections and are associated with geologic anomalies or a rapidly changing geologic environment. Most falls are associated with anomalies such as clay dikes, differential compaction faults, shear zones, joints and fractures, and geologic structure. Therefore, a better analysis of the geology with emphasis on mapping anomalies such as joints and fractures and their application is necessary to improve ground conditions and safety. Such studies within Illinois Coal Basin are limited. This paper presents a mapping study of the joint orientations with documentation of dip of the several lithological units that lie directly above the Herrin No. 6 Coal seam around southern Illinois. These units include the Energy Shale, Anna Shale, and Brereton Limestone. The mapping was done in three mines and the data were plotted on a stereographic projection to determine the average characteristics of joint planes. The study mines included two deep underground mines and one shallow highwall mine. This study has helped identify favorable mining and support orientations in areas of highly variable geology. These are currently under review by the cooperating coal company for experimental roof control studies.

Jan 1, 2011

By Zhang Liandong

Roof control in thick-seam mining has long been a difficult problem in mine design and coal mining. In the early 1980's, through ground control instrumentation and investigation the rules of roof strata movement were determined, based on which the sliding roof bar powered support was properly designed and successfully applied to roof control to thick seam mining with caving. Underground trials in Huating Coal Mine produced satisfying results. The technique is being employed in many Chinese coal mines and won the national price for technical advance.

Jan 1, 1990

By Wenjun Ju

It's very important to determine the limitation of bed separation in entry roof in order to predict roofs stability and take some necessary precautions. Based on numerical modeling and monitoring in the field, the new concept of Limitation of Roof Layer Separation (LRLS) and its method of determination LRLS is given in this paper

Jan 1, 2001

By M. Zaki Rashad

In this paper, if three slant distances are measured from three given points of known coordinates to an unknown point, the position of this point (X, Y, and Z coordinates) can be determined. A computer program was designed for observing the movement of a point in the roof in Abu-Tartour mine (western desert, Egypt) to measure the convergence. This method, for general using, is very fast and can be applied without any restrictions.

Jan 1, 1990

By David Newman

The latest version of the Surface Deformation Prediction Software (SDPS) package is presented through its application to a number of case studies. The package now exploits all the benefits of the Microsoft Windows platform and has a direct interface to CAD software, thus facilitating input and output. Also, the package features advanced calibration routines to handle predictions over specific regions. The case studies presented highlight the features and capabilities of the new package.

Jan 1, 2001

By Jiacheng Wang

The current state of thick seam coal mining methods is illustrated in three typical cases of thick seam longwall mining, each pointing out the outstanding achievements in top-coal caving mining and large cutting height mining in China. At the same time, based on current thick seam mining practices in China, some problems urgently need to be solved. These include improving the recovery ratio of coal reserves, determination of overburden strata pressure, shield support selection, supporting methods for large cross-section in-seam roadways, top coal bulk flow theory, and gas flow and control technology. In addition, the rational production capacity should be studied from both economic and technological perspectives in order to optimize coal recovery and efficiency.

Jan 1, 2011

By Kent D. Mctyer

Mining commenced at Tasman Mine in late 2006. The current method of mining is bord and pillar using continuous minerbolters and shuttle cars for first workings and secondary extraction using mobile roof supports. The two stage process was chosen to accommodate irregularly shaped coal deposits, allowing adjustments to be made to extraction ratios for better management of subsidence and to maximize the efficiency of the operation. Following the completion of the first three full/partial extraction panels, the mining method was changed due to variable caving. Another partial extraction method, referred to as the Duncan Method was adopted; it involves stripping the developed square pillars on four-sides on retreat to leave a load-bearing remnant coal pillar. The system of partial extraction has been successful in meeting safety, productivity, and subsidence targets.

Jan 1, 2011

By William B. Beerbower

When full length resin grouted bolts were introduced into U. S. coal mines some 11 years ago, it was assumed that safety would be the primary benefit. There is ample testimony to substantiate the safety factor as an important reason for using resin roof bolts. More recently, it has been determined that many coal operators not only concur with the safety consideration but also benefit economically from the safer working conditions resulting from a reduction in roof falls. IMPROVED SAFETY Some typical roof fall reduction experiences with resin bolting follow: ? Western Kentucky Mine - Resin bolting reduced roof falls by 80%. ? Southern West Virginia Mine - Experienced 141 roof falls between 1971 and 1972. In 1973 with resin bolting, roof falls were reduced 85%. ? Western Pennsylvania Mine - In 1973 had 59 roof falls with mechanical bolts. During the same period of 1974 with resin, only 19 falls were experienced - a reduction of 66%. The overall experience with resin bolted roof support has been an average 50% reduction in roof falls. Long term, this can translate into improved worker morale. As a matter of fact, a general mine foreman at the same Southern West Virginia mine mentioned above commented that improved miner morale was noticeable following the introduction of resin bolting. HOW GREATER SAFETY CAN BE COMBINED WITH INCREASED PRODUCTIVITY IN COAL MINING OPERATIONS Experience has also shown that a reduction of roof falls with resin bolting has resulted in increased production for many coal operators. Some typical examples with mines using resin bolting follow: ? Western Kentucky Mine - Resin bolting contributed to increasing production from 7000 to 11,000 tons per day, due to a 80% reduction in roof falls. This was accomplished in part by a reduction of 50% in timbering requirements and the reassignment of three roof fall cleanup crews to coal production. ? Pennsylvania Mine - Following resin bolting introduction the section showed a 50% increase in production. ? Central West Virginia Mine - Continuous miner production averaged 6 cuts per unit shift prior to resin bolting. Reduction of cleanup time, resulting from improved roof support with resin bolting, increased production to an average of 9 cuts per unit shift. PROPERTIES OF RESIN BOLTING SYSTEMS Several of the important properties of resin bolting systems which contribute to its improved performance are listed below: ? Full length anchored resin bolts and many resin anchored, point anchored bolts provide effective resistance to changing rock strata stresses throughout their length, thereby improving stabilization of rock strata over long periods of time. ? Individual layers of rock are anchored into a single high strength beam providing excellent resistance to vertical and lateral strata movement, the most common cause of roof failure. ? Moisture cannot corrode the fully grouted bolt. ? Has twice the pull strength of mechanical bolts (See Figure 1)

Jan 1, 1982

By E. Hosca

Alternative tailgate loading conditions due to conventional and conventional/yield pillar combinations were evaluated to define roof loading. In order to isolate different loading situations, the tailgate entry was considered as part of the roof-pillar-floor system, which is controlled largely by the structural integrity of each member. A data bank of case studies was compiled, which contains details of roof and floor lithology, structure, support, and mining data, including any failure modes. The range of loading conditions that any tailgate can be subjected to was identified to form the basis of numerical modeling. Numerical modeling was then conducted on tailgate openings to determine roof, floor, and rib responses, including failure modes to a variety of loading conditions. Trends developed from the numerical modeling were modified using field data to form the basis for support evaluation and application. The effects of support were modeled numerically to determine optimum load capacity and points of application for reducing the potential for entry failure.

Jan 1, 1994

By Fred Leighton

Outbursts, or the sudden and sometimes catastrophic expulsion of coal and gas into the working area of a coal mine, are a little understood and growing problem. The AE/MS (Acoustic Emission/Microseismic), or rock talk, activity prior to and associated with several outbursts has been recorded using ultrasonic and micro- seismic equipment and has been analyzed by the Bureau of Mines. Analysis of these data indicates that outbursts are comprised of several failures in rapid succession and that anomalous AE/MS data suitable for providing failure warning are generated 15 to 45 minutes prior to the failure. This paper discusses the techniques for recording and analyzing these data and presents sane case histories.

Jan 1, 1982

By Daniel W. H. Su

This paper presents a retrospective assessment of ground control research and its successful application by CONSOL Energy over the past 25 years. Recognizing the need to improve the understanding of coal pillar design mechanics under shallow, medium, and deep covers, an extensive underground coal pillar instrumentation program was initiated in the mid 1980s. Combined with sophisticated numerical modeling technique and laboratory tests, results from this instrumentation program were used to advance the concept of coal pillar design and to demonstrate that numerical modeling can be used to predict in situ coal pillar strength, especially under non-ideal conditions where interface friction and roof and floor deformation are the primary controlling factors. To evaluate the impact of longwall mining, a 10-year subsidence research program was initiated in 1986 that encompassed surface, subsurface, and underground measurements, as well as a sophisticated numerical modeling technique. Results from the research revealed that interface friction and shearing between rock layers within the overburden had a profound effect on surface subsidence development and the resulting strain. Recognizing the impact of horizontal stresses on CONSOL?s mining operations, extensive horizontal stress measurements were conducted using the USBM Borehole Deformation Gage and the CSIRO Mininfrac system in the mid to late 80s. Roof extensometers and CSIRO HI Cells were used to measure roof displacement and roof stress concentration in longwall headgates and tailgates, which resulted in the definition of right-hand and left-hand longwall panels. Pillar design modifications, combined with cable bolt application and innovative standing support installation, were then successfully employed to mitigate horizontal stress impact on the right-hand longwall headgates and left-hand longwall tailgates. An in-house floor bearing capacity test apparatus was fabricated and employed underground to determine mine-specific floor bearing capacities which, along with other geotechnical parameters, were then used in the mine-specific shield design. To promote caving of thick, massive sandstone immediately behind longwall face supports and to improve face conditions, the hydraulic fracturing technique was successfully employed by CONSOL to pre-fracture the sandstone prior to longwall mining. Face monitoring and numerical modeling confirmed the effectiveness of the hydraulic fracturing technique. A roof geology reconnaissance program was initiated by CONSOL in the early 1990s to improve roof control, safety, and production. Information obtained from the reconnaissance program was used successfully to guide mine planning decisions and daily roof support practices.

Jan 1, 2011

By Yundong Ma

In this paper the dynamic process of rock deformation and failure in the roadways of the #9 coal seam, Wuhushan Coal Mine by was simulated using the software "Rock Failure Process Analysis System (REPA)" [I]. Based on the simulation results, this paper analyzes the failure process, characteristics and weighting of the overburden as the longwall retreat mining proceeds. The roadway's behavior including deformation characteristics, failure mechanisms, stress distribution in the surrounding rocks and the effects of various support systems on roadway stability were analyzed and studied. While simulating the rock failure process with REPA, the model stresses can be obtained through two approaches: (1) the stress is reflected by the grayscale of pictures in the model failure process and (2) the model stress can be obtained by muti-element information, which is one of the commands in RFPA system. Data files of stress can automatically be created, and curves of stress and displacement of each element can be displayed in each step of computation. Keywords: Rock Failure Process Analysis System (RFPA); roof control; roof to floor convergence; combined support

Jan 1, 2005

By Ross W. Seedsman

The feasibility of 6 m high single pass longwalling has been studied using Newlands Mine as a case study An intensive geological investigation identified a range of hazards but a mine layout was possible that minimised the risks with respect to joints, cleats, angular bedding, coal shears and horizontal stress. Geotechnical design input was required for support capacity, subsidence, gate to face transition, face span, and roof support. It is concluded that longwall face equipment can be made available by modifying existing designs, gate to face transitions will require a number of steps, and a mid-face sprag should not be required for routine operations.

Jan 1, 1992

By Leigh J. Wardle

Significant coal reserves in New South Wales lie under areas with steep topography such as valley slopes and cliffs. Subsidence predictions are difficult to make for these locations as existing empirical databases cannot cover all permutations of topography and mining geometry. Two dimensional numerical model alternatives are of limited applicability. Consequently a novel three-dimensional numerical model is developed including the detailed panel layout, realistic rock mass properties including the full stratigraphy, goat behaviour and actual surface topography. Model results show the same trends as subsidence measurements from Baal Bone Colliery in the Western Coalfield of New South Wales

Jan 1, 1992