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By Bruce Hebblewhite

An ACARP-funded industry research project in Australia recently investigated the problem of premature rock bolt failures in underground coal mines. The project carried out limited sampling from 9 different collieries in NSW and Queensland, followed by a more comprehensive, controlled sampling from one colliery. Over 50 broken bolts were collected and subjected to metallurgical analysis at UNSW to determine both the steel composition and to analyse the fracture surfaces. Prior to this study, there had been anecdotal evidence of similar bolt failures from a number of sites within the industry. The sampling and analysis conducted by this project - whilst being below a statistically representative sample of the entire industry bolt population - highlighted the fact that bolt failures were continuing to occur at a number of sites, in both older and recently installed bolts. The failures were almost entirely attributable to stress corrosion cracking (SCC). It appears that a combination of factors - including steel composition, bolt manufacturing process, loading environment, geological and hydrological environment, bacterial corrosion - can all contribute to this problem. There is no simple solution, however, as with similar research findings from the UK, it appears that steels with higher values of fracture toughness, or Charpy Index, are less susceptible to SCC. Testing to date has been limited to bolt failures below the resin encapsulation horizon. A real concern is the potential extent of the problem at higher horizons, within the encapsulation. These may require some form of non-destructive testing (NDT) for detection. Initial trials in Australia of the ultrasonic device developed by DMT in Germany have proved encouraging. Further testing using this device is anticipated in future research. This paper describes the outcomes of the project, completed in 2002.

Jan 1, 2003

By David J. Reddish

Longwall systems are inflexible in layout, intolerant of disruption and represent a substantial investment. It is therefore essential drat all understanding of the likely strata behaviour and support requirements is obtained prior to panel development. For many tears rock mechanics engineers have applied numerical modelling based kill finite element techniques to the design of the support and layout of underground excavations. Whilst such design methodology has worked well in civil engineering and hard rock situations, it has been less reliable when applied to excavation design in the soft rocks of the coal measures. This lack of reliability has been identified largely as being the result of the utilisation of unsatisfactory input data, This paper describes the results of an ongoing research programme at the University of Nottingham in which a novel rock mass rating system known as the UK Coal Mine Classification system was developed to enable the prediction of the strata properties that are required as input parameters for use in numerical modelling of the deformation of soft rocks around excavations such as roadways and longwall faces in coal mines. The paper describes the development of the system. the way in which classifications are applied to derive input data for numerical models and gives two case studies ill which deformations and stress re-distributions around coal mine excavations have been simulated by numerical modelling using the FLAC finite difference code.

Jan 1, 2000

By David Oyler

Unusual circumstances may require that a longwall retreat into or through a previously driven room. The operation can be completed successfully, but there have been a number of spectacular failures which exposed miners to serious roof fall hazards. To help determine what factors contribute to such failures, the National Institute for Occupational Safety and Health (NIOSH) compiled a comprehensive international database of 130 case histories. The cases include five failures where major rock falls occurred in front of the shields, and six even more serious failures involving major overburden weighting. This suggests two room failure mechanisms. The first is a roof fall type failure caused by loading of the immediate roof at the face as the fender narrows. The second is a weighting type failure caused by the inability of the roof to bridge the recovery room and face area, and affecting rock well above the immediate roof. The data indicate that the roof fall type of failure is less likely when intensive roof reinforcement (bolts, cables, and trusses) is employed together with higher-capacity shields. The overburden weighting failures, in contrast, occurred when the roof was weak and little standing support was used.

Jan 1, 1998

By Chris Mark

Pillar recovery continues to be a significant ground control hazard. During the past decade, 10 miners were killed during pillar extraction operations in southern West Virginia. Studies conducted during the past decade have identified a number of "risk factors" that can be used to evaluate pillar extraction plans: Cut sequence Final stump Timber or Mobile Roof Supports Roof bolting Intersection span Depth of cover Roof quality Age of workings For each of these factors, rock mechanics science suggests which alternative would be expected to be more risky. For example, numerical models were used to evaluate different cut sequences, and indicated that less roof convergence occurred with the outside lift method than with the Christmas tree in the particular environment simulated. For many of the risk factors, accident statistics confirm the science. One finding was that currently almost 70% of the retreat coal in southern WV is being mined with MRS. In contrast, timber supports were used in 70% of the past decade's pillaring fatalities. This paper discusses each of the risk factors in turn, presents the relevant accident statistics, and shows how the risk factors can be combined to estimate the overall hazard. It also addresses the use of pillar design to minimize the risk of global stability hazards including squeezes. massive collapses, and bumps.

Jan 1, 2002

By K. Y. Haramy

Deep coal mines with strong roof and floor strata frequently encounter face and rib bursts. The burst problem becomes more severe with increased depth. While the exact causes of bursts are often difficult to determine, localized high stress zones are common to their occurrence. This Bureau of Mines report describes a study at a mine using longwall systems that experienced both face and floor bursts. The study correlates shield loading, entry closure, and forward abutment pressure increases to observed burst occurrences. The results indicate that strong roof strata that overhang behind the longwall supports create additional abutment stress. When caving lags behind the face and the abutment zone does not advance ahead of the face during mining, stresses on the face may increase to a critical level, resulting in a burst. The critical level occurs when the stress in the abutment zone exceeds the ability of the coal and/or adjacent strata to store strain energy. Caving characteristics may be improved by inducing fractures in the roof or by providing sufficient support resistance to shear the roof. Data analysis of a major burst indicated a dramatic increase in floor heave, with a maximum occurring 80 to 100 ft inby the face, and a lag in the cave line behind the face supports. Analysis of the effects of caving characteristics and floor heave on burst occurrences is presented.

Jan 1, 1987

By Thomas M. Barczak

The initial Support Technology Optimization Program (STOP), Version 1.0, was released at the 19th Gmund Control Conference. This original program has since been updated to Version 2.3 which was released in May, 2001. This paper describes the additional features of Version 2.3, focusing primarily on the following three aspects: (I) including uncontrolled convergence into the design requirements for standing roof supports, (21 the addition of design procedures for cable bolts as an alternative secondary roof support, and (3) the addition of new standing roof support technologies. Another new feature which should facilitate the development of design criteria for standing roof support is the capabilily to define ground reaction curves through convergence measurements alone wthout having to make support loading measurements. There are several other new features provided in Version 2.3 which are only briefly addressed in the paper. These include: (I) additional graphics capabilities to facilitate rapid assessment of support comparisons. (2) an East-West designation for support selection to more accurately provide cost and support availability data, (3) additional safety measures including a safety factor to identify support design near the peak support capacity, checks to see if the support dimensions comply with aspect ratio requirements, and measure of roof coverage for the design layout of the support system These new features of STOP provide more capabilities to design and analyze secondary roof support systems for conditions whlch could not be fully analyzed in the original version of the program.

Jan 1, 2001

By Thomas Barczak

Pumpable roof supports provide an alternative longwall tailgate roof support and have grown in usage during the past few years. Heintzmann Corporation has been installing pumpable roof supports at the RAG Resources Emerald Mine in western PA since 1998, where they have provided effective roof control in their longwall tailgates. Despite the success of these supports in this application, questions remain regarding critical design issues for optimizing the use of this support technology. The support loading profile is characterized by a high initial stiffness with peak loading occurring at less than one inch of convergence, followed by significant load shedding with a post failure capacity comparable to that of wood cribbing. Therefore, the key to optimizing the support utilization is to provide sufficient load density to prevent convergence from occurring beyond the peak loading capacity. This requires an understanding of how the supports interact with the ground conditions, hence measurement of the ground reaction curve. In order to obtain this information, pumpable roof supports were instrumented to measure support loading. Roof deformation and roof-to-floor convergence measurements in the vicinity of the instrumented supports were also made. The experimental parameters for the installation were the support spacing and water¬to-solids ratio of the grout, which controls the grout strength and ultimately the maximum capacity of the support. The study clearly shows that at this mine a 24-inch diameter support is fully capable of providing adequate ground control under depths of cover of 750 ft as only 50 pct of the available support capacity was utilized outby the longwall face. It was also shown that the 2.00 to 1 water¬solids ratio, despite providing a slightly softer support response, is sufficient for maintaining the same degree of roof control provided by the traditional 1.75 to 1 grout mix. The 10-ft spacing of the supports did not cause any ground control problems outby the face. However, inby the face the performance of the support is degraded once the peak capacity of the supports is exceeded. It appears that the large load shedding behavior, which is characteristic of this support following peak loading, allows the immediate roof to separate. When this happens, the support is unable to regain control of the roof in time to prevent failure of the immediate roof beam.

Jan 1, 2003

By John Stankus

Regional high horizontal stress has long been recognized as an important factor affecting roof stability However, localized high horizontal stress, although causing many ground control problems, has not received due attention, Through many ground control studies and underground experiences, it has been round that localized horizontal stress can be created by abrupt changes of overburden and stream valleys. The magnitude of localized horizontal stress depends on the valley configuration and overburden depth. Utilizing the Jennmar Finite Element Analysis System (JFEA), this paper analyzes the mechanism of the localized horizontal stress concentration based on two cast studies.

Jan 1, 1999

By K. Goshtasbi Goharizi

This paper discusses experimental results of physical scale models employed to study the stability of a pillar between rectangular roadways. A programme of laboratory investigations utilising a bi-axial test rig has been undertaken to examine the influence of various stressfields on the stability of pillars between mine roadways. The effect of pillar height/width ratio on the overall stability is studied. Observations of failure characteristics in and around pillars are discussed especially in relation to the various stressfields applied to the models.

Jan 1, 1992

By Qiang Fu

ABSTRACT This paper aims to identify the main issues concerning top-coal loss and the optimum drawing interval of LTCC (Longwall Top-coal Caving) mining by underground tests and DEM (Distinct Element Method) modeling. By extracting a new roadway in the goaf of the mined LTCC Panel No.4318 in Wang Zhuang coal mine of Lu-an CMA (Coal Mining Administration), and underground observation of the distribution of loss coal and broken rock in different sections of the roadway, this paper analyses the composition and presents a 3-D distribution map of the loss coal in the exploration roadway. This study indicates that the DEM modeling of broken top-coal is reasonable and scientific as it can simulate the movement of broken coal and waste rocks and determine the optimum drawing interval of top coal.

Jan 1, 2002

By B. T. Wells

This work concerns the stress fields encountered around mining tunnels. A critical review is undertaken of the various analytical assessments of the support requirements for mining tunnels. Comments are made regarding degree and type of support together with method and timing of installation. The results have been used to indicate the type of support required in different mining and geological conditions.

Jan 1, 1981

By John J. Bowders

The stability of surface slopes undergoing subsidence due to longwall mining was investigated to examine the effect the subsidence had on the stability of the slopes. Stability analyses were performed on all slopes along both longitudinal and transeverse sections of the longwall panel before and after subsidence. Both finite- and infinite-slope analyses were performed with both drained and undrained cases. The results indicated that the changes in the stability of slopes due to subsidence were insignificant for the cases analyzed. Although significant lateral ground movements were recorded, these were not attributed to slope failure. Explanations for these movements are presented in the paper.

Jan 1, 1988

By A. Bortolussi

The paper discusses the state of the progress reached in the development of the diamond wire tool for cutting hard rocks. To the advances of this relatively new technology a substantial contribution has been provided by the research being carried out at the University of Cagliari on samples of granite rocks from different Sardinian stone quarries. On the basis of field experiments and laboratory test data, the mechanism of diamond wire cutting has been assessed. In particular the influence of the rock features and operational variables an tool performance (cutting rate and bead wear) has been elucidated.

Jan 1, 1992

By Collin L. Stewart

A summary of the results of a limited subsidence monitoring program at Cyprus Amax Coal Company's Shoshone Mine No. 1 in the Hanna Basin of south central Wyoming is presented. The monitoring program was designed to meet regulatory requirements for confirmation of subsidence predictions made in the mining permit. The longwall panels were 380 ft to 600-ft wide. Mining heights varied from 11 ft to 13.5 ft. Gate road development included three-entry and two-entry head and tail gates. Overburden depth varied from 150 to 945 ft. The coal seam and overburden dip at approximately 11 degrees. The overburden is very low to low strength mudstone and carbonaceous shale with low to moderate strength, silty sandstone interbeds. Panel W/h ratios varied from subcritical to supercritical. Smax was 0.9 of the mining height. The subsidence ratio was about 0.6 for subcritical width. The draw angle on the up-dip side averaged 20 deg. The draw angle on the down-dip side averaged 12 deg.

Jan 1, 1995

By M. O. Serbousek

In 1975 the Bureau of Mines Spokane Research Center began a series of laboratory and field tests to investigate the feasibility of developing a substitute for the organic chemicals used in resin roof-bolting systems. An inorganic system, comprised of calcinated gypsum, an accelerator, and microcapsules of water was developed and patented by the Bureau in 1977. The system has several advantages including economics, non- toxicity, nonallergenic, inflammability, and is not based on petroleum products. Two roof-bolting systems utilizing inorganic cements are under development at the Spokane Research Center--a cartridge system, using the microcapsules of water and accelerator, and a slurry system, using a mix-inject method which combines dry powder cement with water for simultaneous injection into a predrilled roof-bolt hole. Several underground field tests have been conducted in eastern coal mines. Test results indicate that inorganic cartridge systems are adequately supporting mine roof in several test sections. Economic potential for the bolt systems appears favorable.

Jan 1, 1981

By J. M. Girard

Ground control problems at surface mining operations can occur for a variety of reasons. Stress, gravity loading, rock strength, geology, pore pressure, weather effects, underground workings, and many other factors contribute to slope instabilities that range from small rock falls to massive slides of material. While some of these failures can he predicted or controlled by preventive measures, each year many completely unexpected failures occur. Current methods for monitoring generally involve measuring displacements at a few, selected points in and around the suspected area of instability. While most of the displacements along these points will be in a downslope direction, freeze-thaw cycles of water-filled joints, horizontal stresses or pressure, buoyancy in saturated soils, human measurement errors, or other situations can produce deformation in almost any direction, even without any instability in the slope. Determination of which, if any of the observed movements represents a potential hazard is essential.

Jan 1, 1998

By Erdal Unal

In gate roads where supports are subjected to dynamic loading four times due to retreating longwall panels, it is a real challenge to replace the rigid arches with flexible reinforcement systems and yet to provide safety and stability of these openings within better economical limits. The results presented in this paper are based on a three years of research project aimed at developing safe and economical support systems for gate roads in Çayìrhan coal mine. Çayìrhan -Mine Project includes laboratory and field studies, design analysis, pilot scale design applications and performance monitoring. Based on outputs of the design approach developed, four types of bolts, namely, Split Set, Yielding Super Swellex, Sis-Resin, and Fosroc-Resin bolts were installed in a gate road together with widely spaced yielding steel-arches. In order to compare the performance of these flexible support systems and to provide feed back for the design approach, a monitoring program was initiated.

Jan 1, 1993

By S. Kramadibrata

It has been well accepted that water content significantly changes the physical and mechanical properties of rocks. Experimentation data of Uniaxial Compressive Strength (UCS), Modulus of Elasticity (E) and Brazilian Tensile Strength (UTS) of sandstone showed that increasing water content from oven dried to saturated condition will decrease their values. Ultrasonic tests carried out together with the UCS tests also indicated that the sonic wave traveled faster as rock specimen became saturated. During triaxial tests, the sonic velocity changed with the applied load. It was found that constitutive behavior of dried rock specimen was dependent on the confining pressure, whereas saturated rock specimen did not seem to be dependent on the confining pressure. Dependency of failure criteria such as Mohr-Coulomb. Bieniawski and Hoek & Brown on the water content was conformed as well. In order to show the influence of water content on the stability of an underground opening, a numerical modeling, based on finite element method, was carried out. The model was run using the mentioned failure criteria.

Jan 1, 2000

By Yi Luo

The potential influences of mining subsidence on structural stability, integrity and functionality of guyed tower structures have not been systematically studied before. This paper presents a ease study where a guyed steel tower of a telecommunication company was mined under by a longwall operation. Prior to mining, every aspect of the subsidence influences to this structure was studied. The pre-mining assessment indicated that the structure could experience vibration problem in the second half of the dynamic subsidence process. Bawd on the study, a simple mitigation measure was recommended and implemented and the structure was successfully protected.

Jan 1, 2003

By M. Guana

Island Creek Corporation is predominately an underground mine operator deploying room-and-pillar and retreat longwall mining methods for extracting coal. Wide variations are encountered in overburden depth, stratigraphy, and insitu stress fields. This requires major adjustments in mine design among mines and occasionally within a mine. Simple rock mechanics principles are utilized in mine design to optimize safety and productivity. Physical observations and micro deformation monitoring of roadways are the primary techniques employed. Pillars and artificial support systems are the primary design elements modified. The specific case histories reviewed in this paper include narrow panels with high extraction for conventionally mined rooms, pillar sizing for longwall gate entries at great depths, and bolt type selection for difficult ground conditions.

Jan 1, 1988