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By Ihsan B. Tulu, Keith A. Heasley, Aanand Nandula

"The objective of this paper is to present the development of a software tool that will assist in an area calculation of the Analysis of Roof Bolting Systems (ARBS) support intensity while incorporating more detailed geology, stress, and intersection span input. In this area ARBS tool, a detailed CMRR distribution can be determined from the available geologic database at the mine. In addition, the overburden and multiple-seam stresses, as obtained from the boundary-element program LaModel, can be converted to a “pseudo-depth” which is then used as the depth input to the area ARBS calculation. Finally, the section-specific intersection spans can be determined from the mine design for input to the calculation. All this detailed area input information for ARBS is handled by the latest version of the Stability Mapping (StabMap) program, which now outputs an area ARBS suggested support density given the required input parameters. This final ARBS grid can be plotted, analyzed and overlaid on the mine map for optimum use in support design and for presentation to production personnel.INTRODUCTIONSince the inception of roof bolts in the 1940s, there has been a lack of a universally accepted roof bolting design method (Mark, 2002; Mark and Pappas, 2012). To address this situation, the National Institute of Occupational Safety and Hazard (NIOSH) developed roof support design guidelines, which were incorporated into a computer program called the Analysis of Roof Bolting Systems (ARBS). These guidelines were based on the effects of roof quality, stress, and mine geometry (Mark, Molinda and Dolinar, 2001). The roof rock quality in ARBS is represented by the Coal Mine Roof Rating (CMRR), the depth is used as a surrogate for the stress, and the intersection span serves to represent the mine geometry (Mark, Molinda and Dolinar, 2001). These three input parameters are used in the fundamental, empirically derived equation of ARBS to calculate a required support density factor, called ARBS, (Mark, Molinda and Dolinar, 2001):"

Jan 1, 2018

By Dan Payne

Roof falls and other ground control incidents make up a significant portion of fatal accidents in underground mines. BHP Billiton has undergone a review of all fatalities in their operations and divided these into 10 fundamental categories of which underground ground control is one. Each underground roof fall fatality was investigated and the root causes workshopped to develop a protocol for the control of fatalities due to falls of ground. This protocol sets out 22 requirements which each BHP Billiton underground mine site must comply with. This paper describes the development, requirements and application of the protocol within BHP Billiton.

Jan 1, 2006

By Ben Mirabile, Alan Campoli, Rodney Poland

"The majority of U.S. underground coal mines use some form of cable bolt as supplemental support to a primary roof bolting system. Many coal operators employ the high load capacity of non-tensioned cable bolts, cable roof trusses, or solid bar trusses in an attempt to suspend the primary bolted horizon to overlying strata. However, in cases where many discontinuities exist in the roof strata or where pre-existing fractures are present, these passive supplemental support systems often cannot adequately control roof deflection. This paper examines the application of tensioned cable systems to minimize roof deflection in very laminated and fractured strata. Three underground case studies are presented to demonstrate the application of tensioned cable bolts. The first case study involves the rehabilitation of a longwall head gate in the Eagle Seam with highly laminated and fractured sandstone. The second case study examines the recovery of a longwall face in the Alma Seam. The final case study addresses rehabilitation of a longwall head gate in the Pittsburgh No. 8 Seam that was developed in a sandstone channel margin area and subjected to horizontal stress concentration.INTRODUCTIONDuring the previous decade, Jennmar Corporation has recognized coal operators' increasing interest in tensioned cable bolts. The majority of these tensioned cable bolt systems have been applied in adverse roof conditions including broken ground, high horizontal stress areas, and highly laminated roof strata. In most cases, qualitative data on roof conditions after application of tensioned cable bolt support indicate successful implementation of these systems. The qualitative evidence suggests that the combination of high load' capacity and applied tension provides superior supplemental roof support in adverse roof conditions. This research aims to quantify the effects of applied tension to cable bolts. This paper will review the types and installation of tensioned cable bolt systems, present initial laboratory test data, discuss successful case studies, and propose an instrumentation system for quantification of tensioned cable bolt performance."

Jan 1, 2010

By Daniel Su

Following several roof falls in the E3 longwall development of a Consol Pennsylvania Coal Company underground coal mine, an underground roof geology reconnaissance program consisting of twenty-nine 20-foot deep roof holes had identified the presence of thick sandstone layers with abundant shale and micaceous streaks. The base of the sandstone was 4 to 8 feet above the roof line within the fall areas. The presence of micaceous streaks created extremely pronounced planes of weakness in the sandstone. A structural irregularity, which extends into the E4 and E5 developments, exists between crosscuts I 1 and 30,where all roof falls occurred. Immediately to the east of this structural irregularity lies a huge syncline. The presence of thick sandstone units within the typically all shale environment and within such a structural irregularity helps to maximize stress concentration and creates closely spaced joints in the sandstone, which explains why many cracks are present in the macroscopically massive sandstone upon development. The approaching E2 longwall gob to the south and on the down-dip side, on the other hand, provided horizontal stress relief in the sandstone, which helps to explain the many roof falls in the E3 development that occurred during or immediately after the formation of the E2 gob. Finite element modeling confirmed the presence of such horizontal stress relief. Since the E3 longwall was a right-handed face, stress concentration was expected at the longwall headgate T -junctions, and further delamination of the sandstone was expected upon arrival of the E3 longwall face. Based on the results of the roof geology reconnaissance program and a roof stability analysis that took into account up to eleven parameters, supplemental headgate and track roof support consisting of 16-foot and 20-foot long cable bolts was successfully implemented within the zone of thick sandstone with micaceous streaks and structural irregularity and at other potential roof instability areas along the E3 development. The cable bolts were designed to be anchored at least 4 feet above the sandstone unit into the mostly competent sandy shale layers. No headgate roof control problem was reported within the sandstone and structural irregularity zone during E3 and the subsequent E4 longwall extractions.

Jan 1, 2002

By Kenny D. Basnett

An analysis of the Roof Falls Data Base (RFDB) for underground coal mines in Illinois for the period 2004?2008 indicated that about 80% of the roof falls occur at intersections and are associated with geologic anomalies or a rapidly changing geologic environment. Most falls are associated with anomalies such as clay dikes, differential compaction faults, shear zones, joints and fractures, and geologic structure. Therefore, a better analysis of the geology with emphasis on mapping anomalies such as joints and fractures and their application is necessary to improve ground conditions and safety. Such studies within Illinois Coal Basin are limited. This paper presents a mapping study of the joint orientations with documentation of dip of the several lithological units that lie directly above the Herrin No. 6 Coal seam around southern Illinois. These units include the Energy Shale, Anna Shale, and Brereton Limestone. The mapping was done in three mines and the data were plotted on a stereographic projection to determine the average characteristics of joint planes. The study mines included two deep underground mines and one shallow highwall mine. This study has helped identify favorable mining and support orientations in areas of highly variable geology. These are currently under review by the cooperating coal company for experimental roof control studies.

Jan 1, 2011

By K. Itakura

Information about geostructure for effective rock bolting must be obtained before setting the bolt into rock in a coal mine roadway. Especially, rock type variation and distribution of discontinuities such as layer boundaries, and separation of strata and cracks all represent indispensable knowledge. To achieve in-situ evaluation of roof rock, we developed an MWD (measurement while drilling) system using mechanical data from a drilling machine for rockbolt. System hardware detects torque, thrust, revolution, and stroke of the machine, and software analyzes the mechanical data log and displays locations of discontinuities using Neural Network techniques. Also, this system is able to estimate roof rock 3-D geostructure for varied rock types and discontinuity distribution in cases of a drill hole array arrangement in roof rock. We conducted a feasibility study for it using hydraulic drilling at roadways of the Taiheiyo Coal Mine, Japan. Simultaneously, some core drillings were also performed to confirm system analysis data. For core samples, distribution of cracks was observed and the borehole wall was investigated by OFS (optical fiber scope). Must discontinuity locations estimated from roof logging were conformed with data obtained from core inspection. However, the core sample contained a number of cracks which were not found in the mechanical data log. This indicates that additional cracks were generated during and/or after core drilling by the influence of rotational force of the machine and water supply. Furthermore, in comparison with the OFS image, resolution of the discontinuity's log estimated by this system was clarified; that is, it is difficult to detect small size cracks (hairline cracks) from a mechanical data log and the core inspection by OFS. This paper outlines the new hardware system for hydraulic drilling machine and compares the discontinuity map obtained from the software system with drilling core inspection.

Jan 1, 2001

By Thomas L. Vandergrift

Multiple-scam mining is becoming more common in the United States, especially in the East, where primary reserves arc being depleted, forcing mining companies to develop secondary seams, Although mine engineers and planners are aware of the basic effects of seam interaction and generally follow guidelines for columnizing entries and avoiding load abutments, mine design earn greatly benefit bum more quantitative estimates of scam interaction effects. This paper presents case studies from two eastern mines subject to seam interaction effects where pre-mining stresses were quantified: the Mingo Logan Coal Company's Mountaineer Alma A Mine, and Peabody Group's Lightfoot No. 2 Mine. In each of these mines. mining is being carried out within 60 to 90 ft of overlying longwall and pillar panels under maximum cover of about 1,100 ft As part of overall mine planning and design efforts al these mines. NSA Engineering performed numerical modeling analyses using the non-linear boundary-element code MULSIM/NL Input parameters were calibrated by modeling areas where ground conditions resulting from seam interaction were well documented. By incorporating actual cover loading based on topography and seam dip, and the integrity of remnant, gate road, arid harrier pillars in the overlying mines, the pre-mining vertical stress in the underlying scants was estimated based on the pre-mining stress, plans for mains, gate road, bleeder, and pillar panels were developed. Although mining experience in these areas has beers somewhat limited to date, she designs developed as a result of these analyses have so far proven successful.

Jan 1, 2000

By Keith Heasley

An eastern longwall mine was required to mine through a set of three entries located midway down the length of a panel. Of course, the mine desired to design a successful support system for the mine-through area at the least cost. To further complicate the design process, the mine-through entries were located directly under the end of a previously mined, overlying longwall panel and the entries were close enough to be subject to stress interactions from the overlying seam. In order to accurately evaluate the multiple-seam stresses and modify the mine-through support system appropriately, the boundary element program LaModel was used to model the stress and displacements in the two mines. Using the results of the model's stress analysis, the support density was systematically varied throughout the mine- through entries for optimum sup@ efficiency and economics. During the actual mining of the cross panel entries, the longwall experienced few, if any, additional delays, and overall, the support and mine through of the cross-panel en- tries at Harris #1 was very successful, saving the mine confineable time and money.

Jan 1, 2003

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 Stephen C. Tadolini

The risk of injury caused by minor roof falls continues to increase in underground mining. Most ground control injuries result from small rock falls that occur in a supported area, but do not involve the failure of the support system. In the roof, these are the "skin failures" that occur between previously installed primary or secondary bolting systems. To minimize skin failures and the associated accidents, several devices were designed and tested in the laboratory to maximize stiffness, minimize material thickness and weight, and ease material storage and handling on roof bolting machines. The goal of this project was to design and evaluate devices that can be installed with traditional primary or secondary roof supports, and that arc easy to handle/install under temporary or previously supported roof.

Jan 1, 2001

By Thomas Barczak

Hydraulic cylinders are used in several roof support systems, such as longwall shields and mobile roof supports, to provide critical ground control Many state-of-the-art hydraulic support system operate at pressures as high as 7,000 psi to provide the necessary support capacities Pressure in the upper stages of multi-stage cylinders can be intensified by a factor of 2 or 3 providing pressures of 20,000 psi or greater. A proper understanding of the operation of these high pressure systems is necessary to safely maintain and repair these support systems. Likewise, the performmce characteristics need to be fully understood to evaluate the impact of design parameters on the capability of the support to maintain effective ground control This paper examines the basic operating principles of state-of-the- art hydraulic cylinders and discusses relative issues pertaining to. (1) setting loads, (2) support stiffness, (3) yielding behavior, (4) errors in assessing support loading, and (5) hydraulic failure mechanisms and how to detect them

Jan 1, 1998

By Rajendra Singh

In India, pillar mining is contributing more than 90% of underground production which involves extraction of a part of coal from bords with pillars around as natural support in the development stage. #Liquidation of these pillars with splitting and stooking, broken nature of face during mining under complex geomining factors like massive roof, varying treatment of goaf and over or underlying workings result in high stresses and deformation in and around the neighbouring piilars, causing operational problem and hazards to mine safety. This paper suminarises the results of three case studies of depillaring under shallow depth with or without stowing. Assessment of nature and amount of mining induced stresses and deformation was done under these conditions, by insitu instrumentation and observation at three depillaring panels of Girmint. Samla and Lachhipur collieries of ECC at 54, 90 and 110 m depth cover respectively. The value of induced stresses and associated deformations was comparatively low in well caved and stowed waves, whereas for massive roof the roof fall followed overriding of pillars and stooks, with very high value of induced stresses and deformaton in absence of stowing. The stress in the centre and outer edges of the pillars changed with respect to depillaring face advance or splitting of pillars. The caving panel of Girmint colliery under weak coal measure formation, showed maximum induced stress in the pillar at 4 m distance from the face which was 1.2 times (16.6 kg/sq.cm) the preminlng stress of the area. The main roof fall over partially stowed goaf of Lachhipur colliery under massive roof strata caused overriding of pillar after crushing 39 stooks of 7.5 x 7.5 sq.m in rythmic sequence with development of 133lsq.cm maximum induced stress. Well stowed Waf of the same panel reduced this value of abutment stress to 2.3 times (65 kg/sq.cm.) body stress after 40 m further advance of the face from the crushed zone. The depillaring at Samla colliery even under massive roof strata induced low stress, equal to 0.8 times field stress only under the same condition with stowed goaf. The studies revealed the effectiveness of stowing in the goaf over loading of pillars and stooks.

Jan 1, 1990

By Andre Vervoort

The roof in coal room and pillar panels is normally stratified and can be thought of as beams or plates supported by the pillar sides in the roadways and by pillar corners in the intersections Underground measurements showed the effect of the face advance on the additional total movement of the roof skin and on the relative movement between various layers in the roof. The latter consists of the opening of bedding planes and of the lateral sliding of layers over one another. Both phenomena increase locally the axial load along full column anchors. Four models have been used to come to a better understanding of the roof behavior: a beam either clamped above the pillar side or laying on the pillar sides and a three-dimensional boundary clement model with and without bedding planes. The shape ref the curvature of the roof deflection is host approached by a clamped beam: the other models overestimate the movement at the sides. The largest roof deflection for a given Young's modulus is estimated by the model incorporating bedding planes. This model estimates that the bed separation occurs close to the face (within the first meter) and within 0.2 m of the sidewalk. In some simulations, openings are even recorded just ahead of the face. Further underground measurements should focus on these findings, as the effectiveness of the support is directly related to the amount of bed separation and sliding, which have occurred prior to the support installation and which still cart occur abler installation.

Jan 1, 2000

By Fred Leighton

Outbursts, or the sudden and sometimes catastrophic expulsion of coal and gas into the working area of a coal mine, are a little understood and growing problem. The AE/MS (Acoustic Emission/Microseismic), or rock talk, activity prior to and associated with several outbursts has been recorded using ultrasonic and micro- seismic equipment and has been analyzed by the Bureau of Mines. Analysis of these data indicates that outbursts are comprised of several failures in rapid succession and that anomalous AE/MS data suitable for providing failure warning are generated 15 to 45 minutes prior to the failure. This paper discusses the techniques for recording and analyzing these data and presents sane case histories.

Jan 1, 1982

By Pier Paolo Manca

The investigated area lies close to a large town and covers an area of about lkm2. The mine was worked at depths ranging from 5 to 100 meters but mining operations actually ceased some 50 years ago. There are seven coal seams, each one to two meters thick, overlain by sedimentary rocks in a strongly faulted area. Different kinds of subsidence have occurred, crown holes, shallow depressions and hummocky ground. The results of rock mechanical characterisation (RMR method) in the area have been used to define three different types of numerical model (FLAC 2D). A large numbers of simulations has been performed to solve three different problems at three different scales: to determine the, mechanical properties of the roof rock and overburden involved in the subsidence process, to assess the crown hole subsidence potential and to define the properties of the beam foundations.

Jan 1, 1999

By Nikolaos Polysos

To drive and utilise gate roads economically requires accurate planning and risk assessment considering the variable geomechanical requirements. The geomechanical part of roadway planning is secured by geotechnical investigations of drill cores and geophysical borehole logging. The rock specific parameters determined in this way allow a statement about the quality (defomtation and strength characteristics) of the rock that has to be cut across. They set up the basis for a standardised rock assessment. Combined with the prevailing rock mechanical situation, technical boundary conditions for advance and support techniques can be identified already in the planning phase. Detailed geotechnical investigations prior to the planning and driving of roadways can lead to a technically and economically optimised roof support system. Geotechnical core logging and a standardised assessment scheme based on the data acquired will be introduced. Geophysical borehole logging methods provide additional physical data. Different methods and their validity, which may lead to a further improvement of the planning reliability, will also be presented. The geomechanical information gained and their relevance fir the planning will he illustrated. Keywords: rock classification, geotechnical rock parameters. geophysical well logs. support planning

Jan 1, 2002

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 Steve Signer

Researchers at the National Institute for Occupational Safety and Health (NIOSH) have developed several new tools for evaluating roof support performance. A miniature data acquisition system (MIDAS) was developed that can collect readings from up to 16 strain gauges at regular time intervals and store the readings for later retrieval. Three LED lights change from green to yellow, then red, based on the reading levels and/or rates of change of rock movement. This feature can be used to warn miners of rock instabilities. The system can be used with any strain-gauged support (resin bolt, friction bolt, cable bolt) or with a newly developed rock strain strip (ROSS) developed by NIOSH to measure rock strain. A ROSS is grouted in a hole drilled adjacent to the rock support to be evaluated; the deformed shape of the ROSS provides anchorage on either side of the strain gauges. Data from both laboratory tests and field evaluations are presented that describe the capabilities of both the MIDAS and ROSS instruments.

Jan 1, 2003

By Ezzeddin Bakhtavar

There are several geotechnical challenges and risks inherent in the interaction between the open-pit and block cave mining environments. This is especially true when they have to work simultaneously for a short time, as well as when only the caving operation is continued. Some of these challenges and risks include the induced zones of stress due to the large and deep open-pit, instability of the slope, instability of the crown pillar and water inflows, subsidence, and sometimes instability of the main accesses to the block cave portion of the mine. These challenges usually occur due to cave propagation. In this paper, these geotechnical challenges and associated risks have been studied and discussed. It was concluded that the majority of the mentioned geotechnical risks could be decreased, and often eliminated, by considering the appropriate dimensions of a stable crown pillar, especially in simultaneously working open-pit and block caving operations.

Jan 1, 2011

This paper presents a two-dimensional numerical procedure which incorporates concepts related to the mechanics of strata deformation' as well as' empirical indices associated with subsidence engineering. Regional deformation data were employed in order to establish overburden deformation zones, while material properties were kept constant. Scaling of final displacements was based on empirically predicted values for subsidence and strain for the Appalachian coalfield. The model was validated with a number of regional case studies and predicted subsidence and strain curves were compared to those obtained using the semi-empirical influence function formulation developed by VPI&SU.

Jan 1, 1988