'{-L'NO'f. STATE GEOLOGICAL SURVEY 3 3051 00006 4158 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/surfacesubsidenc17youn STATE OF ILLINOIS STATE GEOLOGICAL SURVEY FRANK W. DE WOLF, Director Cooperative Goal Alining Series BULLETIN 17 SURFACE SUBSIDENCE IN ILLINOIS RESULTING FROM COAL MINING BY LEWIS E. YOUNG ILLINOIS COAL MINING INVESTIGATIONS ( Prepared under a cooperative agreement between the Illinois State- Geological Suxv« v. the Kngineering Experiment Station of the University oi Illinois, and the U. S. Bureau of Mines.) I'KINTKO BY AU'I IIOKITY OF THE STATE OF ILLINOIS ILLINOIS STATE GEOLOGICAL SURVEY UNIVERSITY OK ILLINOIS URBANA L916 CONTENTS PAGE Chapter I — Introduction 11 Area investigated . , 11 Scope and object of report , 11 Acknowledgments 13 Illinois coal field and production data 15 Chapter II — Geologic conditions affecting subsidence 18 Description of Illinois "Coal Measures" 18 Strata associated with Illinois coal beds 21 Cleat 26 Faults and rolls 27 General relations to subsidence 27 Coal No. 2 27 Coal No. 5 27 Coal No. 6 28 Coal No. 7 29 Chapter III — Damage caused by removal of coal 30 Contrasting effects of longwall and room-and-pillar methods 30 Surface evidences of subsidence 30 Surface cracks and displacements 30 Pit holes or caves 34 Sags 42 Agriculture and surface subsidence 50 Importance of agriculture 50 Extent and value of farm lands 50 Value of coal lands 54 Nature of damage to agricultural lands 56 Restoring agricultural lands 58 Transportation and surface subsidence 61 Railroads 61 Wagon roads 64 Streams and canals 64 Buildings and improvements affected by subsidence 65 Buildings 65 Streets, pipe lines, and sewers 69 Water supply 71 Municipal waterworks 71 Reservations of coal 72 Chapter IV — Subsidence data by districts 73 Introductory statement 73 District 1 75 District II 78 District III 78 District IV 79 District V 81 (5) PAGE District VI 83 District VII 8S District VIII 86 Chapter V— Protection of surface 88 General considerations °° Pillars 88 Crushing strength of coal °° Angle of break and angle o f d raw °9 Safe depth 9 1 Theory as previously advanced 93 Increase in volume of rock by breaking 93 Compressibility of broken rock 94 ! Shaft pillars 10 ° Room pillars Filling methods. 103 Gobbing 103 Hydraulic filling 104 Griffith's method of filling 104' Artificial supports ^ 5 Chapter VI— Investigations of subsidence 106 European 10° United States '. 106 Suggested Illinois investigations 106 Considerations 106 Typical mines 107 Monuments and surveys 107 Underground work 107 Possible benefits 10° (6) ILLUSTRATIONS PLATE PAGE I. General geological sections for southwestern Illinois 18 II. General geological sections in coal field of Williamson and Franklin counties 20 III. Graphic sections in Longwall District 22 IV. Cross-sections CD and EF showing structure across the southern and north- ern parts of the Danville field 24 FIGURE 1. Map of Illinois showing extent of the "Coal Measures" 12 2. Map of Illinois showing distribution of thick, medium, and thin coal 14 3. Map showing area underlain by principal coal seams 16 4. Graphic section showing persistent nature of limestones in the McLeansboro formation 20 5. Diagrammatic illustration showing angle to break and angle of subsidence. ... 31 6. Crack in the soil caused by room-and-pillar mining at Streator 32 7. Plan of two panels of Franklin County mine showing relation of subsidence to underground workings 33 8. Crack due to subsidence over north panel shown in figure 7 34 9. Cracks due to subsidence over south panel shown in figure 7 35 Cracks and breaks due to shear 36 Buckling of frozen sod due to compression along the axis of a sag 36 Surface breaks in a field near Carterville 37 Surface breaks over a mine near Harrisburg 37 14. Plan showing workings of a mine and location of subsidence movements ad- j acent to a dike 38 Plan of a mine showing details of movement near a dike IS Breaks in a pasture over an abandoned mine west of Danville 39 Side-hill breaks near Cuba caused by room-and-pillar mining 39 Large area covered by pit holes in the suburbs of a town in central Illinois. . . 40 Cave-in south of Dewmaine 40 Detail of a cave on a hillside near Streator 41 |21. Side-hill breaks over shallow workings near Streator 41 22. Plan of a 40-acre tract in southern Illinois showing relation of approximately 60 pit holes to underground workings 42 23. Topographic map of the Coal City-Wilmington area 43 24. Pond in a sag due to longwall mining at Coal City 44 25. Plan of a mine in Perry County showing relation of sags to underground workings 45 Telephone poles tipped toward a sag over a mine in Franklin County 46 Swamp in a typical sag in Randolph County 47 Pond in a sag near Clifford, Williamson County 48 Flooding of a large tract due to subsidence in Franklin County 48 (7) FIGURE PAGE 30. Pond due to subsidence at Westville, Vermilion County 49 31. Open space in a cornfield south of Danville 49 32. Drowned-out area of corn near Nokomis 49 33. Map showing the relation of different thicknesses of coal to values of farm lands in 1910 51 34. Map showing location of tile drains laid by a mining company to drain sags. . 58 35. Pit hole filled with rubbish at Electric mine near Danville 59 36. Pit holes and cracks over shallow mine near Streator 60 37. Pit holes near Streator filled with mine rock and soil 60 38. Railroad track in Franklin County lowered by subsidence over room-and- pillar mine 62 39. Plan showing relation of railroad in figure 38 to mine workings 63 40. Repaired railroad trestle in Franklin County 64 41. House near Coal City lowered 9 inches at one corner 65 42. Brick house near Danville abandoned on account of danger by room-and- pillar mining 66 43. Plan showing relation of house in figure 42 to the pillar in the mine 67 44. Houses in Springfield for which claims were paid by a mining company 68 45. Small house near Streator beside which a pit hole has formed 68 46. Flooded streets, broken sidewalks and foundations, and damages to plaster- ing resulting from subsidence in Franklin County 69 47. Barn of mining company lowered 4 feet at one end 69 48. Broken sidewalk caused by subsidence in Franklin County 70 49. Map showing division of State into districts 74 50. Cracks in the immediate roof of a longwall mine in the La Salle area 90 51. Stress diagram of an ideal homogeneous cantilever 91 52. Stress diagram of a low-down neutral surface 92 53a. Diagrammatic illustration through the gob and parallel to the face of a long- wall mine 96 b. Plan showing pack walls and loose gob between roadways and cross entries in the same mine 96 54. Diagrammatic illustration showing the flow of roof shale under pressure in a mine near Peoria 98 55. Diagrammatic illustration showing the size of shaft pillars for given depths as recommended by various mining engineers 56. Diagrammatic illustration showing the extent of subsidence resulting from the removal of adjacent pillars 102 99 (8) TABLES PAGE 1. Output of Illinois coal by coal seams 15 2. Output of Illinois coal by depth of shaft 17 3. Character of roof and floor of the commercial coal beds throughout Illinois.. 24 4. Value of farm lands, April 15, 1910 53 5. Value of surface and of coal rights by counties in Illinois 55 6. Assessed value of coal rights and of agricultural lands by counties 56 7. Districts into which the State has been divided for the purpose of investigation 73 8. Alphabetical arrangement of coal-producing counties 75 9. Longwall mines in Illinois 76 10. Data on subsidence in District IV 80 11. Data on subsidence at typical mines in District V 83 12. Data on subsidence at typical mines in District VI 84 13. Data on subsidence at typical mines in District VII 85 14. Compression tests of Illinois coals 89 15. Volumes of different materials after crushing as compared with volumes "in the solid" 94 16. Compressibility of different materials after having been crushed 94 17. Compressibilty tests upon crushed materials made by the United States Bureau of Mines 95 (9) SURFACE SUBSIDENCE IN ILLINOIS RESULTING FROM COAL MINING By Lewis E. Young CHAPTER I— INTRODUCTION Area Investigated During the summer of 1914 there was made a preliminary study of surface subsidence resulting from coal mining operations in Illinois. Mines were visited in 24 of the 52 counties in which shipping mines are located. These 24 counties produced 94 per cent of the coal mined in the State in 1913. Within these 24 counties are 324 of the 371 shipping mines listed in 1913 and 293 of the 469 local mines. The counties visited were Bureau, Christian, Clinton, Franklin, Fulton, Grundy, Jackson, La Salle, Livingston, Macon, Macoupin, Madison, Marion, Montgomery, Peoria, Perry, Randolph, St. Clair, Saline, San- gamon, Vermilion, Washington, Will, and Williamson. In all but three of these counties coal mining has resulted in subsidence of such in- tensity as to result in substantial damage to surface property. Data were secured from 108 companies operating 149 separate plants, and in addition, the field notes of the Cooperative Investiga- tion upon 100 Illinois mines supplied data on 8 mines not visited for this particular purpose. Scope and Object of Report Prior to this preliminary survey no work upon the subsidence problem in Illinois had been undertaken by any scientific bureau. For over a quarter of a century there has been litigation between the operators of coal mines and the owners of the surface when surface subsidence has been attributed to coal mining operations. There is a need for more definite knowledge concerning (1) the extent to which the complete removal of coal may disturb the overlying rock strata, (2) the percentage of coal that may be mined without disturbing the surface, (3) the methods of mining that should be employed in order to minimize the surface movement following the removal of the coal, (4) the methods of protecting the surface by artificial supports, and (5) the best methods and policies to be employed in conserving the mining and agricultural resources of the State. At the present time there is more or less unrest in certain coal mining districts in Illinois on account of minor damage to the surface. (11) map or ILLINOIS Fig. 1.— Map of Illinois showing extent of the "Coal Measures." INTRODUCTION 13 At a number of points mine operators are being forced to pay claims for damages much in excess of the actual cost of restoring the injured property. There is an urgent need for definite data upon the experience of operators in the various districts, so that those who mine coal in the future may be able to recover the maximum percentage of the coal with the least damage to the surface or with the briefest inter- ference with the use of the surface property. It is undoubtedly to the interest of the State to conserve the coal supply by securing as complete extraction as possible from the areas in which mining is now being carried on. Almost one-half of the coal is being left in the ground and made unavailable for future mining, and in some places a large part of this abandoned coal is left because the mine operators fear that by removing a larger per- centage they may cause surface subsidence and be required to pay damages out of proportion to the real value of the surface. In many places the surface may be restored at little expense. But until the prevailing conditions are known accurately and the essential facts are understood and studied, the State cannot consider or propose meas- ures to relieve the mining industry of this severe burden. With these problems and difficulties in view, this preliminary survey was made, and the data collected show in an impartial manner the situation as it exists in Illinois at the present time. Acknowledgments The writer wishes to acknowledge his indebtedness to Mr. G. S. Rice, Chief Mining Engineer, U. S. Bureau of Mines ; Professor H. H. Stoek, head of the Department of Mining Engineering, University of Illinois; and Mr. F. W. DeWolf, Director of the Illinois State Geo- logical Survey, under whose combined direction the work of the in- vestigation has been carried on. Mr. S. (..). Andros of the De- partment of Mining Engineering, University of Illinois, lias been very helpful. The various State and county mine inspectors have been uniformly courteous in furnishing much useful in- formation. Messrs. II. I. Smith and J. R. Fleming, Assistant Min- ing Engineers, U. S. Bureau of Mines, have made valuable suggestions and have furnished numerous photographs for which acknowledg- ments are made in proper place. Mr. R. Y. Williams, formerly mining engineer, U. S. Bureau of Mines, furnished much of the data on Saline County. To the following representatives of the coal mining industry in Illinois the author is deeply indebted for many courtesies and for close cooperation : Mr. W. M. Cole, Mining Engineer, Spring Valley Coal Company; Mr. J. Quaid, General Superintendent, Big Creek Coal Company; Mr. C. II. Nicolet, Engineer, Matthiessen and Hegeler Coal Company; Mr. R. D. Brown, Chief Engineer, O'Gara Fig. 2. — Map of Illinois showing distribution of thick, medium, and thin coal. (After Bement.) INTRODUCTION 15 Coal Company; Mr. R. Williams, Mining Engineer, Saline County Coal Company ; Mr. R. Forrester, Superintendent, Paradise Coal Com- pany ; Mr. A. J. Moorshead, President, Madison Coal Corporation; Mr. G. E. Lyman, Chief Engineer, Madison Coal Corporation; Mr. John P. Reese, Superintendent, Superior Coal Company; Mr. S. F. Jorgensen, Chief Engineer, Superior Coal Company ; Mr. T. P. Brew- ster, General Manager, Mount Olive and Staunton Coal Company; Mr. James Dubois, Superintendent, Dering Coal Company; Mr. Thomas Moses, General Superintendent, Bunsen Coal Company; Mr. M. F. Peltier, Chief Engineer, Peabody Coal Company; Mr. J. A. Garcia, Mining Engineer; Mr. D. J. Carroll, Chicago, Wilmington & Franklin Coal Company. Illinois Coal Field and Production Data Estimates made by the Illinois State Geological Survey show that the known coal areas of Illinois contained approximately 175,000,000,- 000 tons of coal in beds not less than three feet in thickness. The coal-bearing formations underlie part or all of 86 counties, an area of about 36,800 square miles (fig. 1). It may be noted that 674 square miles are underlain by several coal beds over 3 feet thick or an aggre- gate thickness of 15 feet; 3,883 square miles are underlain with an aggregate thickness of 11 feet; 12,546 square miles with an aggre- gate thickness of 7 feet; and 10,184 square miles with 3 feet of coal. Figure 2 shows approximately the extent of the thick, medium, and thin coal; and figure 3 shows the area underlain by the principal coal seams. Practically the entire output of Illinois coal in 1913 was pro- duced from 5 seams, as shown in the accompanying table. 1 Table 1. — Output of Illinois coal by coal scams (Year ended June 30, 1913) Tons Coal No. 1 500,000 Coal No. 2 5,650,000 Coal No. 5 13,500,000 Coal No. 6 41,400 000 Coal No. 7 750,000 Total from 5 seams 61,800,000 n^otal for the State as given by Illinois Coal Report, 1913, was 61,846,204 tons. Fig. 3.— Map showing area underlain by principal coal seams. (After Bement.) INTRODUCTION 17 The greater portion of the output comes from shafts 100 to 400 feet in depth. Table 2 shows the relation of tonnage to depth. 1 Table 2. — Output of Illinois coal by depth of shaft (Year ended June 30, 1913) shaft Number of shafts Output Tons 489 6,500,000 159 17,400,000 73 11,500,000 46 11,500,000 33 8,750,000 18 2,750,000 12 2,350,000 6 720,000 2 180,000 1 105,000 1 75,000 Depth of Feet 0- 99 100- 199 200- 299 300- 399 400- 499 500- 599 600- 699 700- 799 800- 899 900- 999 1,000-1,099 Total 61,830,000 CHAPTER II— GEOLOGICAL CONDITIONS AFFECTING SUBSIDENCE It is evident that whatever movement may result from under- ground mining will be influenced greatly by the nature of the rock over- lying the coal seam being worked. As noted in Table 2, the greater part of the output of Illinois coal is from shafts not exceeding 400 feet in depth, and practically 50 per cent of the output is from shafts not over 300 feet deep. Throughout the greater part of the State the coal beds lie practically flat. Description of Illinois "Coal Measures" The following extracts 1 give a concise statement of the essential facts concerning the coal-bearing formations. Plate I shows general geological sections for southwestern Illinois, Plate II shows sections in the Williamson-Franklin field, Plate III shows sections in the coal fields of northern Illinois, and Plate IV shows sections in the Dan- ville field. The coals of Illinois exist as widespread beds or as local pockets among layers of shale, sandstone, and limestone which together make up the Pennsyl- vanian series. There are five coals known to be important enough to have special names, and numerous other beds occur at various depths. The maximum thickness of Pennsylvanian rocks is known to be at least 2,200 feet, though it is not to be assumed that a single bore hole could penetrate all of the various forma- tions at the place of extreme thickness. The Pennsylvanian series has been divided for convenience of description into three formations which present different characteristics as to time of deposi- tion, physical composition, and economic importance. The divisions from the bottom upwards are the Pottsville, Carbondale, and McLeansboro formations. POTTSVILLE FORMATION The lowermost formation of the Pennsylvanian in Illinois is composed chiefly of massive sandstones interrupted by thinner beds of shale and beds or pockets of coal and fire clay. In Illinois this formation carries plant fragments which indicate, according to White," that deposition took place during late Pottsville time of the Appala- chian coal basins. The early sediments are thinner to the north and west and are nearly or quite absent over much of the State. Massive sandstones in the Rock Island region are of Pottsville age, but later than the first sediments of southern Illinois. There are a number of occurrences of coal in the Pottsville in southern counties but at present they have no commercial importance, because Preliminary report on organization and method of investigations, Illinois Coal Mining Investigations, pp. 48-51, 1913. 2 White, David, 111. State Geol. Survey Bull. 14, p. 293, 1908. (18) KM 19 ooa 008 ILLINOIS STATU GEOLOGICAL SURVEY COOPERATIVE COAL MINING SERIES BULLETIN 17, PLATE I I £ f> \^_Z:\ Lime Shells P=l Shel. WJM "«J8 >ia >ia cos oo r j e N ■ ■ m ounced to Du- ti anti- ase the :d have letailed vorable border :xtreme band) already ata of >ortant iidence nation, )w this Sm largely YTHUl No. 2 etween not as a r.p : ■ tion in e sand- )0 feet. .3 e.a .3 r n shown, [ackson 40 feet 5 to 45 een de- ed as a 'erlying i which beds of r inter- Bulletins aro-Herrin ILLINOIS STATE GEOLOGICAL SURVEY COOPERATIVE COAL MINING SERIES BULLETIN NO. 17, PLATE II 1 Bk ■v. Bk Bk N 1 Wi %- 1 J 82 Base & -6 ~ 5 — — - -7-_ Jr*"" 16-- "16 103 of -at Bk Bk Bk Bk - £=^ Pi ~ ""-— r :;1 '- ':- : "" : Prll :w_ -■ --::- SB W ^~, , ri — - ( ~i ]Pi Bk Bk - Bk »— HI < H 44 Coal Nb.5 10 14 LEGEND 1 I ' I 1 Limestone E=dEJ| Shale |.';'.;';-;'| Sandstone IV^rj Drift |iS^ Coal ^ggj Fire clay ^=±t| Calcareous s :r- ■"->- _- ale ^ Bk No.6 Bk JEFFERSON COUNTY 1 Sec. 24. T. 2 S..R. 1 E. 2 Sec. 30, T. 4 S..R.2 E. SHELBY COUNTY 3 Sec. 8, T. 10 S..R. 1 E. FRANKLIN COUNTY 4 Sec. 35, T. 5 S.,R. 4. E. 5 Sec. 19.T.6 S..R.3 E. 6 Sec. 32. T. 7 S..R. 1 E. FEET 800 S 91 54 Coal No.5 Graphic sections of borings showing the general character of the McLeansboro formation in District VI and Shelby County CONDITIONS AFFECTING SUBSIDENCE 21 the axis of the basin, is much steeper than the east flank. Another pronounced anticline has been traced from the region 5 miles west of Murphysboro to Du- quoin and thence northward to Centralia and Sandoval. This Duquoin anti- cline is also steeper on the side facing the axis of the basin, in this case the east side. Folds and faults of appreciable size other than those mentioned have been discovered, and doubtless they will be found to be numerous, as detailed surveys proceed. The structural relations in Illinois are on the whole unusually favorable to coal mining. The first effect is of course to make the coal around the border of the field more easily available than that in the deeper portion, but the extreme depth necessary to reach the most important coal (No. 6, Herrin, Blue band) probably will not greatly exceed 1000 feet, and the shaft at Assumption is already operating successfully to approximately this depth. Strata Associated with Illinois Coal Beds It is important that attention be directed to the thick strata of limestone, shale, and sandstone, which lie above the various important coal seams, as these may affect the rate and amount of subsidence when underlying seams are mined. 3 As previously noted coal No. 1 lies in the Pottsville formation, but the important beds of sandstone and of conglomerate lie below this seam. The Illinois strata in the Pottsville above this seam are largely fire clays and shales. The Carbondale formation extends from the base of coal No. 2 to the top of coal No. 6. In northern Illinois the boundary between the Carbondale and the overlying McLeansboro formation is not as clearly marked as in southern Illinois. The Carbondale formation in southern Illinois is composed chiefly of shale but includes some sand- stones and limestones. The thickness varies from 200 to 300 feet. In several typical bore holes, both limestones and sandstones are shown, but the thickest single beds of each are about 8 feet. In Jackson County, however, the Vergennes sandstone lies from 20 to 40 feet above coal No. 2. This is persistent but irregular in thickness (15 to 45 feet) and varies from a sandstone to a sandy shale. It has been de- scribed as micaceous, loose, and friable and can not be regarded as a bed that will be strong enough to check subsidence of the overlying beds. 4 Overlying coal No. 6 is the McLeansboro formation in which only coal No. 7 is of commercial importance. The following beds of this formation are reasonably persistent, whereas some of their inter- vening beds vary greatly in character. 8 For detailed reports on the geology of the districts see the Cooperative Bulletins Nos. 10, 11, 14, and 15. 4 Shaw, E. W. and Savage, T. E., U. S. Geol. Survey Geol. Atlas Murphyshoro-Herrin folio (No. 185), p. 7, 1912. 22 SURFACE SUBSIDENCE IN ILLINOIS The well-marked lithologic units of the McLeansboro (in District VII) may be enumerated as follows 5 : 5. New Haven limestone, 200 to 250 feet above Carlinville limestone (thickness about 25 feet). 4. Shoal Creek limestone, about 100 feet above the Carlinville (thick- ness 12 to 25 feet). 3. Carlinville limestone, from 200 feet to a little more than 300 feet above coal No. 6 (average thickness 7 feet). 2. A bed of pink, red, or variegated shale, variable in thickness, aver- aging 35 to 50 feet above coal No. 6 (seldom exceeds 15 feet in thickness). 1. A hard limestone overlying or slightly above coal No. 6 (averages 7 feet in thickness). The New Haven limestone is quite persistent, as indicated graph - ically in the records from Moultrie, Shelby, Montgomery, and Fayette counties in District VII. Mr. G. H. Cady reports it is present over parts of Franklin County from 500 to 550 feet above coal No. 6. It is a solid bed given in most logs as 25 feet thick. The Shoal Creek limestone is from 12 to 25 feet thick and lies 275 to 350 feet above coal No. 6 in District VII. It is described as lacking the homogeneity of the Carlinville and in places consists of a series of more or less argillaceous limestone layers. 6 This limestone apparently does not occur continuously over District VI. "The Carlinville limestone is one of the most widely distributed in the 'Coal Measures' of Illinois. It has been traced from north of Carlinville, Macoupin County, southeast to the Indiana line in Gallatin County. In the type localities this limestone is, according to Udden, 'generally bluish gray, compact, close textured, and very hard, break- ing into irregular, splintery pieces. It averages about seven feet in thickness'. In most of District VII the interval between this limestone and coal No. 6 averages from 275 to 325 feet." 7 In District VI this stratum occurs from 250 to 300 feet above coal No. 6 but is thin and has not been mentioned in many of the records. 8 In parts of Saline and Gallatin counties there occurs a hard sand- stone, known locally as "Anvil Rock", a sandstone about 10 feet above coal No. 6. In several of the small mines on coal No. 6 in Gallatin 5 Kay, Fred H., Coal resources of District VII: 111. Coal Mining Investigations Bull. 11, p. 23, 1915. 6 Lee, Wallace, acknowledgements to, Illinois Coal Mining Investigatons Bull. 11, p. 26, 1915. 7 Kay, Fred H., Coal resources of District VII: 111. Coal Mining Investigations Bull. 11, p. 25, 1915. 8 Cady, G. H., Coal resources of District VI: HI. Coal Mining Investigations Bull. 15, p. 34, 1916. 1 a t y n e il iS ie le s- a le iy 6 fr- ill c- 7. :et lie ry nd lis lb id. b nil. ull. 10, !ull. 11 1 INOIS STATE GEOLOGICAL SURVEY COOPERATIVE COAL MINING SERIES BULLETIN NO. 17. TLATE III 13 10 6 McLeansboro •: Fig. 25.— Plan of a mine in Perry County showing relation of sags to under- ground workings. The subsidence occurred over an area of 44 rooms in a panel in which the entry pillars were 20 feet wide. The cut-throughs in the rooms were staggered 60 feet apart and were 20 feet wide. Where the 46 SURFACE SUBSIDENCE IN ILLINOIS room barrier on the west had been broken through, the squeeze worked over into the zone which had previously subsided. Where the barrier pillar was still intact, it was sufficient to check the movement, and directly above on the surface tension cracks appeared extending in the same general direction as the barrier pillar. There was a gentle sag to the east between the room barrier and the entry pillar. As this pillar was small and broken by cross-cuts, the squeeze rode over it, Fig. 26. — Telephone poles tipped toward a sag over a Franklin County mine where an 8-foot coal is being removed at a depth of 450 feet. and some subsidence resulted, although the deepest portion of the sag was transversely across the panel of rooms. The underground movement covered an area approximately 500 by 1,000 feet. In April, 1915, at the same mine, squeezes occurred in two ad- joining panels each of which consisted of 32 rooms. The rooms were DAMAGE BY COAL REMOVAL 47 turned with wide necks and were worked out quickly. No pillars were drawn. About 3 months after the rooms had been finished, a severe squeeze began. The easterly panel was badly crushed in 3 days, and the squeeze rode over the small barrier to the next panel on the west. Falls began in the west panel on the fifth day and continued for about one week. At the end of two weeks no evidence showed that any considerable surface movement had occurred. In July, 1915, the next panel east caved, and in a few days several sidewalks were cracked and tension cracks appeared in the dry earth. In many districts room-and-pillar mining has caused extensive sags which have eventually become swamps. The trees, unsuited to the changed conditions, have been killed and the entire area has become •> ia'TjE:. ?-.*'•" ukJ Fig. 27. — Swamp in a typical sag in Randolph County where a 6- foot coal is being mined at a depth of 150 feet. The rooms are on 50- foot centers; pillars 18 feet. a waste. Somewhat typical illustrations of such swamps are shown in figures 27 to 31. Frequently swamps of this nature are formed in the spring within fields and interfere with the plowing and planting. During the sum- mer these ponds may dry up, and as a result there may be within fields large untilled and wasted areas. In figure 31 is shown a Vermilion County cornfield containing an unworked area at least 200 feet square. Similar conditions have been noted in many of the coal districts (fig. 32). Not only may occa- 48 SURFACE SUBSIDENCE IN ILLINOIS sional ponds be created, but at times large sections may be lowered beneath the flood levels of river bottoms. There has been considerable litigation in northern Illinois where coal is being mined in the Illinois River bottom. It has been claimed by property holders that lands Fig. 28. — Pond 4 feet deep and covering an area of about an acre, due to subsidence near Clifford, where a coal bed about 7 feet, 6 inches is being mined at a depth of 140 feet. Considerable quicksand occurs in the vicinity. The rooms are driven 20 to 22 feet wide on 40-foot centers, and no pillars are drawn. Fig. 29. — Flooding of a large tract in Franklin County caused by mining of a 8-foot coal at a 460-foot depth. Rooms were carried 30 feet wide on 45-foot centers ; no pillars were drawn. were inundated on account of the increased amount of water flowing into Illinois River due to the discharge through the Chicago Drainage Canal. On the other hand the officers of the Chicago Sanitary Dis- DAMAGE BY COAL REMOVAL 49 Fig. 30.— Pond caused by subsidence at Westville, Vermilion County, where 6 feet of coal was removed by room-and-pillar mining at a depth of 210 feet. Levels at surface show maximum depth of sag to be 4.7 feet. (Photo by R. Y. Williams.) y. ■«■**>- v Fig. 31.— Open space south of Danville about 200 feet square in cornfield, indicating the area is not tilled because water stands over the sag in the spring. Fig. 32. — Drowned-out area of corn near Nokomis where the 8-foot coal bed is worked at a depth of about 625 feet. 50 SURFACE SUBSIDENCE IN ILLINOIS trict claimed that the lands were flooded because coal-mining operations lowered the lands beneath the former drainage levels. Agriculture and Surface Subsidence importance of agriculture Illinois is preeminently an agricultural and manufacturing state. To the manufacturing interests an ample and continuing fuel supply is of prime importance. In few states are both agriculture and manu- facturing developed to so great an extent, and in practically no other state do the coal beds underlie so extensive and so fertile farm lands. In a large part of the Appalachian and the western coal fields the sur- face is almost valueless for agricultural purposes, but where the over- lying surface is tilled it is of much less value than the farm lands of Illinois. EXTENT AND VALUE OF FARM LANDS In the accompanying tables are given data from the United States States Census for 1910 showing the percentage of the area in farm lands, the average value of the land per acre, and also the value of the coal produced in 1910. The farm lands of Pennsylvania constituted 64.8 per cent of the area of the State, whereas only 68.2 per cent of the farm land was improved, or 44.2 per cent of the total area of the State. In the lead- ing bituminous county of Pennsylvania (Table 4) 62.6 per cent of the land was in farms, and in Washington County, which, among the coal counties ranks high in the fertility of soil, the percentage of total area in farms was 91.3. In the three counties important as anthracite producers, the percentage of area in farms ranges from 43.5 to 47.2. The average value of land per acre in Pennsylvania was $33.92 ; in the five leading bituminous counties (excluding Allegheny 2 ) the range was from $22.01 to $55.21 ; whereas in the anthracite counties the range was from $25.03 to $33.68. In the leading coal-producing county in West Virginia only 37.8 per cent of the land was in farms, and 13.6 per cent of the farm land was improved or 5.14 per cent of the area of the county. The average value of the land per acre was $33.42. In one important coal-producing county 93.1 per cent of the land was in farms, yet the average value was only $43.81 per acre. In Kentucky 86.3 per cent of the land was in farms, and 64.7 per cent of the farm land was improved or 55.84 per cent of the total 2 The census reports the average value of land per acre in Allegheny County to have been $146.21. This high value is due to the fact that Pittsburgh and Allegheny are located in this county. Fig. 33. — Map showing relation of different thicknesses of coal to the values of farm lands as given in the 1910 census. 52 SURFACE SUBSIDENCE IN ILLINOIS area. The average value of the land was $21.83 per acre. In one of the leading coal-producing counties of Kentucky 81.2 per cent of the area is in farms, and 29.2 per cent improved, or 23.71 per cent of the total area. The land in this county has an average value of $11.72. In Illinois 90.7 per cent of the area was in farms, and 86.2 per cent of the farm-land area was improved, or the improved land was 78.18 per cent of the area of the State. The average value of the land per acre was $95.02. In Sangamon County, an important coal pro- ducer, 87.33 per cent of the land was improved, which is 9 per cent better than the average for the State. The average price of land in Sangamon County was $138.30 per acre, a value of $43 more than the average for the State. The average price in Vermilion County, another important coal producer, was $138.85. The average price for La Salle County was $142.92. In the southern coal counties the farm lands are less valuable, but in Franklin and Williamson counties the percentage of land improved is greater than in the best counties of any of the states previously mentioned, whereas the average prices per acre, $38.43 and $30.61 respectively, compare favorably with the average prices of lands in the other states. With the "Coal Measures" covering 66.9 per cent of the entire area of Illinois, and over 90 per cent of the area of the State in farms, the average value of which is at present greater than the average price of coal rights, the problem in Illinois of protecting the surface, par- ticularly agricultural lands, is much different and much more important than the problem in most of the other coal-mining states. As pre- viously noted, over 15,000 square miles are underlain by coal beds over 4 feet thick, and at least 10,000 square miles additional carry a bed 3 feet thick. Beds no thicker than 3 feet are being mined in Illinois and in other states ; and it is only a matter of time until the problem of lowering the surface of one-half the State by an appreciable amount must be considered, if at least a fair extraction of the coal is desired. The average value of land per acre in the counties in the coal districts of Illinois is shown in Table 4 and on figure 33. DAMAGE BY COAL REMOVAL 53 Table 4. — Value of farm lands, April 15, igio, (U. S. Census) States and counties Approx. land area Illinois 35 Bureau Christian Clinton Franklin Fulton Gallatin Greene Grundy Henry Jackson Jefferson Knox La Salle Livingston Logan Macon Macoupin McDonough . . . McLean Madison Marion Marshall Menard Mercer Montgomery ... Moultrie Peoria Perry Randolph Rock Island .... St. Clair Saline , Sangamon , Shelby Stark Tazewell Vermilion , Warren Washington .... White Will Williamson Woodford Acres ,867,520 563,840 448,000 309,120 284,800 565,760 216,320 329,600 277,120 527,360 376,320 385,920 455,040 733,440 667,520 394,880 374,400 550,400 376,320 762,240 471,680 364,160 253,440 202,880 345,600 440,960 216,320 407,040 288,640 375,680 271,360 424,320 255,360 560,640 494,080 185,600 414,080 589,440 349,440 359,040 324,480 540,160 287,360 337,920 Land in farms Acres 32,522,937 524,455 422,520 280,440 222,578 506,222 162,693 308,579 249,984 504,927 305,759 336,340 424,381 662,755 646,551 381,478 356,946 511,225 353,776 733,161 408,487 335,624 232,456 192,910 326,311 426,398 207,249 353,206 234,915 323,237 237,936 364,523 213,831 520,999 461,878 175,719 374,528 534,385 326,653 329,135 285,027 498,651 227,642 316,064 n^O-S Value of coal* production in 1910 90.7 93.0 94.3 90.7 78.1 89.5 75.2 93.6 90.2 95.7 81.2 87.2 93.3 90.4 ! 96.9 96.6 95.3 92.9 94.0 96.2 86.6 92.2 91.7 95.1 94.4 96.7 95.8 86.8 81.4 86.0 87.7 85.9 83.7 92.9 93.5 94.7 90.4 90.7 93.5 91.7 87.8 92.3 79.2 93.5 86.2 87.9 96.4 87.2 86.8 70.6 86.0 79.3 91.6 90.6 71.7 85.2 81.6 91.3 97.5 95.7 95.9 80.2 85.7 96.0 86.8 85.5 84.2 91.7 83.2 89.4 94.2 80.3 80.2 76.8 77.7 84.9 85.9 94.0 92.1 91.4 87.7 93.6 86.5 82.6 92.1 89.2 84.4 87.4 $95.02 114.53 123.63 44.59 38.48 88.18 48.60 72.52 124.50 112.03 31.27 34.62 112.69 142.92 161.76 156.49 161.29 69.74 116.89 171.85 70.53 39.45 123.92 122.04 104.63 73.49 154.95 107.67 30.62 36.11 87.97 81.57 39.88 138.30 88.22 123.10 144.21 138.85 129.80 34.02 55.44 104.08 30.61 154.27 52,405,897 1,488,070 1,322,162 1,092,752 2,312,342 2 253,307 85,000 14,330 968,563 225,018 776,363 15,000 54,174 2,032,002 262,056 469,657 387,713 3,479,049 61,194 29,470 4,222,078 801,117 466724 464,375 343,115 1,907,006 3 800 1,042,478 1.411,553 1,065,969 109,433 5,763,249 2,713,514 5,014,237 179,291 53,056 210,824 2,691,574 5,086,928 22,500 27,172 126,362 5,086,928 121,131 Mineral Resources of United States for 1910, U. S. Geol. Survey, 1911. 54 SURFACE SUBSIDENCE IN ILLINOIS Table 4.— Value of farm lands, April 15, igio, (U. S. Census)— Concluded 1 States and counties Approx. land area Land in farms en v rt c3 £"3-9 Per cent of farm land improved Average value of land per acre Value of coal* production in 1910 Kentucky Henderson Hopkins A cres 25,715,840 278,400 349,440 302,080 373,760 498,560 28,692,480 464,000 458,886 730,880 508,800 551,680 664,960 288,640 570,880 497,280 25,767,680 339,840 268,800 15,374,080 426,880 266,240 550,400 341,120 201,600 382,080 Acres 22,189,127 231,677 298,263 245,210 338,211 481,370 18,586,832 308,342 228,004 271,094 318,475 503,923 493,491 134,160 269,486 216,348 19,495,636 270,581 122,874 ; 10026,442 110,142 247,835 252,402 128,784 173,529 139,134 86.3 83.2 85.4 81.2 90.5 96.6 64.8 66.5 49.7 37.1 62.6 91.3 74.2 46.5 47.2 43.5 75.7 79.6 45.7 65.2 25.8 93.1 45.9 37.8 86.1 36.4 64.7 86.7 60.7 29.2 55.1 26.3 68.2 79.4 57.2 59.5 66.4 85.7 74.9 43.8 51.1 65.8 50.6 60.7 41.8 55.1 48.8 84.6 53.8 13.6 76.0 46.9 21.83 36.08 20.28 11.72 9.97 8.82 33.92 146.21 32.59 22.01 55.21 48.03 42.03 33.68 28.64 25.03 20.24 34.58 22.79 20.65 24.72 43.81 23.45 33.42 39.91 27.15 14,405,887 239,332 2 202,299 Muhlenberg Ohio Pike Pennsylvania Bituminous — Allegheny ........ 2,503,371 746,611 869,501 313,304,812 20,359,650 17,566,903 Clearfield Fayette Washington Westmoreland . . . Anthracite — Lackawanna 8,048,056 31,210,480 17,567,634 22,389,051 36,868,765 52,759,185 Schuylkill Virginia Tazewell Wise 28,998,199 5,877,486 1,169,981 3,274,809 West Virginia Fayette Harrison Kanawha McDowell 56,665,061 10,135,369 3,814,791 6,518055 12,767,998 4,165,737 Raleigh 3,309,67c 1 ^Mineral Resources of United St ites for 1910, u. s. c i'eol. Sur vey, 191 1. VALUE OF COAL LANDS Although it is impossible to give an average value for coal lands in Illinois, yet it may not be out of place to indicate at what prices coal lands and coal rights are being sold. From these data some idea of the relation between the present value of the surface and of the coal can be obtained, and possibly a better idea of the prospective value of Illinois coal may be secured by a study of these data. In a report 3 on the value of coal land made by the United States Geological Survey in 1910, the following royalty rates per ton of mine- :! Ashley, G. H., Valuation of public coal lands: U. S. Geol. Survey Bull. 424, p. 1910. DAMAGE BY COAL REMOVAL 55 Table 5. — Value* of surface and of coal rights by counties in Illinois County Value of coal per acre Number of coal bed Average surface value, census of 1910 Bond ! $ 25 Bureau 10-100 Christian 10-50 Franklin 35-100 Fulton 15-100 Gallatin 20-25 Grundy 10-25 Henry 135 Jackson 25- 75 La Salle 10-100 Livingston 10- 50 Logan 20-50 Macoupin 15- 50 Madison 10-40 Marion 20 Marshall 15 McLean 15 Menard 25-30 Montgomery 25- 50 Morgan 20-30 Peoria 20-50 Perry 25 Putnam 15 Randolph 25 St. Clair 10-100 Saline 50-150 Sangamon 20-100 Scott 10- 40 Shelby 10-25 Vermilion 100-150 Warren 15 Will 15 Williamson 50-150 Woodford 15 6 2 6 6 5 5 2 6 2,6 2,5 6 5 6 6 6 2 5 6 6 6 5 6 2 6 6 5 5,6 2 6,5 6,7 1,2 2 6 2 $ 45.43 114.53 123.63 38.48 88.18 48.60 72.52 112.03 31.27 142.92 161.76 156.49 69.74 70.53 39.45 123.92 171.85 122.04 73.49 124.28 107.67 30.62 104.69 36.11 81.57 39.88 138.30 83.21 88.72 138.85 129.80 104.08 30.61 154.27 *These prices are not offered as an authoritative basis for valuation but indicate in a general manner the prices at which coal has been sold or at which it is held in some of the important counties. run coal were given for Illinois : northern Illinois, 5 cents ; southern Illinois, 2 cents ; La Salle district, 10 to 25 cents per ton of screened coal. In 1914, the leasing rates were reported in various counties as follows : Franklin, 3 cents ; Fulton, 3 to 4 cents ; Henry, 12^2 cents ; 56 SURFACE SUBSIDENCE IN ILLINOIS Jackson, 3 to 5 cents; La Salle, 4 10 to 15 cents; Peoria, 8 cents; Perry, 3 cents ; Rock Island, 20 cents ; Vermilion, 3 cents ; and Williamson, 3 cents. Table 5 shows the range of prices for coal rights, depending upon the proximity to operating shafts and developed lands. The United States Geological Survey 5 gives the following sale prices for 1910: Illinois, $10-150; Grundy district, $40-110; Rock Island district, $50-75; Springfield district, $10; and southern Illinois, $25-50. In response to an inquiry regarding the assessed value of coal lands in Illinois in 1913, the various assessors furnished the data given in the accompanying table. Table 6. — Assessed valuation of coal rights and of agricultural lands by counties County Assessed value of coal rights in 1913 ssed valut arm lands including rovements coal Remarks Highest Lowest Average Asse of f not imp and Christian .... $ 5.00 $ 5.00 $ 5.00 $77.18 Includes improvements, but Fulton 35.00 35.00 35.00 60.00 La Salle 7.00 1.53 4.89 20.51 Livingston . . 10.00 10.00 10.00 27.50 Reduced for 1914. Logan 90.00 Coal rights not assessed. Madison .... 7.00 7.00 7.00 22.00 Menard 7.00 7.00 7.00 20.00 Mercer 18.00 11.00 14.50 11.00 Average of land for which coal right is assessed separately. Perry . .... 30 00 Improved ; coal rights not assessed. Putnam 3 50-5.00 Randolph .... 11.00 8.00 8.69 23.46 Full value. St. Clair 60.00 3.00 25.00 50.00 Not including E. St. Louis. Saline 10.00 3.00 6.00 25.00 Including improvements. Scott Coal rights not assessed. Washington . 5.00 3.30 3.80 8 09 NATURE OF DAMAGE TO AGRICULTURAL LANDS As previously noted the removal of the coal may cause (1) the caving of the surface with the formation of cracks and pit holes; (2) 4 In the La Salle district most of the coal mined is owned by the mining companies, and very little is leased. "Ashley, G. H., Valuation of public coal lands: U. S. Geol. Survey Bull. 424, p. 35, 1910. DAMAGE BY COAL REMOVAL 57 the sagging of the surface, resulting in the derangement of natural and artificial drainage; (3) the fracturing of beds containing or preserv- ing water supply; or (4) damage to surface improvements. With the proper care on the part of the mine operator some of these damages may be prevented or reduced. (See Chapter V.) In the main it may be said that damage to farm property is only temporary, and the land and property can be restored. On the other hand when a portion of the coal is left in the ground as support for the surface, it is irretrievably lost, at least according to our commercial standards of the present time. In discussing mining wastes in Illinois, Mr. G. S. Rice 6 said : "If it were possible to systematize mining (longwall) so that the land near- est the water courses was first undermined and then in succession the land farther 'away, the damage done to farming would be minimized. However, until the agricultural land of the United States becomes insufficient to fill the needs of the population, which would be reflected in a continual increase of price for farming land, the money loss from temporarily destroying the surface in places is rela- tively small as compared with the selling price of the coal mined under the same. Taking the average value of the surface at $125.00 per acre, if 80 per cent be rendered worthless, the immediate money-loss would be $100 per acre. A seam 6 feet thick would contain per acre 11,000 tons of coal in place, yielding at 90 per cent, 9,900 tons. The damage done by practically destroying the sur- face would be only 1 cent per ton. If the land prices should rise two or three times above the value stated, this loss would still not prohibit mining." It may be suggested that under average conditions in Illinois at present 45 per cent of the coal is left in the ground in certain portions of the State largely to prevent surface subsidence. If an additional 35 per cent were recovered and damage to the surface should thereby result to the extent suggested by Air. Rice, the increased output per acre, 3,850 tons, may be charged with the surface damage, assuming that ordinary mining costs remain the same (they would probably be reduced) with the increased tonnage. At a price of $125 per acre, the cost per ton for surface damage would be 2.6 cents. Many mining op- erators figure on this basis, and their experience has been that as a rule they are obliged to pay damages greatly in excess of the actual depreciation in value of the property. The suggestion has been made frequently that the mining company should purchase the surface as well as the coal right, and that as soon as the coal has been mined completely under each tract or farm, the prop- erty should be restored as nearly as possible and sold. Under such conditions the surface would propably be as valuable when restored after the coal had been mined as when first purchased The income from the use of the surface for agricultural purposes would have "Rice, G. S., Mining wastes and mining costs in Illinois: Til. State Geol. Survey Bull. 14, p. 218, 1909. 58 SURFACE SUBSIDENCE IN ILLINOIS been sufficient to pay the interest on the money invested in the land. If the company suffers any loss through the depreciation of the surface it would be small as compared with the damages that would be paid to the surface owner under conditions such as now exist in the coal-mining districts. 7 RESTORING AGRICULTURAL LANDS Where the removal of the coal causes local sags or depressions, water may accumulate to such an extent that artificial drainage must be provided. If the sags are of small area the problem may not be particularly difficult or expensive. Figure 34 shows the position of 3 sinks with regard to the ""1 GG ODoooaaCj dd c3QDaQDDD,JQoaC f" jnaao o r»owr?nofV«nnfi innnnnnjin |Qrp ifl'J! WitfnS 2l)o^ t^iiaOaZiiia^lSufl/lfllliiLii -' o o ■;:?, o o ci o o coi ! L.I r^o<:>f:K:^ooc-7c-jM=xa) 1 fl! Fig. 34. — Map showing location of tile drains laid by a mining company to drain sags caused by mining a 7-foot coal at a depth of 330 feet. workings of a room-and-pillar mine. At a depth of 330 feet a 7-foot coal was mined. Rooms were carried 30 feet wide with 30-foot pillars. The floor is fire clay, and it is thought the squeezes were due largely to the soft bottom. In order to remove the water in the ponds form- ed after the subsidence, the mining company laid 4,800 feet of tile at a cost of $647.57. The sags were from 2 to 3 feet deep. The coal was mined from 3 to 5 years before the subsidence occurred. 7 Most of the mining companies are not now amount necessary to purchase the surface. a position to invest the additional DAMAGE BY COAL REMOVAL 59 At a longwall mine in the northern part of the State it was found advisable to construct a sump and to install a pumping plant in order to drain a pond which was caused by surface subsidence. As a gen- eral rule, it is found more economical to lay drain tile at possibly greater first cost. Where breaks or pit holes result from mining operations, if the surface is valuable, it will usually pay to fill the holes partly with re- fuse and then to surface with a layer of soil sufficiently deep to support vegetation. The Agricultural Department of the University of Illinois recommends that the soil cover should be not less than 4 feet in depth. If the soil in its natural state is less than 4 feet thick over the entire area, it would seem equitable to make the soil layer of the filled area of equal thickness with that of the undisturbed area. Fig. 35.— Pit hole filled with rubbish at the Electric mine near Danville. In figure 35 is shown what frequently occurs in the mining district — the dumping of all kinds of rubbish into the cavities without regard to the ultimate dressing of the surface with soil. Figure 36 shows the practice in an area adjacent to Streator where the holes are filled partly with mine rock. Figure 37 shows a field near Streator in which the holes have been filled and then dressed with soil. The darker area in the foreground shows the fresh filling. The hole filled was 20 feet in diameter and averaged about 6 feet in depth. Occasionally a subsided area extends across a road. In many places water collects and forms a pond that extends over the road. 60 SURFACE SUBSIDENCE IN ILLINOIS In order to make the road passable it may be filled to grade with the material most easily accessible. In order to prevent surface water from entering pit holes and thence flowing into the mine below, it frequently becomes necessary to construct dikes around the largest holes and those holes located Fig. 36. — Pit hole and cracks caused by mining at a shallow depth near Streator. Mine rock near the hole has been hauled for filling; the surface will then be dressed with soil. Fig. 37. — Pit holes near Streator caused by room-and-pillar mining filled with mine rock and surfaced with soil. The dark lines border the area of filling. in the deepest part of the sags or in the course of the drainage of storm water. A dike 3 feet high built around a pit hole 60 feet in diameter and 28 feet deep has been constructed near Dewmaine. It may be stated in general that, except where the "Coal Measures" are overlaid with thick beds of quicksand or other material that will flow DAMAGE BY COAL REMOVAL 61 easily, there will be little irreparable damage to the surface on account of coal-mining operations, particularly when the coal is removed com- pletely. If a considerable portion of the coal is left permanently in pillars, the surface will probably be thrown into hummocks and sags, with occasional breaks and ponds. A complete removal of the coal will leave the surface in a much better condition for farming purposes. If the rock cover is thin and the overburden has a tendency to flow into the mine, special precautions must be taken or the surface will be con- siderably broken, due to the flow of sand into the workings. At present attempts at filling the mine workings by flushing seem to be impracticable for the greater portion of the Illinois coal fields owing to the scarcity of material suitable for filling. If surface material is taken, a considerable area will be rendered unfit for agricultural purposes due to the removal of the soil. In portions of the State, however, it is possible that in the future market con- ditions may warrant higher mining costs, and under such conditions hydraulic stowing of crushed material from surface quarries may be feasible. Transportation and Surface Subsidence railroads The practice of separating the mining right from the title to the surface has frequently resulted in mining beneath roads, streets, and railroads without any consideration of the protection of the surface. In many instances the deed for the mining rights specifies that the coal may be removed completely and without a liability for damage to the surface. In other instances in which damage to the surface has resulted, an effort has been made to remove as large a portion of the coal as possible without injury to the surface. Certain rail- roads have permitted mine entries to be driven across the right-of- way and have forbidden the opening of rooms within a specified distance of the center of the track. In several cases mining has been carried on according to the regulation of the railroad, but the removal of coal outside the reserved area has resulted in "draw" or "pull" which has threatened the railway tracks. Railways have been constructed across tracts which have previously been under- mined by the room-and-pillar system and on which no subsidence has occurred. In time the pillars have weakened, and the added burden and the vibration due to the passing trains have resulted in the sinking of the tracks. In the opinion of a number of experienced railroad engineers the problem of subsidence as affecting railroads 62 SURFACE SUBSIDENCE IN ILLINOIS is much different from that affecting other types of property, due principally to the intermittent and moving loads. Few instances have been recorded in Illinois of the loss of life or injury to patrons or employes of railroads resulting directly from subsidence due to mining beneath the right-of-way. Figure 38 shows the partially regraded railway track over a mine in which 9 feet of coal has been worked at a depth of 425 feet. In the distance the work train can be seen as filling material is being ';:■ ,..■■■..*■'■■ * ** 1j *0h > M* *.±^_*m- Fig. 38.— Railroad track in Franklin County lowered by surface subsidence resulting from room-and-pillar mining of a 9-foot coal at a depth of 425 feet. The maximum subsidence was 4 feet. The track has been partially repaired. unloaded for the regrading of a switch. Figure 39 shows the nature and extent of the workings beneath the surface tracks which have subsided. The rooms were carried 30 feet wide by 300 feet long; pillars were 20 feet wide. The squeeze covered an area 1,200 feet square, and the depression at the surface was in places 4 feet. The underclay is quite soft in parts of the mine. One of the large railroad companies reports that in the La Salle district it has been necessary to raise and surface the main line track every year, the total subsidence being estimated at about 3 feet. It is reported by two railroad companies that subsidence result- ing from longwall mining in the vicinity of Decatur has necessitated regrading and filling amounting to 3 to 4 feet. Room-and-pillar mining in southern Illinois has caused consider- able damage to railroad tracks. Near Duquoin a track to a mine DAMAGE BY COAL REMOVAL 63 subsided 3 feet. A branch line of a railroad in Williamson County was damaged by a sink hole 6 feet in diameter and 20 feet deep. This occurred over the abandoned workings of a mine 90 feet deep. Im- portant tracks have subsided amounts varying from a few inches to a few feet at numerous places. Important bridges have been threat- ' V * J 1 1 \ ; /•") r^:~j f o_o r— — - ; r : V /' _ "\ ' i i ■ ; ' ■*-'"! / i i '-.-_< ■' i ; - ! — -_/ v.. rt ' — "> '*_'' o 1 * ' r — -^ « — . / ! n P~- ~> ^— 1 17 ; J { j ; ( j \J i ! ! ! / / , ; ( J L '' l « J - — « <•- - <- 'i j j _ } I i O 1 { u / /-> ', i ;' ) ( ,J N r ! ; S : ! i ! i >:/ U p— f— If - 1 r~ i /I I / I $ i 1 i 1 1 . ( . ,__n ^ Scale 1 inch = 200 feet Fig. 39.— Plan showing relation of railroad in figure 38 to mine workings. ened; one pier of a bridge is reported to have subsided 18 inches but without much tilting (fig. 40). It is now the practice of the railroad companies whose lines extend through mining districts to secure annually from their division engineers complete reports showing the extent of mining beneath the right-of-way. 64 SURFACE SUBSIDENCE IN ILLINOIS WAGON ROADS Generally the title to the coal beneath wagon roads is not reserv- ed by the district maintaining the roads. Frequently where there is a likelihood that the roads may be damaged by mining operations an agreement exists between the mining company and the local officers to the effect that any damage to any road will be repaired by and at the expense of the mining company. In the Longwall District of northern Illinois a general subsidence following the advance of the longwall face in many places results in the inundation of the roads during part of the year. At a number Fig. 40. — Repaired railroad trestle in Franklin County. One end subsided 18 inches more than the other, the total having been reported as about 4 feet. Timbers were placed at A, B, C, and D in order to reduce the sag of the track over the trestle. of places the mining companies responsible for the subsidence have constructed adequate ditches paralleling the sunken road in order to remove the water. It is not uncommon for the mining companies, as important users of the roads, to haul mine refuse and ashes to fill the road to the desired grade. STREAMS AND CANALS Mining has been carried on beneath several navigable streams and canals, but in no instance has the water been let into the mine, nor has subsidence caused any appreciable damage to the water course DAMAGE BY COAL REMOVAL 65 — at least not to the extent of interfering with its usefulness for the passage of watercraft. Buildings and Improvements Affected by Subsidence buildings The subsidence of the surface over a large area may be so uniform and so gradual as to cause no serious damage. In parts of the longwall field mining has been reported to have had no appreci- able effect upon the frame buildings at the surface. The effect is less if the mining face advances rapidly and across the shorter axis of the building. As previously noted, the building is in a ten- sion zone as the mining face approaches and passes under the building, and the section of the building under which the coal has been removed first tends to tilt toward the mine and to tear itself free from the remainder of the building. If the building is constructed of masonry, Fig. 41. — House near Coal City lowered 9 inches at one corner, caused by mining a 3-foot coal at a depth of 125 feet. serious cracks may form and afterward close when the mining face has advanced completely beyond the building. When the advance of longwall mining has been stopped under or adjacent to a building, the surface is likely to be tilted enough to throw the building out of plumb. Figure 41 shows a house in the Coal City district which was thrown out of plumb by the stop- ping of the longwall face near the house. The coal beneath had not been mined. The seam of coal is 3 feet thick and lies at a depth of 125 feet. One corner of the house was almost one foot lower than the other corners. 66 SURFACE SUBSIDENCE IN ILLINOIS The removal of part of the coal by room-and-pillar mining is more likely to damage buildings seriously than is the complete removal of the coal by the longwall system. This difference results from the probable formation of pits and from the unequal subsidence that may tear to pieces a building that happens to be located over a pillar. If it is located in the middle of a small sag, the compression or squeezing may be so great as to destroy the building. Figure 42 shows the part of a brick house still standing above a pillar in a mine near Danville. Approximately 6 feet of coal was taken out at a depth of 200 feet, The position of the house with regard to the mine workings is shown in figure 43. Fig. 42. — Brick house near Danville abandoned on account of danger by room-and-pillar mining of a 6-foot coal at a depth of about 200 feet. Figure 44 shows a portion of a row of houses in Springfield These brick houses were damaged by subsidence resulting from room- and-pillar mining. The coal is 5 feet 9 inches thick and lies at a depth of approximately 200 feet. A pillar 10 feet wide was left along the street and this apparently was responsible for the crack- ing of the houses. If all the coal had been removed the houses probably would have settled uniformly. The houses have been re- paired at the expense of the mining company. In figure 45 is shown a small frame house in the suburbs of Streator. A pit hole 10 by 20 feet and 5 feet deep has been formed along one side of the house. In southern Illinois has recently occurred a movement affecting a large area in one of the coal-mining towns. Eight feet of coal DAMAGE BY COAL REMOVAL 67 was being mined by the room-and-pillar method at a depth of ap- proximately 450 feet. Rooms were carried 30 feet wide with 15-foot pillars. Figure 46 gives a general view showing the flooded streets resulting from the subsidence. The maximum depth of the sag was about 3 feet. The foundations of houses were cracked, and in a ~---_ -J ? / — ^ -1 r i i L. / 1 ! i i "-V->W-^ J i i i 1 I i / i / v >. v- ^s i -J " ' i "~""~~-*-.^* - -- — -j f> N 1 N "I L- i n i j 1 i ! n 1 ,--1 LJ i V ' L- i i „- J l Scale 1 inch = 200 feet Fig. 43. — Plan showing relation of house in figure 42 to the pillar in the number of houses the plaster fell. Another depression at the same mine resulted in lowering one end of the company barn over 4 feet. The barn has been repaired, but one end is still approximately 15 inches lower than the other. The tilting of the barn is shown in figure 47. 68 SURFACE SUBSIDENCE IN ILLINOIS F IG _ 44.— Houses in northwest Springfield for which claims were paid by a mining company for damages caused by mining a coal 5 feet 9 inches thick at a depth of about 200 feet. .'-.' ••*•<<. i . <*:... IB Fig. 45.— Small house near Streator beside which a pit hole 10 by 20 feet and 5 feet deep has formed. DAMAGE BY COAL REMOVAL 69 STREETS, PIPE LINES, AND SEWERS Mining within the limits of cities and towns and the construction of towns upon lands which have previously been undermined have been attended with more or less danger owing to the subsidence of the streets. Where coal at shallow depth has been mined by the room-and- Fig. 46. — Flooded streets, broken sidewalks and foundations, and damages to plastering resulting from subsidence in Franklin County. Figure 48 shows the broken sidewalk at A. (Photo by H. T. Smith, U. S. Bureau of Mines.) \ /. Fig. 47. — Barn of mining company lowered 4 feet at one end. partially restored. It has been pillar system, a sudden collapse of the overlying beds into the worked- out rooms may result. Passing teams have at several times been reported to have dropped with the surface into a pit hole. No fatal accidents are known. Where the coal is at greater depth, or where the coal has been mined by the longwall system, the movement will 70 SURFACE SUBSIDENCE IN ILLINOIS probably not be attended by the formation of holes into which build- ings or creatures may fall. The gradual sinking of the surface may do serious injury to pavements, sidewalks, car tracks, pipe lines, and sewers. Numerous instances of such damage have been reported but as repairs are usually made immediately there has been little opportunity to secure photo- graphs illustrating this type of damage. Figure 48 shows a broken sidewalk caused by subsidence. Mining was being carried on by the room-and-pillar method at a depth of 450 feet on an 8-foot seam of coal. g. 48.— Broken sidewalk caused by subsidence in Franklin County; the location is shown in figure 46. (Photo by H. I. Smith, U. S. Bureau of Mines.) In order to secure reliable information regarding the extent of mining operations within the towns of the State, and the nature and amount of damage which has been attributed to mining, a general letter was sent in August, 1914, to the mayor of each of 80 more or less typical incorporated towns and cities in the coal districts. Replies 8 were received from 56. 8 The towns reporting were Auburn, Belleville, Bloomington, Braidwood, Breese, Car- bondale, Carterville, Christopher, Colchester, Coulterville, Duquoin, Edinburg, Fairbury, Farmington, Galesburg, Geneseo, Ilarrisburg, Hillsboro, Jacksonville, Kewanee, Lewistown, Lincoln, Litchfield, Lovington, Macomb, Marion, Marissa, Mascoutah, Mattoon, Minonk, Morris, Mount Olive, Moweaqua, Murphysboro, Nashville, Nokomis, Norris City, Odin, O'Fallon, Pana, Peoria, Peru, Pinckneyville, Pontiac, Riverton, Salem, Seneca, Shelbyville, Sorento, Springfield, Spring Valley, Staunton, Streator, Virden, West Frankfort, and Witt. DAMAGE BY COAL REMOVAL 71 The questions and answers were as follows: 1. Has coal ever been mined within the city limits? Yes, in 48 towns of 56 replying. 2. Has coal ever been mined under the streets and alleys? Yes, in 39 towns of 56 replying. 3. Has any damage resulted to the streets and alleys on account of the mining of the coal? Yes, in 5 towns of 56 replying; no, in 49. 4. Does the city now own or control the right to mine coal under any or all of the streets and alleys? Yes, in 17 of 56 replying; no, in 35. The replies to question No. 3 received from the five cities and towns reporting damage to streets and alleys were as follows: 1. Yes. 2. Not recently, but in former years some subsidences caused some trou- ble. These have been fixed except one that gives some trouble, but nothing serious. 3. A little. 4. Some cave-ins. 5. Yes. Sink on North Main Street and property adjoining it; also in east and west part of city. The mayor of one town in which a 7-foot seam of coal lies at a depth of less than 450 feet reports that "the city has deeded the right to mine coal under all its streets to the Coal Company, and they have been mined as far as Main Street." The coal company reports that 50 per cent of the coal is left unmined, the rooms being from 30 to 35 feet wide on 55-foot centers. The mining company has not been released from surface damage. There has recently occurred in this town a movement which damaged a railroad right-of- way. Another mayor wrote as follows : "The right to mine coal under streets and alleys was voted to - - Coal Company several years ago." At this place the coal is 7 feet thick and is over 600 feet below the surface. A city in the Longwall District of northern Illinois receives a yearly rental for the privilege of mining coal under the streets. A city ordinance granted this privilege conditional upon the payment of the rental. WATER SUPPLY At a number of places in the State it has been reported that wells have ceased to furnish the usual supply of water owing to the cracks and fissures resulting from subsidence. In some instances water-bearing rocks have been shattered, and in others gravel beds and catchment basins have been tilted or disturbed so that they no 72 SURFACE SUBSIDENCE IN ILLINOIS longer serve as reservoirs for water. However, in a number of in- stances after the subsidence movement has stopped, the surficial beds have become compact and have again furnished water in quantities nearly, if not quite, as great as previously. MUNICIPAL WATERWORKS The protection of city waterworks is a matter which merits serious attention. At several points, notably at two cities of over 50,000 population, mine workings are advancing toward the waterworks. In these two large cities the undermining of the wells, reservoirs, and plants will cause serious damage. Reservations of Coal The topic of reservations may logically be considered under the discussion of protection of the surface and directly in connection with pillars. It has been more or less customary for the owners of farm lands when selling the coal right to reserve a tract of the coal under the dwelling, the well and cistern, the barn, and any important farm buildings adjacent to the dwelling. The advisability of leaving a small tract of coal for the protection of the surface is seriously questioned. If the coal is not thick and can be removed rapidly and completely it may be much more economical to have the coal tak- en out and to make such minor repairs as may result from subsidence. The presence of faults and beds of quicksand would complicate matters. Moreover under some conditions the angle of draw may be so great that for the depth at which the coal occurs the size of reservation necessary for adequate protection would be entirely out of proportion to the value of the objects on the surface for which protection is desired. There are no statutes in Illinois forbidding mining under any particular type of structures, public utilities, or other buildings, al- though there are such statutes in some states. When it can be shown that irremediable damage would result to property used for public purposes, or when mining might seriously threaten life through damage to property used for a public purpose, an injunction may be secured restraining mining in the area where subsidence is feared. CHAPTER IV— SUBSIDENCE DATA BY DISTRICTS Introductory Statement As the reports upon the mining practice and upon the geology have assembled the data by districts, it has been thought advisable to review the data on subsidence by districts so that they may be correlated with the geological and mining data. The location of the various districts is shown in figure 49. Table 7 gives the districts of the State by counties and Table 8 gives the counties arranged alphabetically. Table 7.— Districts into which the State has been divided for the purpose of investigation Coal Method of mining Counties I II III IV V VI VII 2 1 and 2 6 (East of Duqtioin anti- cline) 6 (West of Duquoin anti- cline) 6 and 7 (Danville) Longwall Room-and-pillar Room-and-pillar Room-and-pillar Room-and-pillar Room-and-pillar Room-and-pillar Room-and-pillar Bureau, Grundy, La Salle, Marshall, Putnam, Will, Woodford. Jackson. Brown, Calhoun, Cass, Fulton, Greene, Hancock, Henry, Jersey, Knox, Mc- Donough, Mercer, Morgan, Rock Is- land, Schuyler, Scott, Warren. Cass, DeWitt, Fulton, Knox, Logan, Macon, Mason, McLean, Menard, Peo- ria, Sangamon, Schuyler, Tazewell, Woodford. Gallatin, Saline. Franklin, Jackson, Perry, Williamson. Bond, Christian, Clinton, Macoupin, Madison, Marion, Montgomery, Moul- trie, Perry, Randolph, Sangamon, Shel- by, St. Clair, Washington. Edgar, Vermilion. (73) utiles Fig. 49. — Map showing division of State into districts. SUBSIDENCE DATA 75 Table 8. — Alphabetical arrangement of coal-producing counties County Bond . . . Brown . Bureau . Calhoun Cass . . . Christian Clinton . Edgar . . Franklin Fulton . Gallatin Greene . Grundy Hancock Henry . . , Jackson . Jersey . . . Knox . . . La Salle. Logan . . . Macon . . Mason . . Macoupin Madison Marion . . Coal seam District 6 2 2 2 2,5 6 6 6,7 6 1,2,5 5 1,2 2 1,2 1,2 2,6 1,2 5 2 5 5 5 6 6 6 VII III I III III, IV VII VII VIII VII III, IV V III I III III II, VI III IV I IV IV IV VII VII VII County Marshall . . . McDonough McLean . . . Menard .... Mercer Montgomery Moultrie . . . Peoria Perry Putnam Randolph . . . Rock Island. St. Clair .... Saline Sangamon . . Schuyler . . . Scott Shelby Tazewell . . . Vermilion . . , Warren Washington . Will Williamson . , Woodford . . . Coal seam District 2 I 1,2 III 5 IV 5 IV 1,2 III 6 VII 6 VII 5 IV 6 VI, VII 2 I 6 VII 1,2 III 6 VII 5 V 5,6 IV, VII 1,2,5 111,1V 1,2 III 6 VII 5 IV 6,7 VIII 1,2 III 6 VII 2 I 6 VI 2,5 I, IV These districts do not contain quite all the mines operating in Illi- nois because there are a few which do not fall into the arrangement such as the Assumption mine, 1004 feet deep, operating in coal No. 1 at Assumption in Christian County and a few small room-and-pillar mines in coal No. 2 in the longwall held. From the mines included in the eight districts of the Coal Mining Investigations, however, there is produced 98.3 per cent of the tonnage of the State, and 97.6 per cent of all the employes in coal mines in Illinois work in these districts. District I Practically all the longwall mines of the State are included in District I. In Table 9 is given a list of longwall mines arranged ac- cording to depth and showing the average thickness of coal worked. It will be noted that the majority of the mines are being worked through shafts more than 300 feet deep. Of the 36 shafts only 2 are more than 600 feet deep, and these are outside of the longwall district proper. 76 SURFACE SUBSIDENCE IN ILLINOIS Reference has previously been made to the character of roof and floor in District I. Attention may be called again to the extension over a part of the district of a heavy bed of limestone 25 to 30 feet thick and lying 375 to 400 feet above coal No. 2 and 175 feet above coal No. 7. Data are not available to show to what extent this bed influences the surface movement resulting from longwall mining, but it is reported that where the limestone bed is known to occur, the sinking of the surface does not follow so soon after the coal has been removed as in those areas where the limestone is known to be missing Table 9. — Longwall mines in Illinois Post office rt *o <4- (/) Operator and mine address of mine O u "oIj £° y .5 £^f, n! ^ y .■S ; §»a « *° y ~-0 ^ K g » 4) rt 3 2 C Clay .... Shale .... Sandstone Coal 100 100 100 100 196 213 219 207 209 210 214 224 226 221 211 199 225 -216 224 229 310 214 223 202 COMPRESSIBILITY OF BROKEN ROCK Experiments have been made upon crushed material to determine to what extent it may be compressed. In general it is known to what extent rock in place, when crushed to a specified size, may occupy an increased volume when the crushed material is subjected to pressure. Fayol's results of compression tests upon crushed material are given in Table 16. Table 16. — Compressibility of different materials after having been crushed c Rocks having been previously crushed or broken occupy space indicated, under pressure* to o o Space occu- pied before being broke I Pressure 1,422 1b. per sq. in. II Pressure 2,844 lb. per sq. in. Ill Pressure 7,110 1b. per sq. in. IV Pressure 14,220 1b. per sq. in. Clay Shale Sandstone .... Coal 100 100 100 100 100 90 128 116 136 125 130 125 75 110 120 118 70 97 105 109 The following conclusion was drawn by Fayol : "The material which ordinarily fills the graves of mines always occupies a larger space than it did ordinarily. After an expansion of about 60 per cent, it appears to undergo in workings of from 300 to 900 feet in depth, a compression of about 30 per cent, which leaves a volume of about 12 per cent larger than the volume of the unbroken rock." 5 * Pressure I corresponds to a depth of strata of 1,638 feet; II, 3,276 feet; III, 8,190 feet; and IV, 16,380 feet. B Colliery Engineer, vol. 33, p. 548, 1913. PROTECTION OF SURFACE 95 The United States Bureau of Mines had made tests upon crushed material from the Pennsylvania anthracite districts (Table 17) to de- termine its compressibility (1) when confined in steel cylinders, (2) when built into cogs, and (3) when piled in heaps. Table 17 .—Compressibility tests upon crushed material made by the United States Bureau of Mines Lbs- P er Corresponding depth, Compression SQ- £t - 140 lbs. per cu. ft. (Reduction in height) Feet Per cent A. Broken mine rock and breaker refuse compressed in a steel cylinder 16% inches in diameter and 25^ inches high : 20,000 143 11.4 30,000 215 15.5 90,000 645 24.0 120,000 860 26.2 B. Mine rock passing 1^-inch ring, lying loosely in a conical pile and free to flow : 23,824 170 60.0 40,256 290 62.3 92,445 660 65.0 165,000 1,180 67.0 C. Rock cog, built of mine rock and shoveled material. Pyramidal form; base 5 by 5 feet ; top, 3 by 3 feet ; height, 1 foot 1 1 inches. 22,000 167 22.0 30,000 215 27.0 90,000 645 36.0 120,000 860 37.2 From these data it may be stated that for the conditions of long- wall mining where the gob is well filled and the walls well built, the compression of the gob will be only 33 ]/$ per cent more for a mine 645 feet deep than for a mine 215 feet deep. Data are not available to show how much the broken shale, commonly forming the gob in the longwall district, will be compacted under pressure. The weight of 100 feet of overburden is sufficient to make it deform and flow. Where the mine roof falls into a free space it may be more or less shattered, and as overlying beds fall successively, the worked-out volume may be filled eventually. The material which can not fall sinks upon the fallen material which may have been already compressed to such an extent that it may be able to check further subsidence. In longwall mining the argument has been that, as the beds over- lying the gob subside, they compress the gob to a fraction of the thick- "Unpublished data. 96 SURFACE SUBSIDENCE IN ILLINOIS ness of the coal, and that as they sink they increase in volume suffi- ciently to prevent the movement extending to any great height above the coal horizon. As previously noted, observations in the Longwall District of northern Illinois tend to prove that as the roof along the roadways sinks, it breaks, and that cracks are formed extending parallel to the working face. These may be from 2 inches to 6 feet apart and usually extend up into the roof shale for a distance not exceeding 15 to 20 feet. Above this height no cracks are in evidence as the con- fined rock flows or is deformed. On the basis of these observations there is little justification for presumptions or theories that the increase in volume of the beds sub- siding over longwall workings is sufficient to compensate for the total height of material mined. Apparently the increase in volume is limited to the strata, 10 to 20 feet thick, immediately overlying the coal bed. Center line of road Center li O Fig. 53. — a, Diagrammatic illustration through the gob and parallel to the face of a longwall mine ; b, plan showing pack walls and loose gob between road- ways and cross entries in the same mine. The overlying beds are traversed by breaks or cracks "every 2 inches to 6 feet," and it is evident that the greatest increase in volume that can arise will result from the visible breaks or cracks; the increase in volume resulting from invisible cracks may probably be ignored without serious error. If the visible cracks occur every 2 inches to 6 feet, the material, along the roads at least, is obviously broken into slabs 2 inches to 6 feet long, and these slabs settle in fairly orderly fashion upon the filling. Under such conditions any considerable in- crease in volume of the overlying beds is not conceivable. If, however, the strata higher up do not sink gradually but remain undisturbed for a time, while the beds immediately underlying subside several feet, in time such resistant beds may fail, and large falls may occur with a considerable increase in volume of the broken material. In northern Illinois the average distance between room centers is 42 feet, the road- PROTECTION OF SURFACE 97 ways are not less than 8 feet wide, and the pack walls are 4 yards wide on each side of the road. The distance between the walls should there- fore average about 10 feet. This space, called the "gob," would be filled more or less completely with waste material thrown back as the face is advanced. A cross-section along the face would then show the roof supported as indicated in figure 53 a. There is little probability that the vertical amount of settling over the gob, if not more than 10 feet wide, will be materially greater than the settling over the roads which are 8 to 10 feet wide. Moreover an examination of old roads protected by pack walls or buildings shows that as the roof settles the pack walls are compressed and they tend to bulge and to spread horizontally, as is evidenced by the reduction in the width of roadways. It may logically be inferred that some spreading of the pack walls occurs on the side toward the gob. In time the gob would offer more resistance to settling than do the roadways although these are probably better protected at first, inasmuch as the pack walls are built more substantially on the roadside than on the gob side. Where roads have been driven through abandoned longwall work- ings in which the old roads have been closed for years, it has been found on brushing to the necessary height for the new road, that there is but little undulation in the previously horizontal roof shales on ac- count of the settling over the pack walls. If such undulation exists it is undoubtedly much more marked in the strata immediately overlying the coal than in those at a greater height above the coal horizon. From the observations that have been made in the deeper longwall mines in Illinois, where the measures immediately overlying the coal are thick beds of shale, it is apparent that minor irregularities in filling and pack walls have but little effect upon the general subsidence movement; in fact, so little that only the most precise observations may indicate that such irregularities have influenced the general movement. When it is so impracticable, as is usually the case, to secure complete data on the continuity and uniformity of the overlying beds, it seems unwise to attempt to discuss the effect of minor irregularities in the filling and pack walls. The conditions in the average longwall mine may be, for the pur- pose of discussion, considered to be as shown in figure 53,/;. As the longwall face is advanced new cross entries are started and the old ones are abandoned. Where the cross entries are turned at an angle of 45 degrees to the main entry, the distance between cross entries may be from 225 to 300 feet. 7 Preliminary report on organization and method: 111. Coal Mining Investigations Bull. 1, p. 18, 1913. 98 SURFACE SUBSIDENCE IN ILLINOIS From these cross entries are turned the roadways to the rooms or working places. These working places will average 42 feet in length along the working face, and the distance from center to center of roadway will be 42 feet. As shown in figure 53, the worked-out area is laid out more or less regularly in a series of parallelograms approximately 225 by 32 to 34 feet. Around the four sides of each parallelogram is a wall of mine rock built 9 to 12 feet thick. Within the four walls the space is at first partly filled with shoveled material, A[ Cross entry Cut for fire wall JB Main entry Plan Fig. 54.— Diagrammatic illustration showing the flow of roof shale under pressure in a mine near Peoria. and later the unfilled space, if any, may be filled by falls of roof rock. After the working face has been advanced a short distance, the roof settles upon the pack walls, and in time as these are pushed down into the underclay the roof within the pack walls may sag and bear upon the gob (fig. 54). When this state is reached, the parallelogram bounded by four pack walls, becomes in reality a long cog with walls built of mine rock, the center being filled with shoveled material. PROTECTION OF SURFACE 99 These cogs tend to prevent surface subsidence. Their success in accomplishing this will depend upon the extent to which the clay bottom heaves in the roads, the amount the pack walls bulge, and the power of the pack walls and the gob to resist compression. As usually built these walls are not rigid. Normally, when the main entries have been advanced 225 to 300 feet beyond a cross entry, a new cross entry is started, and in time the old cross entries are abandoned. Until they are abandoned suffi- cient height is maintained to permit the passage of mules. The roads are brushed and the bottom is lifted as long as it is necessary to keep the road open. Though the amount of material thus removed is occa- sionally large, relatively it represents but a small volume of the material within the parallelograms of rock filling on each side of the road. The bulging of the packs and the flow of the bottom may eventually cease where the material within the cog has been compressed to such an extent that it does not flow easily. After the roads have been aban- doned, the amount of flow is limited to the volume of the roadway itself. For the purpose of this discussion it may be assumed that two longwall mines are operated under identical conditions except that the coal seam in one lies at a depth of 200 feet and the other at 600 feet. The plan of mining, dimensions of rooms, and other factors are the same, and the same amount of material is built into walls and stowed in the gob. Considering only the matter of compressibility of pack walls and gob, it should be noted that the maximum amount of subsidence which can result in the mine 200 feet deep, is determined by the amount the gob and walls are compressed under the load of 200 feet of overlying strata. If the load were less, the distance through which the roof would sink would be less ; and if the load were increased, the distance through which the roof would sink would be increased up to the limit of compressibility of the material in the gob. As the weight of the overburden increases directly with the depth, therefore the compression of the gob is greater for deep mines than for shallow mines ; it is greater for a mine 600 feet deep than for a mine 200 feet deep, other conditions being the same, providing of course that the limit of compressibility of the material has not been reached. It follows logically that so far as the factor of compressibility of filling controls — and in a majority of cases it is the controlling factor — the total amount of vertical movement tends to increase rather than to de- crease with the depth of mining. To this general statement there may be exceptions, and there may be depths beyond which this would not apply; but it is the opinion and 100 SURFACE SUBSIDENCE IN ILLINOIS the experience of British mining engineers that subsidence will follow longwall mining irrespective of depth. It is generally supposed that there is more or less flow in the rock beds overlying the coal after these beds have stood for a time depending for support upon the pillars or filling. However, it is difficult to find evidences of such flowage on a large scale. In a mine near Peoria it became necessary to open a section of the mine that had been abandoned and build a fire wall extending up to an overlying limestone. Immediately overlying the 5-foot coal bed is a bed of shale approximately 10 feet thick, and resting on the shale is a stratum of limestone. The roadway from 12 to 15 feet wide, was Scale of Feet o joo eoo a i i— Fig. 55.— Diagrammatic illustration showing to scale the size of shaft pil- lars for given depths as recommended by various mining engineers. filled tightly with shale from the overlying bed. A cut through the shale to the limestone showed that the shale had under pressure prac- tically flowed to fill the opening, whereas the overlying limestone showed no breaks or cracks where it was possible to make an examina- tion. The conditions are illustrated by figure 54. Shaft Pillars Commonly in determining the size of pillars necessary to protect mine openings of a given width it is customary to assume a span of roof and overlying rock to be supported, to estimate the total weight of such PROTECTION OF SURFACE 101 a block for the depth of the workings, and then with the known or assumed unit crushing strength of the material to be left in the pillar, the cross-section may be calculated readily. The dimensions may then be proportioned in order to secure the most economical and safest working conditions ; on the same general plan shaft pillars or other important pillars may be determined. Numerous rules have been formulated for the calculation of shaft pillars in flat seams ; a great diversity of opinion prevails among en- gineers as to the required dimensions at various depths and with dif- ferent thicknesses of coal seams. This diversity of opinion is well shown 8 graphically by figure 55. In determining the size of pillar necessary to protect objects upon the surface, as has been noted previously, the ability of the pillar to carry the load is not the only uestion to be considered. Among the most important of the other problems is that of draw or pull over the pillar and the ability of the underlying bed to sustain the load concentrated upon it by the pillar. Quite frequently the underlying bed is less stable and has less crushing strength than the pillar. It seems logical then to proceed as follows in determining the size of pillar necessary to protect an object upon the surface: 1. Determine the lateral extent of pillar necessary in order to prevent dam- age by draw. 2. Determine whether the pillar thus outlined is sufficiently large to support the burden of the overlying beds without crushing. 3. Determine whether the load upon the pillar will cause the pillar to be forced down into the underlying bed, or cause a flow of the under- lying material. Room Pillars At various points in Illinois it has been necessary for the coal mining companies to assume responsibility for any surface damage which may result from coal mining and it has been deemed advisable to leave in the ground a sufficient amount of the coal to prevent move- ment, at least to prevent the movement from extending to the surface. Where the roof is strong it may stand for some time without breaking, but eventually in parts of the mine at least, movements of the top or bottom may cause a considerable area to be affected. The general theory of the arching of strata has been presented by a French engineer, Fayol. 8 Knox, G., Mining subsidence: Proc. International Geological Congress, vol. 12, p. 798, 1913. 102 SURFACE SUBSIDENCE IN ILLINOIS In his discussion of methods of protecting the surface, Fayol re- ferred to the use of pillars between the working places. "The meshes of the network consisting of pillars with working places between them should be made smaller as the workings are shallower. As the depth becomes greater the size of the meshes can be enlarged, and the dimen- sions of the areas worked can be increased relatively to the sizes of the pillars that are abandoned, regard being had to the height and width of the zones of subsidence, so that the various zones may be kept dis- tinct from each other. This general rule is susceptible of many com- binations according to the thickness, the inclination, the number, and the depth of the seams worked. If the excavation is of small dimen- sions the subsidences which take place above them are restricted in size and become enlarged both in width and height as the excavation increases in area. If the pillars at 1, 3, 5, and 7 be taken out (fig. 56), zones of subsidence similar in Z x , Z 3 , Z 5 , and Z 7 would be produced ; but when pillar 2 is taken out the line of roof subsides on to the floor, and the zone of subsidence rises in Z 2 . The same thing happens when &*~M?''m. z Zk\ Vsr^rferSfer t-\ ; — r — r-< — •) ^~ N \ J. i» 1 i L 12 3 4-5 6, 7 8 3 JO II /Z f3 14 J5 Fig. 56.— Diagrammatic illustration showing the extent of subsidence result- ing from the removal of adjacent pillars (after Fayol). No. 6 pillar is taken out, and if No. 4 pillar is taken out, the space comprised between the zones Z 2 and Z 6 is set in motion and deter- mines the formation of the zone Z 4 ." 9 It follows then from this statement of Fayol, that if the room pillars are properly proportioned and properly spaced, the disturbance of the strata may be limited to the volume within the zones. The material outside these zones throws no weight upon the material within the zone. Necessarily then any vertical pressure must fall upon the unmined material forming the pillars, and the pillars must be large enough to withstand this pressure. In stratified beds the problem is more complicated, owing to the fact that the beds act as single members and frequently sag under their own weight. 9 Proc. South Wales Inst. Eng, vol. 20, p. 340, 1897. It should be noted that these zones outline the dome through which the movement extends, and not the limit of the "falling zone" as described by the Austrian engineer, Rziha. PROTECTION OF SURFACE 103 In a paper before the Pennsylvania State Anthracite Mine Cave Commission, 1913, Mr. Douglas Bunting said, "The application of a formula for determining the safe size of coal pillars for various thick- nesses of veins and depths can be considered practical for depths greater than 500 feet, but it is doubtful if the same formula would be of any practical value for application to veins at less depth, and cer- tainly of diminishing practical value with reduction in depth and thick- ness of veins for the reasons that the variable conditions of vein, top, bottom, and other factors are of more consequence with small pillars than with large pillars." 10 The average dimensions of pillars and rooms in ordinary room-and-pillar mining in Illinois are shown 11 in Table 16. Table 16. — Dimensions of rooms and pillars in Illinois coal mines District Average depth Room width Pillar width Feet Feet Feet II 140 26 19 Ill 90 22 18 IV 201 25 9 V 243 26 16 VI 270 22 18 VII 227 31 30 VIII 174 27 8 Average for State 208 26 19 Filling Methods In a number of important mining districts in America and in Europe, filling methods have been used extensively both to increase the percentage of coal recovery and to reduce the surface movement. These methods are adapted particularly to dipping seams. GOBBING Waste material resulting from regular mining operations or broken for this particular purpose may be stowed or packed into the excava- tion. If sufficient or suitable material is not available underground, it may be lowered or dropped from the surface and stowed where needed. 10 Bunting, D., Pillar and artificial support in coal mining with particular reference to adequate surface protection: Pennsylvania Legislative Journal, vol. 5, Appendix, p. 5988, 1913. 11 Andros, S. O., Coal mining in Illinois: 111. Coal Mining Investigations Bull. 13, p. 76, 1915. 104 SURFACE SUBSIDENCE IN ILLINOIS In the longwall field the waste produced at the working face is stowed in the gob, but usually a large amount of rock resulting from falls and brushing in the roads is hoisted and piled on the surface. Owing to the nature of longwall work it is claimed that it is imprac- ticable to stow much of this material in the gob. It might be done at an increased total cost per ton of coal mined. In room-and-pillar mining the waste material can be stored under- ground much more readily than in longwall mining, and in many Illi- nois room-and-pillar mines practically no rock is hoisted. The coal beds over the greater part of Illinois lie practically flat and the stowing of material in both room-and-pillar and longwall mining would be expensive on account of the necessity for hauling the filling long distances and because a large amount of shoveling would be required underground. HYDRAULIC FILLING The stowing of material by means of water 12 has been employed extensively in a number of mining districts, but is not being employed at any Illinois coal mines. Hydraulic filling seems to be impracticable at present on account of the flatness of the beds, the clay floor, and the difficulty in securing suitable filling material in the coal district. As previously noted, little stone, sand, or gravel is available, and the surface is so valuable for agricultural purposes that the cost of material secured from surface pits would be prohibitive in most places. Deep pits would be impracticable generally, as considerable pumping would be necessary during a number of months in order to keep the pits unwatered. In parts of the State there might be a shortage of water during several months of the year. Griffith's method of filling It has been suggested by Mr. William Griffith that worked-out portions of mines be filled by blasting up the bottom and shooting down the roof. This suggestion was made in connection with a report to the 12 The Copper Range Consolidated Copper Company, Painesdale, Michigan, is using compressed air to carry filling material for short distances through iron pipe. The material is the J4 -inch "stamp-sand" or crushed waste rock from the concentrating plants which is dropped from the surface through steeply inclined raises to the levels where filling is being carried on. On these levels the material is dropped into cars and hauled to some convenient point above the stope to be filled. The filling material is dumped from the cars into pockets or chutes and is distributed horizontally in the stopes from the bottom of these chutes. It has been found economical to carry the filling material by compressed air not to exceed 300 feet through the pipes. The pipe is shifted laterally, lengthened, shortened, or raised as may be necessary to distribute the material. There are no bends in the pipe and the material is handled dry. PROTECTION OF SURFACE 105 Scranton Mine Cave Commission, and Mr. Griffith has secured a patent (U. S. Patent No. 1,004,418) covering this method. 13 It has been suggested that by this method an increased percentage of coal could be recovered. So far as known this method has never been applied to beds as flat as those in Illinois. The conditions in Illi- nois do not lend themselves toward making this scheme practicable as the immediate roof is generally not hard enough to make the proper kind of filling and the bottom is underclay which is too soft to serve as filling that will increase in volume when blasted and in that condi- tion support any weight without being reconsolidated to occupy prac- tically the original volume. Artificial Supports At only a few places have artificial supports been introduced to protect objects upon the surface. The construction of cogs was neces- sary at one place to prevent the collapse of the beds under a railroad right-of-way. The workings were at shallow depth and complete filling seemed to be impracticable though the workings were still accessible. Two types of cogs were used, the more successful being a timber cog filled with mine rock. Masonry, concrete, reinforced concrete, iron, and steel structures have been employed in American and European districts to protect important structures and roads. 13 Messrs. Griffith and Connor say, "It is a well-known fact that loose rock occupies from If to twice the volume of the same weight of rock in place. Your engineers have conceived the idea of taking advantage of this fact, well known to engineers, for the purpose of cheaply producing an adequate support of the rock and surface above certain classes of coal beds under the city of Scranton. So far as we know, this method, in its entirety, has never been used before in any coal-mining district, and the suggestion is here made for the first time. "The process is applicable to beds less than 6 feet in thickness and consists simply in blowing up the floor and shooting down the roof of the mine, each to a depth equal to the thickness of the coal bed. This produces a total thickness of loose rock equal to three times the thickness of the coal. The rock would be well packed together and have great supporting power, and moreover the desired ends would be attained in a comparatively in- expensive manner."— Griffith, William, and Conner, E. T., Mining conditions under the city of Scranton: U. S. Bureau of Mines Bull. 25, p. 57, 1912. CHAPTER VI— INVESTIGATIONS OF SUBSIDENCE European As previously noted a number of investigations have been made in Europe to determine the amount of subsidence which results from min- ing at various depths and under different geological and mining condi- tions. Some of these investigations have continued over long periods of years, and a few of them have been organized so that additional data will be secured from year to year. It has been possible, in districts where such observations have been made, to adapt the system of mining so that the maximum recovery of mineral may be made with least dam- age to the surface. Knowing what effect mining will have upon the surface, it has been possible for several countries to formulate laws and rules for the protection of the owner of the surface and at the same time to secure for the mine owner a just consideration of his rights. The problem of subsidence in Europe has been discussed by the author and Mr. H. H. Stoek in Bulletin 91 of the Engineering Ex- periment Station, University of Illinois. United States In the United States very little work has been done along these lines — practically nothing in the bituminous coal districts. Very few of the data that have been collected are available for the use of the public. The scarcity of data in the bituminous fields may be due largely to the fact that the most extensive mining operations for bituminous coal have been carried on where the surface is mountainous or is of little value for agricultural purposes. If the percentage of extraction from the flat coal beds of the Middle West is to be increased it seems timely that there should be secured, in more or less typical mining areas, data which will serve to show the amount and extent of subsidence which may be expected to result from mining operations. Suggested Illinois Investigation considerations It has been suggested that in Illinois investigations be made in- cluding measurements to determine the surface movement under va- rious conditions and also laboratory tests to determine the strength of rocks forming mine roofs and of materials used for supports. The bearing power of the materials forming the mine floor should also be investigated. The nature and amount of surface subsidence should be studied when different methods of mining are used and when various methods of filling are employed. (106) INVESTIGATIONS OF SUBSIDENCE 107 TYPICAL MINES The idea has been advanced by George S. Rice, 1 that typical dis- tricts should be selected giving special consideration to the longwall fields. For this work the following may be suggested : 1. Wilmington district where the coal occurs at a depth of 50 to 150 feet. 2. La Salle district where the depth to coal No. 2 is from 300 to 500 feet. 3. Decatur district where coal No. 5 lies at a depth of 500 to 600 feet. 4. Assumption district where coal is mined at a depth of 1,000 feet. For the investigations in connection with room-and-pillar and panel work the following may be suitable : 1. Springfield district where the coal is mined at a depth of 150 to 200 feet. 2. Williamson County district where mining is at varying depths up to 300 feet. 3. Gillespie-Staunton district with depths of 300 to 400 feet. 4. Franklin County district where the depth of mining is 400 to 700 feet. MONUMENTS AND SURVEYS It has been suggested that substantial surface monuments be es- tablished in parallel lines across the working face in longwall mines and in room-and-pillar mines across panels that are to be worked. The monuments should be established over the solid coal before any move- ment has commenced, and the location of these monuments with regard to points underground should be determined accurately. At regular intervals levels should be run, and profiles made along each line of monuments showing the position of the working face, the original position of the surface, and the elevations at the time of the successive surveys. The monuments should be located where they will be easily acces- sible to the surveyor but where they will not be interfered with, and they should be so constructed that they will not be lifted by frost. In addition to the levels, measurements should be made from time to time to determine the amount of lateral movement or draw. UNDERGROUND WORK It would be essential that a careful record be made of underground conditions. In room-and-pillar and panel work the date of opening rooms, the actual dimensions of pillars, and the position of the rooms with regard to surface monuments should be noted. When rooms are finished and the pillars are drawn, particular attention should be given to movements of roof and floor, and if a squeeze follows, a detailed report should be made as a basis for the study of surface movement and of other squeezes. ^hief Mining Engineer, U. S. Bureau of Mines, one of the representatives of the Bureau in this cooperation. 108 SURFACE SUBSIDENCE IN ILLINOIS After the movement underground has ceased, whatever observa- tions are possible should be made in the vicinity, noting particularly the angle of break in the roof, the condition of pillars, the amount of open space above the falls, and the condition of the roof material which is standing. In longwall mines the rate of advance should be noted regularly as well as the stability and width of the pack walls, the amount of material placed in the gob, the angle of fracture of roofs, the height to which breaks extend into the roof, the rate of settling of the roof, and the ratio of compression of pack walls. Observations should be made to discover the amount of sag in a direction parallel to the face and between the pack walls. If practicable the amount of filling in one section of the mine should be kept constant, but a different amount should be used regularly in another part, in order to discover the effect that filling has upon sub- sidence. 2 POSSIBLE BENEFITS From the data secured it should be feasible to determine the amount of subsidence which may be expected per foot of material ex- cavated at different depths and with different geological sections. It should be possible to predict in longwall mining whether the wave of subsidence will move in advance of the mining face or lag behind it. The amount of draw after the advance has ceased could undoubtedly be predicted with more accuracy than is possible at present. The effect of leaving pillars could probably be shown with greater certainty, and it would undoubtedly be possible to protect such structures as must be left undisturbed with less interference with mining operations. As previously noted, nearly one-half the coal is being left in the ground and made unavailable for future mining. At present the lost coal in the mines of southern Illinois represents a larger tonnage of coal per acre than there is in the virgin coal seam now being mined in northern Illinois, and each ton of this abandoned coal has a heating value of 7 to 8 per cent greater than the coal mined in northern Illinois. 3 2 This might be feasible in a mine in which the undercutting is done in part by hand and in part by machines. In one mine the ratio between the volume of material removed in the undercutting by machine is about one-fourth as much as in undercutting by hand. s The average analysis of 58 samples of coal No. 6 from east of the Duquoin anticline showed a heating value of 11,825 B. t. u. The extraction of 56 per cent of the seam which averages 9 feet in thickness results in leaving 6,732 tons to the acre. In northern Illinois the average thickness of the coal mined is 3 feet 2 inches, and the average heating value, as indicated by 38 samples, is 10,981 B. t. u. In other words, in northern Illinois the virgin coal seam contains per acre 5.700 tons of coal of 10,981 B. t. u., whereas in District VI the average worked-out mine contains 18.1 per cent more coal of a heating value 7.7 per cent greater than does the unworked tract in northern Illinois. INVESTIGATIONS OF SUBSIDENCE JQ9 It is hoped that a careful study of the subsidence problem in the various mining districts will lead to a better realization of the necessity to the State for extraction of the coal where the damage to the surface will be reparable. Where the damage to the surface is reparable and in excess of the value of the coal, the facts should be made known and the proper steps taken. Where the damage to the surface is irreparable and in excess of the value of the coal, it seems advisable to stop the mining of coal completely until such time as the increased value of coal may be sufficient to warrant the renewal of mining and the com- plete extraction of the coal possibly under improved methods of min- ing. 4 The State should give attention to the problem of conservation in those extensive areas of coal fields where the right to the coal is held by one party and the surface by another, but under the existing deed the owner of the coal is held liable for any surface damage resulting from mining operations. Under such conditions it may be expected that nearly 50 per cent of the coal will be left in the ground. Against this practice a protest may well be raised and the State of Illinois should find or create a remedy. 5 t ♦•* t Ge 7™- S ' R ^ C ' ^ a " informal address in N ew York, in 1913, before the American Institute of Mining Engineers, stated that he had been shown leveling data in Upper Silesia, Germany relating to surface elevations in and about a certain important iron works, which showed that where granulated slag had been hydraulically stowed in coal-mine excavations in a bed 20 feet thick in which there had been practically complete extraction of coal the subsidence had not been appreciable, at the maximum point showing only 2^ inches Also that since the hydraulic stowing system had been inaugurated in Westphalia the German government had allowed mining under important munition manufacturing buildings at the Krupp works in Essen, whereas formerly under the old dry-filling methods there had been serious subsidence in and about parts of Essen resulting in cracked walls and buildings which he had observed in 1911. "It has been suggested that a tax be levied upon coal left in the ground. This might be done by assessing the land or coal right upon the full tonnage originally in the ground less the amount which has actually been recovered, and continuing this assessment against the land until the unmmed coal is recovered. If this were to become the general practice it would undoubtedly tend to induce the mining companies to remove more of the coal INDEX PAGE Agriculture, effect of subsidence on 50-61 Angles of break and of draw. 31, 89-93 Assumption, mining at 75 B Belleville coal, see coal No. 6 "Blue band" coal, see coal No. 6 Bond County, subsidence in.. 73, 85-86 Brown County, subsidence in. 73, 78-79 Buildings, effect of subsidence on. 64-72 Bunting, Douglas, acknowledg- ments to 103 Bureau County, subsidence in. 73, 75-78 Calhoun County, subsidence in... 73,78-79 Canals, effect of subsidence on. ..64-65 Carbondale formation, description of ... : .. 19 Carlinville limestone, description of 22 Carterville, subsidence at 37 Cass County, subsidence in. . .73, 78-79 Caves, description of 34-42 Christian County, coal No. 6 in... 25 subsidence in 73, 85-86 Clark County, structure in 20 Cleat, effect of, upon subsidence. .26-27 Clinton County, coal No. 6 in 25 subsidence in 73, 85-86 Coal City, subsidence near 43, 65 Coal field, extent of 15 "Coal Measures", description of. 18-21 Coal No. 1, production from 15 roof and floor of 24 stratigraohic position of 19, 21 Coal No. 2, faults and rolls in 27 production from 15 roof and floor of . ; 24 strata associated with 21 stratigraphic position of 19, 21 Coal No. 5, faults and rolls in... 27-28 production from 15 roof and floor of 25 Coal No. 6, depth of 21 faults and rolls in 28-29 production from 15 roof and floor of 25-26 strata associated _ with 22-23 stratigraphic position of 19, 21 subsidence above 32 Coal No. 7, faults and rolls in. . . . 29 production from 15 roof and floor of„ 26 strata associated with 23 PAGE stratigraphic position of 19, 21 Coal rights, values of, in Illinois. 55-56 Compressibility of rock 94, 95 Compression tests of coals 89 Conservation of coal 11, 13 Cracks, surface 30-34 Crushing strength of coals 88-89 Cuba, subsidence near 39 Damage caused by mining 30-72 Danville, subsidence near 39,49,59,66,67,86-87 Dewitt County, subsidence in. .73, 79-81 Dewmaine, subsidence at 40 Dikes, subsidence associated with. 38 Duquoin anticline 21 E Edgar County, subsidence in.. 73, 86-87 Edwards County, structure of.... 19 Europe, study of subsidence in.. 106 Faults, relation of, to subsidence. 27-29 Fayol, acknowledgments to 93-94,101-102 Filling methods 103-105 Franklin County, coal No. 6 in... 25 structure in 20 subsidence in ... .32, 44, 46, 62, 64, 69, 70, 73, 83-85 value of land in 52 Fulton County, subsidence in. 73, 78-81 G Gallatin County, subsidence in. 73,81-83 Gobbing, relation of, to subsidence ... 103-104 Greene County, subsidence in. 73, 78-79 Griffith, William, acknowledgments to 103-105 Grundy County, subsidence in. 73, 75-78 H Halbaum, acknowledgments to... 91-93 Hamilton County, structure of . . . . 19 Hancock County, subsidence in.. . 73,78-79 Harrisburg, subsidence at 37 Henry County, subsidence in. 73, 78-79 Herrin coal, see coal No. 6 Jackson County, subsidence in J ...73,78,83-85 Vergennes sandstone in 21 Tersey County, subsidence in. .73, 78-79 (110) INDEX— Continued 111 PAGE K Kentucky, value of land in. . .50, 52-54 Knox County, subsidence in. ..73, 78-81 Lands, values of 50-61 La Salle anticline 20 La Salle coal, see coal No. 2 La Salle County, subsidence in... , • 73,75-78,90 value of land in 52 La Salle _ limestone, stratigraphic position of 23 Lawrence County, structure in....' 20 Logan County, subsidence in . . 73, 79-81 Longwall District, geology of... 23-24 subsidence in .... 42-43, 48, 64, 71, 73, 75-78, 95-96 Longwall mining, relation of, to subsidence 30 M Macon County, subsidence in. .73, 79-81 Macoupin County, coal No. 6 in . . 25 subsidence in 73^ 85-86 Marion County, coal No. 6 in . . ! . 26 structure of 19 subsidence in '.'.'.73, 85-86 Marshall County, subsidence in... 73 75-78 Mason County, subsidence in.73,' 79-81 McDonough County, subsidence in 7 '3 78-79 McLean County, subsidence in.' ^ T _ ••••• 73,79-81 McLeansboro formation, descrip- tion of 19 Madison County, coai No. 6 in... 25 subsidence in 73, 85-86 Menard County, subsidence in.73, 79-81 Mercer County, subsidence in.73, 78-79 Montgomery County, coal No. 6 in 26 subsidence in 49, 73, 85-86 Morgan County, subsidence in.73, 78-79 Moultrie County, coal No. 6, in. . . 26 structure of 19 subsidence in ....73, 85-86 Murphysboro coal, see coal No. 2 N New Haven limestone, strati- graphic position of 22 Nokomis, subsidence near 49 PAGE Pillars, relation of, to subsidence. p . ,• •••••........; 100-103 Pit holes, description of 34-42 Pottsville formation, description „ , of ;••••• 18-19 r roduction of coal 15-17 Protection of surface .88-105 Putnam County, subsidence in.73, 75-78 R Railroads, effect of subsidence on Randolph County, coal No*. 6 in.'. 26 subsidence in 7^ 85-86 Kice US., acknowledgments to.. 107 Kichland County, structure of 19 j Roads, effect of subsidence on 64 I Rock Island coal, see coal No. 1 Rock Island County, subsidence in p „ ••••••;..... 73,78-79 Kolls, relation of, to subsidence. .27-29 Room-and-pillar mining, relation of, to subsidence 30 Peoria County, subsidence in. .73, 79-81 Pennsylvania, value of land in.. 50, 54 Pennsylvanian series, description o£ 18-21 Perry County, coal No. 6 in 26 subsidence in 43, 45, 73, 83-86 Piatt County, structure of 19 "Safe depth" theory 93 Sags, description of '.'.42-50 St. Clair County, coal No. 6 in . 26 subsidence in 73,85-86 Valine County, subsidence in.. 73, 81-83 structure in 20 Sangamon County, coal No. 6 in. . 26 structure of 19 subsidence in '.'.73, 79-81, 85-86 value of land in 52 Schuyler County, subsidence in... 73,78-81 Scott County, subsidence in.. 73, 78-79 Sewers, effect of subsidence on.. 69-71 Shafts, depths of 17 ig Shawneetown fault ' 20 Shelby County, coal No. 6 in 26 subsidence in 7^ 85-86 Mioal Creek limestone, description . of 22 Springfield, subsidence in 68 Streams, effect of subsidence on. 64-65 Streator, subsidence near. .41, 60, 66, 68 Streets, effect of subsidence on.. 69-71 Structure of "Coal Measures". .. 19-21 Subsidence, damage caused by... 28-72 data on 73-87 evidences of WW. .30-50 geological conditions affecting.' 18-29 investigations of 106-109 protection against 88-105 T Tazewell County, subsidence in. 73,79-81 Third Vein coal, see coal No. 2 Transportation, relation of, to sub- sidence 61-65 112 INDEX — Continued PAGE U United States, study of subsidence in 106 Vermilion County, subsidence in.. 39,47,49,59,73,86-87 value of land in 52 Vergennes sandstone, strati- graphic position of 21 Virginia, value of land in 54 W Wachsmann, acknowledgments to 31,91 Warren County, subsidence in. 73, 78-79 Washington County, coal No. 6 in 26 subsidence in 73, 85-86 Water supply, effect of subsidence on . . 71-72 Wayne County, structure of 19 West Virginia, value of land in.. 50, 54 White County, structure of 19 Will County, subsidence in 73, 75-78 Williamson County, coal No. 6 in. 25 subsidence in 63, 73, 83-85 value of land in 52 Woodford County, subsidence in. 73,75-78,79-81