Syd S. Peng Department of Mining Engineering College of Engineering and Mineral Resources West Virginia University Morgantown, WV 26506 USA Copyright 2007 by Syd S. Peng Department of Mining Engineering College of Engineering and Mineral Resources West Virginia University P.O. Box 6070 Morgantown, WV 26506-6070 USA E-mail: syd.peng@mail.wvu.edu Website: http://www2.cemr.wvu.edu/~speng/ The cover was designed by Andre Zingano Printed in the United States of America First printing May 2007 All rights reserved. This book, or parts thereof, cannot be reproduced or stored in any form without the written permission of the author. Library of Congress Card Number 2007900578 ISBN 978-0-9789383-1-4 - I -
To my friends in the mining industry (coal and industrial minerals) who shared their ground control problems with me in the past three decades. Their trust in me helped establish and greatly advance the field of GROUND CONTROL. - II -
PREFACE WHY THIS BOOK? What is a ground control failure? In answering this question, we need to start with the definition of ground control and failure. Ground control is the science of studying and controlling the behavior of rock strata in response to mining operations, or simply, the application of rock mechanics principles to mining operations. Failure in rock mechanics occurs when the stress field in a rock material meets the failure conditions as defined by the chosen failure criterion. Therefore, ground control failures refer to the failure of rock strata associated with mining operations and the technology for controlling the strata for safe and economical mining operations. Failures of rock strata include entry failure, surface subsidence, and slope failure, while failures of strata control technologies include all types of roof supports, hardware, and methodology. In rock mechanics, all failure criteria are borrowed from those developed for continuous materials that are homogeneous and isotropic. They are most suitable for man-made materials for which those criteria have been developed. Consequently, when a failure criterion predicts a failure to occur, the strength and mode of failure can be predicted fairly accurately. In fact, failure defined as such is clear cut and occurs as perceived. In ground control, however, the strata surrounding the mining operations exist as it occurs. Their state of occurrence is not known in advance. Furthermore, they are seldom homogeneous and isotropic, and they change constantly. Consequently, the application of those failure criteria for ground control operations is very complicated and often met with prediction failure. The most common way to handle this dilemma, for researchers and practitioners alike, is to look for general or rough trends, while simply ignoring the fine details of what actually occurs in practice. For instance, when one refers to a cutter roof, what does he/she actually mean? There are many stages and forms of cutter roof development as illustrated in this book. What causes these differences if the same failure criteria are applied to them? During the past 33 years, I have worked on more than 200 cases of underground ground control problems, including cutters, floor heave, failures of pillars, roof bolting, shields, and other forms of standing supports, and roof falls. I have published many of these case studies. One thing I remembered very well was that several reviewers of my papers did not believe what I described as seen underground. Furthermore, the different forms and stages of failures in what appeared to have occurred in a similar setting (i.e., in a mine or a panel/section) are very difficult to describe in detail in writing. It was obvious to me that many researchers/practitioners have not seen and/or do not have a clear concept or vision about certain types of failures underground. Many coal mine operators, having worked only in one or a few mines, are not aware that there are other forms of failures in other mines. This is the major reason that I published this book. Unfortunately, in most cases, a camera was either not allowed or deemed unsafe for taking pictures, and being an amateur, many photographs I took underground were of poor quality. Therefore, I selected the more representative cases for this book. In each case, the year of occurrence is stated in the beginning so that the environment or technological background under which it occurred are reflected. The narrative then begins with the mining and geological conditions, followed by a description of the ground control problems and recommended solutions and results, if any. In some cases, follow-up visits were made to study the results of implementing the recommended remedial measures. It is obvious that those cases occurred in my early career (1970s 1980) were not well-described, as compared to those in the 1990s and 2000s. This trend reflects the progressive development in my understanding of ground control failures during the past 33 years. There are many cases that I provide a very detailed description of the events. In those cases, the forms and severity of failures vary in various parts of a mine structure - III -
and change over time. It is incomplete nor appropriate to pick any one of them for validation of computer models and/or support designs or to develop remedial measures thinking that those measures would be applicable to all other parts of the structure or other parts of the same section or mine. Consequently, it is my hope that the many forms of failures as shown by the photographs that I took at different parts of a case study will inspire new thoughts and approaches on the complicated underground ground control failures. Furthermore, I hope this will set an example for what an underground inspection of a failure case should be. Unfortunately, majority of the cases presented did not have quantitative geomechanics data, underground visual observation with simple measurements was the major source available for analysis. Because, when things happened, coal companies were looking for quick answers. Many cases used in this book involve small coal or industrial mineral mines. A small mine refers to one that employs one to three continuous miner units in room and pillar mining. Small mines are easily accessible, requiring only a few hours for a study trip to the mine. Any mine design concept/ground control techniques can be implemented quickly and results known in a few months. Due to more complicated geological and mining conditions, varying practices of MSHA district offices, and/or lack of ground control professionals, there are many more ground control failures in small mines that can be used to verify existing ground control theories/designs. Experience has demonstrated in many cases that the mine design and ground control practices employed in small mines are more liberal than those developed for longwall mines. The concept of failure in mining operations is usually defined in relation to either safety or production or both. A situation, that could be safe to miners, but uneconomical from a production point of view, is considered a failure. Since mining practices vary considerably from mine to mine, sometimes even from shift to shift, and there are many stages in the failure development of each and every one element of the underground structures, the definition and perception of failure vary with people and the mine. Therefore, it is my hope that the collection of various types of failures in this book will help with the development of a more uniform concept or definition of failure. Failure could be sudden. It could also be slow, or time dependent, or it may stop somewhere during the failure process. Why? This book shows, in pictorial views, many forms and/or stages of types of failures, for instance, cutter, roof falls, and cribs. I do not have definite answers for each and every one of them. So I present them here, hoping some of you may be interested in pursuing the answers. It is important to point out that it may appear that there are many repetitious and look-alike photographs. They are not. As I said, there are many different forms of what appear to be failure in underground mines. This book merely tries to document them. Many failures, especially cutters and roof falls, do not occur in massive, strong roof, such as sandstone and sandy shale. Rather, they occur in weak roof. A weak roof is not restricted to those rocks that have a low uniaxial compressive strength (UCS) as determined in the laboratory. As presented in this book, stack rock accounted for the majority of massive and ugly roof falls. Stack rock is thin layers of sandstone or sandy shale interbedded with thin films of carbonaceous (black) materials. The thicker the total thickness, the poorer they make the roof. Its UCS, as determined by the current testing standards in the laboratory, does not represent its behavior underground. Stack rock, being composed of sandstone or sandy shale, have high UCSs. But underground, the thin films of carbonaceous materials are where stack rock breaks easily into slabs. The thinner the sandstone/sandy shale layers and the denser the thin films of carbonaceous materials, the sooner and worse the roof will fall. For this type of roof strata, stability tests of thin beams or cantilevers are more representative, not the UCS as conventionally obtained from standard rock mechanics tests. Another weak roof that appears to be contrary to the strength obtained in the laboratory is laminated clayey shale. When it is dry, under which the laboratory strength is determined by following the current prevailing testing standards, its strength is high, thereby normally projecting a stable roof. But once they are exposed underground and subject to the wet and dry annual cycles of the ventilation air, their laminations become active and rock materials begin to crumble. The larger the clay content, the sooner - IV -
and worse the roof will fall. For this type of rock, its sensitivity to weathering (moisture) is the most important property for stability evaluation, not the conventional UCS. Another very obvious, but always ignored, issue is the effect of time element. For mine operators, it is common sense to support the entry roof as soon possible after excavation. An entry deforms with time, and the roof converges and/caves continuously in the gob. Both events increase the loads on support with time. The entry deteriorates with increasing deformation, transforming, for instance, the cutter roof into various stages and leading eventually to roof falls in some cases. The photographs of various types of failures presented in this book clearly show that failures in bedded strata, such as the coal measures rocks, are controlled by the planes of weakness, including bedding planes, laminations, and cleats. Accordingly, in order to be realistic, this factor must be considered in the development of ground control theories and designs. Unfortunately, this has not happened so far! Finally, Chapter 10 documents the first two projects that I worked on right after I joined West Virginia University in 1974: one was shortwall mining, a hybrid of longwall and continuous mining in which the tailgate was built as mined, not pre-driven. The other one was thin seam (less than 48 in. thick) plow longwall. Unfortunately, due to difficult conditions, both projects were not successful. However, I do believe that with today s advanced ground control technologies, both projects should have a good chance to succeed. Syd S. Peng Morgantown, WV March 2007 ACKNOWLEDGEMENT I am indebted to my current and former graduate students. Without their assistance, publication of this book is not possible. Among them, Dr. Steve Tadolini reviewed the draft and made extensive comments. Dr. Andre Zingano developed the book format and was in charge of putting the whole book together. Dr. Khaled Morsy, Thomas Du, Reddy Kallu, Jun Lu, and Anil Ray reviewed and edited individual chapters. Thomas Du, Reddy Kallu and Jun Lu also assisted in preparing the illustrations. Charles Howard prepared a portion of the text in Section 8.3, including Figure 8.3.3. - V -
You may be surprised to learn that occasionally even the oldfashioned transportation method found its use in modern longwall mines. The donkeys are carrying several pallets of concrete in 80 pounds bags more than 3,700 feet into the bleeder to fix a seal that was only accessible by foot. (2005) - VI -