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 or brittle processes or by any combination of these method will serve equally well for all cases. For processes. Subsurface deformation leading to sur example, the prediction scheme used in the face subsidence includes the local lateral and up United Kingdom with excellent results for ward displacements of rock above unmined areas longwall coal mining2 does not work as well in (near mine boundaries or barrier pillars) caused other areas largely because of different geologic by the downward movement of overburden into conditions. The great diversity of mining condi mine cavities. Strains induced by mining and tions in the United States partly accounts for the transmitted through intervening strata to the sur circumstance that subsidence prediction is not far face may be compressive or tensile and may have advanced in this country. A systematic concen both horizontal and vertical components. trated effort is needed to develop more broadly CONCEPTUAL DESCRIPTION OF SUBSIDENCE applicable and accurate methods of subsidence prediction in the United States. The more general The void created by the underground extraction and widely used analytical techniques are dis of coal or other resources causes significant cussed here; the reader is cautioned that many cir changes in the magnitude and orientation of the in cumstances will require specific expert advice. situ stress field and results in deformations both The term "mining" as used in this report in in the remaining coal and in the surrounding cludes all extractive processes for recovering or rocks. In general, the sides of the excavated area ganic and inorganic resources. Thus, we are con move inward, the floor upward, and the roof cerned with deformations caused by several min downward. The initial deformations may be elas ing methods in a variety of geologic environments. tic, that is, they may disappear if the deforming Most emphasis in this report, however, is placed forces are removed. Nonelastic deformations, on longwall and room-and-pillar mining of coal. It however, occur with time and, as the region of has been estimated that coal extraction is respon cavity influence increases with continued extrac sible for over 90 percent of worldwide mining-in tion, rock strengths are exceeded and irreversible duced subsidence (Alien, 1978), Longwall mining block movements take place. Large bending mo currently accounts for less than 10 percent of the ments in the mine roof strata will ultimately cause coal mined in the United States, although it has local roof failure and collapse, and the mined-out long been the most popular method in Europe. area will fill with overburden materials. The However, because longwall mining, which re downward movement of overlying rock will induce moves a complete tabular section of coal, is more lateral movement of rock toward the cavity. These efficient than room-and-pillar mining, it is being deformations are illustrated in figure 1. Rock out promoted increasingly in the United States. Also, side the vertical limits of the mined area will also longwall mining induces a generally uniform and subside. The affected ground will lie typically 10°- contemporaneous surface subsidence that can be 35° outside the vertical limits of the mined area (angle of draw). Deformations eventually reach the ground surface and may form subsidence de Technical terms used herein are defined in the glossary, p. 25. pressions, open fractures, pits, and troughs. Com- Original surface -§ybsided surface^
^ P-'^'--vy : '^s-:'-A'::v:-^: :.^_- *<"> ; '.' /.' -.
rTTT
FIGURE 1. Deformations produced in rocks above and below an extracted coal seam (modified from Shad- bolt, 1978).
pression features including doming and thrust there is a stabilizing compression (or pressure) faulting have also been reported. arch in the solid rock above and below the mined- Depending upon several factors including min out area. The duration of this arching effect is ing methods and rock properties, the changes at controlled by the height, width, and length of the the surface may occur almost concurrently with mined opening, and subsidence will not begin until mining or they may be delayed and take place a critical void size is exceeded at which the arch with dramatic suddenness more than 100 years will no longer span the excavated area. Con after mining. sequently, there is often a delay between the onset of a change in state underground and the SUBSIDENCE PRODUCED first appearance of land subsidence at the ground BY LONGWALL COAL MINING surface. The arching effect may be limited, how The mechanical aspects of the subsidence pro ever, by very weak overburden rocks or by poor cess in flat-lying coal bearing rocks mined by mining practice that significantly weakens the longwall methods are reasonably well known, overburden. Geologic conditions, mining depth, compared to room-and-pillar mining, and are and seam thickness also affect arching behavior. documented abundantly in the literature. This After subsidence has begun, it will develop knowledge may be summarized as follows. When progressively and continue so long as there is a an underground coalbed of a given thickness is ex progressive enlargement of the underground tracted over a wide area, the immediate roof will opening. Surface effects from longwall mining in collapse. Rock displacements are transferred to mountainous terrain in New Mexico are illustrated the ground surface when the ratio of the width of in figures 2-4. These conditions occurred essen the extracted material to the depth of overburden tially contemporaneously with mining. (w/k) exceeds a value which varies from 0.1 to 0.5 The concept of a "critical" area (width and (Wardell and Eynon, 1968); the ratio value is con length) of extraction is closely related to the abil trolled largely by the strength and structure of ity of the strata above the excavated area to sup the rock overlying the mined-out area. Field mea port loads across the mined openings. Assuming surements and theory support the concept that an infinite length for an extracted area (in practice meager; however, the lower values of the depth coefficient appear to be associated with overbur den containing thick, strong sandstone and limes tone beds whereas the higher values pertain to overburden containing a large percentage of thin- bedded shales, mudstones, siltstones, sandstones, and unconsolidated deposits. Maximum subsidence is also a function of the thickness of the extracted layer or the volume of material extracted, the mining methods, and several other factors dis cussed in the following sections.
SUBSIDENCE PRODUCED BY ROOM-AND-PILLAR COAL MINING Room-and-pillar mining is the most frequently used mining method in United States coal mines. The coal is mined in entries (rooms) separated by pillars which may or may not be partially ex tracted later. Initially, a series of parallel entries
FIGURE 2. Compression ridges produced contemporaneously with longwall mining near Raton, New Mexico. a length equal to or greater than the depth of min ing), there is a critical width of extraction for which the subsidence (vertical lowering of the sur face) reaches a maximum value. Subcritical widths of extraction produce a trough-like subsidence area with vertical subsidence less than the maximum. At supercritical widths of extraction the subsidence area has an essentially flat bottom at approximately the maximum subsidence. These relations are illustrated in figure 5. The critical width of extraction is normally ex pressed in terms of the mining depth (fig. 5). In European coal fields it ranges from 1.0 h to 1.4 h, '^^Snc where h is the average depth of mining. This FIGURE 3. Large open fissure associated with longwall mining range has been attributed to differences in the in mountainous terrain near Raton, New Mexico. Such fea types of overlying rock. Quantitative studies are tures can divert surface runoff, increasing landslide potential. FIGURE 4. Rockfalls and tension cracks produced during longwall coal mining near Raton, New Mexico. are driven through the seam with interconnecting method. In States such as Illinois, however, openings (breakthroughs) driven at right angles where farm and industrial land are extremely val through the pillars between the rooms. Such a uable, only about 50 percent of the coal may be checkerboard pattern of openings is advanced mined to prevent surface damage from subsi through the coal seam to the limit of the area dence. planned for mining. At this point approximately Because of the several stages of coal removal 50 percent of the coal will have been mined. The and the slow pillar deformation and deterioration coal pillars between adjacent rooms may be fully in room-and-pillar mining, surface settlement is or partially removed (robbed) during final, or re not as uniform and immediate as it is in longwall treat, mining. After full pillar removal, the rock mining; rather, it may be erratic, intermittent, above the mine collapses and the overburden and long delayed. Figure 6 shows a representative gradually settles, creating surface fissures and example of delayed subsidence features in Wyom subsidence. ing resulting from old shallow underground room- The percent extraction by room-and-pillar min and-pillar mining operations. ing depends upon several factors including the number and size of pillars deemed necessary to temporarily support the mine roof during retreat SUBSIDENCE FROM MINING mining and the need to prevent or limit surface OTHER SEDIMENTARY RESOURCES subsidence. In some sections of West Virginia and Other sedimentary resources which are ex in other States where surface land is owned by tracted from considerable depth (hundreds of met the coal producer, nearly 100 percent of available ers) include salt, potash, sulfur, trona, and phos coal can be mined using the room-and-pillar phate. These minerals occur in bedded, usually Surface and datum
EXPLANATION a Angle of draw h Depth m Mining thickness S Subsidence (vertical deformation) ^'max = Maximum subsidence FIGURE 5. Description of subsidence over longwall mining. Face advance direction is perpendicular to plane of page. (Adapted from National Coal Board, 1975). flat-lying deposits, and have been mined by sev subsidence causes unique problems (Marsden and eral methods: conventional room-and-pillar, Lucas, 1973). Drilling through aquifers and salt longwall, and solution. Pillar and roof collapse and beds in search of oil, gas, and water has induced resultant surface deformations are typically de salt dissolution and subsequent subsidence (Fader, layed and difficult to predict. In solution mining of 1975). As Ege (1979) points out, the construction salt, for example, neither the time nor the location of highways, dams, and reservoirs over saline or of subsidence can be predicted with confidence be gypsiferous rock has caused subsidence, water cause salt deforms slowly in a complex manner loss, and dam failures. A comprehensive discus and the deformations are different from those of sion of subsidence associated with solution extrac overlying shale and sandstone. Furthermore, solu tion of sulfur is given by Deere (1961). tion extraction of salt and other soluble evaporites The subsidence produced by room-and-pillar and is a specialized mining technique and the resulting longwall mining of potash, phosphate, and most
FIGURE 6. Southeastward-looking aerial view showing the surface effects of past (room-and-pillar) and present (open pit) coal mining along the Tongue River in Wyoming (July 1977). Pits, troughs, depressions, and cracks have formed because of subsi dence over the south part of the Acme mine. The mine was operated from the early 1900's until 1943. Overburden thickness ranges from 15 m in the middle ground to approximately 30 m in the foreground and consists of alluvium and soft interbedded shales, claystones, siltstones, and discontinuous sandstones. The dam in the right foreground across Hidden Water Creek rup tured because of subsidence. The water now is diverted into subsidence depressions, pits, and cracks upstream from the dam. Garbage from the town of Acme was dumped into the large pit at the left side of the photograph. Pits and troughs in the middle of the photograph are in alluvium. Note that the pits near the road and left of the draw do not occur in any noticeable depression; these pits are located above collapsed areas in haulageways of the Acme mine where adjacent coal pillars are strong enough to support the overburden. Alluvium is being removed at the Big Horn surface coal mine to extract the coal (back ground). Regrading is beginning near the river (right background). (From Dunrud and Osterwald, 1980.) sedimentary rocks is grossly similar to that of intersects both the ground surface and min mining coal, being dependent upon the thickness ing horizon or where the angle of repose of of the bed mined, the depth, the mining methods, the broken rock mass is reached, whichever and the properties of the overburden. The reader is flatter. may want to consult Miller and Pierson (1958) for It is common practice to report an initial and a discussion of subsidence over potash mines. final angle of draw for block-caving-induced subsi dence. The initial angle of draw marks the extent of subsidence effects at the time the surface is SUBSIDENCE FROM MINING breached. The final angle of draw occurs at the CRYSTALLINE ROCKS limit of measurable subsidence effects after min A very different type of subsidence is produced ing ceases. Negative initial angles of draw, that is, by the extraction of irregularly shaped metallifer those inside the mining area, have been reported; ous ore bodies by block caving or by leaching of they were probably derived, however, from insuf disseminated deposits where the extracted width ficient field measurements. Such a negative angle is small compared to the depth. A prominent col of draw refers to the angle between the vertical lapse structure, as much as tens to hundreds of and a line connecting the side of the chimney with meters in depth, may develop rapidly. The subsi the nearest side of the mining level. Initial angles dence mechanism in jointed crystalline rocks pro of draw in crystalline rocks range from 5° to 40° ceeds approximately as follows (Abel and Lee, and final angles of draw between 5° and 65°. 1980): 1. Rock collapses progressively upward from the IMPACTS OF SUBSIDENCE mining horizon (undercut level) as ore is withdrawn from below. Damage from subsidence over underground 2. The ground surface does not begin to subside mines has been a serious problem in urban areas measurably until the collapse has so thinned for many years and will become more widespread the intact rock above the mined-out area as the demand for resources, particularly coal, in that it cannot support the load of the over creases. Continuing subsidence has recently posed lying rock (arching effect). The overlying hazards in parts of Colorado and Wyoming where solid rock will then deflect downward to urban areas have spread onto land underlain by ward the collapsed rock. Lateral movement abandoned coal mines. Delayed subsidence has of adjacent rock into this collapsed rubble caused extensive damage in urban areas estab column is resisted by the active pressure of lished over coal mines in the Eastern States. the rubble. Economic impacts of subsidence in rural areas 3. Further extraction of caved ore from below re can also be significant. Fields must be regraded to sults in increased subsidence of the ground eliminate ponding of water, and, as in urban surface above and outside the area of ex areas, roads must be regraded and homes must be traction. This initial trough subsidence is repaired. Water wells may become dry when similar in shape to the trough subsidence aquifers are disturbed by rock movements. Gas observed over coal mines. mains are especially vulnerable to subsidence and, 4. Continued extraction of ore will result in if ruptured, can catch fire and explode. breaching of the surface. The initial breach Damage from surface subsidence can be caused is typically in the form of a circular depres by changes in surface slope, differential vertical sion, or chimney, that is roughly centered displacements, and horizontal strains. Planners over the mining area; it may be offset a must know whether these changes are complete or minor distance because of preferential col in progress, permanent or temporary; further lapse along geologic discontinuities. mining may restore the original slope or close ten 5. The rock adjacent to the chimney either slides sile fractures thereby rendering some remedial along geologic weaknesses, such as joints or measures unnecessary or even harmful. The mag faults, or topples into the collapsing upper nitude of structural damage in buildings and man- part of the chimney. made structures will depend to a large extent 6. The final, or ultimate, angle of draw is located upon details of design and materials; therefore any where either the flattest geologic weakness damage classification must be a general one. EFFECTS ON SURFACE STRUCTURES 7.3 m deep developed in 24 hours in a then unoc cupied part of a trailer court (Ivey, 1978). The effects of subsidence on surface structures Engineers in PolancP classified structures into are controlled to a large extent by the mining four categories, on the basis of importance and method. The long-term, delayed nature of subsi sensitivity to surface movement, and designated dence over room-and-pillar mines can make the acceptable values of tilt and normal strain for each task of repair and maintenance of surface facilities category (Hutchings and others, 1978, table 4). A intermittent and not predictable. Longwall min classification of subsidence damage based on ing, however, is associated not only with greater structural length changes and horizontal ground recovery of coal but with increased surface stabil strain caused by longwall coal mining was de ity after mining and, hence, earlier construction or veloped by the British National Coal Board (NCB) resumption of previous surface activities. and is given in table 1. The NCB has also de Many accounts exist of severe surface deforma veloped a useful nomogram for estimating defor tions that occurred, often abruptly, long after mations of surface structures in the United King mining ceased. With only a few exceptions, the dom (fig. 7). notable delayed residual subsidence has taken Figures 2-4 show surface effects produced by place in room-and-pillar mined areas rather than longwall mining in mountainous terrain in New in longwall mined regions. At Farmington, W. Mexico. Because subsidence is an inevitable conse Va., intermittent episodes of subsidence occurred quence of high-extraction mining, planners must because 2-m to 3-m high coal pillars were gradu be able to forecast its impact not only on surface ally forced into the weak claystone mine floor activities but also on subsurface structures such as (Gray and others, 1977). Surface deformation that aquifers. damaged dozens of homes and buildings began while the mine, which was 85 m below the sur face, was active. Subsidence movements continued HYDROLOGIC IMPACTS intermittently for more than 4 years after mining Subsidence depressions and associated fractures stopped until the mine was injected with coal may disrupt surface and underground water flow, waste. At the Geneva coal mine in Colorado, com causing diminished well production, aquifer con pression features such as fractured bulges and tamination, and decreased property values. In the small anticlines formed in massive sandstone 274 dry areas that typically overlie western energy re m above a mined-out area about \Vz years after sources, the loss of springs and other surface mining was completed. Measurements showed water is especially critical (fig. 6). Proper planning that the ground surface was shortened locally by and development of underground mining will les as much as 0.92 m (Dunrud, 1976). sen these detrimental effects. Few detailed Damage to surface structures over room-and- studies exist to adequately document the effects of pillar mines has been particularly noteworthy in mining on the hydrologic regimen. Hydrologic those Eastern States underlain by extensive coal changes may be more subtle than other mining ef deposits. Surface developments overlie room-and- fects; for example, subsurface aquifer disruption pillar coal mines in Pittsburgh, Pa., and Birmin may occur because of displacement of rocks above gham, Ala., as well as many smaller cities and mined-out areas without visible surface manifesta towns in 17 States (Alien, 1978). For example, at tion. Some shallow wells in Pennsylvania have ex Scranton, Pa., $29 million worth of property, in perienced reduced production. In one instance, a cluding 2,000 homes, 50 commercial and office mine face passed within 30 m of a 20-m-deep well buildings, 2 hospitals, several schools, and various which went dry. The well was deepened by 12 m utility lines have either been damaged by or are and a good water supply was encountered (Sos- being threatened by subsidence many years after song, 1973). the mines were abandoned. The total cost of sur Of great importance in many areas is the poten face stabilization of this area by hydraulic mine tial hazard that surface waters present to mining. backfill is estimated to exceed $8 million (Dunrud, In Pennsylvania a longwall panel having 200-215 1976). Subsidence occurred dramatically in 1974 in m of overburden was successfully mined directly Lafayette, Colo., above a coal mine abandoned for below a reservoir (Sossong, 1973). Normal mining several decades when a pit 4.5 by 5.5 m wide and conditions were encountered with 63-190 x 10 ~ 5 Seam thickness extracted in meters
200 180 160 140 120 100 80 60 40 20 20 40 60 80 100 120 140 180 200 Length of structure in meters Change in length in millimeters
FIGURE 7. Nomogram for the estimation of change of length of a surface structure for various mining conditions. Example given (dashed line) is for structure with length of 30 m, depth of coal seam of 150 m, and seam thickness of 1.3 m, which results in change of 172 mm in length of structure. (Modified from National Coal Board, 1975.) TABLE 1. Subsidence damage classification for horizontal ground strain and changes in length ofmanmade structures'from longwall mining
[|ic = microstrain; 1 ft= 0.305 m]
Change of length Description of Class of damage of structure typical damage
Very slight or negligible Up to 0.03 m Hair cracks in plaster. Perhaps isolated (example: 15-m long slight fracture in the building, building extended 50 |xe). not visible on outside.
Slight 0.03-0.06 m Several slight fractures showing inside the (example: 33-m long building building. Doors and windows extended 1,600 |xe). stick slightly. Repairs to decoration probably necessary. Appreciable 0.06-0.12m Slight fracture showing on outside of building (or (example: 27-m long building one main fracture). Doors and windows sticking; extended 3,700 |xe). service pipes may fracture.
Severe 0.12-0.18m Service pipes disrupted. Open fractures requiring (example: 67-m long apartment rebonding and allowing weather into the house compressed 2,300 jjie). structure. Window and door frames distorted; floors sloping noticeably; walls leaning or bulging noticeably. Some loss of bearing in beams. If compressive damage, overlapping of roof joints and lifting of brickwork with open horizontal fractures. Very severe More than 0.18 m As above, but worse, and requiring partial or (example: 55-m long apartment complete rebuilding. Roof and floor beams lose house extended 6,000 jxe). bearing and need shoring up. Windows broken from distortion. Severe slopes on floors. If compressive damage, severe buckling and bulging of the roof and walls.
'Modified from National Coal Board, 1975.
m3/s (10-30 gal/min) of water developed from the determinants of subsidence. The significance of entire 183-m-wide panel. Maximum subsidence in these factors is increased when mining under duced by an adjacent panel not under the reser bodies of water or under productive aquifers; in voir was 0.65 m or 47 percent of the 1.37-m min such circumstances an adequate monitoring pro ing thickness with an average width/depth ratio of gram is essential to warn of a possibly hazardous 0.85. Numerous shallow workings have taken water inflow. place under large bodies of water. Many of these Guidelines for mining near surface and under workings are essentially dry, although some ground bodies of water have been published by mined coal seams were less than 30 m below the U.S. Bureau of Mines (Babcock and Hooker, water or saturated alluvium (Orchard, 1973). In 1977). the United Kingdom, undersea longwall extraction In addition to conventional coal mining, the un is permitted with a minimum cover of 105 m and derground coal gasification process and in situ re a maximum tensile strain of 0.01. torting of oil shale can also affect ground-water We have discussed the development of in supplies in complex ways. When ground water creased rock mass permeability through mining- reenters a gasified coal bed, the residual reaction induced fracturing and the structural and products (coal ash, tars, and gases) may undergo lithologic characteristics that may be important leaching, dissolution, and hydrologic transport 11 (Mead and others, 1978). Subsidence effects such drawing in of oxygen and the exhaustion of as extensive fracturing or mine roof collapse may smoke, steam, and noxious gases through subsi cause contaminants to find their way into aquifers dence cracks and pits. As the coal burns, more that lie above the mined zone. Any meaningful cavities are created causing more cracking and evaluation of this potential hazard must be based collapse which allows greater access for air, on long-term monitoring of typically slow flow thereby accelerating coal burning. This uncontrol rates. As a consequence of the sorptive properties led "in situ gasification" process is destroying a of the rocks associated with coal, the contaminants valuable resource. In addition, gaseous combus may move more slowly than the ground water and tion products locally pollute the air; soil changes they may react chemically with each other and are produced, and vegetation is retarded or killed with the rocks, potentially lengthy and poorly un by near-surface fires. derstood processes. FACTORS GOVERNING SUBSIDENCE EFFECTS ON VEGETATION AND ANIMALS The magnitude, rate of development, and sur As we have noted previously, collapse over face expression of the subsidence process are con deep mine cavities may induce tension fractures in trolled by several factors, most of which are inter nearby overlying beds and at the ground surface. dependent. These include mining method, depth of Methane gas may leak out of shallow coal seams extraction, size and configuration of openings, or mines through these fractures and kill trees rate of advance or extraction, seam thickness, to and woody plants, leaving only grasses unaf pography, lithology, structure, hydrology, in situ fected. Garner (1974) found that certain bacteria stresses, and rock strength and deformational in the soil use methane to produce hydrogen sul- properties. We point out the significance of these fide and nitrous oxide. These gases disrupt the factors, if only in a cursory fashion, because, root transpiration of woody plants, ultimately kil taken collectively, they demonstrate the complex ling them. Noxious or toxic gases may also over ity of the subsidence process. come animals grazing on the surface. Another source of damage to plant life and as MINING METHOD sociated wildlife is the trough-like subsidence Although any underground void will potentially areas formed over areas of longwall mining. Un induce subsidence, the manner in which a resource less drainage is maintained, these depressions is extracted exerts a large influence on surface de may fill with water, creating swampy, tree-killing formations. Thus, room-and-pillar, longwall, and conditions and new types of habitats. Conversely, in situ extraction techniques will affect the sur in some western areas (for example, North rounding rock differently; pillar layout, cavity Dakota) collapse pits may be the only places wet shape, and the volume of material removed gov enough to sustain the growth of cottonwoods and ern the timing and configuration of surface ex willows. Collapse pits and open fractures may trap pressions. For example, longwall mining generally animals or interrupt their migration patterns. produces contemporaneous subsidence, whereas Losses of soil water or water in deeper aquifers room-and-pillar mining may prolong or delay the through fractures created by subsidence could be deformation of walls and roofs for many years. equally harmful to plant and animal life, especially Compulsory recording of mine workings was not in semiarid areas. introduced in the United Kingdom until 1872, and there are at least 30,000 unrecorded shallow work COAL MINE FIRES ings, some over 400 years old, where even today Coal mine fires are an indirect result of coal ex gradual deterioration of coal pillars leads to pillar traction and contribute to long-term subsidence. collapse and uneven surface subsidence According to Dunrud and Osterwald (1980) sev (Littlejohn, 1979). eral fires are burning in long-abandoned under ground mines near Sheridan, Wyo. Many of the MULTIPLE SEAM MINING OF COALBEDS fires appear to have been started by spontaneous The sequence in which individual coalbeds of a ignition when air and water were introduced multiple-seam deposit are mined determines the through subsidence cracks, pits, and unsealed stress concentrations elsewhere in the mining mine openings. Combustion is supported by the area, in beds both above and below the bed being
12 mined. According to Dunrud (1976) mining beds 0.3-1.2 m per day, and "high-speed" longwall face from top to bottom generally is safer and more ef advance in coal is about 1.5 m per day (Whetton ficient than other procedures, particularly if the and King, 1961; National Coal Board, 1952, 1963). final geometry is uniform; uniform geometry pre The maximum longwall face advance reported was vents stresses from being concentrated in isolated approximately 5.5 m per day in a bedded quartzite pillars and barriers and transmitted to underlying in South Africa (Cook, 1967). Several studies indi coalbeds. Several exceptions exist, however, to cate that subsidence is transmitted rapidly from this generalization. If the stratigraphic interval is the workings to the surface. For instance, when 7.5-15.0 m between two coalbeds, for example, mining ceases for holiday periods subsidence stops concurrent, uniform extraction of both beds may almost instantaneously; subsidence continues as be the safest and most efficient mining procedure. soon as mining resumes. A smooth, consistent Mining from bottom to top might be less hazard rate of advance promotes consistent, predictable ous and more productive in those areas where surface settlement. water or methane are present because mined voids in upper beds may store large amounts of THICKNESS OF SEAM OR DEPOSIT these substances, later to become hazardous if In some mining areas the relationship of seam tapped by subsidence fractures induced by mining thickness and overburden depth to vertical dis lower beds. placement of the ground surface is well estab lished. A compilation of more than 150 measure DEPTH OF EXTRACTION ments made above longwall mines in the United Because rocks are not perfectly elastic and do Kingdom showed that a maximum depth of the not deform as homogeneous, intact bodies but as subsidence basin of 0.9 times the seam thickness a jointed or layered media, the depth of extraction is reached when the span of the void (the area of governs subsidence development, particularly in complete extraction) exceeds 1.4 times the depth room-and-pillar mines. The deeper the mining of the deposit (National Coal Board, 1975). The level, the greater the length of time required for depth of the subsidence basin is less for thinner rock deformations to reach the surface; thus the seams or for shorter void spans. For example, the earliest surface deformation occurs above the shal maximum subsidence for a void span of 0.7 times lowest coal mines. This phenomenon is frequently the mining depth is 0.45 times the seam thickness. observed in areas of level topography underlain by These relations were derived from empirical ob dipping coalbeds. In the Boulder-Weld County servations gathered by the NCB, and although coal field in Colorado, for example, subsidence they are voluminous and have been verified re problems began in the 1860's when room-and-pil peatedly, their applicability to other geologic envi lar mining of shallow coal started. Subsidence was ronments and different mining methods has not relatively complete above these shallow mines. been demonstrated. Today, many years after mining ceased, the ef fects of deeper mining continue to create new sur LITHOLOGY AND STRUCTURE face hazards. However, poor information as to the The strength and deformational properties of amount of pillar support that remained after min rock masses are largely controlled by rock type ing and the dates of mining make it difficult to ac and structural features such as joints, faults, and curately estimate the potential for further subsi bedding and foliation planes. These geologic condi dence. tions are responsible for significant variations in subsidence development and particularly affect RATE OF ADVANCE the surface extent and timing of subsidence. The British National Coal Board has found that Knowledge of geologic conditions in advance of for longwall coal mining the time taken for subsi mining can aid in subsidence prediction and in the dence to occur depends primarily on the time it development of a monitoring plan. For example, takes for a coal face to be worked through the the angle of draw changes with the dip angle, and critical area. This time in turn is controlled by the this effect can be readily estimated (National Coal depth and the angle of draw (because these factors Board, 1975, p. 16-18). In addition, monument determine the critical area) as well as by the rate line layout can be shifted down dip according to of advance. Typical longwall advance rates are the dip magnitude.
13 A gradual lowering of the surface is associated when the mining face was only 20 m past the frac with weak overburden rocks whereas violent, ture. often delayed, collapse is more typical of strong In fractured crystalline rocks, the final angle of overburden rocks. The presence of through-going draw is typically controlled by faults and joints faults or dikes may limit the lateral growth of a and is usually located where the flattest geologic surface depression, particularly in crystalline wealgiess intersects the mining level and the rocks (Lee, 1966; Crane, 1931). Such discon ground surface. tinuities apparently act as barriers because of lat eral contrasts of rock properties. IN SITU STRESSES Several examples of the role of lithology and AND OTHER GEOTECHNICAL FACTORS structure may be cited. In the county of Lanark High horizontal stresses, which are common in (Lanarkshire) in Scotland, room-and-pillar mining many shallow crustal rocks, act to inhibit the de had ceased 118 years before sandstone beds col velopment of a surface depression by maintaining lapsed abruptly over workings that were only 16 a strong ground arch in the immediate mine roof. m deep. At the surface, structural damage to Arch height and stability, however, are very sen apartment buildings was so severe that the ten sitive to the ratio of vertical to horizontal in situ ants were evacuated and several blocks of build stresses. A highly stressed arch may fail violently ings were demolished (Thorburn and Reid, 1978). as a result of progressive thinning as happened at Orchard and Alien (1965) noted that 9 percent of the Urad mine in Colorado (Kendrick, 1973). At total potential subsidence occurred during a 6- that mine, a molybdenum ore body in country rock year period after a 166-m-deep longwall face ad of rhyolite and coarse-grained granite was mined vance stopped at Peterlee in the United Kingdom. by block caving. Horizontal stresses near the mine A thick dolomitic limestone bed apparently re average 10.2 megapascals (MPa) (1,478 lb/in2) tarded complete subsidence. which is 4.6 times greater than the horizontal According to Kent (1974) roof falls in the stress induced by gravity loading (Hooker and Pittsburgh, Pa., room-and-pillar coal mines show others, 1972). These "anomalous" stresses may two distinct patterns. In some mine areas where have caused the extreme difficulty in breaking shale directly overlies the coal, nearly all roof falls down the arch and initiating caving even though occur along northeast-trending mine passageways very high powder factors were used. An unsup parallel to the butt cleat (joint) direction in the ported stable arch 100 by 150 m existed approxi coal. In other mine areas where shale overlies the mately 100 m below the surface. The caving oc coal but where thick sandstone lenses overlie curred spontaneously and violently; an airblast nearby coal, severe and frequent roof falls may from rock bursting penetrated 60-75 m of broken occur in the shale roof with no consistent orienta muck and was still strong enough to knock people tion relative to the joints or passageways. Region down. The entire back came down breaking al jointing appears to control the first type of roof through to the surface and forming a "glory hole" fall, and sandstone channel deposits control the 150 m in diameter and 30-100 m deep. Similar vio second type. The joint spacing is governed by the lent deformations have been reported in other rock type: joints in the coal are well developed brittle, highly stressed rocks including sandstone, and closely spaced; joints in shale are less well de quartzite, and coal. veloped and have a spacing of less than 0.3 m; and In many room-and-pillar mines little or no subsi joints in sandstone are typically more than 0.3 m dence is anticipated, and surface use is planned ac apart and are well developed. Through careful cordingly. Most pillars deteriorate and deform mine layout it may be possible to control roof falls with time, however, and depending upon bulking, and to more accurately predict the nature and tim deformation may extend to the surface. Lack of ing of subsidence. knowledge of the physical properties and rock At a longwall mine in New Mexico, surface sub stresses can lead to poor mining practices which in sidence fractures indicated that the overburden turn create stress problems in other parts of a was breaking primarily along the major east-west mine, bringing about further uneven extraction joint system and to a lesser degree along the sec procedures. In some mining regions of the United ondary north-south joint set (Gentry and Abel, States, particularly those in the West (Dunrud, 1978). These tension fractures started to close 1976), concentrations of earth stresses cause rock
14 bursting and coal bumps which induce roof falls plied to coal mining. It has long been known that and pillar failures. These rock failures have forced the deeper the seam, the longer the duration of changes in mining plans or abandonment of mine surface movement, although the reasons are not areas before mining is completed in a uniform well understood. manner, causing greater subsidence damage be In longwall mining, the subsidence of a point at cause of uneven mine geometry. Knowledge of pil the surface theoretically begins when a longwall lar rock strength and creep properties is most face enters the "critical area" (fig. 1) and ceases critical in areas of high potential impact. In Al when the face leaves the critical area. The surface legheny Plateau coal mines, for example, high in point actually continues to subside (residual subsi situ horizontal stresses and the orientation of the dence) for a variable period, perhaps months, al maximum horizontal stress are related to roof in though over 90 percent of the total subsidence oc stability and floor heave, making mine layout a curs while the face is within the critical area. Most major concern (Aggson, 1978). investigators point to rate of advance and depth of- mining as the factors governing the rate and tim TOPOGRAPHY ing of surface subsidence. As we have stated ear Several investigators have noted the complicat lier with respect to the areal limits of subsidence, ing effects of topographic variations on subsidence geological/geomechanical properties influence development. In contrast to a level ground surface strain rates and modes of deformation. Orchard where the stresses produced by overburden on and Alien (1974) contend that when the face ad subsurface rocks are uniform, regions with irregu vances out of the "critical area," further ground lar topographic relief will have irregular stress movements occur because of complex time-depen distributions that vary with the height of the col dent stress redistribution processes in the overly umn of rock above a particular underground point. ing rocks. The influence of depth of mining and Measurements made above three panels at the face position on time-dependent subsidence be York Canyon mine in New Mexico, a longwall op comes more significant in room-and-pillar mining eration, show that maximum subsidence occurs primarily because of the difficulty of predicting under ridgetops and minimum subsidence under deterioration of pillars. draws or topographic lows (Gentry and Abel, 1978). Subsidence decreased from 25 to 30 percent SUBSIDENCE ANALYSIS of the maximum under a draw; subsidence was The objectives of subsidence analysis are to pre only 1.6 m or 50 percent of the seam thickness dict the occurrence, timing, and magnitude of ver mined. Peak subsidence of 2.0 m occurred below a tical and horizontal components of surface defor ridgetop. At the same mine, greater horizontal mation induced by underground resource extrac ground movement resulted when the direction of tion. The analysis should consider the likelihood of mining was in the downslope direction than when and the consequence of the impacts discussed in mining was in the upslope direction. This behavior the previous section. Depending on present and disagrees with the calculations of Kapp (1973), future land use plans, how much subsidence is to who showed by geometrical considerations that lerable and, in light of resource conservation, how horizontal strain would be greater when the much of the resource should be extracted? Al ground surface rose in the direction of mining and though it is desirable to recover the total deposit, less when the ground surface fell in the direction this can rarely be done; recent United States un of mining. More field measurements should help derground coal mining practice extracted only ap resolve this discrepancy. proximately 57 percent of the coal (Lowrie, 1963). The natural stability of steep slopes may be af The person concerned with assessing subsidence fected by subsidence-induced deformations, trig potential should determine whether subsidence gering landslides. The definition of stress distribu has occurred previously in the area and its sever tions related to topography is necessary for accu ity. Is future subsidence from old mining a possi rate slope-stability assessment prior to mining. bility? We have mentioned some of the many fac TIME tors that control subsidence, and each of them should be evaluated in a comprehensive analysis in The time factor in mining-induced subsidence order to determine the likelihood of significant has been investigated in the past, mainly as ap harmful environmental impact.
15 DATA REQUIREMENTS The first category includes rock strengths, The following discussion is intended as an over cohesion, angle of internal friction, and elastic view of the broad data needs for subsidence analy properties including, for example, Young's mod sis rather than as an attempt to specify a complete ulus and Poisson's ratio. Creep behavior data for suite of geotechnical properties. Considerable the overburden materials are needed to forecast latitude is necessary to allow for site-specific delayed subsidence. Knowledge of in situ stresses, needs. Knowledge of premming conditions is especially the magnitude and direction of the hori needed for prediction of disturbances that may be zontal stresses, is needed for planning a safe and caused by mining. efficient mining operation and for applying realis tic limits to mine deformation and subsidence cal GEOLOGIC DATA culations. The second category includes horizontal and vertical strains and displacements and their Geologic maps, sections, and core logs are nec magnitudes, locations, and duration. essary to define topography, depth of mining, rock types and thicknesses, jointing, faults, and varia MINING PLANS tions in the attitudes of beds, including folds. A A proposed mining plan is necessary for early drilling program should include at least one hole input to the subsidence analysis. The plan should drilled to below the coal seam in the vicinity of the be based not only on geologic, hydrologic, and surface monuments and the panel. This drilling geotechnical data but also on mining methods, ex should be done for the first panel of a new mine traction location and sequence, location of panels, and for one panel in an operating mine. The hole barrier pillars, and, in the case of multiple-seam should be logged from the surface. Rotary drilling deposits, the seams to be mined and the proposed is satisfactory if cuttings are collected and iden sequence. The mining plan should anticipate the tified for every 1.5 m of the drill run and if the magnitude, location, and timing of surface distur hole is geophysically logged to produce an electri bance. If the plan is revised, initial subsidence cal resistivity (lithologic) log of the rock overlying predictions may be altered. the panel. The location of any methane gas in coal beds should be defined as well as the rank of the coal and its composition, including sulfur content. ANALYTICAL TECHNIQUES Knowledge of premming seismicity of an area is FOR SUBSIDENCE PREDICTION needed to determine the likelihood of complica BACKGROUND AND STATE-OF-THE-ART tions such as severe bumps and roof falls. The accuracy of subsidence prediction varies
HYDROLOGIC DATA from country to country, and is controlled mainly by knowledge of geologic and topographic condi Studies should be done to define premining sur tions and the length of mining experience. In the face-water and ground-water volumes, flow rates, United Kingdom, for example, data has been col and quality. Aquifers and aquicludes should be lected over a long period of time from more than identified. It is very important to identify all sur 150 coal mines in similar geologic environments. face springs, streams, or bodies of water. Well The results of these observations have been pre data for the region should be systematically sented as empirical formulas and procedures for analyzed throughout the mining period. The sorp- prediction of the nature, area! extent, and sever tive properties of the rocks may be closely related ity of subsidence and related events. Subsidence- to subsidence-induced ground-water changes that induced surface deformations can be predicted and were discussed previously. described but only under conditions similar to
GEOTECHNICAL DATA those of the original observation. Particularly in Europe, new methods of extraction such as har Two categories of rock-behavior data are monic mining and stepped-face layout have been needed for subsidence analysis. One category developed to minimize surface deformations and deals with the behavior of the underground mine reduce damage to structures. It has been possible and the surrounding rock and water environment. through integrated systems of prediction, mining The other category deals with the behavior of the techniques, and monitoring procedures to control ground surface above the mine. the development of subsidence basins so that shal-
16 low mining may proceed under buildings and even tion is plotted (fig.SD). From the subsidence pro towns. file the values of subsidence for points at recom In the United States, methods of subsidence mended intervals of h/20 are tabulated. Values of control and estimation are less well developed. ground slope, differential ground slope (
17 81
STRAIN, IN MILLIMETERS PER METER WIDTH IN TERMS OF DEPTH (w/h) WIDTH OF OPENING, IN METERS
-» W CO £» Ol o o o o o o o o o o o
o H m O 3 3 g O o
SUBSIDENCE (S). SLOPE, IN SUBSIDENCE (S), SLOPE, IN SUBSIDENCE (S), SLOPE, IN IN METERS PERCENT IN METERS PERCENT IN METERS PERCENT
DISPLACEMENT, DISPLACEMENT, DISPLACEMENT, IN METERS IN METERS IN METERS
STRAIN, IN MILLIMETERS PER METER STRAIN, IN MILLIMETERS PER METER STRAIN, IN MILLIMETERS PER METER 2 percent, which are four times larger than manipulate elastic or viscoelastic continuum exca those in the United Kingdom, were mea vation models. As more realistic model properties sured. and boundary conditions are defined, predictions Although a few other measurements indicate should become more accurate. Voight and similar gross trends (Gentry and Abel, 1978), in Pariseau (1970) were not able to reconcile field ob sufficient observations have been made at other servations of surface displacements over British U.S. longwall mines to confirm or deny a general coal mines to subsidence profiles calculated on tne trend. basis of isotropic theory. They did find, however, The NCB method is not relevant to mining in that transversely isotropic theory was in good crystalline rocks or to in situ extraction tech agreement with field data. niques. In general, subsidence-prediction methods are most seriously deficient where subsidence defor OTHER EMPIRICAL METHODS mations are controlled by structural discon Another empirical prediction method is based tinuities such as faults, joints, and folds, or by on the so-called "stochastic" or random media rock type. theory discussed by Voight and Pariseau (1970). The method is seriously restricted because it does SELECTION OF AN ANALYTICAL TECHNIQUE not incorporate knowledge of material properties The NCB method is based on the largest sys prior to mining but rather transfers field measure tematic analysis of subsidence data available. It is ments from known areas to areas of new mining accurate enough to predict trough-like subsidence via an empirical procedure. This approach may re from longwall mining where the overburden rock sult in the uncertainty of results, and an exceed is largely shale, siltstone, or marlstone, where it ingly large number of studies would be required to does not contain thick, strong beds of sandstone or demonstrate the general validity of the method. limestone, and where the topography is subdued. The same difficulty applies to the NCB system. Thus, the NCB method is applicable to many en Statistical methods that treat a broad collection of vironmental impact analyses of underground min data concerning geologic controls on subsidence ing. are more broadly applicable. We have found, for Where coal is extracted by room-and-pillar example, that as the percentage of shale in the methods, subsidence prediction will be very diffi rock mass decreases and the amount of sandstone cult, particularly where there is irregular room increases, the angle of draw and the area of poten development, nonuniform barrier pillars, and poor tial subsidence decreases (Abel and Lee, 1980). definition of panels. Under such circumstances, frequently associated with old mining areas, it is MATHEMATICAL TECHNIQUES usually impossible to predict the time, magnitude, The mathematical approaches to subsidence pre or occurrence of subsidence. diction attempt to define the conditions that lead In modern room-and-pillar coal mining, at to subsidence and, hence, to develop general pre tempts are usually made to control subsidence by dictive models. In these approaches the rock mass leaving a larger percentage of the coal in place, surrounding the mined opening is assumed to be often 40 percent or more. In Illinois (Hunt, 1979), an ideal material that deforms elastically. Wide the magnitude, shape, and position of the subsi applicability and improved understanding are the dence profile is delineated on the basis of case-his main advantages claimed for these approaches tory comparisons in which extraction ratio, depth, (Voight and Pariseau, 1970). Included in this panel width, and mining thickness are the princi group are elastic, viscoelastic, and plastic idealiza pal factors. Where bedrock overburden is less tions. Finite-element analysis is widely used to than 50 m thick, smaller sinkhole features pre-
FIGURE 8. British National Coal Board method of estimating subsidence. A, Relation between maximum subsidence, width, and depth. B, Lines of equal subsidence for various width/depth ratios. C, Strain prediction graph. D, Subsidence profiles and associated slopes and horizontal strains for subcritical width of extraction. E, Subsidence profiles and as sociated slopes and strains for critical width of extraction. F, Subsidence profiles and associated slopes and strains for supercritical width of extraction. (Modified from National Coal Board, 1975.)
19 dominate; if it is more than 50 m thick, subsidence amount of extraction. The findings of Hunt con is trough-like. Sinkhole development is also re trast with those of O'Rourke and Turner. Again, lated to the hydraulic connection between the sur more data from longwall coal mines in the United face and the mined-out area. Caving of the roofs States is needed in order to assess applicability of of shallow mines allows seepage of surface waters the NCB method. into the mine, decreasing roof and wall stability Pennsylvania's Bituminous Mine Subsidence and and promoting caving to the surface. The graphs Land Conservation Act of 1966 provides for pro in figure 9 show the relationship between percent tection of certain structures including public build subsidence and panel width for several extraction ings and residences. Protection is defined in terms ratios in Illinois. These examples would be typical of support provided by coal left in the ground. trough-like subsidence. Hunt (1979) has shown From April 1966 to January 1969 only 1 percent of that where extraction is greater than 80 percent, the 5,500 protected structures were damaged by the NCB method accurately predicts subsidence. subsidence, and in 1977 only six cases of damage Figure 10 indicates that surface tilting is directly to the protected structures were reported (Bise, proportional to subsidence and increases with the 1980). The formula by which these structures are
Development and low extraction
20
u. 40 O z 111 A ccO 111 CL 60 C)
A Medium extraction
O High extraction O 80
Retreat mining C)
A Longwall
0.5 1.0' 1.5 2.0 PANEL WIDTH/MINING DEPTH, IN METERS
FIGURE 9. General relation between percent subsidence and panel width to mining depth ratio. Curves indicate approximate range of control of mining method on subsidence. (From Hunt, 1979.) 20 A Longwall mining 10 H Retreat mining
O Room-and-pillar mining
o so LJJ a O LJJ O O LJJ O 2 LJJ a co Kn QQ 50 D CO o o 60 10 20 30 SURFACE TILT, x 10' FIGURE 10. Relation between subsidence and surface tilt (from Hunt, 1979). protected was derived from many subsidence inci lars which can be no smaller in plan than 6.1 by dents over the Pittsburgh seam. We quote Bise 9.1 m. In addition, no mining may be done where (1980, p. 1): the overburden thickness is less than 30.5 m Where a structure is located on terrain that is level or slopes under a protected structure. Barrier pillars cannot less than 5%, the lateral distance (LD) of the support area be extracted where the pillar width is less than from each side of the structure is equal to the tangent of 15 de the cover. As Bise (1980) points out, however, this grees (0.27), multiplied by the depth of cover (D), plus a safety factor of 4.6 m, or LD = (0.27) x(D)+4.6m. To find the total method fails to consider the effect of mining length or width of the support area, double the result found for methods on subsidence and does not allow for re LD and add the length or width of the structure. covery above 50 percent. This guideline deals only with defining the sup Studies done in Europe have resulted in general port area. Within this area, 50 percent of the coal schemes for controlling subsidence damage in par must be left in place in uniformly distributed pil tial extraction mining. Orchard (1964) found that if 21 both horizontal and pillar dimensions are equal to toring be conducted to measure ground deforma at least 10 percent of the mining depth, and if the tions produced by these types of mining opera pillars are uniformly spaced, subsidence will.be tions so that their subsidence characteristics can negligible while allowing 50-percent recovery of be verified. the coal. Wardell (1969) presented data to show that there is a consistent relationship between the MONITORING METHODS percent of subsidence and depth, extraction ratio, Geotechnical measurements are made in order seam thickness, and pillar width. The findings of to establish the validity of model predictions and these researchers are in general agreement. to safeguard surface structures. The monitoring At the York Canyon mine near Raton, N. Mex., methods and types of instruments used must be Gentry and Abel (1978) compared vertical and carefully selected so that their data output is com horizontal strains resulting from longwall mining patible with the data needed for subsidence calcu with values calculated from the NCB method. lations. Much of the overburden there is weak shale, grossly similar to the British coal fields. Maximum SURFACE INSTRUMENTATION compressive and tensile horizontal strains deter Our intent is to briefly describe current moni mined at the mine were more than twice the pre toring practice, rather than to present a specific dicted values. The NCB method predicted 25 per set of guidelines. A premining survey should be cent less vertical tensile strain than was actually made to identify geologic conditions or surface fa measured. The York Canyon mine subsidence pro cilities that would warrant changes in a conven file was similar in shape, however, to the NCB tional monitoring scheme. model. Presence of a strong 12-m-thick sandstone, The most widely used and important type of strong jointing, and mountainous terrain may ac subsidence-monitoring measurement is surface count for some of the observed differences. leveling. Monuments are installed before mining begins, and readings are taken until stability is SUBSIDENCE MODELS reached after mining ceases. FOR IN SITU ENERGY EXTRACTION A typical subsidence monitoring layout for a The subsurface voids remaining after in situ 762-m by 168-m longwall panel below mountainous processes such as coal gasification and oil shale re terrain is shown in figure 11. In this example the torting will not closely resemble the voids created seam thickness is approximately 3 m and the aver by coal mining. The change in load-supporting age overburden thickness is 107 m. One row of ability of the surrounding rock mass in such "par monuments was placed along the centerline of the tial extraction" processes may be difficult to de planned panel and another row perpendicular to termine, and until field measurements are avail the centerline. Two diagonal lines were estab able, the prediction of subsidence effects for in lished radiating from the centerline at 45°. These situ extraction will be based mainly on mathemati lines were added to define subsidence effects cal analyses such as computer solutions of finite above the corners of the loitgwall panel. element codes. Examples of current practice in For monitoring most coal mine panels in flat this area are given by Langland and Fletcher terrain, three lines of surface monuments are ade (1976) and Advani and Lin (1977). quate, two perpendicular to the long axis of the The mathematical solutions have several limita mining panel and one directly above the centerline tions. Among them are: of the panel. The cross-panel lines should extend 1. The solutions are valid only for geometrically 0.9 times the depth of the seam outside the panel simple openings. on both sides and completely across the panel. The 2. The material is assumed to be isotropic, centerline monuments should also extend 0.9 homogeneous, and elastic. times the depth of the seam outside the starting 3. The effects of water and geologic strucure are position of the panel and the same distance past not considered. the planned end of the panel. If the panel is termi Mead and others (1978) discussed prediction and nated because of poor roof conditions or faulting, measurement of subsidence produced by in situ for example, the centerline monuments can be coal gasification. It is important that early moni shortened accordingly. These recommendations 22 EXPLANATION CONTOUR LINE 50-FOOT INTERVAL ENTRY EXISTING REFERENCE POINT O TURNING POINT MONUMENT LOCATION X MPBX FIGURE 11. Location of surface monuments, turning points, and reference points for part of the York Canyon mine, Raton, New Mexico (from Gentry and Abel, 1978). 23 are in general agreement with current practice in terline monuments should be surveyed when min the United States (Wade and Conroy, 1980; Gen ing of the panel has advanced 1.9 times the seam try and Abel, 1978). depth. Each row of crossline monuments should be surveyed when mining has progressed 1.0 SPACING OF MONUMENTS times the depth past the crossline. All monuments The spacing of monuments directly influences should be resurveyed after panel completion and measurement precision. Monuments that are yearly until no further changes in monument posi spaced too closely may result in measurement of tions are detected. Survey procedures and the local anomalous ground movements caused, for ex need for long-term monitoring are addressed by ample, by the displacement of individual joint Collins (1978). Gentry and Abel (1978) give de blocks in which the monuments are anchored. tailed specifications for monument construction Widely spaced monuments may reduce costs but and monitoring equipment. could fail to adequately define the subsidence pro file, particularly in mountainous terrain. The NCB SUBSIDENCE CONTROL AND REDUCTION (1975) suggests a spacing of 0.05 h (where h is An obvious conflict exists between minimizing equal to the depth of the overburden) or one- subsidence and maximizing resource recovery. In twentieth of the depth of mining. Panek (1970) re some areas more surface settlement can be toler commended a monument spacing of 0.05-0.1 h. ated than in others. In urban areas, for example, The presence of critical surface structures may less than 50 percent of the resource may be reco also dictate the location and spacing of monitoring vered in order to prevent subsidence. In many positions. For most situations the monument spac European coalfields where overburden thicknesses ing should probably not exceed 0.1-0.2 h. This rel range from 60 to 900 m, deformation arches are atively wide spacing allows determination of verti stable within the overlying rocks, and subsidence cal ground movements and permits a rough check does not reach the surface if the widths of mined- of the horizontal strain measurements obtained by out areas are held from one-fourth to one-half of extrapolation from the relatively smooth vertical the overburden thickness (Zwartendyk, 1971). subsidence profile. Spacing monuments more Much coal must be left in the ground in order to closely in order to better define horizontal defor obtain this high degree of surface stability. mations is probably not justified. According to Or In the United Kingdom, a "panel and pillar" chard and Alien (1965, p. 622), for example, "It is method has been used with success when mining a fact that although levels taken before and after beneath towns, factories, railroads, and utility subsidence will usually produce a smooth subsi lines. The panels were mined without backfilling, dence profile, the strain diagram obtained from and little subsidence damage occurred. Subsidence horizontal measuring between survey stations is ratios were less than 20 percent of the coal thick often erratic." They attribute this situation to "ir ness above mined panels whose widths were about regularities" in the subsoil. At the York Canyon one-third the average overburden depth. mine (Gentry and Abel, 1978), horizontal move In sedimentary iron ore deposits of the Lorraine ment measurements were less reliable than verti area of France, support pillars were left in a cal measurements, apparently because of shifting checkerboard pattern; this method was aban and tilting of the joint blocks on which the con doned, however, because of subsidence problems crete monuments were anchored; movement of the associated with severe pillar bursts. The joint blocks affected horizontal measurements geometry of the mining plan was changed to more than vertical measurements. Additional de create sturdy barrier pillars separating panels tails of generally accepted monument layout and whose widths were 0.42 times the overburden construction are given by Wade and Conroy (1980) depth. These pillars sustained the compression and Gentry and Abel (1978). arch within the overburden, minimizing subsi Monuments should be surveyed at least twice dence and allowing safe recovery of 60 percent of before starting panel extraction, and any dis the total reserves. crepancies should be resolved by an additional Another mining technique developed in The survey. The frequency of readings is determined Netherlands coal mines is known as harmonic min by the rate of movement of the face, seam thick ing. This technique involves mining in such a way ness extracted, and monument spacing. The cen- that the final vertical and horizontal surface 24 strains produced by mining in one area are cancel ruptive effects of mining on water supplies have led by strains produced by mining in another area. been studied at only a few domestic locations. For example, mine extraction panels might be Some of the factors that influence the develop offset and mined concurrently so as to produce ment of ground movement cannot be quantified negligible final horizontal strain in the overburden precisely. The differences in rock-mass behavior or at the surface perpendicular to the mining caused by site conditions alone would indicate that panels. Several drawbacks exist, however, to this subsidence prediction and engineering cannot be approach. Damaging transient strains and com treated in purely mathematical terms. Although pression arches can be produced at the surface. the NCB has developed quantitative, practical as The method might also cause high stress concen sessments of mining effects in the United King trations in the rock between coal beds at the mine dom, there is no generally applicable subsidence boundaries which could cause rock bursts and seri model for the United States, nor are there ous roof falls. Further, mining schedules must be adequately tested, empirical models for any of the very precise under this system to maintain the re major U.S. coalfields: the Appalachian, Interior, quired geometry. Other factors that would lessen or Rocky Mountain. The influence of local geologic the effectiveness of this method are uneven topog environments and mining methods on subsidence raphy and steeply dipping coal beds. in the United States will require the collection of Backfilling of mined-out areas in room-and-pillar large masses of data in order to construct several mines has been effective in preventing subsidence. accurate prediction models. Virtually no data base This method is used primarily in mines below exists from which to forecast the surface effects of urban areas where surface stability is critical. the various proposed in situ extraction techniques. Backfilling materials may be placed manually, The behavior of rock units above longwall oper hydraulically, or pneumatically and may consist of ations has been shown to be more accurately pre sand or larger grained mine waste. Hydraulic dictable than for room-and-pillar mining. An ac backfilling is generally most efficient, and cumulation of quantitative data in the United maximum subsidence is frequently less than 10 States similar to that of the NCB for longwall percent of the seam thickness extracted. The ex mining would be valuable in establishing sound ample given previously of backfilling with coal domestic empirical or mathematical subsidence waste at Farmington, W. Va., illustrates the ef models. The tools, techniques, and knowledge fectiveness of this method. exist to conduct a coordinated nationwide subsi dence research program. If the development of DESIGN PRECAUTIONS subsidence technology can lessen costly environ Buildings and other structures can be designed mental or structural damage, the cost of this de to resist or tolerate subsidence deformations. velopment will be repaid. Very flexible structures remain intact (and con formable) despite subsidence-caused distortion and, alternately, very rigid foundations have been developed that can be releveled by the use of GLOSSARY jacks as subsidence progresses. The design of sup port mechanisms for facilities built over old mine workings involves a careful appraisal of subsur Angle of draw. The angle formed by the vertical and a line face conditions. Support by fill, caissons, or piers drawn from the edge of the underground workings (rib- using boreholes may be feasible. side) to the point of zero subsidence on the ground surface. Also called limit angle. Arching effect. The mining-induced process by which the roof CONCLUSIONS bows upward spanning the mined-out area and tending to prevent roof falls. The span width is controlled by many Although the damaging surface effects of under factors including depth, seam thickness, mining method, ground mining have been widely documented, lit rock strength, structure, and in situ stresses. tle research has been carried out to determine ac Back. Roof of a mine or tunnel. Backfilling. Placement of material in underground workings curately the effect of subsidence on surface struc in order to retard collapse of roof and pillars and to reduce tures under specific geologic conditions in the subsidence. United States. Further, the potentially broad dis Barrier pillar. A solid block or rib of coal left unmined be- 25 tween two mine workings for protection against water in Residual subsidence. That amount of the total subsidence that flow or squeezing conditions. occurs after the face leaves the critical area. Bay. See panel. Retreat mining. A mining method by which a pillar of solid Block caving. A sloping method of mining in which a thick coal or ore is left until the final mining while pulling out block of ore is partly isolated from surrounding blocks by of a room-and-pillar panel. a series of drifts. The block is undercut by removing a Rib. The side of a pillar or the wall of an entry. slice from under the block causing it to cave under its own Ribside. Edge of mine workings. weight. The broken ore is removed from below, and as the Room. An excavation driven from an entry from which coal or caved mass moves downward, it is further broken by pres ore is produced. sure and attrition. Room and pillar. A method of mining coal or other resources Breakthrough. A passage cut through a pillar to facilitate ven in which the seam is mined in rooms separated by narrow tilation from one room to another. pillars and in which 50 percent or more of the resource is Bulking. The increase in volume, by virtue of increased void removed during initial mining. volume, of mined rock. Solution mining. The in-place dissolution of mineral salts of Butt cleat. The minor cleat system or jointing in a coal seam. an ore with a leaching solution. Also called end cleat. Subcritical subsidence profile. Subsidence profile drawn at a Chimney. Initial surface collapse above a mined-out area in subcritical width. crystalline rocks. Subcritical width. Width of extraction of coal seam at which Cleat. A joint system in a seam along which the coal fractures. the bottom of the subsidence area is trough-like and has There are usually two cleat systems developed perpen less than the maximum value. dicular to each other. Subsidence. Vertical component of ground movement. Critical subsidence profile. Subsidence profile drawn at the Subsidence area. The entire surface area affected by subsi critical width. dence over a high-extraction panel. Critical width. Width of extraction of coal seam at which the Subsidence basin. The depression at the surface above high- subsidence at the bottom of the trough has the maximum extraction mining panels. value. Subsidence profile. A curve depicting subsidence of the Delayed subsidence. A variable amount of residual subsidence ground surface on a section drawn parallel to the direction that occurs long after mining, usually controlled by of advance of an underground excavation. lithologic properties. For example, a strong rock layer be Supercritical subsidence profile. Subsidence profile drawn at tween the mine and the surface may retard the collapse a supercritical width. process months or years. Supercritical width. Width of extraction of coal seam at which Dip angle. The inclination from the horizontal of a seam or the bottom of the subsidence trough is approximately flat bed, measured perpendicular to the strike of the struc at the maximum subsidence value. ture. Transition point. The point of transition between concave and Extraction ratio. Ratio of mined-out area to the total planned convex curvature of a subsidence profile. mine area. Face. A surface on which mining operations are in progress. Face cleat. The major cleat system or jointing in a coal seam. In situ. In the natural or original position; in place. Leaching. The extraction of soluble metals or salts from ore by means of percolating water or other solutions. Longwall. A method of mining coal or other resources in REFERENCES CITED which the seam is removed in one operation by means of a long working face or wall. The workings (face) are ad Abel, J. F., Jr., and Lee, F. T., 1980, Subsidence potential in vanced in a continuous line which may be several hundred shale and crystalline rocks: U.S. Geological Survey Open- meters long.. File Report 80-1072, 52 p. Mining Horizon. The level at which a deposit is mined. Advani, S. H., and Lin, Y. T., 1977, Subsidence and roof re Monument. A stake, rod, or concrete structure that is used to sponse studies related to underground coal gasification: mark ground location points for mining surveys. Annual Underground Coal Conversion Symposium, 3d, Overburden. The rock and (or) soil above a coal seam. Fallen Leaf Lake, California, June 6-9, 1977, Proceedings, Panel. Areas of extracted coal, separated by long, solid bar p. 422-429. rier pillars. Aggson, J. R., 1978, Coal mine floor heave in the Beckley coal Panel length. Dimension of a panel measured in the direction bed, an analysis: U.S. Bureau of Mines Report of Investi of face advance. Also called face length. gations 8274, 32 p. Panel width. Distance across a working coal face. Also called Alien, A. S., 1978, Basic questions concerning coal mine subsi face width. dence in the United States: Association of Engineering Pillar. Solid coal or ore left either temporarily or permanently Geologists Bulletin, v. 15, no. 2, p. 147-161. to support the roof or prevent water inflow. Babcock, C. 0., and Hooker, V. E., 1977, Results of research Powder factor. The amount of explosive used to mine a ton of to develop guidelines for mining near surface and under rock or ore. ground bodies of water: U.S. Bureau of Mines Information Percent recovery. The proportion of coal or other resource Circular 8741,17 p. mined from a seam or deposit. Bise, C. J., 1980, Pennsylvania's subsidence-control guidelines- 26 should they be adopted by other States: American Insti Kapp, W. A., 1973, Mine subsidence, in Subsidence in mines: tute of Mining Engineers Preprint no. 80-6, 9 p. Australasian Institute of Mining and Metallurgy Sym Collins, B. J., 1978, Measurement and analysis of residual min posium, Wollongong, 1973, p. 1-1 to 1-10. ing subsidence movements, in Geddes, J. D., ed., Confer Kent, B. H., 1974, Geologic causes and possible preventions of ence on large ground movements and structures: Cardiff, roof fall in room-and-pillar coal mines: Pennsylvania Wales, July 4-7, 1977, Proceedings, p. 3-29. Geological Survey Information Circular 75, 17 p. Cook, N. G. W., 1967, The design of underground excavations: Kendrick, R. E., 1973, Induction caving URAD mine, in Cum Symposium on Rock Mechanics, 3d, University of Min mins, A. B., and Given, L A., eds., SME mining engineer nesota, Proceedings, p. 167-193. ing handbook: American Institute of Mining, Metallurgi Crane, W. R., 1931, Essential factors influencing subsidence cal, and Petroleum Engineers, v. 1, p. 12-203 to 12-210. and ground movement: U.S. Bureau of Mines Information Langland, R. T., and Fletcher, D., 1976, Predicting subsidence Circular 6501, 14 p. over coal gasification sites: Lawrence Livermore Labora Deere, D. U., 1961, Subsidence due to mining-A case history tory Report UCID-17326. from the Gulf Coast region of Texas, in Mining engineer Lee, A. J., 1966, The effect of faulting on mining subsidence: ing series-Symposium on Rock Mechanics, 4th, Univer Mining Engineer, p. 735-745. sity Park, Pennsylvania, Proceedings: Pennsylvania State Littlejohn, G. S., 1979, Surface stability in areas underlain by University Mineral Industries Experiment Station Bulle old coal workings: Ground engineering, v. 12, no. 2, p. 22- tin 76, p. 59-64. 48. Dunrud, C. R., 1976, Some engineering geologic factors con Lowrie, R. L., 1968, Recovery percentage of bituminous coal trolling coal mine subsidence in Utah and Colorado: U.S. deposits in the United States: U.S. Bureau of Mines Re Geological Survey Professional Paper 969, 39 p. port of Investigations 7109,19 p. Dunrud, C. R., and Osterwald, F. W., 1980, Effects of coal Marsden, R. W., and Lucas, J. R., 1973, Specialized under mine subsidence in the Sheridan, Wyoming, area: U.S. ground extraction systems, in Cummins, A. B., and Geological Survey Professional Paper 1164, 49 p. Given, I. A., eds., SME mining engineering handbook: Ege, J. R., 1979, Surface subsidence and collapse in relation to American Institute of Mining, Metallurgical, and Petro extraction of salt and other soluble evaporites: U.S. leum Engineers, v. 2, sec. 21, p. 1-118. Geological Survey Open-File Report 79-1666, 33 p. Mead, S. W., Campbell, J. H., Ganow, H. C., Langland, R. Fader, S. W., 1975, Land subsidence caused by dissolution of T., Greenlaw, R. C., Wang, F. T., and Homsy, R. V., salt near four oil and gas wells in central Kansas: U.S. 1978, LLL environmental studies of in situ coal gasifica Geological Survey Water-Resources Investigations 27-75, tion: Lawrence Livermore Laboratory Report UCRL- 28 p. 50032-78, 63 p. Federal Register, 1977, Surface mining reclamation and en Miller, E. H., and Pierson, F. L., 1958, Underground move forcement provisions, December 13, 1977, 30 C.F.R. 700: ment and subsidence over United States Potash Company Federal Register, v. 42, no. 239, p. 62639-62712. mine: Society of Mining Engineers, AIME, Preprint Garner, J. H. B., 1974, Death of woody ornamentals associated 5819P9, 3 p., illus. with leaking natural gas: Highway Research Abstract, v. National Coal Board [United Kingdom], 1952, Investigation of 44, no. 3, p. 5. mining subsidence phenomena: Production Department Gentry, D. W., and Abel, J. F., Jr., 1978, Surface response to Bulletin 52/78. longwall coal mining in mountainous terrain: Association of 1963, Principles of subsidence engineering: Information Engineering Geologists Bulletin, v. 15, no. 2, p. 191-220. Bulletin 63/240, 27 p. Gray, R. E., Bruhn, R. W., and Turka, R. J., 1977, Study and 1975, Subsidence engineers' handbook: Mining Depart analysis of surface subsidence over the mined coal bed: ment, London, 111 p. U.S. Bureau of Mines Open-File Report 25-78, 362 p.; Orchard, R. J., 1964, Partial extraction and subsidence: Mining available from U.S. Department of Commerce, National engineer, v. 123, no. 43, p. 417-427. Technical Information Service, Springfield, VA 22162, as 1973, Some aspects of subsidence in the United Kingdon, report PB 281 511. in Hargraves, A. J., ed., Subsidence in mines-Proceed Hooker, V. E., Bickel, D. L., and Aggson, J. R., 1972, In situ ings of Symposium, 4th, Wollongong University, February determination of stresses in mountainous terrain: U.S. Bu 20-22, 1973: Australasian Institute of Mining and Metal reau of Mines Report of Investigations 7654, 19 p. lurgy, p. 3.1-3.11. Hunt, S. R., 1979, Characterization of subsidence profiles over Orchard, R. J., and Alien, W. S., 1965, Ground curvature due room-and-pillar coal mines in Illinois: Illinois Mining Insti to coal mining: Chartered Surveyor, v. 97, p. 622-631. tute, 86th Annual Meeting, Proceedings, p. 50-65. 1974, Time-dependence in mining subsidence, in Jones, M. Hutchings, R., Fajdiga, M., and Raisbeck, D., 1978, The ef J., ed., Minerals and the environment: Proceedings of In fects of large ground movements resulting from brown ternational Symposium, Institution of Mining and Metal coal open-cut excavations in the Latrobe Valley, Victoria, lurgy, London, p. 643-659. in Geddes, J. D., ed., Conference on large ground move O'Rourke, T. D., and Turner, S. M., 1979, Longwall subsi ments and structures: Cardiff, Wales, July 4-7, 1977, Pro dence patterns A review of observed movements control ceedings, p. 136-161. ling parameters, and empirical relationships: Ithaca, New Ivey, J. B., 1978, Guidelines for engineering geologic investi York, Cornell University, Civil and Environmental En gations in areas of coal mine subsidence-a response to gineering School Geotechnical Engineering Report 79-6, land-use planning needs: Association of Engineering 82 p. Geologists Bulletin, v. 15, no. 2, p. 163-174. Panek, L. A., 1970, Methods and equipment for measuring 27 subsidence, in Rau, J. L., and Dellwig, L. F., eds., art in subsidence engineering: American Society of Civil Northern Ohio Geological Society symposium on salt: 3d, Engineers Journal of Soil Mechanics and Foundation Divi Cleveland, 1969, Proceedings, v. 2, p. 321-338. sion, no. SM2, p. 721-750. Shadbolt, C. H., 1978, Mining subsidence-historical review Wade, L. V., and Conroy, P. J., 1980, Rock mechanics study and state of the art, in Geddes, J. D., ed., Conference on of a longwall panel: Mining Engineering, v. 32, no. 12, p. large ground movements and structures: Cardiff, Wales, 1728-1735. July 4-7, 1977, Proceedings, p. 705-748. Wardell, Kenneth, 1969, Ground control and subsidence: Min Sossong, A. T., 1973, Subsidence experience of Bethlehem ing Congress Journal, v. 55, no. 1, p. 36-42. Mines Corporation in central Pennsylvania, in Hargraves, Wardell, Kenneth, and Eynon, P., 1968, Structural concept of A. J., ed., Subsidence in mines-Proceedings of Sym strata control and mine design: Mining Engineer, v. 127, posium, 4th, Wollongong University, February 20-22, p. 633-645. 1973: Australasian Institute of Mining and Metallurgy, p. Whetton, J. T., and King, H. J., 1961, The time factor in min 5.1-^5.5. ing subsidence, in Clark, G. B., ed., International Sym Thorburn, S., and Reid, W. M., 1978, Incipient failure and de posium on Mining Research, Rolla, Missouri, 1961, Pro molition of two storey dwellings due to large ground ceedings, v. 2, p. 521^539. movements, in Geddes, J. D., ed., Conference on large Zwartendyk, Jan, 1971, Economic aspects of surface subsi ground movements and structures: Cardiff, Wales, July 4- dence resulting from underground mineral exploitation: 7, 1977, Proceedings, p. 87-99. Pennsylvania State University unpublished Ph.D. thesis, Voight, Barry, and Pariseau, William, 1970, State of predictive available from University Microfilms, 411 p. 28