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By Soheil Bahrampour

Development of measurement while drilling (MWD) systems for ground control applications in mining applications has recently been an active research topic. The main goal is to utilize the drilling information for characterizing the ground condition. Such characterization is the key to the accurate and efficient mapping of hazards and proper planning of ground support. However, most of the existing drilling units and measurement while drilling systems are mainly developed for joints and voids detection, and much less research is done to estimate rock strength. This paper focuses on rock classification based on estimated strength from the drilling data. For this purpose, data from J.H. Fletcher roof bolters instrumented with the vibration and acoustic sensors as well as torque, thrust, penetration rate, RPM and position signals is used. All the available data was utilized for rock classification using pattern recognition algorithms. The algorithms were developed based on data from drilling, and into three different rock types. The classification is performed using a simple feature extraction algorithm with a well-known pattern recognition algorithm. The results demonstrate the suitability of the proposed algorithms in identifying the rock types based on their strength properties, which can be adopted in measurement while drilling systems.

Jan 1, 2014

By Denise Weber, Jürgen Kateloe, Marku Papst, Laurenz Preusse, Axel Preusse

"The angle of draw (in Germany, referred to as Limit Angle) plays a significant role in the field of mining subsidence engineering. This applies both to the assessment of the affected area by underground mining activities as well as applied methods for the prediction of ground movements.Recent observations using remote sensing show that ground movements outside the affected area defined by the angle of draw have been detected in German hard coal mining. Since the affected area by underground mining activities also plays an essential role regarding claims of landowners with damages to their houses and infrastructures, possible solutions for limiting the size of the affected area will be presented in this paper.Ground movements which exceed the affected area of an underground coal mine, as defined by the angle of draw will be shown in an exemplary manner. Present knowledge indicates an expectation that ground movements outside the affected area will not cause damage of major significance.This paper outlines the consequences for all affected parties regarding ground movements due to underground coal mining that exceed the extent of the area defined by the currently assumed angle of draw. This will be visualized in a case study.INTRODUCTIONGermany has a long tradition of coal mining, especially in the mining area of the Ruhr district. Following a long period of mining activity, the area is now highly populated and densely developed. In such an environment, it is of utmost importance to calculate and predict ground movements initiated by mining activities; therefore, the research here focused on the prediction of ground movement for the last decades. Special attention was given to the development of accurate ground movement models, which can be utilized in all mining areas. These models are based on different influencing factors such as depth of mining, worked thickness and the angle of draw.Depending on these models, the mining companies calculate the area of land subsidence; the predicted results are used for financial calculations and planning. For structures on top of zones with a high risk of deformation, structural measures can be applied, construction warnings can be given and, in cases of damage, compensation can be planned. The technical and economic planning was performed for all areas of mining subsidence. The most important factor for the outer limits of mining subsidence is the angle of draw (Ger. Grenzwinkel), which plots the border of the area of mining subsidence."

Jan 1, 2015

By Kristine L. Pankow, Keith D. Koper, Michael K. McCarter, Jared R. Stein

"The Wasatch Plateau region of central Utah contains a number of active underground coal mines in an area of well-documented natural seismicity. As a result, there are several hundred seismic events recorded in this region every year that may or may not be attributed directly to mining activity. Previous studies have linked the majority of this seismicity to mining activity through various means, but have not attempted to develop a method for categorizing individual events within a seismicity catalog. This study uses both relative relocation and waveform cross-correlation to separate events based on hypocentral depth and waveform similarity, respectively, in the area surrounding a single longwall coal mine within the Wasatch Plateau. We conclude that relative relocation can help with depth discrimination, but there are some unavoidable limitations caused by sparse seismic network coverage. The cross-correlation technique is more promising, with the ability to group events by waveform similarity and track mining progress over both space and time. With cross-correlation based clustering, we confirm that the vast majority (97.6%) of events in the study area are directly related to mining activity, but that a small population of natural seismicity is present as well. Eliminating this natural seismicity from the catalog of seismic events, leaving only those events related to mining, can give operators valuable information about the changing conditions as mining progresses and eliminate concern over events unrelated to mining activity.INTRODUCTIONA number of underground coal mines are located in the Wasatch Plateau region of central Utah, and have been in operation since the late 1800s. This activity has resulted in a well-known history of mining induced seismicity (MIS) (Wong, 1985). Additionally, this area is situated near the border of the Basin and Range and Colorado Plateau physiographic provinces, where naturally occurring tectonic earthquakes also occur (Arabasz et al, 1980). Seismicity separating these physiographic provinces is part of a larger band of seismic activity known as the Intermountain Seismic Belt (ISB) that runs from Montana to Arizona and cuts through the western boundary of the central Utah coalfields (Smith and Arabasz, 1991) (Figure 1)."

Jan 1, 2015

By Jinwang Zhang, Zhaohui Wang, Jiachen Wang

"Thick coal seams (seam height larger than 3.5 m) account for a large portion of the proven coal reserves of China, and underground thick coal seam mining provides about 50% of the Chinese annual coal production. Such seams are mainly mined using the longwall top-coal caving (LTCC) method. For safe and successful mining, a number of analyses on the stability of surrounding rocks in the LTCC face area have been extensively performed, and certain research achievements have been made. This paper introduces the main findings on rock pressure occurrence in the LTCC face during the last few decades and presents face and roof stability control techniques. INTRODUCTIONUnderground thick coal seams in China provide 1.8 billion tons of coal every year, which accounts for approximately 50% of the total Chinese annual coal production and 25% of the world, indicating that thick coal seams stand an important position in Chinese coal mining industry (Wang, 2005). Before the 1980s, thick coal seams were mined mainly by using the multiple-slice mining method. With the improvement of mining techniques and equipment in last two decades, longwall top-coal caving (LTCC) and high-seam, single-cut longwall are employed for thick coal seam extraction, offering advantages over multiple slicing from entry excavation and production perspectives (Wang, 2013).. The primary author of this paper introduced the most popular thick coal seam mining methods in China at the 30th ICGCM (Wang, 2015). This paper presents the stability of surrounding rocks in face area and ground control techniques for mining thick coal seams. For traditional longwall faces (seam height less than 3.5 m), the well-developed VoussoirVoussoir beam theory has been successfully used to analyze the movement of overlaying strata, which indicates that ground control techniques have become mature for this mining system. In such faces, the four-leg support with loading capacity of 4000–8000 kN is considered adequate for safe and successful mining. These faces normally have an annual yield of 1–2 million tons (Wang, 2007). For thick coal seams, however, the traditional support becomes inadequate and the ground control problem change to be more difficult and complex. Thus, development of large-scale mining equipment and ground control techniques for enlarged mined-out space and entries have been recognized as the most basic technical problems in mining thick coal seams."

Jan 1, 2017

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 Jun Lu, Greg Hasenfus

"In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh seam are introduced. The longwall headgate T-junction experienced very high face convergence (up to 2 feet), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded.Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter, and Borehole Pressure Cell (BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment, were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.INTRODUCTIONIt is well known that high horizontal stress will affect the stability of roof, floor, and ribs within underground mines. In general, the headgate entries will experience stress concentration for the right-handed longwall face. On the contrary, the stress concentration will be in the tailgate entries for the left-handed longwall face (Hasenfus and Su, 2006). Without the stress shadow, the stress concentration in the headgate for the first longwall panel may be more significant (Figure 1).A mine in the CONSOL Pennsylvania Operation was extracting the first right-handed longwall panel in the Pittsburgh seam, which created high horizontal stress concentrations at the longwall headgate. Because of concern over soft wet floor within the syncline, floor coal had been left in place on development. Early on, upon retreat, the longwall headgate T-junction experienced very high face convergence, accompanied by roof sag, floor heave, and rib loading. The floor heave was exacerbated by the presence of floor coal, which buckled upward near the center of the beltline. Roof movement was occasionally aggravated by laminated roof conditions, but was kept in check by aggressive supplemental support including cable bolts, yielding props, cribbing, and roof mesh. Rib bolting and rib mesh helped to further maintain the entryway stability by keeping sloughage at a minimum. Nevertheless, despite the heavy support, headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded."

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 Hongwei Wang, Hao Zhang, Panshi Xie, Xicai Gao, Youfu Zeng, Yongping Wu, Juan Dou

"The reserves and production of steep coal seams account for approximately 15–20% of the total coal reserves in China and 5–10% of the total annual coal production. More than 50% of the coal is high-quality coking coal and anthracite, based on reviews of steeply dipping seam mining at home and abroad. Fully mechanized mining is considered to be the only safe and efficient way to mine steeply dipping seams. To achieve fully mechanized longwall mining, four issues must be addressed: ground control, equipment stability control, development of complete sets of equipment, and safety. Engineers must focus on many factors of steeply dipping seams, such as the following: stope, roof asymmetrical structure, stability control, “roof-support-floor” system dynamics control, multi-region big-range ground control technology, adjustment false-inclined method, nonlinear layout method, control technology, safety, and development of complete sets of equipment. Engineering practices in China introduce these concepts systematically: fully mechanized mining of medium-thickness seams, fully mechanized high mining of thick seams, fully mechanized caving of extremely thick coal seams, fully mechanized mining of coal seams, roadway support, and so on. A steeply dipping seam is one with a dipping angle in the range of 35°–55°. This accounts for about 15%–20% of the total coal reserves in China. Because of the environment, more than 50% of the coal is high-quality coking coal and anthracite. Mining steeply dipping coal seams is known to be difficult in the mining industry. After the study conducted by Germany, Poland, and the former Soviet Union in the1970s–1980s, the fully mechanized mining basic theory, core technology, and key equipment research have not been a breakthrough. Being fully mechanized is a technological “restricted area” at home and abroad. A lot of steeply dipping seams are excavated by non-mechanized mining, leading to harsh job environments, labor-intensive work, low productivity, frequent accidents, and significant security hazards. The death rate per million tons of coal reaches above 4.0 and is far higher than slow inclined coal seam mining. Making steeply dipping seam mining safe and efficient is a global problem that urgently needs to be solved. Fully mechanized mining is the only way to overcome these problems (Wu, 2003; Wu, Yun, and Zhou, 2000). The realization of fully mechanized coal seam mining must solve four problems. (1) Examine the disaster mechanisms induced by overburden “key strata” mutation under steeply dipping seams as a basis for ground control methods and technical problems. (2) Work to solve the dynamic stability control technical problems of thousands of tons of complete sets of equipment under the steeply dipping seams. (3) Complete sets of equipment must be developed to adapt to the conditions of steeply dipping seams. (4) Overcome the steeply dipping face safety protection technical problems. In the last two decades, a number of universities, research institutes, and enterprises have realized the importance of safety and efficiency of steeply dipping seams in China. This is especially true of Professor Wu Yongping’s “hard coal layer safe and efficient mining” team at Xi’an University of Science and Technology. After several generations’ research and technical work, the team puts forward China’s longwall mechanized coal mining of steeply dipping seams strata control basic theory. This paper puts forth a series of key technologies of fully mechanized mining, develops and manufactures complete sets of equipment by independent innovation, and establishes a perfect face security system, forming a theory and technology system for fully mechanized longwall mining of steeply dipping seams."

Jan 1, 2017

By Pierre-Yves Declercq, Andre Vervoort

"After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—an upward movement. Although most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method, surface movement still occurs many decades after mining is terminated. After the closure and flooding of underground excavations and surrounding rock, this movement is reversed. This paper focuses on quantifying the upward movement in two neighboring coal mines (Winterslag and Zwartberg, Belgium). The study is based on data from a remote sensing technique: interferometry with synthetic aperture radar (INSAR). The results of the study show that the rate of upward movement in the decade after closure is about 10 mm/year on average. The upward movements are not linked directly to the past exploitation directly underneath a location. The amounts of subsidence at specific locations are linked mainly to their positions relative to an inverse trough shape situated over the entire mined-out areas and their immediate surroundings. Local features, such as geological faults, can have a secondary effect on the local variation of the uplift. The processes of subsidence and uplift are based on completely different mechanisms. Subsidence is initiated by a caving process, while the process of uplift is clearly linked to flooding. INTRODUCTION After the mass closures of entire coal mine districts in Europe at the end of the last century, a new phenomenon of surface movement was observed—upward movement in the area above the past exploitation and in the immediate surroundings. Of course, most surface movement (i.e., subsidence) occurs in the months and years after mining by the longwall method (Peng, 1986). This downward movement continues to occur at a smaller rate many decades after mining has been terminated when the water in the underground excavations is pumped to the surface. This is the case as long as a mine is in operation. However, after the closure and flooding of the underground excavations and the surrounding rock, this movement is reversed, and the surface is uplifted. A total uplift of about half a meter has been recorded to date. This phenomenon has been described in several different coal basins in Europe, for example, in Belgium (Devleeschouwer et al., 2008), France (Samsonov, d’Oreye, and Smets, 2013), Germany (Baglikow, 2011), the Netherlands (Caro Cuenca, Hooper, and Hanssen, 2013), and Poland (Preusse, Kateloe, and Sroka, 2013). To date, research has focused mainly on understanding the phenomenon (e.g., Herrero et al., 2012) and identifying general trends, whereby the link with the rise in water levels is an important issue (Caro Cuenca, Hooper, and Hanssen, 2013). The most accepted explanation is linked to the swelling of clay minerals in the argillaceous rocks in the coal strata (Herrero et al., 2012). Without doubt, the full explanation is complex, and swelling is probably not the only cause of the residual movement. Most coal strata rock is weakened upon contact with water. The increase in water pressure also results in a decrease in the effective stress. Both aspects facilitate the fracturing of rock, which normally would result in further subsidence. However, decreases in the effective stresses also result in the relaxation or expansion of the rock, which induces an upward movement (Fjar et al., 2008). All of these aspects, including the long-term residual subsidence due to compaction under dry conditions, result in a net upward movement. The phenomenon discussed here is clearly different from the upsidence, sometimes observed on other continents (e.g., in Australia, Galvin, 2016). When upsidence occurs, the rock near the surface bends and buckles upwards due to the overstressing of the floors of the valleys."

Jan 1, 2017

By Peter Zhang, Daniel Su, Essie Esterhuizen, Mark Van Dyke, Berk Tulu, Dave Gearhart

"Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site. The roof behavior in the headgate near the face was carefully observed during longwall retreat. Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.INTRODUCTIONLongwall mining is the primary underground coal mining method in the United States and currently accounts for more than 60% of the underground coal production. The longwall headgate, as a passageway for the longwall crew, intake air, material supplies, and coal belt transportation, is critical for both safety and continuous production of the longwall panel. A roof fall in the longwall headgate would not only result in substantial interruption of production but could potentially cause injuries or fatalities. Rehabilitation of failed roof in the headgate would also expose miners to the risk of injuries. Roof falls in the headgate, though infrequent, mostly occur in the belt entry near the face. Considerable research has been conducted to determine the effects of various factors on headgate roof stability, as well as effective measures to support the roof in longwall mines in the Pittsburgh seam (Mark, Mucho, and Dolinar, 1998; Chen et al., 1998; Su and Hasenfus, 1995; Su, Thomas, and Hasenfus, 1999; Su, Morris, and McCaffrey, 2002; Su, Draskovich, and Thomas, 2003; Hasenfus and Su, 2006; Van Dyke et al., 2015). Previous studies have demonstrated that high horizontal stress and transitional roof geology are primary factors that cause headgate roof failure. These studies also showed that panel orientation, retreat direction, pillar sizes, and roof support played an important role in the stability of a headgate.In underground coal mines located in the eastern United States, the magnitude of the maximum horizontal stress is typically three times greater than the vertical stress, and about 40% greater than the minimum horizontal stress. Mark, Mucho, and Dolinar (1998) studied seven cases of headgate failures caused by high horizontal stress in different coal seams in the United States and stated that roof stability is affected, to a large extent, by rock type, entry orientation, and longwall orientation. The effects of horizontal stress can be summarized in these statements: (1) A laminated roof is very vulnerable to high horizontal stress. (2) Entries that are aligned with the maximum horizontal stress will suffer less damage on development than those perpendicular to it. (3) Horizontal stress concentration and relief depends on panel orientation, the direction of retreat, and the sequence of longwall panel extraction."

Jan 1, 2018

By Peng Zhang

In a recent research effort, a laminated overburden loading model (as implemented in LaModel3.0 was compared with the results of ARMPS 2010, and the laminated overburden model was found to more accurately classify a ARMPS database of 52 deep cover case histories by a factor of 60% versus 54%. However, the LaModel analysis of each case study required several days to complete, as compared to a requirement of only several minutes for each ARMPS analysis. In the research presented in this paper, a computer code (ARMPS-LAM) has been developed to effectively implement the laminated overburden model into the ARMPS program. This program takes the basic ARMPS geometric input for defining the mining plan and loading condition and then automatically develops, runs, and analyzes a full LaModel analysis of the mining geometry to output the stability factor on the Active Mining Zone(AMZ), all without further user input. The present laboratory version of ARMPS-LAM can be run in batch mode; therefore, the ARMPS 2010 database of 645case histories can be analyzed very quickly. In an initial logistic regression analysis of this database, the ARMPS-LAM program was seen to be slightly more accurate than ARMPS 2010 by a factor of 71% versus 63%.

Jan 1, 2013

By Gennaro G. Marino

As part of the development of a shopping center in West Virginia, a significant bench had to be cut into a mountainside that was over 700 ft. high. As a result, a steeply stepped, over 400-ft.- high wall was created. The high wall exposed old abandoned room-and-pillar coal workings at the toe of the cut. Because of exposure to the elements, the mine openings began to severely deteriorate with significant potential of undermining of the rock cut face and causing a large rock fall(s). Exacerbating this wall problem was a set of fairly planar sub-parallel and sub-vertical joints creating, in essence, thin rock slabs. Also present were stress relief fractures that were observed in the exposed pillars. Given the proximity of the project building to this high rock face, mitigation of the rock fall potential was deemed necessary. In order to prevent continued damage and deterioration of these exposed mine entries, the rubblized openings were filled with bulkhead grout and pressure injected with grout. The total amount of grout installed in these openings was about 3,945 CY. Also, a rock drape was attached to the entire face to mitigate small rock falls from natural causes. There also existed another level of mining below the ground surface and under an existing building at a depth of about 160 ft. Under a portion of the building, the pillars in these old works were second mined, and the building seemed to have been already affected by limited subsidence above this area. Also, calculated pillar safety factors in this high extraction section of the mine were too low. Consequently, these old works were stabilized by saturation grouting. An indication of overall roof distress was the significant fracturing encountered in the bedrock overburden during the drilling of the injection holes and the volume of grout placed in these holes. The total grout installed in the high extraction section of the lower mine was about 6,660 CY. This paper describes the investigation and the grouting techniques employed at the rock cut face and under the existing building to mitigate continued damage to avoid future subsidence problems at the face and the building premises.

Jan 1, 2013

By Darin Cotton, Tim Burgess, Tim Martin

"As the leading US supplier of roof control drills, J. H. Fletcher and Company® has encountered an extensive set of roof strata challenges and has overcome various drilling issues by varying machine parameters and working with customers and tool manufacturers to develop solutions. Every mine wants to drill and install roof bolts as fast as possible with maximum safety at the lowest possible installation cost for their conditions. In most circumstances, the intuitive method of higher speeds and pressures can actually have a detrimental effect on the roof drill safety and performance.This paper will outline recent tests carried out at several different coal mines in the Eastern United States. The tests focused on optimizing the drilling parameters of feed pressure, feed rate, rotation speed, and drill torque.The drill feed, rotation speed, and rotation torque, as well as the type of bit selected, have a definite impact on the performance of the roof drill including drilling speed, safety, and tool life. The drilling process can be improved while also improving operator safety and reducing bolt installation costs by maintaining the appropriate roof drill settings for the specific strata conditions.The advantages of improving the drilling process includes increased bit life, increased drill steel life, reduced hanging drill steels, reduced clogging of the vacuum system, decreased whipping drill steel potential, reduced noise generation, and increased cutting size which reduces fine dust generation and dust collection system servicing.INTRODUCTIONIn recent times, the economic pressures that coal producers are facing have created an increased emphasis on controlling operating costs. Several coal operators have pointed out their dissatisfaction with low bit life, and there is a renewed interest in improving tool life and machine performance as part of an overall cost control strategy. Operators are looking for ways to reduce tooling cost, maintenance costs, and drill downtime without decreasing drill performance. This renewed interest in reducing operating costs, along with the ongoing desire to improve safety, created a perfect environment to take a fresh look at the existing fleet of machines to see what could be accomplished. The goal was to take a practical approach to reviewing machine parameters to determine what changes could be made. The emphasis was on changes that would be feasible to roll out to an entire fleet of machines in a short period of time with minimal capital expenditure. The thought process was that significant improvements could be made by simply tuning up the machine parameters to provide a more efficient and safer bolting process."

Jan 1, 2015

By Steven Derycke, Stephen C. Tadolini, Anand Bhagwat

"The tensile and shear strength of intrinsic bolting support systems has always been a major concern of designers. A comprehensive laboratory testing program was designed to evaluate the tensile and shear strength of individual wires and completely wound PC-strand cables. PC-strand cables with smooth wires and the recent anchorage enhancement innovation of indentation were evaluated and compared. The testing protocol detailed in ISO Standard 15630 utilizes a mandrel system that was investigated at 3 different diameters which alters the wire to mandrel ratio from 2:1 to 9:1. The results demonstrate that the difference between smooth and indented wires is statistically insignificant when larger diameter mandrels are used, and that indentation does not adversely affect strand properties and performance. Insight into the shearing mechanism and evaluation techniques are discussed with the introduction of triaxial loading to describe the PC-strand tensile and shearing mechanisms. Another important result indicates that the shear strength of PCstrand cable bolting systems has a greater shear strength value than traditional steel bar bolting systems.INTRODUCTIONPC-strand cable bolts have proven to be a global cost effective ground control solution in metal/non-metal and coal mines. The inherent strength to weight ratio of cable supports, combined with flexibility has been used in underground entries that range from 20 to less than 1.2 m in height (65 to 4 ft). In coal mining, the systems are used as primary and secondary support installed both passively or actively with installation tensions that range from 38 to over 178 kN (8,500 to 40,000 lbs) . While cable can be specified in several diameters, the most predominant sizes in the US remain 15.2mm (0.6 in) and 18mm (0.7 in). The capacity of the systems defined in the ASTM 432-13 is 90% of the minimum strand strengths of 260 kN (58,600 lbs) and 320 kN (72,000 lbs), respectively.Cables are not only suitable to support vertical loads, but are also often used to effectively support fractured and jointed materials and can be exposed to shear forces across the wound PC-strand structure. In fact, Goris et al. reported that cable bolts placed across jointed surfaces could more than double the shear resistance. These results were duplicated for both smooth and rough jointed surfaces (Goris et al., 1996). An example of tensioned cable bolts used to support a fractured and sliding roof in a longwall tailgate entry is shown in Figure 1."

Jan 1, 2015

By Yongping Wu

Taking the longwall mining face area in a steeply dipping seam as the research object, the stability of an asymmetrical rock mass structure is analyzed using shell theory and masonry beam theory. The results show that an asymmetrical shell structure is formed at the top strata of the mining stope and its magnitude is mainly determined by the mechanical characteristic of the overburden strata of working face, mining space size and so on. Localized instability of the shell structure is the main factor that leads to the overall structural instability in the stope. The instability modes includes: the shell roof instability dominated by tensile failure, the shell shoulder instability initialized by compression-shear failure, and shell bases composite instability occurs in tensile and compression-shear modes. The shell structure stability dedicates rational strata control measures to choose, and the caving height of overburden strata is the key for shell structure instability.

Jan 1, 2013

By Russell C. Frith

This paper examines the design of longwall shields, focusing on those aspects that are critical to either their successful application or contribution to failure during longwall extraction. In many instances, longwall shields are assessed and designed along lines similar to that of roadway ground support systems, namely according to typical or normal geotechnical conditions. However it will be contended that for longwall shields, this is inappropriate because unlike roadway ground support systems, longwall shields cannot be supplemented with additional or secondary support in localised areas of adverse geotechnical conditions. In other words, the longwall shield needs to be designed according to what are judged to be worst case or adverse geotechnical conditions?the outcome being that a well-designed shield will be significantly over-rated for the majority of its working life. Therefore, it will be argued that the additional capital cost of such shield design can be readily justified as a prudent riskbased outcome that is essential to minimising future business risks due to strata instability on the longwall face. Shield geometry will be discussed, including such relevant factors as leg angle, inclination of the top caving shield, canopy ratio, operating height range, and tip to face distance. These are all well-established longwall shield design considerations and, most importantly, areas whereby inadequate design can render a longwall shield as highly ineffective. The issue of tip to face distance will be considered in detail, in particular, the extent by which it is an important geotechnical design consideration, the conclusion being reached that, in fact, it is not. The critical importance of maximising set to yield ratio within practical operating limits will be justified, the wisdom of this having been subject to questioning in more recent technical literature. Overall, the aim is to provide the industry with a set of suggested guidelines for future use when designing longwall shields and, hopefully, to initiate discussion on a subject that, unlike roadway ground support design, is not well covered in its entirety in the published technical literature.

Jan 1, 2013

By Weihua Song, Hongwei Zhang, Feng Zhu, Sheng Li, Guoshui Tang

"Rock burst is one of the most serious dynamic disasters in coal mining. Rock burst prevention involves two difficult tasks: systematically evaluating the occurrence conditions and effectively forecast for rock burst. This paper systematically studies the effects of natural geological dynamic conditions and mining engineering on and forecast for rock burst. It establishes the evaluation method and index system of rock burst geological dynamic conditions and proposes a method of forecasting rock bursts by monitoring fault activity with anchor cables. Natural geological dynamic conditions are a necessary condition while mining engineering is a sufficient condition for the occurrence of rock bursts. Rock bursts in mine must have suitable geological dynamic conditions, and mining engineering activity is merely an inducing factor. There is a good correlation between anchor cable stress variation and fault activity, when the anchor cable dynamometer indicates that the fault is active. It can forecast a rock burst a few days in advance. This technique has been successfully applied to Panel 83003 of a coal mine in China, which improved the safety production level of the mine and found a new way of rock burst evaluation and prediction.INTRODUCTIONRock burst is one of the most serious dynamic disasters in coal mining. With the increase of mining depth, rock burst disaster is becoming more and more serious. Systematically evaluation and effective forecasting of the occurrence of rock burst in the coal mines are important and difficult work in the control of rock bursts. Chinese researchers have performed numerous researches on rock bursts, Qi et al., established the rock burst model of frictional sliding instability, explained the occurrence mechanism of rock bursts by stick-slip phenomenon in frictional sliding, gave full consideration to the influence of primary rock stress and mining-induced stress on rock burst occurrence, and proposed the stress controlling theory of rock burst. Pan et al., [3] put forward the criterion of perturbation response, explained and analyzed the problem of rock burst in circular openings, put forward the ratio of coal-rock elastic modulus and reduced modulus is the significant parameters for the determination of stabilization; systematically summarized the rock burst distribution, type, mechanism and control in China. They classified rock bursts into three types: coal compression, roof fracture and fault movement. By applying energy transfer principle and conservation of energy, combined with the analysis of the weakening of rock fabric damage, Zhou and Jiang put forward the theory and equation for the occurrence of coal-rock mass impact effect. They analyzed and discussed the formation mechanism of rock burst by combining impact effect theory and energy equation. Wang et al., recognized that the mechanism of rock burst which induced by fault, fold and phase transition is the superposition of tectonic stress and mining stress and that the axes of syncline and anticline and their flanks are the location where rock bursts mostly occur. To prevent stress superposition is the efficient way to control the structural rock burst [7]. Jiang et al., summarized the research achievement of rock burst mechanism and controlling techniques and existing problems in recent years in China, and pointed out the direction to improve the level of prevention of controlling rock bursts in coal mines."

Jan 1, 2015

By Thomas W. McClain

This paper describes the importance of considering ground control measures when importing U.S. continuous mining technology in foreign countries. Important ground control factors are discussed. Each year the USA exports a significant amount of continuous miner equipment (i.e. continuous miners, roof bolters, coal haulers, shuttle cars, continuous haulages, scoops and feeder-breakers). International customers see the opportunity to significantly improve their productivity and increase their longwall gateroad development rates by implementing the room-and-pillar (place changing) mining systems used throughout the USA. However, a number of issues are often overlooked or not thoroughly evaluated when selecting the equipment, often resulting in lower than expected productivity. Ground control is one of the critical factors that must be addressed. Since 1993, the author has assisted numerous international customers and manufacturers with implementing or improving the performance of continuous miner and room-and-pillar systems in Australia, India, Mexico, South Africa, and the United Kingdom. An essential first step is to conduct a thorough ground control study, and then integrate its findings into mine layout, equipment selection and section operating plans. Based on actual case histories, the author will present a number of examples and "Do's and Don'ts." Another key to success is the involvement of manufacturers and suppliers with customers when implementing new systems and technology. The importance of allowing for cultural evolution by both management and workers for acceptance and successful implementation of any new system is also discussed. Over the past ten years, the global coal industry has gone through some very dramatic changes. Countries that previously experienced a freight advantage, such as Australia shipping coal to Japan and Korea versus the United States, are finding they are now in a world coal market similar to other commodities and products. Today, coal prices arc stated F.O.B. at the consumer, and the origin is often irrelevant. Also, previously state-run coal industries have been privatized, such as the United Kingdom, and others such as Poland and India are currently going through the same transition from state-run to the private sector. In all cases, the key to continued viability, or survival, depends largely on reducing costs in order to compete for a market share. Two ways exist to reduce costs. First, almost everyone tries to reduce expenditures. Depending on the circumstances at a given mine or in a specific country, this helps to some point. However, most find that secondly, production must be increased to achieve the necessary cost reductions to compete in today's global coal market. As a result, over the past ten years, foreign companies and countries have looked to the USA for new mining methods and systems to significantly increase productivity. Thus, a major portion of American equipment manufacturers and suppliers sales have been exports, which has played a major role in helping to "keep-the-doors-open" for a number of companies through the lean 1990's, when American coal companies were postponing capital expenditures and expansions. As foreign companies and countries have looked to the USA for ways to improve productivity, they have become particularly interested in continuous miner equipment systems and most often "room-and-pillar," "bord-and-pillar" or "place changing" mining systems for four reasons. First, in countries such as the United Kingdom and Germany, who have been longwall mining for generations, there remain numerous remnant small pockets of reserves in existing mines that are either too small or too difficult to longwall mine, and are conducive to the flexibility of continuous miner systems. Second, for countries such as Australia, who have experienced the constant problem of developing longwall gateroads in a timely manner to avoid longwall move delays, new or alternate methods are required. Third, companies and countries using roadheader machines on coal production are recognizing the increased productivity potential of continuous miner equipment. Fourth, countries such as South Africa and India, where a substantial amount of the production comes from conventional mining methods, "drill-and-blast", companies are recognizing the opportunity to improve by implementing continuous miner technology. Colliers & Associates, Inc. has found that over the past eight years too often foreign companies or countries purchase American continuous miner equipment without first conducting a thorough geologic, geotechnical and ground control study for the reserves to be mined. The problems are compounded by not developing mine plans, and submitting them to the regulatory authorities for approval prior to purchasing and even implementing the equipment.

Jan 1, 2001

By A. H. PH. D Wilson

Compared to the rocks encountered in, say, metalliferous mining or tunnel drivage in igneous and metamorphic rocks, the strata associated with the coal measures are relatively soft. If excavations are made at depths greater than one or two hundred metres, zones of fractured or yielded rock develop around the openings. The stress which can be taken by yielded rock at the boundary of an opening is low, but because of the friction present between the fragments or particles, the sustainable stress rises rapidly with distance from the opening until sufficient confinement has been developed to prevent failure of the rock in the first place. Thereafter the laws of elasticity will apply and increasing distance will reduce the stress field until conditions consistent with the cover load are reached. For rocks which maintain their elastic condition right to the edge of an opening, finite element analysis and similar techniques have proved successful in determining the distribution of stress around the opening. This approach, however, is not successful when a yield zone is introduced between the elastic zone and the opening. Nevertheless, some estimate is required if help is to be given in the siting of roadways and in the determination of adequate pillar sizes. This matter is of particular importance in the planning of new mines especially if retreat faces are involved, as extensive roadway drivage must be planned and executed before experience of the effects of face extraction is gained. In 1977, the author published [I] a method based on 'stress balance'. The total aggregate downward load over a large area will remain that of the cover load even after part of the coal seam has been removed. Any rise in stress in ribsides and pillars must be offset by an equivalent stress reduction across roadways and other areas of extraction, and vice versa. Knowledge of one can lead to an estimate of the other, provided the general form of the stress distribution can be postulated. It is the object of this paper to revue the method in more detail and to give further examples of its successful application.

Jan 1, 1981

By Madan M. Singh

Mining under bodies of water such as oceans, lakes, rivers, streams, ponds, and other impoundments, is not a new technique. However, in the United States it has been fairly restricted to date. In order to recover the maximum amount of the nation's reserves, in the future, it is important that further mining under such bodies of water be planned. However, it is imperative that this is not done at the expense of danger to the nation's miners or appreciable property damage. If all the bodies of water are considered large, and it is assumed that these constitute a hazard when mining in their vicinity irrespective of their size and geometry, a significant percentage of the coal reserves are likely to be sterilized. How- ever, by adopting special mining plans and procedures, it may be possible to release reserves for exploitation that would otherwise be unmineable since the surface of bodies of water below which they lie are relatively small in size. In 1977, the Bureau of Mines issued Information Circular (IC) 8741, entitled "Results of Research to Develop Guidelines for Mining Near Surface and Underground Bodies of Water". This document was based on the work done by Skelly and Loy (under Contract No. H0252083) and K. Wardell and Partners (under Contract No. H0252021). How ever, these guidelines have limited application, because these did not (1) define the bodies of water of concern, and (2) take into account the nature of the strata being encountered. These guidelines assumed that all bodies of water which would be undermined were large. For the purposes of this discussion, water bodies may be divided into three categories, depending upon their potential to cause damage to the underlying mining activity: ? catastrophic ? major ? limited Bodies of water which pose the threat of catastrophic destruction in the mine, should there be an inrush, include oceans, large lakes, big inland reservoirs, and rivers that could flood the mine complete- ly in a short period. Those with major potential for damage would include lakes and streams which might present a danger to both life and property if not considered in the mining plans. Finite bodies of water which have a volume considerably smaller than the mine volume offer a limited potential for damage in the case of an inrush. These include flowing streams which have a relatively small rate of replenishment and which can be dealt with by pumping or isolation in the event of a breakthrough into the mine. Such bodies of water may be inconvenient and cause a temporary disruption of production, and could even cause some damage to equipment, but only in rare instances would personal injuries occur. The scope of this paper is limited to the exploitation of stratified mineral deposits with overlying -face bodies of water. Vein and dessiminate ore deposits are not discussed, nor are breakthroughs into adjacent mines in the same horizon. Overlying, water-logged strata also pose a serious threat to mining, but are not treated in this discussion. Pending further investigation these strata may be considered as bodies of water with catastrophic potential for damage, with the base of the strata being considered the bottom of the water body. When the body of water is large, so that it may cause catastrophic damage to the mine in the event of an inrush, the guidelines suggested in IC 8741 are applicable. In order to develop criteria and procedures for mining under smaller bodies of water, however, it is necessary that the nature of the strata disturbance due to underground mining be examined.

Jan 1, 1981