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By William B. Beerbower

When full length resin grouted bolts were introduced into U. S. coal mines some 11 years ago, it was assumed that safety would be the primary benefit. There is ample testimony to substantiate the safety factor as an important reason for using resin roof bolts. More recently, it has been determined that many coal operators not only concur with the safety consideration but also benefit economically from the safer working conditions resulting from a reduction in roof falls. IMPROVED SAFETY Some typical roof fall reduction experiences with resin bolting follow: ? Western Kentucky Mine - Resin bolting reduced roof falls by 80%. ? Southern West Virginia Mine - Experienced 141 roof falls between 1971 and 1972. In 1973 with resin bolting, roof falls were reduced 85%. ? Western Pennsylvania Mine - In 1973 had 59 roof falls with mechanical bolts. During the same period of 1974 with resin, only 19 falls were experienced - a reduction of 66%. The overall experience with resin bolted roof support has been an average 50% reduction in roof falls. Long term, this can translate into improved worker morale. As a matter of fact, a general mine foreman at the same Southern West Virginia mine mentioned above commented that improved miner morale was noticeable following the introduction of resin bolting. HOW GREATER SAFETY CAN BE COMBINED WITH INCREASED PRODUCTIVITY IN COAL MINING OPERATIONS Experience has also shown that a reduction of roof falls with resin bolting has resulted in increased production for many coal operators. Some typical examples with mines using resin bolting follow: ? Western Kentucky Mine - Resin bolting contributed to increasing production from 7000 to 11,000 tons per day, due to a 80% reduction in roof falls. This was accomplished in part by a reduction of 50% in timbering requirements and the reassignment of three roof fall cleanup crews to coal production. ? Pennsylvania Mine - Following resin bolting introduction the section showed a 50% increase in production. ? Central West Virginia Mine - Continuous miner production averaged 6 cuts per unit shift prior to resin bolting. Reduction of cleanup time, resulting from improved roof support with resin bolting, increased production to an average of 9 cuts per unit shift. PROPERTIES OF RESIN BOLTING SYSTEMS Several of the important properties of resin bolting systems which contribute to its improved performance are listed below: ? Full length anchored resin bolts and many resin anchored, point anchored bolts provide effective resistance to changing rock strata stresses throughout their length, thereby improving stabilization of rock strata over long periods of time. ? Individual layers of rock are anchored into a single high strength beam providing excellent resistance to vertical and lateral strata movement, the most common cause of roof failure. ? Moisture cannot corrode the fully grouted bolt. ? Has twice the pull strength of mechanical bolts (See Figure 1)

Jan 1, 1982

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 Morgan M. Sears, Gamal Rashed, Jim Winfield, Brent Slaker

"Over the past decade, ground falls have resulted in five fatalities, representing over 40% of all fatalities occurring in underground stone mines in the United States. In conjunction with recent high-profile pillar collapses, the need for further ground control research in the nation’s stone mines is evident. Ground conditions are growing more difficult. Therefore, this research is focused on deep cover, steeply dipping, and multi-level underground nonmetal mines. The objective of this paper is to introduce a geomechanical study of pillar development in a steeply dipping limestone deposit. To accomplish this, a numerical model was developed using input from modified laboratory test data and in situ stress measurements provided by the mine to simulate the development of a pillar under these conditions. The model was then verified against field observations and stress mapping. The calibrated model was then used to calculate the estimated stresses in the pillar and immediate roof as development progresses around an instrumented pillar.INTRODUCTIONGround falls represent a significant hazard in the nation’s underground stone mines. In response to recent large-scale pillar collapses, as well as stakeholder interactions, researchers from the National Institute for Occupational Safety and Health (NIOSH) are currently conducting detailed investigations into the complex loading conditions at mines operating in challenging operational environments. These include deep-cover, steeply dipping, and multi-level mines where there is the potential for an increased risk of ground failure. The Pleasant Gap Mine, shown in Figure 1, operates in the Valentine Formation, which is typically light grey, extremely fine-grained limestone, and approximately 21.3 m (70 ft.) thick. The Centre Hall Formation makes up the immediate and main roof members and is approximately 9.1 m (30 ft.) thick, consisting of medium-dark-gray limestone (Murphy et al., 2018). Situated in a syncline, the top of the Valentine Formation dips at approximately 19° with a strike of approximately N54E in the area of interest (see Figure 2)."

Jan 1, 2018

By Peter Zhang, Todd Minoski, Mark Van Dyke, Daniel W. H. Su

"This paper presents the results of a 2017 study conducted by the National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Mining Research Division (PMRD), to evaluate the effects of longwall-induced subsurface deformations within a longwall abutment pillar under deep cover. The 2017 study was conducted in a southwestern Pennsylvania coal mine, which extracts 1,500-foot-wide longwall panels under 1,185 feet of cover. One 650-foot-deep, in-place inclinometer monitoring well was drilled and installed over a 150-foot by 275-foot center abutment pillar. In addition to the monitoring well, surface subsidence measurements and underground coal pillar pressure measurements were conducted as the 1,500-foot-wide longwall panel on the south side of the abutment pillar was being mined.Prior to the first longwall excavation, a number of simulations using FLAC3D™ (Itasca, 2017) were conducted to estimate surface subsidence, increases in underground coal pillar pressure, and subsurface horizontal displacements in the monitoring well. Comparisons of the pre-mining FLAC3D simulation results and the surface, subsurface, and underground instrumentation results show that the measured in-place inclinometer casing deformations are in reasonable agreement with those predicted by the 3D finite difference models. The measured surface subsidence and pillar pressure are in excellent agreement with those predicted by the 3D models. Results from this 2017 research clearly indicate that, under deep cover, the measured horizontal displacements within the abutment pillar are approximately one order of magnitude smaller than those measured in a 2014 study under medium cover.INTRODUCTIONDue to a recent shale gas boom, approximately 1,400 unconventional shale gas wells have been drilled through active and future coal reserves in Pennsylvania, West Virginia, and Ohio over the past 15 years. These shale gas wells have penetrated many coal seams, such as the Sewickley, Pittsburgh, Upper Freeport, and Kittanning seams. These unconventional gas wells, whether tapped into the Marcellus or Utica formations, contain very high gas pressure. Strata deformations associated with longwall and other high extraction techniques could induce high stresses and deformations in the shale gas well casings. This could seriously compromise the mechanical integrity of the production, intermediate, and coal protection casings (Figure 1). Damaged well casings could potentially introduce high-pressure, explosive gas into underground mine workings, particularly during an extended well shut-in period, which could seriously jeopardize underground miners’ safety and health."

Jan 1, 2018

By Kevin Andrews, Scott Nelson, Terry Hamrick

"In the process of Geologic Hazard Mapping (GHM) for underground coal mining, geologists evaluate the effects of various geologic and geotechnical conditions to assist with optimization of mine productivity and with selection of more favorable areas for future development. The methodology involves correlation of multiple geologic factors (such as overburden depth, proximity to sandstone channels, and areas with weak immediate roof strata) with conditions encountered during mining (roof falls, floor heave, pillar spalling, etc.). Such studies yield maps that identify areas where mining is likely to be difficult and allow mining engineers to foresee favorable and unfavorable mining areas when planning new or expanding mines. To expand the benefits of GHM, work has been done to correlate geologic factors with room-and-pillar mining advance rates. Average mining advance rates are mapped and then correlated to different areas, as characterized by GHM. This process exposes the geological characteristics that have a material impact on mining rates of advance, improving the rates of advance into future mining areas with similar geological conditions. The final result of the work allows mine planners to not only design the mine qualitatively accounting for areas of unfavorable conditions, but also allows the mine planner to more accurately apply rates of advance for mine timing and production sequencing. The use of GHM to predict mining advance rates in unmined areas enhances the benefits of traditional hazard mapping and provides quantitative data that can be incorporated into mine sequencing models.INTRODUCTIONThe role of a geologist in the coal mining industry has evolved significantly over the last 30 to 40 years. Geologic Hazard Mapping (GHM) mapping for coal mines began in the late 1970s and early 1980s (Chase, Newman, and Rusnak, 2006). Since its beginnings, the GHM process has evolved significantly and continues to take on new levels of sophistication. Numerous case studies of GHM application to predict future mining conditions exist in the literature. For example, Artrip, Nelson, and Newman (1993), documents the geotechnical characterization of roof and floor for mining in the Imboden Seam in Virginia where previous mining efforts had been abandoned due to adverse mining conditions. In addition, Keim and Miller (1999) summarize evaluation of geological influences on a longwall mine in West Virginia. In both examples, significant geotechnical and geological information was available, including the opportunity to compare predicted mining conditions against actual conditions as mining progressed."

Jan 1, 2018

By Jamal Rostami, Asok Ray, Samer S. Saab Jr., Wenpeng Liu

Ground instability, such as roof or rib failures, is one of the most serious and frequent safety hazards that occur in underground mining, tunneling, and underground construction. The ground-related parameters and features of interest, in assessing roof or rib failures, are location, frequency, and orientation of the joints, voids, as well as rock-strength. These parameters are the main components of rock mass characterization that offers effective strategies for ground support, which, in turn, allow one to mitigate the risks related to ground instability. The main objective of this study is to determine the location of joints and classify rock-strengths by analyzing the drilling data recorded from an instrumented roof bolter while drilling for rock bolt installation during the operational cycle. For this purpose, computer programs based on updated pattern recognition algorithms were developed for joint-detection and classification of rock types to offer an estimated strength. This paper briefly introduces ongoing research on joint detection, especially for joints with an aperture less than 0.125in (3.175mm), by employing an instrumented roof bolter in controlled environments. To improve the capability and precision of joint detection programs in detecting the joints from the drilling data and reducing the number of false alarms, many laboratory tests with various simulated joints and rock-strengths were carried out at a J.H. Fletcher & Co. facility. This paper reviews testing procedures, data analysis, updated algorithms used for joint detection, and discusses the latest round of testing in samples with simulated joints at various angles along the borehole.

Jan 1, 2017

By Stankusm John C., Xiaoting Li

"Quantitatively identifying the load on a steel king beam set is a complicated issue in designing the king beam structure at the slope bottom area. From an engineering perspective, understanding the nature and support mechanism of the king beam structure as a standing support is critical in identifying the calculation input parameters for the structure design software. Based on design practices in recent years and successful applications in several slope bottom intersections, the authors present a king beam design methodology, including qualitative analysis, quantitative calculation with a structural design software, and computer Finite Element Analysis (FEA) verification.Especially important is the concept of interaction and the confinement effect between the king beam structure and surrounding rocks, which is essential in designing a proper steel structure in terms of maintaining the roof stability, as well as saving the steel materials and costs of manufacture, labor, and delivery. This paper details the design methodology and applications in several underground slope bottom intersection designs, reaching the goals of improving roof stability and reducing costs.INTRODUCTION AND DESIGN METHODOLOGYCurrently, the load on a square or arch set is typically determined by either past empirical design experiences or known heights of roof falls in current or adjacent mines. These methods work but sometimes are too conservative from an engineering perspective. For example, when designing a king beam structure for an intersection, these methods may lead to a design with extremely large and heavy steel beams, which are not convenient for delivery and installation on-site, not to mention, costly. Therefore, accurately identifying the potential maximum load on the king beam structure is important for designing a safet, effective, and cost-efficient steel structure. Based on past several years’ practices and fundamentals of ground control and structural design engineering, the authors propose a design methodology including the following components:The nature and mechanism of effective roof support (qualitative analysis),Ground reaction curve,Interaction and confinement effect between bolts and steel standing support and surrounding rocks,Factors affecting the load conditions of the king beam set"

Jan 1, 2018

By Shuren Wang, Wenbing Guo, Paul C. Hagan, Yanhai Zhao

"The overlying strata is often destroyed in large scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. The typical shallow coal mining in Shendong Mining Area in China was used as engineering background; the load distribution characteristics of the overlying strata were analyzed based on the monitoring data of the support resistance. Through theoretical analysis and numerical simulation, the formation and changing process of the pressure-arch in the overlying strata were studied, and the instability mechanism of the overlying strata was revealed by focusing on the accumulation and release law of the elastic energy in the pressure-arch. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The elastic energy was accumulated in the pressure-arch, and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining, and the shear zone could be formed. Therefore, the sliding of the caved zone along the shear zone induced strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct similar engineering practice.INTRODUCTIONThe instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju and Xu, 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al., 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mine and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch."

Jan 1, 2018

By Wenbing Guo

Surface subsidence for fully mechanized thick seam top coal caving was measured, and surface subsidence characteristics were then analyzed. The subsidence initiation distance from the set-up room and the advance influence distance ahead of the advancing longwall face were studied. The maximum surface subsidence velocity and the delay angle of the maximum subsidence velocity were determined. The results indicated that surface movement was intensive, the surface subsidence velocity was very high, the ground surface was badly damaged, and the subsidence initiation distance from the set-up room was smaller than that in general geological and mining condition.

Jan 1, 2011

By Behdeen Oraee-Mirzamani

Since the advent of New Austrian Tunneling Method (NATM), shotcrete as a primary means of support in tunnels has been widely applied. It?s most important features are durability, speed of application and cost effectiveness. This paper introduces some tables that provide guidelines for the thickness of shotcrete required in some common situations of mine roadways. In order to devise such tables, two different arch sections, together with three different overburden types, were considered. Geotechnical parameters such as apparent cohesion and angle of internal friction of surrounding rocks were chosen, based on the five-category classification of Bieniawski. Two K0 factors (the ratio of horizontal stress to vertical stress) and an average rock density were utilized. Using numerical methods, 60 models were then devised in this way. By applying interaction diagrams of axial force and the bending moment for different thicknesses of shotcrete, appropriate shotcrete thickness for these models were calculated. The results of this research, as well as the methodology applied, can be used in mining roadway support design and all types of civil engineering tunnels.

Jan 1, 2011

By Brian E. Travis

The recent development of polymer grids for the underground coal mining industry has created new alternatives for supplemental ground control practices. The grids utilize strong, lightweight polymers - usually special grades of polypropylene. High tensile strengths and resulting load support characteristics are achieved by molecular orientation of these polymers in the manufacturing process. One of the most important applications of polymer grids as supplemental ground control is in longwall shield recovery. When shields are moved from one face to another, the determining factor in the success of the recovery is the ground control provided by roof support structures along the old face. Whereas primary support is usually provided by roof bolts and cables which run the full width of the panel, supplemental support is often provided by metallic meshes of welded wire or chain-link fence. lightweight, high-strength polymer grids replace these heavy, cumbersome metallic meshes, giving the operation increased productivity by decreasing installation time and reducing injury downtime.

Jan 1, 1990

By Michael Gauna

Ground fall injuries and fatalities in United States underground coal mines have shown a diminishing trend. Advances in bolting and standing support systems have led to improved safety in underground coal mines. However, the Crandall Canyon Mine disaster in 2007, in which 9 individuals were fatally injured as a result of a widespread collapse, emphasized that proper mine design is a critical issue that must be addressed. The Mine Safety and Health Administration (MSHA) has instituted technical review procedures structured to facilitate the implementation of safe mine designs in underground coal mines. The technical review process and the typical information required are outlined in this paper. The range of mining situations that lead to a technical review, common design methods submitted for review, and their relevant guidelines and limitations are also discussed. Additionally, the relationship between design criteria and mine site historical experience is discussed.

Jan 1, 2011

By E. Hosca

Alternative tailgate loading conditions due to conventional and conventional/yield pillar combinations were evaluated to define roof loading. In order to isolate different loading situations, the tailgate entry was considered as part of the roof-pillar-floor system, which is controlled largely by the structural integrity of each member. A data bank of case studies was compiled, which contains details of roof and floor lithology, structure, support, and mining data, including any failure modes. The range of loading conditions that any tailgate can be subjected to was identified to form the basis of numerical modeling. Numerical modeling was then conducted on tailgate openings to determine roof, floor, and rib responses, including failure modes to a variety of loading conditions. Trends developed from the numerical modeling were modified using field data to form the basis for support evaluation and application. The effects of support were modeled numerically to determine optimum load capacity and points of application for reducing the potential for entry failure.

Jan 1, 1994

By T. J. McMahon

This Bureau of Mines study describes the development of a large-scale, three-dimensional, finite-element model of a deep-vein Coeur d?Alene District mine. The three-dimensional model was prepared from combined digitized mine maps and generated levels that may also be analyzed as independent two-dimensional models. A comparison of the stress changes and displacements was made following a simulated vein excavation in both the three-dimensional and two-dimensional models. A one-cut excavation was simulated with the two-dimensional model and a multiple-cut excavation was simulated with the three-dimensional model. The compared results from the two analyses are discussed in terms of the inherent differences between the models.

Jan 1, 1989

By Kazem Oraee, Mehran Gholincjad, Navid Hosseini

"Chain coal pillars are pans of the structure of longwall mining system that play a significant role in the stability of the entries. With mechanization and developments in the various aspects of the method, higher efficiency in optimization of the design of chain coal pillars seems appropriate. In this paper the three main methods of chain pillar design, namely the empirical analytical and numerical methods arc compared. Real data from the Tabaslran coal mines have been used in order to make the comparison process reliable. It is concluded that the most apparent advantage of the empirical method is the reliability of the results while the use of numerical methods enjoys the advantage of flexibility. On the other hand the analytical methods are complex unless simplifying assumptions arc made that can substantially decrease the accuracy of result which is thought to be the main advantage of the design method. A new method is therefore introduced here that combines all of the three presently used methods and by doing so the new method has the advantages of the three while minimizing complexities and inaccuracies associated with the use of the individual methods.INTRODUCTIONLongwall mining is a high production method that can use mechanization to its full extent. Its initial development can be traced to 17th century European collieries but it was not until the second half of the 20th century with the development of self advancing support systems that the method gained popularity in the USA (Oraec. 2001). During the last decade, the production of longwall faces has increased significantly (Peng. 2006). The ability for mechanization along with good safety condition is the main reasons for such development. In the longwall mining method a good design for panel and entries on both sides, are two essential parameters. In this method, the thickness of overburden (depth of mine) nom1ally ranges from 60 to 20 m (197 to 2.7 9 ft) (Hartman, 2000) and with increasing mine depth designing for stabilizing the entries on both sides of the panel become more difficult. Thus, for safety and stability the size of chain pillar must increase. This causes the overall coal recovery to decrease and increase mining costs per ton. As such wide pillars hinder the air flow and hence ventilation costs increases too. Therefore with accurate identification of loading conditions and stress analysis. pillar that are a minimum size and sufficient safety factor are carefully designed."

Jan 1, 2010

By Fernando O. L. Xavier

"The Barro Branco coal mine was located in the southern region of Brazil, in the state of Santa Catarina. In 2002, an incident occurred at the mine, leading to a violent, massive pillar failure, resulting in almost 100 failed pillars in less than 3 hours, totaling nearly 700 collapsed pillars after a period of 6 weeks. The paper outlines the geological and operational conditions before the accident and analyzes the primary factors leading up to the event. This paper also discusses and examines the unusual characteristic of violent, uncontrolled pillar bursts that occur over a longer timespan, in comparison to other equally violent but rapid events, such as those that occurred at the Crandall Canyon Mine, at a Book Cliffs District mine, and at the Solvay Mine. INTRODUCTION The southern region of Brazil holds 100% of the coal reserves for the country, and half of it is located within the state of Santa Catarina. Almost the entire Brazil’s coal production, around 8 million tonnes/year, is consumed for electricity generation. Most of the coal mines in Santa Catarina are underground room-and-pillar mines, and most are shallow mines, with depths of cover ranging from 20 to 400 m. The Barro Branco mine, near the city of Criciuma, experienced a violent mine collapse at the end of March 2003, which was analyzed and discussed by Zingano, Koppe, and Costa (2004). The event consisted of violent failure of 100 pillars in less than 3 hours, and a subsequent failure of approximately 700 additional pillars over the next 6 weeks. After the panel collapsed, the mine was closed due to unsafe conditions as a consequence of the seismic event. A map of the collapsed area is shown in Figure 1. GEOLOGY AND MINING METHOD There are two coal seams mined at Barro Branco mine. The first is the 1.5 m to 2.3 m thick Barro Branco Seam, under about 75 m of cover. The second is the Bonito Seam, where the collapse occurred. The Bonito Seam is 2.5 to 5.5 m thick located at a depth of approximately 80 m, with an average dip of 0° to 15°. Various major faults and inherent joints are present in the area."

Jan 1, 2017

By Liu Xiao, Lin Xiaoying, Ma Geng, Tao Yunqi

"The soft coal in China has several issues, low permeability, high gas content and prone to gas outburst, making it difficult to drill and improve its permeability. Using the rock mass loading test, the relationship of stress-strain-permeability-time of sandstone was investigated. The fracture network that communicates with coal seams to provide the pathways for gas migration can be established by drilling, hydraulic fracturing or loose blasting in the roof and floor about 5m from coal seams top and bottom. The gas migration can be summed up as ""desorption-diffusion-two levels of seepage"" for gas drainage of the quasi-protection-layer in hard coal seams, and ""desorption-two levels of diffusion-seepage"" in soft coal seams. In the quasi-protection-layer, different gas drainage techniques were used during different stages of mining. Gas drainage of the quasi-protection-layers was demonstrated at Hebi Zhongtai Mining Co., Ltd. The results showed that the technique promotes the gas seepage flow, and improves the efficiency of drainage. The influence zone of the quasi-protection-layer was 1601 m2 and the average amount of gas drainage was 328m3/d, with the maximum being 661m3/d. INTRODUCTIONGas is one of the main factors that cause disasters and influence safety production in underground coal mines (SACMS, 2009). The coal-bearing formations in China have been subjected to multiphase tectonic stress field during the period of geological evolution. Coal body was often crushed into fragmented coals and mylonitic coals under the tectonic stresses (Su and Lin, 2009). The gas permeability of coal is generally poor. So in underground, drilling and hydro-fracturing technology have been used to improve the permeability of coal, such as intensive drilling, hydraulic punching, hydraulic cutting, hydraulic extrusion and deep hole blasting. But the gas drainage rate was low, only 23% (Liu, Li, and Li, 2006; Li and Wei, 2006; Henan Polytechnic University, 2008; Ma, et al., 2014; Lin and Wu, 2009; Zhou, et al., 2011) that was much lower than the rates achieved in the main coal-producing countries such as America and Australia whose average rate of gas drainage was 50%. The protection layer mining technology has been used widely as an effective technique to improve the permeability of coal in the mining of multiple coal seams (Cheng and Yu, 2003; Diaz and Gonzalez, 2007; Karacan, et al., 2007; Palchik, 2003; Latham, et al., 2013; Yuan, et al., 2013; Shi and Liu, 2007; Wang, et al., 2008, Guo, et al., 2015; Lu, et al., 2015; Slazak, Obracaj, and Swolkien, 2014 ). In the single low permeability coal seam, there is no protection layer. This kind of coal seam has low permeability, high gas content, soft coal body, and is prone to serious gas outbursts. During drilling, the holes very often spray, collapse, stuck and wedged the drilling tool. The hole length drilled is difficult to reach the required minimum, the service life of holes for gas drainage is short, the construction cost is high, and the holes are difficult to maintain. The gas drainage efficiency of single low permeability coal seam, especially soft coal seams, cannot be improved even though hydro-fracture technology has been implemented, because its permeability is not improved."

Jan 1, 2016

By R. Kaund

"Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence, preventative measures can be taken to avoid severe hazards to mine safety.INTRODUCTIONWith the increasing demand of mineral resources and depletion of near-surface ores, the depths of underground mines have made the mining activities one of mankind’s most dangerous types of work (Ortlepp, 2005). The Mine Safety and Health Administration (MSHA) defines rockburst as “a sudden and violent failure of overstressed rock resulting in the instantaneous release of large amounts of accumulated energy.”The types of rockburst can be classified into three types: (1) strain burst, (2) pillar burst, and (3) fault slip (Müller, 1991). Strain burst is the most common type of unstable rock failure in underground openings, where the intensity and scale are usually smaller than pillar burst and fault slip (White and Whyatt, 1999; White, Williams and Whyatt, 2002; Ortlepp, 2005). The cause of strain burst can range from shattering of rock under high stress concentration to buckling of discontinuities parallel to underground openings. The occurrence of pillar burst also depends on the stress and discontinuity conditions in the rockmass, but pillar burst usually involves the sudden loss of strength in the core or foundation of a pillar (Whyatt and Blake, 2002; Kias and Ozbay, 2013; Khademian et al., 2016). Fault slip or slip burst can be either slip along pre-existing discontinuities or shear failure in the rockmass, when the shear stress is larger than shear strength in the fault (Rice, 1983; White and Whyatt, 1999; Sainoki and Mitri, 2014). The consequence of fault slip can vary from very small rockmass damage to consequential rockmass damage, but large seismic magnitude does not always result in large rockmass damage (White and Whyatt, 1999; Whyatt and Blake, 2002)."

Jan 1, 2018

By Yvette Heritage

"The risk of fatalities from rib failure is still prevalent in the coal mining industry. This risk has prompted further research to be conducted on rib deformation in order to understand the mechanisms of rib failure, with the long-term objective being to improve rib support design. This paper presents the results of ACARP research project C25057, which investigated the mechanics and drivers of rib failure. The results of rib deformation monitoring at three different mines in Australia provides rib deformation characteristics for overburden depths ranging from 160 m to 530 m. Monitoring includes deformation during development drivage conditions and during the longwall retreat abutment stress environment. The rib deformation monitoring covered three different seams: the Goonyella Middle Seam, Ulan Seam, and Bulli Seam in the Bowen Basin, Western Coalfield, and Southern Coalfield, respectively. The observed mechanisms driving the rib deformation ranged from bedding shear failure along weak claystone bands to vertical shear fractures to kinematic failures driven by shear failure dilation. The variation in mechanisms of rib failure, together with the seemingly consistent method of rib support design, prompts the question: What exactly is the role of rib support?INTRODUCTIONThe current risk of rib fatalities in the coal mining industry has prompted further research into understanding the mechanisms of rib failure. Three Australian mines assisted further research by providing sites for rib deformation studies as part of the Australian Coal Association Research Program (ACARP) project C25057. The mines included in this study were Moranbah North Mine, Ulan West Mine, and Appin Mine. This paper provides key results of the rib deformation monitoring relevant to assessing the mechanisms of rib deformation.The approach of monitoring rib deformation and rib support loads was used to measure and characterise the dynamic rib deformation present in both development and longwall retreat stress environments. The characterisation of rib deformation, together with rock failure modelling, was used to determine the mechanisms for failure within the rib and to assess the interaction of rib support with the failure process. The mechanisms of rock failure are considered an important factor in understanding the drivers for the failure. Consideration of the site-specific failure mechanisms can provide for a more tailored support design."

Jan 1, 2018

By Ross W. Seedsman

The feasibility of 6 m high single pass longwalling has been studied using Newlands Mine as a case study An intensive geological investigation identified a range of hazards but a mine layout was possible that minimised the risks with respect to joints, cleats, angular bedding, coal shears and horizontal stress. Geotechnical design input was required for support capacity, subsidence, gate to face transition, face span, and roof support. It is concluded that longwall face equipment can be made available by modifying existing designs, gate to face transitions will require a number of steps, and a mid-face sprag should not be required for routine operations.

Jan 1, 1992