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By Hamid Maleki

This study presents a historic overview of the role of mobile roof support (MRS) technologies in improving stability and worker safety and presents the results of recent field evaluations of the MRS load rate monitoring device and other remote deformation-monitoring techniques. Field studies were implemented at two sites in cooperation among researchers from the National Institute for Occupational Safety and Health (NIOSH), Maleki Technologies, Inc., and J. H. Fletcher & Co. The objective of the field programs were to (l) study the interaction between MRS's and coal mine strata and (2) develop and test suitable monitoring systems for assessing roof and pillar stability. An MRS consists of a roof canopy, four hydraulic cylinders, a caving shield canopy, and associated electromechanical systems mounted on crawler tracks. The machines are controlled by radio from a remote location and operate on self-contained power units. Typically, MRS's have capacities of 5,340 and 7,120 kN (600 and 800 tons). In comparison to posts, an MRS is capable of maintaining the yield load after significant amounts of roof-floor deformation. Because the mining cycle is accelerated, MRS's help reduce the potential for time-dependent roof falls. MRS performance has been monitored in the laboratory under controlled static loading conditions and in the field under deep, two-scam mining conditions. Laboratory studies have quantified support capacity and system stiffness as a function of machine height. Field investigations have focused on determination of optimum operating conditions and development of warning systems that indicate excessive load on the machine and/or impending roof-pillar stability problems. Analyses of field data show that roof instabilities are influenced by (1) pillar failure, (2) pillar yielding, (3) mine seismicity, (4) geologic structures, and (5) panel layout designs and mining practice. Pillar yielding and failure (unloading) and seismicity can be conveniently monitored by the load rate monitoring device, but for consistent detection of roof falls, additional deformation measurements directly within the cuts ate needed.

Jan 1, 2001

By Ding Xu Chu

There are many methods developed for measuring the crustal stress in China. Among them only two methods have been adopted in practice, i.e., overcoring method (for which the sensors include: Piezo-magnetic stress gauge, steel-ring strain cell, 3-dimensional hollow-inclusion stress ,gauge, and Leeman's-like stress gauge) and hydraulic fracturing method. For the former method, one of those sensors is installed in a borehole, 36 mm in diameter, then prestressed, and finally overcored with a drill bit of 130 mm in diameter. After that the deformation of the borehole wall is measured, from which the in-situ stresses are determined. In order to avoid the errors resulting from the difference in elastic modulus for various kinds of rocks as well as from the installment of the cells, we have developed a technique to calibrate the sensor on-spot using a confining pressure calibrator and produced a very good result. For the later method, the measurements are achieved by isolating a certain section of a borehole and then injecting water to pressurize it till a tensile break is induced. It is identical to the typical technology used internationally. For many years, through field observations and a large number of laboratory experiments, we believe that the most efficient way of stress measurement for engineering purposes is the overcoring method using Piezo-magnetic stress gauge. It is characterized by stable and reliable measurement values, high accuracy and high adaptability. Taking the measurements in the intact rock mass as an example, the measured stress magnitudes are within accuracies of 8% and the errors in orientation are about 5 degrees. It can even be used perfectly in deep water. Up to now, we have performed in-situ stress measurements in more than 70 sites and obtain more than one thousand sets of data. The deepest depth used in the overcoring method is 80 m in vertical borehole and 780 m in tunnels.

Jan 1, 1987

By Jim Sandford

The South Bulga Colliery longwall operation, owned and operated by Cyprus Coal Australia. Oakbridge Pty. Ltd., and located near Singleton, NSW, is currently operating under an average 80-ft-thick (24-nt) sandstone unit residing immediately above the coal seam, resulting in sudden and severe face weightings. To date, nine severe weightings have occurred at the property (within Panels 3, 4. and 5), three of which resulted in face closures accompanied by significant equipment damage and several weeks of downtime. The remaining six events nearly resulted in face closures, incurring single-pass vertical convergence of up to 8 in (200 mm) along the pan line. In an effort to mitigate and/or eliminate the occurrence of these severe weighting events, mine management has established an in-house program to immediately address the geotechnical and operational mechanisms underlying rapid weightings, as well as specific measures to mitigate these events on future panels. In support of this in-house effort. NSA Engineering Pty. Ltd., is assisting the engineering staff at South Bulga in developing a more aggressive face monitoring program. The aim of this program is to provide weighting occurrence predictions sufficiently ahead of time to ensure that operational controls are optimized prior to attempting to mine through potential weighting areas. This paper reviews the occurrence of weightings to date at the South Bulga Colliery as well as the latest developments in shield monitoring technologies to forecast these events in near-real time.

Jan 1, 1999

By Steve P. Signer

Researchers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health installed 39 instrumented, fully grouted bolts at six test sites in a trona mine retreat panel to study mine roof stability for the improvement of workplace safety. Variables at each test site included bolt spacing, bolt length, roof span, and location in the panel layout. At most test sites, two rows of instrumented bolts were installed, one in or near the intersection created during development and the other in or near the intersection created during second-pass mining. The most significant factor affecting bolt load was roof span. The highest loads were on the bolts installed in the intersections of the 6 m-wide entry. Mining-induced stress resulting from panel layout was the next most significant factor affecting bolt loading. Minor varia¬tions in bolt loading could be attributed to changes in bolt spacing and bolt length. Gas pressure release in the immediate roof contributed to some bolts showing compressional loading during gas bleed-off.

Jan 1, 2001

By Hani S. Mitri

The behaviour of grouted cable bolts used for rock reinforcement as a means of ground control in hard rock mining is investigated. The study utilizes an analytical model which is based on the variational principle to calculate the shear bond stress distribution along the embedded length of cable bolt. In this paper, a comprehensive parametric study which covers both geometric and mechanical design parameters involved is presented. Special emphasis is placed on studying the characteristics of the grout model represented by Winkler springs. This involved varying the maximum shear strength of the grout, its residual strength and the slip occurring between the cable and the borehole wall.

Jan 1, 1990

By Paul A. Lu

This paper proposes an unconventional, simple, and practical method of measuring mining-induced around stresses with hydraulic borehole pressure cells. The method is a direct ground stress measuring technique. It does not require the value of deformation modulus for back-calculation of stresses; instead, it directly measures the ground stress in situ. The applicability and practicality of the technique are shown by two case studies. The technique may be applied to monitoring a stress build-up that may lead to mine roof failure or to rock and coal outbursts.

Jan 1, 1984

By Kanaan Hanna

The integrity factor approach was developed by the Bureau of nines to assess the stability of mine pillars and has been applied primarily to longwall chain pillars. Recently this approach was applied to a room-and-pillar mine in the Illinois Coal Basin. A special configuration of hydraulic borehole pressure cells was installed in a pillar to determine the in situ strength-and- stress relationship of the coal pillar and to verify the applicability of the integrity factor approach. Results indicate that the integrity factor approach is applicable to the assessment of pillar st6b:lity and nay bs a realistic and effective approach for room-and-pillar mine design.

Jan 1, 1988

By R. J. Morrell

The Bureau of Mines developed a ground support system for use with an underground coal auger. The system consists of inflatable airbags that are inserted in the holes as they are drilled, and then gradually deflated to provide controlled roof caving. This paper presents results of laboratory and field tests of the system that show it to be technically feasible.

Jan 1, 1988

By Francis Kendorski

A longwall coal mine in Appalachia about 1,500 ft deep encountered a fault while developing a new longwall panel. The fault extended from mining depth to the surface near a secondary road and drainage. The fault was located inside the anticipated angle of draw within the mined panel and gob. The fault extended vertically out and up, away from the panel, caved zone, and gob at nearly the angle of draw: the fault very nearly following the angle of draw. It was initially thought that "fault reactivation" could possibly occur. Fault reactivation is the phenomenon of having mining subsidence localized along a fault leading to a "reactivation" of the fault and shearing and displacement along the fault. Such fault reactivation would disrupt and deform the fault plane beyond the normal angle of influence of the subsidence trough, and may provide a conduit for any ground and surface waters to reach the mine. We contacted all operators of longwall mines in Appalachia to determine if any Appalachian longwall mines had ever experienced fault reactivation, and teamed that none had experienced the phenomenon. After studies of possible water intrusion quantities and rates based upon in-fault pump tests, which indicated that water intrusion rates should be manageable, and the prior experience that faults in this particular area were usually barriers to water flow, mining proceeded with caution and monitoring. Mining was successful with no noticeable increase in water inflow rates, and no measurable off-setting of the fault exposure on the surface. It can be concluded that the fault did not reactivate due to its relationship to the mining sequence.

Jan 1, 2003

By Lu Jiam Zhang

Coal and gas outburst is one of the serious natural disasters in the coal mines of China. Since 1950, more than 10,000 coal or gas outbursts have occurred in more than 200 underground coal mines of different types. The outburst prone coal mines are widely spread out. It occurs in different types of coal -ma and is very serious in some areas. Most outbursts occurred at the time of roadway driving in coal seam, serious outbursts occurred mainly when entering the coal seam during the cross-cut driving; of the outbursts, coal and gas outbursts We UP major proportion, while pressing out and pouring out ah account for a large proportion; outburst. occur under different operating conditions, but mostly occur during shotfiring. This paper introduces the prevention and control measures as practiced in China and illustrates the three types of outburst cla8sification. Both technology and management are equally important in outburst prevention and control. Measurement of the volume of drilling cuttings and the initial gas outflow rate fm- boreholes have proven to be effective. A special team and organization must be set up to deal with the problems.

Jan 1, 1987

By George Mishra

In mid-1979, J&L Steel Corporation experienced rapid failure of pillars after only two weeks' operation with a combination of full retreat and partial mining in the 154 Road Section of its Nemacolin Mine, Greene County. The rate of advance of the "squeeze line" was very rapid for the first 500 feet, but it slowed down considerably as it approached some large coal blocks and gas well barrier pillars. After moving all section equipment out tc a safe distance, all accessible headings and crosscuts were filled with cribs and tri-set posts to prevent further spread of the squeeze. At one time, there was great apprehension by mine management that without implementation of some immediate corrective measures the safety of #3 airshaft and the only haulage and access road to the 8 Road mining area would be in jeopardy. In September, 1979, the U.S. Bureau of Mines established convergence monitoring stations outby the squeeze area. Readings were taken at weekly intervals until January, 1980, and monthly thereafter. Mathematical calculations, plot- ting of contour maps, and graphing of total convergence at different stations were used to follow and analyze the progress of the squeeze and provide guidelines for mining plans in the adjoining areas.

Jan 1, 1981

By George Gardner

The rapid growth of highwall mining in the Appalachian coalfields has resulted in unique safety concerns. Due to the concentration of activity at the base of the highwall and the potentially destabilizing influence of the mining process, proper mine planning and continual monitoring of the ground conditions is essential to ensure a safe and productive operation. This will also help to minimize the need to recover equipment that has become entrapped underground, and the subsequent hazards associated with a recovery operation. Adequate geotechnical site investigation and rational design methods should enable mine planners to properly account for the site-specific conditions that influence web and abutment pillar, roof, floor, and highwall stability. This paper examines the growth of highwall mining, provides a general overview of the various safety concerns, and reviews the modes of highwall failure as they relate to a highwall mining operation. Gcotechnical considerations that should be taken into account before and throughout the mining operation are emphasized. The importance of closely examining and monitoring highwall conditions for subtle signs of highwall instability will be discussed.

Jan 1, 2002

By James G. Schotsch

Unmapped abandoned mines are hazards for under¬ground mines, surface mines and surface structures. The Radio Imaging Method (RIM) provides a cross borehole technique to remotely detect abandoned mine works. RIM was used at the Buck Creek Mine, an underground coal mine, to determine the extent of unmapped mine works. The information provided by the RIM survey assisted the mine planners in developing their mine.

Jan 1, 1990

By A. W. Khair

This paper presents an analysis of ground movements recorded from longwall operations in the Northern Appalachian region in West Virginia. The site chosen for the investigation was selected due to varying heights of overburden and topography and was instrumented for both surface and subsurface measurements. The instrumentation areas consisted of two panels covering various sections with unique geologic and topographic features. Instrumentation included surveying monuments and shallow boreholes using an inclinometer for measuring horizontal strata displacements. Vertical strata deformation was monitored utilizing a unique design of the mechanical grouting method. Full length borehole profile extension was measured using Sondex displacement rings in the inclinometer installation. Subsurface displacements, i. e. caving zone, fractured zone, and highly deformed non-fractured zone were identified. The nature of surface disturbances with respect to varying geologic settings were compared, and an attempt was made to simulate final strata caving and bed separation using finite element analysis.

Jan 1, 1987

By Xiaolin Wu

Most coal bump occurrences in the U.S. are directly related to mining operations under strong roofs. An understanding of the behavior of bridging or cantilevering strong roofs over working longwall faces is a key to a safe and productive longwall operation. In this paper, an attempt has been made to study the behavior of strong roof beds using the elastic beam theories. Four structural models are formulated to simulate the locations of strong roofs and their distinct weighting stages. For each model, analytical solutions for the deflection, bending moment, stress, and the strain energy stored in both the coal scam and the strong roof are obtained. Solutions for the critical spans, which are controlled by the caveability of the strong roof beds, are developed. The effects of rocks' differing elastic moduli, under tension and compression, on the actual flexural rigidity, and stress redistribution are analyzed.

Jan 1, 1993

By S. S. Peng

The hydraulic powered supports have been employed in U. S. longwall mining for the past two decades. Yet the sciences of roof control at the powered supported faces remain largely unknown. In order to improve longwall roof control and increase production, a series of questions in longwall roof control should be studied and answered: what is the basic rules of roof behavior in longwall coal faces equipped with the hydraulic powered supports? What is the interaction between the roof, supports, and the floor What is the contact condition of the canopy with the roof, and consequently the load distribution on the canopy? Des the periodic roof weighting really exist in the powered supported face and if so, how does it affect the face area? This paper presents the preliminary analysis of underground observations and instrumentations in a longwall panel with 4-leg shield supports designed to address the questions mentioned above.

Jan 1, 1982

By M. Bouteldja

A finite element modelling approach is developed for the evaluation of the mechanical performance of cable bolts supports in underground mine structures. The modelling approach permits the simulation of any arbitrary number or pattern of cable bolts, that can be either partially or fully grouted, with or without end plates. The modelling technique used treats each cable bolt as a set of bar elements representing the steel cable. The bond stiffness characteristics of the grout bolt interface are represented by shear springs uniformly distributed along the cable length. Coupled with a simple linear elastic analysis, this technique can serve as a dedicated numerical model for cable bolted excavations. The application of the model is illustrated with two case studies: one for a hard rock mine and another for a coal mine.

Jan 1, 1999

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 Gexin Sun

This paper overviews the recent research progress in predicting cutting performance using fracture mechanics principles. It is emphasised that rock fragmentation due to cutting is mainly a process of chip formation resulting from crack initiation and propagation or crack interaction for multi-tool cutting. This process can be simulated by using the well developed fracture mechanics criteria coupled to numerical methods. Fracture toughness is shown to be an important and useful parameter in quantifying cutting performance and in designing cutting tools. The paper concludes with discussions on the dynamic effects involved in rock cutting processes which should be taken into account for optimum cutting efficiency and tool design.

Jan 1, 1992

By Bruce K. Hebblewhite

Underground pillar extraction continues to be practiced in Australia in various forms, in spite of the dominance of the longwall method, which accounts for over 90% of underground production. Australia has been a leader in various forms of pillar extraction, particularly since mechanisation was introduced to the industry during the middle of the 20th century. Historical developments have included a range of thick seam mining pillar extraction methods; the widely adopted Wongawilli method; early adoption of the use of mechanized breaker line supports; and a number of more recent partial extraction systems including what is known as rib stripping. Current pillar extraction practice varies from total extraction panels to various forms of partial extraction, driven by either surface subsidence constraints or by underground conditions. This paper reviews a number of different pillar extraction methods and mining layouts, and also highlighting a number of fundamental geotechnical principles which have been found to be critical for safe pillar extraction ? often through lessons learned the hard way, as a result of back analysis of different accident scenarios. These principles include the all-important understanding of the location of the goaf (gob) edge at any point in the operation, and the management of that goaf edge; the impact of geological structures on local and regional stability; the critical importance of extraction lifting sequences; the role of stooks (remnant pillars); and a number of other factors. The paper will also briefly reference the educational and training practices adopted in Australia aimed at the continuous improvement of pillar extraction safety.

Jan 1, 2011