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By Benjamin Mirabile

"Current and emerging practices to ventilate longwall gobs are prompting increased design and analysis of standing support requirements related to longwall mining. Specifically, additional design methodologies are being employed to evaluate standing supports in the #2 entry of a 3-entry longwall system.Numerical modeling will be used to evaluate the performance of standing support in the #2 entry between gobs of longwalls in the Herrin No. 6 seam. The performance of standing support in the #2 entry will be evaluated based on the following: (1) The effectiveness of standing support in maintaining an open airway in the #2 entry; (2) The interaction of standing support in the #2 entry with standing support in the longwall tailgate; and (3) Loading conditions of longwall pillars. Initial numerical modeling activities are focused on evaluating the roof to floor convergence in the #2 Entry and using the ground reaction curve concept for support design.Numerical modeling indicates that, like standing support in tailgates, supports in the #2 entry require some type of yield mechanism to withstand initial uncontrolled convergence. Numerical modeling also suggests that support density can be lower than that used in a tailgate support application. The numerical models used for initial investigation of support performance in the #2 entry are limited in their ability to evaluate support performance. The magnitude of uncontrollable convergence can be estimated with LaModel software. Near-seam conditions have not been investigated in this research since the LaModel software does not have the ability to evaluate inelastic rock behavior.IntroductionDuring the past three decades, many researchers have investigated standing support performance in longwall tailgate entries. Very little attention was paid to the middle entry of a threeentry longwall gateroad. As longwalls retreated, no personnel would travel the #2 entry, and the entry was considered part of the longwall gob. In response to recent mine disasters, regulatory agencies and operators have devoted more attention to these entries. Prior to the Upper Big Branch mine disaster, little or no secondary support was applied in these entries. Unlike secondary support in longwall tailgates, there is little empirical knowledge or quantified research specifically applied to secondary support in the #2 entry. This paper outlines initial steps used to develop a design methodology for secondary support in the #2 entry."

Jan 1, 2015

By Guorui Feng, Min Zhang, Yi Luo, Jian Yang, Yujiang Zhang, Junwen Bai

"Safe mining of coal seam sandwiched between abandoned upper and lower room-and-pillared mines (ULPM) ·is increasingly becoming the focus of attention in recent years, which is closely related to the damage intensity of the upper and lower interburdens. The #7 seam with ULPM in Baijiazhuang coal mine was selected for this research. According to the mining and geological conditions, the vertical stress and plastic zone distributions were determined first by numerical simulation. Then, a theoretical model based on the elasti-plasticity theory was developed. The stress distribution and failure zone created in the interburden by mining the upper and lower seams were determined. Finally the feasibility of mining #7 seam between the abandoned upper and a lower seam was evaluated. The results show that: (1) the distributions of abutment pressure and plastic zone coincide, both of which can be used to evaluate the failure zone in ULPM. The failure zone penetrates the whole upper interburden, while only parts of the lower interburden fail. (2) the theoretical failure zone coincides with the maximum distribution zone of abutment pressure and plastic zone obtained by numerical simulation: the failure zone height in the lower interburden is less than its thickness, while that in the upper interburden is larger than its thickness. (3) the #7 seam is located in the failure zone created by the upper seam room-and-pillar mining, which leads to well-development fractures and blocky fragments in the roof, thereby it is unstabled. (4) when mining the #7 seam in the interburden, the key is to control the roof strata and proper support measures should be taken timely to ensure safe mining.INTRODUCTIONMany coal mines in China mine the thick, superior and easily mineable resources preferentially, while the thin, inferior and difficult-to-mine resources were abandoned (Chappells and Trentrnann, 2015; Zhenfu, 2012; Xu, et al., 2015; Guorui, 2009). As the results of deficient geological data and shortage of investment capital, the residual coal resources distributes in many mining areas in China.In recent years, many residual coal seams sandwiched between the abandoned upper and lower room-and-pillar mines (ULPM) were explored in Shanxi Province, especially in the mining area of Datang, Xishan and Luan (Figure.1). The reserves of residual coal seams in ULPM are considerable, increasingly becomes the focus of attention among scholars and engineers."

Jan 1, 2016

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 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

By Don Berger

Highwall scaling, bolting and technical blasting are important aspects in the remediation of existing highwall exposures. Proper planning, design and research are important first steps in the development of long term stability. Research into old workings and their influence on highwall stability control the planning and design. These steps are followed by improvements in blasting techniques, highwall monitoring programs and constant evaluation of the results. Recently, due to a number of factors, the highwalls of the Open Cut Mine have started to deteriorate, creating both operational and potential safety problems. Geologic setting, highwall design, mining operations, old workings and weather have combined to create adverse conditions that have allowed sections of the highwalls to become unstable. Homestake Mining Company has developed a program to alleviate the existing ground control problems and to decrease the effect operations has on future highwall stability.

Jan 1, 1996

By William R. Kohl, Craig B. Clemmens

"Since 2010, North Central Resources, Bridgeport, WV, has been using the sonic log to estimate the rockmechanics properties of the coal and adjacent strata in their exploration drilling program. In 68 out of the 85 boreholes they have drilled since 2010, they have conducted shear wave analysis on sonic logs run in these boreholes. By incorporating the rock mechanics logs into the geological model of the coal seam, the engineers can develop a better understanding of the support requirements for the mine.WellCAD Borehole processing software provides vastly improved efficiency in analyzing shear waves from borehole sonic data than previously available methods. The calculation of most rock mechanics parameters requires knowledge of the density and both the compressional wave velocity and the shear wave velocity. Density logs are well known and an industry standard. Sonic logging is equally well known, although less frequently used, as an accurate source of compressional wave velocity. Shear wave velocity is difficult to measure in a geophysical log, but can be estimated from digitally recorded wave form records. This is a time consuming manual procedure which requires the visual examination of thousands of individual wave forms.WellCAD allows the viewing all the wave forms on a VDL display. The observer can create a pair of false logs by manually tracing the interference points on the display for each receiver. He can check himself by observing wave forms in the traditional format. He can then create a shear wave transit time log or “Delta-S” log by subtracting the false log from the near receiver from that of the far receiver. This can be plugged into standard formulae to generate curves for most standard rock mechanic parameters, including bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio.While these parameters do not entirely substitute for destructive testing, they are extremely cost effective. An entire borehole can be analyzed for the cost of a single destructive test. This allows for a more judicious use of destructive testing while permitting the extension of test data into the rock mass with much greater confidence."

Jan 1, 2015

By Raj R. Kallu

Underground gold mines in Nevada are typically hosted in geologic zones with a wide range of geotechnical properties ranging from blocky competent rock to faulted and highly altered soil-like material. Geotechnical engineering design in weak rock utilizes rock mass classification systems as inputs to both empirical and numerical methods. The most common rock mass classification system utilized in the gold mines of Nevada is the easy-to-use Rock Mass Rating (RMR) system; however, in practice it is difficult to apply in weak to very weak rock masses. This paper attempts to improve the sensitivity of RMR in weak rock masses by utilizing W-RMR?a modified procedure for calculating RMR?and evaluates its performance against other methods of determining RMR for predicting in situ moduli. In-situ rock mass moduli were determined using a portable plate loading device (PPLD), and these data were used to quantify the improved sensitivity of the modified RMR versus existing classification systems. The PPLD was designed for performing rapid and economical in-situ deformability tests in a mining environment. Tests were performed at eight sites yielding thirteen rock mass moduli measurements. Statistical analyses indicate Detailed-Discontinuity RMR-89 and W-RMR provide the best correlations to the in-situ moduli compared to other methods of calculating RMR.

Jan 1, 2014

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 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 Yoginder Paul Chugh, Jason E. Tinsley

"Over the last five years, the authors have compiled a database of Short Encapsulation Pull Test (SEPT) evaluations from Interior Basin (Basin) coal mines. Typically, SEPT data are analyzed to obtain peak load carrying capacity per unit length of resin encapsulation designated as the ""Grip Factor,"" or GF. In a previous paper, ""An Analysis of Short Encapsulation Bolt Pull Test (SEPT) Data From Interior Basin Coal Mines,"" authors analyzed tangent value of the load-displacement curve as Anchorage Stiffness at 50% (AS-50) of the peak load, or GF value, to assess reinforcement potential of an installed bolt system. This paper attempts to correlate the secant AS VALUES (of SEPT on 6 ft (1.8 m) long installed bolts at a lower load of about four tons from several different mines in the Basin with their MSHA-approved primary support requirements. Since the current regulatory environment requires plan changes dynamically to ensure that roof falls are minimized, the authors hypothesize that the practiced roof control plan must meet the minimum requirement of adequately supporting immediate roof strata. The analyses are based on 181 SEPT studies performed by Minova professionals. Analyses indicate a macrolevel correlation for both roof support density and roof support cost that may have the potential to be used by mine operators to compare their performance with other mines in the area. Limitations of the analyses include ignoring mining depth, mining height and required secondary supports as part of the requirements for a roof control plan. The authors recommend additional analyses to develop this concept further to develop guidelines that can be confidently used by coal companies.INTRODUCTIONPartially- and fully-grouted bolts are extensively used in coal mines for roof support as part of MSHA-approved roof control plans. Since grouted bolts have a larger area of contact with the rock, they can develop much greater anchorage capacity than conventional expansion shell (point-anchored) bolts. Load transfer characteristics and interaction mechanics between steel rebar, grout and rock mass around the bolt hole for grouted bolts are generally assessed in the laboratory or field though short-encapsulation pull tests (SEPT), which have been extensively researched by Aziz and Jalalifar (2005), Aziz (2004a), Serbousek and Signer (1987), Signer (1990), and Mark et al. (2002), among others. In a SEPT, about 12 in. (30 cm) of bolt length is typically encapsulated in resin and a pull test is performed to collect load-displacement data prior to failure and, in some cases, post-failure. A typical load-displacement data set for a bolt is shown in Figure 1. SEPT characteristics are designated by peak load carrying capacity, or grip factor, which is the peak load carrying capacity per unit length of resin encapsulation. SEPT is routinely used to assess anchorage performance characteristics of different lithologies overlying a coal seam to identify type of bolt, bolt diameter, length and spacing across and along an excavation. Anchorage stiffness, or AS, at any load may also be calculated as the tangent or secant value of the slope of the load displacement curve at a particular load level. Anchorage Stiffness characteristics of bolts from SEPT data have typically not been considered in any depth until recently (Chugh et al., 2014; Chugh et al., 2015)."

Jan 1, 2016