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3-D finite element modeling was performed to study the effect of bedding plane sliding and separation and tensioned bolt in layered root It was found that the major influence zones of a tensioned bolt are restricted to the neighborhood of the anchor points; that bedding plane sliding and separation releases the vertical stress in the roof above entry but greatly increases the horizontal stress within the first layer of the roof, and that there is a triangular zone between the two adjacent bolts at and near the roofline level that is beyond the influence zone of the bolts which is liable to fail if conditions exist. Based on the results, three possible modes of roof failure are proposed for the design of tensioned bolting.

Jan 1, 2001

By Murali Gadde

U.S. coal mines? primary roof supports typically consist of passive resin bolts; however, the use of active bolt systems is increasing. Despite this widespread use, a comparative performance evaluation of these bolt systems has not been done under similar geologic conditions. The performances of three commonly used primary bolt systems were investigated: fully grouted passive rebar, fully grouted tension rebar (using two speed setting resin), and resin-assisted rebar with mechanical anchors. In order to investigate the performance of these bolts, strain gauged bolts of all three types were installed at three different coal mines as part of a larger research project funded by the National Institute for Occupational Safety and Health (NIOSH). This paper addresses the initial installation bolt loads measured at these mines. Analysis of the data showed much lower installed loads on active bolts than is traditionally assumed. It appears that the active load on bolts bleeds off relatively quickly under geologic and operational conditions similar to those at the studied mines. The data also showed that within a short time span after installation, the measured loads on active and passive bolts did not differ significantly. This paper compares the results from over 20 bolts of each bolt type that were all 1.8-m (6-ft) -long, 20-mm (0.804-in) nominal diameter Grade 75 rebar and discusses the relevance and shortcomings of the data and bolt performance.

Jan 1, 2011

By John D. VandeKraats

Yielding rock bolt systems have been developed to provide ground support while accommodating shifting, creeping, or swelling ground movements. The Dywidag Systems, Inc. (DSI) Slip Nut System is one such system. It has been tested in the laboratory and installed and monitored in the interbedded halite and anhydrite at the Waste Isolation Pilot Plant (WIPP) since 1994. The Slip Nut System is installed on a point anchored threaded bar or on a threaded bar coupled to a cable bolt. At a pre-engineered load the nut acts as a die and cuts through the thread on which it rests, thereby reducing the load and coming to rest on the next thread where the process repeats. Two types of laboratory tests were performed using slip nuts on DSI's threaded bars and their Tensionable Cable bolts. Straight pull tests exceeding 0.3m (1 ft.) of displacement were performed on the systems. Also, offset tests were performed on these bolt systems in a test frame designed to simulate WIPP geologic conditions. A room scale test area consisting of 186 threaded bars began in 1994. Select bolts were instrumented to track bolt load. All bolts were tracked through time for operation. Through three complete years since installation over twelve slips, corresponding to approximately 0.15m (6 in.) of ground movement, have been observed in some areas. Since installation strata offset has exceeded 0.08m (3 in.) at a supported clay layer in the roof. No bolt failures have been observed. Slip loads observed underground generally approximate those observed in the lab. To date, the slip nut systems tested and installed have performed as expected.

Jan 1, 1998

By N. P. Kripakov

This paper presents a brief overview of current bump mechanics theories and pillar design methodologies, and relates these concepts to experiences at two mines located in a north-central Utah coalfield where different pillar designs were used to control mountain bumps. Experiences gained at the first mine demonstrated the successful implementation of a two-entry, 9.8 m (32 ft)-wide, yield-pillar design. A U.S. Bureau of Mines field study quantified the timing of chain pillar yielding and resulting load transfer from the gateroad. In-mine pillar response, although apparently sensitive to site-specific conditions, compared favorably to estimates derived using two yield-pillar design methods. A second study conducted at another mine located in the same district, but subjected to different geologic conditions, documents the unsuccessful attempts to employ progressively narrower three-entry pillar designs based on successes achieved at the first mine site. This second mine never achieved a true yield-pillar design. Use of pillar widths ranging between 9.1 m (30 ft) and 27.4 m (90 ft) always resulted in violent bumps in the tailgate pillars. The pillars were either too large to yield, or too small to support peak operational loads. Hypothetical gateroad systems were evaluated using both analytical and empirical approaches for configurations comprised of two rows of conventional pillars, and systems incorporating both yield and abutment pillars. Analysis concluded that yield pillars less than 6.1 m (20 ft) wide and abutment pillars ranging between 30.5 m (100 ft) and 39.6 m (130 ft) square would be required to achieve a stable gateroad design. However, results of a field study conducted on a two-entry, 36.6 m (120 ft)-wide abutment pillar concluded that abutment pressures from the first panel overrode the pillar, and that a still larger pillar may be required to preclude bumps in the tailgate during second panel mining. Without the benefit of a demonstrated in-mine success, it is not clear which design would ensure elimination of gateroad bumps.

Jan 1, 1992

By John C. Stankus

ldentifcation of potential roof problems in coal mines has long been a complex issue due to the wide variety of mining and geological conditions. It is well known that mine roof falls are related to many different factors such as roof strata properties, sandstone channels, regional and localized horizontal stress, vertical stress, and tectonic stress. Many times, a combination of these factors causes roof problems. Through years of ground control experiences, Keystone Mining Services LLC, (KMS) has developed a Roof Instability Rating (TUR) system to predict potential roof problems. The first RIR analysis was presented at the 19th Ground Control Conference (Watts, et al., 2000). The RIR system includes: quantifying each individual factor, weighting each factor, combining individual factors, finalizing the RIR, and setting the criteria for the areas where special support is needed. In this paper, the application of the Roof Instability Rating (RIR) system at Enlow Fork mine will be presented.

Jan 1, 2001

By M. Karmis

mining factors and representative ground movement parameters. Based on this analysis, the previously developed prediction methods were modified and tested for their applicability in the Appalachian coalfield. Finally, in order to facilitate the implementation of the prediction methods, as well as the data processing of the monitoring program, a number of computer software were developed and will be discussed in the paper. Surface deformations above underground mines and their prediction and control have been the topics of an extensive research effort by Virginia Polytechnic Institute and State University over the past few years. During the initial stages of this research, a number of case studies were collected from the literature, the mining industry and various government and state agencies and, based on the analysis of this information, appropriate prediction methods were proposed (Goodman, 1980; Webb, 1982; Hasenfus, 1984; Karmis et al., 1984). Although the prediction methods were based on sound concepts, it became evident that their accuracy and implementation were a reflection of the size and quality of the collected case studies. In this respect, the data bank was rather insufficient and posed certain questions regarding the development of subsidence information due to inappropriate length and spacing of monuments used in subsidence surveys, density and accuracy of such surveys, lack of monitoring horizontal movements, etc. In an effort to alleviate these problems, a comprehensive monitoring program was initiated in the coal mines of southwest Virginia, which included: - Two longwall and five room and pillar mines. - In total, seven longwall and sixteen room and pillar panels. - Monitoring lines in excess of 35,000 feet. - Vertical as well as horizontal measurements. A high accuracy digital computer tacheometer was used in all surveys and the type and spacing of all monuments were in accordance with well accepted guidelines. The data from this monitoring program completed the initially collected data bank and, thus, enabled a much needed statistical analysis between

Jan 1, 1987

By Binchuan Zhang

The combined reinforcement system (CRS) combines the strengths of roof' bolting, resin-anchored cable bolt, and chemical grouting to effectively control the deformation of the strata surrounding an extremely poor entry. It is easy to implement and fast in installation. It is one of the systems that are suitable for the reinforcement and maintenance of extremely poor entries.

Jan 1, 2001

By William Bookshar

The development of increasingly more sophisticated and powerful retreat longwall equipment has led to ever wider and longer panels. To maximize gateroad development efficiency, provide for effective face recovery, andlor reduce ventilation pressures, entries are cut across longwall panels in advance of longwall retreat mining. These entries must be supported to prevent falls of immediate roof in advance of the face. Recently, cut through entries and predriven recovery room entries have been supported with various combinations of roof bolts and passive support. The cost and relative success of these support systems has varied considerably. Eighty-Four Mining Company, Fosroc, and Sub-Technical have developed and implemented an efficient, yielding, multiple component, cement crib to support longwall panel cut through entries and pre-driven recovery rooms. The cutable crib consists of an inexpensive flyash cement pillar poured in a bag, within a temporary metal form. The form is moved to a subsequent cutable crib location after the initial hardening of the flyash pillar. The remaining entry height between the mine roof and the top of the flyash cement pillar is filled with a specially designed bag filled with Fosroc's Tekset cement formulation. The diameter of the pillar and thickness of the Tekset bag are designed to accommodate the anticipated front abutment loading and entry convergence.

Jan 1, 1998

By Graham Daws

State of the Art Room and Pillar mining techniques have been employed at two coal mines in the UK in the last two years. This paper describes the design of pillars and rockbolted support systems for Room and Pillar mining operations in these collieres. Within the last year, rock mass classification systems have been used in the design process of a room and pillar mining layout at one of these collieries. In particular, the roofbolt support system was detailed using these techniques and the support system was linked with operational aspects such as depth of cut. The depth of cut was initially estimated using rock mass classification systems and then verified by a stand up trial which was monitored remotely. The stand up time has a direct influence on the operational aspects of the mine and this is described. The design was verified by performance monitoring and the results are discussed. The importance of understanding the role of rockbolting is emphasised along with some examples of accurately determining in-situ bolt performance. The paper also recommends some modifications to rock mass classification systems when applied to underground coal mining operations.

Jan 1, 1998

By Erik C. Westman

The objective of this Bureau of Mines study is to determine the structural integrity and stress state of coal structure using seismic techniques. Bureau personnel conducted seismic transmission and refraction surveys on yield pillars and seismic tomography on rigid pillars. The pillars studied were in the headgate at increasing distances from the working face of a longwall coal mine, effectively evaluating pillars at lowering stress states and increasing levels of structural integrity. By examining characteristics of the seismic wave, including velocity and frequency, the survey results are used to infer yield zone thickness, core mss state, and stress distribution. An accurate definition of these parameters can not only provide mine engineers a useful tool for evaluating mine design, but also give valuable insight into the inherent safety of the mine structure.

Jan 1, 1993

By David Newman

The Southern Appalachian coalfield has a long history of coal bumps that are attributable to a unique combination of topography, geology, and multiple seam mining. The high pillar stresses that generate a coal bump result from: i. mountainous topography where the overburden commonly exceeds 1,500 feet beneath ridges, ii. multiple seam mining, iii. thin interburden between active and abandoned mines, iv. thick beds of competent sandstone and sandy shale strata throughout the stratigraphic section, v. strong coal seams, and vi. irregular pillar and mining geometry in older room-and¬pillar mines. Coal has been intensively mined for more than 100 years in Eastern Kentucky, Southwestern Virginia, and Southern West Virginia. As a result, it is commonplace to have abandoned mines above and/or below active operations. Therefore, those conditions that generate coal bumps will be more prevalent in the future, as underground mining continues in Southern Appalachia. The focus of this paper concerns two coal bumps that occurred in the Kellioka seam where both undermining and overmining had occurred. The purpose of the investigation is to determine the magnitude and distribution of vertical stress that initiated the bumps, to identify bump-prone areas in the unmined reserve, and to recommend pillar centers and mining layouts to avoid future bumps. The state of stress in three vertically adjacent seams was examined using the LAMODEL boundary element program. Large, detailed numerical models were constructed of both bump areas and of the remaining unmined reserve block. The results of the numerical analyses were compared with those obtained using analytical equations.

Jan 1, 2002

By John Ellenberger

The nature of competition in the coal market tends to deplete the most favorable coal reserves first, and forces subsequent development of mines in more extreme ground conditions such as those associated with multiple-seam mining. In fact, 70% of the United States coal reserves are in multiple-seam situations. The National Institute for Occupational Safety and Health (NIOSH) is conducting research to develop mine design algorithms that will result in safer multiple seam mines. This paper presents an overview of multiple seam issues in the central Appalachian coalfields. To date, more than 50 case histories have been analyzed from 20 operating mines in Eastern KY, Western VA, and Southern WV. Each case history is classified according to the degree of multiple-seam interaction, ranging from no apparent interaction to severe interaction with direct impacts on mine safety and resource recovery. The case histories are also examined with regard to amount and geologic characteristics of interburden and overburden, and design stability factors. The sequence of mining has also been found to be critical when the lower seam is fully-extracted. This paper compares current case histories to traditional rules of thumb. In addition two situations were modeled using LaModel software. LaModel has been upgraded to import pillar grids and topographic data from AutoCAD files. The cases presented demonstrate vertical stress capabilities of LaModel.

Jan 1, 2003

By Asmaa Yassien

Finite element models were developed to study the effect of tensioned and fully-grouted bolts on the stability of the mine opening in a three-entry development system using a typical geological column for the Pittsburgh Coal Seam. The models were used to study the effect of each bolt type on the stress distribution in the immediate roof, yield zone development and the vertical deformations. Comparison of all of those features show that the effect of a tensioned bolt on the stress distribution in the immediate roof is restricted to limited areas, whereas the fully-grouted bolt has a much wider effect on the stress distribution. Both types of bolts can reduce the yielding and deformation in the immediate roof to different degrees. The fully-grouted bolt reduces the sliding along the bedding plane more than the tensioned bolt. Both types of bolts were able to maintain the bedding planes without separation. The induced forces in both types of bolts are analyzed. A rapid increase in the shear force and bending moment occurred in the fully¬grouted bolt due to the relative motion of bedding planes.

Jan 1, 2002

By I. Geldmacher

In-Seam Seismic (ISS) methods have been used extensively worldwide for the past 10 years to assist mine planning, espe¬cially longwall mining in coal seams. The technique is only starting to gain wide acceptance in North America where it is being applied in a much wider variety of applications than was initially considered. Surveys elsewhere have concentrated on the detection of faults (structures) in coal mines prior to longwall mining. The majority of surveys in North America has been the location of abandoned workings which were not adequately surveyed prior to closure. Modeling data based upon the lithological logs, geophysical logs and interpreted sections show whether the method will be successful prior to carrying out the survey. The surveys have been performed to locate abandoned wor¬kings from the surface and underground using both seismic reflection and transmission techniques. Data from coal seams in both Illinois and West Virginia will be presented. A second method where ISS surveys are being applied to detect old workings will be presented. The ISS tomography produces an image of the P-wave and Channel Wave velocities which is representative of the material through which the signal traveled thus identifying the location of the old workings. Data from Texas will serve as an example.

Jan 1, 1990

By Maynard O. Serbousek

The mechanics of load transfer of fully-grouted roof bolts in coal mines has been investigated in laboratory and analytical studies. The purpose of this research is to increase the understanding and better interpret data from pull tests which are routinely performed in the field. Laboratory work includes variation of bolt length, hole diameter and grout type in pull tests. Four-foot bolts were tested in holes of 1- and 1-3/8-inch diameters. Both polyester resins and inorganic grouts were used as the anchorage medium. The bolts were strain gauged at six locations to observe the rate of load transfer from the bolt to the rock mass when a force was applied at the bolt end using standard Jack pull testing procedures and equipment. Analytical studies using closed form solutions and finite-element techniques were compared to experimental results. The models are for the elastic regime only but do include slip elements. A detailed analysis of the load transfer mechanisms from the bolts through the grout to the rock mass is presented.

Jan 1, 1984

This paper describes the experiences with the application of the Coal Mining Roof Rating (CMRR) System to South African strata conditions. Roof classification studies using the CMRR system were undertaken as part of a SIMRAC investigation into falls of ground in South Mean collieries. Colliery roof rating exercises were undertaking in a range of geotechnical conditions from shallow level coal seams under flat topography to workings with cover depths of greater than 300m A comparison is made between the CMRR and other Rock Mass Rating systems used to classify different roof geotechnical conditions. The need for geological discontinuity orientation, blasting and rockmass stress adjustments to the CMRR is discussed. The effects of acid mine water on strata weatherability and the need to incorporate immersion tests using acid mine water are also detailed. In addition limited correlation is given between CMRR values and other geotechnical factors such as joint trace length, joint shear strength, joint orientation, weatherability and roof damage due to horizontal stress. Taking cognizance of these observations, a concept of stress induced roof damage triggering is discussed.

Jan 1, 2001

By Deren Zhu

Based on the results of field observation and physical model analyses, a computer simulation method, FEAEBP, has been developed for simulating the breakage of the main roof by considering it as a Kirchhoff plate on Winkler elastic foundation. By this method various parameters of rock property, different boundary conditions of the working face and the initiation, development and results of the breaking process of the main roof with different geometric dimensions can be effectively simulated. Since the main and immediate roofs, and the coal seams are treated as an integrated mechanical system, the interaction among them can be studied in detail.

Jan 1, 1988

By Tom Wynne

In order to shorten the time required and improve safety for a longwall face move, a 36-ft wide longwall recovery room was predriven. The room was supported with specialty bolts, trusses and concrete cribs. Rock mechanics instrumentation was implemented to evaluate the adequacy of the whole recovery system. This included instrumented roof bolts, entry convergence, load cells, and abutment stress in the surrounding pillars. This paper describes the methodology of support and instrumentation designs, and monitoring and evaluations. The results indicated that this method offers the safest and yet faster longwall recovery so far. But the rock mechanics monitoring also indicated some improvements are desirable in the truss system and concrete crib layout.

Jan 1, 1993

By Peter Cain

Twenty years of research into the design of permanent gateroad support (packs) in coal mines have resulted in two methods of pack design based on engineering principles. The first, the detached block concept, is based on static loading of the support system by an hypothetical detached block. The second, the roof beam tilt concept, is based on the dynamic loading of the support system as the roof beam tilts over the roadway about a pivot point located in the rib-side. The roof beam tilt concept has been used on a number of occasions to design packs in a variety of conditions. However, recent work in the Sydney Coalfield of Cape Breton Island, Nova Scotia, has shown that design criteria based on the roof beam tilt concept in its present state of development are far from universal in application. After a review of published pack performance data, including detailed studies conducted in the UK, France and Canada, it was found that consideration of the mechanical properties of the roof beam produced a design criterion that fitted the available data. Modifications to the design process using the roof beam tilt concept are suggested for use in pack design. The work also revealed that current research in other aspects of coal mine ground control, particularly the areas of powered support design and subsidence prediction, may provide valuable information leading to major advances in this field. Such developments could also extend the application of the roof beam tilt concept to the design of pillar systems.

Jan 1, 1993

By Hani S. Mitri

The behaviour of grouted cable bolts used for rock reinforcement as a means of ground control in hard rock mining is investigated. The study utilizes an analytical model which is based on the variational principle to calculate the shear bond stress distribution along the embedded length of cable bolt. In this paper, a comprehensive parametric study which covers both geometric and mechanical design parameters involved is presented. Special emphasis is placed on studying the characteristics of the grout model represented by Winkler springs. This involved varying the maximum shear strength of the grout, its residual strength and the slip occurring between the cable and the borehole wall.

Jan 1, 1990