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By Bruce Hebblewhite

This paper recognises the 25th year of the Morgantown International Ground Control in Mining Conference, by offering a twenty five year review of ground control in the Australian mining industry ? primarily in underground coal. The paper provides an overview of the developments in ground control design methodologies, practices and products. It not only summarises what has been done, but attempts to highlight at least some of the major issues, significant developments, controversies and areas of contention, plus some of the continuing gaps in our understanding or capabilities as we go forward into the next 25 years. Some of the issues included are in relation to: the development of the high strength bolt and cable reinforcement practices widely adopted today and the contrast between stiff and soft systems; a focus on local detail, at the expense of understanding regional stability concerns; dramatic improvements in the use of, and systems available for underground geotechnical instrumentation and monitoring; some of the ground control management trends, including occupational health and safety concerns, availability of suitably trained staff and legislative requirements; the opposing approaches of the empiricists versus the numerical modellers ? is there some middle ground?

Jan 1, 2006

By Jun Han

Coal has been mined in the Kailuan mining area for more than 130 years. The remaining reserves consist of, in large part, coals that would have been considered economically unminable or technically not feasible by the previous generations. Some of the current operations have to resort to overmining because the lower seams had been extracted in the past. This paper deals with the feasibility and proper layout of mining roadways of overmining carried out in the No. 5 seam of Qianjiaying coal mine of Kailuan mining area using numerical and physical simulation. FLAC2D was used to investigate the characteristics of stress and displacement in the overburden after extraction of the lower seams. It also analyzed the proper layouts of gateroads in the upper seams, i.e. locating them in the stress-relief zone of the gob so that their stability can be easily maintained. In addition, a two-dimensional physical model was constructed for investigation of damages to the upper seams due to mining of the lower seams. Finally, based on the results of investigations, proper gateroads layout was designed and successful extraction was implemented for panel 1357 in the No.5 seam.

Jan 1, 2009

By Ebrahim Ghasemi

Problem statement: One of the most challenging safety problems in room-and-pillar coalmines is controlling roof falls during retreat mining. Roof falls not only cause both fatal and non-fatal injuries to miners, stoppages in mining operations, and equipment breakdowns, but also results in the inefficient recovery of coal reserves. Approach: Sufficient awareness of effective parameters related to roof falls is an important factor in roof control. In this study, the major contributing parameters on roof falls during retreat mining are introduced based upon experience and judgment acquired from extensive retreat mining case histories and a review of relevant literature. Key findings: The main roof fall parameters in retreat mining can be divided into three categories: geological, design, and operational. Key conclusions: The design and operational parameters can be controlled by the engineer and/or operator. By the appropriate selection of these parameters in the design and implementation stages of retreat mining, the roof fall risk can be decreased to an acceptable extent.

Jan 1, 2012

By T. N. Singh

An evaluation was made on the cavability of the roof formation of Dishergarh seam including caving sequence, caving span, and load on powered support.

Jan 1, 1994

By Kourosh Shahriar

The occurrence of rock falls in underground coal mines entails detrimental effects as fatal or non-fatal injuries on workers, stoppages in mining operations and breakdown of equipment. In this paper, a risk assessment approach on the basis of a decision analysis trend is employed in order to assess the possibility of and manage roof falls. Risk is then assessed by determination of likelihood of occurrence and the cost of consequences (outcomes). In this regard, collected real roof fall data from Tabas and Kerman coal regions comprising of several underground coal mines are used. It is concluded that the annual accidents due to the roof falls occurrence in the all investigated mines are so high that it is economically feasible to improve the support systems and to implement a suitable educational program as well as an accurate supervision and other elements of safety management.

Jan 1, 2009

By Hakan Gurgenli

Many coal seams have weak shale immediate roofs that cause ground control problems. Therefore, it is important to know the properties of these shales so that preventive measures can be developed in advance. However, it is sometimes hard to understand this rock type due to its unpredictable behavior. A good example of this situation is found in two shale coal mine roofs located in the Illinois coal basin (Herrin No. 6 seam). Although, these mines are located very close to each other, one of them has many roof falls, whereas the other has no measurable ground control problems. The aim of this paper is to explain why and how one shale roof behaves worse than the other. Laboratory tests, including point load strength index, slake durability index, moisture activity index, weatherability index, water content determination, swelling strain and x-ray diffraction tests were performed to analyze the shale properties. According to these analyses, some useful correlations were found between the engineering properties, moisture-sensitivity indices, and weathering properties. Finally, mine and support design considerations based on the test results were discussed and recommendations to prevent ground control problems were given.

Jan 1, 2006

By Rob Thomas

Main gate roof control during longwall retreat is subject to a significant increase in horizontal stress and risk. The implementation and management of an appropriate roof support design is as a result, an imperative requirement for any longwall mine and must in addition to the likely consequences of a roof fall, also consider the variables of the geotechnical environment, associated support costs and practicalities. This paper will therefore summarise a number of recent advances in main gate roof control in the Australian coal industry. Points considered include; (i) the ?Longwall Acceleration? concept, (ii) macro-scale horizontal stress notching, (iii) micro-scale horizontal stress notching, (iv) bedded versus laminated roof types and (v) the type of cable reinforcement used.

Jan 1, 2006

By Murali M. Gadde

Welded wire mesh is the most commonly used skin control technique in the United States underground coal mines. Currently, selection of the mesh is based on trial and error rather than any rigorous design methodology. Several factors affect a wire mesh?s performance including the amount of rock material to be supported and how the load is distributed, wire gauge, aperture size, grade of steel, weld strength, bolting pattern and sheet size. Although the best way to choose the mesh is to conduct full-scale physical load tests, because of the limitations on the number of variables that can be considered in such tests and the requirement of an elaborate test set-up, it may not always be possible to use this option. An alternative would be to use numerical modeling to study the load-deformation behavior of a wire mesh. In addition to providing a very detailed stress, strain distribution in the entire mesh, the advantage of this approach is the ease with which parametric studies can be conducted. In this paper it is shown that the load-deformation curves generated by nonlinear numerical models match closely with laboratory derived ones. Then, based on parametric studies, design guidelines are developed to choose the mesh appropriate for different loading conditions. The modeling studies also bring forth the limitations of the current lab studies. For example, under some conditions it was noticed that the small sheet size considered in lab tests would produce higher mesh deformations than a full-size one used in actual practice.

Jan 1, 2006

By Jeffrey Whyatt

Mining coal in deep, gassy strata is difficult, particularly in deep mines of the west. Dynamic failure (bumps, bounces, etc.) is commonplace, particularly where strong sandstone strata are encountered in the overburden. The disaster potential posed by dynamic failure in these mines is examined and found to be significant. Recent trends in MSHA reportable bumping of deep western coal mines have been increasing. Recent clusters of bumps are explored, a number of which have ended badly. The mechanisms of these bumps and their interactions with geologic structures are explored. Significant progress in controlling these hazards depends on a full understanding of their interaction with geologic conditions. It is apparent that not all MSHA reportable bumps have the same mechanism. These reports describe bumping of pillars, rolling of ribs, outbursts of coal, heaving of floor, shaking-induced roof falls, etc. Progress depends on understanding the underlying mechanisms of these events, and then determining which are potentially active at a particular location. Designs and other protective measures can then be adapted and deployed appropriately. Explicit consideration of mechanisms also supports extrapolation beyond experience as design extends into new mine geometries, geologies, mining methods and increasing depth. Finally, the goal should not be only to design hazard out of mines, but also to provide assurance that this, in fact, has occurred. Pursuit of this goal has been taken up by a major NIOSH research project. The centerpiece of this project is development of a Dynamic Failure Control Program to monitor evolution of dynamic failure hazards with changing geologic conditions to assure that control and protective measures are appropriately deployed.

Jan 1, 2008

By Jan A. Nemcik

Steel mesh has been used successfully for many years to control friable roof conditions and prevent loose roof and rib material from caving into the roadway. Despite its extensive use, the installation of steel mesh is difficult to automate and many other products have been evaluated as an alternative for skin control. Ideally, the properties of these new products should be similar or better than those of the steel mesh. In particular, development of a strong and resistant shell that minimizes movement along the fractured rock and coal surfaces between the bolt anchors is recommended. A strong surface adhesion and the strength of a reinforced polymer skin can provide the necessary toughening mechanism required to enhance roadway surface support by forming a reinforced polymer/rock surface layer. Most of the Thin Spray-on Liners (TSL) evaluated in the mines are weak with slow curing times, and the plastic mesh currently used to support the coal ribs is relatively weak. Therefore, neither material can seriously compete with steel mesh. Currently, a new polymer product that cures in seconds and forms an instantaneous strata binder that surpasses the properties of steel mesh is under development at the University of Wollongong.

Jan 1, 2009

By Wenquan Zhang

Based on the mining and geological conditions of Wadong Mine, Longkou City, Shandong Province, China, a viscoelastic model with proper creep parameters was established. The 3-D ANSYS was used to simulate surface subsidence, analyze overburden subsidence characteristics, and propose the rational mining and pillar widths for mining in weak rocks and under special mining conditions, and provide the policy makers scientific data for decision making on mining activities.

Jan 1, 2006

By David H. Tang

Massive pillar failures were observed at two underground coal mines. The results of stability analysis using traditional pillar design formulae did not agree with the underground observations. Finite element models were used to analyze the causes and mechanisms of the pillar failures. The results were confirmed by the underground observations. Finally, the improved mine plan for each case was derived to ensure the stability of the underground structure.

Jan 1, 1990

By Peter Fischer

Reducing mining costs has become such an important factor in the German coal industry that operators are forced to work only those areas where production costs can be kept low. These areas will normally be located close to the mine shafts because they are easily accessible. Even the shaft pillar is now being considered as part of the winning zone. Before working coal in shaft pillar zones it is essential to assess the impact this will have on the shaft lining. There is a huge difference between sliding and strata-anchored shaft systems. However, if the overburden comprises lightweight soil both types of shaft will have a tendency to protrude from the surrounding ground. This paper presents the geomechanical reasons underlying this reaction and the methods used for predicting and measuring it.

Jan 1, 2007

By Khaled Morsy

The tragic incidents at Sago and Darby coal mines in 2006 pointed out the need for stronger seals to resist the blast effects of violent gob explosions. Mine Safety and Health Administration (MSHA) has published the final rule on Sealing of Abandoned Areas in April 2008. According to the final rule, the mine seals for unmonitored gob areas should withstand an instantaneous pressure of 120 psi. Reinforced concrete (RC) seal is one of the feasible options that can be used to meet the final rule. Concrete is a heterogeneous, cohesive-frictional material and exhibits complex non-linear inelastic behavior under multi-axial state of stress. The increased use of concrete as primary structural material requires the development of sophisticated material models to have an accurate prediction of the material response to a variety of loading situations. The constitutive modeling of concrete has advanced considerably in the past three decades. Choosing a reasonable concrete material model is important in order to obtain an accurate mine seal design. This paper attempts to address the effect of different concrete material models on the response of RC seal subjected to dynamic loads from methane explosion. Three concrete models, from ABAQUS library, were selected to address the effect of non-linear behavior of concrete and damages due to tensile and compressive stresses. In addition, a typical and verified concrete material model was used for conducting 3D finite element models simulating the RC seal by ABAQUS, EXPLICIT ver. 6.6.

Jan 1, 2008

By David Oyler

Sonic travel time logging of exploration boreholes is routinely used in Australia to obtain estimates of coal mine roof rock strength. Because sonic velocity logs are relatively inexpensive and easy to obtain during exploration, the technique has provided Australian underground coal mines with an abundance of strength data for use in all aspects of ground control design. However, the technique depends upon reliable correlations between the unconfined compressive strength (UCS) and the sonic velocity. This paper describes research recently conducted by the National Institute for Occupational Safety and Health (NIOSH) aimed at developing a correlation for use by the U.S. mining industry. At three coreholes in Illinois, Pennsylvania, and southern West Virginia, sonic velocity logs were compared with point load tests for a broad range of coal measure rock types. For the entire data set, the relationship between UCS and sonic travel time is expressed by the following equation, where UCS is in psi and t is the travel time of the P-wave in microsec/ft. UCS = 468,000 x e-0.054t (1) The r-squared value for this equation is 0.87, indicating that a strong correlation between sonic travel time and UCS can be achieved with this technique. The paper also addresses the steps that are necessary to ensure that high-quality sonic logs are obtained for use in estimating UCS.

Jan 1, 2008

By Nic Slatalla

Acoustic emission monitoring techniques are a powerful tool for analysing deformation behaviour and fracture characteristics of rock samples under uniaxial loading. Uniaxial compression tests were carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage develop-ment. All sandstone samples revealed congruent characteristic acous-tic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages proved to be entirely different due to variations in their microstructure.

Jan 1, 2008

By Guo Junfeng

The strength of coal mass saturated with gas is an essential parameter in studying the mechanism of gas and coal outburst, and its prediction method. Up to now, there has not been a method for determining the strength of coal mass saturated with gas. This paper describes a new method whereby the strength of in-situ coal mass can be determined and stored continuously and automatically. The simulation results obtained in the laboratory indicate that the new method allows a fast, reliable, inexpensive and convenient determination of the strength of coal mass.

Jan 1, 1992

By Kot F. Unrug

Strength of coal is one of the governing factors in size determination of mine pillars, and therefore affects the percentage of extraction and thus, utilization of reserves. Coal strength is also important from the subsidence control standpoint, where mine pillars with appropriate safety factors should be designed for long term stability. Determination of coal seam strength has been addressed through the extrapolation of the coal strength obtained from a small number of samples on the whole section of the seam. This procedure often gives unreliable results. Presented in this paper is a method of in-situ strength testing of coal, allowing for a realistic determination of seam strength. Results of a field case study dealing with the coal seam, where samples for the laboratory strength tests could not be obtained due to the dense cleat, are presented. However, in these conditions, the in-situ testing was successful. A comparison of both conventional and the in-situ penetrometer method was carried out. It was found that the penetrometer data not only improve the accuracy of strength determination of the entire seam, but also prove to be much more cost effective than the conventional method on a per test basis.

Jan 1, 1995

By Paul Singh

Blasting is an integral part of mining activity and is an inherently destructive process. The quality of the rock mass in which an excavation is being driven has a strong influence on the level of dynamic forces to which the rock can be subjected and sustain damage. Though each specific rock mass occurrence is unique, yet there are common rock mass features which influence the outcome of controlled blasting in a similar way. A field study was conducted which involved assessment of blast damage at several operating mines and blasting of small scale models. The effects of block size and strength, joint parameters, ground water, weathering of rocks and rock mass rating on the results of controlled blasting have been investigated and discussed in this study. The influence of number and thickness of weak layers on overbreak control has been discussed in the light of practical data. Critical particle velocity ranges for damage to rock masses with different qualities have been outlined. An approach for the judicious design of perimeter hole pattern and charge concentration has been proposed.

Jan 1, 2007

By David Hill

The Australian underground coal mining industry is increasingly operating in an environment that challenges traditional concepts of coal pillar design, against a background of heightened expectations and demands from other stakeholders with regard to the protection of both natural surface features and burgeoning civil infrastructure. The result is greater possibility of conflict between mine operators, developers and regulatory bodies, with potential for unnecessary sterilisation of Australia?s coal resources and/or escalating surface development / infrastructure protection costs. On the positive side some significant advances have occurred in the general level of understanding of pillar behaviour and stability, both in Australia and overseas. This paper examines some current issues in coal pillar design, focussing particularly on the application and limitations of analytical models of pillar loading. Established methodologies are reviewed and alternative approaches explored, including examples of key geometrical, geological and probabilistic concepts. Practical experiences from the Southern Coalfield of New South Wales (NSW) are examined for the purpose of highlighting critical variables. Finally, some suggestions are made with regard to the direction and opportunities for future research, with particular regard to coal pillar loading, in the interests of all stakeholders.

Jan 1, 2008