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By Ben Fahrman

The classical approach to engineering design involves the purely deterministic calculation of the factor of safety, the ratio of strength to stress. The factor of safety is an easy calculation to perform in many circumstances, and it is very easy to interpret the results. The simplicity of the method is both a strength and a weakness. The results of the deterministic factor of safety calculation are simple to determine and interpret, but they have no ability to account for the uncertainty present in the inputs or outputs. Uncertainty is prevalent in geotechnical engineering applications. This uncertainty can stem from spatial variability, poor sampling, a dynamically changing environment, etc. Probability analyses allow input parameters to take the form of a distribution rather than a single value, making them uniquely equipped to handle the uncertainty inherent in geotechnical engineering design considerations. A stochastic approach also has the benefit of providing results as a probability of failure, which is more meaningful than a factor of safety. Though stochastic modeling is becoming more common in many geotechnical applications, it is still not widely used for coal pillar design. The objectives of this study are to justify the concept of using a stochastic approach to coal pillar design and to compare the results of a simple stochastic analysis to those of standard industry practice. A simple synthetic study was performed to compare the results of a probabilistic analysis of coal pillar design to the results obtained from running ARMPS, which is published by NIOSH. Normal distributions were assigned to each input parameter that is required to determine the factor of safety of a single coal pillar. The probability of failure resulting from the synthetic study was found to be more conservative than the ARMPS stability factor. A probabilistic approach to coal pillar design is discussed in comparison to the traditional deterministic approach. The greatest strengths of the probabilistic approach were determined to be the effective handling of uncertainty and the increased meaning of the inputs and outputs inherent to a probabilistic approach.

Jan 1, 2013

By Christopher Mark

Coal bursts are typically associated with highly stressed coal. Most bursts occur during retreat mining (longwall mining or pillar recovery) in highly stressed locations like the tailgate corner of the longwall panel. Others are associated with multiple seam interactions. However, a small but significant percentage of coal bursts have occurred during development or in outby locations unaffected by active mining. Most development bursts have been relatively small, but some have been highly destructive. No theory of coal bursts can be complete if it does not account for this type of event. This paper focusses on the development mining coal burst experience in the US, putting it into the context of the entire US coal burst database. The first documented development coal burst occurred almost exactly 100 years ago during slope drivage at the Sunnyside Mine in Utah. Sunnyside subsequently had a long history of bursts, mainly during retreat mining but also during development. Several Colorado mines have also experienced multiple development bursts. Some, but by no means all, of the development bursts in these western US coalfields have been associated with faults. In the Central Appalachian coalfields, most development bursts have occurred in multiple seam situations. In some of these cases, however, there was no retreat mining in either seam. The paper closes with some lessons from this history, with implications for preventing such events in the future.

Jan 1, 2017

By Michael Gauna

In 2010, the number of fatal rib falls in U.S. underground coal mines exceeded fatal roof falls for the first time ever. While recent trends have seen roof fall fatalities reduced by about 62% since the 1990s (from an average of 9.6 per year in the 1990s to 3.6 per year during the last five years), the incidence of rib fall fatalities has remained approximately constant (about 1.5 per year). In addition, injury incidents associated with pillar rib failure have been steady at approximately 100 per year. The persistence of rib fall injuries, underlined by the recent increase in rib fall fatalities, has led to this investigation of the coal rib failure parameters and methods available to mitigate rib failure in U.S. underground coal mines A detailed study of the 21 fatal rib fall incidents that have occurred since 1995 (excluding coal burst incidents) indicates that the two most important risk factors are the mining height and the depth of cover. The depth of cover was 700 ft (213 m) or greater in 81% of the fatalities, and the mining height was at or exceeded 7 ft (2 m) in every instance but one. Other risk factors included multiple seam interactions, rock partings in the seam, and retreat mining. The accident history reveals that joints or slickenside features within the rib increase the hazard. Approximately threequarters of the rib fatality victims were roof bolting machine operators and continuous mining machine operators. This paper includes a brief overview of techniques for rib control; including bolts, standing support, bent steel straps, and rib strapping. The available mining equipment features for installing rib support and protecting operators from rib falls are also discussed. A proactive approach to rib control using available technologies will lower the risk to miners and is an endeavor that must be pursued to reduce rib fall accidents.

Jan 1, 2011

By R. Karl Zipf

Multiple seam mining interactions caused by full extraction mining, whether due to undermining or overmining, frequently involve tensile failure of the affected mine roof. The adverse ground control conditions may prevent mining for both safety and economic reasons. Prior researchers have identified the geometric, geologic and mining factors controlling multiple seam mining interactions. This numerical study examines the mechanics of these interactions using a modeling procedure that 1) incorporates the essential constitutive behavior of the rock such as strain- softening of the intact rock and shear and tensile failure along bedding planes and 2) captures the geologic variability of the rock especially the layering of weak and strong rocks and weak bedding planes. Specifically, the numerical study considered the effect of vertical stress, interburden thickness, and the immediate roof quality of the affected seam in both undermining and overmining situations. The models show that for overburden-to-interburden thickness (OBAB) ratios of less than 5, interactions do not occur, and that for OBAB more than 50, extreme interaction is a certainty. In between, the possibility of an interaction was found to depend on gob width-to-interburden thickness ratio, site specific geology and horizontal stress to rock strength ratio in addition to the OBIIB ratio. The models also showed that horizontal stress was profoundly altered well above or below a full extraction area and that these changes are likely to influence the success or failure of multiple seam mining. The role of horizontal stress in multiple seam mining interactions has received little attention in prior investigations. Four factors control the mechanics of multiple seam mining interactions: 1. vertical stress concentration 2. horizontal stress concentration 3. stress re-direction 4. bedding plane slip bands A combination of vertical and horizontal stress increase and high stress gradients in the vicinity of full extraction areas re-orients principal stresses into a very unfavorable direction. This seemingly small stress re-orientation has a profound adverse effect on bedded rock.

Jan 1, 2005

By Hanjie Chen

A mine panel collapse may occur when pillar sizes are too small or the surrounding rock strata (roof or floor) yields. After a small pillar fails, its loading is rapidly transferred to adjacent pillars that in turn fail. If the surrounding rock strata yield, the loading on pillars will redistribute. which may result in some pillars under high stress. This paper analyzes the causes of two cases of mine panels failures in two room and pillar mines. One case of massive pillar failure occurred in a phosphate mine and the other, mine panels failure, is in a coal mine. In the phosphate mine, both the roof and floor strata were a thick competent shale stratum. The finite element analysis shows that the major reason for the massive pillar failure is that the pillar sizes were too small in a panel. In addition, the high cliff topography has some influence on the failure. In the coal mine, the failed area was about 1,000 ft long by 760 ft wide (covering more than 3 panels) involving 390 pillars. The harrier pillars between the panels were punched on retreat into small chain pillars and the extraction ratio in this failed area was more than 80%. The roof remained intact, and pillars were crushed and punched into the floor. Based on the stress analysis, it is found that multiple panel collapse occurred because the barrier pillars were punched into small pillars and the loading on panel edges increased due to floor failure.

Jan 1, 1999

By Hideki Shimada

In underground coal mines, grouting method is employed in order to reinforce the floor and to control the movement of groundwater into the roadway. However, the seepage and the reinforcement behavior of the grouting material into the floor are not clearly understood. Therefore, grouting is today normally based on empirical relationships and on the experience of the grouting personnel. To assist grouting projects, it must be acknowledged that many of the flow and the hardening processes occurring during grouting so far defy mathematically analysis. From this point of view. this paper discusses the behavior of grouting material, when it is injected into the roadway floor, by means of 3-D Eulerian-Lagragian seepage analysis. Moreover, the effectiveness of reinforcement grouting on the mechanical properties of slaking-prone mine tunnel floor are studied by means of several laboratory tests. Keywords: Mine tunnel floor, Grout, Seepage, Reinforcement

Jan 1, 2002

By David Bigby, Tim Ross, David Conover

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

Jan 1, 2010

By Stephen C. Tadolini, John Bolton, Bre-Anne Sainsbury, Tom Meikle

"The capacity of ground support components which have been affected by corrosion is reduced and may ultimately lead to dynamic failure of the component and the strata. In order to maintain an effective, long-term ground support system, significant campaigns of rehabilitation are often required in corrosion affected areas which also expose the workers to hazardous conditions. The most common corrosion protection for steel ground support utilises sacrificial systems such as galvanising. Galvanising has previously been proven to be susceptible to some corrosion processes. Stainless steel is the most effective in resistance to corrosion, but can be cost prohibitive, and its mechanical properties often make it unsuited to use in ground support components.Providing an outer protective plastic coating to bolts has proven to be an effective means of protecting the inner steel bar from corrosion. However, these support systems tend to be susceptible to coating damage, and require post cement grouting to provide full encapsulation. In comparison to a standard bolt/resin system, they can be slow to install and expensive. These systems have also been shown to reduce overall load transfer performance of the bolting system.In order provide a higher level of corrosion protection whilst maintaining current installation practices and bolting cycle times, Minova has developed the Enduro™ Steel Ground Support Range. The Enduro™ range consists of standard Minova steel ground support components which have been treated with a unique coating process. The Enduro™ coating has been tested in the harshest of conditions, in laboratory controlled conditions and in underground trials. It has been proven to effectively resist or completely eliminate the formation of corrosion, even in the most aggressive environments. This paper explains the process and provides the details of the laboratory and underground corrosion performance testing carried out on Enduro™ ground support products.INTRODUCTIONTraditionally, the most common form of corrosion protection in steel ground support consists of a sacrificial protective coating such as galvanising (Aziz, et al., 2013). Such coatings have proven to be ineffective in extreme high and low pH conditions with corrosion of the support system commonly encountered. There is also evidence that common forms of galvanising may actually increase the rate of corrosion in certain pH environments. Figure 1 shows typical corrosion and failure of a standard re-bar roof bolt."

Jan 1, 2016

By Zach G. Agioutantis

The prediction of ground movements due to underground mining using the influence function method is a mature technology, widely used by researchers and planning engineers around the world. Surface strain, either tensile or compressive, is an important indicator of potential impacts on structures such as buildings, bodies of water, pipelines, railway and power lines, and tailings dams. Calculation of static or dynamic surface strains, for varying surface terrains and slopes, should be accurately predicted and be independent of the methodology applied for the calculation. This paper discusses and compares the development of horizontal and ground strains on the surface due to underground mining for different surface topologies (sloping terrain, monitoring lines, etc.) and presents the steps needed to ensure accurate calculations. Examples are given to demonstrate how ground strains can be calculated for one- and two-dimensional gridded surfaces.

Jan 1, 2013

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 Adam Mackenzie, Dan Payne, Yass-Marie Rutty

"Historically, the Crinum mine has experienced significant falls of ground when longwall production slowed to prepare for recovery in weak roof areas. These conditions continue through the recovery process and result in both safety concerns and delays. When mine plans and exploration revealed that most of the mine's future longwall recoveries were located in weak roof areas, a decision was made to try pre-driven recovery roads as a solution to the problem. After completing eight pre-driven recovery roads with varying degrees of success and numerous lessons, the Crinum North mine now utilizes a modified Pre-driven Recovery Roadway (PDRR) to improve the longwall take-off process in weak roof areas. During mine development, a standard roadway is driven where the final recovery location of each longwall is planned. After the installation of secondary support, the PDRR is backfilled with a cement-flyash mix to provide support to the roof, and confinement to the ribs and floor of the roadway. The method has been refined over the last four years to provide greater strata stability and improved operational and safety performance compared with conventional take-offs at Crinum, and has resulted in a site record of longwall relocation in 11.5 days (pull mesh to picks in coal). This paper describes the evolution of the PDRRs from Crinum East to Crinum North, including lessons from initial attempts and changes to the secondary support regime, the operational approach during the final stages of retreat, the backfill strategy and plans for the future.INTRODUCTIONThe Crinum mine is the underground component of BHP Billiton Mitsubishi Alliance's (BMA) Gregory Crinum mine, located northeast of Emerald, Queensland (Figure 1).The Crinum mine has a history of roof control problems coming into and during longwall recoveries. Weak roof, combined with the slow mining, which is a consequence of preparing for equipment recovery, pulling mesh (which also creates tip-to-face issues), and the lengthy bolt-up process, often results in loss of immediate roof and subsequent roof falls. In fact, approximately 50% of the first 13 longwall recoveries (which were at depths of 180 to 200m) experienced roof fall delays, the longest of which took four months to mine the last pillar and recover the longwall. This recurring problem prompted the mine to investigate options to reduce or eliminate the problem."

Jan 1, 2016

By Hai Rong

EH-4 Electrical conductivity imaging system has been playing an important role in prospecting, looking for water resource, and determining coal mine gob boundary by virtue of its significant advantages such as collection of large volume of data, light weight, fast analysis, high-precision, and low cost. However, it has little application in determining the overburden failure zone and development of water flowing fractured zone and bed separation space in complex extra thick coal seam conditions. In order to confirm the overburden failure range and evolution of water flowing fractured zone and bed separations in complex extra thick coal seam conditions the panel 63005 of Laohutai coal mine in China was selected for study in this paper,. EH-4 magnetotelluric method was used to determine the overburden failure zones and bed separations in the overburden. Theoretical analysis and numerical simulation were employed to verify the EH4 survey results. The results show that the failure height in the overburden of the three coal splits was 120m to 170m before panel mining, and increased to 150m to 220m after panel mining. The overburden failure height was on average 7.4 times the equivalent mining height in complex extra thick coal seam condition of Laohutai coal mine when the sand backfilling and fully mechanized top coal caving methods were employed. After mining of panel 63005 started, bed separations were found on the hanging wall and footwall of the F18 fault where water bodies existed. The results obtained in this research not only provide a theoretical guidance for safe mining at Laohutai coal mine but also serve as a reference for predicting and preventing water inrush accidents for coal mines under similar condition in China.

Jan 1, 2014

By Timothy Batchler

"Standing roof supports help to provide a stable working environment in coal mine gateroads. NIOSH participates in the development of these support systems by conducting full-scale tests in their Mine Roof Simulator. These tests have established the performance characteristics of every coal mine support system currently in use in the United States. Through these tests, mine operators and regulatory officials have acquired a fundamental understanding of the various types of support performance that guides the assessment and use of these support systems. However, some little known factors that influence support behavior can lead to misconceptions and, perhaps, a poor assessment of support behavior at times. This paper highlights several of these factors and discusses their significance. The paper also examines a current longwall tailgate support and discusses the inefficiency with this approach. Finally, the NIOSH Mine Roof Simulator (MRS) is the world’s most powerful load frame built specifically to test full-scale roof supports, including longwall shields. are addressed in this paper to provide a more in-depth assessment of the limitations of the MRS and what the MRS testing protocol is designed to do. INTRODUCTION Over the years, considerable research has been conducted to develop standing support technologies and to evaluate their performance characteristics. The load and displacement characteristics of the standing supports have been determined from full-scale testing conducted using the National Institute for Occupational Safety and Health (NIOSH) Mine Roof Simulator (MRS) (Barczak and Tadolini, 2008) located in Bruceton, PA. Performance traits, installation patterns, and ground responses are observed in various geological and mining conditions and are evaluated to determine the support performance in the field (Dolinar, 2010; Campoli, 2015; Zhang et al., 2012). Despite widespread use of standing supports, not every aspect of their performance and resulting ground support capability are always well understood. This paper discusses 10 factors, several of which are surprising, that impact standing support performance."

Jan 1, 2017

By Winton Gale

"The aim of this paper is to summarise the results and conclusions of Australian Coal Association Research Project (ACARP) Report Cl3013 which relate to water inflows into a mine which occur through the overburden above and adjacent to longwall panels. The study assessed available data of inflows into underground coal mines and utilised computer simulation of water flow through fracture networks. The study concluded that flow into mines is typically via an interconnected network of pre-existing and mining induced fractures. The height above the coal seam that mining induced fractures extend is typically related to the width of the panel. However the potential for those fractures to form a connected network which can facilitate flow, is related to the amount of subsidence and the depth of mining. The study compares model simulations with measured data and provides guidelines to estimate the average hydraulic conductivity of the overburden above extracted longwall panels in Australia.INTRODUCTIONWater inflow into coal mines has been a design issue for many years. Guidelines as to the potential for water inflow have been developed in many countries based on local experience and the form of mining being undertaken. In most instances, the guidelines relate to inflows which would endanger underground personnel and operations.In more recent times, water inflow criteria for mines has been widened to include lesser inflows which may not impact on mine safety or operations, but have the potential to reduce water flow within streams and surface aquifers. For the purpose of this paper the larger inflows which are readily measured in a mine or may impact on mining safety are defined as mine inflow and the lesser inflow relating to aquifer water loss as environmental inflow.The water losses referred to are those which enter the mine from the surface or from near surface aquifers.The empirical relationships developed as mine design guidelines or regulations typically relate to high flow (mine inflow) situations and in this paper, these have been reviewed as to their application to environmental inflow. In many cases environmental inflows were unlikely to have been noted or recorded in many mining operations in the past."

Jan 1, 2010

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

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

Jan 1, 2015

By D. Newman

The objective of this is paper is to review the current state of knowledge and practice in highwall mining (HWM). HWM has become a standard method in surface mining, commonly used alone or in conjunction with contour or slot mining. It provides 800-feet to 1,200-feet of additional recovery when the economic stripping ratio is reached in contour mining or in slot mining when surface access to a reserve is limited. A significant attribute of the highwall miner is its versatility. HWM has been used successfully to mine abandoned pre-reclamation law highwalls, points or ridges uneconomic to mine by underground or other surface methods, outcrop barriers left adjacent to underground mines, separate benches of the same seam where the parting thickness or quality differences between benches render complete extraction uneconomic, previously augered areas containing otherwise inaccessible additional reserves and close or widely spaced multiple seams. The theory and design methods to assess roof; pillar, and floor stability are presented followed by three case histories. Simple design charts for sizing HWM web and barrier pillars are also presented. A recommended web pillar width may be obtained from the design charts given the overburden depth, the HWM cut width, and the mining height. Given the depth and panel width for a set of HWM cuts, another set of charts gives a suggested barrier pillar width. The case histories, hm Northern and Southern Appalachia are used to illustrate the application of rock mechanics to quantify the stability of the highwall, roof, web pillars, and floor. The case histories involve 1) mining through a previously augered highwall, 2) mining under back-stacked spoil and 3) selective mining of closely spaced benches of the same seam. Because each site is unique, the appropriate pre-mining geotechnical analyses range from the calculation of roof, web pillar, and floor bearing capacity stability factors to detailed numerical modeling of the auger and underground mine workings. When operating in the vicinity of existing underground mine or auger workings, the determination of ground deformation and strains resulting from highwall mining is a necessary facet of a ground control investigation.

Jan 1, 2005

By Joe Zelanko

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

Jan 1, 2011

By Alexandra Garcia

A measurement-based ground control management system, commonly employed in coal mines, has been successfully implemented in a UK potash mine operating at a depth of over 800 m. This system includes the routine deployment of telltales in roadways throughout the mine to mitigate against the risk of large-scale roof falls and the deployment of detailed ?design monitoring? instrumentation for ongoing design verification. The routine monitoring systems used in coal mines have not been, to date, commonly applied in potash mines, despite the similar, variable weak geology and associated ground control hazards. Monitoring information from the site indicates that telltales can be deployed to monitor roof conditions, with action levels based on differential rates of strain, as opposed to absolute movement.

Jan 1, 2014

By John Kucish

Roof bolting in the Pittsburgh Coalbed takes many forms today: coupled partially grouted bolts, fully grouted rebar, passive cable bolts, tensioned cable bolts, and torque tension rebar. The Federal No. 2 Mine of Peabody Energy has recently made a significant advance in longwall headgate primary roof support, with the change fiom an 8 ft long coupled partially grouted bolt to a 6 ft long torque tension system. The coupled partially grouted bolt consists of a 4 ft long 0.804 in diameter rolled deformation bar thread coupled to a smooth 718 in headed bar. The coupled partially grouted bolt is anchored into a 1 318 in borehole with 4 ft of high speed resin. The 718 in diameter smooth bar is tensioned by rotation into the treaded coupler, upon setting of the resin, compressing the immediate roof. The change was initially driven by occasional failure of the coupled partially grouted bolt at the threaded coupler, due to lateral movement caused by horizontal stress. A fully grouted % in diameter torque tension bolt, in a 1 in borehole, was proposed as a cost effective primary bolting for the high horizontal stress conditions. The upper 2 112 ft of the rebar is anchored in fast set, medium viscosity resin and the bottom 3 K ft is anchored in slow set, special low insertion force resin. The lower end of the rebar is threaded with a nut that permits the counter clockwise spinning of the bolt to mix the resin, at maximum rotational torque. Upon setting of the high speed top resin, the nut is spun clockwise to tighten the nut against a metal dome plate. The plate is forced against the roof to compress the immediate roof, A complete headgate entry has been successfully supported with the torque tension system. The mining of the adjacent longwall panel was completed in September 2004. The torque tension system provided a 25% savings over the previously employed coupled partially grouted bolt system.

Jan 1, 2005

By Shuangsuo Yang

Coal ribs can roughly be divided into three types: (1) roof and floor rocks are similar to ribs, (2) roof and floor rocks are stronger than ribs, and (3) roof and floor rocks are weaker than ribs. Different types of roof/floor and ribs will exhibit different types of ground pressure around an entry. Most coal mines belong to the 2" type with ribs fractured or even broken. In this case the mine ground pressure depends on the occurrence and development of deformation and fracture of the ribs. If it's not properly controlled, the following deteriorating process will follow: the roof bent downward > the ribs squeezed and broken > rib sloughing and collapse > rib support to the roof reduced > the roof bent downward further > the ribs broken further. In this paper rib instability is divided into four types: compression/shear sliding rib fall, gravity sliding rib fall, cross arch rib fall, and cleavage rib fall. Since failure in thin walls is largely of tension type whereas that of thick walls by compression or shear, a concept is proposed here to utilize a thick bolted rib to stabilize entry ribs, i.e. the thickness to height ratio of the thick bolted ribs

Jan 1, 2005