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By C. W. Bollier

In recent years, Peabody Coal Company has enjoyed a great amount of success using roof trussing as a secondary support system. Since 1975, over 180,000 trusses have been installed in 12 different Peabody mines with varying geological conditions. Projections from Peabody, and throughout the mining industry, indicate a continuous growth in the trussing principle for roof control. Peabody's primary supplier has sold over 1,000,000 trusses to 50 different mines throughout the country. (1) Roof trusses provide an improved means of controlling difficult roof conditions which, in turn, improves worker safety and mine production. Since the original development of the truss, (2) the only major modification to the trussing procedure has been the addition of the resin point anchor system in the mid-70's. The resin point anchor was the catalyst needed to provide a reliable anchor and create acceptance of the roof truss by both management and labor. Peabody began using trusses in their mines in 1975. For the first few years, most of the truss effort was geared toward determining proper anchorage, spacing, and gathering historical data. In recent years, Peabody's activities have centered on the development of improving trussing procedures. Through monitoring and trussing activities, a number of areas of improvement have been found.

Jan 1, 1982

By James J. Scott

Over the past two years research, development ad field tests have been conducted on a new product called the "Scott Cable Sling". This work resulted in the awarding of U.S. Patent No. 4265571 on May 5, 1981, to the inventor. Work on this' project was done by the staff of Scott Mine Technical Services, Inc., and is in keeping with its philosophy of advancing mine grad control technology. Recent acceptance by industry has resulted in volume purchases of the product and indications are that it will gain wide acceptance by the mining industry. The system features flexibility in its application and is useful in both roof and rib control. This paper describes the product, discusses its range of applications and cites specific field examples of its use.

Jan 1, 1981

By Joseph C. Zelanko

Cable bolt support techniques. including hardware and anchorage systems, continue to evolve to meet U.S. mining requirements. For cable support systems to be successfully implemented into new ground control areas, the mechanics of this support and the potential range of performance need to be better understood. To contribute to this understanding, a series of 36 pull tests were performed on 10 ft long cable bolts using various combinations of hole diameters, resin formulations, anchor types, and with and without resin dams. These tests provided insight as to the influence of these four parameters on cable system performance. Performance was assessed in terms of support capacity (maximum load attained in a pull test), system stiffness (assessed from two intervals of load-deformation), and from the general load-deformation response. Three characteristic load-deformation responses were observed. An Analysis of Variance identified a number of main effects and interactions of significance to support capacity and stiffness. The factorial experiment performed in this study provides insight to the effects of several design parameters associated with resin-anchored cable bolts.

Jan 1, 1995

By Denise Y. Dixon

The objectives of this study were to (1) document the hydrologic impacts of longwall mining on ground water, (2) identify the factors which affect the extent of dewatering, and (3) develop empirical trends to predict the extent of dewatering and recovery in advance of mining. The study was confined to three mine sites in north-central West Virginia. At each site, available ground water supplies were identified and monitored for dewatering effects and recovery. Mine A, located in Upshur County, West Virginia, has relatively thin overburden (approximately 715-280 feet). At this site domestic water supplies and specially constructed monitoring wells were monitored, continuing with an earlier study done by Cifelli arid Rauch. Most such supplies were completely dewatered over the longwall panel. Some of these wells have shown significant recovery, especially near a stream. Extensive monitoring of two nested piezometer wells revealed that dewatering generally started in advance of undermining in the lowermost monitored overburden zones first and then progressed upwards. Most dewatered wells completed in bedrock have shown no recovery, except. for valley drilled wells located within 100 feet of a Stream. Affected dug wells completed in valley alluvium rend located within 200 feet. of the nearest stream have shown significant recovery. Some wells have partially collapsed following mine subsidence. Mine B, having relatively thick overburden (approximately 585 to 900 feet), is located in Monongalia County, West Virginia. Ground water supply surveys have been conducted there in an attempt to study past dewatering effects. Owners reported Chat several wells were dewatered by longwall mining arid vertical air shafts that preceded longwall mining. Dewatered wells over longwall panels hove shown some recovery within 3 ½ years of mining. Mine C also has relatively thick overburden (approximately 585-900 feet), and is located mostly in Greene County, Pennsylvania. In addition to a survey of domestic water supplies at this site, five specially constructed monitoring wells were tested. The shallow monitoring wells suffered drops in water level due to mining but totally recovered within one day to three weeks after dewatering occurred. However, they did occasionally exhibit partial collapse during mine subsidence. Subsidence fracturing was beneficial in increasing the lateral hydraulic conductivity by a factor of about S to 22 for rock strata located within 124 feet of the surface, indicating enhanced shallow aquifer potential. One deep well suffered both partial dewatering and structural damage, with the state of water recovery bring uncertain. Also, two shallow domestic supplies out of five located over the mine were reportedly dewatered, but this claim could not, be substantiated.

Jan 1, 1988

By K. Hanna

This manuscript describes an extensive ground control study at the Inland Steel No. 2 Mine near McLeansboro, IL, conducted by the U.S. Department of the Interior's Bureau of Mines (USEM) in cooperation with Inland Steel Coal Co. The mine experiences severe ground control problem primarily due to high horizontal stress, particularly at entry intersections. Mine experience has shown that reorienting mine entries, modifying the bolting pattern, and altering the mining sequence are effective in reducing the severity of roof falls and out-of-seam dilution. However, the frequency of roof falls has not been significantly reduced. This research project was designed to provide a thorough explanation of the intersection failure mechanism through combined analyses of company in-mine experience and Bureau field measurements. Stresses and deformations were monitored in a four-way intersection during all stages of development and were related to site observations and theoretical analyses. The results of the combined analyses indicate that high horizontal stress is the predominant factor contributing to intersection roof falls in the Inland Steel No. 2 Mine. The horizontal stress induces shear zones to form along the outby 1 ribs, causing the immediate roof layer to detach 1 and behave as a cantilever beam oriented diagonally across the intersection and parallel to i the maximum horizontal principal stress. As the shear zone progresses upward, roof layers successively detach and fail, forming a dome-type roof fall.

Jan 1, 1986

By Al Campoli

One of the critical variables affecting resin bolt performance is the size of the resin annulus, defined as the difference between the hole and bolt radius. The vast majority of the millions of resin roof bolts installed each year in the United States employ an annulus between 0.125 and 0.188 inch. As the annulus size decreases the resin flow around the bolt becomes more difficult, increasing the resin pressure induced by bolt insertion. As the resin pressure increases the loss of resin into bedding planes and vertical fractures in the rock increases. As the annulus size increases the mixing of the resin paste and mastic becomes less efficient. Thus, the lower and upper limits of the optimum annulus size is determined by insertion pressure and mixing considerations, respectively. Previous U.S. Bureau of Mines researchers have concluded that a 0 125 inch annulus is optimal. However, others have recently reported that a 0.095 inch annulus with special rolled deformation (SRO) bolts provide superior anchorage to standard rebar with a very large 0.250 inch annulus, attributing SRD resin loss in weak ground to oversized holes. The effect of annulus size on unit length pull strength and resin loss to fractured rock was investigated. Short encapsulation pull tests with resin annuluses of 0.125 and 0.095 inch resisted an average of 2.25 tons per grouted inch, with no significant variation. However, annulus size has shown a dramatic effect on the magnitude of resin lost to simulated rock fractures. No. 5 standard . No. 6 standard, and No 7 J-Bar rebar were inserted into two different viscosity resin cartridges designed for 4 foot boll encapsulation into a one inch inside diameter metal tube, To simulate a rock fracture, a 0.250 inch diameter orifice was located near the capped end of the tube Resin loses increased as annulus size decreased, with a minimum loss of 35% and a maximum loss of 60% associated with 0.125 and 0.095 inch annuluses, respectively.

Jan 1, 1999

By Brian Clifford

Thoresby Colliery is in the process of recovering an old roadway that was flooded following the closure of an adjacent mine. The aim is to re-use the roadway as a gate road for a longwall retreat panel and it is therefore important to establish the condition of the existing rockbolt support during the recovery operation. Two NDT methods, ultrasonics and RMT's newly developed RF (Radio Frequency) system were used in combination to interrogate the rockbolts to determine their condition. This paper describes the condition of the rockbolt supports, the NDT methods used and the problems that had to be overcome to successfully apply the two methods.

Jan 1, 2002

By Bingjie Huo

"China is a major coal producer and consumer. It is necessary to mine the steeply pitching coal seams due to development of the coal industry to meet the increasing needs for coal. Flexible shield supports mining is a safe and efficient mining method for steeply pitching coal seam that was first applied in HuaiNan mining area in the 1970s. The coal seams are steeply pitching in DaAnShan Colliery, so mining is mainly done by the flexible shield supports method. The roadway lay-out and extraction technology are researched in order to improve the safety, yield and work efficiency in the flexible shield supports working face. Optimization of the flexible shield supports mining method is discussed. The main optimization aspects are as follows: (1) the angle of working face and mining track roadways are increased from 23° to 28°, and (2) the technology and parameters of coal breaking are optimized. Applicable conditions, as well as advantages and disadvantages are also discussed. Finally, a comparison is made of the two different flexible shield supports mining methods, namely ""bench blasting . technology"" and ""piecewise blasting technology"".INTRODUCTIONChina is the largest producer and consumer of coal in the world and relies heavily on coal for energy production. At present, Chinese production is about 42% of the worldwide production, and coal has accounted for 76% of primary energy production. Coal plays a dominant role in the structure of energy production and consumption, which is fuel and an important industrial raw material but it is non-renewable. In recent years, the high inclined seams (35°< angle of coal seam < 45°) and steeply pitching coal seams (angle of coal seam >45°) have drawn increasing attention in the mining sector along with the growth of coal mining. There are two reasons. First, there are abundant reserves of high inclined and steeply pitching coal seams in China, which account for 15- 20% of total reserves (Xie, Dong-hai, 2007). Highly inclined and the steeply pitching coal seams are mined in more than 50% of the western mining provinces such as Sichuan, ChongQing, YunNan, GuiZhou, XinJiang, GanSu, NingXia, etc. As the mining of mineral resources continues west in the 21st century, the study of coal mining in high inclined seams and steeply pitching coal seams will be of great importance. Secondly, better quality coal has become scarcer in the eastern mining areas after years of exploitation, so steeply pitching seam mmmg is growing more prevalent. In short, carrying out research on steeply pitching seam mining technology plays an important role in sustaining the development of the Chinese coal industry."

Jan 1, 2010

By Li Yang, Andre Cezar Zingano, Alberto Fronza

"Immediate roof control is very important to the security and stability of entries in coal mining. It is also the highest cost in the coal mining operation, and roof spalling is directly responsible for coal contamination. The immediate roof in the coal mines in the State of Santa Catarina varies in lithology and in the presence of structures, such as bedding planes, vertical fractures that cut lamination, sandstone, and siltstone boulders. The presence and crossing of these structures should influence the decision of the specification of the roof support. Furthermore, the variation of rock layer and quality of the roof must be immediately followed by the variation of support specification. This work aims to study the behavior of the immediate roof under different geological and structural conditions during the excavation of entries, including the changing of the geometry of the entry as a function of time. This includes the effect of vertical fractures that cross the bedding planes of the roof and the position of those fractures on the roof. The effect of time between excavation and installing support to stabilize the roof is also examined. This study uses empirical methodologies for support design and numerical methods to study the behavior of the roof when anchored to the different scenarios proposed. The results of this work show the importance of quality characterization of the roof in terms of the type of rock and its thickness and the presence of vertical fractures that cut the bedding planes of the roof immediately.INTRODUCTION The immediate roof in a coal mine can varies in terms of typology and structural features, like bedding plane spacing and vertical joints. The roof support must be specified based on the features of the roof layer and its classification. Unal (1983) and Mark, Molinda, and Dolinar (2001) suggest that the roof support design should be based on the rock mass rating (RMR) and coal mining roof rating (CMRR), which look at rock mass classifications. These methods are based on the beam-building approach, and the bedding planes are tied together using fully grouted bolt. The bolt can be pre-tensioned or not depending the time of the curing of the resin."

Jan 1, 2017

By Robert M. Cox

Abrupt tailgate roadway ground failures, described as floor bumps, sometimes occur in the tailgate entry outby the face during the high-speed extraction of coal from mechanized longwall panels. These floor bumps typically damage the tailgate entry support structures for a distance of from 12.2 to 24.4 m (40 to 80 ft) outby the face. The relative closure of the tailgate entry may be a key indicator of potential tailgate ground control problems. Regression analyses of tailgate entry convergence data conducted by the U.S. Bureau of Mines (USBM) indicate that entry closure may be characterized as an exponential function of the distance outby the longwall face. Assuming that the entry closure is caused by the abutment loads created by the excavation of the longwall panel, the resulting convergence equation can be integrated to locate the centroid of abutment loading. This information can be used to document zones of potential ground failure ahead of the operating face.

Jan 1, 1994

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 Jinhua Wang

Based on engineering examples and theoretical analysis, the mechanism of chemical consolidation for fractured roadway surrounding rock was discussed. The chemical consolidating materials and three consolidating methods, that is, capsule, forced infusion and spraying, were introduced. Finally the application effects of the chemical consolidating technique to reinforce the fractured surrounding rock of roadways were analysed.

Jan 1, 1996

By Rudrajit Mitra, Tien Dung Le, Chengguo Zhang, Bruce Hebblewhite, Joung Oh

"Geomechanical and geotechnical characteristics of coal seam are important parameters that directly affect the cavability of top coal in the longwall top-coal caving (LTCC) method. The impact of these parameters such as coal strength, coal deformation modulus, coal discontinuities and top-coal thickness on cavability has not been fully understood in previous numerical analyses. The limitation and selection of the numerical modelling codes were two of the main reasons. In this paper, detailed caving simulations are performed using a 2D distinct element method (DEM) code, UDEC, to study the effect of various critical parameters on top-coal cavability. The numerical analysis evidently shows that the coal deformation modulus, coal discontinuities, and top-coal thickness have considerable influences on cavability. Coal strength is also a factor but to a lesser extent. The results presented in this paper contribute to the understanding of top-coal cavability in different mining environments and facilitate the development of an advanced cavability evaluation criterion. INTRODUCTION The longwall top-coal caving (LTCC) method was originally developed in the 1950s in the former Soviet Union and France (Tien, 1998). The method was nearly abandoned in Europe after the mid-1980s; however, it has been significantly improved and widely applied in China since the late 1980s. A conceptual model of the LTCC method is shown in Figure 1 in which a thick coal seam is divided into two sections including the lower or cutting section, and the upper or top coal section. The lower section of coal seam is mined by conventional longwall method. Meanwhile, the upper section is recovered by caving through the face support. The LTCC method is considered to be the most viable option for mining general thick seams. Compared to the multi-slice longwall (MSL) method, LTCC has the major advantages (Zhongming, 2001; cited by Vakili, 2009):"

Jan 1, 2017

By Zhengdong Liu, Kaizhong Zhang, Zhaonan Jiang, Yuanping Cheng, Biao Hu, Qingquan Liu, Ranpeng Wang

"Major coal mining areas in eastern China have entered into the deep mining state. Gas pre-drainage, which has been successful at shallower depths, is not suitable at greater depths because of complex conditions. Coal permeability, which is closely related to the damage behavior of coal, is the most important parameter in determining gas migration and gas drainage in a coal seam. To achieve an optimal gas drainage design in a deep mining state, the damage and permeability evolutions of coal should be further studied. The progressive failure process of coal has been analyzed by conducting a series of triaxial loading and unloading tests. The development of coal fractures has been successfully quantified, and the corresponding characteristic stress points have been obtained by using Martin’s fracture model and the stress-strain curve. Coal will produce plastic deformation, and its permeability will rapidly increase when the stress level reaches damage expansion stress. INTRODUCTION Coal mine methane (CMM) is a general term for all methane released during and after mining operations. CMM is both a potentially valuable energy and a serious hazard in active coal mines. Degassing coal seams is important for mitigating this hazard and results in the beneficial recovery of a cleanburning, low-carbon fuel resource (Karacan, Ruiz, Cotè, and Phipps, 2011). The permeability of coal is a key parameter in determining CMM production, coalbed methane (CBM) production, CO2-enhanced coalbed methane (CO2-ECBM) production, and CO2 storage in coal seam reservoirs (Connell, Lu, and Pan, 2010; Chen et al., 2011; Perera, Ranjith, and Choi, 2013). Significant research has been condcuted to the study of the evolution of coal permeability. These studies revealed that coal permeability evolution is controlled by the competing influences of effective stress and sorption-induced coal deformation. Chen et al. (2011) investigated the influence of an effective stress coefficient and sorption-induced strain on the evolution of coal permeability. Robertson and his team (Robertson, 2005; Robertson and Christiansen, 2005) measured sorption-induced matrix strain and permeability changes with regard to CO2, CH4, and N2. They found that the compressibility is not constant in a natural coal sample. Karacan (2007) conducted a series of experimental tests to study the swelling-induced volumetric strains internal to a stressed coal associated with CO2 sorption. Liu et al. (2016) investigated the effects of matrix swelling area propagation on the permeability evolution by using both natural and reconstituted samples. Many coal permeability models have been developed to describe the permeability evolution under uniaxial loading, biaxial strain, hydrostatic confining pressure, triaxial strain, and triaxial stress. Recently, some analytical permeability models have been developed to describe the anisotropic permeability behavior (Palmer, 2009; Pan and Connell, 2011; Robertson and Christiansen, 2005; Wang, Zang, Wang, and Zhou, 2014; Chen, Pan, Liu, and Connell, 2012)."

Jan 1, 2017

By Stanley J. Michalek

Good planning is critical to operating a safe and efficient mining operation. As the shallow coal scams become depleted, attention turns toward deeper mineable seams or remnant coal reserves. It is becoming more common to place a mine in a scam that is below or adjacent to existing mine workings. Significant research has been conducted regarding the ground control issues of multiple scam mining. However, safety aspects of the operation may not be fully considered. Safety issues include working below or adjacent to a flooded mine and controlling gas Communication between the active mine and the nearby mine. This paper will discuss the safety issues a mine operator should consider when working under or adjacent to mines that may be flooded with water.

Jan 1, 2000

By Larry Stolarczyk

The mining industry would benefit greatly by imaging geologic conditions well in advance of mining. In layered deposits such as coal, trona, quartz, and potash, natural waveguides form and enable electromagnetic seam waves to travel great distances. In the early 1980s, the Radio Imaging Method (RIM) was developed to capitalize on electromagnetic seam wave propagation in the assessments of seam conditions in longwall panels. The initial applications of RIM tomography and comparisons to in-mine mapping of geologic conditions proved that faults, paleochannels, and rapidly thinning coal could be detected and imaged with seam waves. This paper describes the application of electromagnetic seam waves in mapping the margins of a paleochannel crossing a longwall panel and the interesting possibility of adopting the newly developed Full Wave Inversion Code (FWIC) )Newman 1995) to significantly improve imaging resolution. When tomography images are combined with ground control science, a significant reduction in cost, roof fall potential, and waste rock in underground mining may be realized.

Jan 1, 1999

By Ken W. Mills

The ground movements associated with underground coal mining and, in particular, longwall mining, are recognised to include horizontal subsidence movements, but the mechanics of the processes that cause these horizontal movements are not well understood. Over the last two decades, three-dimensional subsidence monitoring has become routine in Australia and has provided a wealth of measurements of horizontal movements caused by mining subsidence. These measurements and other sub-surface observations allow the processes that cause mining-induced horizontal movements to be inferred and, subsequently, verified. In this paper, the mechanics of the processes that cause horizontal movements, particularly those in sloping topography, are described and discussed on the basis of field observations. There are several processes recognised to generate horizontal subsidence movements. In flat terrain, systematic horizontal movements cause the surface to move initially toward the newly created goaf and, subsequently, in the direction of mining. Tectonic energy stored as horizontal stress is released by mining, and, when the horizontal stresses are high, the magnitude of this horizontal stress relief movement is large enough to be perceptible for some kilometres from the panel. In sloping terrain, there is an additional component of horizontal movement that occurs in a down slope direction. This movement, sometimes referred to as valley closure movement, has a magnitude that is typically much greater than systematic or stress relief movements.

Jan 1, 2014

Tailgate pillar and tailgate-face corner humps have been a major threat to longwall coal mines where hump-prone conditions exist. The yield pillar design concept, in which the longwall chain pillars are designed in such way that they can yield in a nonviolent or controlled manner during longwall retreat, has been used in many longwall mines to prevent tailgate pillar humps. However, this practice often shifts the side abutment load onto the tailgate-face corner especially when a massive and strong sandstone member is present within the caving zone or in the vicinity of the coal seam. Therefore, the abutment pillar design concept has also been used in some hump-prone coal mines to minimize the stress concentration on the tailgate panel rib for the control of tailgate-race corner humps. The disadvantage of using the abutment pillar design concept is that it is more costly and puts additional pressure on mine operations for gate entry development and significantly reduces the recovery of coal reserve. In addition to pillar size, panel layouts. such as the panel width and panel orientation also play a critical role in pillar or face humps and should he taken into account. Based Upon the Cyprus mine design and operational experiences at Willow Creek and case studies at other bump-prone longwall nines in the U.S., the authors attempt to identify the critical factors causing coal mine bumps and to introduce some guidelines and criteria for the design of both yield and abutment pillar systems and panel layouts.

Jan 1, 1999

By Weijia Guo

With the exploitation of coal resources, a large area of land had been affected by surface subsidence, causing conflicts between man and land to become more conspicuous by the day, affecting the life of general public, and constraining further development of the land. This paper predicts the subsidence areas according to the 10-year mining plan of the Jibei mining district as an example. A plan of environmental management was developed based on sciences and practical economics.

Jan 1, 2006

By Tom Blevins, Roland Walker, Jinsheng Chen

"The Dywidag-Systems International DSI Tandem Anchor bolt, with an improved expansion shell-washer design, can substantially reduce potential entry roof falls caused by twisted or deformed expansion shells of the traditional mechanical anchor bolts during installation. In addition, the improved anchorage system can also ensure that a consistent and high installed load is achieved, which is critical for effective beam building under thinly laminated or layered roof conditions. This paper discusses the genesis and anchorage performance of the Tandem Anchor bolt, and analyzes the tension/torque relationship of different mechanical shell anchors using data obtained from both lab and field tests. It also discusses roof bolting mechanisms, roof bolting systems, and factors influencing effective beam building. Finally, numerical modeling was used to analyze the effect of bolt pretension magnitude on entry roof stability.INTRODUCTIONThroughout the history of underground mining, keeping the roof from uncontrollable collapse has been the greatest concern of the miner. Tremendous strides have been made in better understanding, predicting, and supporting the mine roof. This conference has spent the last twenty-eight years sharing information and distributing it to the mining community. A paraphrased statement from former Secretary of Defense Donald Rumsfeld adequately describes the challenges to the mining engineer as it pertains to roof control: it is a study of known knowns, known unknowns, and unknown unknowns. It is the hope of the authors to address some of these unknowns.The paper discusses the development of bolt anchorage systems from dry shells to shells with resin-assisted anchorage. It looks at some of the issues faced with the anchorage shells being used when introducing the insertion pressures caused by resin cartridges. The rationale behind the shell design changes was to compensate for this resin induced stresses.An improved expansion shell assembly (a slotted expansion shell base, a tapered cramming plug, and a novel support nut with tapered top) for mine roof bolts makes up the Tandem Anchor (U.S. Pat. No. 6742966); this minimizes the possibility of twisting seen with traditional mechanical shells and thus prevents springy bolts and shell leaves from breaking. A fully grouted Tandem Anchor bolting system has the advantages of both the pure mechanical point-anchor and the non-pretention fully grouted bolting systems. Once installed, the installed load or pretension will be locked in and never bleed off. Compared with the fully-grouted (2-speed resin) tension rebar bolting system, the fully-grouted Tandem Anchor bolting system can (a) reduce bolting cycle time (about 15 seconds per bolt), boosting coal production and productivity; (b) provide a greater installed load or pretention; and (c) achieve a consistent tension/torque ratio, which gives better bolting quality control. Therefore, a fully grouted Tandem Anchor bolting system is the most effective means to reinforce a weak rock mass intrinsically by the theory of beam building. For thinly laminated or layered roof conditions, a higher installed load or pretention (50% or more of the bolt's yield strength) can make a roof bolting system much stiffer, which results in less roof movement. Numerical modeling results show that the magnitude of the roof's tensile stress and relaxation zone can be substantially reduced under a higher installed load or pretention."

Jan 1, 2010