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By Anthony S. Spearing

The vast majority of the estimated 100 million rock anchors installed in mines in the USA per year use resin cartridges (Tadolini and Mazzoni, 2006). About 4.5 million per annum of these are bolts with a mechanical shell that if properly installed become effective active supports on installation. In addition, cable anchors (mostly passive) are installed with typically 4 ft of effective resin anchorage length sometimes with mechanical shells, which further makes the installation more difficult. This is because the cable (even with a stiffener tube) is not very rigid. When installing bolts with mechanical shells and cable bolts in the holes, the back-pressure can be so high that the installation is unsuccessful often creating a ?spinner? (an un-tensioned bolt) at best or buckling the bolt thus requiring another bolt/cable to be installed, or leaving a poorly functioning unit. The installation problems can create a significant potential safety hazard and waste time and money. Using a purpose designed test rig that physically inserts different rock anchors into pipes with resin and measure the back-pressure developed with time during the insertion/ installation. The test installation procedure is designed to mirror that used underground by rockbolt operators in coal mines. In addition a computer model has been calibrated that can predict the approximate back pressure of new anchor and resin combinations acting as a screen before physical testing. This research has lead to an understanding and quantification of the insertion back-pressures and this already resulted in improved prototype designs that could lead to more reliable mechanical shell and resin bolt installations.

Jan 1, 2009

By Mark K. Larson

Researchers at the National Institute for Occupational Safety and Health?s (NIOSH) Spokane Research Laboratory are developing a borehole profilometer to dynamically measure the roughness pro-file of boreholes used to install roof bolts fully encapsulated with resin. Numerical measures of roughness were calculated from profiles of borehole surface roughness to determine potential roof bolt anchorage capacity. The concept of dynamically profiling was proven through the following series of laboratory tests: a sensitiv-ity test using various grits of sandpaper, a known-profile test along a section of a concrete hole, and a simulated borehole test in a pipe with machined grooves. Two profilometer prototypes were devel-oped that measure slight changes in the radius of a borehole using either strain-gauged, spring-steel arms or rigid, steel arms attached to a spring-loaded cone and a linear potentiometer. Both instruments performed reasonably well in the laboratory and were subsequently tested in boreholes drilled in two concrete blocks using different drilling techniques to obtain three different borehole surface roughnesses. A Z2 roughness parameter was calculated with data from the first profilometer, and an average radial increment was calculated from the test results of the second profilometer. Although both instruments were able to readily distinguish the difference between smooth and rough boreholes, the correlation coefficient (r2) for a linear relationship between Z2 and average radial increment was 0.577. Pull tests were conducted with resin bolts installed in the concrete blocks, but maximum pull-out strength could not be correlated to borehole roughness because in each of these tests, failure occurred at the resin-bolt interface. A more sophisticated prototype has been designed to measure surface roughness and also accurately determine the location and tilt of the instrument in the borehole. However, this instrument has not yet been fabricated or tested.

Jan 1, 2007

By V. V. Nazimko

Abandoned coal mines cause subsidence and structural damages. Shallow abandoned mines induce the most severe and harmful damages. In addition, they produce time-dependent subsidence which is difficult to predict. Backfilling of underground cavities with waste materials have been used to prevent the time-dependent subsidence or to mitigate its effect. However, this procedure is expensive. A new drill and blast technique has recently been proposed to prevent surface damage from time¬dependent subsidence over abandoned mines. It utilizes the principle that blasting in the caved rock and roof in the vicinity of a cavity will dilate the ground and create a self-supporting effect owing to increase in the volume. Physical modeling has demonstrated that artificial dilation of caved rock using the drill & blast technique creates positive effects due to the following sequence of events: the dilated rock creates a supporting effect for the suspended rock strata and reduces its span which decelerates 3.4 times the process of caving. The roof span is reduced by the next caved layer, which causes the hole boundaries to incline to each other. This effect converts the process of caving from a developing mode to a reduction mode. Finally, the sinkhole zone assumes a triangular shape with a negative angle of draw 64-700 in contrast to a rectangular shape of the chimney zone for natural caving. Vertical dimension of the triangular zone is 3.87 times of the room height. Final surface subsidence decreases at least 2.09 times; displacement 1.31 times, slope 2 27 times; curvature 3.42 times, horizontal strain 1 21 times in comparison to natural caving of the ground over an abandoned mine.

Jan 1, 1999

By N. P. Kripakov

Results of a complex three-seam interaction analysis in a western U.S. coal mine are presented. The physical situation simulated by the models represents the development of a set of entries underneath two previously mined seams with all three seams in proximity to each other. The layout and the geometry of the remnant pillars in the upper seams were nonuniform and not always directly superimposed over each other, creating a very challenging modeling problem. The U.S. Bureau of Mines' MULSIM/PC computer code was upgraded and enhanced to perform the required three-seam interaction calculations. Analysis was limited to simple linear-elastic assumptions as only peak stresses were required. Results clearly indicated that stress concentrations in the lower seam increased approximately 100 percent when compared to cases simulating no extraction in the two upper seams. These stress concentrations generally occurred in zones directly below points of highest abutment pressures in the upper seams. Overall, the model results compared favorably with actual documented observed behavior where roof failures were known to have occurred.

Jan 1, 1996

By J. Willan

This paper addresses the problem of designing a destress blasting procedure to reduce the incidence of rockbursting in hard rock mines. A review is made of the developed theory and practice of destress blasting in rockburst-prone ground. Specific reference is made to a recent survey of past and current practice with particular reference to Ontario mining operations. The need is identified for a more scientific basis for destress blast design and monitoring. Reference to developments in blasting technology as well as rock mechanics are then made to support a proposed, integrated design procedure and research strategy.

Jan 1, 1984

By Michael Alber

Resin grouted rock bolts are the major support measure in coal mining. They are rather easy and foolproof to install, take immediately load and are the least time consuming support measure for controlling gateroad deformation. The use of only resin grouted bolts allows fast gateroad development which is essential for economical resource exploitation. The design of reinforcement / support systems for coal mining typically bases on empirical methods, which work fine when encountering known or usual ground conditions. However, coal mining approaches depths where little experience exists and numerical modeling is often the only method to estimate ground behavior. This paper discusses the geotechnical properties relevant to numerical model and focuses particularly on input values for modeling resin grouted rock bolts for safe gate design.

Jan 1, 2006

By Yi Luo

In dealing with various subsidence cases, accurate predictions of mining induced subsidence are the basis for correct assessment of subsidence influences and subsequent mitigation efforts. There is a good demand for a reliable tool to perform the subsidence predictions and related tasks. A comprehensive and integrated subsidence prediction model, CISPM, developed by the senior authors in the later 1980?s and early 1990?s (Peng and Luo, 1992) is one of such tools. Built on solid mathematical models and calibrated by a large number of collected subsidence cases, CISPM provides a rich set of features needed in subsidence engineering and research works. Its reliability and usefulness have been proven by numerous field applications. This upgrade of CISPM will not only preserve its original features and reliability but also provide some new needed features and a more user-friendly working environment.

Jan 1, 2008

By Hamid Maleki

In this paper, the authors present a review of data gathering programs and stress analyses for improving mine layout designs at BRL mines located near Paonia, Colorado. BRL and other companies have used both longwall and continuous miner methods for the extraction of the Upper D, Upper B, Lower B and C seams from the multiple-seam reserves in the North Fork Valley. Historic mining in the B and C seams has created stress footprints in the neighboring unmined seams compounded by stress transfer from recent longwall extraction in the Upper D Seam. Thus optimum positioning of workings in the remaining reserves requires careful collection of pertinent data and consideration of load transfer from previous workings through remnant abutment pillars. Based on long-term subsidence monitoring, underground mapping and stress analyses, we have confirmed initial load transfer analyses finding mild interactions between the UD and LB workings, and the mining conditions have proven to be good and manageable during the development stage of the LB Seam. This has been achieved through prudent mine planning, prudent positioning of the gateroads in multiple seam workings and support allocations.

Jan 1, 2007

By J. Nielen van der Merwe

Rock engineering research and technology development in South Africa only received serious attention in reaction to the Coalbrook disaster in 1960. At the time, a number of gaps in knowledge of rock related matters were identified and in the ensuing years, these were progressively attended to. The paper describes the events that led up to the Coalbrook disaster and reviews the developments that took place afterwards. The effectiveness of the research outcomes are tested by referring back to the disaster. It is concluded that tremendous advances were made but that not all the unknowns were attended to at the same level. The strength of nominally square internal pillars is the one issue that received the most attention, but the strength of barrier pillars, overburden behaviour and loading systems are still largely unknown. Due to the complexity of the remaining unknowns, they can and should be addressed by the application of back calibrated numerical models. There are no readily available simple analytical solutions. The state of mining rock engineering research in South Africa borders on an emergency. More than a 1000 years of aggregate research experience has been lost in the last three years and funding is under severe pressure. There are disturbing parallels between the current positioning of the industry as a whole and the Coalbrook mine in the years preceding the disaster.

Jan 1, 2006

By A. M. Richardson

The significance of a massive sandstone unit in the immediate roof to longwall support weighting is well known. If the unit lies in the main roof above the zone of caving, its influence becomes more difficult to calculate using traditional methods. Much of the reserves at Cyprus Plateau's new, multi-seam Willow Creek Mine are located in the A Seam. The massive Kenilworth Sandstone is typically situated 21.3 m (70 ft) above the A Seam in the main roof and is on average 14.6-m (48-ft) thick, ranging to 29.0-m (95-ft) thick. This paper describes the application of distinct-element modeling to determination of the appropriate yield capacity for supports at Willow Creek. The modeling was determined to be useful for understanding support loading mechanisms and as an aid to engineering judgment in selecting supports.

Jan 1, 1996

By Xinzhi Du

Coal strength is essential to analyze the stability of excavation in coal. It is well known that the strength of coal tested in the laboratory is size-dependent, and different size specimens will produce different strength. Many researchers have studied the uniaxial compressive strength of coal and developed a number of empirical formulae to describe the strength variation with specimen size. According to a comprehensive data base that includes most of the laboratory testing results in the last century in U.S. and South Africa, the size effect and shape effect were reviewed and analyzed. The results show that different groups of coal specimens have different size effect on the compressive strength. The critical size for the size effect from the data analysis is about 40 in. The shape (W/H) of the specimens has a large effect on the compressive strength. The pillar strength formulae obtained from the data regression have a larger strength than all other existing formulae. Most pillar strength formulae have their own place in coal pillar design field. In order to analyze the difference of those formulae, twelve design formulae have been selected to compare using the Pittsburgh coal seam. The intention is not to determine the preferred formulae, but only to compare them, and see how do the W/H ratio, width, length, angle, cross-section area and effective width affect different pillar strength formulae?

Jan 1, 2008

Historically, the selection of mining sequence for multiple seam conditions has been based on many factors other than ground control considerations and minimization of interaction effects. Today, numerous operations are finding themselves mining above existing or abandoned operations, some of which exhibit large scale caving, pressure arches and subsidence troughs. Development of design criteria for the upper seam that would alleviate damage due to lower seam interaction effects can contribute substantially to the safety and productivity of many mining operations. The primary objective of this re search has been to identify methods of improving ground control conditions in the upper seam when mining through either passive or active subsidence waves. These methods concentrate on improved ground control practices for the upper seam, rather than reducing the amount of subsidence in the upper seams by improved practices in the lower seam. Design factors such as pillar size, pillar and opening orientation relative to the subsidence trough, offsetting intersections, and different support methods have been studied for the upper seam in a variety of typical geologic environments. Analysis has demonstrated criteria for ameliorating negative interaction effects and facillitating design in the superincumbent seam.

Jan 1, 1986

By G. M. Molinda

Ground Penetrating Radar (GPR) is being investigated for the potential to determine roof hazards in underground mines. GPR surveys were conducted at four field sites with accompanying ground truth in order to determine the value of GPR for roof hazard detection. The resolution of the current system allows detection of gross roof fractures (>63 cm (s1/4 in) zone) or rider beds in coal measure roof. Data quality is not yet sufficient to detect small bed separations or subtle lithologic changes in the roof. Differences in data quality are discussed, as well as suggestions for collecting improved data.

Jan 1, 1996

By Vincent A. Scovazzo

Benchmarking is a business management process that seeks improvement through the study and reapplication of practices conducted by industry leaders. This discussion presents objectives and organization of benchmarking applied to mining geomechanics. Similar procedures have been successfully applied to mining companies, laboratories, and research organizations involved in mining geomechanics throughout the world. Benchmarking is a structured process that allows an organization to learn from the "best" in the field. The objectives of benchmarking vary depending on the company's targeted areas for improvements. Typical goals include ground control cost reduction, enhanced safety, optimization of geotechnical staffing, development of state-of-the-art design procedures, or the adoption of the "best in the field" geotechnical data management. Once the objective of the study is established and the practices and procedures to be benchmarked are identified, a benchmarking team is formed from internal and outside specialists. Benchmarking in geomechanics is fundamentally different than in most other fields because of the effects of local geologic conditions and regulations, both of which must be understood by all on the benchmarking team, before a comparative analysis can be made.

Jan 1, 2000

By Clive Chadbourne

Investigations of world wide coal face strata control practice, the study of theoretical calculations and detailed engineering analysis have been combined to design and select a powered roof support suitable for use in conjunction with the extraction of 4.5-m (14.7- ft) of the 7.5-m (24.6-ft) thick Warwickshire Thick Coal Seam. The interface between geotechnical criteria and engineering design has been closely considered along with the following 3 important factors: Firstly, the need to further improve strata control and operational characteristics whilst producing a wall integrated overall face design. Secondly, to enable the supports to be rapidly transported from face to face in roadways driven at the minimum size for operational requirements. Finally, to provide hydraulic power, automatic control and monitoring systems which will improve operations reduce manpower requirements and improve management information. Clear design requirements were determined in order to allow selection on a commercial basis. A method of tendering is described with the subsequent analysis and selection process. In the event oversupply of coal in the UK resulted in the order not being placed. Brief consideration has been given to supports suitable for reserves to be mined In a few year's time where there is potential for extraction well in excess of 4.5-m (1 4.74.

Jan 1, 1994

By Christopher Mark

Pillar recovery continues to be one of the more hazardous activities in underground coal mining. Safety requires that the roof above the intersection remain stable until after the pillar has been extracted. Artificial supports (timbers and Mobile Roof Supports) arc essential to roof stability, but so is the final remnant stump or pushout. Traditional mining practices usually called for the complete extraction of the final stump, but the recent trend (both in the U.S. and internationally) seems to be towards mining plans that leave a remnant stump. For this study, a sample of roof control plans from the Mine Health and Safety Administration (MSHA) Coal Districts were analyzed to determine current pillar recovery practices. Both full- and partial- extraction plans were included. Special attention was paid to whether plans require that a final stump be left, and whether requirements regarding the dimensions of the remnant stump are included. Foreign experience with final stumps is also summarized. It seems that the risk of major pillar falls can often be reduced by leaving final stumps that are large enough to protect the intersection, but small enough that they do not inhibit the caving of the gob. Because final stumps are often irregular in shape, a new approach for estimating their strength is described. Analyses were conducted to assess the effect of seam height and depth of cover on the potential variation in the size of remnant stumps.

Jan 1, 2001

As the surface reserves are being depleted, more and more stone operators are seeking underground mining options. Stability of underground openings is a major concern for the safety and productivity. This paper relates to a field study conducted at a large underground limestone mine in Central Pennsylvania. Identification of hazards leading to ground failures and assessment of roof fall risks are important in mitigating injuries. Field observations at this mine were integrated with Roof Fall Risk Index (RFRI) mapping software recently developed by West Virginia University. The system is designed to combine field study data related to geologic factors, mining-induced damages, roof profile, and ground water influx to generate RFRI maps. The mine has adopted a proactive ground control program consisting of comprehensive examination of drilling and bolting logs; precision surveying of roof and rib; accurate mapping of cutters, falls, and geologic structures; and installation of extensometers and microseismic monitoring systems. Investment of time and resources has eliminated fatalities and lost-time injuries due to ground fall. This paper presents the geologic settings, mining conditions, ground control issues along with risk management and control techniques adopted at the study mine.

Jan 1, 2009

By John C. Stankus

Jennmar Corporation. in an effort to improve mine roof control, has conducted extensive study and research comparing tensioned point anchor resin systems versus the use of non-tensioned fully grouted rehar. Preliminary indications are showing that in thinly laminated, weak strata, tensioned point anchor resin systems are providing more effective roof control than the non-tensioned fully grouted rehar. In this paper two case studies are presented in which tensioned point anchor resin systems were utilized to stabilize and control the mine roof in two different mines. Both mines are alike in many ways, but yet very different in other aspects. However, the mines do share one main common factor. and that being the non-effectiveness of the non-tensioned fully grouted rebar in controlling the thinly lamininated strata in each respective mine.

Jan 1, 1990

By Tristan H. Jones

Beginning in 1987, miniature dataloggers approved by the Mine Safety and Health Administration (MSHA) were publicly available for sale. The principal among these was the model CR21XQM, manufactured by Campbell Scientific, Inc. These dataloggers had been tested and were deemed permissible by MSHA and thus could be installed in the return air of an underground coal mine. In 1996, production of all permissible Campbell Scientific dataloggers ceased due to lack of sales. Data collection with these dataloggers has been a staple of ground control research in the United States for decades, and, without them, or a replacement, the capability to monitor ground behavior is severely impacted. Since that time, any party requiring a permissible datalogger has needed to borrow one that was purchased during the production years. In order to fill this technology gap and ensure its own data-gathering capabilities, the National Institute for Occupational Safety and Health (NIOSH) has developed and refined its own miniature data acquisition system (MIDAS). On February 16, 2010, MSHA deemed the MIDAS to be permissible and approved it for use in the return air of an underground coal mine. The MIDAS is an intrinsically safe, 16- or 24-bit datalogger, compatible with a wide range of resistive, single-ended, and differential-type instrumentation. It contains enough data storage memory to operate for over a year with typical data rates and is efficient enough to do this on a single C-sized alkaline battery for that entire period. It is menu-driven and requires no programming knowledge to operate. It supports wireless or hardwired communication for datalogger setup and data download. This paper describes the design, field and laboratory testing, and capabilities of the MIDAS datalogger. While not as fully -functioned as the CR21XQM, it is the only new permissible datalogger available, and there is intent to make it available to the mining industry for purchase.

Jan 1, 2012

By Kazem Oraee

In underground coal mining, particularly room-and-pillar methods, the coal pillars play a significant role in roof stability. If the pillar dimensions are increased, the pillar bearing capacity also increases, but more coal remains in the pillar and therefore the recovery of mining operation decreases. Therefore, determining the optimum pillar dimension based on technical, economical and performance parameters is important. In this paper, the coal pillar is designed by using the concept of ground reaction curve (GRC). For this purpose, the GRC for coal pillars is drawn and then the elastic and plastic zone of coal pillar behavior is determined. Based on elastic and plastic zone, different equations are presented to draw a pillar reaction curve and the process of coal pillar design based on GRC is explained. As a case study, this new technique is applied to study the coal pillar state in Tabas coal mine of Iran. The results show that the presented new approach is successfully used in the design of the optimum coal pillar dimensions, and also determines the elastic and plastic behavior limit, which is important especially for the logical design of yield pillars.

Jan 1, 2009