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By Houquan Zhang, Yu Wu, Yanlong Chen, Zhenfu Luo, Aihong Lu

"A non-destructive detection scheme for bolt installation quality was designed by adopting the method of non-destructive detection on bolt installation quality. Based on the field measurements, a random non-destructive detection method for determining the characteristic length including anchoring length, free segment length and initial axial load of bolt was discussed, and the characteristics of increasing bolt axial load in the initial stable stage was obtained. The results show that the bolt axial load and deformation of surrounding rocks correlate each other. Compared with the traditional detection technique for bolt installation quality, the non-destructive detection method has obvious advantages such as random, accuracy, and economic. The method provides a reliable basis for bolt support optimization and stability control of surrounding rocks.INTRODUCTIONBolt support in roadway can make the surrounding rocks and bolt form a structure in the early stage of roadway development through the bolt pre-tension, which can maximize the self-bearing capacity of surrounding rocks. Thus it is a form of active support (Kang, 2007). In order to make the bolt play an active supporting role, it needs to ensure the installation quality of bolting. Thus the bolts can supply a high initial anchoring force and control the deformation of surrounding rocks (Shan, 2009; Wei, 2012). Moreover, bolt supporting is a concealed work. Once installation quality problems appear; it will be difficult for effective detection and screening (Xie, 2008).THE PROBLEMS OF BOLT INSTALLATION QUALITY IN COAL MINE AND ITS DETECTIONThe Problems of the In Situ Bolt InstallationDue to the problems in installation environment, machine, and technique, and workers proficiency (Li, 2003), there are many problems exist in bolt installation. If the borehole depth is too long, the anchoring agent will accumulate at the bottom of the bore, which reduces the anchoring length. Furthermore, if the borehole depth is short due to the poor geological condition, the exposed part of the bolt will be too Jong and the pallet cannot cling to the rock face, which prevent the pre-tension from being applied. The pre-tension is very important for bolt installation. If pre-tension is insufficient the rock bolt cannot control the deformation of surrounding rock efficiently and the bolt anchoring force will decrease considerably. The bolt length is another important parameter for anchoring quality, which will affect the control area of bolt supporting. In addition, the cleanness of drill hole has a great influence on the effective anchorage length of bolt. If the drill hole cleaning is not enough, it will directly affect the anchoring agent and the contact area of surrounding rocks, which will decrease the effective anchorage length and initial anchor-force."

Jan 1, 2016

The U.S. Bureau of Mines (USBM) has developed the coal Mine Roof Rating (CMRR) to aid in ground control. This paper describes the application of the CMRR at four coal mines operated by a major U.S. coal company. The mines are widely separated geographically, and cover a wide variety of geologic conditions and mining methods. The study found that the CMRR was a useful predictor of roof performance. In mines where the CMRR varied, and other parameters held constant, roof falls were associated with low CMRR values. Important similarities in roof performance were found in mines with similar CMRR values, despite radically different geologic environments. The analyses also indicated that statistics can be used to evaluate ground control performance. For these mines, a "roof fall probability" can be estimated from the CMRR and the intersection span. A spreadsheet program has been developed to facilitate treatment of CMRR data, and it is briefly describe

Jan 1, 1994

By Selmar A. Oliveira

"Safe mine closure is an important step in the mining lifecycle. Plans for safe and efficient closure should begin at the inception of mining. Ideally, closure occurs when the reserve has been depleted. However, when the abandonment of a mine occurs prior to the depletion of the reserve, several problems arise which need to be treated. This situation occurred at Verdinho Mine, located in the state of Santa Catarina, southern Brazil. Verdinho Mine is a coal mine that operated for more than 40 years without had backfill technologies applied. Soon before the exhaustion of the reserves, it was abandoned by the owners without proper closure. As a result, the Federal Public Ministry filed a public civil action to promote proper closure. Several parties are currently involved, including the National Department of Mineral Production (DNPM), the mining regulator of the Brazilian Government. To meet the judicial determination, a cooperation agreement was initiated between DNPM and the Federal University of Rio Grande do Sul (UFRGS) to identify and to monitor parameters which could result hazard due flooding mine, to model the various conditions to minimize the impact of flooding caused by total abandonment. Many processes must be planned and planned early in mining operations, and these processes must be performed over the life of the mine. Financial resources must be guaranteed by a specific fund fueled by operating revenues. When these resources are not properly employed, costs and technical difficulties prevent proper closure progress. If the removal of subsoil equipment, treatment and control of groundwater, regeneration of the surface, and closure of openings are not done correctly, there can be environmental, social, and stability impacts over time. To avoid this in the future, a detailed study is being performed at the Verdinho Underground Coal Mine on how to appropriately implement mine closure and monitoring procedures. This study is being conducted in response to a lawsuit filed by the Federal Public Prosecutor’s Office. INTRODUCTION The nature of the Mining Industry is such that all mining is a temporary activity, where the operational life of a mine is defined by the final project scope. The operating life of a mine lasts from a few years to several decades. Mine closures occur once the mineral resource at a working mine is exhausted, or operations are no longer profitable. Mine closure plans are required by most regulatory agencies worldwide, such as the Mineral Production National Department (DNPM) in Brazil."

Jan 1, 2017

By Vincent A. Scovazzo

"The most significant study on the effects of longwall mining on gas and oil well integrity was completed by a consortium of coal and shale gas industry participants. A field study was conducted in 2013 and 2014 and is referred to by its location on Well Pad NV-35. The experiment site was located at the CONSOL Enlow Fork Mine in Washington County, Pennsylvania.The experiment was completed to verify John T. Boyd Company (BOYD) calculations and studies completed under contract by for the Pennsylvania Department of Environmental Protection Gas Well Pillar Study Update. The findings of the original study were presented by BOYD at the 32nd Conference, while numerical analysis of the Well Pad NV-35 field study was presented by Dr. Su at the 35th Conference.This paper addresses the findings of the experiment and how it confirmed the conclusions of the original study.INTRODUCTIONA field study was conducted in 2013 and 2014 by a consortium of coal and shale gas industry members to test the effects of longwall-induced ground movement on the stability and integrity of wells drilled through a gateroad abutment pillar. The project was initiated to test initial John T. Boyd Company (BOYD) findings of the Gas Well Pillar Study Update (Scovazzo and Moran, 2014), completed for Pennsylvania Department of Environmental Protection, Bureau of Oil and Gas Management (PDEP). Background and initial findings on this project were presented at the 32nd Conference (Scovazzo and Moran, 2012). The field work and numerical analysis were spearheaded by CONSOL Energy, Inc. (CONSOL) with participation by Chevron, EQT, Noble Energy, Range Resources, Marcellus Shale Coalition, and the Pennsylvania Coal Alliance.Dr. Wen H. Su of CONSOL was the leader of the field experiment with input from the participants. Dr. Su also completed numerical analysis of the results and model casing deformation. Analytical results were presented by Dr. Su at the 35th Conference (Su, 2017)."

Jan 1, 2018

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 Kewal K. Kohli, Stefanie Self

"Three cases of surface subsidence investigated over abandoned coal mines after flooding in one case and dcwatering in two cases are presented in this paper. Mining was conducted by the room and- pillar method at all the mines leaving stable pillars. This paper will discuss the background chain of events and the likely causes of the pillar failure resulting in surface subsidence.CASE NO.1BackgroundThe Pittsburgh scam was mined in 1960 by the room-and- pillar method, leaving pillars (Figure I) through Mine A. located in Monongalia County. West Virginia.The mining height in U1is area was 1.83 m (6 ft) with an average mining depth of 140 m (460 ft). The mining recovery was 62% and the safety factor of the pillars was calculated at 1.15 using the Bieniawski ( 1984) fom1ula. For stable pillar design. a 1.5 safety factor is recommended. A 1.15 safety factor is considered marginal. Pumping continued after the mine was shut down in 1960 to prevent water seeping into an adjoining mine owned by the same company. The pumping ceased on December 12. 2003, and the mine was allowed to flood. The water level rose about 17 m (55 ft) by August 3 1, 2004, based on observations in a nearby borehole. On September 3, 2004 surface subsidence occurred above this area causing damage 10 a residence (Figure 2).Summary of InvestigationThe floor of the Pittsburgh coal scam played a strong role in the pillar failure. In this area, the immediate floor of the Pittsburgh seam consists of about two feet of fireclay. Extensive damage to the surface house was an indication that a major collapse below the surface had occurred. The pillars supporting the overlying strata were evaluated to find the cause of their failure. The pillars in the Pittsburgh seam (sec Figure I) had a marginal safety factor of 1.15 before flooding and were stable for 26 years. Once the mine was flooded the water level in the Pittsburgh seam rose by 17 m (55 ft). Shortly thereafter the fireclay in the floor weakened, resulting in several pillars punching into the floor unevenly making the immediate roof unstable. The overburden strata supported by these pillars also subsided do"" n into the mine, causing damage to several house on the surface."

Jan 1, 2010

By Kazem Oraee-Mirzamani, Nikzad Oraee-Mirzamani, Arash Goodarzi

"Unexpected movement of ground can potentially endanger lives, damage equipment or destroy property. Occupational accidents frequently occur with fatal consequences in developing countries with significant economic dependence on industries such as mining. Therefore, there is increasing need for miners' protection against such hazards. Slope stability and roof support accidents are two of the major causes of fatalities at surface and underground mining operations. According to national codes, employers are obligated to protect workers against accidents; however, these rules fall short of the standards in developed countries. National safety regulations should clearly specify support systems. The International Labor Organization (ILO) prepared two codes of practice, aiming to guide those responsible for improving standards of safety and to provide guidelines for drafting of safety regulations for the coal mine industry and quarry open cast mines. The practical recommendations of these codes in the ground control area have been analysed and the advantages and disadvantages of ILO codes concerning ground control are summarized.INTRODUCTIONTwo million people die every year from work-related accidents and diseases. An estimated 160 million people suffer from work-related diseases, and there are an estimated 270 million fatal and non-fatal work-related accidents per year. In economic terms, the ILO has estimated that 4% of the world's annual GDP is lost as a consequence of occupational diseases and accidents (ILO, 2009).Mining has always been dangerous due to many potential risks such as those associated with using machinery in enclosed spaces, rock falls, blasting operations and explosion of flammable gases, which endanger miners' safety and health. However, it is suggested that implementing new laws along with strict adherence would reduce such risks in the workplace.The severity and frequency of accidents are trending downward in developed countries, where the focus is on large private companies who invest heavily in mechanization and developing economies of scale. But in many parts of the world, particularly in developing countries, minerals are extracted by people working with simple tools and equipment; they work on a smaller scale, are more flexible, and can exploit irregular ore bodies and steep, dipping seams."

Jan 1, 2010

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 Joe Zelanko

In order to prevent roof fall accidents and injuries, U.S. underground coal mine operators must develop and implement effective roof control strategies. Various tools are available to assist engineers in developing designs. For example, criteria to address global stability issues (e.g., pillar stability) are well established in the U.S.; effective tools (e.g., empirical and numerical) are available to diagnose potential problems and avoid catastrophic failures. However, most miner injuries occur as a result of local instability (e.g., a roof fall in a single heading or a rib roll from a pillar corner). Hazard recognition and the availability of support plans suitable to those conditions are key to maintaining stability and safety on the working section.

Jan 1, 2011

By Zhaohui Wang, Jiachen Wang

"Effective surrounding rock control is a prerequisite for realizing safe mining in underground longwall faces. In the past three decades, longwall top-coal caving mining (LTCC) and single pass large height longwall mining (SPLL) found expanded usage in extracting thick coal seams in China. The two mining methods lead to large void space left behind the working face, which increase the difficulty of ground control. Longwall face failure is a common problem in underground mining, which is induced due to low strength and high fracture density of the coal seam. Moreover, the stiffness of main components included in the surrounding rock system also greatly influences longwall face instability. Correspondingly, the surrounding rock system is developed for LTCC and SPLL faces in this paper. Mechanical conditions for simultaneous balance of roof structure and longwall face are put forward by taking the stiffness of coal seam, roof strata, and hydraulic support into account. The safety factor of the longwall face is defined as the ratio between its ultimate bearing capacity and the load imposed on it. The influence of coal strength, stiffness of the coal seam, roof strata, and hydraulic support, as well as the movement of roof structure, are analyzed. Finally, the key elements dominating longwall face stability are identified for improving surrounding rock control effectiveness in LTCC and SPLL faces.INTRODUCTIONThe concentration of coal mining in China is gradually shifting to the west region, and there are a large number of thick coal seams in the newly formed coal mining area. Therefore, the surrounding rock control problems related to mining of thick coal seams will become the focus of research in the field of mining engineering in the future. There are two kinds of mining methods for thick coal seams in China, including single pass large height longwall mining (SPLL) and longwall top-coal caving mining (LTCC). Both methods can accomplish the goal of excavating the full thickness of thick coal seams at one time. SPLL mining benefits from the improvement of the development level of associated equipment. The appearance of large hydraulic supports and high power shearers ensure that the height protected by the hydraulic supports and the height of the sheared coal are the same as the coal thickness. LTCC mining benefits from mining technology advantages. The thick coal seam is divided into two sub-layers, and the working face is normally installed in the bottom layer, while the top coal is caved under the compressive forces due to mining. Therefore, there is a scraper conveyor at the front of the hydraulic support for transporting the sheared coal and another one at the tail for transporting caved top coal. Both SPLL and LTCC methods realize one-off recovery of thick coal seams."

Jan 1, 2018

By Gregory M. Molinda

Difficult mining conditions in deep western U.S. Iongwalls have necessitated the use of novel tailgate support systems. Extreme ground movement in these yielding gate systems has caused operators to utilize secondary support systems which match yield and strength characteristics with expected ground reaction. Other benefits include ventilation and safety gains, as well as lower costs. Recently, the use of novel support has been tried at longwall mines in other U.S. coal basins. One of the main considerations is the capability of the support to maintain an adequate airway for gob ventilation in gassy eastern coal mines. In one large Pennsylvania mine working the Pittsburgh seam, a number of support configurations were monitored over a 2,300 ft tailgate test area. The test included 4-pt wood cribs, Link-N-Lock cribs, Propsetter yieldable posts, cable trusses, and vertical cable bolts. Ground reaction, and thus support performance, was documented by roof sag and convergence instruments, ventilation measurements, and photographs. Various combinations of standing support and cable truss and vertical cable bolts provided adequate support for ventilation and safe escape. A cribless configuration consisting of only cable trusses and vertical cable bolts was inadequate to provide inby gob ventilation to the next open crosscut. Instrument ground movement data and visual documentation are presented.

Jan 1, 1997

By Rob Thomas

"In Australia, a wide roadway is commonly defined as any roadway that is driven to a width greater than 5.5 m (18 ft). These roadways are typically required to enable the installation of longwall equipment (termed installation roads); however, other applications include the need to house conveyor driveheads and pre-driven recovery roads.As a result of the increasing size of longwall equipment and mine infrastructure, in some mines, roadway widths of between 9 and 12 m (30 and 39 ft) and are being developed.Accepting that these almost always constitute strategically important and often long-term roadways, this paper considers the use of: (i) an empirical roof support design model, which is based on over one hundred case histories, (ii) various roof support design principles, and (iii) commonly accepted strata management techniques.The paper demonstrates that wide roadways can be driven and supported to an acceptable standard providing due consideration is given to the prevailing ground conditions (including the quality of the immediate and upper roof strata, the Depth of Cover and the horizontal stressfield), the final width of the roadway and the required use of the roadway.INTRODUCTIONRoof behaviour and the associated roof support requirements in wide roadways, which are roadways typically driven to a width that is greater than 5.5 m (18 ft), are controlled by a number of key geotechnical and operational factors. The recognition of these factors is an important first step in the design process and it is imperative that each is considered in conjunction with local mine site information.The main points of note in this regard can be summarised as follows:• The greater the final width of the roadway, the greater the resulting surge in roof displacement on widening - note: a) see Figure 1 for a schematic illustration of the roof displacement trends typically measured during the development of the first pass and roadway widening, b) since displacement increases with roadway width, it is always recommended that the roadway width is kept to the minimum required and c) general buckling beam theory indicates that the capacity of any beams present in the roof will reduce in inverse proportion to any increase in roadway width squared (i.e., [final width2 -first pass width2] I final width2)."

Jan 1, 2010

By Andrzej Walentek

In polish coal mining due to specificity of seams lying the multiple extraction is used. In this kind of mining, it is essential to take into account all geological and mining conditions which have an influence on stability and functionality of workings during their using. In view of above, for one of polish coalmines, by mean of Phase2 program, the numerical calculation were made for assessment of the influence of advancing excavation front on convergence of roadway located 30 meters above longwall was analysed. In order to verify developed prognosis, in analyzed roadway the underground investigations in scope of convergence measurement and changes of load exerted on working support were carried. As a result of these investigations it was found that real convergence on certain part of roadway was significantly larger then determined in prognosis. Additional calculations and analysis showed that the main cause of differences was underestimation of influence of dynamic load, which was an effect of numerous low energetic rockmass tremors. These tremors, related to excavated longwall, were located in close proximity of roadway in its part where the biggest convergence occurred. In current practice, in this type of convergence prognosis, the maximal value of dynamic load exerted on roadway resulted from single rockmass tremor is taken into consideration. Quantitative consideration of all tremors including their location is impossible at the stage of convergence prognosis because of lack of this type of data. This paper presents results of roadway convergence prognosis compared to results of underground investigation. Conclusions formulated on that base allow in future similar designing works for obtaining more accurate results.

Jan 1, 2013

By Anthony Spearing

There are approximately 68 million rockbolts installed annually in underground coal mines in the USA (Tadolini and Mazzoni, 2006). The most utilized is the passive fully grouted resin rebar bolt, which constitutes almost 90% of the primary support in US underground coal mines. Resin-grouted rockbolts are a very effective and safe support, but their performance still needs to be further optimized and improved. This paper investigates their performance and suggests operating conditions to optimize the performance of resin-grouted rockbolts. In addition, this paper discusses a small-scale test that can also be used to evaluate the performance of rockbolts that is quicker, easier, and less costly than short encapsulation tests.

Jan 1, 2014

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 Kourosh Shahriar, Kazem Oraee, Ezzeddin Bakhtavar

"In this paper, a relationship between dependant parameters and the crown pillar thickness is first introduced. This relationship defines geotechnical problems caused by thin pillars and economic considerations created by pillars that are thicker than the optimum size. For this purpose, a dimensional analysis as an effective physico-mathematical tool was used. This technique restructures the original dimensional variables of a problem into a set of dimensionless products using the constraints imposed upon them by their dimensions. A model is hence introduced that calculates the optimum pillar thickness. The relationship introduced here and the method applied can be used by mining engineers in all situations where a combined open-pit and block caving method is deemed to be the most appropriate mining method.INTRODUCTIONMany deposits can be mined entirely with the open-pit method; others must be worked underground from the very beginning. In addition, there are the near-surface deposits with considerable vertical extent. Although they are initially exploited by the open pit method, there is often a ""transition depth"" where decision has to be made about changing to underground methods.Some of the biggest open-pit mines worldwide will reach their final pit limits in the next 10 to 15 years. Furthermore, there are many mines planning to change from open-pit to underground mining due to increasing extraction depths and environmental requirements. In this way, it is likely that block and/or panel caving will enable the operations to continue achieving a high production rate at low costs as an underground method (Bakhtavar et al., 2009).In these cases, it is often necessary to consider a crown pillar beneath the transition depth (open-pit floor) before starting an underground caving stope method (Figure 1).There are generally two kinds of crown pillar: a ""surface crown pillar"" and ""crown pillar between open-pit and underground mining."" In general, they both play the similar role in mining. Since the primary purpose of a surface crown pillar is to protect surface land users, the mine, and those working in it from inflows of water, soil, and rock; it is vital that the surface crown pillars remain stable throughout their life. When a crown pillar could potentially remain between open-pit and underground mining it is specially proposed to prevent water entering from the open-pit floor into the stope, as well as to reduce open-pit wall and floor caving."

Jan 1, 2010

By C. J. H. Brest van Kempen

The techniques developed should provide a useful tool, not only during the initial formulation of a suitable bolting plan for a new section, but also for periodic check¬ing on the utilization factor of hardware installed in the roof. Indications are that with present practice, utilization seldom exceeds 507.. Retro-fittable bolter equipment now available (FCBS?) allows doubling of the utilization factor in many cases. We are also introducing a computer pro¬gram based on the material outlined in this paper. This program can save a great deal of time in examining the real effect of implementing alternate bolting plans or using different installation techniques. It allows fine-tuning of bolt lengths, spacing and installation tension to mini¬mize roof support cost, while maximizing safe life of the mine opening. We have developed a procedure which allows easy, site-specific quantification of the need for reinforcement of a mine roof. The concept is based on the forma¬tion of a direct link between theory and empirical data to custom-fit each mine section. In its simplest form, the proce¬dure requires only recording of roof bolt torques in a sample working place (16 to 25 bolts) several times during the first week or so after initial bolting. The way in which the torque pattern changes during this time can reveal whether or not the bolts carry enough tension to maintain stability in that specific roof. From the same data, the nature of the anchorage horizon is derived. Specifically, shale or limestone can be distinguished from sandstone. If the procedure is repeated a few times (within the same mine section) with different bolt installation tensions, we can numerically characterize the roof in terms of one single value: an "effective angle of internal friction". This number can then serve to calculate trade-offs in bolting plan design, such as bolt length, bolt spacing and optimum bolt tension. Additional refinement is possible if a sagmeter is installed in the sample work¬ing place being monitored and if bolt anchorage capacity is determined. The lat¬ter can be done automatically on each bolt as it is being installed, using a system we developed and patented. Alternately, a number of pull tests could be performed. Using the sagmeter, bolt torque and anchor¬age data, it is possible to project the safe life of the supported opening. The details of the procedures described apply specifically to tensioned roof bolting, but a brief discussion paints the way for a similar analysis of full-column resin bolting also.

Jan 1, 1986

By Christopher Mark

After the United States and Australia, South Africa has the largest. modern underground coal mining industry in the world. Historically, South Africa has been the cradle of many innovations in ground control, particularly in the area of pillar design. Today, South African mines are grappling with many of the same issues faced by their U.S. counterparts, including safety during pillar retreat operations, selecting the proper roof support, and maintaining long-term pillar stability. The author recently visited seven underground mines in the Mpumalanga, Free State, and KwaZulu-Natal provinces. Mining methods included longwall, room-and-pillar with continuous miners, and drill-and-blast. The mines represented a cross-section of geology and ground support practices. At each mine evaluations were made of the Coal Mine Roof Rating (CMRR), roof support practices, pillar design methods, and general ground control experience. The coal seams observed were generally thick (2-5 m), the depths of cover moderate (less than 200 m), and the roof rocks relatively strong (CMRR greater than 45). Unfortunately, the South African roof fall fatality rate is approximately three times greater than in U.S., perhaps in part because roof bolts seem to be more widely spaced in most South African mines. Roof bolting equipment was generally outdated, and the quality of bolt installation was a major concern. Pillar design, on the other hand, is quite advanced, and pillar failures are rare. Several novel partial pillaring techniques are employed, including "checkerboard" mining in which every other pillar is removed, 3-cut systems, and high-extraction mining where engineered final stumps are left by design. The paper also offers observations on ground control management techniques (including recently introduced Codes of Practice), the role of the mine inspectorate, and the status of the research community.

Jan 1, 1999

By David Bigby, Tim Ross, David Conover

"Extensive arrays of telltale roof monitoring instruments were recently installed in a Mexican copper mine and a large underground mined storage facility in the eastern U.S. The data were used to evaluate roof stability during development and retreat mining and after installation of supplemental supports. Both manual-reading and automated systems were employed and consisted of up to 64 instruments. This paper will discuss the practical issues involved with installation and monitoring, including reliability and maintenance. Examples will be given for telltale response in relation to known events affecting roof stability, including nearby pillar extraction roof caving, installation of supplemental cable bolts, and separation of the immediate roof layer. The strategy for processing the large quantity of data, presenting the data for review, monitoring the system remotely, and identifying and reporting critical events will be described.INTRODUCTIONTelltales have been used for many years with many varied applications. They are relatively inexpensive, easy to install and interpret, robust, and adaptable. The term ""telltale"" generally denotes a strata extensometer that incorporates a visual indication (and warning) of strata movement. The most common dual-height telltale (Figure 1) was developed by British Coal in the early 1990s as roof bolting was replacing steel arch support (Bigby, 2001). Dual-height telltales provide immediately visible measurements, distinguish between movements above and below the bolted height, and are an established means for providing preemptive warnings of roof falls.Many improvements and modifications on the basic design have been developed to adapt to different mining conditions. For example, triple-height telltales are commonly used where roof bolts of different lengths are installed. The standard manuallyread telltales are limited by the difficulty of obtaining frequent, consistent, and accurate readings, especially in high openings. The instruments may also be located in unstable or inaccessible areas. Consequently, an intrinsically safe electronic telltale system was developed that combines high accuracy and both local and remote reading options. The system supports a network of up to 100 dualheight telltales on each of four separate branches. The instruments are connected with a two-conductor cable and can be read both underground, using a portable readout, and on a surface computer (Figure 2). The automated system still provides the visual warning indication underground."

Jan 1, 2010

By Joseph Hirschi, Naga S. Y. Tirumalaraju, Aditya Mishra, Anthony (Sam) J. S. Spearing

"Primary roof support in underground coal mines in the USA consists mostly of fully grouted passive rebar or grouted (partially or fully) tensioned rebar. Research conducted at two room-and pillar coal mines in the Illinois Basin and one longwall mine in Colorado monitored the performance of these passive and active rock bolts under similar geologic and mining conditions and found no significant initial loads in either passive or active rock bolts (Spearing et al., 2011). Active rock bolts showed significant loss of initial pre-load, although the exact reasons for this were not fully investigated. Subsequent laboratory testing has also shown that in weak rock, resin creep occurs, caused by slippage of the mechanical key between resin column and rock interface. These creep tests were studied in the laboratory in an effort to quantifythis phenomenon.INTRODUCTIONResin-grouted rock bolts are the first and main line of defense to counter roof and rib falls in underground coal mines of the United States. Rock bolts are categorized as active or passive. Active bolts are tensioned during installation whilst no (or limited) tension is applied to passive bolts, which need subsequent strata movement to generate reactive loads. During installation of passive bolts, some reactive pre-load may be generated by a significant up-thrust from the rock bolter boom.The selection of active or passive bolts is critical and is mostly dependent on the conditions and properties of the immediate roof (Unrug, 2009). This decision is frequently made based on personal experience, bias, and most recently cost. Active bolts are generally used in cases of highly laminated roof rock. When active bolts are tensioned, they develop compressive forces between these layers forming a beam, which helps to stabilize weak individual layers thereby increasing frictional forces between layers and decreasing effects of horizontal stresses. The fully grouted tensioned rebar system is high in stiffness as compared to mechanical bolts and provides higher surface area of contact along the length of the bolt due to the grout, which permits it to develop a strong anchorage mechanism."

Jan 1, 2015