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By Morgan M. Sears, lhsan B. Tulu, Ted M. Klemetti

"Why do some room and pillar retreat panels encounter abnormal conditions? What factors deserve the most consideration during the planning and execution phases of mining and what can be done to mitigate those abnormal conditions when they are encountered? To help answer these questions, and to determine some of the relevant factors influencing the conditions of room and pillar (R&P) retreat mining entries, four consecutive R&P retreat panels were evaluated. This evaluation was intended to reinforce the influence of topographic changes, depth of cover, multiple-seam interactions, geological conditions, and mining geometry.This paper details observations made in four consecutive R&P retreat panels and the data collected from an instrumentation site during retreat mining. The primary focus was on the differences observed among the four panels and within the panels themselves. The instrumentation study was initially planned to evaluate the interactions between primary and secondary support, but produced rather interesting results relating to the loading encountered under the current mining conditions. In addition to the observation and instrumentation, numerical modeling was performed to evaluate the stress conditions. Both the LaModel 3.0 and Rocscience Phase2 programs were used to evaluate these four panels. The results of both models indicated a drastic reduction in the vertical stresses experienced in these panels due to the full extraction mining in overlying seams when compared to the full overburden load. Both models showed a higher level of stress associated with the outside entries of the panels. These results agree quite well with the observations and instrumentation studies performed at the mine.These efforts provided two overarching conclusions concerning R&P retreat mine planning and execution. The first was that there are four areas that should not be overlooked during R&P retreat mining: topographic relief, multiple-seam stress relief, stress concentrations near the gob edge, and geologic changes in the immediate roof. The second is that in order to successfully retreat an R&P panel, a three-phased approach to the design and analysis of the panel should be conducted: the planning phase, evaluation phase, and monitoring phase."

Jan 1, 2016

By Brian E. Perrine

Underground mining requires a variety of specialized equipment. With ever-changing mining conditions and methods, some mines have had to resort to purchasing existing industrial equipment and adapting it for underground use. Since this equipment is not made for underground mining or their specific task, the result is often lost time, money, and sometimes an unsafe working condition. One such application is the setting of a CAN. A CAN is a type of cylindrical secondary roof support made from a cement composite and encased in steel. The CAN has become popular because of its simple one-piece design that can handle heaving and sagging better than other available designs. ?Because of this exceptional ability to sustain load through extended convergence and displacement, the CAN is one of the most robust and stable standing supports in use today.?It also promotes better air flow due to the size and shape of the CAN. CAN size and weight have increased over time to handle more difficult mining conditions. Significant down time and inefficiencies with existing CAN setting machines at Twentymile Coal were directly due to the item in use not being designed to be task-specific. They were undersized with very limited movement. The miners had to take extra steps and precautions when setting CANs. Maintenance delays were significant. Twentymile Coal Company realized that they required a safer, more reliable and more efficient machine to handle and set CANs, and looked for a partner to develop a new design-specific machine. They worked jointly with J.H. Fletcher® engineering and sales to design and manufacture a machine specifically made for setting CANs. The objective was to produce a machine that would handle the conditions and methods that the mine required for setting CANs during longwall retreat. The resulting design had to be safe, stable, have a wide range of motion, be ergonomic, simple to use, and easy to maintain. The machine also had to improve efficiency. The resulting machine has been operating for approximately one year at Twentymile Coal and has been proven to improve safety, reliability, and efficiency. In conclusion, with the task-specific CAN Manipulator, the mine is able to better plan daily tasks with the knowledge that the machine will be available throughout a shift, and the employees will be safer because of the machine?s ergonomics, stability and safety. This has resulted in better productivity and a safer environment.

Jan 1, 2013

By Stephen C. Tadolini, Peter A. Gray, Robert Hawker, Andrew Sykes

"Self-drilling rock bolts (SDRBs) that use pumpable cement grout to anchor bolts have been used successfully in the tunnelling and civil engineering industries for many years. Pumpable cement grout offers a considerable advantage in that the SDRB can be fully encapsulated right down to the bearing plate, including filling cracks and cavities in the borehole. However, cement grout cannot provide the immediate rock support required for the coal industry. This paper describes a new SDRB system that uses pumpable resin to anchor the SDRB in a borehole, which not only provides all the advantages of a pumpable grout anchoring system, but also provides immediate roof and rib support as required in the coal industry. This paper further outlines the latest developments in SDRB technology including (1) extendable self-drilling rock bolts, which can almost double in length during the installation process and are particularly suited to low seam heights and rib support and (2) pre-tensioned SDRBs, where a pre-tensioned load can be applied during the installation process. These new SDRB systems offer both considerable advantages in rock support and improvements in mining productivity.INTRODUCTIONThe mining industry is continually striving to develop more efficient methods to support the roof and walls of roadways and tunnels, and rock bolting has proven to be one of the most significant improvements in ground support over the last 50 years. Rock bolting is now almost universally used in the underground mining industry worldwide, and there are many different rock bolts available.Rock bolts are traditionally made from steel or fiberglass and are anchored into a borehole with a friction-anchoring device, such as an expansion shell mechanism; cement grout; or chemical resin. Friction-anchored bolts are widely used in the Australian metal mining industry, predominantly as short-term support. They are used in conjunction with resin-anchored bolts where there is a requirement for longer life. Resin-anchored bolts are used in the coal mining industry.Friction-anchored bolts provide immediate rock support, but their load transfer capacity is poor compared to resin or cement anchored bolts. In addition, friction-anchored bolts are more susceptible to corrosion than resin or cement anchored bolts."

Jan 1, 2010

By Esterhuizen S. Gabriel, Su W. H. Daniel, Tulu B. Ihsan, Klemetti M. Ted, Mohammed M. Khaled, Gearhart F. Dave

"A numerical-model-based approach was recently developed for estimating the changes in both the horizontal and vertical loading conditions induced by an approaching longwall face. In this approach, a systematic procedure is used to estimate the model inputs. Shearing along the bedding planes is modeled with ubiquitous joint elements and interface elements. Coal is modeled with a newly developed coal mass model. The response of the gob is calibrated with back analysis of subsidence data and the results of previously published laboratory tests on rock fragments. The model results have been verified with the subsidence and stress data recently collected from a longwall mine in the eastern United States. INTRODUCTION In 2015, there were 40 longwall mines operating in the United States, each producing an average of 4.5 million tons of coal per year, and they supplied 60% of the U.S. underground coal production. This represents a substantial increase from 50% three years earlier (Sears, Tulu, and Esterhuizen, 2017). During this period, reportable roof fall rates in U.S. longwall mines also increased. Large roof falls that can block the gateroads are not only a ground-fall hazard, they can disrupt the ventilation system, block the escapeways, and increase the potential for elevated methane levels in the gob. To address these hazards, the National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Mining Research Division (PMRD) is conducting research to improve the design of ground control systems in longwall gateroads. Gateroad stability is primarily determined by the longwall pillar design. Generally, the required dimensions of the pillars around a longwall panel are determined first, which then dictates the location of the gate entries relative to the mined panel. The Analysis of Longwall Pillar Stability (ALPS) method is the most accepted gateroad pillar design procedure in the United States (Mark, 1992). The ALPS method accounts for local roof geology in the gateroad stability assessment by including the Coal Mine Roof Rating as an input parameter (Mark, 1992). The key assumption in the ALPS method is that unstable pillars result in unstable gate entries. However, experience provides examples of mines where pillar stability and gateroad stability are only loosely correlated (Su, Draskovich, and Thomas, 2003)."

Jan 1, 2017

By P. Oliver

Rockburst conditions were first encountered at Inco's Sudbury area operations in the mid 1930's when mining advanced to below the 610 m (2000 ft.) horizon. Now, mining is being carried out at depths down to 2130 m (7000 ft.) at Inco'a Creighton Mine. While rockburst potentials have increased with depth. improvements in mine planning, methods, support systems and developments in destressing have alleviated the condition to where it is less of a problem now than it was in the early years. Hang of the improvements were developed, of necessity. through observation and reasoning because these were the only tools available at the time. Now, we are making use of stress - measurements, numerical modelling and microseismic source location technology to - help further reduce rockburat hazards.

Jan 1, 1987

By Douglas Tesarik, Mark K. Larson, Habte Abraham, Heather E. Lawson

Dynamic failures, or "bumps", remain an imperative safety concern in underground coal mining, despite significant advancements in engineering controls. While many factors have been empirically linked to the occurrence of dynamic failure events, identifying a consistent, repeatable set of criteria within a field setting has proven elusive; conditions generally associated with dynamic failure might produce an event at one site, but not another. Conversely and more troubling, dynamic failure could occur where relatively few of these factors exist. The presence of spatially discrete, stiff roof units, such as paleo channels, are one such feature that has been linked to the occurrence of dynamic failure events. However, an empirical stratigraphic review investigating the relative frequency of discrete units in bumping versus non-bumping deposits indicates that no significant difference exists based on this criterion alone, and that instead an apparent relationship exists between reportable bump history and the overall character of the host rock with respect to stiffness. Due to the complexity of the bump problem, however, these results are not conclusive, as they do not take into account any variable other than the presence or absence of stiff members in the roof lithology; To weight the relative impact of changes in a single variable, such as the thickness or location of sandstone members, it must be examined in isolation-i.e. in a setting where all other variables are held constant. Numerical modelling provides this setting, and the effects of variability in a stiff discrete member in a hypothetical longwall mining scenario are investigated within the context of three stratigraphic "types'', as determined by the ratio of stiff to compliant stratigraphic members; Compliant, Intermediate and Stiff A modelling experiment examines changes in rupture potential in stiff roof units for each stratigraphic type as discrete unit thickness and location are manipulated through a range of values. Results suggest that the stiff-to-compliant ratio of the host rock has an impact on the relative stress-inducing effects of discrete stiff members. In other words, it is necessary to consider both the thickness and the distance to the seam, within the context of the host rock, to accurately anticipate areas of elevated rupture induced hazard; acknowledging the presence of a discrete unit within the overburden in general terms is an insufficient indicator of risk. Through modelling of anticipated changes in the placement and dimensions of discrete units within their stratigraphic setting, elevated rupture-induced bump hazard can be anticipated on a case by case basis. Were similar modelling studies conducted at mine sites in tandem with tracking of problematic discrete stiff units, areas of elevated rupture risk could be anticipated in advance of mining. Developing this predictive capability beyond identifying rupture potential in discrete roof members is essential to the eventual elimination of dynamic failure related worker injuries and fatalities. As stress is a necessary component in the occurrence of dynamic failure events, this finding helps to refine our understanding of the role of individual stiff, strong roof members in bumping phenomena, and suggests that a more holistic view of overburden lithology, combined with site-specific numerical modelling, may be necessary to achieve greater miner safety.

Jan 1, 2016

By Changshou Sun

The geotechnical properties of mining zones at Leeville Underground mine (or simply called Leeville mine through this paper) are different from one zone to another. The typical ground condition is poor to very poor. Therefore, an appropriate ground control practice is the key for the successful and smooth ore production at Leeville mine. Based on the available geotechnical data, the empirical and numerical methods are used to design the right ground support systems. The main ground control practice at Leeville mine includes geotechnical data collection, evaluation of the stiffness of the support system, selection of rock bolt types, design bolt length and bolting patterns, development of the ground support standards, and performance of quality controls on bolt installation to ensure proper rock anchorage, and strength of shotcrete and backfill. Through all these activities, an appropriate ground support system has been established and applied at Leeville mine for the past four years. So far, a good result related to successful ground control and smooth ore production has been achieved, although the system is still being improved. The authors of this paper will summarize the ground control practice conducted in the last four years and will discuss the procedures and standards of establishing the appropriate ground support systems for Leeville mine.

Jan 1, 2013

By Bruce K. Hebblewhit

Australia is well endowed with extensive reserves of thick underground coal scams, particularly in the range of 4.5m to 9m thicknesses. (For the purposes of this paper, thick scams are defined as being greater than 4.5m thick.) At the present time, the thickest, high production, high productivity underground mining operation in Australia operates at a maximum of approximately 4.8m using single pass longwall methods. In a number of instances, conventional' height operations are mining thick seam coal, effectively sterilising considerable reserves - either because of financial or technical risk concerns, or in some cases poorer quality coal at some horizons in the scam, UNSW has been involved in thick scam mining research over many years, most recently through an ACARP -funded project, jointly with the CMTE in Australia. A key part of that project involved the assessment of the relative geotechnical risks associated with the various methods under consideration. The methods were: single pass longwall; multi-slice descending longwall; soutirage and related caving methods (including the Chinese Longwall Top Coal Caving method (LTCC)): and hydraulic mining. A formal risk assessment process was adopted to review the risks, relative to a conventional 4m high longwall operation. This paper presents the findings of that risk review and discusses the geotechnical issues (and some possible solutions for risk management) which need to be dealt with in order to bring these methods to fruition in the context of the Australian coal industry environment.

Jan 1, 2001

By Michael J. Peacock

The Powhatan No. 4 Mine is located in southeastern Ohio along the Ohio River in Monroe County. The mine produces approximately 3.2 million tons of steam coal annually from the Pittsburgh No. 8 seam. The Pittsburgh No. 8 seam in this area dips to the southeast in varying amounts from 10-feet to 40-feet per mile. The seam is made up of a main bench coal (4' to 5f' ) containing several thin and variable partings and a rider ("roof") coal (0" to 18") which is separated from the main bench by a parting (0" to 18") that is reasonably consistent in occurrence and position. Strata overlying the seam varies from a bedded to nonbedded calcareous mudstone-claystone (6' to 12') in the north and east regions of the mine to a sandstone (Pittsburgh Sandstone C10' to 40'1 in the south and west regions. The Redstone Limestone (10' to 18' ) overlying the mudstone-claystone in the north and east thins and rises toward the area of sandstone deposition in the south and west. From its opening in April of 1971 until late 1981, primary roof support in the long-1 ived entries at Quarto Mining Company's Powhatan No. 4 Mine had been achieved with 8-foot and 10-foot. 518-inch mechanical roof bolts using a Single expansion shell. These bolts were installed so that the expansion shell achieved a competent anchorage in the limestone. However, as the mine developed westward, the limestone began to trend upward becoming inaccessible as an anchorage medium at the 8-foot and 10-foot horizons. Anchorage achieved in the underlying mudstone-claystone strata was failure-prone due to weathering (after exposure to air and moisture) and inconsistencies in the anchorage horizon, leading to an increase in the incidence of reportable roof falls at the mine. Responding to the threat to safety and productivity posed by the increase in roof falls. Mine Management through its Safety and Engineering Departments, sought solutions to the problem. In seeking a solution to the problem, the safest and en engineering groups examined the reportable roof falls to determine the cause of those failures. Their examination pinpointed two items which were found to be a common link in the majority of those falls; namely, anchor integrity and geologic trends. Analysis of the roof falls indicated that due to effects of air and moisture on the anchoraae zones in the mudstones and clay- itones, the integrity of the anchorage over time could not be maintained. Geologic trends noted included the upward trend of the -main limestone as mining progressed westward, inconsistencies in the amount of roof coal and draw slate in the immediate mine roof and increasing evidence of heavily slickensided immediate mine roof. In the absence of the protective barrier provided by the roof coal, the immediate strata and particularly the draw slate, is susceptible to deterioration due to the weathering effects of air and moisture and thus is prone to "eat out" around the roof bolts and cause roof falls. The presence of heavily slickensided roof creates a need for increased contact surface such as that provided by plank and header blocks to aid in holding up the roof and tends to make the behavior of such roof very unpredictable. Having determined the causes of the roof falls. Mine Management began to identify the kind of roof support which would best solve these problems, while at the same time providing the mine with a safe, cost effective and efficient roof support system. This paper will provide an analysis of how this decision was reached, the background that was researched to aid in forming the decision, the considerations that played key roles in the formulation of the plan, the mechanics of following through and implementing the new roof support system, and an assessment of the resultant gains in safety, productivity and improved costs derived from the implementation of the new roof support system.

Jan 1, 1986

By Kudret Tutuk, Serkan Saydam, Sungsoon Mo

"This paper presents an overview of the floor heave management at the Glencore Bulga Underground Operations and investigates the contributing factors to the behaviour of the floor. The mine experienced a number of major floor heave events in gateroads on development. As the longwall face approached the roadways, the magnitude of floor heave frequently increased, while new floor heave also developed. Furthermore, severe floor heave events took place along the longwall face. The most observed failure mode was buckling. While regular floor measurements were conducted to better understand the nature of the phenomenon, and various measures were considered to control the deformation of floor, the mining height was increased for the predicted floor heave domains, which facilitated effective management of the floor issues. The experience in the mine indicates that mainly high horizontal stresses with greater depths of cover and certain types of floor lithology configuration are likely to contribute to the failures of floor strata.INTRODUCTIONThe Glencore Bulga Underground is located in the Southwest of Singleton in the Hunter Valley of New South Wales, Australia. Blakefield South, part of Bulga Underground, commenced longwall mining in 2010, extracting coal from the Blakefield seam, while the overlying longwall panels were previously mined from the Whybrow seam. The coal seam ranges from 4.5 m to 8.0 m thick with the mining heights varying from 2.9 m to 3.5 m. The longwall panels are 320 m and 400 m wide and up to 3.5 km long.The mine experienced a wide range of floor issues around longwall panels LW7 and LW8, both 400 m wide. For instance, the concrete floor in the travel road failed and lifted as the longwall retreated in close proximity as can be seen in Figure 1. Figure 2 shows the beam stage loader (BSL) buried into the lifted floor, and Figure 3 shows a timber prop distorted due to the movement of the floor. In addition, severe deformation of the floor along the longwall face impeded the longwall operation, as shown in Figure 4."

Jan 1, 2018

By Gift Makusha

The Witbank Coalfield in South Africa contains up to five economically viable coal seams. These seams have been mined continuously for over 100 years and the reserve in now becoming depleted. The normal mining method has been bord and pillar with examples of stooping. The situation now occurs where the formed pillars, left to maintain stability, have a higher quality than the coal that is left in the unmined reserves. Where the reserves are less than 70m deep and continuous over several kilometers, opencast mining is being successfully applied. After several years of investigation, Anglo Coal has identified blocks of coal that warrant investigation with regard to pillar extraction by underground methods The proposed method is pillar extraction using Mobile Roof Supports (MRS) It is proposed to mine three panels at a depth of 120m, by keeping the panels narrow and sub-critical. The sub-critical layout does not allow caving to surface and more stress is thrown onto the barrier pillar rather than the face pillars. The sub-critical span is anticipated to be 100m. The paper describes the full geotechnical investigation, including numerical modelling, instrumentation and drawing on the local South African experience of shortwall mining. The project will be unique in that pillars were not designed for pillar extraction and have low factors of safety (l.4 - 1.8) and width to height ratios of less than 4. The mining height will be approximately 4-4 5m, and this represents "new" operating heights for Mining Roof Supports. The required equipment was ordered in late 2002 and pillar extraction is anticipated to begin in June 2003.

Jan 1, 2003

By Paul L. Tyrna

The Fola Coal Co., LLC No 2 surface mine, located in Nicholas and Clay Counties, WV, exploits up to nine coal seams by contour, highwall, and mountain top removal mining methods. After encountering faults, areas of highly fractured coal, and subsidence failures. the mine requested a lineament analysis from the Mine Safety & Health Administration's (Pittsburgh Safety and Health Technology Center) Roof Control Division. The 31 km2 study area was viewed on a LANDSAT-5 band 4 (near infrared, 0.76-0.90 µm wavelength) satellite image with ERDAS Imagine 8.4 software. The software allows image examination under artificial sun azimuths and elevations, resulting in variable tonal characteristics. The study area image was viewed in 45° azimuthal increments from 000° to 315° at artificial sun angles of 10°, 30°, 50°, and 70°, yielding 32 separate images. The interpretation process developed by MSHA Roof Control personnel identified 16 northeast-striking and one northwest-striking lineament that intersected mine property. Faults and areas of roof instability corresponded well with lineaments identified in the study. Successful lineament identification prompted mine management to incorporate lineament analysis as a mine-planning tool for adjacent property, thereby identifying in advance areas that could adversely affect safety and production.

Jan 1, 2001

By Alan Campoli

Approximately 100 million fully grouted rebar roof bolts are installed each year in the United States, 80% are B-Series (5/8 inch fully grouted rebar in a one-inch borehole). The lesser used more expensive A-Series (3/4-inch fully grouted rebar in a one-inch borehole) maximizes resin performance. Standard B-Series provides only 60% of the pull out resistance of A-Series. The Eclipse System was developed by Minova USA Inc. and marketed with Excel Mining Systems to improve the performance of B-Series. A 1/8-inch offset bolt head combined with a specially formulated Eclipse Lokset resin cartridge results in increased mixing efficiency and more uniform resin anchorage. Over two million units have been sold to date; the majority of these Eclipse bolts have been sold to coal mines in the Illinois Basin. Pull testing has demonstrated that a Grade 60 Eclipse bolt provides equal or superior pullout resistance as compared to a Grade 40 No. 6 rebar. Short encapsulation pull tests have yet to consistently demonstrate the superior pullout strength of Eclipse to standard B-Series, however 12 inch Eclipse grouted columns have successfully held in excess of the bolt yield load. The successful substitution of Eclipse bolts for A-Series has significantly reduced the primary roof support costs by approximately $0.18 per foot of bolt. Recent over coring analysis of various anti-gloving methodologies performed in New Zealand has confirmed the superior mixing of the Eclipse system (5).

Jan 1, 2004

By Hamid Maleki

Results of geologic investigations, in-mine instrumentation, and numerical modeling are presented for two longwall mines in which pillars with large width-to-height ratios (8 to 9) are used. These investigations revealed that wide pillars unload when subjected to sufficient stress and that pillar unloading is influenced by geologic conditions. Field measurements in mine 1 clearly indicated load transfer from the longwall area toward the gate roads, resulting in pillar failure near the margin of a fluvial channel. Loads were shown to transfer to a harrier pillar (width height ratio equal to 9), contributing to progressive failure of this pillar. Unloading of the barrier pillar was associated with significant load transfer to the two-seam submains to the north of the harrier, contributing to roof falls, floor heave, and rib spalling within the submains. Stress analysis results addressed pillar peak strength and unloading behavior that was in agreement with the measurements and underground observations. Underground observations and stress analyses were completed before and after longwall mining near a six-entry submain in mine 2. This submain was intended to he used as a temporary bleeder and tailgate for a small longwall block consisting of one to two panels, depending on the development schedule in other areas of the trine and observed ground conditions. Numerical modeling using both elastic and inelastic pillar behavior for submain pillars with a width-to-height ratio of 8 was completed before mining began. Results indicated that pillar unloading was likely. During retreat of the first longwall panel, pillars unloaded. as evidenced by progressive rib spalling along the cleats, This behavior was mostly confined to the immediate face area, an observation that was in agreement with the modeling results,

Jan 1, 2000

By Stephen C. Tadolini

A joint research program was designed and conducted at Lodestar Energy?s Baker Mine to investigate the performance of a high-capacity tensioned cable supported gateroad. The secondary system consisted of a 4.5 m (14.5 ft) 7 strand cables that were 23 mm (.9 in) and 18 mm (.? in) in diameter with a designed capacity of 515 kN (58 tons) and 355 kN (40 tons), respectively. The cables ware installed with an effective resin column length of 1.25 m (4 ft) and then pre-tensioned using a hydraulic jacking system. Roof separations, caving characteristics, and support performance was monitored and evaluated during the critical extraction of the adjacent panel, complex geological conditions of the west Kentucky No. 13 seam were mapped and the support performance recorded. This paper will describe the theory of tensioned cable systems and present an assessment of the support performance during the longwall retreat mining process.

Jan 1, 2000

By S. M. Hsiung

A model (support selection model) for the selection of suitable types of powered supports under various geological conditions is proposed. This model is developed by evaluating the support, performance under different roof conditions at longwall faces in terms of five controlling parameters: (1) roof Falls, (2) front abutment pressure, (3) Face convergences, (4) gob materials entering into working area, and (5) roof caving line. This model suggests that more than one type of powered support. may be used under the same roof condition and the best selection should he based on the magnitude of suitability index of each support- type. The development and application of suitability index arcs discussed. Fourteen panels in seven coal seams are discussed in the paper to illustrate the applicability and effectiveness of this model. A roof classification system at longwall faces developed specifically for the construction of the selection model is also discussed. A simple example fur determining roof classification index is given.

Jan 1, 1988

By Michael J. Friedel

The fundamental problems associated with ground control reflect the inability of mining personnel to "see" the development of anomalous conditions. Toward this end, the U. S. Bureau of Mines is investigating the application of seismic refraction tomography for imaging the process of pillar yielding, at the Foidel Creek coal mine, near Steamboat Springs, CO. The inherent three-dimensional problem of imaging coal pillar stress is reduced to a single plane by considering only the stress transmitted from the pillar to the adjacent floor rock. This approach exploits the fact that refracted seismic energy is essentially confined to the adjacent higher velocity sandstone strata. The tomographic analysis using ray-tracing based on wavefront migration resulted in a robust P-wave tomogram from which regions of high velocity indicate zones of high stress concentration, whereas low velocity zones indicate regions of stress relief. The interpreted region of high stress concentration occurs at the pillar end nearest to the working face of the adjacent longwall panel. This finding is corroborated by vertical pressure cell measurements recorded in the pillar. The stress diminishes toward the pillar perimeter with local minima correlating to the locations of observed pillar spalling. Time-sequence monitoring, as the longwall face advances, is recommended for evaluating changing stress conditions.

Jan 1, 1993

By Jian Yong Chen

The results of a laboratory investigation of pullout tests conducted on grouted, plain cable bolts are reported. The main purpose of the study is to determine experimentally the shear bond stiffiess of cement grouted and resin grouted cable bolt systems, which is an important input parameter for numerical modeling of cable bolt support. Three nominal embedment lengths of 152mm (6 inch), 203mm (8 inch) and 254mm (10 inch) were used and two types of grout namely cement and Anchortite resin were used. Two water- to-cement ratios of 0.3 and 0.4 were employed for the cement grout. The concepts of effective shear stress and effective shear stiffiess are explained and adopted to calculate the shear bond strength and the elastic shear bond stiffiess per unit length. A comparison of results with previous research is presented to reveal the effect of water-to-cement ratio and embedment length on the shear bond characteristics of grouted cable bolts.

Jan 1, 2005

By L. Munsamy

The bulk of primary mining at the Anglo Coal's Bank Colliery was carried out by drill and blast method, in a bord and pillar layout exploiting the No. 2 Seam. Secondary extraction, top/bottom coaling, also took place using both the drill and blast (D&B) method and continuous miners (CM). Extensive mining in this seam resulted in the risk of sterilising the No. 4 Seam reserves. Therefore, multiseam design guidelines established by Salamon and Oravecz (8) were re-evaluated in light of the low safety factors and the low pillar width to mining height ratios of pillars in the No. 2 Seam. Two and three dimensional (2D & 3D) numerical modelling was carried out to quantify and verify the parameters that influence the multiseam mining. The modelling was conducted primarily in two dimensions (plane strain), with 3D models to confirm the validity of the 2D results. A panel risk assessment flow chart was developed to assess the panels with respect to pillar failure and multiseam mining. This flow chart was extended into the evaluation of risk of surface subsidence using the current rock engineering knowledge. The aim of this risk assessment tool is to assess all underground workings at Anglo Coal collieries simply and rapidly. The identification of high-risk areas would warrant further investigation and possible backfilling to reduce the risk of pillar failures ensuring that reserves in the No. 4 and 5 Seams above are intact.

Jan 1, 2004

By Daniel Su

Following several roof falls in the E3 longwall development of a Consol Pennsylvania Coal Company underground coal mine, an underground roof geology reconnaissance program consisting of twenty-nine 20-foot deep roof holes had identified the presence of thick sandstone layers with abundant shale and micaceous streaks. The base of the sandstone was 4 to 8 feet above the roof line within the fall areas. The presence of micaceous streaks created extremely pronounced planes of weakness in the sandstone. A structural irregularity, which extends into the E4 and E5 developments, exists between crosscuts I 1 and 30,where all roof falls occurred. Immediately to the east of this structural irregularity lies a huge syncline. The presence of thick sandstone units within the typically all shale environment and within such a structural irregularity helps to maximize stress concentration and creates closely spaced joints in the sandstone, which explains why many cracks are present in the macroscopically massive sandstone upon development. The approaching E2 longwall gob to the south and on the down-dip side, on the other hand, provided horizontal stress relief in the sandstone, which helps to explain the many roof falls in the E3 development that occurred during or immediately after the formation of the E2 gob. Finite element modeling confirmed the presence of such horizontal stress relief. Since the E3 longwall was a right-handed face, stress concentration was expected at the longwall headgate T -junctions, and further delamination of the sandstone was expected upon arrival of the E3 longwall face. Based on the results of the roof geology reconnaissance program and a roof stability analysis that took into account up to eleven parameters, supplemental headgate and track roof support consisting of 16-foot and 20-foot long cable bolts was successfully implemented within the zone of thick sandstone with micaceous streaks and structural irregularity and at other potential roof instability areas along the E3 development. The cable bolts were designed to be anchored at least 4 feet above the sandstone unit into the mostly competent sandy shale layers. No headgate roof control problem was reported within the sandstone and structural irregularity zone during E3 and the subsequent E4 longwall extractions.

Jan 1, 2002