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IC 9116 
 
 Bureau of Mines Information Circular/1986 
 
 Thick-Seam Mining in the Western 
 United States— Geological 
 Considerations 
 
 By D. L. Boreck 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
^ CUn^ StDfe*. 3uLT€CUi_ of Mrr**) 
 
 Information Circular 9TI6 
 
 Thick-Seam Mining in the Western 
 United States— Geological 
 Considerations 
 
 By D. L. Boreck 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Donald Paul Hodel, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 

 <\0' 
 
 Library of Congress Cataloging in Publication Data: 
 
 Boreck, D. L. (Donna L.) 
 
 Thick-seam mining in the western 
 siderations. 
 
 United States - 
 
 -geological con- 
 
 (Information 
 
 circular ; 9116) 
 
 
 
 Bibliography 
 
 p. 17-18 
 
 
 
 Supt. of Docs 
 
 . no.: I 28.27: 9116. 
 
 
 
 1. Coal mines and mining- West (U.S.) 2. Coal- 
 Information circular (United States. Bureau of M 
 
 Geology-West (U.S.) I. Title. II. Series: 
 nes) ; 9116. 
 
 TN295.U4 
 
 [TN805.A5] 622 s [622'.334] 
 
 86-600298 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Occurrence of thick and split coal seams 2 
 
 Thick-seam mining methods 4 
 
 Geology as a scientific tool 5 
 
 Geologic factors affecting thick-seam development 5 
 
 Cleats 5 
 
 Joints and fractures 7 
 
 Coal quality 7 
 
 Petrography 8 
 
 Roof 1 i thology 8 
 
 Floor li thology 10 
 
 Thickness and three-dimensional configuration of coal seam 10 
 
 Continuity of the coal seam 11 
 
 Rolls 12 
 
 Spontaneous combustion 13 
 
 Gas 13 
 
 Water 15 
 
 Conclusions 17 
 
 References 17 
 
 ILLUSTRATIONS 
 
 1. Map of coal-bearing regions in Colorado, Utah, and Wyoming 2 
 
 2 . Two thick-seam mining methods 4 
 
 3. Generalized log of corehole showing depth and the degree of cleat develop- 
 
 ment throughout the core 6 
 
 4. Cross section through thick coal in the Danforth Hills Field, western 
 
 Colorado 10 
 
 5. Example of small fault 11 
 
 6. Structure map drawn on top of coal seam 12 
 
 7. Fire in strip operation in southern Colorado 14 
 
 TABLES 
 
 1. Summary of thick- and split-seam occurrences and development in Colorado, 
 
 Utah, and Wyoming 3 
 
 2. Known active or proposed underground mines working thick or split seams in 
 
 Colorado, Utah, and Wyoming ; 3 
 
 3. Importance of geologic factors for four thick-seam mining methods 16 
 

 UNIT OF MEASURE 
 
 ABBREVIATIONS 
 
 USED 
 
 IN THIS REPORT 
 
 cm 
 
 centimeter 
 
 ra 2 /d 
 
 
 square meter per day 
 
 ft 
 
 foot 
 
 pet 
 
 
 percent 
 
 in 
 
 inch 
 
 St 
 
 
 short ton 
 
 m 
 
 meter 
 
 std ft 3 
 
 
 standard cubic foot 
 
THICK-SEAM MINING IN THE WESTERN UNITED STATES- 
 GEOLOGICAL CONSIDERATIONS 
 
 By D. L. Boreck 1 
 
 ABSTRACT 
 
 Thick coal seams are common in the Western United States. Many seams 
 are over 50 ft thick (some are over 200 ft thick) and are too deep to 
 extract using surface methods. Currently, such deposits are developed 
 using standard "eastern" mining methods which only extract a few feet of 
 total seam thickness, often rendering the remaining coal unminable with 
 current technology. Novel methods that increase thick-seam recovery 
 have been developed and are currently being used in Europe. These 
 methods — high-face single-pass and multislice longwall, longwall caving, 
 and hydraulic mining — have great potential for use in the United States. 
 Successful use of these methods is an objective of the Bureau of Mines. 
 Their use requires, among other things, a full evaluation of geologic 
 features common to thick coals. The objective of this report is to pre- 
 sent and summarize those features that will affect the introduction of 
 the methods into thick-seam mines in the Western United States. 
 
 The geologic elements delineated are three-dimensional configuration 
 of the seam, cleat development, joints and fractures, roof and floor 
 lithology, and faulting. 
 
 Geologist, Denver Research Center, Bureau of Mines, Denver, CO. 
 
INTRODUCTION 
 
 Thick (which includes closely spaced) 
 coal seams are common in the Western 
 United States. The "thick coal reserve" 
 base is made up of seams that are more 
 than 15 ft thick. Also included in the 
 reserve base are seams split by from less 
 than 1 ft to greater than 30 ft . of in- 
 terburden. As the seam height commonly 
 mined in the United States is less than 
 10 ft, 45 to 95 pet or more of a de- 
 posit's original reserve may be lost un- 
 less improved methods are introduced into 
 western mines (_1_). 2 This western coal 
 has high export potential and is a major 
 source of royalty revenue. 
 
 The Bureau of Mines is evaluating the 
 feasibility of introducing European 
 thick-seam mining methods into the United 
 
 States. To predict the degree of success 
 of such an introduction, geologic, geome- 
 chanical, mining, economic, and safety 
 factors that will affect the thick-seam 
 methods are being examined. This report, 
 which was derived from an analysis of 
 published material, concentrates on de- 
 fining the geologic aspects of thick-seam 
 development. As one of several publica- 
 tions scheduled to be completed by the 
 Bureau on thick-seam mining technology, 
 it is a first step toward the identifica- 
 tion and development of appropriate min- 
 ing systems for such deposits. 
 
 The report addresses underground devel- 
 opment of thick-seam deposits and does 
 not cover surface mining methods or 
 techniques. 
 
 OCCURRENCE OF THICK AND SPLIT COAL SEAMS 
 
 Because of differing mining conditions 
 and methods across the world, a thick 
 seam is often defined as a seam whose 
 full thickness cannot be efficiently ex- 
 tracted using the available equipment. 
 In the Western United States, the maximum 
 extractable mining height is 14 ft. For 
 this report, a thick seam is defined as 
 any minable coal seam 15 ft high or 
 higher. Closely spaced or split seams 
 are also included in the report, as one 
 of the two seams is usually lost from the 
 reserve base due to subsidence of the up- 
 per undeveloped seam when present-day 
 mining practices are used. 
 
 Because of the manner in which coal re- 
 source figures have been compiled, it is 
 difficult to estimate the western re- 
 source base for coal seams greater than 
 15 ft thick or for minable seams split by 
 a parting. For this reason, the author 
 chose to look at "occurrences" in three 
 States instead. Colorado, Utah, and Wy- 
 oming (fig. 1) were evaluated for re- 
 ported occurrences of thick seams at 
 3,000 ft or less of overburden. Although 
 data available to the author were more 
 limited for Utah and Wyoming, both States 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 did have a substantial number of occur- 
 rences reported in publications summariz- 
 ing their coal resources. In Colorado, 
 the accessible data were more extensive, 
 consisting of over 1,500 records on inac- 
 tive coal mines in the State and ba- 
 sin analyses on major coal regions that 
 
 200 
 
 I 
 
 LEGEND 
 
 I I Coal-bearing region 
 
 QE Area known to contain minable thick 
 
 or very closely spaced coal seams 
 
 FIGURE 1.— Map of coal-bearing regions in Colorado, Utah, 
 and Wyoming; adapted from Trumbull (2). 
 
included listings of coal "calls" from 
 oil and gas logs. Given the foregoing 
 data density, the occurrence of thick 
 coal seams in Colorado and past develop- 
 ment in those seams were better charac- 
 terized for Colorado than for the other 
 two States. 
 
 Table 1 lists the coal regions in Colo- 
 rado that have reported occurrences and 
 past or present mining activity in thick 
 coal seams. The limited available infor- 
 mation for Utah and Wyoming is also in- 
 cluded. The numbers represent separate 
 occurrences spread over a large area. 
 In the past, any development at these 
 points often resulted in a loss of ton- 
 nage from the reserve base because of 
 incomplete extraction. Without a change 
 in the mining method, the same will be 
 true in the future. The mines given in 
 table 1 include both historic and modern 
 operations. 
 
 Table 2 lists active or proposed under- 
 ground mines in thick coal seams in the 
 the three States (3-4). It should be 
 
 noted that Wyoming is known for its thick 
 seams which, in places, exceed 200 ft. 
 In the past, many of the mines produc- 
 ing from thick seams were underground. 
 At present, only one active underground 
 mine is in a thick seam; the remaining 
 
 TABLE 1. - Summary of thick- and split- 
 seam occurrences and development in 
 Colorado, Utah, and Wyoming 
 
 Region or basin 
 
 Occurrences 
 
 Mines 
 
 Colorado: 
 
 6 
 
 68 
 
 16 
 
 121 
 
 3 
 
 
 9 
 15 
 
 Uinta 
 
 42 
 
 
 
 
 211 
 52 
 36 
 
 69 
 ND 
 ND 
 
 
 
 ND Not determined. 
 
 Available data 
 
 were inconclusive on the number 
 that have, in the past, worked 
 or split coal seams. 
 
 'Undifferentiated by region. 
 
 of mines 
 in thick 
 
 TABLE 2. - Known active or proposed underground mines working 
 thick or split seams in Colorado, Utah, and Wyoming (4) 
 
 Region 
 
 Mine 
 
 Seam 
 
 Thickness, 
 ft 
 
 Colorado: 
 
 Canon City 
 
 Green River 
 
 Do 
 
 Uinta 
 
 Do 
 
 Do 
 
 Do 
 
 Do 
 
 Do 
 
 Do 
 
 Do 
 
 Do 
 
 Utah: 
 
 Book Cliffs 
 
 Do 
 
 Do 
 
 Kaiparowits Plateau. . . 
 Wasatch Plateau 
 
 Do 
 
 Wyoming: 
 
 Hanna 
 
 'Multiple seams — minimum 
 
 Dorchester No. 1. 
 
 Eagle No. 5 
 
 Little Bear Creek 
 Coal Ridge No. 1. 
 
 Deserado 
 
 Dutch Creek No. 2 
 
 Fruita No. 1 
 
 King 
 
 McClane Canyon. . . 
 Munger Canyon. . . . 
 Orchard Valley... 
 Somerset 
 
 Price River Coal. 
 Soldier Creek. . . . 
 
 Sunnyside 
 
 John Henry 
 
 Skyline 
 
 Valley Camp 
 
 CCCC No. 1 
 
 Van guard No. 2 . . . 
 of 2. ^~3 seams. 
 
 Red Arrow and Dirby Jack' 
 
 F 
 
 Seymour 
 
 Wheeler 
 
 A, B, and C 1 
 
 Dutch Creek 
 
 Cameo' 
 
 B 
 
 Cameo 
 
 ...do 
 
 D 
 
 B 
 
 Castlegate' 
 
 Rock Canyon' 
 
 Sunnyside' 
 
 A and B 1 
 
 ' Conner 2 
 
 . . .do 
 
 No. 80 
 
 No. 50 and No. 51 
 
 5-6 
 
 8-22 
 
 15 
 
 42 
 
 0-17 
 
 4-20 
 
 10-25+ 
 
 16 
 
 17-22 
 
 5-26 
 
 4-27 
 
 14-21 
 
 5-12 
 13 
 4-12 
 6-18 
 4-24 
 0-24 
 
 20+ 
 4-22 
 
operations are surface mines. In the 
 future, as economic stripping ratios 
 are exceeded, the surface operations in 
 Wyoming may again move underground, 
 
 necessitating the incorporation of more 
 efficient extraction methods to develop 
 the thicker seams economically. 
 
 THICK-SEAM MINING METHODS 
 
 In the Bureau's research on thick-seam 
 mining, four methods are being investi- 
 gated for use in western thick-seam 
 mines: (1) high-face single-pass long- 
 wall, (2) multislice longwall, (3) long- 
 wall caving, and (4) hydraulic mining. 
 The following brief summaries of the 
 first three methods are derived from 
 Oitto (5). Data on hydraulic mining were 
 taken from information published by Kai- 
 ser Resources (6) . 
 
 The high-face single-pass longwall 
 method is set up similar to a standard 
 longwall, the main difference being ex- 
 traction height. While a standard face 
 may be 7 to 9 ft high, a high-face long- 
 wall extracts up to 18 ft of coal through 
 the use of specialized equipment. The 
 high-face longwall method was developed 
 for coals dipping less than 25°. 
 
 The second method, the multislice long- 
 wall method, mines thick seams in slices 
 or lifts (fig. Ik ) • Mining may be simul- 
 taneous (concurrent mining) with two or 
 more lifts being worked, or nonsimultane- 
 ous (nonconcurrent mining) where all pan- 
 els in the upper lift are mined first, 
 one at a time, before proceeding to a 
 lower lift. This method is practical in 
 coals from 14 ft to greater than 100 ft 
 thick. The method is especially adapta- 
 ble to thick seams that contain partings 
 or other impurities that make part of the 
 seam uneconomic. Seams separated by less 
 than 30 ft of interburden will probably 
 require multislice mining methods. 
 
 Longwall caving is a hybrid that com- 
 bines traditional longwall mining and 
 sublevel caving mining methods. In this 
 method, a 7- to 10-ft slice is made at 
 the base of the coal seam, and the re- 
 maining upper part of the seam is allowed 
 to cave downward (fig. 2B). The caved 
 coal is drawn through the shields by us- 
 ing gates or chutes set on the gob side 
 of the shields and transported from the 
 longwall face. The method was developed 
 
 for coal seams 18 ft thick or greater. 
 In very thick coal seams , longwall caving 
 can be done in successive 30-ft slices. 
 
 Hydraulic raining is a method used to 
 extract thick, dipping (greater than 4°) 
 coal seams. This system extracts coal 
 from retreat panels using hydraulic moni- 
 tors which discharge water at high pres- 
 sure from nozzles. Coal dislodged by the 
 water is washed into feeder-breakers and 
 then into an inclined flume for gravity 
 transport. Advantages of this method may 
 include mechanical simplicity, dust con- 
 trol, and safety (due to removal of work- 
 ers from the face areas since the con- 
 trols are often located 20 to 40 ft outby 
 the face). 
 
 ,4, Multislice 
 
 
 
 
 mm 
 
 J o 
 
 Waste® 
 
 B, Longwall caving 
 
 FIGURE 2.— Two thick-seam mining methods. 
 
GEOLOGY AS A SCIENTIFIC TOOL 
 
 Many features that affect the develop- 
 ment of a mining property are geologic in 
 nature. The presence of these features 
 can be delineated by a thorough geologic 
 analysis of the property prior to devel- 
 opment. Due to geology's interpretive 
 power, it is an essential tool in setting 
 up a mine property; however, this tool is 
 not often used to its fullest capacities 
 during mine development. 
 
 The geologic analysis of a coal prop- 
 erty for development begins during the 
 first stages of an exploration program. 
 During exploration, the structure and 
 stratigraphy are determined for an area. 
 This often includes a widespread evalua- 
 tion of a part of the coalfield to pick 
 up general trends, followed by site- 
 specific studies of the properties of 
 interest. The exploration phase deter- 
 mines the location, continuity, thick- 
 ness, change in thickness, and quality 
 of prospective coal mining targets. Mul- 
 tiple methods of obtaining data are 
 used — surface mapping, aerial photogra- 
 phy, drill and coreholes — all of which 
 help to give a three-dimensional view of 
 the subsurface. Other methods, such as 
 
 seismic, gravimetric, magnetic, and other 
 surveys, have their place in filling in 
 the unknowns in this subsurface view. 
 It is during exploration that the first 
 idea is formed of what geologic hazards a 
 thick-seam mine will face during its 
 development; although these hazards may 
 not be fully realized until mining is 
 underway. 
 
 After development begins, the geologist 
 can obtain data from another point of 
 reference — inside the developing mine. 
 In-mine mapping and subsurface coring al- 
 low the geologist to fill in information 
 and further verify trends and features 
 seen in exploration. 
 
 The following section describes factors 
 that need to be examined and, as far as 
 possible, quantified. Many of these fac- 
 tors are affecting thick-seam development 
 on the international level and will prob- 
 ably affect the success of thick-seam op- 
 erations in the Western United States. 
 The process of subsidence is important in 
 thick-seam mining, but it is not within 
 the scope of the paper to discuss this 
 topic. 
 
 GEOLOGIC FACTORS AFFECTING THICK-SEAM DEVELOPMENT 
 
 The geologic factors affecting thick- 
 seam development are often the same fac- 
 tors that affect standard longwall devel- 
 opment. In thick-seam mining their dele- 
 terious effects may be magnified owing to 
 a greater face height. 
 
 CLEATS 
 
 Cleats are essentially fractures in 
 coal. They are usually present in a set 
 of two: the face or primary cleat, and 
 the butt or secondary cleat. The two 
 cleats are usually 90° to each other, but 
 this angle may increase or decrease, due 
 possibly to deformation of the seam. 
 Cleat surfaces are usually smooth and 
 planar, yet like the surfaces of frac- 
 tures of joints, they can also be rough. 
 Mineralization is common along the 
 cleats. 
 
 In the past, the presence of cleats was 
 essential to mine development. The coal 
 was mined either parallel or perpendicu- 
 lar to the cleat, so that it broke away 
 from the face during mining (7^). Contin- 
 uous and longwall mining methods , when 
 they were introduced, allowed for effi- 
 cient extraction of coal independent of 
 the cleat orientation to the coal face. 
 Nevertheless, the cleat and its orienta- 
 tion and development are still important 
 factors to consider in mine layout, es- 
 pecially in thick-seam mining owing to 
 the increase in mining height and surface 
 area at the face. The characteristic 
 that will have the greatest effect on 
 thick-seam mining is cleat development, 
 or, the degree of formation of a cleat 
 surface, which is dependent on many fac- 
 tors: (1) the maceral makeup of the coal 
 
(.8), (2) the degree of coalif ication, and 
 (3) the content and composition of ash 
 within the coal (9^» 
 
 In a coal seam, the development of the 
 face and butt cleats varies. The face 
 cleat is, by standard definition, the 
 more continuous, well-developed, fracture 
 that cuts across bedding planes within 
 the coal. The butt cleat may or may not 
 be as well developed as the face cleat. 
 The degree of development of both sur- 
 faces has a direct effect on thick-seam 
 extraction in three major areas: 
 
 1 . Cleat development in the coal de- 
 fines the macropermeability or fracture 
 permeability of the seam. The variable 
 development of the face and butt cleats- 
 causes the coal to be anisotropic in 
 its transmission of water and gas. This 
 would have a direct effect on their move- 
 ment at the face and in the panel(s) be- 
 ing mined. 
 
 2. According to Vaninetti (10) , "If 
 cleats are prominent, then strata pres- 
 sure on the coal face often results in 
 slabbing." This slabbing causes jams at 
 the face conveyor and stage loader trans- 
 fer points (5 ) , which increases downtime 
 and decreases productivity. Also, the 
 higher the face or rib, the larger the 
 slab will be, which in turn increases the 
 hazard to miners in the working sec- 
 tion. Vaninetti (10) also noted that, 
 due to slabbing, the stability of entries 
 and faces can be reduced. In the same 
 line, Bottrill noted (11) that faces at 
 right angles to the cleat show a greater 
 tendency to burst conditions than faces 
 parallel to the cleat, where constant 
 "spalling" of the coal relieves the 
 situation. 
 
 3. In longwall caving, the development 
 of cleats is critical, as their develop- 
 ment coupled with their density can be a 
 determinant of size of caved material 
 dropped through the caving chutes. 
 
 Just as cleat development is important, 
 changes in the development of the face 
 and butt cleat through the seam can 
 determine the success of a mining method, 
 especially longwall caving. Cleat devel- 
 opment may vary due to changes in maceral 
 composition, the presence of partings, or 
 possibly the deformational history of the 
 seam. As an example, cleat development 
 
 was logged on the core of a 202-ft sub- 
 bituminous coal seam (fig. 3). The data 
 (12) showed a change in cleat development 
 down section in the Anderson deposit; 
 there was poor to no cleat development in 
 the upper 69 ft, increasing downward to 
 poorly to well developed cleats in the 
 
 CORE OF 
 
 ANDERSON 
 
 DEPOSIT 
 
 r- 1030 
 
 - 1,100 
 
 - 1,050 
 
 CLEAT DEVELOPMENT 
 
 FACE BUTT 
 
 - 1,150 
 
 -1,200 
 
 1,250 
 
 FIGURE 3.— Generalized log of corehole showing depth and 
 degree of cleat development throughout the core. Partings 
 designated by dashes. For cleat development, N = none, 
 P = poor, M = moderate, W = well. Adapted from Boreck {12). 
 
lower 133 ft of the seam. Oitto (5_) 
 noted that longwall caving was suitable 
 for friable coal that caves naturally by 
 gravity. Given the above example, the 
 upper 69 ft of the seam may not cave 
 spontaneously during caving unless frac- 
 tures other than the cleats are present. 
 This would necessitate the use of explo- 
 sives or other methods to induce caving 
 and decrease block size of the caved ma- 
 terial. On the other hand, a thick coal 
 with well-developed cleat sets near the 
 top may be more prone to cave spontane- 
 ously. For this reason, it is important 
 to determine not only cleat development 
 but also changes in both the development 
 and density through the seam. 
 
 JOINTS AND FRACTURES 
 
 Joints are similar to cleats in that 
 they are vertical to near-vertical frac- 
 ture sets. They can occur singly; but 
 are more commonly found in two or more 
 sets, distinguishable from each other by 
 dissimilar orientations. The difference 
 between joints and cleats is that joints 
 are found predominately in rock. 
 
 The main characteristics of joints that 
 will affect thick-seam mining are devel- 
 opment, orientation, and spacing. Well- 
 developed joints, depending on their con- 
 tinuity, act as vertical zones of 
 Weakness at which failure can occur. 
 Their presence can be detrimental, where 
 roof joints at the face allow for pre- 
 mature movement in response to stress 
 on the longwall panel, or beneficial, 
 where joints allow a strong roof to cave 
 easily. 
 
 The joint orientation is as important 
 in thick-seam mining as it is in stan- 
 dard mining. The difference between 
 joint orientation and the orientation of 
 the longwall face and gate entries will, 
 in part, determine the stability of the 
 roof and its cavability. The orienta- 
 tions of surface joint sets have been 
 used to predict the orientation of sub- 
 surface joints, as have lineation studies 
 of the property and analysis of major 
 structural trends in the area. The pos- 
 sibility of error in utilizing these 
 methods varies with the property and its 
 geologic history. 
 
 The joint spacing is important in that 
 it, in part, determines the competency of 
 the roof rock and the block size of the 
 caved material. The spacing can vary 
 substantially, causing changes in roof 
 conditions across a panel. Joint spacing 
 in the subsurface often cannot be effec- 
 tively predicted unless the roof rock is 
 exposed and mappable within the mine. 
 
 Joints can be open or closed, have 
 smooth or rough surfaces, and be filled 
 with minerals such as calcite, pyrite, 
 gypsum, or clay. These characteristics, 
 in part, determine the degree of weaken- 
 ing of roof rock above the seam. Joints 
 act as conduits for water and gas enter- 
 ing the mine workings. 
 
 Fractures are breaks within the coal 
 and surrounding strata that are not di- 
 rectly related to either cleat or joint 
 sets. For this report, breaks both with 
 movement on the plane of separation 
 (slickensides) and without such movement 
 will be considered as fractures. Frac- 
 tures that displace the coal (faults) 
 are considered under another heading. 
 
 Fractures can result from tectonic ac- 
 tivity or postdepositional compaction. 
 They may or may not be vertical. The 
 fracture planes can be flat or curved and 
 can have variable orientations. Frac- 
 tures, like joints, can be open or 
 closed, have smooth or rough surfaces, 
 and be mineralized. They may also act as 
 conduits for water and gas. 
 
 The effects of high-angle fractures are 
 similar to those of joints and cleats. 
 Low-angle fractures, because of the shal- 
 low dip, can separate prematurely during 
 the mining process. This can be a crit- 
 ical problem in hydraulic mining (13) . 
 Individual fractures are often localized 
 discontinuous feaures, although a group 
 or zone of fracturing may extend for a 
 long distance, as in differential compac- 
 tion features (slickensides or slips) 
 that may parallel a sandstone channel. 
 
 COAL QUALITY 
 
 Coal quality is one of the most subtle, 
 yet important, controls on development of 
 a thick-seam mine. In the past, quality 
 helped determine which part of the seam 
 was mined. High ash or sulfur zones 
 
ruled out mining parts of a thick seam or 
 one of the two closely spaced coal seams. 
 Changes in quality through a seam may 
 determine the mining method. A thick 
 seam containing partings or bone near the 
 center is amenable to development using 
 a multislice longwall setup, especially 
 where mining the parting or bone in- 
 creases the ash content above acceptable 
 limits. 
 
 One example of the control of quality 
 is the Rienau property in the Danforth 
 Hills Coalfield, western Colorado. The 
 seam worked on the property is the Rienau 
 Bed. The seam reportedly ranges from 11 
 to 24 ft thick and dips at 18° ( 1_4 ) . The 
 thick-seam property was first developed 
 in 1928 and has been worked sporadically 
 since that time. The mine was originally 
 designed to accommodate a bone split 
 (parting) that occurred two-thirds of the 
 way up the seam. Rooms were driven at 
 the top and bottom of the coal, avoiding 
 the split. Given these conditions, the 
 multislice method may be the most effec- 
 tive means of increasing recovery in 
 mines similar to Rienau property, while 
 at the same time keeping the product's 
 quality high. 
 
 PETROGRAPHY 
 
 combust. Bacharach (15) reported that 
 petrographic consituents differed in 
 their tendencies to combust. The exinite 
 maceral was reported as having a higher 
 oxidation rate than the vitrinite or in- 
 ertinite macerals, while fusain was the 
 least reactive. The maceral content of 
 the coal and the relative abundance of 
 the more reactive macerals help determine 
 the liability of a seam or parts of a 
 seam to spontaneous combustion and help 
 explain why some seams will catch on fire 
 while others do not. 
 
 Also of major importance is the maceral 
 makeup's effect on the mining of a thick 
 seam. An example of this condition was 
 given by Ahcan (1_6) in his description of 
 a longwall operation producing lignite in 
 Yugoslavia. Ahcan noted that, for the 
 lignite, breakage often decreased and 
 blasting of the coal was necessary. One 
 reason given for the coal's tenacity was 
 the presence of xylite intercalations in 
 the lignite. Xylite, macroscopic bands 
 within brown coals derived from stumps 
 and stems that have been vitrified (17) , 
 increased the lignite's strength by ap- 
 proximately 15 pet. This increase, along 
 with other factors, resulted in a higher 
 consumption of explosives to stimulate 
 caving. 
 
 The petrographic composition of the 
 coal also plays a subtle but important 
 role in thick-seam mining. The maceral 
 type and percentage can affect cleat 
 development. Coals rich in a certain 
 type of maceral will have a substantially 
 different cleat density than coals rich 
 in other macerals. Vitrinite bands in a 
 bright coal commonly have a higher cleat 
 density than the constituents of a dull 
 coal. Dull coal (composed of fine- 
 grained exinite, inertinite, vitrinite, 
 and fine-grained disseminated mineral 
 matter) tends to be hard, compact, and 
 blocky (8). Cleat density, in turn, 
 helps determine the friability of the 
 coal, its tendency to cave (effects of 
 block size), and its permeability to 
 gases and water. 
 
 The petrographic makeup of the coal has 
 a subtle, yet important effect on the 
 tendency of the coal to spontaneously 
 
 ROOF LITHOLOGY 
 
 Lithology of the roof is an essential 
 consideration for any mining method. In 
 planning a thick-seam mine, an analysis 
 of the roof lithology should contain in- 
 formation in three major areas: (1) a 
 description of the rock type or types 
 present in the roof, (2) documentation of 
 vertical changes in the roof (bedding, 
 low-angle planes of separation, rooted 
 zones, and abrupt horizontal or low-angle 
 changes in lithology), and (3) evaluation 
 of lateral changes in rock type, the na- 
 ture of the contacts between different 
 roof rock, and the presence, orientation, 
 and development of high-angle fractures 
 and joints, over the panels to be mined. 
 
 The roof rock in western coal mines can 
 often be highly variable. Roof lithology 
 is determined by the type of sedi- 
 ments deposited under changeable energy 
 
regimes involving the termination (often 
 followed by the reestablishment) of peat 
 deposition. Some of the most common 
 western rock types and their effects on 
 mining follow: 
 
 Sandstone . — A massive sandstone roof 
 is, in most cases, the strongest, most 
 competent roof type in western coal 
 mines. It is a good roof for development 
 mining and for main entries that need to 
 stay open for transport of coal, equip- 
 ment, and personnel. However, for most 
 other phases of thick-seam longwall min- 
 ing, the sandstone roof makes a poor 
 roof. 
 
 Due to their competency, sandstones, as 
 a rule, do not cave well in longwall or 
 retreat mining. Ghose (18) reported on a 
 mine using thick-seam single-pass long- 
 wall development in Czechslovakia. The 
 immediate roof in the mine was made up of 
 laminated sandstone with alternating con- 
 glomerate beds. The roof type was char- 
 acterized by difficult caving and a lia- 
 bility for bumps. Khanna (19) noted in 
 his study that the caving of an immediate 
 sandstone roof was fraught with dangers 
 such as crushing, collapse of pillars, 
 and airblasts due to delayed caving of 
 the competent roof. 
 
 The sandstone body may also be quite 
 variable internally. It can be thick, 
 strong, moderately homogeneous, and rel- 
 atively massive. Or the sandstone can be 
 weakened by poor cementation, the pres- 
 ence of shale lenses or coal stringers, 
 pebble lag, well-developed crossbedding, 
 or thin flaggy sandstone beds, often 
 found at the base of the sandstone. Ow- 
 ing to their relatively high permeabil- 
 ity, sandstones are potential aquifers 
 and both store and transmit water (and 
 gas). Even if the sandstone is not in 
 direct contact with the mined coal, frac- 
 tures in the roof can bring water and 
 gases into the mine. 
 
 Shale . — Shale is a laminated sediment 
 made up predominantly of clay. It is of- 
 ten characterized by fissility. A shale 
 roof can be either a good or bad roof in 
 all mining methods. Thick sequences of 
 shale without fractures, slickensides, or 
 organic debris can make a good roof. It 
 caves easily — an advantage in longwall 
 
 as a 
 Be- 
 
 mining. Major problems with shale are 
 its fissility and tendency to slake when 
 exposed to humidity. 
 
 Claystone . — Claystone is defined 
 rock consisting of indurated clay, 
 cause of the high clay content, the rock 
 tends to soften and decompose in the 
 presence of excess water. Moebs (20) 
 noted that claystone makes poor roof when 
 it is massive and unlaminated. 
 
 At least one thick-seam mine has re- 
 portedly experienced problems with the 
 presence of claystone in the mined sec- 
 tion. Ahcan (21) reported that, in the 
 Kreka Lignite Basin, the longwall face 
 mining methods were not successful be- 
 cause the lignite seams, ranging up to 
 10 m thick, were "inbedded into very soft 
 clay layers." 
 
 To evaluate the cavability of the roof, 
 researchers have recommended analyzing 
 the properties of the roof rock up to 
 four times the thickness of the seam to 
 be mined (22). The geologic considera- 
 tions are not only the type of roof rock, 
 but changes in the rock vertically above 
 the seam and laterally across the panels. 
 The thickness of individual layers or 
 beds making up the roof strata should 
 also be included. Figure 4 is a north- 
 south cross section through a thick coal 
 in western Colorado. The section shows a 
 two-dimensional representation of litho- 
 logic changes vertically and laterally in 
 the roof, coal (including partings), and 
 floor. Vertically, the lithology of the 
 roof is variable. Horizontal zones or 
 planes of weakness (H) occurring along 
 bedding planes, contacts between differ- 
 ent lithologies (as in coreholes A-D) , 
 rider seams and coal stringers (core- 
 holes A, C, D) , rooted zones, and layers 
 of carbonized plant material (coreholes 
 A, C) are common throughout the section. 
 
 Workings in the center of the seam 
 would be subject to roof failure owing 
 to the presence of partings in the up- 
 per part of the seam. Rider coals and 
 stringers in the roof, besides becoming 
 zones of weakness, will also contribute 
 to the total gas emission that must be 
 handled in gassy thick-seam mines. 
 
 The lateral lithologic changes in fig- 
 ure 4 are as variable as the changes 
 
10 
 
 South 
 
 North 
 
 H=lO 
 
 Scale, ft 
 
 FIGURE 4.— Cross section (coreholes A-D) through thick coal in the Danforth Hills Field, western Colorado. H = horizontal 
 planes of weakness; V = vertical planes of weakness. 
 
 through a vertical section. For this ex- 
 ample, low- to medium-angle fractures 
 (V), although not evident from the logs, 
 are also hypothesized to be present. 
 Thick sandstone would possibly make a 
 competent roof in the area intersected by 
 section B. Yet, the roof adjacent to the 
 sandstone, because of the large number 
 of horizontal zones of weakness (H) and 
 postdepositional compactional features 
 (V) that often develop under and adjacent 
 to sandstone channels, may be unstable, 
 caving easily (possibly before it should 
 cave). 
 
 FLOOR LITHOLOGY 
 
 Evaluating the lithology of the floor 
 rock is often not emphasized as much as 
 evaluating the roof. However in a long- 
 wall operation, the floor must bear the 
 weight of the shields and their load, 
 and its lithology and three-dimensional 
 changes in lithology need to be evalu- 
 ated. The strength of the floor, the 
 
 reactivity of the immediate floor to 
 water or increases in humidity, and the 
 presence of gas-bearing coal seams or 
 gas- or water-bearing strata in the floor 
 must be determined. 
 
 THICKNESS AND THREE-DIMENSIONAL 
 CONFIGURATION OF COAL SEAM 
 
 The majority of western coal seams 
 are lenticular bodies. The seams can 
 vary significantly in thickness within a 
 single mining property. An example was 
 given by Ryer (23) . The thickness of the 
 I coalbed, of the Ferron Sandstone Mem- 
 ber, Emery Coalfield, Utah, shows a lat- 
 eral change of approximately 23 ft over 
 a distance of 1.2 miles. Less signifi- 
 cant changes can also be present in a 
 coal seam as a result of depositional 
 basin topography or postdepositional com- 
 paction. Depending on the seam thick- 
 ness, it is critical that changes in the 
 thickness across a property be deter- 
 mined prior to selection of a mining 
 
11 
 
 method. This ensures an adequate match 
 between equipment and thickness of avail- 
 able reserves. 
 
 CONTINUITY OF THE COAL SEAM 
 
 Continuity of the coal seam is related 
 in part to its configuration and in part 
 to features that displace or thin it as a 
 result of structural and depositional 
 controls. Three features that are impor- 
 tant in thick-seam mining are faulting, 
 washouts, and partings. 
 
 Faults cutting the coal and displacing 
 the mined seam cause the same problems in 
 thick-seam development as they cause in 
 standard mining: (1) displacement of the 
 coal seam, limiting reserves, (2) frac- 
 turing and weakening of the coal and 
 surrounding rock, and (3) migration path 
 for water and gas into the mine workings. 
 The effects may not be critical to long- 
 wall caving or hydraulic mining unless 
 the coal and roof are highly fractured 
 over a large area. The effects would be 
 more evident in multislice or single-pass 
 mining, where a slight displacement may 
 throw the panel off. If the fault was 
 active during deposition of the coal, the 
 area may be marked by an abrupt thinning 
 or thickening of the coal seam. In the 
 West, some faults may be located by sur- 
 face mapping and by using remote sensing 
 techniques. 
 
 Smaller faults (growth faults or slump 
 features) are more subtle (fig. 5) and 
 may not be detectable until they are 
 encountered in the mine. Their effects 
 would depend on the amount of strata dis- 
 placement and orientation of the faults 
 with relation to the longwall face. 
 
 Depositional features that hinder mine 
 development are similar in both thick- 
 seam and standard mining. As in fault- 
 ing, the percentage of the seam affected, 
 its orientation, and the lateral extent 
 of the features determine the effects 
 on face development. A feature related 
 to deposition is a washout (channel 
 erosion) . 
 
 Washouts, or wants, are defined as 
 areas where the coal has been eroded by a 
 channel above the coal seam, often re- 
 sulting in rapid thinning of the coal. 
 
 The coal seam may be partly or completely 
 cut out by the channel. The effects 
 on thick-seam mining would depend on 
 
 (1) the mining method being utilized, 
 
 (2) the initial thickness of the seam, 
 and (3) the percentage of the seam eroded 
 or missing. An unexpected decrease in 
 seam thickness may not bring a longwall 
 caving operation to a standstill, yet 
 the multislice and high-face single- 
 pass longwall methods would be seriously 
 affected. Channels may or may not be 
 located during preliminary exploration. 
 Areas believed to be affected by channels 
 require intense exploration to evaluate 
 the extent of the washouts. 
 
 The above two features are important to 
 thick-seam development. A third factor, 
 the presence of a parting or split in the 
 seam, is critical to thick-seam mining; 
 the parting alone may determine the min- 
 ing method used. 
 
 
 
 FIGURE 5.— Example of small fault (growth type). 
 Downthrown block to the right. 
 
12 
 
 Thick seams split into two or more 
 seams are common in the Western United 
 States. The parting occurs when deposi- 
 tional processes interrupt coal swamp 
 development. This usually affects one 
 part of the swamp. The swamp then rees- 
 tablishes itself on the newly deposited 
 sediments. As a result, the coal is 
 split into two or more seams separated by 
 from <1 ft to >30 ft of rock parting. 
 The depositional model helps determine 
 the thickness and aerial extent of the 
 coal seams and their interburden, charac- 
 teristics that are essential considera- 
 tions in determining the feasibility of 
 adopting any of the subject methods. 
 
 One of the methods most adaptable to 
 the above condition is the multislice 
 method. Incorporation of multislice min- 
 ing requires that substantial minable 
 reserves exist in both the upper and 
 lower seams. Both seams need to be 
 relatively continuous without appreciable 
 faulting or washouts. The lithologic 
 makeup of the parting is important in 
 that it is the floor of the upper lift 
 and the roof of the lower lift. 
 
 cannot be adequately predicted during the 
 exploration phase prior to mining. 
 
 Although the rolls studied in mines are 
 usually small-scale features in coals of 
 standard thickness, Law (24) presented 
 proof of large-scale compactional fea- 
 tures in the coal-bearing Fort Union and 
 Wasatch Formations. Law noted that the 
 structures were the result of differen- 
 tial compaction. Law also wrote that the 
 magnitude of the folds had been intensi- 
 fied by the unusually thick coal in the 
 section. 
 
 Similar features were noted by the 
 author in parts of the Anderson Deposit 
 (fig. 6). In figure 6, a domal structure 
 (shaded area) is present. The most ob- 
 vious problem would be an increase in 
 grade or dip of the seam, a problem that 
 could be dealt with in mine planning. A 
 second, less obvious problem could be the 
 effect of the feature on fluid flow 
 through the strata, causing a buildup of 
 
 ~l 
 
 ROLLS 
 
 Rolls, as they occur in the West, have 
 been defined by Vaninetti (10) as small- 
 scale folds in coal and enclosing strata 
 formed in response to differentially com- 
 pacted sandstones (or other lithologies) 
 pushed into the coal during compaction. 
 The effects are seen in local changes in 
 grade in the coal seam with possible 
 slight thinning or thickening of the coal 
 associated with the roll. In a thick 
 coal seam, the presence of the small- 
 scale rolls may not cause significant 
 problems during mining. But, in split 
 seams, the unexpected presence of rolls 
 can cause significant damage. Oitto (_5) 
 noted that multislice longwall mining 
 requires at least 14 ft of coal for two 
 longwalls to progress at different eleva- 
 tions without being forced out of their 
 horizons by undulations in the roof or 
 floor. The existence and trend of rolls 
 or other smaller scale features usually 
 
 ^ H 
 
 10,000 20,000 
 
 J 
 
 FIGURE 6.— Structure map drawn on top of coal seam in 
 part of the Anderson Deposit. Shaded portion denotes area of 
 structural interest. Contour interval is 100 ft. 
 
13 
 
 gas and water in different parts of the 
 structure. 
 
 SPONTANEOUS COMBUSTION 
 
 Spontaneous combustion is a critical 
 factor that needs to be considered in 
 western coal development; it has caused 
 and continues to cause a substantial 
 loss of minable reserves. Western thick 
 seams often have a high susceptibility 
 for spontaneous combustion. A main rea- 
 son for this susceptibility is the pres- 
 ence of coal remaining in the section 
 during or after mining. 
 
 The process of spontaneous combustion 
 is complicated, involving both geologic 
 (seam thickness or presence of multiple 
 seams, rank, sulfur content, petrography, 
 moisture, particle size, and caving char- 
 acteristics) and mining factors. Several 
 of the geologic factors that directly af- 
 fect ignition potential in western thick 
 seams are discussed below. 
 
 Seam thickness is an important consid- 
 eration in spontaneous combustion (15). 
 Where seam thickness is greater than the 
 mining height, excess coal left in the 
 workings has a high potential for igni- 
 tion. The thicker the coal, the greater 
 the volume of coal remaining in the mine 
 and the higher the risk is for spontane- 
 ous combustion. For split seams, Bach- 
 arach (15) noted that — 
 
 Where a multi-seam situation ex- 
 ists, both during the working of 
 the first seam or subsequent seams, 
 situations can arise with spontane- 
 ous combustion hazards for the seam 
 currently being worked and any 
 other seam above or below it. For 
 example, where a seam has been 
 worked with another unworked seam 
 underlying it, leakage paths can be 
 created into the lower seam, with 
 consequent risk of heating. In 
 other circumstances, where a seam 
 is worked under an overlying un- 
 worked or worked seam, the later 
 mining operations can result in 
 fire hazards in the upper seam. 
 Rank can be an important variable 
 in determining ignition susceptibility. 
 
 Although coals of any rank are subject to 
 spontaneous combustion, lower rank coals, 
 due to a higher tendency to oxidize, are 
 more prone to spontaneous Ignition than 
 the higher rank coals. Many of the thick 
 coals in the Western United States are 
 lower rank subbituminous coals. As a 
 result, fires in surface and underground 
 mines occur frequently (fig. 7). 
 
 Moisture content in the coal is consid- 
 ered to be an essential factor in sponta- 
 neous combustion in the West. Bacharach 
 (15) discussed how changes in moisture 
 content can lead to heating. The mois- 
 ture content of the unmined coal and its 
 surrounding environment is in a relative 
 state of equilibrium. With mining, an 
 increase in moisture content in the air 
 and a higher vapor pressure can lead to 
 absorption of water and a rise in temper- 
 ature in the coal. Dunrud (25) in his 
 studies on underground fires in abandoned 
 mines noted that fires may ignite by 
 spontaneous combustion where air and wa- 
 ter reach exposed coal through subsidence 
 cracks, pits, or open or poorly sealed 
 mine portals. 
 
 The area exposed to air and water is 
 another factor determining the liabil- 
 ity of the coal to ignite. Exposed sur- 
 face area increases with increased mining 
 height and friability of the coal. The 
 friability may be due to the petrographic 
 makeup of the coal and the presence of 
 faulting or intense fracturing in the 
 coal. 
 
 Other factors besides the above men- 
 tioned that affect the ignition potential 
 of the western coal seam will not be 
 covered. Some, like the geothermal gra- 
 dient, may be more important in some 
 areas (like Colorado) than others. Min- 
 ing factors such as mining method, rate 
 of advance, and mine layout also affect 
 the potential for sponteneous ignition to 
 take place (15). 
 
 GAS 
 
 Explanations as to how gases form and 
 migrate are complex, and gas continues to 
 be a main threat to miners working under- 
 ground. The gases commonly encountered 
 
14 
 
 
 FIGURE 7.— Fire in strip operation in southern Colorado. (Courtesy Colorado Geological Survey) 
 
 underground are CH 4 , C0 2 , CO, H 2 , N 2 , 
 and H 2 S. This section will discuss the 
 effects of CH 4 on thick-seam develop- 
 ment, although the other gases, naturally 
 occurring or otherwise, can have equally 
 deleterious effects. 
 
 Methane (CH 4 ) is a major constituent of 
 natural gas, occurring as a byproduct 
 during the decay and upgrading of carbon- 
 aceous material. Although the maximum 
 yield of gas from coal occurs in the 
 high-volatile A to low-volatile bitumi- 
 nous range, CH 4 can still be present in 
 lower rank coals. This point is critical 
 in the discussion of western thick seams 
 because many of these seams are low rank. 
 Reported occurrences of CH 4 in water 
 wells and gas problems or blowouts on 
 coal exploration rigs are common (26) ; 
 often the rigs are drilling at relatively 
 shallow depths. The gas is associated 
 not only with the coal, but also with 
 permeable sandstones within the coal- 
 bearing section. Core desorption from a 
 
 202-ft-thick seam (part of the Anderson 
 deposit) resulted in gas contents of 56 
 to 74 std ft 5 per short ton of in-place 
 coal. Although lower than expected for a 
 higher rank coal, the gas content coupled 
 with the size of the source and reservoir 
 can cause difficulties during development 
 of a thick-seam operation. 
 
 The presence of CH 4 (or any other gas) 
 in thick-seam mines can cause multiple 
 problems. One such problem was noted by 
 Bise ( 27 ): 
 
 In thick-seam workings, particu- 
 larly during the phase when the 
 whole seam is extracted, the wide 
 and high roadways create conditions 
 which may lead to low air veloci- 
 ties. In gassy seams, methane lay- 
 ering along the roof may result. 
 Another problem that can be encountered 
 in a thick gassy coal seam is a gas out- 
 burst. The outbursts can be caused by 
 CH 4 , C0 2 , or other gas under pressure 
 within the coal. 
 
15 
 
 The Bureau has been highly involved in 
 the study of methane and its genesis 
 and migration since the early 1970' s 
 (28). This research, along with previous 
 research done by the Bureau and other 
 organizations, has resulted in a substan- 
 tial number of tools that assist in de- 
 termining the quantity of gas, its qual- 
 ity, and controls on migration through 
 the coal and rock into the mine. The 
 tools are as usable on thick-seam prop- 
 erties as they are in seams less than 15 
 ft thick. The projected gassiness of the 
 mine can be determined utilizing standard 
 desorption methods (28). Probable migra- 
 tion paths can be delineated by an analy- 
 sis of orientation of cleats. In thick 
 seams, it is known that cleat development 
 may change throughout the seam's thick- 
 ness. To what extent this would affect 
 gas emissions in thick-seam mining is not 
 known. It is important to document all 
 gas sources and their content and the 
 degree of cleat development prior to mine 
 development. 
 
 WATER 
 
 Water is a significant concern in any 
 underground mining operation, and the 
 problem is magnified in thick-seam opera- 
 tions owing to increased seam thickness 
 and the tendency for some western coals 
 to act as aquifers. In the West, water 
 is an essential resource, as was pointed 
 out by Van Voast (29) in his report on 
 the hydrologic characteristics of coal 
 mine spoils in southeastern Montana. The 
 report dealt with the coal-bearing Fort 
 Union Formation. The uppermost unit of 
 the formation, the Tongue River Member, 
 contains 26 coal seams with thicknesses 
 ranging from 3 to 78 ft. In the report, 
 Van Voast noted — 
 
 Energy is not the only resource 
 that southeastern Montana can pro- 
 vide. The coal beds, because of 
 their generally fractured nature 
 and large areal continuities, are 
 commonly the most accessible and 
 widely used aquifers of the region. 
 In this semiarid climate, many in- 
 habitants are almost totally de- 
 pendent upon ground water for stock 
 and domestic supplies and in many 
 
 places they obtain it (water) from 
 coal beds that will be removed by 
 mining. 
 
 The coalbeds from the Tongue River Mem- 
 ber had transmissivities of 0.4 to 41.0 
 m 2 /d. Often they were not considered 
 viable wells by standards set in nonarid 
 environments. Yet, the wells are con- 
 sidered essential to the inhabitants of 
 an area where other easily obtainable 
 water sources do not exist. 
 
 Water can be a hazard in thick-seam 
 mining, whether it drains from surface 
 sources through subsidence cracks or 
 fractures, is tapped into from confined 
 aquifers in either the overburden or 
 strata directly under the coal, or is de- 
 rived from the mined seam. Ahcan (30) 
 discussed the effects of water collecting 
 in gob areas during use of the vertical 
 concentration method (longwall caving of- 
 ten combined with multislice mining). In 
 Yugoslavian mines, inrushes consisting of 
 water or mud occurred during winning 
 from the overhanging face section. Ahcan 
 noted that the water inrushes caused min- 
 imal difficulties as compared to the mud 
 inrushes. The mud, resulting from a mix- 
 ture of water and clay originally de- 
 rived from the coal and overlying strata, 
 flowed into the face area, causing casu- 
 alties and damage to the longwall. Ahcan 
 also noted that all the mud inrushes and 
 many of the water inrushes occurred 
 at faces using the vertical concentra- 
 tion method. The horizontal concentra- 
 tion method (standard longwall mining 
 with a face height of approximately 10 
 ft) was considered safer where imperme- 
 able clay strata separating the workings 
 from water-bearing formations in the roof 
 were too thin to act as a barrier. 
 
 Another example demonstrated the effect 
 of water below the mined seam. Strong 
 (31) noted that in the South Wales Coal- 
 field, coal was mined above a highly 
 jointed limestone aquifer. The water in 
 the aquifer was artesian. As such, early 
 excavations in the field experienced in- 
 rushes of water from the aquifer when the 
 intermediary strata were disturbed. 
 
 The majority of the factors previously 
 discussed were derived from actual case 
 studies of thick-seam operations and from 
 research conducted on western thick 
 
16 
 
 TABLE 3. - Importance of geologic factors for four thick-seam 
 mining methods 
 
 Geologic factor 
 
 Cleats 
 
 Joints and/or fractures.. 
 
 Coal quality 
 
 Petrography 
 
 Lithology of roof rock... 
 Lithology of floor rock.. 
 Thickness and 3-D 
 
 configuration 
 
 Continuity 
 
 Rolls 
 
 Spontaneous combustion. . . 
 
 Gas 
 
 Water 
 
 C Critcal to development 
 
 Multislice 
 
 Longwall 
 
 Single 
 
 Hydraulic 
 
 
 caving 
 
 pass 
 
 mining 
 
 X 
 
 C 
 
 X 
 
 c 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 C 
 
 X 
 
 X 
 
 - 
 
 X 
 
 - 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 c 
 
 X 
 
 c 
 
 - 
 
 c 
 
 X 
 
 X 
 
 X 
 
 c 
 
 X 
 
 X 
 
 - 
 
 X 
 
 C 
 
 X 
 
 c 
 
 X 
 
 X 
 
 X 
 
 X 
 
 X 
 
 C 
 
 X 
 
 X 
 
 X Important. 
 
 deposits. Table 3 summarizes the fac- 
 tors and their hypothesized importance to 
 the different mining methods. 
 
 All the factors listed will affect 
 development in thick seams differently, 
 depending on the geology of the site and 
 the mining method. In high-face single- 
 pass longwall, the thickness and change 
 in thickness across the panel are impor- 
 tant. Anything that thins the coal or 
 displaces the seam, such as faulting 
 or washouts, can have a deleterious 
 effect on the face. Some features cannot 
 easily be picked out prior to actual 
 development. 
 
 In multislice mining, thickness, change 
 in thickness, and continuity of both 
 seams are important. Another determinant 
 of multislice use is coal quality, as 
 zones of low quality in the coal may 
 be preferentially left as web between 
 the upper and lower slice. Rolls were 
 previously mentioned. Also, when the 
 panels are being developed, heating in 
 the remaining coal may cause problems. 
 
 Many factors are important in longwall 
 caving. Cleats are hypothesized to be a 
 critical factor in the caving operation 
 in that the cleat development, density of 
 major cleat planes cutting through the 
 coal to be caved, and relative smoothness 
 of the cleat surfaces help determine 
 
 cavability as well as the block size of 
 the caved material. Another important 
 characteristic is the degree of fractur- 
 ing in the seam to be mined; a highly 
 fractured coal will cave more easily. 
 Fractures will also help control block 
 size. Coal quality is important from the 
 marketing standpoint. Also, caving in a 
 section of the coal that is high in ash 
 (bone) may vary owing to both changes in 
 the strength of the material and fracture 
 and cleats within the section. Spontane- 
 ous combustion is a critical considera- 
 tion, depending on the amount of coal 
 remaining in the gob after caving is 
 completed. 
 
 In hydraulic mining, cleat development 
 helps determine the ease with which coal 
 can be cut. As in longwall caving, spon- 
 taneous ignition of coal remaining in the 
 gob is also an important consideration. 
 
 Hydraulic mining is viable because of 
 its flexibility. The method can handle 
 changes in thickness and has worked well 
 in highly faulted discontinuous seams. 
 Water, which is often a hazard in other 
 operations, is often advantageous in 
 hydraulic mining. Water that is derived 
 from the surrounding strata can be added 
 to the water used in mining, replenishing 
 water lost from the supply due to evapo- 
 ration and the raining process. 
 
17 
 
 CONCLUSIONS 
 
 Thick coal is common in many of the 
 coal regions in the Western United 
 States. In Colorado alone, 211 separate 
 occurrences of thick, coal or seams split 
 by less than 30 ft of interburden were 
 found in 4 of the 8 coal regions. A sub- 
 stantial number of occurrences are also 
 evident in Utah and Wyoming. 
 
 Ongoing Bureau of Mines research indi- 
 cates that four mining methods — high-face 
 single-pass longwall, multislice long- 
 wall, longwall caving, and hydraulic 
 mining — can be used to effectively ex- 
 tract these seams. It is essential to 
 the success of these methods that, on 
 each site, a thorough geologic evaluation 
 be performed. The resulting data are 
 critical in determining the selection of 
 
 the mining method and predicting problems 
 that will be encountered in the use of 
 the method. The principal factors that 
 will affect the different mining methods 
 follow: high-face single-pass longwall - 
 thickness and changes in thickness across 
 the panel; multislice longwall - thick- 
 ness and changes in thickness and the 
 presence of faults, washouts, and rolls; 
 longwall caving - cleat development, coal 
 quality, fracturing, and the tendency for 
 spontaneous ignition; hydraulic raining - 
 thickness, cleat development, and tend- 
 ency for spontaneous ignition. All other 
 factors listed in this report can also 
 cause problems and must be considered in 
 the initial mine assessment. 
 
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