Search Documents

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 Bob J. MacDonald

The Cape Breton Development Corporation is a federally owned Crown corporation in Canada that operates two coal mines the Prince and Phalen Colliery in the Province of Nova Scotia. Retreat longwall mining was first introduced to the shallower workings (I 80m) of Prince Colliery in the early 1980's. Phalen Colliery started longwall retreating in 1987 and since that time has extracted twelve panels. In 1993 there had been a total of eight longwalls varying in widths from 140-230m mined in Phalen Colliery that is on the Phalen Seam. Up to this point there were only minor ground control problems on the faces except for one of the earlier longwalls that was mining a split seam. There is some interaction on Phalen workings from remnant structures in the overlying Lingan and No. 26 Collieries which worked the Harbour Seam. Induced abutments loads along pillar edges have been observed and resulted in poor face conditions when mining the Phalen Seam. The ninth panel, 6 East was the first panel to suffer a major roof fall from the First Major Weighting. This was also the first panel to undermine a paleo sandstone river channel named the Lower Sandstone Unit (LSU). For the first time in Phalen's history subsequent longwalls began to experience periodic weighting events. The 6 East panel suffered a number of periodic weightings and on two occasions had to use foam cement to fill the cavities that resulted. On the next east side panel, 7 East, there were numerous periodic weighting events, but interestingly there was no significant First Major Weighting. 7 East was the first 260m wide panel and all others were 230m or less. This longwall was 3,400m long and began producing in December of 1994 and was still cutting coal in May of 1997. There were five significant roof fall events on this panel, three of which required foam cement to fill the cavities. Of these three the interruption to production varied from four days to seven months. There were definite precursors to each of the weightings including: 1) an increase in audible noise (termed bumping) on the face, 2) supports went rapidly into yield, 3) roof convergence increased, 4) water percolated from the face roof strata, and 5) severe spalling of the coal occurred resulting in increased tip to face distance. Longwall face weightings are a result of geomechanical factors and cannot be avoided However, the loss of roof control is a different matter and highly subjective to operational procedures. The causes of weightings, their magnitude and impact on the mining at Phalen have been investigated. The following factors have been identified as relevant to these occurrences. 1. The influence of pillars in the overlying Harbour Seam workings. 2. Location of the LSU relative to the Phalen Seam. 3. Thickness of the LSU. 4. Pressure arch effect from the adjacent gobs. 5. Poor operational practices on the longwall including: a) slow rate of retreat; b) inadequate supply of pump pressure to the hydraulic supports; c) premature yielding of the yield valves; and d) poor face alignment practices. This paper will review the events that took place on the 7 East longwall and show that the Lower Sandstone Unit is the primary cause of the weightings. However, other factors contributed to the loss of roof control and the formation of the roof cavities.

Jan 1, 1997

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 S. MacGregor

The role of horizontal stress, its orientation and magnitude, in defining the behaviour of strata in underground coal mines has been well established. Poor panel layouts have led to gate end stress concentrations, roof falls and lost production. The ability to define the horizontal stress regime over a mine site has historically been limited to point measurements, in part due to technology and cost. Recent advances in the application of geophysical tools, notably the acoustic scanner (borehole televiewer) have resulted in a new technique to conduct stress measurements. By quantifying the nature of borehole breakout and the mechanical properties of rocks in which they occur, this technique provides the ability to: obtain a vastly greater number of measurements, both at different depths and spatial distribution, than other techniques such as overcoring or hydraulic fracturing readily obtain depth versus stress relationships define geotechnical domains on the basis of stress direction and in-situ stress magnitude for mine planning purposes This paper presents an overview of the technique and presents case histories in its application at a mine site in the Sydney Basin, Australia.

Jan 1, 2002

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 Shiva P. B. Kolli

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

Jan 1, 2001

By J. R. Stoltz

To date, there has been limited formal research dealing with the use of Mobile Roof Supports (MRS) for pillar recovery. This paper, which is a portion of a larger project, presents convergence data from 4 different mine case studies. MULSIM was used to create a numerical model. The resulting information from this model was then compared to the actual measured values. MULSIM was also employed to analyze the industry-accepted pillar/barrier layout for mining underground reserves with the Joy Highwall Mining System. This layout was examined for both a low cover and a high cover extreme,

Jan 1, 1999

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 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

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 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