Search Documents

By Stephen C. Tadolini, Anand Bhagwat

"Intrinsic supports installed in mines with limited seam or mining heights have always proven to be difficult when the required support length is longer. To date, three solutions have been used to ensure that the correct length bolt is installed to achieve the required anchorage or to maintain immediate roof material: combination bolts, milled-notch bolts, and hot-notch bolts. The combination bolts use threaded sections and couples, which can be screwed together to form the required length. The primary bolt of choice, to minimize costs and achieve the desired support, has been milled- or hot-notch rebar bolts. These notches can be placed at strategic (required) locations along the bolt axis, so the bolt can be successfully bent and straightened during the installation process. However, these notches can reduce the strength of the bolting system; special reductions are detailed in ASTM F432-13 (2013). Technical advances in steel rolling and forged heading processes have led to the development of a deformed rebar bolt termed, the BORE bolt, which stands for Bendable Oval REbar. The BORE bolt can be bent at any location and straightened during the bolt installation cycle. This support exceeds the minimum designed capacities described in ASTM F432-13 and meets the yield and tensile strength requirements of a full diameter no. 6 rebar bolt. The profile and shape also reduces the resin annulus during the installation process to enhance mixing and shreds the cartridge film to help eliminate glove fingering.INTRODUCTIONThe height or depth of competent strata material, in the immediate mine roof, can be greater than the developed mine openings heights. This is especially true in coal mine openings where 50% of the minable seams in the US are less than 50 inches (132 cm). When this scenario occurs, it can be difficult to insert steel bolts in a solid single length. Figure 1 illustrates what difficulties can be experienced when a roof bolter operator is working and supporting a thin seam coal mine. There are three viable options currently available. Lengths of the bolt can be threaded and a steel coupler can be used between the sections to reach the required depth of anchorage. These systems, called “coupled bolts,” usually require a larger diameter hole to facilitate the larger coupler diameter and can be anchored with an expansion shell, resin, or a combination of both. To ensure that the coupler will develop the tensile capacity of the bolting system, required by ASTM 432-13 Standard Specifications for Roof and Rock Bolts and Accessories (2013), the threaded bars should be completely rotated and engaged into the coupler. Premature failures can occur when the threads “strip out” if not inserted completely."

Jan 1, 2018

By Denise Müller, Axel Preusse

"Due to multilateral underground use, mining subsidence is discussed more and more with regard to issues other than mining, such as post-mining topics, storage, or geothermal use. These discussions and the influence of the energy policy is why the situation in Germany is described as a transition from active to abandoned mining. Many PhD theses have been written within the last 20 years—every single one with a different topic but all with the same issue—the effects and challenges of mining subsidence.These challenges start with the research regarding geological and hydrogeological conditions, which is the basis for calculations like the angle of draw or predictions of subsidence due to water table drawdown. Furthermore, the research investigates issues like risk-management and financial planning. One of these issues is post-mining calculations; others include potential hazards of storage or the use of geothermal energy.This paper outlines an overview of German mining and the consequences due to subsidence. It deals with an overview of the circumstances related to hard coal mining, lignite mining, the use of geothermal energy, gas, and opportunities regarding underground storage. Basically, it describes the essential steps of processing mining subsidence, starting with monitoring to forecast and up to safety measures.With a glance back to the applied research of the prior two decades, we compare given forecasts with experiences already gained.Even though mining decreases, ground control does not.INTRODUCTIONAfter the Second World War, an economic recovery started in Germany, during which the mining industry provided an advantage. Areas like the Ruhr District or Saarland were characterized by hard coal mining. In addition, there was lignite mining in the Rhenish mining area and the middle German coal district. In the Lausitz area, there were gravel and sand companies for the construction industry and many other medium-sized mining companies. Especially in the Ruhr District and in Saarland, where large coal fields were built, mining played a major role. The number of mines increased, and the negative effects on the terrain got bigger. While minor open pits or quarries have influences on the landscape only, large open pits and underground mining lead to significant ground movements. This results in a larger importance of mining subsidence engineering. Calculations of subsidence and forecasts have been developed and refined within the last several years. Due to the fact that the mining areas were and still are densely populated, the forecasts are getting more and more precise."

Jan 1, 2018

By Murali Gadde

Design of underground excavations in coal mines involves two major tasks: estimation of stresses and determination of rock mass strength. Owing to the complicated nature of rock mass, progress in its strength determination has lagged significantly compared to advancements in stress analysis techniques. Also, because of the impracticality of conducting field-scale strength tests, experimental approaches have very limited applicability in estimating rock mass strength. As a result, many empirical rock failure criteria have been developed based on observed performance of in situ rock structures. Unfortunately, all the popular rock mass strength criteria are based on data from large-scale ?isotropic? non-coal structures and thus can not be directly applied to coal mines. In this paper, an attempt has been made to adopt the popular Hoek-Brown (H-B) criterion for use with coal measure rocks. First, the H-B criterion is examined against laboratory triaxial strength data for different types of coal measure rocks. Then, procedures to estimate the H-B rock mass strength parameters relevant to coal measure rocks are suggested. Finally, a case-study has been given to show the usefulness of the proposed strength estimation approach.

Jan 1, 2007

By Rafal Misa

In order to prevent the formation of mining damage or to reduce its reach, it is advisable to look for technical solutions which would enable effective protection of construction sites from the impacts of underground mining. So far, other authors have mainly focused on developing methods of securing construction sites in order to minimize the damage resulting from underground mining; the issue of geotechnical methods for protecting buildings against mining damage has not been thoroughly covered in those articles. It should be noted that there are relatively few publications directly related to this issue, nor are there practical examples of geotechnical protection. In this paper, a geotechnical solution available for securing building structures is discussed. Trenches presented in this research reduce the size and range of mining deformation. The results of numerical modelling for sample protective solutions, aimed at reducing the impact of mining activity on the terrain surface under various conditions, are presented. The calculations were performed with the aid of the Abaqus 6.10-2 software, based on the Finite Element Method.

Jan 1, 2014

By Thomas Barczak

A cooperative study was conducted with Emerald Coal Resources, L.P., an affiliate of Foundation Coal Corporation, and the National Institute for Occupational Safety and Health (NIOSH) to evaluate the effectiveness of (partially) pre-driven longwall recovery rooms supported with pumpable roof supports. This paper evaluates the load transfer mechanics associated with the advancement of the longwall face into the pre-driven recovery room and the widening of the room to recover the longwall shields. The results show that the yielding of the panel fender produced uncontrollable convergence that caused yielding of the shields and stiff pumpable roof supports when the face was approximately 10 ft from the recovery room. The shields continued to yield until the face advanced into the recovery room. Convergence in the pre-driven recovery room typically reached 6-8 in once the pumpable supports yielded and shed load through a declining residual load capacity prior to being cut out by the longwall shearer. Roof deformations occurred beyond the 32-ft horizon with total deformation ranging from 2 to 4 in, indicating that standing support was necessary to help control the span as the longwall face advanced into the pre-driven recovery room. Stability improved as the pumpable supports were cut out, thereby reducing the span between the shields and the outby pillars. In the end, the shields were successfully recovered under stable ground conditions. However, a disconcerting discovery was the load shedding of the outby pillars as the recovery room was mined into. It is postulated that this may be due to the progression of the rear ?abutment? moving toward the recovery room as the panel fender and standing support in the recovery room yield and shed load. This behavior was not expected with a relatively narrow room that is fully supported with standing support. If this mechanism is indeed occurring, the width of the room and performance of the standing support becomes even more critical. Premature failure of the support can lead to excessive convergence on the shield line that cannot be controlled even with modern shields, resulting in roof instabilities that can lead to catastrophic results.

Jan 1, 2007

By Stephen C. Tadolini

The U.S. Bureau of Mines (USBM) is conducting research to provide alternatives for traditional secondary support methods. These cost-saving methods are proving to afford safer installation, improve entry stability, eradicate material-handling injuries associated with crib supports, and enhance ventilation. In cooperation with Mountain Coal Company, the USBM replaced timber cribbing with high- strength cable supports to provide stability in a gate road that was used for two longwall panels. The support design consisted of 2.1-m (7-ft) full-column resin-grouted bolts and 4.8-m- (16-ft-) long high-strength resin-grouted cable supports. Three support concepts were evaluated: passive, stiff, and tensioned systems. Cable loading and roof deformations were monitored to evaluate the behavior of the immediate and main roofs during panel extraction. The stress and loading histories for both panels and the abutment and yield pillars were monitored to assist in evaluating the stress transfer and pillar performance in conjunction with the roof behavior. The test results indicated that resin- grouted cable support systems successfully maintained the immediate and main roofs during the extraction of two longwall panels. This paper describes the theory, application, and advantages of resin-grouted cable supports and presents the mine measurements made to assess the cable performance during the retreat process of longwall mining.

Jan 1, 1994

By Stephen C. Tadolini

PC-Strand tendons, commonly referred to as cable bolts, have become a major rock reinforcement device for underground mining and civil applications. The most common configuration is a 1+6 strand, which designates a center king-wire, with six outer wires wound around it to form a 0.6-in (15-mm) diameter cable. The capacity of this tendon approaches 30 tons, but the performance is largely dependent on the anchorage medium and the ability of the system to transfer stresses back into the rock mass while reinforcing and suspending the immediate roof. Recent developments to improve the transfer mechanisms of the PCstrand via the anchorage medium by impregnating the individual wires with uniquely designed deformations have resulted in improved anchorage and subsequent cable bolt performance. This paper describes a series of laboratory tests used to develop the initial indentation patterns and numerical modeling simulations that were undertaken to refine and optimize the final designs. The new, indented PC-Strands, offered in both bright and properly galvanized, have been tested in the field and are being used effectively in two new cable bolt products. The results of the laboratory, computer modeling, field testing and in-situ installations provide a glance at the next generation of cable bolts. This suite of testing shows how they will improve the safety and serviceability of openings under adverse ground conditions.

Jan 1, 2012

By David F. Gearhart

The Burrell Can1 is a thin, steel, tubular shell filled with aerated concrete that is used as a roof support in coal mines. The Can height is always shorter than the mining entry, so it is capped with wooden timbers and wedges when it is installed. This combination of the Can and the wooden header is a standing support system that is designed to yield in a controlled manner and exhibit a constant yield, or elastic-plastic behavior. The Burrell Can is capable of withstanding 20 in or more of vertical convergence and 15 in or more of horizontal displacement. Because of this exceptional ability to sustain load through extended convergence and displacement, it is one of the most robust standing supports in use today. The National Institute for Occupational Safety and Health (NIOSH) conducted testing on seventeen Burrell Cans in 1995, during development of the support where the ratio of the height to the diameter, or the aspect ratio, was between 2:1 and 4:1. Based on the results of that testing, it was suggested that the standard aspect ratio of the Can be less than 5:1. This suggestion is intended to preserve the overall stability of the Can, which will help to maintain the initial stiffness and prevent load shedding. Recently, the necessity of the aspect ratio has been questioned, and this issue will be addressed in this paper. Since the initial development and introduction, more than 130 tests of the Can have been conducted at the NIOSH Mine Roof Simulator (MRS), and 25 of those specimens had aspect ratios greater than 5:1. The 18-in-diameter by 10-ft-tall specimens had the largest aspect ratio, 6.67:1.However, the aspect ratio is not the only parameter that affects the performance characteristics of the Can. Multiple factors, such as the use of different wood species, the use of different header or footer configurations, and the use of pedestals, as well as biaxial loading conditions can also decrease the stiffness and capacity of the Can. Data from full-scale tests that include these other factors are analyzed to show that the initial stiffness is primarily affected by the roof and floor contact configurations. Furthermore, load shedding is more likely to occur or begin earlier when the aspect ratio is greater than 5:1 and one or more of the other factors are present.

Jan 1, 2012

By Andre Zingano

The objective of this paper is to compare the behaviors of different types of roof bolts: (1) One-Step Bolt (OSB), (2) Fully grouted Bolt (RB), (3) Combination Bolt (CB), and (4) Torque tension Bolt (TB). Two dimensional numerical models were built for both bolted and unbolted roofs. The geological sequence and rock properties used in the numerical models represent the typical roof condition for Pittsburgh coal seam. Using the unbolted roof as a base case, a comparison is conducted by monitoring the roof vertical displacement at the center of the entry and at the bolt positions, the axial force in the roof bolts, and stress distribution in the bolted area. The grout/rock interface is assumed to be perfect. The estimated roof displacements showed that the differences among the four types of roof bolts are small. The major differences are in the shear stress at the grout/rock contact and in the shear stress along the immediate roof bedding interface as well as in the bolt axial stress. There is less axial stress for the OSB compared to RB and CB as the diameter of the OSB is much bigger and the contact area between grout and rock is twice as large as for RB and CB. The higher strength of the OSB contributes to keep the bedding planes in the roof closed.

Jan 1, 2007

Recent improvements are described in several techniques for ground control in underground mining. Promising methods for analysis and mitigation of rockbursts are discussed. Methods of assessing damage to underground excavations from repetitive seismic loading are reviewed. Analysis of mining-induced surface subsidence is discussed.

Jan 1, 1989

By Peter Zhang

The immediate roof at the test site consisted of laminated clay shale that is likely to be susceptible to weathering. With seasonal dry-wet cycles in the mine ventilation air, the roof in the intake air would deteriorate and lose integrity over time. In many instances, the weathering-prone roof would gradually scale off piece by piece between bolts and over the entry corners. In some long-term entries, roof weathering could develop up to 2-3 ft deep with columns of bolted roof hanging under the upper intact roof. If wire mesh is used, the roof bags down and reduces the entry height, and the weight of the bagged roof tends to pull the bolts out and poses a safety hazard. An effective measure is always sought to support this type of weak roof that is sensitive to moisture degradation. One solution is to seal the roof shortly after entry development to prevent moisture penetration so that roof integrity can be maintained for the long term. In this application, a polymer-based sealing material was tested in the Rockspring mine for weathering protection and reinforcement. The material is a one-component polymer powder for spray application onto the roof and rib. The sealant material is mixed with water in the spraying nozzle to apply to the roof and rib as a paste. The product sets within 5-10 minutes and forms a membrane as thin as about 1/8-in. The membrane will quickly develop tensile and bonding strength within minutes and completely cure within a few days. Through sealing, bonding and lateral reinforcement, the integrity of the weak roof can be maintained. The material has been tried in a long-term evaluation over several months in a set of main entries in the Rockspring mine owned and operated by Rockspring Development, Inc., affiliate of Foundation Coal. Preliminary evaluation has shown that the material is an effective and economical solution to ensuring the long-term stability of weathering-prone roof.

Jan 1, 2009

By David R. Hanson

Seismic tomographic imaging, based on the same principles as a medical CAT Scan (Computer-Aided-Tomography). has been used for many years in the oil industry for large-scale subsurface stratigraphic characterization. and has most recently been applied to various structure- and stress-related problems in the coal industry. New developments in signal processing have greatly enhanced the speed, resolution, and range of applications of topographic imaging in underground settings, and recent innovations in data acquisition hardware have further increased data reliability and repeatability. Whereas past structure mapping applications relied on Seismic velocity and/or attenuation tomopraphy within an enclosed seismic source gram receiver array, recent developments in "True Reflective Tomography- (TRT TM) have expanded the application of this technology considerably. For example, in-seam TRT TM seismic reflection tray now he used to image structure, and/or old workings from one general location in the mine face areas of mains and panel developments) well ahead of planned developments largely eliminating the need to probe-hole drill on regular intervals. This new reflective technology bas also been combined with traditional cross-hole seismic tomography-providing a comprehensive, powerful tool for velocity attenuation. and reflective imagine utilizing the same source/receiver array data sets (borehole-to-borehole. borehole-to-entry and entry-to-entry). These new developments in data acquisition and image processing have greatly expanded the variety of applications for tomographic site characterization allowing the mine operator considerable flexibility to cost-effectively characterize previously inaccessible areas of the property. This paper presents examples of 2-D and 3-D .seismic topographic imaging applied to (1) the location of old works in Appalachian room-and-pillar operations and (2) the identification of anomalous zones ahead of mining/tunneling developments these applications demonstrate the state-of-the-art in seismic ground imaging using velocity, attenuation. aril reflection techniques land combinations thereon, and further illustrate the flexibility of data acquisition from a variety of tinning arid tunneling settings. Recent examples are presented from projects in the United States and Europe.

Jan 1, 2000

By Takashi Sasaoka

A great deal of coal will be left under the final end-walls in Chinese surface coal mines because of lower slope angle in shoveltruck mining systems and larger mining depths. In traditional surface mining systems, the coal under the end-walls will be buried by the inner dumping site, and these coal resources are generally abandoned. Considering these situations, the application of end-wall mining systems was considered in order to recover the residual coal around the end-wall. This mining system can improve economic benefits by exploiting haulage and ventilation roadways from end-walls and utilizing the existing transportation systems. In order to develop the appropriate pillar design methodology for the end-wall mining system, several empirical formulas and methods were discussed based on the formulas developed so far and the mechanics properties of coal and rocks in the Chinese coal mine. From the results of a series of theoretical and numerical analyses, it can be concluded that the coal pillar width calculated by the Mark- Bieniawski formula may serve as an important reference for pillar design in end-wall mining systems, and the end-wall mining system can increase the coal recovery from residual coal resources around highwall and reduce the effect of underground mining operation on slope stability

Jan 1, 2013

By Alan D. Smith

[A standardized checklist in a questionnaire format, was established to investigate selected factors associated with coal nips roof falls, as derived from an intensive review of the literature. A plot study was undertaken to aspect and explore possible statistical relationships that may exist with these parameters. the Purposes of toe study were to: 1. Study in a systematic and empirical fashion selected parameters that may be capable of differentiating characteristics of roof falls; 2. Obtain some predictive indices cased on selected physical and management related variables so that predictions of future problems associated with mine roof falls can be completed, prior to its actual occurrence. Accomplishment of these goals should result in valuable guides to the mining engineer in scheduling exploration work and 4 determining economic feasibility of certain ground control and .lining pro and practices. The parameters associated with mine roof failure that where included in the questionnaire are: working section, county, spar., orientation degrees), pillar dimensions width, length), seam thickness, roof fall dimensions (length, width, surface area, volume), shape of fall (arch, dome, laminar), thickness of thinnest immediate layer, lithology of the first four i.e.-diate roof layers, thickness of h e first four immediate roof layers, type of initial support systems before failure, type of resupport system after failure, length and spacing of bolts in use before failure, length and spacing of bolts planned after allure, presence of .cater, time failure after It coal excavation, distance t3rest ace, physical operation during, time of coo failure, location of the roof fall entry failure occurred ence of sloughing of ribs before failure, and presence of floor heave condition before failure. The typical roof fall had an average roof spar of 20.3 feet, -06 feet depth o coal seam, occurred 136 feet from nearest face, coal seam thickness of inches, p--ar :width off 45 feet surface area D' roof fall 1151 ft2, mainly lamin- shaped, previously rat fore fall between good to very good, little floor heave presence, usually cracks before the fall about 40 percent of reported cases, shales in most of the immediate layers, and usually located in an entry or intersection. in addition, multiple linear regression techniques were employed in the testing process of 18 major research hypotheses in a search for relationships among the various parameters studied of which were found to be significant, once corrected for multiple comparisons.]

Jan 1, 1984

By Michael Amick

Due to the importance of maintaining open airways on a longwall panel, mine management decided to install cribs made of foamed cement. Foamed cement had been used at other company operations on several projects. A recent test at a sister operation had proven the crib installation technique and established that foamed cement cribs could provide better support than wooden cribs. The area to be supported was in a remote location; the limited material needs of foamed cement cribs offered a substantial advantage. In mid February 1993, 75 foamed cement cribs were installed on the tailgate of the active longwall panel. The 75 cribs resisted both horizontal and vertical stresses while successfully supporting the entry.

Jan 1, 1993

By Gabriel S. Esterhuizen

"Cable bolts are sometimes used in low-seam coal mines to provide support in difficult ground conditions. This paper describes cable bolting solutions at two low-seam coal mines in similar ground conditions. Both mines used support systems incorporating cable bolts as part of the primary support system. Two original cable bolt based support systems as well as two modified systems are evaluated to estimate their ability to prevent large roof falls. One of the support systems incorporated passive cable bolts, while the other used pre-tensioned cable bolts. The results and experience at the mines showed that the modified systems provided improved stability over the original support systems. The presence of the cable bolts is the most important contribution to stability against large roof falls, rather than the details of the support pattern. It was also found that a heavy steel channel can improve the safety of the system because of the ‘sling’ action it provides. Additionally, the analysis showed that fully-grouted rebar bolts load much earlier than the cable bolts, and pre-tensioning of the cable bolts can result in a more uniform distribution of loading in the roof.INTRODUCTIONCable bolting is sometimes used as primary support in coal mines experiencing difficult roof conditions. In low-seam mines the flexibility of the cable bolts allows greater length supports to be installed near the advancing face without the use of couplers. When used as primary support, the cables are typically installed in the same row as fully grouted bolts, replacing two or more of the bolts in each support row. A heavy steel channel may be used as a strap to spread the support load over a greater portion of the roof. Historically, the Mine Safety and Health Administration (MSHA) has not allowed widespread use of partially grouted un-tensioned bolts (e.g., passive cable bolts) for primary support; however, pretensioned cable bolts have been accepted.Various solutions using cable bolts as primary support were attempted at two low-seam coal mines in Western Pennsylvania that were experiencing difficult roof conditions. Both mines originally used fully grouted rebar bolts as primary support and cable bolts as supplementary support. It was found that when a large roof fall occurred, the cable bolts may be contained within the dome of fallen rock. As problematic roof conditions continued to exist, both mines decided to use cable bolts as part of the primary support system. The cable bolts were located near the ribs of the entry, to increase the likelihood that they would be anchored outside the dome of potentially unstable roof. The cable and rebar bolts were installed on a heavy steel channel that acts as a ‘sling’ to distribute the load across the width of the entry. At the first mine, Mine A, pre-tensioned cable bolts were used while at Mine B, un-tensioned cables were used. The two support systems consist of essentially the same support components installed in different patterns and with varying degrees of pre-tension."

Jan 1, 2015

By Scott A. Shapkoff

High strength cable strand supports are predominately used as a supplemental means of ground control in longwall and development operations in the U.S. The advantages of cable strand supports have been documented and include flexibility, strength, and ease of installation. Since the majority of cable supports installed in the U.S. today are partially grouted, and this same segment of supports has no inherent installed tension, the supports are considered currently arbitrarily strictly supplemental and therefore, cannot currently be deemed primary support (in most mining districts). This paper details and discusses two distinctly different configurations and the technical advantages of active partially grouted cable supports for certain primary support applications. The evolution of the designs for both systems is highlighted. Field data illustrates the amount of active load in relationship to installed torque and the affects that component variables have on this load measurement. Roof control patterns using active cable supports are examined as alternatives for combination patterns using primary and supplemental systems. The importance of maintaining consistent installation procedures plays an important role in the level of performance of the system. Economics comparing the installation of active cables with other primary/supplemental patterns is also included.

Jan 1, 2006

By Charles Steed

The design of pillars in room and pillar operations is always a compromise between factor of safety and ore recovery. In actively mined and accessed areas the stability of pillars must be sufficient to support the roof, providing safe working conditions for men and equipment, while optimizing ore recovery. As a reserve is mined out, and if conditions are favourable, pillars are often removed to increase recovery. Considerations for pillar removal include value of the ore, the effect on immediate and overall mine stability and the safety of the mining operations as well as the consequence of disruption of overlying strata and surface infrastructure as the support of the pillars are removed. Many parameters influence pillar strength and the contribution of the pillar to ground support. These include seam height, seam depth, seam inclination, room span, quality of rock, time, etc.. Empirical, analytical and numerical modeling methods are commonly used to optimize pillar sizing and placement and predict influence of pillar extraction or reduction on ground stability. Practical examples for pillar design and optimization in low seam gypsum mining operations and optimizing sequencing of pillar extraction in thick seam metal mining operations are outlined.

Jan 1, 2003

By Ted Klemetti

Roof deterioration in weak moisture-sensitive rock leads to roof falls in coal mines. An observation site was set up in intake air at an underground coal mine in central Ohio to evaluate the time-dependent deterioration of moisture-sensitive roof rock. By periodically collecting and weighing the rock falling from the roof, the National Institute for Occupational Safety and Health (NIOSH) was able to quantify the deterioration of the moisture-sensitive shale roof. The measurements showed that more roof rock is falling in the dry winter months. Movements in an intersection were monitored in order to determine if deformation correlated with seasonal changes in temperature and humidity. The rate of intersection roof-to-floor convergence was found to be cyclical and related to seasonal changes, with more convergence during the months of April through September. At the study site intersection, the roof beam did not show deflection indicating movement most likely due to mine floor deformation. The bolt load also increased in the warmer months before shedding load, possibly due to rock failure. Temperature and relative humidity sensors, placed in holes in the roof show that mine air does not mix with the air in the holes. Consistent saturation levels of relative humidity were measured in the holes and it appears that high levels of moisture may have migrated into the bolt holes from roof shales. A roof sealant applied to a portion of the test roof appears to be preserving the roof and reducing the amount of rock fall into the entry, while the screened area virtually eliminated rock falling from the roof.

Jan 1, 2009

By John D. VandeKraats

Yielding rock bolt support systems have been developed to accommodate ground movement in shifting ground such as in coal operations; in creeping ground such as salt, trona, and potash; and in swelling ground associated with some clays. These systems, designed to remain intact despite ground movement, should enhance mine safety and help contain costs in areas where rebolting of rigid non-yielding systems is typically required. Four such systems were tested in straight tensile pulls in the laboratory. They include the Slip Nut System from Dywidag Systems International USA, Inc., Ischebeck's bolt mounted Titan Load Indicator, Rocky Mountain Bolt Company's Yielding Cable Bolt, and a rock bolt installed variation of the yielding steel post developed by RE/SPEC Inc. The first two systems are currently marketed products and the latter two are prototype systems. Each system responds to load and displacement by yielding in a unique manner. All are designed to yield at predetermined loads. A description of each system and its yield function is provided. Each system was tested over its prescribed yield range in a test machine. At least five tests were performed on each system. Each system yielded and continued to provide support according to its design. Each shows promise for ground control use in shifting or creeping rock. This work helps to illustrate the comparative differences in performance between these specialized systems and the applications where they may be most useful.

Jan 1, 1996