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By Thomas Barczak

A new generation of hydraulic mine support prestressing devices has been developed. These thin-walled steel shells are machine-welded and can be inflated with water or any liquid to provide prestressing for a wide variety of roof support products ranging from Can supports to props and cable bolts. With an expansion capability of several inches, depending on the geometry and size of the unit, they can also establish roof contact eliminating the need for wooden wedges or crib blocks that are commonly employed with several standing roof support products. Capable of withstanding pressures from 1,000 to 6,000 psi, these cells are able to transfer roof loading into the support structure without rupturing. A recent development has been the addition of an inexpensive yield valve that provides control of the maximum pressure and load development on the support. This paper examines the performance capabilities of these inflatable prestressing units and the impact they have on the performance of various support systems, including an evaluation of the overall stiffness of the support system and the load control during yielding of the prestressing unit. Recommendations are made regarding the design requirements of the prestressing unit to optimize the support performance. A summary of mine applications using this technology is provided with general comments regarding their impact on ground control.

Jan 1, 2004

By Keith A. Heasley

Pillar recovery in deep coal mines with competent roof and floor can concentrate stresses and generate hazardous bumps. Actual calculation of the geologic strain energy released in association with these coal bumps has been rare, although energy release values have been utilized successfully for indicating burst potential in numerous hard- rock mines. This paper advances the current knowledge of energy release calculations as applied to coal bumps through an extensive analysis of an actual bump occurrence. The U.S.B.M, displacement- discontinuity code, MULSIM/NL, is used to produce a numerical model which generates stress, displacements and energy values associated with a pillar retreat section. This model is calibrated with actual field data, and then dissipated energy values from the model are examined as to their suit- ability for indicating bump potential. Ultimately, it is determined that prudent application of dissipated energy calculations can be used as a tool to investigate the coal bump potential of a mining plan or cut sequence.

Jan 1, 1990

By M. Bouteldja

A finite element modelling approach is developed for the evaluation of the mechanical performance of cable bolts supports in underground mine structures. The modelling approach permits the simulation of any arbitrary number or pattern of cable bolts, that can be either partially or fully grouted, with or without end plates. The modelling technique used treats each cable bolt as a set of bar elements representing the steel cable. The bond stiffness characteristics of the grout bolt interface are represented by shear springs uniformly distributed along the cable length. Coupled with a simple linear elastic analysis, this technique can serve as a dedicated numerical model for cable bolted excavations. The application of the model is illustrated with two case studies: one for a hard rock mine and another for a coal mine.

Jan 1, 1999

By Robert A. Stansbury

The No. 14 Mine of United States Steel Corp- oration is located at Munson, West Virginia, in &Dowell County. The mine was originally assigned 7,530 acres of Pocahontas 3, 4 and 5 Seams. Production began in 1948 with a listed reserve of 64,500,000 in place tons. It was originally intended to superimpose the 4 Seam workings with the 3 Seam and mine the two simultaneously to minimize the effects of one seam on the other. Roof control problems and economic conditions led to the discontinuance of 4 Seam workings in 1958 and again in 1980, after reopening in 1972. With room and pillar mining continuing in the 3 Seam, the majority of the 4 Seam reserves have been undermined. The mine began operation using track mounted conventional equipment. Off-track conventional equipment was next used, and in 1957 continuous miners were introduced. Since this tie, continuous miners on room and pillar work have been exclusively used with the exception of a brief shortwall trial in the 4 Seam in 1975. By mid 1982, 3 Seam mining will have been completed, leaving an estimated 42 million in place tons of 4 and 5 Seam coal as the focus of mining efforts. Plans for the subsequent reopening of the 4 and 5 Seam workings have been scheduled for June of 1981. With the resumption of mining, management is faced with solving the roof support problems caused by both the inherent physical conditions and undermining.

Jan 1, 1981

By John L. Ellenberger

The National Institute for Occupational Safety and Health (NIOSH) has continued the research role of the former U.S. Bureau of Mines to develop techniques that will reduce the hazards in the mining work place associated with coal bumps. Current research focuses on both analyzing historical seismic data from bump-prone operations and utilizing a mine-wide seismic network to investigate the exact strata failure mechanics associated with bump-prone geology. The anticipated outcome of this research will be reduced bump incidences through advanced engineering concepts and designs which implement the new understanding of strata behavior. The analysis of the historic seismic data consists of correlating observed mining seismicity with the geologic and geometric parameters at the sites. The primary seismic parameters are the timing, location and magnitude of a recorded seismic event. These parameters are correlated with such mining parameters as: the overburden, the size of the immediate gob, the size of the district gob area, etc. This detailed analysis of historical seismic data has provided an informative quantifiable relationship between many of the specific mining parameters and the induced seismicity. The second aspect of the coal bump research is the instrumentation of an appropriate field site to determine the main roof, floor, and gob behavior associated with hump behavior. The chosen field site is a deep-cover longwall mine with competent geology in a historically bump-prone area. The primary field instrumentation is a three-dimensional, full-waveform, seismic array with both surface and underground sensors surrounding an active multi-panel district. The purpose of this seismic array is to determine the timing, the exact location, and the mechanism (tensile fracture, bedding plane slip. etc.) of the failure of the strata surrounding the active and multi¬panel gobs. The preliminary results presented in this paper help to define the strata failure areas around the longwall panel.

Jan 1, 2000

By Kazem Oraee-Mirzamani

Blasting operations generate seismic effects in underground mines. These effects apply additional dynamic loads on the support system, which should bear both static and dynamic loads. Static loads are caused by the weight of the superincumbent strata, while dynamic loads occur as a result of blasting in the mining area. Identification of the origin and determination of the support system behavior in natural frequencies is crucial in assessing the stability of underground mines. This is because resonance occurs when a support is vibrated with its natural frequencies, which can cause a vibration with the maximum amplitude and subsequently cause extreme deformations. The mechanism of support system deformation during dynamic load displacement has been studied and numerical simulation for the impact of the dynamic loads on stability of supports is carried out using finite element method. The paper introduces a simple technique for improving stability and safety of mining operations. Results obtained and the methodology adopted in this research can help mining design engineers make decisions on adequate support for active mining operations.

Jan 1, 2011

By Andrew Starkey

Ground anchorage systems in the form of rock bolts are used extensively throughout the world as supporting devices for civil engineering structures such as bridges and tunnels. It is widely recognized that non-destructive testing methods for ground anchorages need to be developed as a high priority [1 ], with only a small proportion of anchorages currently being monitored in service [2]. The condition monitoring of anchorages is a relatively new area of research, with the objective being a wholly automated or semi-automated condition monitoring system capable of repeatable and accurate diagnosis of faults and anchorage post tension levels. The GRANIT tm system developed by the University of Aberdeen is capable of measuring the integrity of ground anchorages and has won two major UK Awards - the National John Logic Baird Award for Innovation in 1997 and a Design Council Millennium Product Award in 1999. The normal procedure with the GRANIT tm system [3,4] is to train a neural network on dynamic response data that has been sampled from an anchorage over a range of post-tension levels. Further data is needed in order to test the neural network. The paper presents neural networks that have been trained on data taken from a number of anchorages on different sites, and the results of the diagnosis of these neural networks on new test data is presented. The results show that the cross-diagnosis of anchorages is possible, where a neural network trained on one anchorage is capable of successfully diagnosing the post-tension level for an anchorage of similar construction, and the high level of accuracy that can be attained with the GRANIT tm technique is shown. The paper also introduces a mathematical model of the GRANIT tm process.

Jan 1, 2001

By Klaus Opolony

The German coal industry operates a multi-seam extraction system, with average working depths currently around 1,000 meters. The gate roads serving the longwalls are often used for a second time - a practice which has the advantage of allowing several panels to be extracted with a minimum of roadway drivage work. This preference for re-using gate roads is based on a number of factors: such an arrangement a) prevents the strata pressures in the subsequent panels from being adversely affected by the presence of support pillars, b) helps achieve the target of full mineral extraction and c) allows the roadways to be used for air conditioning and ventilation purposes as part of an advancing longwall system. This paper explains the planning procedure required for a rectangular rockbolted roadway, using as an example a gate road in the 3 meter-thick D2/C seam at Lippe colliery, and describes the subsequent operating experience. Here special attention is focused on modem procedures and techniques for mine roadway planning. The investigation also examines the part played by the mine ventilation system and the available machine technology, which are particularly important parameters for the design of the T-junction areas and support system. Keywords: multiple seam extraction, longwall mining, roadway drivage, roof bolting, roadside pack system, hydraulic frame type support.

Jan 1, 2002

By D. M. Shu

A two-dimensional finite element modelling technique is applied to predict the subsidence movements on a sloping ground surface above a completely mined panel. The modelling is carried out for the surface both directly and using a combination of an equivalent horizontal surface and a rays projection method and the result compared. The rays projection method provides the subsidence components on a sloping surface from the corresponding ones on an assumed equivalent horizontal surface through the point of mean elevation of the relevant part of the sloping surface. This method is used next to compare the subsidences, horizontal displacements and horizontal strains from the mining of a given panel at the surface for different inclinations. The results show that the magnitudes of the subsidence components are greater on the dip side of the slope from the point of the mean elevation, than the corresponding ones on the rise side and the asymmetry increases with the inclination of the surface. The findings are also compared with those of other research workers.

Jan 1, 1992

By Leo L. Van Sambeek

Specification of properly sized pillars to support the overburden is one of the most challenging aspects of salt mine design. Interest in salt pillar design is heightened by the recent flooding of the Retsof salt mine, water inflows to the K2 potash mine, and closing of the Belle Isle salt mine because of ground control problems. A series of numerical modeling analyses show that a simple pillar design equation for salt pillars adequately reproduces results of extensive modeling efforts. The paper describes (1) the basis for the pillar design equation, (2) equation verification by comparison with numerical modeling results, and (3) equation validation by comparison with measurements and observations of salt pillar behavior.

Jan 1, 1997

By John P. Dunford

Ground support continues to be the most important safety consideration in underground mining. The Bureau of Mines has developed a yielding steel post with a capacity of 45 tons and b c1osure allowance of 24 inches in a 6 to 7 foot seam height. The post mechanism in a three piece unit consisting of a top and bottom leg made from standard steel pipe and a separate foot bracket. It develops its load carrying ability when the lower pipe, with an attached interference ring, is forced into the larger top leg. The walls of the top leg are deformed radially and along the axis. Yield continues until the unit bottoms out. Other advantages of the post is that due to its simplicity it can be made at the mine shop level and its cost should be about one-hundred dollars. Prototype units were tested in the laboratory. Based on these results, modifications were made that would make the post perform better as a mine support. Two field tests were conducted. The first test used the units as supplemental support for the wood crib rows in a longwall tailgate. The second and larger field teat had 84 posts placed on salvaged track rails. They were installed on two foot centers in the middle of the entry. Overall the supports functioned well during use in the headgate entry, but problems with the quality of some of the pipe caused localized failures. However, these problems can be attributed to one manufacturer of pipe and through tighter specifications can be eliminated. The use of the posts with the rails is showing additional benefits as a ground support system. By providing continuous support between the posts a beam is created that has much more load bearing. The additional support can reduce floor heave and in some cases eliminate the need for the rib side crib line.

Jan 1, 1986

By Kot Unrug

Fully grouted tensioned bolts provide a desirable compressive of laminated roof and bondage along the entire bolt length, at the same time the entire surface of the hole is protected against penetration of moisture. This is particularly important in shale strata, where, when left unprotected by a partial grout column, moisture penetrates to the annular space, and weathers rock around the bolt hole, having an easy access to the bedding planes which are strongly hydrophilic. The most desirable are thrust tensioned rebar bolts that can be installed with one procedure step, and can secure a desired pre-stressing. Such bolts have a forged conventional head and offer no hazard to miners in low clearance mines, whose number increase constantly due to the depletion of reserves in thicker seams. The presented field-testing program has been based on instrumented bolts allowing for monitoring of tension at installation and at a chosen time interval after. The tested system is based on rebar bolts installed with two speed Fosroc resin (fast at the top and stow at the bottom), and mechanical compression by the bolting machine at installation. Combination of several different washer -- plate arrangements have been tested. The results are reported with their interpretation, and recommendations directly towards strata control requirements, as well as the operational considerations for efficiency of installation.

Jan 1, 2002

By Al Campoli

The majority of the 100 million roof bolts installed each year in the United States are 5/8 inch deformed rebar fully grouted into a one inch borehole. Commonly referred to as B-Series or No. 5 bar, this system is proven effective and cost efficient in coal mines with average to above average roof conditions. One of the critical variables affecting resin bolt performance is the size of the resin annulus, calculated as the difference between the hole and bolt diameters. The optimum annulus has been proven to be '/4 inch, which provides for effective mixing of the two polyester resin components and incorporation of the plastic film packaging within the cured resin. Glove fingering occurs when the plastic film is forced against the borehole wall potentially interfering with the mechanical interlock of the resin and the rock mass. Glove fingering is not a problem with the optimum '/n inch annulus but has been know to occur with the B-Series 3/8 inch annulus. Laboratory testing correlated the degree of Glove fingering with resin formulation and the bolt head to shaft axes alignment. Glove fingering occurred over 45 % of the grouted length of standard B¬Series bolts. The newly formulated Eclipse B-Series resin when combined with a 1/8 inch offset between the bolt head and shaft axes reduced the glove fingering 70%. The Eclipse system allows the 3/8 inch B-Series annulus to achieve the positive mixing characteristics associated with the / inch optimal annulus systems.

Jan 1, 2002

By Winton Gale

"Rock and coal fractures and micro seismic vibration are common occurrences during development mining. It is very uncommon for coal and rock to be propelled into the roadway during normal mining operations. However, such occurrences do occur and appear to require significantly more energy than is available from strain energy release during coal cutting. The sources of energy, which can contribute to the propulsion of coal from the face or ribs, are typically strain energy from the surrounding ground, seismic energy from a rapid rupture of the ground in the vicinity, or rapid expansion of gas from within the burst source area.The aim of this paper is to briefly review the bursts that could be related to strain energy or seismic energy. However, the greatest emphasis is placed on the effect that gas within the coal could play in moderate to gassy mines.It has been found that the bursts related to the expansion of gas can occur in coal and stone. The volume of gas involved in coal bursts is typically lower than in gas outbursts; however, the process is generally similar.INTRODUCTIONA coal burst is defined as a rapid expulsion of coal (and potentially gas) from the boundary of the roadway. The volume of a burst can be variable, but volumes above 10-50m3 are noted and cause significant disruption to operations.Rock and coal fracturing is common about roadways, particularly under the influence of elevated stresses, either tectonic or related to mining abutment stress. Shear fracture is very common; however, it is very uncommon that the coal is propelled from the face or ribs as a result of fracture of the material. The nature of rock fracture about roadways under elevated stress is presented in Figure 1.A burst requires energy to propel the coal or rock from the rib or face of the roadway. The depth of material impacted may vary from 0.5m to greater than 3m inside the roadway boundary. The strength of the ground in this zone is variable depending on the nature of the coal seam (or surrounding material). Typically, this material has significant residual strength as confinement increases into the ribside.The energy required to propel this material is typically in addition to that which is associated with normal fracturing about the roadway."

Jan 1, 2018

By Kot F. v. Unrug

The depletion of thicker coal reserves has resulted in the thinner seams being economically attractive. Surface protection against subsidence limits the extraction ratio to 50% for the room and pillar method as a "no subsidence condition". That rule was developed for pillars with smaller width/height ratio because seams mined at the time were commonly 6 feet thick. Over time pillar ribs deteriorate around the pillar perimeter and thus decrease the area of the pillar. Using 50% extraction as the no subsidence condition in the squat pillar category (below 42 in with w/h > 7) causes unnecessary losses of reserves because squat pillars are stronger. In this paper a method is presented for design of long-term pillars. Based on the analysis of pillar deterioration and field observations, an increase of the "no subsidence" extraction rate to 60% for squat pillars is proposed.

Jan 1, 2001

By John Cassie

The condition of roadways in the Deep Main seam at Asfordby mine, operated by RJB Mining (UK) Ltd, is strongly dependant on drivage direction relative to the maximum horizontal stress. The mine is laid out with gate roads driven approximately parallel to the maximum horizontal stress giving favourable drivage conditions. This has led to particulary difficult conditions for face installation headings which have to be driven to an increased width and at a high angle to the horizontal stress. A solution to the formation of face headings is described making use of stress relief with the face heading being driven in the zone of reduced stress surrounding a previous heading. In implementing this solution use was made of results from computer modelling and detailed monitoring of roadway behaviour. High reinforcement densities including cable bolts were used in supporting the headings. The experience provides an example of a traditional technique being applied more scientifically. The results obtained demonstrate the effect of stress relief and reinforcement methods in altering drivage conditions and suggest possible alternative strategies for the formation of future face headings in these conditions.

Jan 1, 1997

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 Keith Heasley, Christopher Newman, Zach Agioutantis

"Previously, in 2013, a laboratory version of the computer code ARMPS-LAM, introduced at the 32""d International Conference of Ground Control in Mining, successfully integrated the laminated overburden model of LaModel into an ARMPS-style pillar stability analysis. The ARMPS-LAM analysis has now been fully integrated into the ARMPS program allowing users to more accurately classify the stability of a given mine design utilizing a laminated overburden model for the determination of pillar loading within theAMZ. ,This paper introduces the new Windows-based ARMPS-LAM program. The program's ease of use, output result generation, and limitations are highlighted through the parametric analysis of three hypothetical case studies where traditional ARMPS output is compared to output generated by LaModel. As mining operations continue to produce at greater depths and in more complex geometric and geological conditions, flexible and efficient analyses of mine stability for underground room and pillar mining has become more and more essential. The ARMPS-LAM program provides the mining industry with an empirically backed, numerical design tool for the evaluation of underground room and pillar mine plans.INTRODUCTIONIn the Southern Appalachian coal fields of the United States, retreat room-and-pillar methods have allowed mining operations to maintain high rates of production in the face of adverse geologic conditions and shrinking coal reserves. Over the years, research has been dedicated to understanding and mitigating mining-related dangers due to second mining practices, such as: pillar squeezes, floor heave, roof falls, pillar bumps, etc. However, as mining operations continue at deeper depths and with more complex geometric and geologic conditions, there is an inherent industry need for more accurate, flexible, and faster evaluations of stability for underground room-and-pillar mining."

Jan 1, 2016

By Yoginder Paul Chugh, Jason E. Tinsley

"Over the last five years, the authors have compiled a database of Short Encapsulation Pull Test (SEPT) evaluations from Interior Basin (Basin) coal mines. Typically, SEPT data are analyzed to obtain peak load carrying capacity per unit length of resin encapsulation designated as the ""Grip Factor,"" or GF. In a previous paper, ""An Analysis of Short Encapsulation Bolt Pull Test (SEPT) Data From Interior Basin Coal Mines,"" authors analyzed tangent value of the load-displacement curve as Anchorage Stiffness at 50% (AS-50) of the peak load, or GF value, to assess reinforcement potential of an installed bolt system. This paper attempts to correlate the secant AS VALUES (of SEPT on 6 ft (1.8 m) long installed bolts at a lower load of about four tons from several different mines in the Basin with their MSHA-approved primary support requirements. Since the current regulatory environment requires plan changes dynamically to ensure that roof falls are minimized, the authors hypothesize that the practiced roof control plan must meet the minimum requirement of adequately supporting immediate roof strata. The analyses are based on 181 SEPT studies performed by Minova professionals. Analyses indicate a macrolevel correlation for both roof support density and roof support cost that may have the potential to be used by mine operators to compare their performance with other mines in the area. Limitations of the analyses include ignoring mining depth, mining height and required secondary supports as part of the requirements for a roof control plan. The authors recommend additional analyses to develop this concept further to develop guidelines that can be confidently used by coal companies.INTRODUCTIONPartially- and fully-grouted bolts are extensively used in coal mines for roof support as part of MSHA-approved roof control plans. Since grouted bolts have a larger area of contact with the rock, they can develop much greater anchorage capacity than conventional expansion shell (point-anchored) bolts. Load transfer characteristics and interaction mechanics between steel rebar, grout and rock mass around the bolt hole for grouted bolts are generally assessed in the laboratory or field though short-encapsulation pull tests (SEPT), which have been extensively researched by Aziz and Jalalifar (2005), Aziz (2004a), Serbousek and Signer (1987), Signer (1990), and Mark et al. (2002), among others. In a SEPT, about 12 in. (30 cm) of bolt length is typically encapsulated in resin and a pull test is performed to collect load-displacement data prior to failure and, in some cases, post-failure. A typical load-displacement data set for a bolt is shown in Figure 1. SEPT characteristics are designated by peak load carrying capacity, or grip factor, which is the peak load carrying capacity per unit length of resin encapsulation. SEPT is routinely used to assess anchorage performance characteristics of different lithologies overlying a coal seam to identify type of bolt, bolt diameter, length and spacing across and along an excavation. Anchorage stiffness, or AS, at any load may also be calculated as the tangent or secant value of the slope of the load displacement curve at a particular load level. Anchorage Stiffness characteristics of bolts from SEPT data have typically not been considered in any depth until recently (Chugh et al., 2014; Chugh et al., 2015)."

Jan 1, 2016

By Robert Kimutis, Ke Yang, Yi Luo, Jianwei Cheng

"Surface subsidence processes, especially those associated with a longwall mining operations, could cause operational and/or safety problems to railroads. For railroads with normal amounts of traffic, both the limited time window between traffic and the limited space available make the efforts to return the railroad back to its original state nearly impossible. In order to ensure the safe operation of a subsiding railway, the partial lifting method has been developed and applied. This paper describes the principle and application of the method. A case of applying this method to mitigate subsidence effects on a long section of railway has been demonstrated.INTRODUCTIONMaintaining an operational railway as it is experiencing a longwall subsidence process is not an easy task. The subsidence process may induce sufficient strain and curvature to cause problems to the railroad structures. Localized subsidence-induced slope often is significantly larger than the permissible ruling gradient of 0.7% for a locomotive to haul a loaded train reliably. In order to ensure safe operation, it would be ideal to instantly lift the railroad track back to its original level as it is subsiding. However, it is often impractical to do so due to the limited time windows available between railway traffic and the available space on the railroad base.A partial lifting method has been developed to keep the railroad operational under such limitations. The subsiding railroad track is raised by only a determined, partial amount of the occurred subsidence at a given time. By making such a determined adjustment, the railroad track can be maintained on a smooth and operational profile, and the high strain and curvature on the railroad structures can be reduced as well. To define a smooth railroad track profile in a subsided section the following two conditions should be met: (1) the maximum railroad gradient should be less than the permissible ruling gradient, and (2) the recommended amount of adjustment should be able to be made by the repair crew within the available time windows between scheduled railway traffic.The subsidence effects on a long section of railroad over two longwall panels have been successfully mitigated using the partial lifting method. The railroad experienced a maximum subsidence 89 of about 5 .0 ft (1.5 m) with the average subsidence along the entire length being about 3.45 ft (1.05 m). However, the average adjustment due to lifting that was made along the entire length of the subsidence-influenced railway was only 0.5 ft (152 mm)."

Jan 1, 2010