557.09773 I IL6cr 1982-2 ISGS CONTRACT/GRANT REPORT: 1982-2 U.S. D.O.E. ET-76-G-01-9007 Geologic Investigation of Roof and Floor Strata: Longwall Demonstration, Old Ben Mine No. 24 Robert A. Bauer Philip J. DeMaris April 1982 Illinois Department of Energy and Natural Resources STATE GEOLOGICAL SURVEY DIVISION Champaign, Illinois Bauer, Robert A. Geologic investigation of roof and floor strata: longwall demonstration, Old Ben Mine No. 24, final technical report: part 1 / Robert A. Bauer and Philip J. DeMaris. - Champaign, III. : Illinois State Geological Survey, April 1982. 49 p. ; 29 cm. - (Illinois-Geological Survey. Contract/Grant report ; 1982-2) (DOE/ET/12177-1) (FE/9007-1) Also published by U.S. Department of Energy, Division of Fossil Fuel Extraction. 1. Coal mines and mining. 2. Coal balls. 3. Mine roofs (stability). I. DeMaris, Philip J. II. Title. III. Series 1. IV. Series 2. V. Series 3. Printed by authority of the State of Illinois/ 1982/250 ISGS CONTRACT/GRANT REPORT 1982-2 FE/9007-1 DOE/ET/12177-1 GEOLOGIC INVESTIGATION OF ROOF AND FLOOR STRATA: LONGWALL DEMONSTRATION, OLD BEN MINE NO. 24 FINAL TECHNICAL REPORT: PART by Robert A. Bauer and Philip J. DeMaris Principal Investigators: Heinz H. Damberger Harold J. Gluskoter (to 6/78) April 1982 Contract No. U.S.D.O.E. ET-76-G-01-9007 (Formerly U.S.B.M. G0166207) This report represents work on a program that was originated by the Interior Department's Bureau of Mines and was transferred to the Department of Energy on October 1, 1977. Illinois State Geological Survey Natural Resources Building 615 East Peabody Drive Champaign, I L 61820 Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/geologicinvestig19822baue CONTENTS Abstract v Acknowledgments v Introduction 1 Geology of Herrin (No. 6) Coal in Franklin County 2 Geologic Characteristics of Old Ben No. 24 Mine Area Setting 6 Roof 9 Coal 14 Floor 14 Rolls 18 Coal balls 21 Mining Problems 25 Prediction of coal balls 25 Roof stability 27 Subsidence 34 Physical strength results 35 Further Research Directions 37 References 38 Appendix A. Testing Procedures 41 Unconfined strength and elastic modulus 41 Indirect tensile strength 41 Water content 41 Specific gravity 42 Shore hardness 42 Slake durability 42 Appendix B. Core Descriptions and Test Results 43 ISGS Drill Hole LW-1 43 ISGS Drill Hole LW-2 46 ISGS Drill Hole LW-3 48 TABLES 1. Identified plant material by plant group from coal ball vertical section 3 in Area L 21 2. Occurrence of fallen intersections for mapped area B by categories of roof lithology and structure 34 3. Number of mapped falls by location for mapped area B 34 FIGURES 1. Distribution of the Herrin (No. 6) Coal Member in southern Illinois and the location of the study area, Franklin County 3 2. Composite stratigraphic section of the roof of the longwall panels 4 3. Thickness of the interval between the top of the Springfield (No. 5) Coal and the base of the Herrin (No. 6) Coal in Franklin County 5 4. Generalized thickness of Herrin (No. 6) Coal in Franklin County 5 5. Distribution and thickness of the Energy Shale Member in a portion of Franklin County 7 6. Index to mapped areas in the Old Ben Coal Company Mine No. 24 8 7. Plan view of roof geology at the longwall demonstration site in mapped area A 10 8. Schematic cross section showing interrelationships of Herrin Coal, rolls, and immediate roof members 9. Spatial relationship of mapped areas A and B and the sinuous exposure of Anna Shale roof 12 10. Map of the roof geology for mapped area B 13 11a. Clay mineral compositions of the Energy Shale and roll material 15 11b. Clay mineral analyses of the Anna Shale showing differences of clay mineral content between the upper and lower portions of the unit 15 12. Coal thickness variation by roof lithology 16 13. Correlation of macropetrographic units with the Herrin (No. 6) Coal along a 1,000-foot traverse 17 14. Clay mineral composition of underclay samples 18 15. Idealized cross sections of rolls showing variation in fill materials 20 16. Concentrated coal balls near the center of coal-ball area L 23 17. Map of area M showing limits of the "core" of concentrated coal balls 23 18. Coal balls, coal-ball areas, and roof lithologies identified during mapping of the 3 longwall panels 24 19a. Five of the 13 positions where coal and coal balls were measured separately 26 19b. Small compaction fault under round coal ball has displaced thin coal ball about 1 cm. 26 20. Plan view (above) and cross section of roof fall in relation to mine and geologic setting 29 21. Slips in roof shown paralleling black shale roof exposure (area B) 30 22. Roof fall in relation to mine plan and geologic setting 31 23. North-south fractured zone in roof caused by large horizontal compressive forces 32 24. Roof falls in the Energy Shale showing a preferential north-south orientation 33 25a. Subsidence profile above the second longwall panel 36 25b. Subsidence profile above the first longwall panel 36 ABSTRACT In-mine mapping of three longwall panels at the Old Ben No. 24 Mine has revealed both major and minor roof -stability problems and multiple areas of concentrated coal balls within the Herrin (No. 6) Coal Member. The roof-stability problems are related to three interacting factors: variations in roof lithology, various structural features, and mining plan. Major roof-stability problems are rare at the longwall face, but more common in the longwall support entries. Several major falls have occurred in areas where potential problems were identified previously during mapping. Lesser roof-stability problems are associated with "rolls" (linear shale bodies in the top of the seam) containing compaction faults and with a tectonic fault zone running perpendicular to the face of the second panel. A distinctive roof type has been identified in this mine; the roof type varies between Energy Shale and Anna Shale /Brereton Limestone over a short distance. Other mines with this type of transitional roof can expect similar roof problems. Large deposits of coal balls within the seam repeatedly interfere with orderly development of support entries and damage longwall equipment. The second and third panels have been shortened to avoid areas of coal-ball concentrations. The distribution of coal balls is strongly correlated with roof lithology; thus the association of roof and coal balls has been used successfully to predict locations of coal balls in unmined areas. ACKNOWLEDGMENTS This report is based on detailed geologic investigations in the Old Ben No. 24 Mine near Benton, Illinois, in conjunction with the longwall coal mining demonstration project. This work was supported by the U.S. Bu- reau of Mines Grant G0166207 from September 1976 to March 1978, and by the Department of Energy Grant ET-76-G-01-9007 from April 1978 to Decem- ber 1979. The work was administered under the technical direction of the Pittsburgh Mining Technology Center with James R. White, and later, Mary Ann Gross as Technical Project Officers. We wish to thank the officials and employees of the Old Ben Coal Company and of their No. 24 Mine for valuable information and assistance. We also wish to thank W. John Nelson and John T. Popp of the Illinois State Geological Survey for assistance in underground mapping and Peter R. Johnson for petrographic study of the Herrin (No. 6) Coal seam in the study area. We are grateful to Tom Phillips of the University of Illi- nois, Botany Department, for identification of the plant material in the coal balls found in the mine. We also thank Sue M. Rimmer for clay mineral analyses of roof, floor, and in-seam materials. INTRODUCTION In the 1960s and early 1970s, ten attempts at longwall mining were made in southern Illinois in the Herri n (No. 6) Coal Member. All attempts were abandoned before the completion of one entire panel. The most serious problems were related to the chock supports, which were inadequate to hold the roof at the face. Since geologic conditions encountered during these first attempts were not documented, how geologic conditions related to the stability problems is not entirely known. Moroni (1974) described several of the longwall attempts of the Old Ben Coal Company; Harrell (1974) described similar attempts by Freeman Coal Mining Company. In 1975, the U.S. Bureau of Mines and the Old Ben Coal Company began a longwall demonstration project on a cost-sharing basis. The Illinois State Geological Survey, with financial support from the U.S. Bureau of Mines and later the U.S. Department of Energy, documented the geologic conditions encountered during the longwall mining demonstration. This report summarizes the observations made by Illinois State Geological Survey staff during geological mapping, sample collection, and testing conducted in connection with the three longwall demonstration panels and other selected areas in the Old Ben Coal Company No. 24 Mine. It also contains information from past State Geological Survey investigations and drill hole logs. Our tasks and goals were 1. to map lithologic variations and geologic structures in roof, floor, and coal in and around the longwall panels and other locations in the mine (methodology generally follows that discussed in Krausse et al . , 1979a; 1979b); 2. to relate geological features to the stability of the roof during the longwall mining operation; 3. to drill several boreholes into the roof and floor and conduct mechanical strength tests on the cores; 4. to study the nature and occurrence of coal balls found in the coal seam in the demonstration area for the purpose of predicting other coal -ball occurrences. INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 GEOLOGY OF HERRIN (NO. 6) COAL MEMBER AND ASSOCIATED STRATA IN FRANKLIN COUNTY This longwall demonstration project was conducted in the Illinois Basin Coal Field at the Old Ben No. 24 Mine, which is located in southern Illi nois within Franklin County and operates in the Herri n (No. 6) Coal Mem- ber (fig. 1) of the Carbondale Formation of the Pennsylvanian System (fig. 2 J: A large river was contemporaneous with the deposition of the peat that is now the Herrin (No. 6) Coal. The position of the river is now repre- sented by the sediment-filled Walshville channel, located about 7 miles west of the longwall site. Shale splits within the coal seam along the channel indicate that the river associated with the Walshville channel remained active throughout deposition of the Herrin peat (Johnson, 1972). Furthermore, an investigation of the thickness and type of strata below the Herrin (No. 6) Coal showed that the river responsible for the Walsh- ville channel was active even before deposition of coal -forming peat began in Franklin County. The thickness of the interval between the Herrin (No. 6) Coal and the next major coal seam below, the Springfield (No. 5) Coal Member, varies greatly in the county. The interval is thin- nest near to and thickens away from the channel. The thickness of the interval was used as an indicator of the relative topography upon which the peat forming the Herrin (No. 6) Coal was deposited. Whereas the Herrin peat may have been slightly time transgressive, the effect within the study area was judged to be insignificant. Further investigation of strata below the Springfield (No. 5) Coal showed that no variations in thickness existed that might have influenced these upper units. In western Franklin County the interval between the Herrin (No. 6) and the Springfield (No. 5) Coals thins abruptly within a mile-wide, crescent- shaped depression that has both ends terminating near the Walshville channel; it appears to be an abandoned meander (fig. 3). The coal is thickest in the meander (fig. 4). Shape, location, and increased coal thickness all indicate that the meander was formed and abandoned before peat deposition began. After deposition of peat ceased, the river persisted a while, forming the first layer on top of the coal— a gray silty shale called the Energy Shale Member. This shale, believed to be an overbank deposit of the river, is found in several large lobate deposits along the former course of the river; they all thin away from the channel. The next layer deposited on the gray shale, or directly on the coal where the gray shale is missing, is a black carbonaceous shale known as the Anna Shale Member. The Anna Shale here is a widespread, black, highly carbonaceous shale averaging 2.8 feet thick with a low-diversity marine fauna suggestive of a restricted environment. Where the Energy Shale exceeds about 35 to 45 feet in thick- ness, the Anna Shale generally pinches out. 2 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 Walshville channel; coal missing Energy Shale associated with the Walshville channel iil&j Clastic deposits associated with the New Goshen channels * Low-sulfur coal reported (< 2.5% S; each point generally represents several analyses) -x"^* Limit of Herrin (No. 6) Coal N New Goshen channels Franklin Co. Figure 1. Distribution of the Herrin (No. 6) Coal Member in southern Illinois, and the location of the study Franklin County. (From Treworgy and Jacobson, in press.) INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 I V Named Members r . 1 . 11 h-6 4 -2 -0 Figure 2. TTTTT 5& 1 , 1 , 1 15=° r~n Bankston Fork Limestone Lawson Shale Conant Limestone Jamestown Coal Figure 3 Thickness of the interval between the top of the Springfield (No. 5) Coal and the the base of the Herrin (No. 6) Coal in Franklin County. Brereton Limestone (4 to 5 ft) Anna Shale (2.8 to 3.7 ft) Energy Shale (0 to 18 ft) Herrin (No. 6) Coal (Top) Figure 4 Generalized thickness (in feet) of Herrin (No. 6) Coal in Franklin County. Composite stratigraphic section of the roof of the longwall panels, representing a portion of the Carbondale Formation, Kewanee Group, Pennsylvanian System. ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 The Breretcn Limestone Member is the next younger unit. Here the Brereton Limestone is dark gray, fine grained, and argillaceous. It contains an open-marine fauna, averages 5.9 feet thick near the mine, and overlies the Anna Shale or the coal, if both the Energy Shale and the Anna Shale are absent. Where the Energy Shale is thicker than 35 to 45 feet, the Brereton Limestone also pinches out. Each of these units may form the immediate roof within a small area; therefore, this type of roof is considered transitional between the thick Energy Shale roof found locally near the Walshville channel, and the Anna Shale/Brereton Limestone roof found well away from the channel. This variability was first recognized during in-mine mapping in Old Ben No. 24 (DeMaris and Bauer, 1978), and was defined as that area where the roof varied from Energy Shale to Anna Shale/Brereton Limestone and back to Energy Shale over a short distance. On the basis of further in-mine mapping and analysis of nearby drill-hole data we now recognize that Energy Shale was originally deposited over the Herrin (No. 6) and then partially or completely eroded. The term "transitional roof" has since been extended to include areas with similar variability, which were produced primarily by initial deposition (Nelson and Nance, 1980). Based on drill-hole data we have extended the area of transitional roof within part of Franklin County (fig. 5) using the 25-foot thickness line as the break point between thick Energy Shale and the area of rapidly varying thickness. A corridor of this roof type separates two lobes of thick Energy Shale and appears to extend to, or very near to the Walshville channel. The north-facing slope of the southern lobe is much steeper than the opposite slope of the northern lobe, suggesting that erosion has accentuated the slope. The erosion may have been caused by temporary diversion of the south-flowing river waters toward the east-southeast, forming the corridor. The genesis of the transi- tional roof on the east side of figure 5 is uncertain because no active mines exist there. GEOLOGIC CHARACTERISTICS OF OLD BEN NO. 24 MINE AREA Setting The mine is situated beneath the bottomlands of a small river. The Pennsylvanian bedrock over the longwall panels is covered by 50 to 60 feet of Pleistocene material, which is largely composed of silts and clays deposited on a lake bottom. The longwall demonstration panels, located in mapped area A (fig. 6), are about 620 feet below the surface, with bedrock representing 565 feet ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 -25— Thickness (ft) of the Energy Shale Member Energy Shale slightly over 25 ft (point not honored by 25 ft thickness line) W£$ Transitional roof (Energy Shale thickness from ft to < 30 ft) ■ Coal-ball sites X Entrance shafts Figure 5 Distribution and thickness of the Energy Shale Member in a portion of Franklin County, IL. Areas of transitional roof and all known coal-ball sites are also plotted. Townships and ranges are indicated along the map border. INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 33 OLD BEN NO. 24 Y/, g # a. 14 R2E R3E 35 *4r 36 I 31 12 I 6 miles N 1 V/2 Mapped on reconnaissance X, Mapped in detail 5? Entrance shafts Figure 6_ Index to mapped areas in the Old Ben Coal Company Mine No. 24. (Numbers on the map are section numbers, defining 1 square mile areas.) ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 of that distance. The overburden consists of about 5 percent limestone, 80 percent shale, claystone, and siltstone, 1.3 percent coal, 3.5 per- cent sandstone, and 10 percent Pleistocene cover. Roof The immediate roof of the longwall demonstration panels is the transi- tional type described in the preceding section; about 65 percent of the roof is Energy Shale (fig. 7 ). The Energy Shale is a fine-grained, medium-gray shale that is usually poorly bedded and has some thin iron- stone bands and ironstone nodules roughly an inch in diameter scattered throughout. The gray shale rests conformably on the coal; in many areas the top of the coal and the bottom of the Energy Shale interfinger slightly. Clay mineral analysis of samples of Energy Shale from the longwall panel area show that it is homogeneous both laterally and vertically (fig. 11a). The contact between the top of the gray Energy Shale and the black Anna Shale is unconformable, dipping 10° to 14°. The angle decreases as the top of the Energy Shale is approached (fig. 8). This unconformity is the result of erosion that removed the gray shale and exposed the top of the coal seam, forming long sinuous areas from which gray shale is absent (fig. 9). Two different facies of the Energy Shale are found as immediate roof; a lower, carbonaceous shale facies of limited distribution, and an upper, widespread, light- to medium-gray shale facies. The carbonaceous facies ranges from medium- to dark-gray in color, is rarely more than 0.4 feet thick and locally contains a lycopod-dominated compression flora. This carbonaceous facies is generally similar to the dark-gray unit described from a nearby mine (Edwards et al . , 1979). The light- to medium-gray facies locally reaches over a hundred feet in thickness, but only rarely contains recognizable plant material. This facies is sometimes silty, varies from finely laminated to weakly laminated and often contains sideritic bands or bands of small pyritic nodules near the base. The next younger unit, the Anna Shale, overlies the eroded top of the Energy Shale, or the coal where the Energy Shale is absent. The bedding of the Anna Shale parallels the surface on which it was deposited. The lower part of the Anna Shale is black, very carbonaceous, fissile, hard, and typically has a prominent set of joints that strike about N 80° E with occasional minor joint sets perpendicular to it. Large disc-shaped concretions up to 3 feet across occur in this lower portion of the unit. It grades upward into a massive, very dark-gray claystone. The Anna Shale reaches a maximum thickness of about 4 feet but locally thins to zero near the central axes of sinuous areas lacking Energy Shale (fig. 10) Clay mineral analysis of the Anna Shale shows that the characteristic INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 ^^ Brereton Limestone ] Black Anna Shale ] Gray Energy Shale <£?•) Coal -ball areas — Shale -filled rolls \ Normal fault, downthrown side indicated 100 ft Figure 7 Plan view of roof geology at the longwall demonstration site in mapped area A. 10 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 2*s lilLKlI'M O "co *. o II II £ E INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 □a 2? AS U. CO ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 O ■£ £ o INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 upper and lower portions differ slightly (fig. lib). Field exposures and preliminary chemical analyses suggest that the lower portion of the Anna Shale is missing locally due to nondeposition. In some small areas no Anna was deposited and Brereton Limestone, the next higher unit, became the immediate roof of the coal. The Brereton Limestone overlies the Anna Shale and the coal where the Anna Shale thins to zero. It is a gray, argillaceous, fossil iferous, fine-grained limestone, ranging in thickness from about 4 to 6 feet. Coal The Herri n (No. 6) Coal in the demonstration area is of high volatile B bituminous rank. Based on six face-channel samples from this mine, the coal averages 7.8 percent moisture, 36.2 percent volatile matter, 46.2 percent fixed carbon, 9.9 percent ash, 2.6 percent total sulfur, and about 11,900 Btu/lb on an as-received basis. The seam dips at only 17 feet per mile to the northeast. The cleat in the coal is poorly developed with a face cleat that strikes N 35° W, while the butt cleat varies between N 35° E and N 58° E. Both cleat surfaces have some localized calcite coatings. The coal seam ranges from 7.2 to 8.7 feet thick. Measurements of the coal seam thickness show a bimodal distribution (fig. 12) with two popu- lations corresponding to the two major roof types. The thicker coal occurs under the gray shale roof whereas the thinner coal is under the black shale roof. On the average, coal below the black shale roof is nearly three-fourths foot thinner, with the thinnest part located near the center of the channel -like black shale roof exposures. The thinning of the coal is related to the erosional cutout of the Energy Shale (fig. 8). This was confirmed by petrographic study of a series of full -seam columns traversing the linear Anna Shale/Brereton Limestone roof exposure in study area A (fig. 9). Johnson (1979), in a detailed petrographic study, recognized four distinct petrographic units within the seam along a 1,000-foot traverse (fig. 13). Significantly, the petrographic units are continuous except where the top unit is truncated or missing under Anna Shale roof. This loss of coal is attributed to erosion, and/or degradation due to oxidation of the peat (Johnson, 1979). Floor The floor immediately below the coal seam consists of an underclay vary- ing in thickness from 1 to 3 feet. The underclay usually contains many ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 Figure 11a Clay mineral compositions of the Energy Shale and roll material • Energy Shale O Roll material Kaohnite and chlor ite Expandable clay minerals Figure 11b Clay mineral analyses of the Anna Shale showing differences of clay mineral content between the upper and lower portions of the unit. Base of Anna Shale unit Top of Anna Shale unit Expandable clay minerals INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 ■o £ §60 feet. 20 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 slake and tend to fall out around roof bolts after mining. These falls usually end within the overlying Anna Shale and are fairly shallow. Coal balls Coal balls are masses of peat that have been permineral ized in place during or soon after peat deposition. Carbonate minerals were introduced into the tissues of the peat before the peat deposit was coalified, thus preserving the original plant materials. Identification of the plant mate- rials in the coal balls by T. L. Phillips of the University of Illinois (table 1) shows the typical peat composition for the Herri n (No. 6) Coal with strong dominance of lycopod trees (Phillips et al., 1977; Phillips, 1979) Coal balls are locally a serious obstacle to mining. While we now have a short-range ability to successfully predict these deposits, general pre- diction of coal-ball locations must await a more thorough understanding of their origin. A fresh, broken surface of these coal balls has a light brown color which darkens only slightly with oxidation. Size and shape of coal balls vary considerably. In general, coal balls are less than 1 inch to more than 3 feet wide and are often elongated horizontally. Smaller coal balls (fist-sized and smaller) are usually spherical or slightly ellipsoidal in shape. Medium-sized coal balls (up to 1.5 ft long) typically have height- width ratios from 1:3 to 1:6. Where coal ball material exceeds 60 to 70 percent of the seam, the coal balls have varied shapes and range from 1 to 4 feet thick. Clarification of the spatial distribution of the coal balls was accom- plished by mapping coal balls both on pillar ribs and in the roof. When Table 1. Identified plant material by plant group from coal ball vertical section 3 in Area L (T. L. Phillips, personal communication) Lycopods 76.8 Ferns (primarily tree ferns) 12.6 Pteridosperms 7.6 Sphenopsids 2.9 Cordaites .1 100.0 INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 21 coal balls were encountered, special attention was paid to both their association with roof lithology and their density of occurrence. Coal balls occurred individually, in clusters, and in concentrated groups defined as "coal -ball areas," ranging from about 20 feet to more than 150 feet across. The distinction between a cluster and a coal -ball area was made both on the size of the accumulation and the concentration of coal balls. In the absence of a natural boundary, coal balls more than 5 feet apart were considered to be outside of a coal -ball area. The large coal-ball areas with a dense accumulation of coal balls often show an increase in both size and number of coal balls toward a central point (fig. 16), where a core of highly concentrated coal balls may exist (fig. 17). At one point in area L (fig. 18), only 0.25 feet of coal remained in a 10-foot vertical section of coal balls. There is a strong association between the areas of black shale/limestone roof and the distribution of coal balls (figs. 18 and 10). Coal-ball areas B, H, I, and N partially span the boundary between Anna Shale and Energy Shale roof. A stratigraphic approach to coal -ball distribution shows two distinct patterns in the longwall demonstration area. Coal balls near the top of the seam are by far more widespread than those in lower positions in the seam. The mid- and low-seam coal balls are almost exclusively found within coal-ball areas. Where these two patterns of distribution coin- cide, it is not yet clear whether they are independent distribution or are simply preferred areas of permineralization. The origins of these coal balls are not fully resolved. Stratigraphic evidence based on the close association of coal balls with black shale/ limestone roof suggests that mineralization began after the initial ero- sion of the Energy Shale. Mineralization was also probably complete when coal balls with a normal peat matrix were redeposited within the fossil- iferous gray shale found in rolls. Preliminary carbon isotope data from these coal balls (T. F. Anderson, University of Illinois, personal com- munication, 1981) suggest the dominance of freshwater carbonates in their formation. The study of other coal-ball sites in the Herrin (No. 6) Coal should help resolve some of the questions raised at Old Ben No. 24. Within Franklin County there are several other coal-ball sites following the pattern of coal balls under the black Anna Shale roof (fig. 7). These sites are known only from Illinois State Geological Survey mine notes and cannot be mapped or sampled to check their similarity to conditions at Old Ben No. 24. The thickness of the coal seam in the coal-ball areas is greatly increased because the permineralization preserved the volume of the peat from later compaction. Therefore, coal balls provide evidence on the original ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 **%m+ ■ i Figure 16. Concentrated coal balls near the center of coal -ball area L. Strings form a box 0.5 meters on a side. '. Edge of coal -ball area Dense coal balls mm 100 ft \=n Figure 17_ Map of area M showing limits of the "core" of concentrated coal balls, and a minor alteration of mine plan due to concentrated coal balls. INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 23 Coal- ball Coal -ball area Energy Shale roof 1 Anna Shale/Brereton Limestone roof Figure 18 Coal balls, coal -ball areas, and roof lithologies identified during mapping of the 3 longwall panels. ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 thickness of peat at the time of mineralization. Peat-to-coal compac- tion ratios were derived by comparing the thickness of coal next to individual coal balls with the corresponding thickness of permineralized peat in the coal ball (Teichmiiller, 1955; Zaritsky, 1975). The coal balls of Old Ben No. 24 Mine show an average value of 5:1, which is similar to values found by Zaritsky for coal balls of similar age in the Donets Basin (USSR). Another set of measurements between stratigraphic markers in the seam gave a value of about 2.0:1 (fig. 19a). The coal immediately above and below each coal ball often shows extra compaction, and some slippage of coal (shown by slickensides) has also occurred (fig. 19b); these factors produce the lower compaction ratios in measured sections through multiple coal balls. MINING PROBLEMS Prediction of coal balls Problems with concentrations of coal balls occurred early in the develop- ment of the first longwall panel. In mining these areas, the continuous miners had to repeatedly receive new cutting bits due to excessive wear. The belt entry east of the first panel had to be offset to the east at coal-ball area B (fig. 18); and a smaller problem with coal-ball area F was encountered low in the seam on the west side of the first panel. As the first longwall panel was being mined, two large areas of coal -ball concentrations were encountered, areas D and E. The longwall shearer suffered damage as well as excessive wear, and advanced at the low rate of only 155 feet during 105 eight-hour shifts— 1.5 feet per shift com- pared to a normal 10 feet per shift. Explosives had to be used to break up the masses of coal balls in the core of each area. When mine development reached the west side of the second panel, coal- ball area L (fig. 18) was encountered. Because of these coal balls, this panel was shortened by 80 feet (fig. 7). Similar obstacles in area Q also required the third panel to be shortened by nearly 300 feet (fig. 7) Subsequently, the second and third panels had no serious mining problems due to coal balls, but because of their occurrence, approximately 50,000 tons of coal were lost within the two panels. Initial mapping showed that the coal balls were closely associated with black shale/limestone roof, and this predictive link between coal balls and roof type has proven reliable. Because of the many areas of black shale/limestone roof in the mine, the distinctiveness of the exposure crossing the longwall panels was not initially apparent. With the dis- covery in late 1979 of another area of concentrated coal balls more than a mile east of the longwall area associated with the same roof exposure INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 Figure 19 a. Five of the 13 positions where coal and coal balls were measured separately. Lower stratigraphic marker is the "blue band," which is indicated by the arrows. Measured positions are at .5 m intervals. b. Small compaction fault under round coal ball has displaced thin coal ball about 1 cm. (Bar scale is 1 cm.) 26 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 (fig. 10), general prediction of coal- ball occurrences can now be made with some assurance. In general, concentrated coal-ball areas are associated with a continuous sublinear area of Anna Shale/Brereton Lime- stone roof trending N 60° W. Roof stability The variable conditions encountered at the face were a good test of the longwall mining procedure. The face advanced through a number of geolog- ically difficult areas: (1) a massive 130-foot diameter coal -ball deposit; (2) one of the largest rolls documented in Old Ben No. 24 Mine, which was 600 feet long and cut to a maximum of 4 feet into the coal; (3) an unstable, slipped, and fractured roof area 2 to 3 feet high covering an area about 150 by 50 feet over a large roll in the first panel; and (4) a fault zone which ran the entire length of the second panel (perpendicular to the advancing face) with vertical displacement as great as 7.5 feet. Nowhere was roof stability serious enough to stop the advance of the longwall face. Roof control at the longwall face is good, especially if the shield supports are promptly advanced as soon as a cut is made by the shearer. The stability of the face can be a problem when faults, low angle slips, or roof lithology boundaries parallel the face. Several areas of slips associated with roof lithology boundaries paralleled the longwall face. During cutting of the coal at these areas, pieces of roof would fall before the shields could be advanced. After advance of the shields many small pieces would fall from the gaps in front of and between the shields near the face. Neither amount was great enough to hinder any operation of the longwall. Only the roof area just in front of the shields in the tailgate entry posed a continuing problem. Here pieces of the roof shale would pile up on the floor and have to be removed in order to advance the shield and pan assembly. A possible solution to this problem would be to bolt wire mesh to the roof to retain the small pieces. The greatest roof-control problem for the longwall operation exists in the support entries (for men, materials, ventilation, and belts). In the Anna Shale, minor problems in entry ways are associated with rolls, coal balls, and roof concretions. The top surface of a roll body of shale is often slickensided and may also have a thin layer of coal between the body of the roll and the roof shale. In both cases a plane of weakness is created from which the body of the roll can pull away from the roof. Coal balls in top coal left as roof as well as concretions in the Anna Shale are hazards due to slickensides that have developed around these concretions, allowing them to fall. INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 Within the Energy Shale, thin stringers of coal commonly extend from the top of the coal into the shale and form planes of weakness in the roof. Slips and slickensided surfaces associated with the steeply dipping trun- cated upper surface of the Energy Shale often cause serious problems in roof control . The contact between the top of the gray Energy Shale and the overlying black Anna Shale is sharp and occasionally slickensided, particularly where the contact dips (fig. 20). Also, slips in the Energy Shale are common within 150 to 200 feet from the boundary of the black shale roof; generally they parallel the gray shale/black shale boundary (fig. 21). Intersections with such slips become particularly vulnerable to failure (fig. 22). Two factors possibly contributing to the falls in figures 20 and 22 include (1) the belt entry that was cut wider than the rest of the entries, and (2) that they were within 150 feet of the north-south fault system in the second longwall panel. The north-south fault system in the second longwall panel (fig. 7) with a range in vertical throw from 3.0 to 7.5 feet was only a minor problem for the longwall process. The face was graded across the fault, and only small parts of the roof fell out from the footwall side of the fault. This fault system had some large horizontal east-west compressive stresses in the downthrown block, which locally produced bad roof conditions (fig. 23) and slowed development of the entries for the second longwall panel. These entries parallel the fault system. The closest entry, only 30 feet west of the fault system, had the most roof-control problems with large blocks falling out during entry development (fig. 23). In one short section, con- centrated horizontal stress was relieved in the form of sizable mine bumps. The roof in the area of the bumps had a vertical, thin, crushed zone of roof shale produced by failure of the shale in east-west compression. Other mines in the area have also experienced roof problems caused by east- west compression. One mine in the general area has experimented with cut- ting slots in the roof of some north-south entries. The slots have been observed to close halfway in about a week. Additional data from hydro- fracting in the Mississippian strata below the Pennsylvanian shows that southern Illinois is under horizontal compression in an east-west direc- tion (Lindner and Halpern, 1978; Zoback and Zoback, 1980). For mapped area B shown in figure 10, the number of fallen intersections has been tabulated as a function of lithology and the presence of struc- tures that may influence falls of the immediate roof (table 2). Only roof falls where more than one foot of rock had come down were included. Tables 2 and 3 show that mine entries with an Energy Shale roof have roof- control problems. This same study area displays the north-south roof fall problem (fig. 24) associated with other mines in the region; similar roof- control problems in the region have been documented over the past 60 years in the Illinois State Geological Survey mine notes. 28 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 Figure 20. Plan view (above) and cross section of roof fall in relation to mine and geologic setting. INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 30 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 Figure 22. Roof fall in relation to mine plan and geologic setting (plan view above and cross section below). INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 31 Figure 23. North-south fractured zone in roof caused by large horizontal compressive forces. 32 ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 O 1 c IB 4-1 O o °1 «- to o oJ C *J .£ OJ a. 5 <= O) c .E E S o E O C f I £ . QJ to c 4- 0) c .2 5 < V o ^ □ _ o .2 E I? a % • 3 C QJ § Id 5 % en „_ > o o> a) B* 3 .i§| u. cc < INVESTIGATION OF ROOF AND FLOOR STRATA: OLD BEN MINE NO. 24 Table 2. Occurrence of fallen intersections for mapped area B by categories of roof lithology and structure Energy Sh. roof Anna Sh./Brereton Ls. roof Falls e _,. Falls Total no. of Sample size No. Sample size No. % samples Intersections with slips* Total intersections 15 Intersections without slips* 293 41 308 47 6 21 14 191 197 484 505 * Slips are small faults, most of which are of nontectonic origin. Table 3. Number of mapped falls by location for mapped area B No. Falls at intersections 41 85 Falls in entries between intersections Total falls 7 48 15 100 Subsidence Subsidence of the surface over the first two longwall panels was period- ically monitored by Old Ben Coal Company personnel. Conroy (1978), Curth and Cavinder (1977), and Wade and Conroy (1977) assumed a uniform mined-out void height of 7 feet to calculate the percentage of surface subsidence/mining height over the panels. However, the actual mined-out void heights measured during many traverses of the operating second ILLINOIS STATE GEOLOGICAL SURVEY CONTRACT REPORT 1982-2 longwall face vary between 7.2 and 8.25 feet. This introduces a 3 to 17 percent error in calculating the ratio of subsidence/mining height if a 7-foot mining height is used. Even though the coal thickness ranges from 7.2 to 8.6 feet, the variability in mined-out void heights can also be related to the roof rock being brought down by the longwall mining operation. The average mined-out void height in the area (not affected by the end constraints of the panel) is about 7.5 feet. These void measurements were made on the gob side of the face conveyor pan from the floor to the roof exposed between the shields. Using the maximum sur- face subsidence of 5.2 feet over the second longwall panel and dividing it by the 7.7-foot mining height found below the monument gives a ratio of .67 instead of .74 when using 7 feet as the mining height. Figure 25a represents the subsidence profile from the second longwall panel and the mined-out void height measurements below the profile. It shows that the profile is controlled by small variations in the mining height. In figure 25b, the subsidence profile for the first longwall panel shows that the greater amount of subsidence has taken place over the area with thicker coal in the panel. Maximum surface subsidence over the first panel was 4.72 feet, and over the second panel, 5.21 feet (sur- vey, December 4, 1978). It was found that the second panel had a 13 per- cent increase in maximum surface subsidence as compared to the first panel. The comparison was made when the faces had advanced equally from the surface monuments, which show the maximum subsidence of each panel. This difference may be attributable to the presence of a fault system that runs along the length of the second panel. Large differential move- ments can take place along faults (Lee, 1966) and may allow larger blocks of rock to cave, reducing bulking. The amount of subsidence usually increases when a longwall panel is mined next to another longwall panel, as compared to the smaller amount of subsidence produced over a single longwall panel in a virgin coal mining area. Physical strength test results The cores for these tes drilling from the mine icvei. m luiai ui au icei, ui cure wab rauveiKu and tested for its physical strength. Testing procedures, the raw data and core descriptions are in appendixes A and B. The cores for these tests were obtained from the roof of the mine by drilling from the mine level. 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