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By Hani Mitri

A new monitoring device called SAFEBOLT (Stress And Force Evaluator in Bolts) has been recently developed. The device SAFEBOLT, essentially replaces the load cell, it permits the monitoring of bolt axial load from the time of its installation underground. The SAFEBOLT system consists of a portable readout unit, connecting cable and the SAFEBOLT device, which is installed in the bolt head. SAFEBOLT can be installed on any industry standard rockbolt. No special installation method is required; the conventional method of bolt installation by the stoper or roofbolter remains unchanged. The device is completely encapsulated in the bolt head, which makes it resistant to corrosion, rugged, and free of any electrical wires. Pilot tests at three underground Quebec mines were carried out and helped improve the initial design of the device connector. The paper reports on two of such pilot tests.

Jan 1, 2001

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 H. Gene Wilson

Mobile Roof Supports (MRS) look similar to a longwall shield on crawler tracks. The MRS units offer a safe, productive method of extracting the 40% to 60% product left normally unrecovered in room and pillar mining. The units feature radio control, self-contained power units, crawler track drive, and a 600 ton yield. Cribbing and setting posts in retreat mining has been a necessary burden to production. The MRS reduces this burden. Long blocks of coal can be developed to further reduce move and tram time, thus increasing production. The MRS units have successful safety records. The workers are removed from the immediate fall area, and we feel back injuries have been reduced by the workers not setting posts. While MRS units have been damaged in falls, the other capital equipment has been unharmed. The MRS product line has undergone normal product improvement and evolution. Product evolution is seen in our development of machines for particular mine requirements. Through the continuing development of the MRS, Fletcher presents the coal industry with a safe, productive method of retreat mining with potential to develop new mining methods.

Jan 1, 1990

By Edward S. Custer

Recent investigations of mine roof falls have demonstrated that roof failures are generally associated with geologic conditions. The roof falls can be predicted by using the techniques of depositional modeling for the coal bed and roof rocks, and by lineament analysis. The ability to predict potential roof failure areas will allow the engineer and mine planner to develop a mine plan to avoid the roof failure areas and thus reduce overall mining costs. The ability to avoid roof failure areas also will permit maximum recovery of the coal resources.

Jan 1, 1981

By Xianming Zhou

Mining economics is such that too conservative in safety will result in a considerable penalty in mining cost. However, initial planning and design of a mining project often must be done without complete information. The assurance of economical and safe mining is thus difficult. This study is focused on the improvement of the mine design associated with Zhaoyuan Gold Mine in P.R. China as more information becomes available. Attention is given to the selection of backfill structures. Based on a proposed practical finite element model, a design, which is believed to be not only reliable but also cost-effective, is recommended for further mining operations in this mine.

Jan 1, 1989

By M. E. Duncan Fama

The aim of the paper was to model numerically some aspects of the extraction of coal pillars by split and fendering. A two-dimensional elasto-plastic plane strain model with strain softening and interface modelling was used. The model was able to simulate the improvement of conditions experienced in lifting from the fender, when the roof adjacent to the fender has caved. In contrast, when a stook was left, providing support to a span of uncaved roof, the model predicted the fender would suffer considerably more deformation and strain.

Jan 1, 1992

By Donald P. Richards

West Elk Coal Company operates its Mt. Gunnison No. I mine on the western slope of the Rocky Mountains in Western Colorado. The mine is located along the north fork of the Gunnisson River near Somerset, Colorado as shown in Figure 1. The mine is designed for a sustained production of 1.4 million tons per year out of the "F" coal seam in the Mesa Verde formation. The coal seam is nominally 7 feet in thickness and dips at about 3 ½ degrees to the northeast. Mining of the seam is up dip with the entry portals on the north facing slope of Mount Gunnison. The mine is designed as a five entry system with four entries for ventilation and access and one entry for a coal-handling conveyor. The mine entry layout is shown in Figure 2. Initial mine planning began in 1979, with "permitting" complete and construction starting in the fall of 1981. A "turnkey" contract for design and construction of all five parallel mine entry tunnels was awarded by West Elk Coal Company to a Joint Venture of Jenny Engineering Corporation (design and contract management responsibility) and Affholder, Inc. (construction responsibility). The first entry access into the coal seam was completed in the early spring of 1982. The mine has been operating at full service (all five entries) for nearly 3 years.

Jan 1, 1984

A procedure fm the estimation of pilla1 strength is presented where the design of underground coal pillars is emphasized. An analytical approach has been developed, proposing that the effect of end constraints on the pillar strength and the ductile behavior of coal material should be incorporated in the computation of the stability of coal pillars. Bad on this approach, it has been found that end constraint has a significant influence on the pillar strength. The study indicates that when the end constraint is zero the computed strength of the pillar is the same as the unconfiied compressive strength of the coal mass and will not increase with the increase in the width to height ratio. However. when the end constraint is greater than zero the pillar strength will increase exponentially as the width to height ratio increases.

Jan 1, 1990

By Lu Jiam Zhang

Coal and gas outburst is one of the serious natural disasters in the coal mines of China. Since 1950, more than 10,000 coal or gas outbursts have occurred in more than 200 underground coal mines of different types. The outburst prone coal mines are widely spread out. It occurs in different types of coal -ma and is very serious in some areas. Most outbursts occurred at the time of roadway driving in coal seam, serious outbursts occurred mainly when entering the coal seam during the cross-cut driving; of the outbursts, coal and gas outbursts We UP major proportion, while pressing out and pouring out ah account for a large proportion; outburst. occur under different operating conditions, but mostly occur during shotfiring. This paper introduces the prevention and control measures as practiced in China and illustrates the three types of outburst cla8sification. Both technology and management are equally important in outburst prevention and control. Measurement of the volume of drilling cuttings and the initial gas outflow rate fm- boreholes have proven to be effective. A special team and organization must be set up to deal with the problems.

Jan 1, 1987

By S. M. Matthews

The magnitude and orientation of the in situ stressfield has been determined as a key factor in controlling the stability of openings for both coal mine development and extraction. Monitoring of roadway and pillar behaviour in conjunction with numerical analysis has identified specific stress control measures that have been successfully applied to improve productivity in underground coal mining operations. The influence of the major horizontal stressfield on the stability of openings has been identified at a wide range of sites including mines in Australia, Britain, the USA, New Zealand and Japan. Stress mapping, stress measurement and stress monitoring techniques have been applied to define the in situ stress regime and, together with the correlation of measurements of rock deformation and rock reinforcement behaviour, has enabled appropriate stress control measures to be developed. Stress control measures for development headings have required the selection of roadway orientation, roadway sequencing, reinforcement density and pillar size. The monitoring of behaviour at a range of trial sites has confirmed significant improvements in development stability. Stress control measures for longwall extraction have required rational selection of longwall orientation and extraction direction. Where a mine layout design incorporating the optimum longwall layout design has not been possible, the use of sacrificial roadways has been successfully applied to protect key roadways from stress concentration effects. Comprehensive in situ monitoring of behaviour has been carried out to confirm the mechanisms involved.

Jan 1, 1992

By Michael J. Friedel

The fundamental problems associated with ground control reflect the inability of mining personnel to "see" the development of anomalous conditions. Toward this end, the U. S. Bureau of Mines is investigating the application of seismic refraction tomography for imaging the process of pillar yielding, at the Foidel Creek coal mine, near Steamboat Springs, CO. The inherent three-dimensional problem of imaging coal pillar stress is reduced to a single plane by considering only the stress transmitted from the pillar to the adjacent floor rock. This approach exploits the fact that refracted seismic energy is essentially confined to the adjacent higher velocity sandstone strata. The tomographic analysis using ray-tracing based on wavefront migration resulted in a robust P-wave tomogram from which regions of high velocity indicate zones of high stress concentration, whereas low velocity zones indicate regions of stress relief. The interpreted region of high stress concentration occurs at the pillar end nearest to the working face of the adjacent longwall panel. This finding is corroborated by vertical pressure cell measurements recorded in the pillar. The stress diminishes toward the pillar perimeter with local minima correlating to the locations of observed pillar spalling. Time-sequence monitoring, as the longwall face advances, is recommended for evaluating changing stress conditions.

Jan 1, 1993

By Anthony T. Iannacchione

Underground bituminous coal mining is a large and significant industry within the Commonwealth of Pennsylvania, and it has a legacy of environmental consequence. For example, in 2008, 13.5 acres (5.5 hectares) of land were undermined by longwall mining every day. Laws and regulations have been promulgated requiring companies involved in this industry to remedy mining damage caused to homes, businesses, and land; replace impaired water supplies; and repair impacts to streams and wetlands. The regulations and technical standards set by the Pennsylvania Department of Environmental Protection (PA DEP) have been put in place to assure that coal operations comply with ACT 54 of the Bituminous Mine Subsidence & Land Conservation Act and other related legislation. These regulations and standards require operations to address impacts when a mining permit is submitted and as permits are modified and extended. One of the special provisions of ACT 54 is the requirement by the PA DEP to produce an assessment of the surface impacts of mining every five years. The University of Pittsburgh was contracted by the PA DEP to perform the 3rd assessment, which spanned from August 21, 2003, to August 20, 2008. This paper details the findings related to surface structure undermined during this period. A total of 3,735 structures were tracked. Four-hundred-fiftysix reported effects were found, and each required additional information and analysis. The majority of the reported effects were associated with eight longwall mines. Characteristics of the structures were analyzed and discussed, with special attention to those with reported effects. The number of undamaged and reportedly damaged structures (i.e., dwelling, barn, commercial building, etc.) undermined during the assessment period were listed. Lastly, the resolution status and type of resolution were analyzed for all structure reported effects.

Jan 1, 2011

By Karl Brandt

Longwall panel 580 has recently been retreated in the Zollverein 1/2 seam at a depth of approximately 1000m at Auguste Victoria mine with rectangular rockbolted roadways used for both the maingate and tailgate. Although Auguste Victoria mine has been increasingly using rockbolted support since the mid 1990's. this has previously been restricted to tailgates for single use. This panel represents the first use of a rectangular rockbolted roadway as a maingate at Auguste Victoria mine. As such, it represents a significant change in support strategy. As part of the strategy numerical modelling was used to examine expected behaviour of rockbolted roadways at the site, stress changes during face retreat, the feasibility of adopting rockbolted support for the maingate and the reinforcement pattern required. Monitoring instrumentation was installed in both gates to confirm that the support systems were performing satisfactorily as the gates were driven and as the face was retreated. Although the panel is isolated from others in the Zollverein 1/2 seam, markedly different roadway conditions have been experienced in the two gate roads. Favourable conditions were obtained in the maingate, but particularly difficult conditions were encountered in the tailgate. Previous work for the mine had examined interaction with old workings in other seams and shown a vertical stress of 28MPa for the maingate and up to 45MPa for the tailgate. The values from the earlier work were used in the modelling work described, which showed contrasting results given the different stresses. The difference between the gates can be attributed to the interaction. The paper describes the numerical modelling work, the reinforcement systems adopted, the monitoring results, the interaction effects and the comparison between expectations from the numerical modelling results and the conditions actually experienced.

Jan 1, 2002

By Dennis Dolinar

In general, the direction of the maximum horizontal stress in the eastern United States is fairly well defined. However, the variation of the magnitudes of the horizontal stresses is not very well understood. Because the horizontal stresses cause severe ground control problems in underground coal and limestone mines throughout the eastern United States a more complete understanding of how the magnitudes vary would be useful for developing mine design strategies to combat horizontal stress related ground control problems. Therefore, in this National Institute for Occupational Safety and Health (NIOSH) study, the variation of the magnitude of the horizon stresses in sedimentary deposits in the eastern and Midwestern United States are examined with respect to two factors, the elastic modulus of the rock and the site depth. Stress measurements from thirty-seven sites are used in the evaluation. Examining the applied excess strains indicates that the eastern United States can be, separated into high and low strain zones. For most of the eastern United States the maximum applied excess or tectonic strain ranges from only 300 to 550 micro strains. However, there is one area, a portion of the Beckley seam in the central Appalachian region where the strains are significantly higher than the other regions. In this higher strain zone, the maximum applied tectonic strains range from 700 to 1,000 micro strains. Regression models for each zone based on the elastic modulus can explain between 83 to 85% of the variation of the maximum horizontal stress. Because one region, the northern Appalachian district, has strains that are about 20% higher than the other regions in the low strain zone, multiple strain models based on geographic regions were developed for the low strain zone that can explain 87 to 91% of the maximum horizontal stress variation with the elastic modulus. Depth was found not to be a significant causal factor in any increase in the horizontal stress even though the site depths ranged from 275 to 2,500 ft. Beyond a theoretical increase, based on Poisson's effect and gravity, no other increase in the horizontal stress with depth can be justified with this data. The most significant factor controlling the variation of the maximum horizontal stress is the elastic modulus of the rock, not the overburden depth.

Jan 1, 2003

By John C. Stankus

At the 19th International Conference on Ground Control in Mining, a paper was presented detailing new types of Jennmar INSTáL tensionable cable bolts (Stankus and Taylor, 2000). Since that time, significant testing and trials of the various types of tensionable cable bolts have been conducted. Several MSHA districts and state regulatory agencies have now approved these systems as primary support This paper will detail the use of tensionable cable bolts as primary support at two longwall mines in Virginia (MSHA District 5).

Jan 1, 2001

By Xinchuan Xia

Based on the stepwise strain features and the strength reduction law of the surrounding rock in extremely softrock roadway, this paper proposes a dynamic supporting principle. A new kind of stepwise reinforced technology, which consists of immediate shotcreting, timely bolting and delay grouting, was optimized after thorough analysis and illustrated with a successful engineering example in extremely difficult softrock roadway.

Jan 1, 2000

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 J. G. Stewart

Polyurethane injection for the consolidation and control of broken strata is a common practice in the deep mines in Europe. The technology has gained wide acceptance in the coal mines of America since the introduction of RokLok polyurethane binder by Mobay Chemical Corporation in 1977. A popular and growing use for polyurethane consolidation grouting is to control the roof and prevent gob flushing during recovery of longwall face equipment. The technique provides many time and performance-related benefits compared to more conventional methods using wire mesh. Kitt Energy Corporation installed the nation's first 1000-feat longwall face in April and May of 1984. The face completed the first panel in October, 1984, and recovery was begun. Roof Strata consolidation for the recovery was accomplished by a RokLok polyurethane binder infection program designed and supervised by Halbach & Braun Industries. The application provided the most rigorous and demanding test to date of this method's performance and cost-effectiveness relative to bolts and wire screen.

Jan 1, 1984

By Hanjie Chen

Mining experiences have shown that in a pitching seam, roof behavior is significantly different from a flat seam. Uphill mining usually experiences more roof problems than downhill mining. Based on a case study at RAG American Coal Company, Willow Creek Mine, this paper presents the roof control problems experienced in a high pitching seam. Then, finite element modeling is conducted to analyze the roof stress distribution for an entry developed in different directions relative to the seam strike. The results shows that seam inclination has a significant effect on roof stability in terms of tensile stress. Uphill mining experiences higher tensile stress than downhill mining. The modeling technique utilized in this analysis presents a first of kind methodology.

Jan 1, 2000

By H. N. Maleki

This paper presents a field evaluation of six tailgate secondary support systems, with the objective of optimizing support efficiency for maintaining open gate roads. Support systems consisted of a combination of three wooden crib configurations, with either tension rebars or truss bolts. The effectiveness of the support systems was evaluated through measurements of ground movements, support load, and observations along 2000 linear ft of a tailgate. It was concluded that floor heave, crib width to height ratio, cribbing pattern, and support spacing have significant impact on tailgate stability and access after the passage of the first longwall face. Guidelines were suggested for selection of secondary support for other applications.

Jan 1, 1986