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Subsidence from Underground Mining: Environmental Analysis and Planning Considerations

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Subsidence from Underground Mining: Environmental Analysis and Planning Considerations Subsidence from Underground Mining: Environmental Analysis and Planning Considerations GEOLOGICAL SURVEY CIRCULAR 876 COVER PHOTOGRAPH: Subsidence pits, troughs, and cracks above an abandoned coal mine in the western Powder River Basin, Wyoming. Photo­ graph by C.R. Dunrud. Subsidence from Underground Mining Environmental Analysis and Planning Considerations By F. T. Lee and J. F. Abel, Jr. GEOLOGICAL SURVEY CIRCULAR 876 7983 United States Department of the Interior JAMES G. WATT, Secretary Geological Survey Dallas L. Peck, Director Library of Congress Cataloging in Publication Data Lee, Fitzhugh T. Subsidence from underground mining. (Geological Survey circular ; 876) Bibliography: p. 1. Mine subsidence Environmental aspects. 2. Mine subsidence. I. Abel, John F., Jr. II. Title. III. Series. QE75.C5 no. 876 557.3s [622] 82-600123 [TD195.M48] Free on application to Distribution Branch, Text Products Section, U. S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS Page Page Abstract __...____..___...____..___...._ 1 Topography __.__---_-_------------- 15 Introduction .._.___.______..____..___._._ 1 Time _____......______---.....-.-- 15 Overview of subsidence processes ______________ 2 Subsidence analysis __----_.------------- 15 Definition ........................... 2 Data requirements .......-----.--..-- 16 Conceptual description of subsidence __...___. 2 Geologic data _-----.------------- 16 Subsidence produced by longwall coal mining . 3 Hydrologic data ..___------.--_--- 16 Subsidence produced by room-and-pillar coal Geotechnical data ................. 16 mining _.___..__...______.____._ 4 Mining plans _______-_-----.----- 16 Subsidence from mining other sedimentary Analytical techniques for subsidence prediction .. 16 resources __....__.._._....._.... 5 Background and state-of-the-art __________ 16 Subsidence from mining crystalline rocks .. 8 British National Coal Board method .... 17 Impacts of subsidence ...................... 8 Other empirical methods ......._.... 19 Effects on surface structures .___..___..___ 9 Mathematical techniques .....-._-..- 19 Hydrologic impacts ..................... 9 Selection of an analytical technique ..._.-.._- 19 Effects on vegetation and animals .......... 12 Subsidence models for in situ energy extraction 22 Coal mine fires ....................._. 12 Monitoring methods ...__.-.----.-------- 22 Factors governing subsidence ................ 12 Surface instrumentation __-_.---.------ 22 Mining method ........................ 12 Spacing of monuments ------------- 24 Multiple seam mining of coal beds ..__._..__. 12 Subsidence control and reduction .....--_-.-- 24 Depth of extraction .................... 13 Design precautions _......-----.------ 25 Rate of advance .._.................... 13 Conclusions __.__---_-__.-------------- 25 Thickness of seam or deposit .............. 13 Glossary ____.---.-------------------- 25 Lithology and structure ................. 13 References cited .-_--.__---------------- 26 In situ stresses and other geotechnical factors .. 14 ILLUSTRATIONS FIGURE 1. Generalized and idealized sketch of deformations produced in rocks above and below extracted coal seam . 3 2. Photograph showing compression ridges produced contemporaneously with longwall mining near Raton, New Mexico ---........................................--------.----------.-.-- 4 3. Photograph showing large open fissure associated with longwall mining near Raton, New Mexico ..... 4 4. Photograph showing rockfalls and tension cracks produced during longwall mining near Raton, New Mexico ...................................................... 5 5. Generalized sketch illustrating subsidence over longwall mining _______________________ 6 6. Aerial photograph showing the surface effects of past and present room-and-pillar coal mining along the Tongue River in Wyoming ___.____._____._____._______.___._-_---_-__ 7 7. Nomogram for the determination of change of length of a surface structure for various mining conditions . 10 8. Graphic representation of the British National Coal Board method of subsidence estimation ........ 19 9. Graph showing general relation between percent subsidence and panel width to mining depth ratio ... 20 10. Graph showing relation between subsidence and surface tilt _________________________ 21 11. Map showing location of surface monuments, turning points and reference points at the York Canyon mine near Raton, New Mexico ____________________________________________ 23 ill TABLE Page TABLE 1. Subsidence damage classification for horizontal ground strain and changes in length of manmade structures for longwall mining .______._.__.....__.__....___.__.......___..___.. 11 IV Subsidence from Underground Mining: Environmental Analysis and Planning Considerations By F. T. Lee and J. F. Abel, Jr. 1 ABSTRACT cal. Past coal mining practice left much coal in the Subsidence, a universal process that occurs in response to ground as pillars that are difficult to recover. Pre­ the voids created by extracting solids or liquids from beneath sent-day emphasis is on improving extraction per­ the Earth's surface, is controlled by many factors including centage. Because both the methods of extraction mining methods, depth of extraction, thickness of deposit, and and the number of mines will increase overall sub­ topography, as well as the in situ properties of the rock mass above the deposit. The impacts of subsidence are potentially sidence risk, we must be able to accurately predict severe in terms of damage to surface utility lines and struc­ the surface and underground impacts of subsi­ tures, changes in surface-water and ground-water conditions, dence and, if necessary, to suggest means to les­ and effects on vegetation and animals. Although subsidence sen these impacts. It is clear from poorly con­ cannot be eliminated, it can be reduced or controlled in areas trolled mining operations of the past that we no where deformation of the ground surface would produce dangerous or costly effects. longer have the luxury of mining without regard Subsidence prediction is highly developed in Europe where to present and future land use. there are comparatively uniform mining conditions and a long The purpose of this circular is to give an over­ history of field measurements. Much of this mining has been view of subsidence processes and their potentially carried out beneath crowded urban and industrial areas where harmful consequences, the methods of subsidence accurate predictions have facilitated use of the surface and re­ duced undesirable impacts. Concerted efforts to understand prediction, and methods to control and reduce subsidence processes in the United States are recent. Empiri­ subsidence impacts. The report is primarily in­ cal methods of subsidence analysis and prediction based on tended to serve as an introduction and state-of- local conditions seem better suited to the current state of the-art review for those individuals or groups con­ knowledge of the varied geologic and topographic conditions in cerned with assessing the potential environmental domestic coal mining regions than do theoretical/mathematical approaches. In order to develop broadly applicable subsidence effects of underground mining. The major em­ prediction methods and models for the United States, more in­ phasis is on coal mining. formation is needed on magnitude and timing of ground move­ The time to plan for subsidence impacts is well ments and geologic properties. before mining begins, not after surface effects are noticed. Because subsidence due to underground INTRODUCTION mining may be inevitable, the relevant questions The worldwide need for energy resources re­ to be asked are how much, when, and where, and quires increased production of coal and other what abatement procedures are possible and fuels. A large amount of this production will even­ might be necessary. The impacts of subsidence are tually come from underground mining in areas broad, affecting water supplies, transportation where surface mining is impractical or uneconomi- and utilities, vegetation, and farming. In addition, in situ extraction techniques for coal gasification and oil shale retorting are supported by extensive, 'Colorado School of Mines, Golden, Colorado 80401 costly surface facilities. The success of these oper- ations may depend upon accurately estimating the more accurately forecast than subsidence caused extent of subsidence, both in area and strain mag­ by room-and-pillar mining. The current state of nitudes, so that surface plants are not damaged or knowledge of subsidence permits a more detailed located unnecessarily distant from the mining op­ discussion of longwall-induced subsidence than of erations. the other forms. Recent Federal regulations implementing the Surface Mining Control and Reclamation Act of 1977 (Federal Register, 1977) will encourage in­ OVERVIEW OF SUBSIDENCE PROCESSES creased consideration of subsidence effects prior DEFINITION to mining. At present, however, very few mining companies in the United States make routine Subsidence is a time-dependent process, either leveling surveys of the ground surface. Increased natural or man induced, in which there is a lower­ surface monitoring is needed in order to imple­ ing of the ground surface in response to the re­ ment current mining regulations. moval of gas, liquid, or solid matter. Deformation To a greater or lesser degree each mining area of the rock mass may be by either elastic, plastic, is different, and no one subsidence prediction
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