TN295 .U4 I' No. 9072 •^••\.*\. v^^- / \-^--\/ v^^-;o' \'-^^'\^'^ <->m'j '..«' '^^ *ir .^^"^< '.* <>r -^^ ^VW* .-^^^ ^^ .^^-V. V .N- .....°'^^*"''"' ♦' '^^o^ -^0 ^^' ^'^U. -^^ •• ** \ --^K-" ,i>'\ •'^^•' **'"*- '-^K- .?*•%. i^P.- .*^'*. V 3P 'T'j N O ^. 't ,t' ^ ,-. *. .* .>Va-- V.^* :'mm% V.*"^ .-I^/i'. v.<*' *■ ^^^^ ^Co^^ «V> \J, ' • « s * > V j» V^^ "jvs^ -'5^0 .V » o 'bv" 1 ■< ^0^ ° ' .oVc-.^-^o, /V;,;^.X .c^/^;:."- ./Vi';^A o«^^^%% / ^ *o;"<.' .^ A" ..t-. 6^ ^ *,-^T*' /V <"•»* "^o I « ■' ^^ o * , /X.^,,,, r ^^.^12^,^ '^^^^ ov^^^i^'- -^a^ :^mj^^ ^^S ^^X:, ^ov^ -Mm>^\ ^^^..^ (i^- *o. «"^^ ^., 'bv" • o.-?-' " « " «'^ q,. *..•.'■' A.0 V ► i *J o. o> «, IC ^°^2 Bureau of Mines Information Circular/1986 Ground Subsidence and Structural Damage Over an Abandoned Room-and-Pillar Coal Mine at Hegeler, IL By Gennaro G. Marino, James W. Mahar, Larry R. Powell, and Richard E. Thill UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 9072 Ground Subsidence and Structural Damage Over an Abandoned Room-and-Pillar Coal Mine at Hegeler, IL By Gennaro G. Marino, James W. Mahar, Larry R. Powell, and Richard E. Thill UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Model, Secretary BUREAU OF MINES Robert C. Morton, Director Library of Congress Cataloging in Publication Data: Ground subsidence and structural damage over an abandoned room- and-pillar coal mine at Hegeler, IL . (Information circular / Bureau of Mines ; 9072) Bibliography: p. 22-23. Supt. of Docs, no.: I 28.27: 9072. 1, Subsidences (Earth movements)— Illinois— Hegeler. 2. Coal mines and mining— Illinois— Hegeler. 3. Mine subsidences— Illinois — Hegeler. 4. Earth movements and building— Illinois— Hegeler. I. Marino, Gennaro G. II. Series: Information circular (United States. Bureau of Mines) ; 9072. -^?^&5.U4 [QE600.3.U6] 622s [363.3M97] 85-600363 ^ \0 CONTENTS Page Abstract 1 Introduction 2 Acknowledgments 3 Site description 3 Geologic conditions 4 Regional geology 4 Site geology 5 Mining history and practice 6 Surface subs Idence 7 Subsidence history 7 Sag characteristics 7 Sag Interrelationships 13 Structural damage 14 Behavior of house C 14 Behavior of radio station building 17 Behavior of radio towers 18 Building response to sag subsidence 19 Summary 21 References 22 Appendix. — Chronological list of events related to subsidence at Hegeler, IL. . . 24 ILLUSTRATIONS 1 . Area location map. 4 2. Site location map 4 3. North-south geologic cross section through study site 6 4. Mine map showing location of subsidence sags 7 5. Location of subsidence profiles, cross sections, and reference points 8 6. Sketch map of sag 1 tension and compression features In 1967 9 7. Existing and assumed presubs Idence, north-south profile 9 8. Adjusted profiles, slopes, and curvatures for sag 1 10 9. Adjusted profiles, slopes, and curvatures for sag 2 11 10. Adjusted profiles, slopes, and curvatures for sag 3 12 11. Photographs of tension cracks and compression ridges 12 12. Relationship between mine depth and the subsidence factor 13 13. Relationship between the subsidence factor and (.4) profile slope and (B) average maximum curvature 13 14. Relationship between maximum tensile curvature and compressive curvature. . 14 15. Plan of damage to house C 15 16. Vertical-displacement contours for sag 1 15 17. North bearing wall of crawl space In house C In 1978 16 18. Plan of damage to radio station building 17 19. Postulated progression of subsidence associated with sag 3 18 20. Relationships between subsidence profile characteristics and ground deformations 19 21. Comparison of subsidence profiles of radio station building and house C... 19 TABLES 1 . Summary of sag characteristics 8 2. Summary of building damage and associated ground movements 20 3 . Classification of visible damage to walls 21 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT ft foot mm millimeter mi mile pet percent GROUND SUBSIDENCE AND STRUCTURAL DAMAGE OVER AN ABANDONED ROOM-AND-PILLAR COAL MINE AT HEGELER, IL By Gennaro G. Marino/ James W. Mahor/ Larry R, Powell, and Richard E. Thill ABSTRACT The Bureau of Mines and the University of Illinois investigated sur- face characteristics and damage to structures from mine subsidence over a room-and-pillar coal mine in Hegeler. IL. Data on three adjacent sub- sidence sags and associated structural damage were collected, sum- marized and evaluated. The subsidence sags developed over a 10-year period and took place above a modified room-and-pillar operation mining the Herrin (No. 6) coal at a depth of 130 to 135 ft. Surface vertical displacements of 3.0 to 3.5 ft resulted from extracting 6.1 to 6.4 ft of coal. Ground movements associated with sag formation severely damaged three houses and a radio station building, broke numerous utility lines, and structurally distorted three radio transmission towers. The radio sta- tion was remodeled and the towers repaired, but the three houses were subsequently demolished. Surface waters collecting in the subsidence depressions caused failure of radial ground transmission systems. The following subsidence profile characteristics were determined at the ra- dio station and one of the houses, respectively: profile slopes, 0.02 and 0.07; maximum curvatures, 2.3 x 10"'* ft"' and 3.0 x 10~* ft"'; and angular distortions, 6.6 x 10"^ and 62.0 x 10"^. Although the house was more severely damaged than the radio station, both structures are clas- sified as severely to very severely damaged. 'Visiting research engineer, Dept. of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, o Geologist, Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. •^Supervisory geophysicist. Twin Cities Research Center. INTRODUCTION Mine subsidence and damage to surface structures have been problems in Illinois since the start of extensive underground mining in the late 1800' s. The most serious problems are associated with early room-and-pillar practices. Early coal production was obtained from out- crops and shallow seams at depths less than 100 ft (I) A The extraction scheme was characterized by irregular layouts and poorly defined areas (2^). In older mines, although overall recovery averaged about 50 pet, some panel areas are be- lieved to have recovered as much as 80 pet of the coal (3), By 1975, over 4,000 abandoned underground coal mines were re- ported in 70 Illinois counties (4^). A considerable number of unidentified small mines still exist for which no documenta- tion is known. As of 1975, over 800,000 acres of abandoned underground coal mine workings existed in the State (_5). Subsidence creates topographic changes characterized by tilt, curvature, and displacements of the ground surface (6^) . Differential movements of the ground sur- face cause damage to structures and util- ities. By 1976, subsidence was reported in 28 municipalities in 18 counties (7^). During the first 2 years of operation of the Illinois Mine Subsidence Insurance Program, mine subsidence was found to be the cause and origin of damage to the structures in 20 pet of the files closed. Cost data for repairs are incomplete be- cause about 60 pet of the structures still show active movement. Field data reveal that damage estimates average $20,000 to $30,000 per structure, or to- tal property damage of $1 to $2 million per year (8^). Recently, Illinois mining practices and subsidence characteristics were described (2^, 9-11) , structural responses to sub- sidence profile strains were analyzed (12-13) , and the Illinois Mine Subsidence Insurance Program was summarized (JB) . ^Underlined numbers in parentheses re- fer to items in the list of references preceding the appendix. Two reports on subsidence (14-15) have been prepared as public information for homeowners considering subsidence insur- ance. In addition, several reports con- taining data on structural reponses to subsidence at various sites have been published by the Illinois Abandoned Mined Land Reclamation Council. The nature and extent of damage to sur- face structures as a result of subsidence movements are not well known in Illinois because there has been very little sci- entific or engineering documentation of subsidence. To date, few structures have been monitored and little is known about the extent of subsidence effects on the land surface, especially farmland (15). Because no systematic subsidence profile data and accompanying structural damage measurements have been coiiq)letely synthe- sized, it is difficult to predict the critical strains, slope, and curvature prevailing at the time of structural dam- age. Predictions of differential settle- ments and damage potential of buildings rely on structure-ground surface inter- actions (16-17) . Measurements of differ- ential settlement and horizontal strain on the foundation and adjacent ground are necessary (18-19) . As part of a cooperative mine subsid- ence research program with the State of Illinois, the Bureau of Mines is measur- ing the type and severity of subsidence damage and ground profile characteris- tics to determine the effects of subsid- ence on surface lands and structures. These data will be used to develop sub- sidence prevention, control, and repair strategies. First, the mechanics of how a structure is affected by subsidence must be thoroughly understood. Severity of damage with respect to differential settlement and tilt must be assessed to establish a severity index, which must then be verified with respect to the mag- nitude of ground movements. In addition, valuable information is being collect- ed on what causes room-and-pillar mines to become unstable and collapse causing subsidence. The development of subsidence predic- tion methodologies will enable Illinois coal mine operators to comply with regu- lations for protection of surface land while maximizing resource recovery. By combining the capabilities to precalcu- late the subsidence profile and the level of damage that may occur to surface lands and structures, coal mine operators will be able to determine the impacts of un- dermining an area and thus be able to plan an efficient, economic, and safe mine while protecting the surface. Terminology for subsidence depres- sions is adopted from Bauer and Hunt (10). In Illinois, pit subsidence is used to describe a depression with near- ly vertical to belled-outward walls. Pit subsidence is caused by collapse of shallow, abandoned mines with incompe- tent overburden. Sag subsidence is ex- pressed by a large depression with gentle slopes. The term "sag" is used to de- scribe the nearly equidimensional sub- sidence depressions over room-and-pillar mines. The term "trough" is reserved to describe elongate depressions produced over modern longwall and high-extrAction room-and-pillar operations. The three depressions that developed on the Hegeler site are subsidence sags, and this term will be used throughout this report. The purpose of this study is to docu- ment and characterize mine subsidence and related damage that occurred over an abandoned room-and-pillar mine between 1967 and 1981. The approach was to col- lect, summarize, and evaluate the surface ground movements and damge associated with three adjacent subsidence events in Hegeler, IL. The work included col- lecting news articles, personal accounts, and documents related to the subsidence events; determining the sag configura- tions; establishing reference points and measuring the existing differential dis- placements of the ground and structures; and summarizing and evaluating the data. Monitoring of the site is continuing. Future reports will detail the mining and geological conditions relevant to eval- uating the mechanisms of mine collapse and changes in overburden properties sub- sequent to the formation of the subsid- ence sags. ACKNOWLEDGMENTS The results of this report are based on work conducted under Bureau of Mines con- tract J0205071, initiated under the Min- erals Environmental Technology Program. The work was performed by the Civil En- gineering Department, University of Illi- nois at Urbana-Champaign between May 1978 and June 1981. The authors would like to thank James Jessop, geophysicist , Twin Cities Research Center for support and par- ticipation in field activities. The cooperation of Mike Mitzloff , Ralph Cox, and Allan Thomas of the radio station management is acknowledged and great- ly appreciated. Robert Bauer and Tony DeVine, geologists with the Illinois State Geological Survey, supplied infor- mation about the site. We would also like to thank Paul DuMontelle of the Il- linois State Geological Survey for crit- ically reviewing the manuscript and pro- viding protographs and documentation of the early subsidence history at the site. SITE DESCRIPTION The study site is located in Hegeler, IL, which is a few miles south of Danville and about 5 mi west of the Illinois-Indiana border (fig. 1). Heg- eler can be characterized as a light industrial and agricultural area with a population of about 1,600. The area is extensively underlain by roomr-and-pillar coal workings that operated between 1870 and 1974. Note: Shoded area is Vermilion County FIGURE 1.- Area location map. The site is situated in SW 1/4 sec. 29, T.19N, R.UW of the second principal meridian. Subsidence has affected the north side of Spelter Avenue, including a radio station and its three transmit- ting towers (fig. 2). Farmlands lie to the north and east of the property. The west side of the property is bound- ed by an embankment probably used in the past for coal hauling. Other proper- ties along Spelter Avenue are residen- tial. The topography is flat to gently rolling. Property line Spelter Ave. LEGEND ■^ Borehole -•- Guy wire and onctior 100 200 I I I Scale, ft FIGURE 2. - Site location map. GEOLOGIC CONDITIONS REGIONAL GEOLOGY Physiographically , Hegeler lies within the Bloomington Ridged Plain, a region of gently rolling terrain crossed by many glacial recessional moraines that form low hummocky ridges that trend in a gen- eral east-west direction. In east cen- tral Illinois, the bedrock has been cov- ered serveral times by large continental glaciers during the Pleistocene Series. The surficial deposits include glacial drift deposited during the Wisconsinan, Illinoian, and Pre-Illinoian glacial stages (20) . The drift is a complex se- ries of units, including till, which is primarily a heterogeneous mixture of sand, gravel, and pebbles in a compact clay and silt matrix. The thickness of the tills is generally 10 to 20 ft in the study area. Structurally, the area is situated on the northeastern margin of the Illinois Basin. The basin contains marine and nonmarine Pennsylvanian age sediments that thicken toward southeastern Illi- nois. The site is located in a broad gentle depression known as the Marshall Syncline, which is bounded on the east by the Cincinnati Arch and on the west by the LaSalle Anticlinal Belt (21). Most of the anticlines and synclines are wide, gentle, and open, and the strata dip 1° or 2°. However, occasional dips of up to 15° are found on more prominent structures (22). The strata in east central Illinois are essentially flat-lying interbeds of sand- stone, shale, coal, underclay, and lime- stone that were deposited as part of large deltas in a gently subsiding basin (23) » The marine, brackish water and delta plain sediments have complex rela- tions making interpretations of their depositional environments difficult. In the Hegeler area, sandstone and limestone are much less abundant than in adjacent areas (24). Frequent and abrupt changes in rock type over relatively short dis- tances are characteristic of coal measure rocks in Illinois. The rocks immediately underlying the study area are part of the Modesto and Carbondale Formations (fig. 3). The Dan- ville (No. 7) coal is at the top of the Carbondale Formation. It varies in thickness from 2.5 to 5.5 ft and has been mined locally (25) . The Farmington Shale is the lowest named unit of the Modesto Formation, located above the Danville (No. 7) coal. It is commonly a gray shale that becomes coarser grained upward. The Carbondale Formation con- tains the Herrln (No. 6) coal, which is located 120 to 150 ft below the surface in the Hegeler area. It is generally 6 ft thick and overlain by grayish-black shale. Rock units below the Herrin (No. 6) coal consist of an underclay over the nodular Higgensville Limestone followed by a shale and then the Vermilionvllle Sandstone. SITE GEOLOGY The geology beneath the site was de- termined by drilling five exploratory boreholes to depths ranging from 147 to 159 ft. The locations of these bore- holes are shown in figure 2. Split-spoon samples were taken in the glacial till at 5-ft intervals, continuous NX core was obtained in the rock, and thin-walled tube samples were recovered from the underclay. A north-south geologic cross section across the site, developed from the five boreholes, is given in figure 3. Glacial till, 30 to 47 ft thick, lies directly on the bedrock surface, which forms a gentle bedrock valley centered near the north end of the site. The grain size distribution of the till varies across the site, but no particles greater than medium-size gravel were re- covered in the split-spoon samples. Gen- erally, the till consists of two major units. The first unit is a thick layer of dense, coarse-to-fine sand with some clayey silt and medium gravel surrounded by the second unit, which consists of silt and clay. A small lens of dense, coarse-to-fine sand with some fine gravel and silt exists near the bedrock surface at the north end of the site. The Farmington Shale is present below the glacial tills and is the first bed- rock unit encountered in the boreholes. The upper part of the shale, consisting of interbedded shale and siltstone, was encountered only at the south end of the site. Generally, the Farmington Shale is a gray silty shale with both clayey seams and thin siltstone bands that become more frequent toward the base of the unit. In boreholes B-2 and B-5, the lower part of the Farmington Shale above the Danville (No. 7) coal is a thin lens of black car- bonaceous shale. The Danville (No. 7) coal ranges from 4 to 7 ft thick and lies 80 to 88 ft below the surface. It thins and contains more impurities below the black carbonaceous shale. A 6- to 7.5-ft-thick underclay, which grades into a nodular limestone, lies below the coal. The next unit, the Energy Shale, con- sists of a sequence of three grayish siltstones, which change from a laminated siltstone at the top to a thinly bedded siltstone with clay layers at the bottom. The unit is 29 to 35 ft thick, and indi- vidual facies range from 4 to 23 ft thick. The Herrin (No. 6) coal is 6 to 7 ft thick at depths of 131 to 135 ft below the surface. The seam has a westerly dip of about 1.2 pet. The underclay is a green-to-gray clayey shale that contains limestone nodules and grades into a dis- continous argillaceous limestone. Below the limestone is a 9- to 13-ft-thick gray shale. The lowest rock unit drilled into at the site is the Vermilionvllle Sand- stone. It is a gray-to-green siltstone. No 650 625 *". 600 < > LJ _J Id UJ > UJ < 111 en rth B-3 575 550 525 500 - 475"- Silty shale Coal Silt and clay with some sand and^gravei ;iaY wiin some sana ana gravel ^^-^ ^ i, — j Silty shale ^^Carbonaceous shale Nodular limesfo^ Calcareous clayey shale- 5j,ly ghole Siltstone 100 200 _J I Glacial drift Formington Shale nanville INo. ^)ronl Underclay Energy Shole Herrin (No.6) coal Underclay ^ioainsvineXimestone Cioysfone Shale Vermilionville Sandstone r Scale, f1 FIGURE 3. - North-south geologic cross section through study site. The groundwater table generally lies within 10 ft of the ground surface. Based on the borehole data, there are no good soil or bedrock aquifers ft of the ground surface. within 150 MINING HISTORY AND PRACTICE The mine that underlies most of the town of Hegeler, and an extensive area north of the site, worked the Herrin (No. 6) coal from 1946 to 1974. The site was undermined from 1960 to 1967. The mine, the last operating deep shaft in the area, was closed in 1974. The 6- to 7-ft-thick coal seam was mined using a modified room-and-pillar operation (2^) with mine openings oriented north-south and east-west (fig. 4). At the project site, working entries were driven westward from the south main. Panels were then developed southward from east-west working mains. A 250-ft wide barrier pillar borders the north edge of the site. After 1953, pillar robbing was not practiced, but as much coal as possible was taken at the working face, contingent upon the stability of the mine opening. The roof of the mine was supported with timber props, and the ratio of timber to mined tonnage was high, which indicates stability problems in the immediate roof. In the study area, the mine has rec- tangular pillars ranging in width from 10 to 25 ft, rooms 20 to 45 ft wide, and crosscuts 10 to 25 ft wide (fig. 4). The crosscuts were made about every 85 to 160 ft. The pillar height-to-width ratio ranges from 0.24 to 0.64 and panel width to depth ratios range from 1.9 to 2.8. Under the site, the extraction averages 70 pet with some variation due to differ- ent extractions in entries and panels. SURFACE SUBSIDENCE SUBSIDENCE HISTORY Three subsidence sags have developed in the study area between 1967 and 1978. The locations of the sags occurred above areas of high extraction (fig. 4). Sub- sidence has progressed northward starting with sag 1 in 1967, sag 2 in 1968, and sag 3 between 1976 and 1978. The sags range from 240 to 570 ft in diameter. Sag 1 encompasses the radio station building and an area to the south and west. Sags 2 and 3 developed north of the building in the field containing the transmission towers. Sags 1 and 2 sub- sided with one main event, but sag 3 de- veloped in two or more major events in which the ground movements have pro- gressed westward. A summary of the sub- sidence history at the study site can be found in the appendix. Sag 1 started to develop in the vicin- ity of the radio station about noon on July 21, 1967. The subsidence eventually encompassed the access road and parking lot and the surrounding area. The ground movements damaged three houses, the radio station, and service utilities to these structures (26-29) . In addition, the southern guy wires to tower 1 were ten- sioned. The maximum settlement (about 3.5 ft) occurred over an area west of the radio station parking lot and access road. Although most of the displacement occurred within 2 days, ground movements were reported as late as October 1967. Then, in May or June 1969, the northern portion of the radio station building sustained additional damage from resid- ual movements in the northern portion of sag 1. Sag 2 develop in the field north of the radio station building in May or June 1968. The center of the sag is located 40 ft northwest of tower 1. Most of the settlement (3 ft) occurred rapidly within 6 weeks. Tower 1 subsided 2.75 ft, which caused the guy wires supporting the tower to loosen, and they had to be retightened several times. Surface water collected in the sag around tower 1. In November or December 1976, sag 3 de- veloped under tower 2. The tower nearly failed when it settled and tilted 0.05 ft O 100 200 I , 1 Scale, ft Barrier pillar LEGEND o Tower ^ Coal pil lar IT-IT Sag limit FIGURE 4. - Mine map showing location of sub- sidence sags. to the east. In July 1978, residual sub- sidence of sag 3 produced settlement and lateral movement of the southwest guy wire anchors for tower 3. The upper 150 ft of the tower was bent in response to the ground movements. The maximum set- tlement was about 3 ft. SAG CHARACTERISTICS Level surveys were run across each sag to determine apparent maximum vertical ground displacements. Each sag was sur- veyed at least twice. Horizontal ground movements were also measured periodically using a tape extensometer stretched be- tween posts supporting the transmission line and reference points on the radio station building (fig. 5). Because sur- vey results indicated that no significant horizontal displacement has occurred be- tween late 1978 and 1981, no further dis- cussion will be necessary. LEGEND Tower Guy wire and anchor Trough limits FIGURE 5. - Location of subsidence profiles, cross sections, and reference points. The limits of subsidence were deter- mined from (1) a map of perimeter tension cracks and compression ridges prepared by the Illinois State Geological Survey (31) in 1967 (fig. 6), (2) accounts of damage from interviews with radio sta- tion personnel and owners of the damaged structures in the area, (3) measured dif- ferential settlement of the radio station building, and (4) field work by project personnel. Ground surface profiles prior to subsidence were estimated by inter- polating linearly across the access road and parking lot to points outside the sag (fig. 7). The profile for sag 1 is lo- cated 100 ft east of the center of the sag and was measured after resurfacing of the parking lot. For sags 2 and 3, profiles were pre- pared from elevations measured on a 20- to 25-ft grid in order to establish the postsubsidence profiles. The presubsid- ence profiles were determined by linearly interpolating between the apparent limits of sags and known presubsidence eleva- tions of reference points inside the sags. The following were taken into con- sideration in drawing the profiles: the drainage ditch as a low point, measure- ments on the settlement of the tower bases, and estimated limits of the sags as discerned from the postsubsidence elevations. The adjusted displacement profiles of the sags are presented in figures 8 through 10, and their locations are shown on figure 5. The adjusted vertical dis- placement is the estimated elevation prior to subsidence minus the respective subsidence profile elevations. The ad- justed vertical-displacement profile was determined by drawing a smooth curve through points of known settlement. Slopes and curvatures were calculated based on the adjusted vertical displace- ment profile and are plotted below the corresponding settlement profiles. Table 1 summarizes sag characteristics. TABLE 1. - Summary of sag characteristics Sag No, Maximum diameter f t. . Minimum diameter f t. . Maximum settlement f t. . Maximum slope Maximum curvature, ft"^: Compression (+) Tension (-) Extraction pet. . Seam height f t. . Settlement to extraction height ratio Mining depth f t. . 1 1.2 1.2 350 310 3.5 0.055 : 10-3 : 10-5 66 6.4 0.55 130 8.6 7.0 450 240 3.0 0.038 ; 10-4 : 10-4 74 6.4 0.47 130 6.3 6.4 570 410 3.0 0.034 : 10-4 : 10-4 71 6.1 0.50 135 LEGEND — — Tension fractures — - Compression ridges Sag limit FIGURE 6. - Sketch map of sag 1 tension and compression features in 1967. (Courtesy of Illinois State Geological Survey.) ■Spelter Ave < UJ KEY Presubsidence profile South 642 Tower 3 North 200 400 600 800 HORIZONTAL DISTANCE, ft 1,000 1,200 FIGURE 7. - Existing and assumed presubsidence, north-south profile. 10 Vertical displacement Vertical displacement 400 Vertical displacement 100 200 300 400 50 100 160 200 250 300 50 100 150 200 250 Curvature 1 00 200 300 HORIZONTAL DISTANCE, ft Too -4 - -8 - -10 Compression Tension Curvature 50 100 150 200 250 HORIZONTAL DISTANCE, ft 6 4 2 -2 -4 Compression Tension 50 100 150 200 250 300 HORIZONTAL DISTANCE, ft FIGURE 8. - Adjusted profiles, slopes, and 40=ft lengths from survey data. Surveyed Nov curvatures for sag 1. Profile slopes and curvatures calculated on , 11, 1978. Sag 1 is a 290- by 350-ft semirectangu- lar depression with rounded corners. The area of maximum settlement is approx- imately concentric to sag limits. A maximum settlement of 3.5 ft is located 100 ft west of profile A-A' (fig. 8). The ratio of settlement to extraction height, known as the subsidence factor, is 0.55. The maximum slope^ in sag 1 is 0.055 and occurs on the southeast side where ^Slope is a ratio between any like units of length or distance (feet to feet, inches to inches, etc, ) . house C was located (fig. 8, B-B'). Slopes decrease to the north. Around the radio station building, the maxi- mum slope is roughly 0.02. Both structures are located in the tension zone but on opposite sides of the sag. Curvatures , corresponding to the maxi- mum tension and compression, are 1.2 x 10~5 ft~^ on the south side of the sag and 2.0 to 4.0 x 10"^ ffl on the north side. Figure 11 shows tension cracks and compression ridges associated with sag 1. 11 Tower I-, Vertical 200 250 Tower In Vertical displacement X E' Host -L 50 100 150 200 250 300 350 400 450 500 150 200 250 300 350 400 450 500 250 300 10 8 6 4 2 -2 -4 -6 -8 - 1 1 1 1 1 ^^ /^Curvature - - / Compression \ - i / Tension \ > / -J 1 1 1 1 1 - 50 100 150 200 250 HORIZONTAL DISTANCE, ft 300 4 3 2 I I -2 -3 -4 1 1 III III /^Curvature >^ 1 1 f Compression \ - Tension \ - / y^^y^'Vj: V III III 1 1 50 100 150 200 250 300 350 HORIZONTAL DISTANCE, ft 400 450 500 FIGURE 9. - Adjusted profiles, slopes, and curvatures for sag 2. Profile slopes and curvatures calculated on 40-ft lengths from survey data. Surveyed July 9, 1981. Sag 2 is a 450- by 240-ft elliptical depression elongated in the east-west di- rection. The sag enconpasses the base of tower 1 and most of its guy wire anchors. The adjusted displacement profiles (fig. 9) show the sag to be a bowl-shaped de- pression with a maximum settlement of about 3 ft. Slopes for sag 2 are greatest along the north-south profile (fig. 9, D-D') where the maximum slope is 0.038. This value is 1.5 times greater than the maximum slope in the east-west direction. The maximum curvatures are 8.6 x 10"'* ft~^ in the compression zone and 7.0 x 10"'* ft"^ in the tension zone. Both occur along the north-south profile and are about twice those in the east and west portions of the sag. 12 LJ (E < > o 2 - 1 n 1 1 1 1 Curvature - -/I i/\ Compression / v/ TensiorN^ J, v 1 1 1 1 1 1 1 50 100 150 200 250 300 350 400 50 100 150 200 250 300 350 400 450 HORIZONTAL DISTANCE, ft Tension. 50 100 I 50 200 250 300 350 400 HORIZONTAL DISTANCE, ft H Tower Z-\ H' 1.0 2.0 3.0 0.04 .02 -.02 I I I r] r Vertical displacement LWest 75 1 50 275 300 375 450 525 600 1 1 1 1 1 1 1 Slope A/ 1 1 1 M 1 1 1 75 150 275 300 375 450 525 6C I I I I Curvature Compression _ 75 1 50 275 300 375 450 525 600 HORIZONTAL DISTANCE, ft FIGURE 10. - Adjusted profiles, slopes, and curvatures for sag 3. Profile slopes and curvatures calculated on 40-ft lengths from survey data. Surveyed July 9, 1981. FIGURE 11, - Photographs of 0.33-ft-wide tension cracks (A) and 0.25-ft-high compression ridges (B) from sag 1 on June 24, 1967. (Courtesy of Illinois State Geological Survey.) 13 Sag 3 is oval with a maximum east-west diameter of 570 ft and a minimum north- south diameter of 411 ft. The sag encom- passes nearly all of tower 2 and the southern guy wire anchors of tower 3. Figure 10 shows that in the north-south direction, the sag has a uniform smooth profile (F-F' and G-G'), but in the east- west direction, the sag bottom varies by 1 ft or more (fig. 10, H-H'). The maxi- mum settlement is 3.0 ft. The maximum slope for sag 3 is 0.034, which occurs on the south side of profile G-G' (fig. 10); however, slopes greater than 0.02 exist in many other areas of the sag. Curvatures in the compression and tension zones are 6.0 x 10~* ft~^ and occur in the western part of the sag, but in the eastern part they are as great as 4.0 X 10"^ ft"'. Both slopes and curva- tures are erratic in the east-west direc- tion and reflect the nonuniform settle- ment profile across the bottom of the sag. SAG INTERRELATIONSHIPS The three sags developed above the same mine and in similar geologic conditions. Sags 1 and 3 are above working panels , have semirectangular shapes , and are bounded on the north by working entries, and on the south and west by barrier pil- lars (fig. 4). They have displacement profiles with relatively flat centers and nearly equal curvatures in the tension and conq)ression zones. On the other hand, sag 2 occurred over a working entry bounded by a barrier pillar on the north and wide pillars on the south. It has a distinct bowl-shaped profile with the maximum compressive curvature greater than the maximum tensile curvature. Sag 3 is the largest, but probably consists of two overlapping sags or an initial small sag that progressively enlarged. Sag 2 is the smallest and has its long axis parallel to the underlying working entry. 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