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By Chris Mark

The Coal Mine Root Rating (CMRR) was first presented at this Conference nine years ago. Since Its Introduction, the CMRR has been incorporated into many aspects of mine planning. including longwall pillar design, rout support selection, feasibility studies. and extended cut evaluation. It has also become truly international, with involvement in mine designs and funded research projects iii South Africa. Canada. and Australia. The purpose of this paper is to bring the minim community Up to date with recent improvements and applications of the CMRR. The must important new development is a streamlined process for determining the CMRR from exploratory drill core. Just three types of information are Jaw required: Fracture spacing (or RQD) Uniaxial compressive strength for axial Point Load Testing) Diametral Point Load Testing The moisture sensitivity adjustment has also been changed, and new research has related the immersion test to slake durability. Most recently, the CMRR has been implemented in a computer program. which can be obtained from NIOSH free of charge. The program facilitates calculation of the CMRR firm either underground or drill core data. Values from ninny locations can he saved in a single file. and an interface with Autocad allows CMRR contour plots to he integrated 11110 mine planning.

Jan 1, 2002

By A. Wahab Khair

This paper presents results of the experiments carried out in Beth Energy No. 33 Mine for the purpose of alleviating problems associated with cutter roof. Cutter roof problems have delayed the advance of the entry development considerably and maintenance cost of these entries was very high due to the requirement of very extensive and heavy artificial supports. The study includes assessment of the problem, investigation of the geology and lithology of the mines, study of in-situ stresses and corrective measures to combat the problem. Two approaches were selected as the corrective measures: 1) implementing yield pillar concept in order to reduce the effects of in-situ horizontal stresses, 2) designing an effective roof reinforcement system and application procedure. Results of this study revealed that roof strata is weak and in-situ stresses are very high at the problem area which results in development of cutter roof. Implementation of yield pillar concept was found to be ineffective because cutter roof often developed at the entry face concurrent to the development. However, proper design and implementation of roof reinforcement system stabilized the roof and eliminated roof caving during both entry development and retreat of longwall face, despite existence of the cutter roof, at a much lower cost.

Jan 1, 1992

By David A. Newman

Strength and physical property testing of coal is conducted regularly by the Arch Mineral Corporation as part of reserve exploration. Pillar design and mine layout are the primary applications for the coal strength data. Distinct differences in pillar behavior were observed during longwall mining at a mine in Southeastern Kentucky. Horizontal and vertical variation in both triaxial and uniaxial compressive strength was observed within the four (4) benches of the Harlan seam. Similarly, geographical variations in coal strength were noted across the Harlan seam reserve. During the past three decades substantial research efforts have been focused upon coal pillar behavior and the refinement of pillar design methodologies. Ironically, little attention was directed to integrating the variability of coal strength into pillar design. The authors examine, through the presentation of case history data, i) typical industrial practice with respect to establishing a database of coal strength for use in pillar design and stability analysis, ii) approaches in which the observed variation in strength in the Harlan seam is integrated into pillar design and stability analyses, and to iii) propose standards for coal cube testing

Jan 1, 1993

By D. M. Shu

A two-dimensional finite element modelling technique is applied to predict the subsidence movements on a sloping ground surface above a completely mined panel. The modelling is carried out for the surface both directly and using a combination of an equivalent horizontal surface and a rays projection method and the result compared. The rays projection method provides the subsidence components on a sloping surface from the corresponding ones on an assumed equivalent horizontal surface through the point of mean elevation of the relevant part of the sloping surface. This method is used next to compare the subsidences, horizontal displacements and horizontal strains from the mining of a given panel at the surface for different inclinations. The results show that the magnitudes of the subsidence components are greater on the dip side of the slope from the point of the mean elevation, than the corresponding ones on the rise side and the asymmetry increases with the inclination of the surface. The findings are also compared with those of other research workers.

Jan 1, 1992

By Shengli Yang

Solid backfill mining, as a green mining technology, has the advantages of a higher resource recovery, less damage to the environment and the ability to dispose of waste underground. It has been applied successfully in the Xingtai and Xinwen mining areas in China. Strata behavior and the movement of overburden strata in this mining method are very different from fully mechanized longwall mining, which results in a natural caving, because the gob is filled with waste rock, fly ash and other backfill material. The results of field measurements showed that the periodic weighting interval of the main roof is longer; the magnitude of periodic weighting is smaller; the peak and zone of influence of the front abutment pressure are smaller than those in fully mechanized mining; that major deformation of mine roadways occurs within the influence zones of the abutment pressures; that the falling space of immediate roof is smaller, with the lower portion falling more sufficiently than the upper; that the main roof subsides slowly; and that the falling height of the overlying strata is greatly reduced.

Jan 1, 2011

By Hamid R. Nikraz

This paper reports the finding of a comprehensive investigation carried out to evaluate the compaction characteristics of saturated sandstone aquifers in the Collie Coal Basin under conditions anticipated during dewatering operations. A triaxial technique has been adapted to evaluate the compaction characteristics of the sandstone aquifer in the Collie Basin. This technique, which allows the strata stress regime to be reproduced by triaxial loading with zero lateral strain, also provides a precise evaluation of lateral stresses and consequently Poisson's ratio under in situ conditions. The paper contains a description of the equipment commissioned, test techniques, results, analysis and interpretation of the data obtained. The testing evaluation techniques are general in nature and can be applied to field situations in locations where similar weak sandstones occur.

Jan 1, 1993

A two-dimensional time-dependent analysis of a overburden-coal pillar-weak floor strata interaction problem is presented as a beam model consisting of a composite roof beam resting on multiple elastic foundations (pillars) underlain by a composite rock mass representing immediate floor strata. Several different material models may be considered for the immediate floor strata. The analysis can include all openings and pillars in a panel and permits bed separations in the roof composite beam. The model has enabled identification of the relative significance of different geometric and mechanical behavior parameters which govern the system. The paper presents the theoretical background for the model as well as its application to a mine in Illinois where surface and underground geotechnical observations have been conducted over the last two years.

Jan 1, 1988

By Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

Virtually all mineable Appalachian coal seams exist in a multi-seam environment. This makes it inevitable that most seams will eventually experience interaction induced ground control and mining problems. Indeed, it has been estimated that every working face will experience interaction effects from previous or current workings at least once during its lifetime (1), and that these problems are considerably exaggerated when seams are in close proximity -- a distance of less than 110 feet. The continued increase in multi-seam problems has resulted in considerable research activities over many years (2,3,4,1). Theoretical analysis, numerical and physical model studies, statistical and empirical analysis of numerous interaction related field data have been applied to multi-seam mining situations and have identified many useful trends and design criteria (4,5,6,7). To fully utilize research results and transfer multi-seam mining technology to field engineers, a design model -- USEAM was constructed (8). This model, designed for close seam under-mining situations, was computerized to facilitate utilization. In the current research, a new computer program "MSEAM" was developed to produce a model for both under- and overmining close seam situations. It is hoped that both of these software packages will aid field engineers in using important interactive design criteria to gain a better understanding of interaction mechanisms. Through advance planning with these programs, detrimental interaction may be avoided and minimized, and occasionally a design can be selected which will allow interaction effects to be utilized to the benefit of mining operations.

Jan 1, 1987

By Edward McHugh

While less than 1% of reported accidents are associated with slope stability problems. slope failures were responsible for about 15% of all U.S. surface mine fatalities between 1995 and 2001. Small rockfalls, which may involve hand-sized rocks weighing only a few kilograms, can cause fatal injuries to workers away from the protection of large machinery. Massive highwall failures containing a million cubic meters of material or more can be fatal even for heavy equipment operators. As part of an ongoing study at the Spokane Research Laboratory of the National Institute for Occupational Safety and Health, several remote-sensing technol¬ogies are being evaluated as tools to monitor slopes for hazards and to assess slope stability. Field tests of a hyperspectral imager were conducted to assess its value for improving geologic maps of poten¬tially unstable alteration zones on mine slopes. An interferometric radar device capable of detecting very small displacements on slopes awaits final assembly and field testing. Computerized moni¬toring methods using images from digital and video cameras are being assessed for application to mine slope surveillance. These and other techniques will eventually provide new tools to augment current methods for monitoring ground control hazards in mines.

Jan 1, 2002

By Guang Ping Cui

In this paper the imminent relationship between rock burst and its phenomenon with normal rock pressure and its appearance, as well as the factors of one changing into another have been analyzed. Based on this, the necessary and sufficient appearing factors of rock bursts have been put forward, and the term "burst pressure" has been defined. According to the statistics data of tremendous rock bursts which appear in collieries in Hunan Province, the futures of high frequency, great strength and shallow depth of the beginning burst have been found. There ere some special structures in the coal seams, which are closely related with the burst phenomenon. A hypothesis of "star structure" has been suggested for the appearing mechanism of bursts. Inside and around this star structure the distribution rule of burst pressure and rock strength has been researched depending upon the theory of the limited stress field. A new description of the appearing causes of bursts has been raised.

Jan 1, 1984

By J. A. Nemcik

Although floor failure at longwall faces has been traditionally associated with a soft floor, in many csses floor buckling has been observed just ahead of the longwall supports, despite the floor being a competent rock The floor failure is associated with the presence of weak bedding planes in the floor and the movement of strata towards the goaf opening. This paper presents the principles of the floor failure based on numerical modelling and field observations and outlines the practical approach to estimate the risks involved with floor failure.

Jan 1, 1995

A procedure fm the estimation of pilla1 strength is presented where the design of underground coal pillars is emphasized. An analytical approach has been developed, proposing that the effect of end constraints on the pillar strength and the ductile behavior of coal material should be incorporated in the computation of the stability of coal pillars. Bad on this approach, it has been found that end constraint has a significant influence on the pillar strength. The study indicates that when the end constraint is zero the computed strength of the pillar is the same as the unconfiied compressive strength of the coal mass and will not increase with the increase in the width to height ratio. However. when the end constraint is greater than zero the pillar strength will increase exponentially as the width to height ratio increases.

Jan 1, 1990

By Kewal K. Kohli

An investigation was performed to determine the sequence of events which caused the 1987 surface subsidence and related damage to several homes in Walker County, Alabama, USA (see Figure 1). Surface affects compared to mine maps indicated the subsidence to be mine related. However, two coal seams had been worked under this area. The upper seam, the American seam ranged from 250 to 280 feet beneath the surface in the area in question. It was mined-out before 1955 by room-and-pillar method leaving in place narrow-long pillars to support the overburden strata, and abandoned in 1955. The lower seam, the Mary Lee seam, ranged from 650 to 700 feet beneath the surface. The Mary Lee seam had been abandoned in 1966 and subsequently became flooded. The dewatering of the Mary Lee seam workings in 1986 caused the submerged pillars to be exposed to the atmosphere. Due to multiple seam mining and the fact that workings had been inundated then dewatered, a subsurface investigation ensued to determine the sequence and ultimate cause of surface subsidence. Core sample tests with fracture analysis in conjunction with "down-the-hole" TV camera inspections provided necessary information to determine that the subsidence started in the lower seam and progressed through the upper coal seam to the surface. Evidence from the investigation program established that dewatering of the lower seam workings caused the marginally stable support pillars and the roof to collapse. This failure triggered additional subsidence in the upper seam which broadened the area of the influence at the surface.

Jan 1, 1992

By Robert L. Schmidt

The support of underground openings by water is a natural phenomenon. Surface sinkholes, such as those that occur with some frequency in Florida, are attributed to a lowering of the water table resulting in the draining of existing limestone cavities. This withdraw the natural support and causes the surface subsidence. The role of water as a ground support medium was demonstrated in a mining situation during Bureau of Mines research on borehole mining of phosphate in St. Johns County, Florida. The borehole mining tool (BHT) incorporates a water jet nozzle and slurry eduotor in a single down- hole tool. The tool is lowered through a bore- hole into the ore zone, and an underground cavity is eroded by the motion of the water jet with the slurry simultaneously pumped to the surface. In addition to phosphate, the system has been successfully used in coal, oil sands, and uranium. The Florida phosphate ore is located at a depth of approximately 250 ft in strata that produce high water inflows. These factors preclude extraction by conventional methods, either surface or underground, but they present an ideal setting for borehole mining. Previous borehole mining research had led the researchers to believe that the system would only be effective in a void cavity, that is, a cavity where the water level is maintained below the water jet nozzle, so that the water does not impede the action of the let. It was soon discovered, however, that the void cavity presented ground control problems. The overlying strata started to cave soon after the support provided by the water was removed. This was an unacceptable situation as it adulterated the ore and threatened surface subsidence. Further tests wherein the size of the cavity -as reduced by cutting only a 30° arc also resulted in caving. A new concept, air shielding, was then introduced and tested. With this system, the water jet cutter was surrounded with a shroud of compressed air, thereby gaining the advantages of cutting in air while retaining the roof support afforded by a water-filled cavity. This arrangement proved to be highly successful. The air-shrouded jet was able to cut a cavity radius of about 18 ft, and the system production was 35 tph, which compared favorably to the production previously attained in void cavities. The borehole phosphate experience proves the efficacy of remote mining in a water filled opening. This system neutralises the hazards of heavy ground, and may, as in the case of the phosphate deposits, be the only viable alternative. It opens up the possibility of application in a wide range of deposits not only those amenable to hydraulic mining. Remote mechanical System might be developed using present robotics technology and then operated in water-filled openings.

Jan 1, 1986

By Yong-Ming Jiang

In cooperation with Cyprus Twentymile Coal Co., researchers from the National Institute for Occupational Safety and Health (NIOSI-I), Spokane Research Laboratory, conducted a study at the Foidel Creek Mine, an underground coal mine near Oak Creek, CO, to evaluate the stability of underground working conditions as a longwall advanced through a series of backfilled cross-panel entries. Instruments installed in the cross-panel entry pillars, backfill, longwall panel, and adjacent headgate entry were continuously monitored by a computerized data acquisition system as the longwall approached and advanced through the backfilled section of the panel. Data collected from these instruments were analyzed to determine the magnitude and direction of the secondary principal stress changes ahead of the longwall face, deformation and closure of the headgate entry, and stability of the cross-panel entry pillars and backfill. A three-dimensional, boundary element model was calibrated to accurately predict mining-induced stress changes and the distance they occurred ahead of the longwall face. As the longwall advanced through the hackfillcd entries, most of the mining-induced load was supported by the cross-panel entry pillars. The backlill provided stability for the root and Moor of the in-panel entries and also confined the entry pillars significantly improving their load-carrying capacity. Comprehensive information derived from backfill material property tests, instrument data, underground observations, and numeric modeling results document the safety and stability of the 8-Right minethrough and should provide beneficial case study information fur other longwall backfilling applications in the future.

Jan 1, 1998

By Thomas E. Marshall

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) examined and characterized conditions at a majority of the underground stone mines in the United States. Observations at these mines revealed a limited degree of roof monitoring beyond visual inspection and sounding. When monitors are used, they typically require the miner to measure movement at the roof. If conditions are unstable, the miner may be in a hazardous situation while recording data. Based on this scenario, researchers surmised that a simple, inexpensive monitor with the capability of recording data at a distance from the mine roof would be a safer way to gain the information. Additionally, more widespread use of monitors could potentially lead to better understanding of roof movement in general. A monitor to meet this need was designed, tested, and subsequently improved as experience was gained in its' use at a number of underground stone mines. The Roof Monitoring Safety System (RMSS) can provide an initial indication of movement in roof beams. By understanding and measuring roof movement in underground mines, the potential for injuries and fatalities to mine workers From falls of ground can be reduced. Also, officials at a mine with a history of data are better prepared to make a decision on remedial actions in the event of ground falls. This paper will outline the evolution of the RMSS and how it can be used in a comprehensive pro-active ground control safety program. Also included is a cast history describing how the RMSS was used in an evaluation of the effectiveness of a mechanical impact scaling machine at an operating limestone mine.

Jan 1, 2000

By Duk-Won Park

A vast majority of the operating longwall sections use shield-type face supports to provide ground control in the United States. As a co-operative research program between the University of Alabama (UA) and Jim Walter Resources, Inc.(JWR), a joint research program is underway to study interaction of shields with surrounding rock masses. A series of analyses on the response of shields are performed using monitored data at a longwall face in a deep coal mine. In this paper, various effects of parameters, such as leg pressure, setting load, developing load, maximum loading rate, pressure in capsule, and convergence between canopy and base, are discussed and recommended as stability indices. A microcomputer program is being developed for instantaneous interpretation of the shield monitored data, and for giving warnings of potential problems. The preliminary results define the effects of the setting load, developing load, and rate of developing load or leg closure rate. The objective of the study is to understand the shield characteristics and their interaction with surrounding strata in real time, then develop a reliable program for maintaining longwall stability.

Jan 1, 1992

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 Binchuan Zhang

The combined reinforcement system (CRS) combines the strengths of roof' bolting, resin-anchored cable bolt, and chemical grouting to effectively control the deformation of the strata surrounding an extremely poor entry. It is easy to implement and fast in installation. It is one of the systems that are suitable for the reinforcement and maintenance of extremely poor entries.

Jan 1, 2001