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By Peter Cain

Longwall extraction in the Phalen Scam in the Sydney Coalfield of Nova Scotia is carried out under flooded abandoned workings in the Harbour Seam 140 m above, and at depth of up to 700 m beneath the Atlantic Ocean. Careful planning of panel and pillar dimensions is necessary to prevent inflows from the overlying workings and to prevent the development of excessive subsidence strains at the seabed. This paper briefly summarises the research undertaken over the past five years by CBDC and CBCRL into the problems associated with mining under water, and then describes the development and application of a design/planning code of practice developed jointly by CBDC, CBCRL and mine regulatory agencies. The design method employed in the Code of Practise is based on empirical data from the coalfield and parameters developed from subsidence modelling and previous water inflow experience. It includes precautionary measures to be adopted during development and mining, and monitoring procedures to be followed. It is flexible enough to allow future developments in the understanding of inflow mechanisms to be incoporated without major modifications, and represents a major step forward in reducing the potential hazards associated with coal extraction in the coalfield.

Jan 1, 1996

By A. W. Khair

Laboratory studies indicate that the specimen size influences the compressive strength of coal significantly. In the past, the diameter/height ratio of coal and rock specimens with different geometry and its influence on failure characteristics was discussed (Khair, 1993; Khair, 1994; Khair and Xu, 1994). In the current attempt, tests are designed and carried out under uniaxial condition in the laboratory to investigate the influence of specimen size. Cubical specimens of coal with edge dimension ranging from 5 cm (2 in.) through 20 cm (8 in.) were prepared according to the standards and tested uniaxially. Results show that compressive strength of coal is dependent on specimen size until certain level and later the change in specimen size would not yield significant change in compressive strength. Cubical specimens were chosen for testing because they are not only in close resemblance to underground pillars, but they are also the most stable shape under loading. The failure characteristics of these specimens were analyzed and their applicability to pillar design aspects in coal mines is studied. A detailed analysis of the physical and mechanical properties of the tested coal seam is also presented.

Jan 1, 1996

By Anthony Iannacchione

Designing coal pillars to provide resistance against overburden loads has long been an aim of rock mechanics engineers. The need for accurate pillar strength models has become more urgent as greater overburdens are encountered and pillar sizes grow larger. Current pillar design models differ widely in their predictions of the trend in pillar strength with increasing pillar width-to-height ratio. The goal of this paper is to evaluate current pillar strength theories, using a comprehensive data base of stress measurements from coal pillars. The stress measurements indicate that coal pillars maintain relatively high stresses near the ribline, and that the stress gradient within the yield zone can be approximated as a straight line. Several problems are identified for further research, including calibration procedures for different types of stress cells, strain softening behavior in the yield zone, and measurement of stresses in the cores of very wide pillars.

Jan 1, 1992

By Ian Misich

Surface subsidence resulting from "total extraction" of coal in the Collie Basin first occured in 1987 when Western Collieries Ltd trialed the Wongawilli mining method. Because very little quantitative subsidence data existed for the weak, Permian sediments, a research program was set up prior to the extraction of the first panel. The objective of the study was to be able to predict subsidence characteristics and effects prior to mining future total extraction panels. Attempts at predicting subsidence began using existing subsidence models from other coal mining regions, however, as more site information became available, it became clear that it was necessary to develop a separate, site specific predictive model for the Collie Coal Basin. Analysis of extensive field monitoring data for both surface and subsurface subsidence has produced a series of empirical models which can predict the shape of surface subsidence troughs and corresponding tills, and strains for rectangular panels and describe the sequences of ground movement from the mine root through to the ground surface. The fact that both subsurface and surface subsidence characteristics have tied in well, gives greater confidence for predictions made from these models

Jan 1, 1992

By Victor V. Nazimko

Multiple mining generates gob-interaction displacements. They redistribute stress m strata and obey laws of irreversibility thermodynamics. An irreversible state causes inhomogeneity of the stress distribution on the mass. A destressed zone develops along a working panel, and worked out panels are deprived of the destressing. The inhomogeneity is smoothed out during elapsed time.

Jan 1, 1996

By Anastasia M. Suchowerska

The design of effective ground support requires a sound understanding of the pre-mining state of stress in the stratum to be mined. Irregularities in the stress field encountered during multiseam mining are governed primarily by the previous mining in the surrounding strata. The aim of this study is to quantify the effects of rock mass transverse isotropy on the vertical stress redistribution in the rock strata beneath supercritical longwall panels. Wilson?s equations for determining the stress distribution induced in and beyond a single longwall panel after it has been mined have been adopted, and a plane strain finite element analysis of an elastic medium, together with the stress changes predicted by Wilson, was used to calculate the final pre-mining stresses in the underlying strata. Values of the independent shear modulus (G¢) of the rock strata, ranging from 0.1 to 1 times the isotropic shear modulus (Giso) in the plane of the strata, and a Poisson?s ratio (?) of 0.25 have been investigated. The key finding of this study is that the predicted magnitude of vertical stress in the transversely isotropic strata beneath a mined supercritical longwall panel is larger and changes more quickly with horizontal distance than is the case for an isotropic rock mass. This finding is of significance for the design of ground support measures in multi-seam mining operations.

Jan 1, 2012

By Andre Vervoort

Falls of ground, such as falls of roof and sidewall collapses, are a major cause of fatalities and injuries in South African collieries. Based on information obtained from the accident reports of the various Chief Inspectors of nines, a statistical analysis was conducted covering the period 1970 to 1988. This study shows that the volume of rock in a fatal fall of ground is often quite small. In intersections, fatal falls are on average larger than in roadways and at the face. It was also found that in 36 % of all fatal falls of ground no temporary or permanent support was installed at the site of the accident.

Jan 1, 1990

By Wouter Hartman

Xuandong Coal Mine is mining coal in one of the most challenging underground environments in the world. This work has found that it is not only challenging but it also has the composition for some interesting rock mechanics and ground control conditions. The paper highlights the extreme mining and rock mass behavior conditions encountered when mining coal at 900 m below surface. It was found that the ground control environment associated with geological weakness, low coal material strength and stiffness, was reasonably well controlled; however, the chain pillar geometry and design did not take these geological weaknesses into consideration. Some of the chain pillars employed were quite large, which resulted in floor punching and secondary floor heave, which causes extreme difficulty for production. No barrier pillar has been employed except for the center 164-m-wide main headings pillar with a w/h ratio in excess of 40. After assessing the location and geometry of a 200-m-thick dolerite sill overlaying the coal measures, we found the probability of it breaking up into to smaller fragments and producing a larger goaf to be low. Therefore, the influence of the dolerite sill on chain pillar loading is considered small compared to the low coal, immediate floor and roof rockmass strength. It is further recognized that horizontal stresses could be locked up within the dolerite sill, which would likely result in regional bending that would be distributed through the chain pillars. This has, to some extent, already occurred, and the effects have been seen and felt through excessive floor heave, pillar punching, and coal bumps (seismicity). We?ve also found that there was a need to employ basic rock mechanics principals in assessing the stability of future multiple seam mining layouts.

Jan 1, 2012

By Anthony T. Iannacchione

The Roof Fall Risk Index (RFRI) is a new method introduced by the National Institute for Occupational Safety and Health (NIOSH) to assist the underground stone mine operator in 1) assessing defects the mine strata and 2) rating the relative roof fall risk these defects pose. The RFRI utilizes observational techniques to identify defects in the roof strata caused by local geologic, stress, and mining conditions. Assessment values for ten defined defect categories provides the foundation for estimating a range of risk conditions between 0 and 100 that define the potential for a roof fall. This paper examines how the defect information is collected using two field verification sites and proposes methods to analyze the RFRI data. These examples provide information on how well the method replicates observed conditions and how it might be applied in practice.

Jan 1, 2006

By C. Dinis da Cama

In order to reduce the complexity of mechanisms influencing rock- fragmentation by blasting a simulation approach is proposed, using the capabilities of micro-computer interactive graphics. Situations involving cratering and bench blasting are simulated, and both jointed and intact rocks can be subjected to explosive action, taking into consideration its breakage mechanisms. Size analysis curves relating rock distribution before and after the blasts are determined and compared with field data.

Jan 1, 1984

By Greg Smith

This paper documents the use of bench testing equipment developed to enhance the understanding of, and improve upon, common roof bolt support systems. Most roof bolt systems consist of components that are covered within the scope of ASTM (American Society for Testing and Materials) standards. However, ASTM does not specify testing methods or performance standards for complete roof bolt ?systems?. It only designates testing methods and minimum performance standards for roof bolt ?components?. Improved understanding and continuous improvement of these systems is essential given the enormous impact that they have on the safety and profitability of underground mining operations. Bench testing equipment has been developed that effectively measures and records multiple points of deformation in a rock bolt support system while load is being applied. With this testing capability, an improved understanding of component and system stress distribution can be obtained and devices and methods for visual load indication can be developed. This paper is formatted as case studies where testing was performed to demonstrate load versus deformation profiles and to develop system improvement products.

Jan 1, 2012

By Peter Fischer

Between 45,000 and 50,000 claims for subsidence are submitted each year to the German hard coal industry. In order to improve the service to claimants a quality management was established in 2005. This paper describes the process now used for handling these claims. A quality check has also been carried out by way of a customer survey, which was performed by an independent research agency. The results of the survey are presented in this paper. In Germany, research into mining subsidence mainly involves the prediction of ground movements caused by deep mining operations. Special areas of research are subsidence reducing aspects of the extraction panel planning, especially from the point of view of mining dynamics, as well as securing and restoration measures. This work seeks to prevent or minimize mining-induced damages close to the surface, for instance, buildings or infrastructure installations (such as pipelines), or underground mine openings (e.g. shafts). Roof collapses and surface fractures caused by shallow mine workings of an earlier era currently constitute a special area of investigation.

Jan 1, 2006

By Takashi Sasaoka

Chain Conveyor Cutter (CCC) is a chainsaw-type machine and originally developed for the construction of underground walls and/or ground improvements. CCC was developed from the accumulated technology of coal mining, combining the cutting technology of a shearer and the fuctions of chain conveyor. The CCC can cut harder soils and soft rocks, and vertically mixes the cut soils/rocks with cementing material selected through the discharging port installed at the bottom of the machine. In Indonesia, highwall mining trials have already been conducted in several coal mines. However, full-scale and regular operations have not been done yet due to the ground and machine control issues from their geological conditions. In order to overcome these problems, the application of CCC to highwall mining systems was proposed, and its applicability and technical issues to be solved were investigated in this research. This paper discribes the characteristics of CCC system and discusses the application of CCC to highwall mining systems.

Jan 1, 2012

By K. H. Brandt

A standard planning system for the development of roadways was adopted for German coal mines based on the results of geotechnical and rockmechanical investigations. Its goal is to provide for sufficient support systems under conditions of multiple seam mining at great depth. The report points out the required determination of geotechnical parameters and the basis for achieving a high level of planning reliability and resulting high performance in roadway development and longwall mining. The instrumentation and the planning procedure for selecting the support design most appropriate for the geotechnical conditions are described and demonstrated by examples.

Jan 1, 2004

By B. G. D. Smart

An understanding of the nature of yield zones surrounding coal mine excavations is fundamental to the design of mine support systems. Various methods have been designed for investigation of underground rock masses and an extensometric technique, the Magnetic Rod Extensometer, has been developed to overcome inherent design limitations in existing equipment of this type. This paper refers to the prototype form of the extensometer and details changes designed to overcome a variety of drawbacks indicated by routine use of the equipment. The principal change has been one of manual to computerised data acquisition and presentation in the laboratory phase of the system. This has dramatically reduced the time taken to process underground measurements and produce numerical and graphical results. The revised Magnetic Rod Extensometer represents an integrated instrumentation system that combines speed and accuracy in all areas of data handling.

Jan 1, 1984

By Michael Bayerl

The HILTI OneStep rockbolt was implemented for roadway development in German deep coal mines in 2004. The development by HILTI Company is based on a technical joint venture with RAG Deutsche Steinkohle AG. The technical development of a One-Step bolting system was required for weak strata conditions at the roadway development face. Standard resin bolting has reached the limit of performance when boreholes are squeezed and cannot be filled with capsules. In some cases the heading face is brittle and requires additional support for installation of the bolts and a person working immediately next to the borehole. The Hilti One-Step system allows automated and safe processing at the heading front and provides the key technique to avoid these problems.

Jan 1, 2009

By Youzhen Zhang

The drilling boom is the key component of a roof bolter, and it has significant impact on the performance, safety, and reliability of the overall working device. In this paper, the experimental modal parameters of a drilling boom at low frequencies are identified with the Laplace wavelet correlation filtering method. Based on the obtained experimental parameters, a finite element model of the drilling boom is revised using an optimization algorithm to improve its accuracy. The dynamics of the drilling boom are analyzed through its modal and harmonic response. As a result, an improvement for the drilling boom is proposed to fix the weak links and enhance the dynamic performance.

Jan 1, 2012

By Andre Zingano

The objective of this paper is to compare the behaviors of different types of roof bolts: (1) One-Step Bolt (OSB), (2) Fully grouted Bolt (RB), (3) Combination Bolt (CB), and (4) Torque tension Bolt (TB). Two dimensional numerical models were built for both bolted and unbolted roofs. The geological sequence and rock properties used in the numerical models represent the typical roof condition for Pittsburgh coal seam. Using the unbolted roof as a base case, a comparison is conducted by monitoring the roof vertical displacement at the center of the entry and at the bolt positions, the axial force in the roof bolts, and stress distribution in the bolted area. The grout/rock interface is assumed to be perfect. The estimated roof displacements showed that the differences among the four types of roof bolts are small. The major differences are in the shear stress at the grout/rock contact and in the shear stress along the immediate roof bedding interface as well as in the bolt axial stress. There is less axial stress for the OSB compared to RB and CB as the diameter of the OSB is much bigger and the contact area between grout and rock is twice as large as for RB and CB. The higher strength of the OSB contributes to keep the bedding planes in the roof closed.

Jan 1, 2007

By R. D. Yancich

A case study of longwall mining surface subsidence was performed at a mine in Southwestern Pennsylvania. The mine operated in the Pittsburgh coal seam which averaged 70-72 inches with 6-8 inches draw slate in the longwall location. The average overburden depth ranged from 625 8eet to 660 feet. Three adjacent longwall panels were studied and classified as Panel 1, Panel 2, and Panel 3. The following is information on each and can be seen on the General Mine Map (Fig. 1).

Jan 1, 1984

By Christopher Mark

The first International Conference on Ground Control in Mining opened with the topic of pillar design. Two classic papers were presented, one by Bieniwski and the other by Wilson. Unfortunately, the two methods were so radically different from each other that it was nearly impossible to reconcile them. Adding to the confusion were the many other pillar strength formulas (such as the Salamon-Munro, the Holland-Gaddy, and the Obert-Duvall, just to name a few) that were also available. Little wonder that discussions of pillar design in those days often ended with anguished cries of ?but which formula is the right one?? The past 25 years have seen substantial progress in the science of coal pillar design. Indeed, one testament to the improvement is the relative scarcity of papers on the topic at recent Conferences. Two factors have been largely responsible for the progress that has been made. The first has been the collection of large data bases of actual case histories of pillar performance in a variety of settings, from shallow room-and-pillar mines through deep cover longwalls. These have made possible the development of empirical design procedures that are closely linked to real world experience. The second important factor is the development of sophisticated computer models that can accurately simulate pillar behavior and roof/pillar/floor interactions. Together, these two lines of research have led to a new understanding of pillar mechanics that identifies three modes of pillar failure: ? Sudden, massive collapse, accompanied by airblast, for slender pillars (width/height<4) ? Squeezing, or slow, non-violent failure, for most room and pillar applications (4<w/h<10) ? Entry failure or bumps for deep cover and longwall applications (w/h>10) It is particularly satisfying that the insights gained from numerical models broadly support those obtained from the empirical studies. While far less controversial than in the past, pillar design problems continue to arise. One recent example is pillar design for highwall mining. NIOSH has just released a software package, called ARMPS-HWM, which employs a number of modern pillar design concepts. Since highwall mining web pillars are long and slender, the greatest danger is that of a sudden collapse. ARMPS-HWM suggests two possible prevention strategies, one which concentrates on the SF of the webs, and the other which creates a ?pressure arch? using properly sized barrier pillars. The paper will close with a discussion of some current needs in coal pillar design, including: ? Updating older empirical methods, such as ALPS, where changes in technology (new types of roof support, more demanding ventilation requirements, faster retreat rates) may have made some of the original case histories obsolete. ? Methods for determining site-specific coal strengths, focusing on bedding plane strength and other factors that may effect confinement, as input for both empirical and numerical design. ? Improved methods for evaluating coal pillar performance for environmental issues, such as surface subsidence and hydrologic impacts, which consider such factors as depth, w/h ratio, water immersion/drainage, and time dependent seam strength.

Jan 1, 2006