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By Behdeen Oraee-Mirzamani

Tunnel stability has an important role in the production process of an underground coal mine. There are various methods for analysing tunnel stability, such as numerical methods and analytical methods. In this paper, numerical methods (Phase2 software) have been used to determine tunnel wall displacement in a mining tunnel of the Parvade underground coal mine. The Ground Reaction Curve has also been drawn using analytical methods to determine tunnel wall displacement. The comparison of results from the numerical method and the analytical method show a noteworthy difference in the tunnel wall displacement. The displacement calculated by the numerical method shows a lower value than that of the analytical method because the numerical method is more suited to modeling the various coal and rock layers, as well as the shape of the excavations found in coal mines. The methodology used in this paper together with the results obtained from both methods can serve as useful tools for the coal mine design engineer when determining the ground support requirements.

Jan 1, 2013

By Andrew Hyett

Over the last 30 years, a transition away from end-anchored bolts to fully grouted rock bolt systems has occurred in many underground mines. It seems geomechanically intuitive that the success of such bolts, which interfere with rock movements around underground excavations through a variety of complex interactions, is precisely why they remain challenging to instrument and monitor. Whereas measuring the load profile for an end-anchored mechanical bolt is trivial, for a fully grouted system, the load may vary dramatically due to discrete rock movement at cracks and joints. Despite the challenges, measuring the strain (i.e., load) along fully grouted rock bolts remains very appealing, and a lot of valuable research has been conducted using foil resistance strain gauges, particularly in US coal mines (Signer and Jones, 1990). Despite good intentions, more widespread adoption of this instrumentation remains unlikely for the following reasons: (i) The technology is expensive, partly because Intrinsically Safe (IS) requirements limit the number of suppliers and solutions. (ii) The technology is perceived as challenging to successfully implement except by those who are skilled in the art. (iii) The reliability of the technology is questionable in harsh environments. (iv) The technology relies on discrete pairs of strain gauges that may not capture the complete strain distribution along the bolt. No existing alternative technology can offer a solution to all of these limitations. However, in this paper, we introduce results from a new Distributed Optical Sensing technology that promises significant potential. Since the technology is capable of distributed sensing, our results suggest that this new technology can, for the first time, reveal the detailed strain profile along fully grouted rock bolts.

Jan 1, 2013

By Gary Buchan

"Mine-wide ground control stability is essential to any mining operation to prevent catastrophic failure of underground structures. The ability to recognize or indicate the potential for ground control hazards in underground mine operations is highly desirable to reduce or eliminate ground-control-related injuries and fatalities. The acoustic emissions (AE) form of non-destructive testing has been used for decades to determine material or machine deterioration or potential failure. This paper presents the findings of research in AE with the results of laboratory bolt pull tests and comparisons to field experimentation, including AE sensors and bolt load data in an underground stone mine. The goal of the project is to compare laboratory results of AE sensors on roof bolt pull tests with field experimentation to determine the capability of predicting bolt failure or roof deterioration.INTRODUCTIONMine-wide ground control stability is essential to any mining operation to prevent catastrophic failure of underground structures. The ability to recognize or indicate the potential for ground control hazards in underground mining operations is highly desirable to reduce or eliminate ground-control-related injuries and fatalities. Typical ground control monitoring devices involve drilling into roof or rib, grouting or gluing of sensors, and dedicated cabling for data collection. One of the original goals of this research was to develop a methodology for a simple wireless device that could provide an indication or warning of potential roof bolt or roof failure. Various types of nondestructive testing (NDT) have been used for decades to determine material or machine deterioration or the potential failure of either. One type of NDT is acoustic emission (AE) monitoring, which has been studied for use in mining to predict rock bursts and mine failures since the 1940s (Obert, 1941; Obert and Duvall, 1942). The primary goal of the work was to compare laboratory results of AE sensors on roof bolt pull tests with field experimentation to determine the capability of evaluating bolt loading or roof deterioration in an underground stone mine. Bolt pull tests were conducted in the laboratory to establish a repeatable trend of AE activity versus bolt load. The second part of the project was to then install bolt load cells in an underground stone mine and monitor AE activity in the vicinity of mining."

Jan 1, 2018

By Timothy Batchler

"Standing roof supports help to provide a stable working environment in coal mine gateroads. NIOSH participates in the development of these support systems by conducting full-scale tests in their Mine Roof Simulator. These tests have established the performance characteristics of every coal mine support system currently in use in the United States. Through these tests, mine operators and regulatory officials have acquired a fundamental understanding of the various types of support performance that guides the assessment and use of these support systems. However, some little known factors that influence support behavior can lead to misconceptions and, perhaps, a poor assessment of support behavior at times. This paper highlights several of these factors and discusses their significance. The paper also examines a current longwall tailgate support and discusses the inefficiency with this approach. Finally, the NIOSH Mine Roof Simulator (MRS) is the world’s most powerful load frame built specifically to test full-scale roof supports, including longwall shields. are addressed in this paper to provide a more in-depth assessment of the limitations of the MRS and what the MRS testing protocol is designed to do. INTRODUCTION Over the years, considerable research has been conducted to develop standing support technologies and to evaluate their performance characteristics. The load and displacement characteristics of the standing supports have been determined from full-scale testing conducted using the National Institute for Occupational Safety and Health (NIOSH) Mine Roof Simulator (MRS) (Barczak and Tadolini, 2008) located in Bruceton, PA. Performance traits, installation patterns, and ground responses are observed in various geological and mining conditions and are evaluated to determine the support performance in the field (Dolinar, 2010; Campoli, 2015; Zhang et al., 2012). Despite widespread use of standing supports, not every aspect of their performance and resulting ground support capability are always well understood. This paper discusses 10 factors, several of which are surprising, that impact standing support performance."

Jan 1, 2017

By Keith A. Heasley, Jian Yang

"LaModel uses a laminated overburden boundary-element model and can calculate not only seam-level stresses and displacements but also surface subsidence for thin tabular deposit such as coal seams (Heasley, 1998). Up to this point in time, the material property wizards in LaModel have primarily been designed for calculating accurate stress distributions in single- and multipleseam situations, and for investigating and optimizing pillar sizes and layouts in relation to overburden, abutment, and multipleseam stresses. However, the critical input parameters that will give the most accurate seam-level stress distribution do not necessarily produce the best surface subsidence prediction. The objective of this paper is to develop a methodology for calibrating the critical input parameters in LaModel to optimize surface subsidence prediction.For optimum surface subsidence prediction, it was found that the gob convergence (as defined by the final gob modulus) and the overburden stiffness (as defined by the lamination thickness) were the two most critical parameters that needed to be calibrated (Heasley, 2016). By calibrating numerous panels with various widths and depths, a formula that provides the optimum final gob modulus to use for subsidence prediction was developed. Also, three different empirical formulas for optimizing the lamination thickness as a function of overburden depth and/or the panel width to- depth ratio were determined for the three cases: supercritical panels, and subcritical panels with and without offsets. Further, if the user has measured data for subsidence factor and angle-of draw, the optimum final gob modulus and lamination thickness can be determined from the measured data. These new subsidence prediction formulas are being implemented into new material wizards in LaModel.INTRODUCTIONIn the past few decades, with the increased need for highly productive mining techniques, full extraction mining methods have been increasingly employed in the U.S. coal mining industry. These total extraction mining methods, which include both longwall mining and room-and-pillar retreat mining, normally cause immediate roof caving and the associated surface subsidence. These methods can be highly productive and cost-effective, but the associated surface subsidence can cause damage to surface structures and negatively impact the surface environment (Peng, 2008). Therefore, a reliable program for predicting the surface subsidence associated with full extraction mining is required for appropriately engineering and managing the surface subsidence impacts of full extraction mining."

Jan 1, 2016

By Winton Gale

"Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar.INTRODUCTIONA coal burst is defined as a rapid expulsion of coal (and potentially gas) from the boundary of the roadway. The volume of a burst can be variable, but volumes above 10-50m3 are noted and cause significant disruption to operations.Rock and coal fracturing is common about roadways, particularly under the influence of elevated stresses, either tectonic or related to mining abutment stress. Shear fracture is very common; however, it is very uncommon that the coal is propelled from the face or ribs as a result of fracture of the material. The nature of rock fracture about roadways under elevated stress is presented in Figure 1.A burst requires energy to propel the coal or rock from the rib or face of the roadway. The depth of material impacted may vary from 0.5m to greater than 3m inside the roadway boundary. The strength of the ground in this zone is variable depending on the nature of the coal seam (or surrounding material). Typically, this material has significant residual strength as confinement increases into the ribside.The energy required to propel this material is typically in addition to that which is associated with normal fracturing about the roadway."

Jan 1, 2018

By Gabriel S. Esterhuizen

The design of support for underground excavations is a complex process in which the interaction between supports and the rock mass must be evaluated. The strength reduction method (SRM) is useful because it provides a stability factor of the supported excavation that can easily be compared between support alternatives. The method is based on numerical model analysis where the stability factor is determined by reducing the rock mass strength until collapse is indicated in the model. This paper presents the results of validation studies that were conducted using the FLAC3D stress analysis software to compare SRMcalculated stability factors to the empirically based Coal Mine Roof Rating (CMRR) and the Analysis of Roof Bolt Systems (ARBS). The objective was to determine whether the SRM, with its basis in mechanics, would predict similar outcomes as the fundamentally different ARBS and CMRR methods with their basis in empirical observations. A total of 450 different scenarios were evaluated in which the geology, horizontal stress, depth of cover, entry width, support spacing, and support length were varied. The results showed that the SRM-calculated stability factors were able to satisfactorily capture both the ?weak bed? and ?strong bed? adjustments in the CMRR. The SRM results were found to be well correlated to the ARBS stability factors for the 450 cases considered. Further analysis showed that the SRM accurately captures the effect of bolt length, depth of cover, and entry width on the overall stability of an entry.

Jan 1, 2013

By Shu Cheng

Abstract: Based on the consolidation theory of soil dewatering and land subsidence prediction methods in soil mechanics, preliminary analysis of surface subsidence and movement due to groundwater withdrawal was performed using the stochastic theory and the principles of effective stress and the method for subsidence prediction was derived, Surface subsidence due to coal mining was then superimposed to obtain the total subsidence. Keywords: land subsidence, rock and soil consolidation, stochastic theory, effective stress

Jan 1, 2002

By G. Skybey

The Skybolt is a new bolt concept, point anchored with resin in large diameter holes of specific sizes 38 mm and 45 mm. The new design achieves the optimum annular spacing required for mixing the resin anchor components which is essential for optimum resin bonding of the bolt to the surrounding rock. This paper explains the load transfer mechanisms between bolt/resin/rock, unique to this bolt concept. It compares the pull-out test results to those of paddle and expansion shell type bolts, and highlights the versatility of Skybolt over existing, currently used, rock support systems and the benefits mine operators can achieve when using the Skybolt.

Jan 1, 1992

By John C. Stankus

At the 19th International Conference on Ground Control in Mining, a paper was presented detailing new types of Jennmar INSTáL tensionable cable bolts (Stankus and Taylor, 2000). Since that time, significant testing and trials of the various types of tensionable cable bolts have been conducted. Several MSHA districts and state regulatory agencies have now approved these systems as primary support This paper will detail the use of tensionable cable bolts as primary support at two longwall mines in Virginia (MSHA District 5).

Jan 1, 2001

By Frank Kendorski

Several startup or longtime underground stone mines have experienced unanticipated roof control difficulties due to the variable geologic character of the roof strata. Careful mapping of roof conditions, and identification of controlling geologic and mine geometric factors using sedimentological descriptors, lead to an understanding of the mechanisms causing roof problems. Types of bedding plane features can be categorized into geometric, internal structures, bedding-plane markings, deformed bedding, solution structures, and accretionary structures. Factors such as encountering open water-filled master joints; roof strata delamination due to internal water pressures; local karst features with mud, sand, and water hazards; and sudden loss of limestone roof due to paleo-channel erosion are some recent problems described.

Jan 1, 2002

By Liu Changyou

This paper analyzes experimentally the stress-strain relationship and the characteristics and variation trends of acoustic emission signals of the medium- and high-burst-prone coal/rock fracture process. The precursor and sensitive characteristics of the acoustic emissions when the burst-prone coal/rock fracture occurs were obtained. The characteristic index and the feasibility for predicting rock bursts using acoustic emissions technology were proposed. Keywords: burst-prone coal/rock, acoustic emissions, energy, prediction index

Jan 1, 2003

By Ezzeddin Bakhtavar

There are several geotechnical challenges and risks inherent in the interaction between the open-pit and block cave mining environments. This is especially true when they have to work simultaneously for a short time, as well as when only the caving operation is continued. Some of these challenges and risks include the induced zones of stress due to the large and deep open-pit, instability of the slope, instability of the crown pillar and water inflows, subsidence, and sometimes instability of the main accesses to the block cave portion of the mine. These challenges usually occur due to cave propagation. In this paper, these geotechnical challenges and associated risks have been studied and discussed. It was concluded that the majority of the mentioned geotechnical risks could be decreased, and often eliminated, by considering the appropriate dimensions of a stable crown pillar, especially in simultaneously working open-pit and block caving operations.

Jan 1, 2011

By Ebrahim Ghasemi

Roof fall risk is a common problem in coal mines, and it is generally unpredictable due to variability in geological and mining parameters. In this study, a new fuzzy logic model was developed to predict roof fall rate in coal mines. Parameters such as coal mine roof rating (CMRR), primary roof support (PRSUP), intersection span, and depth of cover were considered as the model inputs. The model was developed based on experts? knowledge and also a database including about 109 datasets of roof performance from U.S. coal mines. Approximately 20% of these datasets were used to assess the performance of the fuzzy model. In each case, comparison between results from model and real roof fall rate showed that this model can predict roof fall rate to an acceptable extent.

Jan 1, 2011

By Kikuo Matsui

In a difficult economic environment, the coal mines in Japan have been making extensive efforts in order to survive by rationalizing the mining operations. Introduction of a new longwall mining system is one of these efforts to establish the rational production structure. In the high production longwall mining system, gateroads should be developed rapidly and maintained effectively during their lifetime. Strata bolting support system has been employed in the gateroads to raise their stability levels and reduce their maintenance cost. However, excessive gatemad closure caused by longwall mining is still occurring, impeding the passage of equipment, personnel transportation, and ventilation air in the longwall panels. This paper describes the development system of gateroads and proposes a new mining method to control the excessive roadway closure problem.

Jan 1, 1995

Roof conditions at Southern Colliery, the second Iongwall mine to be established at German Creek, were found to be much stronger and more massive than at the earlier longwall mine on the lease. Investigations were made to ascertain the level of face support required for longwall extraction under the changed geological conditions. Geotechnical information was provided from drillholes and highwall exposures. Downhole geophysical logs, notably the sonic velocity log, were used to obtain a distribution of the major lithological units over the mine area. A representative type section of the strata and geomechanical properties was compiled to form the basis for numerical modelling. Two-dimensional finite element methods were used to predict strata movements and chock loads under cantilevered roof conditions. The model incorporated a 6.3 m thick roof beam either bonded or detached, with either a 2 m or 5 m overhang. The solid rib, immediate roof, chock support and goaf were modelled as distinct parts, in addition a front abutment stress profile, derived from three-dimensional displacement-discontinuity analysis, was superimposed on the model.

Jan 1, 1992

By Dale S. Preece

In 1975 Congress passed the Energy Conservation Act to establish a U. S. Strategic Petroleum Reserve (SPR) with a capacity of 750 million barrels of crude oil. The most economic storage medium was determined to be salt caverns leached in salt domes in Louisiana and Texas. Salt caverns existed at several sites when the reserve was created. These were obtained by the U. S. Department of Energy (DOE) and used to initiate SPR oil storage. In order to meet the storage capacity approved by Congress, new caverns also had to be leached. To support the resulting design effort, finite element computer program have been used to determine the creep closure and structural stability of salt caverns. Using site specific material properties including creep models, elastic moduli and fracture data, the finite element analyses have replaced earlier empirical approaches to cavern design. This report presents results of such finite element analyses to determine the best cavern roof shape and the minium pillar to dimeter ratio. , P/D. These numerical predictions indicate that the current cavern design is safe.

Jan 1, 1984

By H. N. Maleki

Tension-torque relationships were investigated *n a coal mine for bath standard 5/8 in. (16 mm) bolts and a vibrating wire instrumented bolt, using conventional and controlled installation techniques. The instrumented bolts differed from the standard bolts by their head size and hole preparation procedures. The controlled installation technique developed by the U. S. Bureau of Nines (USBN) involved using hardened washers and lubrication in the bolt assembly, and maintaining a minimum thrust level during bolt installations. The object of this study was to determine the tension to torque ratio (lb/ft-lbs; for standard bolts and instrumented bolts for both conventional and controlled installation techniques, and to modify the bolt assembly so that the ratios would be equal for the instrumented and standard bolts for both installation techniques. It was shown that the instrumented bolts installed in faced holes had a similar tension torque ratio td that of standard 5/8 in. (16 mm) bolts installed in unfaced holes for each installation technique, and thus they were representative of other uninstrumentea bolts. This was achieved by evaluating the influence of several factors oh the relationship, including: facing the hale surface, introducing washers and lubrication in the bolt assembly, and applying different levels of thrust at installation due to bolter operator practice. The tension to torque ratio fur conventional bolts was only 21 (lb/ft-lb) for the coal mine, resulting in an installation tension of 4000 lbs (17.8 kg) with the typical installation torque of 150 to 400 ft-lb. This tension level was approximately 50 percent of the required bolt tension, and thus, it was concluded that torque standards are not sufficient to assure adequate bolt tension. .he low tension levels were due to the particular wait hardware in use, operator practice, and bolt fencing.

Jan 1, 1984

By Kevin Beauchamp

In many cases traditional support designs cover the many variations in ground conditions and bolt performance by a conservative factor of safety. The result is that most bolts are ineffectual most of the time and yet the response to occasional roof collapses is an even more conservative design. The probabilistic analysis presented in this paper takes several important parameters of a support design into account such as span, bolt strength, bolt length, water pressure, seismic accelerations, joint orientation, bed thickness and joint strength and allows the effect of variations in these parameters to be evaluated simultaneously. The analysis demonstrates the application of the probabilistic approach to both blocky, wedge-dominated conditions and bedded deposits dominated by laminations. All mine excavations have a designed stand-up time. By taking into account of the possible variations in the design parameters explicitly, it is possible to produce a support system designed to ensure stability over a given period of time. Rather than use a factor of safety criterion, the approach uses a percentage reliability or probability of failure. By understanding the probability of failure of the support system, it is possible to improve the performance of ground support without any increase in cost or conversely to decrease the cost while maintaining the performance of ground support. This approach reduces reliance on empirical experience and leads to a better understanding of the key factors that influence stability and can provide a much more accurate and optimized assessment of support design and performance. The result is a more cost-effective support system and we hope that a broader application of this approach to ground support in underground mines will lead to a greater acceptance in the industry of probabilistic analyses and of terms such as Probability of Failure and Reliability of Support.

Jan 1, 2003

By Nielen van der Merwe

The data base of failed pillar cases as used by Salamon and Munro (1967) in their original analysis to determine the strength of coal pillars empirically, has been updated for this study. It is shown that the Vaal Basin and Klip River coal fields exhibit similar behaviour, i.e. failure at high safety factors within a short period of time after their creation. Those collapse cases have been omitted from the new data base. New failures that occurred after 1967 have been added, except for those that occurred in the Vaal Basin and Klip River coal fields. (1967) a technique to minimise the area of overlap between the populations of failed and intact pillar cases, the new data base was re-analysed in conjunction with the original Salamon and Munro (1967) data base of intact pillar cases. It was found that a linear formula reduced the overlap area by 22%. The new formula predicts higher strength for pillars with a width-to-height ratio greater than approximately 2S and a lower strength for smaller pillars. For most current South African coal mining conditions, using the new formula for pillar strength will result in leaving smaller pillars without sacrificing stability. The reason for this is that the Salamon and Munro (1967) formula underestimated the strength of larger pillars and overestimated the strength of smaller pillars. The potential benefit for the South African coal mining industry amounts to saving reserves equal to the total production of two large mines per year.

Jan 1, 2002