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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 D. W. H. Su

A two-year study was conducted by CONSOL in a northern West Virginia coal mine to evaluate pillar design and support requirements to improve roof control in right-hand longwall headgates subject to high in situ horizontal stresses. Horizontal stress measurements conducted within undisturbed areas of the subject mine revealed a pre-existing regional horizontal stress field with a significant difference in magnitude between the east-west maximum and the north-south minimum stresses. Three¬dimensional stress cells installed to monitor horizontal stress concentration in the headgate roof during mining revealed that a narrow (60-foot center) headgate pillar reduces the east-west horizontal stress concentration in the roof ahead of the face such that the headgate is more stable and requires less support. This 60 foot x 140-foot pillar design, which has been employed in subsequent panels, still provides adequate stress attenuation to protect future tailgates under a maximum cover depth of 1000 feet. Roof geology derived from underground roof reconnaissance and underground bolt load and roof displacement measurements confirmed the effectiveness of 12-foot long cable bolts for controlling the headgate roof subject to high horizontal stresses. With the new headgate pillar design and the supplemental cable bolts, headgate roof conditions in the subsequent panels improved significantly, longwall production delay due to adverse headgate roof condition was totally eliminated, and record coal production was achieved by the subject mine in the subsequent years.

Jan 1, 2003

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 Rene Rodriguez

The initial results of our investigations on the applicability of geophysical exploration methods to map coal seam inclusions were summarized in a paper submitted at the Thirteenth Conference on Ground Control in Mining in 1994. These investigations were continued under a variety of conditions at a number of underground coal mining operations. The results have been encouraging and have fostered a much more detailed understanding of the geophysical principles at work in underground mine mapping. This report summarizes a particularly interesting application, that is, the use of Underground In-Seam Seismic exploration methods to identify and to approximately delineate coal seam inclusions within longwall panels. The methodology relies on the excitation of seismic sources in the coal seam along one side of the panel and the recording of seismic waves by geophones secured to the coal rib on the opposite side. Head-Body waves (compressional and shear), direct and channel waves are generated in the process. While the head waves are critically refracted waves traveling at the coal floor and roof, direct and channel waves are transmitted through the coal body and are dispersive in character. Tomographic techniques applied to the velocity inversion of first arrivals permits a fast mapping of coal seam inclusions in the panel. The process incorporates path velocity and geostatistical intersect analysis. Full wave analysis of the dispersive channel waves, provides an independent and confirmatory information of the coal geometry, including the inclusions within. The inversion technique in this case is the modal analysis of the wave trains. The application of this method to three different problems was confirmed by subsequent mining of the predicted anomalies, proving the accuracy of the technique within limits of experimental error.

Jan 1, 1995

By Stephen Tadolini

Reduction in the time required and improvements in safety during a longwall face move have compelled coal mine operators to examine and use pre-driven longwall recovery rooms. While this concept is not without controversy and risk, the rewards can result in substantial reductions in longwall face move/set-up times and provide safer working conditions. Both of these factors can reduce mining costs and enhance profitability in a competitive market. This project was even more complex because the longwall system operates under a weak roof condition. To examine the feasibility of this concept, a 16-ft wide by 170-ft long experimental pre-driven recovery room was designed using analytical, empirical and 3-dimensional finite element modeling techniques. Based on the projected loading conditions, immediate and main roof geological and strength characteristics, a variety of roof bolt types were selected and installed in the recovery room and adjacent chutes. To supplement the intrinsic supports, high-capacity roof-to¬floor cuttable cribs were strategically placed to reduce the potential of a weighting failure before the longwall shields safely entered the room. To evaluate the behavior and final support performance, the recovery room and chutes were instrumented to monitor the physical response during various loading phases. Additionally, the cuttable cribs were instrumented to evaluate the loading and stiffness characteristics during the extraction process. This paper describes the successful design process, support methodology, monitoring results, final observations and recommendations.

Jan 1, 2002

By Erdal Unal

In order to define the support parameters, by use of empirical approaches, it is particularly important to analyze the rock-load height that should be controlled by supports. The controlling mechanism however, depends on the type of support used there- fore, the rock loads should be treated differently. In this paper, an empirical equation is introduced which relates the rock-load height (ht) to a quantitative rock quality index (REIR), and roof span (B) as fallows:

Jan 1, 1986

By S. M. Hsiung

A model (support selection model) for the selection of suitable types of powered supports under various geological conditions is proposed. This model is developed by evaluating the support, performance under different roof conditions at longwall faces in terms of five controlling parameters: (1) roof Falls, (2) front abutment pressure, (3) Face convergences, (4) gob materials entering into working area, and (5) roof caving line. This model suggests that more than one type of powered support. may be used under the same roof condition and the best selection should he based on the magnitude of suitability index of each support- type. The development and application of suitability index arcs discussed. Fourteen panels in seven coal seams are discussed in the paper to illustrate the applicability and effectiveness of this model. A roof classification system at longwall faces developed specifically for the construction of the selection model is also discussed. A simple example fur determining roof classification index is given.

Jan 1, 1988

By Karl Jr. Zipf

The famous Homestake gold mine in Lead, SD. closed recently alter 125 years of operation. However, the mine may receive a new lease on life as a National Underground Science Laboratory (NUSL) supported by the National Science Foundation (NSF) and dedicated to high-energy physics experiments involving neutrinos. These experiments require the construction of many large underground chambers tells of meters wide at depths of over 21 18 m. Construction of such large openings at such extreme depths has few precedents m civil-engineering-type construction. The Spokane Research Laboratory has a long history of ground control research at the Homestake Mine and has supported NSF and the physics community in their efforts to develop a NUSL at I Homestake or other suitable location. This paper will summarize underground science and describe proposed underground science laboratories ill the United States, in particular tile proposed NUSL at Homestake. Next, the paper describes an empirical design approach using the Q system of rock mass classification for these large Openings. .M initial site investigation that produced three good case histories is described. The actual support system used at the three sites agrees well with support recommendations from current support design charts, giving credibility to estimates of the classification parameters and Q. Feasibility of, and support requirements for, 25-, 50-, and 100-m spans arc estimated in the anticipated rock mass. This analysis indicates that 25- and 50-m spans are probably feasible, but a stable 100-nn span may be unachievable. The empirical predictions are then compared to detailed numerical analyses. The paper concludes with preliminary support recommendations and necessary site investigations needed to make a proposed NUSL a successful reality. These studies have been important input to NSF for their decisions on the technical feasibility of developing the proposed NUSL. If NUSL is developed, it could enable scientific experiments in areas beyond high-energy physics. 'the laboratory could also enable research in rock mechanics. ground water hydrology. mining engineering, nine safety and health, and ground control.

Jan 1, 2002

By Leo L. Van Sambeek

Specification of properly sized pillars to support the overburden is one of the most challenging aspects of salt mine design. Interest in salt pillar design is heightened by the recent flooding of the Retsof salt mine, water inflows to the K2 potash mine, and closing of the Belle Isle salt mine because of ground control problems. A series of numerical modeling analyses show that a simple pillar design equation for salt pillars adequately reproduces results of extensive modeling efforts. The paper describes (1) the basis for the pillar design equation, (2) equation verification by comparison with numerical modeling results, and (3) equation validation by comparison with measurements and observations of salt pillar behavior.

Jan 1, 1997

By D. Newman

An underground limestone quarry, developed in the 1940's, transitioned from a surface quarry to an underground room-and-pillar operation. The quarry expanded to a second underground level when the reserves on the first level reached the property boundaries. The historical basis for the sill pillar thickness between the first and second level, pillar dimensions, and mining height was lost during the course of several changes in ownership. When reserves on the second level reached a five-year life, and plans were initiated for a third level, quarry management embarked upon a thorough evaluation of i. current ground control practices for the support of the immediate roof, ii. roof support in the declines between the surface, Level 1, and Level II, iii. pillar centers and pillar safety factors, iv. pillar alignment between levels, v. mining height within a level, and vi. the locations of declines to access future reserves on level three. The evaluation began with detailed mapping of the location and orientation of calcite veins, joints, fractures, and ground conditions in the immediate roof of Level II. The orientation and location of calcite veins, joints, and fractures on Level II were compared with similar mapping on Level 1, completed in the 1970's to establish whether these features were continuous and would be expected to affect conditions on Level III. Rock strength, physical properties, and fracture spacing were obtained from two (2) diamond core holes drilled from Level I to Level II and two (2) holes drilled from Level II to define the vertical extent of future reserves on Level III. The effort to characterize geological conditions was expanded to a rock mechanics investigation of the: i. performance of the current roof support, ii. optimization of roof and rib scaling, iii. calculation of safety factors for 416 pillars on Level II, iv. evaluation of future pillar centers and mining height within a level based upon pillar strength and pillar geometry, v. development of beam safety factors for the immediate roof on Level II, vi. identification of the most suitable strata for the roof in the future level three, and vii. use of LAMODEL to develop a large numerical model of overburden, inter-level, and total stress on the pillars in levels one and Level II with a stress "footprint" on the proposed level three. The characterization of engineering parameters and geological conditions resulted in changes to the: ? active operations in Level II regarding scaling ? equipment and scaling practices, ? pillar centers, ? mining height, ? raising the immediate roof horizon to avoid ? laminated strata in the current roof, and ? a change in rock bolts from a friction based system ? to active tensioned rebar. The planning and design of level three is ongoing and relies heavily upon the geological and engineering investigations conducted during the past year and one-half.

Jan 1, 2001

By Hamid R. Nikraz

This paper reports the finding of a comprehensive investigation carried out to evaluate the compaction characteristics of saturated sandstone aquifers in the Collie Coal Basin under conditions anticipated during dewatering operations. A triaxial technique has been adapted to evaluate the compaction characteristics of the sandstone aquifer in the Collie Basin. This technique, which allows the strata stress regime to be reproduced by triaxial loading with zero lateral strain, also provides a precise evaluation of lateral stresses and consequently Poisson's ratio under in situ conditions. The paper contains a description of the equipment commissioned, test techniques, results, analysis and interpretation of the data obtained. The testing evaluation techniques are general in nature and can be applied to field situations in locations where similar weak sandstones occur.

Jan 1, 1993

By Chris Strawson

Moranbah North is a modern longwall operation mining premium coking coal in the Northern Bowen Basin, Central Queensland. Australia Moranbah North Coal (MNC) is owned and operated by Anglo Coal Australia. Extracting approximately 4.5m of the Goonyella Middle seam using a single longwall, the mine produces 5 to 6 Mt per annum. The weak coal (4.5 MPa), low strength laminated immediate roof, and strong upper roof sandstones, make operation of the longwall at depths in excess of 250m geotechnically challenging. In the past, there have been a number of production delays due to geotechnical issues. Roof instability resulted in a long stoppage on Longwall 105, and the eventual relocation of the face further outbye in the block. (Figure 1). When mining commenced in LW106, there was a requirement for uninterrupted production, particularly in the more demanding inbye area of the panel. Following discussions with a number of Geotechnical experts, it was decided to narrow the face, and test the theories of reduced face weighting due to spanning of the goaf across the panel. A new on line chock monitoring system, and microseismic monitoring was installed in order to characterise the difference in the rockmass response, as the panel width changed from 175m, to 265m, and then 300m. The nature of the goaf formation and impacts on face loading and production are discussed, having the unique opportunity of three different face widths within the same area.

Jan 1, 2007

By Duk-Won Park

The excavation of a longwall panel causes a continuous stress redistribution in the surrounding strata for every face advancement. The understanding of the behavior of rock to high level of stresses is instrumental to the structural design of underground mines. In order to achieve a realistic analysis of the behavior of strata, the three-dimensional finite element method is frequently used. Also the model developed for the analysis has to be designed large enough to represent the influenced strata in the study area and a sufficient number of elements has to be used to provide detailed information on the stresses and displacements. Due to the size of the model and its numerical solution the CRAY X-MP/24 supercomputer has been used for this study. The mine site selected for the analysis is a coal mine located in the Black Warrior Basin in Alabama. The mine has two longwall sections at a depth of 2,000 feet with various dimensions of chain pillars, including yield pillars. Various research programs at this mine have been carried out by Park et. al. ( 1984 ). Therefore data were already available on the geology, and physical properties of rock and coal. In addition a rock testing program has been carried out to obtain rock mass classification parameters. The progressive failure technique has been utilized to simulate stress redistributions. The failure criterion after Hoek and Brown ( 1980 ) which is based on rock mass behavior was used for the iterative technique. Simulated stress redistributions are therefore very realistic. In order to automate the process involved in the application of progressive failure technique, a computer program has been developed. The combination of a large scale three-dimensional finite element model and the automation of progressive failure simulation provided a tool that can generate a large amount of information related to the behavior of rock and coal as a response to a longwall mining operation.

Jan 1, 1989

By Lawrence R. Artler

The North American Coal Corporation, its subsidiaries and other coal companies, have been mining the Pittsburgh #8 seam in Eastern Ohio for several decades. Underground mining by the "room and pillar" method, and more recently by longwall methods, have been practiced. This presentation deals with ground control problems and subsequent solutions to those problems for both mining methods. But first, let me briefly discuss some pertinent points about the geologic conditions of the coal seam near the southwest edge of the Pittsburgh Coal Basin. The Eastern Ohio region of the Pittsburgh Coal Basin is situated along the western edge of the unglaciated Allegheny Plateau. The local terrain is quite steep, with topographic relief of 430-500 feet being common. The coal thickness in the region ranges from 36"to 120", with the average being 60" in the areas being currently mined. Regional dip of the seam is toward the southeast in an amount of 10 to 40 feet per mile. A cross-section of the mineable seam generally consists of the main bench (60"), a rider coal (6"-12"), and a claystone parting (6"-12") for a total excavated height of 6-7 feet. Greater excavated height occurs where the fireclay floor is so soft and friable that movement of mechanical equipment breaks up the first 4-8 inches. Generally, the main bench and rider coal thin and become more variable toward the southwestern portion of the mining area. The rider coal also grades laterally into a coaly carbonaceous shale. In those areas where the roof coal can no longer be counted on to help support the immediate top, some of the main seam (6") is left to protect the overlying 8'-12' of mudstone/claystone strata. These strata are virtually non-bedded, highly slickensided units and are "hung in suspension" by roof bolts from a more competent limestone stratum above. These mudstone/claystone units are very susceptible to moisture and thus to the weathering cycles. The rider coal or 6" of the main bench is left in place to "seal" the mudstone/claystone units in Mains and Submains development. For many years, it was thought that the strength of the limestone member was detrimental in attempts to fully extract the seam by conventional means; however, longwall methods that have been more recently employed have broken the "myth" that the Redstone Limestone prevents full extraction of pillars.

Jan 1, 1984

By D. Reddish

The combination of free standing steel supports, such as arches, and rock bolts (mixed support) has Frequently been in used in UK coal mine roadways when it is considered that the strata and/or environmental conditions would not be suitable for rock bolts solely. However, how such mixed support systems operate is not fully understood. Due to the differences in the support method provided by rock bolts and free standing steel, the total support generated is not simply a case of adding the contribution to the support of the roadway given by the two systems. This paper describes a numerical investigation into mixed support systems when used in coal mine roadways. A series of 2D and 3D geomechanical models are described and the method of modelling the rock bolts and steel supports in coal mine tunnels is discussed. The numerical models described include rock boltd only roadways, steel supported roadways and mixed supported roadways in a variety of geological and in-situ stress conditions. Conclusions are formed that allow an insight into mechanisms of strata reinforcement provided by the support systems modelled. Further to this, a review of the numerical modelling of pre-tensioned rock bolted systems is described identifying the contribution to support efficiency given by the adoption of tensioning in full column resin anchors. The results obtained highlight that the effectiveness of the pre-tensioned load is dependant on the rock mass quality and in- situ stress level at the site where it is to be used.

Jan 1, 2004

By Yunqing Zhang

Roof failure occurs much more frequently at intersections than entries and crosscuts. Using a commercial finite element package, four-way and three-way intersections reinforced with the tensioned bolts are modeled. The 3-D model considers bolt pre-tension, bolt installation procedure, bedding planes and in-situ horizontal stresses. The modeling results from both four-way and three-way intersections are analyzed with regard to the intersection stability. The mechanisms of tensioned bolting at the intersections are investigated and some tensioned bolting guidelines for intersections are given. Based on this study, a better way to enhance intersection stability is to use longer bolts than entries or crosscuts and increase the bolt density near the intersection corners.

Jan 1, 2003

By Holger Witthaus

The conditions of great mining depth and multi-seam¬longwalling require a special method of rockbolting, It can be described as a control loop which is based on the elements of planning, drivage and measurement. Different planning elements such as the results of the observation of physical models, numerical programmes for calculation of stress-distribution and support success, lab and underground support testing and the knowledge of strata behaviour underground including geotechnical classification are used to get best results. The increasing geotechnical information collected during drivage of roadways are used to estimate and minimise the risks of support-overload. The repeated plan/acutal comparison during the use of roadway and continuously monitoring of support and roadways provided the opportunity of optimising support design during the process of drivage and if necessary in front and after of the longwall face.

Jan 1, 2000

By Uros Bajzelj

When driving mine roadways in the vicinity of tectonic disturbances and fault zones, sudden outbursts of gas, along with ejections of fine coal, occur. Substantial dynamic changes occur around the roadways in the coal and around the support. The problem has several aspects. Studies have been carried out on the technical analyses of the phenomena which occurred in the Velenje Lignite Mine from 1957 to 1988, The second part of the investigation involves systematic laboratory studies of strength and deformability characteristics of lignite samples and "in-situ" measurements of stress changes during the driving of some roadways. Analyses of events (using the model of limit convergence and the finite element method) showed the possibility of gas outbursts, taking into account the deformability conditions when crossing over from compact lignite to the disturbed region. The rate of advance of roadways and the method of support strongly influence the occurrence of the phenomenon. The preventative measures used in driving roadways under such conditions include trial drilling and the use of blasting instead of machine cutting.

Jan 1, 1992

Excessive roadway closure in the gateroads impedes the passage of equipment, supplies, personnel and ventilation air in longwall panels. This pager describes several factors that influence the gateroad stability and presents some measures to alleviate the problem.

Jan 1, 1994

By N. C. Saxena

Subsidence investigations in Indian coalfields have led to development of relationships between subsidence movement parameters and defining of safe limits of subsidence movements for surface properties. The maximum subsidence over caved workings was generally not more than 60% of extraction thickness underground while in case of hydraulically sand stowed workings it was generally not more than 5%. The relationships have been useful in extraction of more than 7-million tonne of coal from underneath and in the vicinity of surface properties.

Jan 1, 1988