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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 Kot F. v. Unrug

In spite of the extensive research and accumulated experience from mining operations, an appropriate pillar design remains an open issue. Due to the shrinking reserve base, present mining operations move to more challenging conditions. There are usually thinner seams and geologic conditions are often unfavorable. For instance, mine roof and floor are weak or deteriorate rapidly when exposed to moisture fluctuations in mine atmosphere. It is common that pillars of current operations are more- squat than several decades ago when the bulk of pillar research have been carried out in thicker seams. The methods of pillars calculations were developed regionally (in particular countries) to suit design needs for the most important seams, from which the majority of coal production was coming (e.g. Pittsburg or Pond Creek seams in the eastern of US). Most of the pillar formulae use similar approach by assigning weights (using constants) to the influence of coal (seam) strength and pillar widthheight ratio on its final strength. That mechanistic approach has sound theoretical base and good data supporting its validity. However, what seems to be missing is a qualification system of the mine conditions, which are outside of the data base or do not comply with the theoretical model applied in pillar strength formulae. This paper attempts to address that issue in two major categories, first when only a drill core data are available, and second during the mine development observations can be made directly on a coal seam. The composition of a coal seam is also addressed, (which often changes rapidly across the reserve), and discussed its influence on pillar's structural performance. In summary a set of practical design rules is presented.

Jan 1, 2005

By Kevin W. Harris

Multiple-seam coal mining is a major issue in underground coal mining that adds additional complexity to the practice of ground control. As reserve exploitation is driven by economics, it is now difficult to mine coal without the existence of previous workings above and/or below the active seam; this is particularly true in the Appalachian coalfields. The frequency in which problematic multiple-seam interactions are now encountered has resulted in significant research regarding the underlying mechanisms of the problems and methods of mitigation. In the last few decades, a great deal has been learned about these interactions and the ability to minimize problems associated with these hazards has greatly advanced, resulting in better economics and improved safety. Minimizing the adverse effects of stress interactions often requires a thorough understanding of site-specific geotechnical parameters and previous mining geometry. This is especially evident in bump-prone ground, where pillar behavior is highly dependent on local conditions. This paper presents a case study in a deep cover, Central Appalachian coal mine where three bump events occurred. Initially, empirical design software was used in an effort to analyze the situation and prevent further instability. The empirical study proved ineffective. A detailed investigation of the bump events using the boundary element software LaModel 3.0 was then performed. This study resulted in a site-specific calibration standard that will be applied in numerical pillar design for the remaining reserve.

Jan 1, 2014

By B. (Mamas) Das

A project to evaluate the suitability of point Load strength tests for testing and classifying coal measure rocks for coal mining application was initiated by Canada Centre for Mineral and Energy Technology (CANMET). A summary of this study with emphasis on the important findings is presented in this paper. The instrument as used in this study is a Robertson Research Rock Strength Index Log. Because of the variation in the correlation ratio between uniaxial compressive strength and point Load strength index for hard rocks and soft rocks it was decided to establish this corre¬lation initially on cement concrete specimens specially prepared for this purpose following ASTM specifications. These specimens are expected to represent soft rocks and at the same time would be of uniform quality. Once a general trend of this correlation was established tests were performed on coal and rock samples from the sub-bituminous and bituminous coalfields in Canada. The most important results may be stated as follows: (i) Generally, the common value of the ratio between the uniaxial compressive strength and the point load strength index ranges between 8 to L4. (ii) The Robertson Research Rock Strength Index Log proves to be quite suitable for testing sandstone, siltstone and some coals from the bituminous coalfields but problems have been experienced when testing weathered samples or samples from the sub-bituminous coalfields. (iii) In some cases tests were successful if Larger samples from weathered zones or from sub-bituminous coalfields were used. However, a magnified load gauge for 0-3KN or 0-5KN is expected to be the best solution for soft and weathered rocks in most of the cases. Some mudstones and some rocks from weathered zone do not appear to be suitable for these tests. This aspect of the study is still under investigation. (iv) Deterioration of rock properties due to weathering has been determined through tests on samples collected from weathered and unweathered samples.

Jan 1, 1984

By David A. Newman

In the 1950s, an underground limestone mine in the Camp Nelson Formation was developed from an outcrop exposure. The Camp Nelson Limestone met the requirements for DOT stone and aggregate. The mine ownership changed several times, which resulted in distinct mine designs in Level 1. Based upon the production requirements and an attempt to maximize limestone recovery from a single level, a variety of pillar dimensions and three mining heights (28 feet, 68 feet and 83 feet) were developed. This was sufficient prior to reaching the mineable extent of Level 1. Level 2 introduced previously unknown ground control challenges. These included: ??Pillar spalling and a significant back fall on the Level 1 - Level 2 decline; ??Laminated immediate back and the need to bolt the back on Level 2; ??Delamination of the back adjacent to the Level 1 - Level 2 decline resulting in bolt shearing, and floor heave in isolated areas of Level 2; ??Multiple level interaction, the need to design pillars based upon numerical modeling as columnization between Levels is infeasible: and ??Water inflow as the mine approached river. To meet the challenges, a testing program to determine rock strength and physical properties was implemented along with numerical (LAMODEL and FEM) modeling, and borescope inspections of the immediate back. The efforts to implement rock mechanics were re-focused as Level 2 approached its mineable limit and the initial attempt to drive a decline to Level 3 was unsuccessful. A second decline site was selected based on inter burden stability, a consistent shale parting for the Level 3 back horizon, results from numerical models of the immediate back and multiple level interactions, and the projected low water inflow. The mine is operating on Levels 2 and 3 with long-term planning and ground monitoring for Level 3.

Jan 1, 2014

By Y. P. Chugh

The paper discusses laboratory and field studies to analyze time-dependent deformation behavior of immediate weak floor strata in an Illinois coal mine. In the laboratory, core samples of the immediate floor strata were subjected to incremental creep tests under unconfined compressive stress. Both axial and lateral creep deformations were recorded. Time-dependent behavior was reasonably characterized by a Burger model with one Kelvin-Voigt unit. The elastic and viscous parameter8 of the model were found to be stress-dependent, but in the range of 40% to 80% of the failure stress, the effect of stress was relatively small. Therefore, a linear visco-elastic model was developed to represent the laboratory time-dependent behavior of immediate floor strata. Such a model may be acceptable for the design of partial extraction mining systems having extraction ratios of 40% to 60%. Field observations of roof-floor convergence (floor heave) at the mine also indicated a time-dependent behavior similar to that observed in the laboratory. Field time-dependent behavior was also characterized in terms of a Burger model with one Kelvin-Voigt unit to obtain scaling factors between the field and laboratory parameters. This approach should help in predicting likely floor heave in the mine based on laboratory creep tests. It was concluded that additional studies in the field are required to arrive at more reliable correlations between the two sets of parameters.

Jan 1, 1987

By Murali Gadde

U.S. coal mines? primary roof supports typically consist of passive resin bolts; however, the use of active bolt systems is increasing. Despite this widespread use, a comparative performance evaluation of these bolt systems has not been done under similar geologic conditions. The performances of three commonly used primary bolt systems were investigated: fully grouted passive rebar, fully grouted tension rebar (using two speed setting resin), and resin-assisted rebar with mechanical anchors. In order to investigate the performance of these bolts, strain gauged bolts of all three types were installed at three different coal mines as part of a larger research project funded by the National Institute for Occupational Safety and Health (NIOSH). This paper addresses the initial installation bolt loads measured at these mines. Analysis of the data showed much lower installed loads on active bolts than is traditionally assumed. It appears that the active load on bolts bleeds off relatively quickly under geologic and operational conditions similar to those at the studied mines. The data also showed that within a short time span after installation, the measured loads on active and passive bolts did not differ significantly. This paper compares the results from over 20 bolts of each bolt type that were all 1.8-m (6-ft) -long, 20-mm (0.804-in) nominal diameter Grade 75 rebar and discusses the relevance and shortcomings of the data and bolt performance.

Jan 1, 2011

By Bruce Hebblewhite, Jim Galvin

"A coal burst occurred on 15 April, 2014 at the Austar Coal Mine, located west of Newcastle, NSW, Australia. The burst resulted in fatal injuries to two men working as part of the mining crew at the development face. At the time, a continuous miner was being used to mine a longwall development gate road through heavily structured coal, at a depth of approximately 550m. A number of pre-cursor bumps had occurred on previous shifts, emanating from the coal ribs of the roadway, in proximity to the coal face.This paper reviews the geological, geotechnical and mining conditions and circumstances leading up to the coal burst event; and presents and discusses the available evidence and possible interpretations relating to the geomechanical behaviour mechanisms that may have been critical factors in this incident. The paper also discusses some key technical and operational considerations of ground support systems and mining practices and strategies needed for operating in such conditions in the future.INTRODUCTIONAustar Coal Mine is an underground longwall coal mine located near Cessnock in the Hunter Valley of New South Wales (NSW), Australia and is the only underground mine still extracting the Greta Seam in this region. The mine was the first in Australia to adopt the Chinese-developed Longwall Top Coal Caving (LTCC) method for thick seam extraction. Typically seam thickness ranges from 4m to 7m and depth of mining from 480m to 560m, with future mining planned down to depths of up to 700m, making it one of the deepest operating coal mines in Australia.On 15 April 2014, a pressure burst occurred in the left hand rib at the active mining face of B Heading, 2 to 3 cut-through, Maingate A9 panel, during development of the gateroads for the ninth longwall top coal caving panel. Strata in the general vicinity was affected by disturbed geology and multiple geological structures. Figure 1 shows a section of the mine plan as at October 2014 (six months after the accident), indicating the current longwall extraction panel (AS) and the development panel A9 where the accident occurred. (Note: Neither development nor longwall face positions changed significantly between the time of the accident and the date of this plan). At the time of the accident the current longwall face was in excess of 1,000m away from the Maingate A9 development panel face position."

Jan 1, 2016

By M. Karmis

Underground mining will disturb the natural equilibrium of the rock mass, causing significant stress redistributions in the vicinity of the excavation with corresponding horizontal and vertical displacements. Subsidence of the ground occurs when these displacements propagate from the mine opening, through the overlying strata, to the surface. Such ground movements may take either of two general forms: a sudden and sometimes violent collapse, or a more gradual and progressive ground movement. The former, characterized by localized failure of roof or pillar in old, shallow room and pillar workings, is rather unpredictable and difficult to analyze. As a result, most control measures are directed toward the latter type of movement, which is associated with the fracture and flow of the strata overlying longwall and high-extraction room and pillar systems, resulting in horizontal and vertical strains in the affected area. Such ground movements can often cause considerable surface disturbances ranging from simple land settlement to severe structural damage. Due to the increasing incidence of this phenomenon in more populated areas and the rising awareness of the problem, it has become necessary to establish a balance between the development of mineral resources and the protection of the environment and society. This paper presents a summary of the major findings derived from a substantial research effort on mining subsidence carried out, during the past few years, in the southern Appalachian coalfield. The research program encompassed the following major tasks: identification of the most significant factor influencing ground movements above mined openings, collection of subsidence case studies for the southern Appalachian coalfield, development of subsidence and strain prediction techniques for that region and initiation of a subsidence monitoring program in southwestern Virginia.

Jan 1, 1984

By John Bullock

Limestone and dolomite quarries were mapped to determine specific failure mechanisms for the various geologic and structural conditions. Algorithms for analyzing these mechanisms were incorporated into a software package together with ranges of physical properties such as joint shear strength, angle of internal friction, and cohesion for specified lithologic units. Reliability options using Monte Carlo sampling and sensitivity studies are included to evaluate alternative highwall designs. The package is designed to facilitate progressive stability evaluation of quarry highwalls as mining continues. This package includes an optimum design protocol that can be followed to avoid the potential of massive failure. The program was developed in conjunction with the quarry industry and is demonstrated using case studies.

Jan 1, 1993

By H. N. Maleki

Extensive measurements and underground observations in three Western U. S. coal mines are integrated in this paper to determine in-situ pillar load-deformation characteristics for narrow (30 it wide, 80 ft long) pillars in two-entry gate road systems. In spite of similarities in the regional geology, coal pillar laboratory mechanical properties and gate pillar geometries, the pillar peak strength, post-failure behavior, and failure mechanism were shown to be significantly different. Pillar peak strength was shown to be dependent on depth at one site, approaching burst-prone stress levels of 4000 psi. At another site, the pillar peak strength was lower because of lower confinement; this was related to the higher frequency of cleats and the lower friction- al properties of the roof/floor and coal contact. Two failure mechanisms were identified - one in the pillar, and the other in the mine floor. The roof stability was good at all three sites because of the thick-bedded nature of the roof strata and limited total gate span in a two-entry system. Existing pillar design techniques were shown to be inadequate for design, requiring adjustments for depth of cover, cleat frequency, and roof /floor frictional properties.

Jan 1, 1988

By P. K. Kaiser

The purpose of deformation monitoring for the evaluation of the stability of underground openings is reviewed briefly and some results from laboratory tests of small tunnels are presented. The ground behaviour is described by comparison with the response of structural systems and illustrated by the data from the testing of small tunnels. It is concluded that the main objective of a deformation monitoring system for stability evaluation is to determine the actual process of deformation and instability in the area surrounding the opening. Only when this process has been established is it possible to use deformation measurements to design support systems or to improve construction or mining procedures. Present monitoring practice is not sufficiently orientated toward the aim of process description.

Jan 1, 1981

By Hamid Maleki

During the last decade, a significant amount of research has been conducted by Energy West Mining Company, government agencies, academia, and consulting companies to develop predictive tools for assessing the stability of the Castlegate Sandstone, which is found approximately 250 m above multiple-scam coal reserves, Energy West Mining uses longwall mining techniques in its operations near Huntington, Utah, and these studies were initiated to satisfy requirements for maintaining the stability of the Castlegate Sandstone and resource recovery. In this study, the authors have used multiple-regression analysis techniques and a wealth of data collected over many years on geology, mining, and escarpment stability. The volume of failed rocks is used as the response variable after several other factors were considered, including measurement of surface deformation and frequency of mining-induced surface fractures. Geologic and geometric variables were obtained along 3.7 km of escarpment exposure at 130 study locations. Regression analysis of data for the first 29 study location, which had been fully' undermined, showed that surface topography played a critical rule in influencing escarpment stability. Preliminary regression analysis results from 70 study loca¬tions identified several other important geologic arid mining factors that influence be stability of the sandstone escarpment These are canyon slope, sandstone thickness, and mining influence ante.

Jan 1, 2000

By Ding Xu Chu

There are many methods developed for measuring the crustal stress in China. Among them only two methods have been adopted in practice, i.e., overcoring method (for which the sensors include: Piezo-magnetic stress gauge, steel-ring strain cell, 3-dimensional hollow-inclusion stress ,gauge, and Leeman's-like stress gauge) and hydraulic fracturing method. For the former method, one of those sensors is installed in a borehole, 36 mm in diameter, then prestressed, and finally overcored with a drill bit of 130 mm in diameter. After that the deformation of the borehole wall is measured, from which the in-situ stresses are determined. In order to avoid the errors resulting from the difference in elastic modulus for various kinds of rocks as well as from the installment of the cells, we have developed a technique to calibrate the sensor on-spot using a confining pressure calibrator and produced a very good result. For the later method, the measurements are achieved by isolating a certain section of a borehole and then injecting water to pressurize it till a tensile break is induced. It is identical to the typical technology used internationally. For many years, through field observations and a large number of laboratory experiments, we believe that the most efficient way of stress measurement for engineering purposes is the overcoring method using Piezo-magnetic stress gauge. It is characterized by stable and reliable measurement values, high accuracy and high adaptability. Taking the measurements in the intact rock mass as an example, the measured stress magnitudes are within accuracies of 8% and the errors in orientation are about 5 degrees. It can even be used perfectly in deep water. Up to now, we have performed in-situ stress measurements in more than 70 sites and obtain more than one thousand sets of data. The deepest depth used in the overcoring method is 80 m in vertical borehole and 780 m in tunnels.

Jan 1, 1987

By Peter Zhang

As simpler and easier reserves are exploited to depletion, underground coal mining continues in more complex geologic settings and conditions. Roof bolting for long-term entry stability is always a challenge. For long-term entries of a mine such as belt, track entries and return airways, roof could be under-supported if one uses the same bolting plan as in short-term entries or even seem over-supported with increasing use of longer, stronger bolts along with mesh, pans, straps or trusses. In most underground coal mines, roof deteriorates over time with cutters developing deeper in entry corners; roof sagging and popping out between bolts; entry width increasing due to rib sloughage; and separations developing deeper up into the roof. In deteriorated long-term entries, roof falls could occur at certain locations and re-bolting could be costly or sometimes unfeasible. Therefore, it is important to choose an appropriate bolting plan during development based on mining and geologic conditions if the entries will be used for the long-term. In both short-term and long-term entries, primary roof bolting is critical in building a strong and effective beam. But in long-term entries, supplementary bolting is necessary in certain cases to ensure roof stability can still be maintained even if a roof failure develops beyond the primary bolted horizon. This paper deals with roof bolting for long-term entries based on studying the progressive roof failure and roof falls under different geologic conditions. The paper presents bolting considerations in terms of bolt type, length, diameter and spacing as well as bolting strategy to minimize the risk of roof falls in long-term entries.

Jan 1, 2011

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 Mohummad Akram

Statistical analysis of multi-seam mining case studies demonstrated that geologic structure and lithological type and sequencing were predominant factors in entry failures. Finite element analysis was used to evaluate the effects of stress field changes on entry instability under differing geologic conditions. Analysis showed the roof to be particularly vulnerable to shear with the location of the failure site changing to meet loading and geologic conditions. Effects of external support on roof stability were examined. Results were correlated with field data to determine stress levels at which failure could be expected to occur. Based on this research design, criteria were developed for roof stability evaluation under specified interactive loading conditions.

Jan 1, 1993

By Winton J. Gale

Coal mine pillar design formulae available to mine planners are typically based on relationships determined for specific site conditions. Application of these methods is difficult when other factors, relating to mining or geological conditions, not specifically addressed in these relationships are encountered. A review of pillar design methods, coal strength properties and relevant field monitoring of pillar behaviour has shown pillar strength and deformational characteristics are extremely sensitive to in situ behaviour of the rock and other material components that make up the coal pillar system. A method has been developed that has been found to provide excellent correlation between existing field data using the strength and deformational characteristics of materials determined in selected laboratory tests. This method is based on definition of the leading conditions, accurate modelling of the strength, confining conditions and deformation occuring in the roof, floor and coal seam and definition of the in situ strength of coal based on underground monitoring of appropriately sized coal pillar. The design method is considered to provide an accurate and rigorous pillar design method that will give mine planners a more rational mine design capability which is based on the mine geometry and geological setting of the operation. It is envisaged that availability of such a method will allow significant optimisation of pillar layouts and more closely define the geological and mining issues that relate to pillar design optimisation.

Jan 1, 1992

By Zhaohui Wang, Jiachen Wang

"Effective surrounding rock control is a prerequisite for realizing safe mining in underground longwall faces. In the past three decades, longwall top-coal caving mining (LTCC) and single pass large height longwall mining (SPLL) found expanded usage in extracting thick coal seams in China. The two mining methods lead to large void space left behind the working face, which increase the difficulty of ground control. Longwall face failure is a common problem in underground mining, which is induced due to low strength and high fracture density of the coal seam. Moreover, the stiffness of main components included in the surrounding rock system also greatly influences longwall face instability. Correspondingly, the surrounding rock system is developed for LTCC and SPLL faces in this paper. Mechanical conditions for simultaneous balance of roof structure and longwall face are put forward by taking the stiffness of coal seam, roof strata, and hydraulic support into account. The safety factor of the longwall face is defined as the ratio between its ultimate bearing capacity and the load imposed on it. The influence of coal strength, stiffness of the coal seam, roof strata, and hydraulic support, as well as the movement of roof structure, are analyzed. Finally, the key elements dominating longwall face stability are identified for improving surrounding rock control effectiveness in LTCC and SPLL faces.INTRODUCTIONThe concentration of coal mining in China is gradually shifting to the west region, and there are a large number of thick coal seams in the newly formed coal mining area. Therefore, the surrounding rock control problems related to mining of thick coal seams will become the focus of research in the field of mining engineering in the future. There are two kinds of mining methods for thick coal seams in China, including single pass large height longwall mining (SPLL) and longwall top-coal caving mining (LTCC). Both methods can accomplish the goal of excavating the full thickness of thick coal seams at one time. SPLL mining benefits from the improvement of the development level of associated equipment. The appearance of large hydraulic supports and high power shearers ensure that the height protected by the hydraulic supports and the height of the sheared coal are the same as the coal thickness. LTCC mining benefits from mining technology advantages. The thick coal seam is divided into two sub-layers, and the working face is normally installed in the bottom layer, while the top coal is caved under the compressive forces due to mining. Therefore, there is a scraper conveyor at the front of the hydraulic support for transporting the sheared coal and another one at the tail for transporting caved top coal. Both SPLL and LTCC methods realize one-off recovery of thick coal seams."

Jan 1, 2018

By Alan Peacock

Shield supports have been in use in the USA underground mines since 1975, and their development in the intervening time has been progressive and inovative. Specialized techniques are used to optimize their performance in a particular application and to design the component parts.

Jan 1, 1981