TN295 No. 9076 .y ,.. '<^. O^ * » « "^.r ^ • 'W&''"' "^ "^ "'Sift'- "^ '^ y^MM" "^ ^^ «.^fflK^ '^ "i^ 'ImS^'o "^o ♦'^ ''i ^ ^'^ 'J .:^^- ^^ *./-i^-?/ 'V--^-/ \-^"\/ V-^'."'' \-^^y' <> *'..«* ,0 V^^'"v^ .0 ^- ' ^^c \/ :^: %.^^ •£&'. \„./ /Jlfe- *^..# ^n ^^ ► ^^ 0^ \3 *?5{T^' ^^ <» V f ' • o- o ."^-^I.^^o y*.C^..\ .'".-^i-"- /"'-^^k'^ <'" -'Mk^- / 'y ^ < -^^ , A '5', ' " ' - "v? A*2^ . ' " , <*_>. o^ , » " » » "^o A . ^ ' » - '<^^ '^MrH o^^^^^a'- <>'^v5^' ^'Ml^^^\ '^^^c^' cV^^^'' O'^C?^'' *MI^^\ ^^juc^ • «.-?- ^AO^ 'bV" "Cp-:.^ <<"^^ q, '^-^^^r ,/\^^-' .*'% -'^P- /\ .^, ^. r .*i.>,L'* <> »«o'* V'" ' • o, O '•^r?'. c<^^ y^K'> "^.. .^^ .'^%i>i'o ^^^_ A"^'' ^ P\^^:rL'* '^ o * •■ ■^ '^^ IC ^°^^ Bureau of Mines Information Circular/1986 Coal Mine Roof Instability: Categories and Causes By Noel N. Moebs and Raymond M. Stateham UNITED STATES DEPARTMENT OF THE INTERIOR i Information Circular 9076 // Coal Mine Roof Instability: Categories and Causes By Noel N. Moebs and Raymond M. Stateham UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Model, Secretary BUREAU OF MINES Robert C. Norton, Director Library of Congress Cataloging in Publication Data: Moebs, Noel N ('oal mine roof instability. (Bureau of Mines information circular ; 9076) Bibliography: p. 12. Supt. of Docs, no.: 1 28.27: 9076. 1. Coal mines and mining— Safety measures. 2. Mine roof control. 3. Coal— Geology . I. Statcham, Raymond M. II, Title, III. Series: Information circular (United States. Bureau of Mines) ; 9076. ^PN^^SrtJt" 622s [622'. 334] 86-600014 4 i CONTENTS Page Abstract 1 Introduction 2 Previous work 3 Categories of roof failures 3 Type S — stress related 4 Subtype S i , in situ stress 4 Subtype S2, induced stress 6 Type G — geologic defects 6 Subtype G\, low rock strength 6 Subtype G2 > moisture sensitivity 7 Subtype G3 , bedding plane spacing 8 Subtype G4 , minor structures 9 Subtype G3, major structures 11 Discussion 11 References 12 Appendix. — Glossary 13 ILLUSTRATIONS 1. Subtype Si roof failure associated with topographic "notch" 4 2. Subtype Si roof failure attributed to high lateral regional stress 5 3. Subtype S2 roof failure attributed to mining-induced stress 6 4. Subtype Gi roof failure attributed to low rock strength 6 5. Subtype G2 roof failure attributed to moisture— sensitive roof rock 7 6. Subtype G3 roof failure attributed to thinly laminated strata 8 7. Subtype G4 roof failure attributed to sllckensides 9 8. Subtype G4 roof failure attributed to kettlebottoms 9 9. Subtype G4 roof failure attributed to clay vein (clay dike) 9 10. Subtype G4 roof failure attributed to paleochannel (roof roll) 10 11. Subtype G4 roof failure attributed to joints 10 12. Subtype G4 roof failure attributed to pinchouts 10 13. Subtype G4 roof failure attributed to concretions 10 14. Subtype G4 roof failure attributed to faults 10 TABLE 1 . Tabulation of roof-fall occurrences 4 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT o degree MN/m^ meganewton per square meter in inch psi pound (force) per square inch COAL MINE ROOF INSTABILITY: CATEGORIES AND CAUSES By Noel N. Moebs and Raymond M. Stateham ABSTRACT Coal mine roof failure Is categorized according to character, trend, or pattern of occurrence. Two principal categories of failure are pro- posed — geology related and stress related. Geology-related failure In- cludes both llthology and structure. Each of several subcategories re- flects the probable cause of failure and thereby provides a basis for the selection of appropriate techniques for reducing the Incidence of failure. These control techniques, depending on local conditions, may Include supplementary support, destresslng, reduction of mine air humid- ity, or a change la the customary support methods. ^Geologist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. ^Geophysicist, Denver Research Center, Bureau of Mines, Denver, CO. INTRODUCTION Roof-bolting practices are largely em- pirical, having evolved from much experi- mentation and trial and error. Bolting theory still is more descriptive than mathematical. The subsurface environment surrounding a coal mine is complex, and the failure of bolted mine roof is diffi- cult to describe fully. Nonetheless, the qualitative identification of the con- ditions leading to roof failure can pro- vide some basis for modification of mine design that should reduce instances of failure under prevailing conditions. Most operators are aware of the sometimes subtle and sometimes pronounced changes in roof conditions that occur during a mining operation, and the difficulty of either predicting these changes or re- sponding with the appropriate support or mine design to prevent roof failure. However, the recognition of a general characteristic of individual falls or a pattern of multiple falls usually will offer some clue as to the cause and, therefore, the appropriate remedial action. The purpose of this paper is to provide the mine operator with a guide for deter- mining the probable cause of persistent roof failure based on the occurrence, character, and distribution of the indi- vidual falls in a mine, and to offer sug- gestions as to how additional roof fail- ure might be avoided. The diagnosis of roof failure is based chiefly on the careful examination of a mine roof-fall map, and recognition of certain charac- teristic patterns of distribution or modes of failure. This requires diligent documentation of a significant number of falls, as occasional widely distributed falls seldom are sufficient for analysis and in most mines would not indicate a need for action. Occasional failure can result from faulty materials or improper bolt installation, but these factors are outside the scope of this paper. Roof- fall documentation at the mine should in- clude the following: 1, A mine map showing all roof falls and indicating not only the location, but also the approximate size and dimensions of the falls. This map will then provide information for determining trends of roof failure. 2. A mine map with surface topography superimposed for correlating roof fall occurrences to topography features. 3. A map showing positions of super- jacent or subjacent mining. 4. A map showing roof structures such as clastic dikes or sandbodies, as well as major geologic features (faults, dense jointing, etc.). The combined informa- tion from maps of this type can be ex- tremely useful in the diagnosis and pre- vention of roof failure. The significance of roof-fall patterns became discernible upon an examination of numerous roof falls in scores of coal mines , followed by a review of supplemen- tary geologic information on each prop- erty and an interview with underground personnel. Some patterns of roof falls were erratic or difficult to explain, and some were found to correlate with a transition in bolt type, mining cycle, or entry design. This paper summarizes the significance of roof-fall patterns at- tributable to structural or stress condi- tions and offers suggestions for remedial action. Before describing roof-fall patterns and their significance, it must be empha- sized that any supplementary information, such as maps showing the character and thickness of roof strata, or discrete roof structures such as rolls or clay veins , may help explain the occurrence of falls and aid in determining the appro- priate remedial action. The improper installation of bolts con- tributes to roof failure but, with re- spect to stress and geologic factors, is a lesser influence on roof-fall occur- rence. There are several reasons for this condition. First, improperly in- stalled, mechanically anchored bolts can be detected prior to failure by torque measurements; poorly installed full- column, resin-grouted bolts are not so easily detected. However, even these im- properly installed bolts tend to be rela- tively effective support devices because some portion of the bolt's length is usu- ally well bonded to the rock. As little as 12 to 18 in of bond can provide an an- chor that will hold a load greater than the yield strength of the steel bolt. On the basis of personal observations and experience, the authors believe improper installation is a lesser problem when compared to the stress and geologic in- fluences discussed in this paper. This is especially true in coal mines because of the following: 1 , Coal mines are located in relative- ly weak, bedded sediments where adverse stress and geologic conditions severely affect ground stability. 2. Coal mines generally use rotary drilling equipment that provides consist- ent hole sizes and spin rates so that the grout is mixed adequately for proper bolt installation. 3. Mechanical bolts are tested ac- cording to procedures defined by safety regulations . Aside from inappropriate support, im- proper installation, configuration or size, or mining sequence for existing conditions , most roof failures can be at- tributed either to high stresses or to geologic defects in roof structures. PREVIOUS WORK Several schemes for classifying mine roof have been attempted, chiefly for the purpose of predicting the occurrence of roof falls from a knowledge of local geo- logic features. For example. Weir (V)^ described six kinds of roof falls as fol- lows: shale dusting or slaking, sand- stone rolls, concretions, slabbing, clay seams, and massive. These categories of falls probably predominate in the spe- cific area of Indiana studied but are of limited usefulness for the entire region. Hylbert (2^) proposed a classification of roof falls based on the structural and compositional character or roof at a mine in eastern Kentucky. This scheme proved useful in projecting trends of roof con- dition as an aid in anticipating problem areas in advance of mining. Some infer- ences can be drawn from this scheme re- garding the appropriate support method for each of the four types of roof condi- tions described by Hylbert. Patrick and Aughenbaugh O) have de- vised a classification based only on the geometry of a roof fall. The categories are dome, arch, minor, and sloughing. This simplified scheme is intended to expedite the reporting of roof falls independent of local conditions and to serve as a first step toward developing a means of predicting the occurrence and extent of future falls. While some in- ferences as to the cause of failure can be drawn from a geometric classification only, further usefulness in analyzing roof control problems is very limited. The classification of roof proposed by the Bureau of Mines requires somewhat more information than that needed in the works of other investigators (3); how- ever, it should provide for broader us- age, supply a sound basis for diagnosing the underlying causes of roof failure, and indicate some appropriate means of reducing the rate of failure. Often, a simple inventory of individual roof-fall occurrences will indicate the probable cause of failure, as shown in table 1, which is based on examples from four selected mines in western Pennsyl- vania and northern West Virginia. This method, however, requires extensive map- ping of roof falls and is too broad in classifying, although it may be useful in conjunction with the classification scheme proposed here. CATEGORIES OF ROOF FAILURES For simplicity and clarity, the illus- trations used here to designate various ■^Underlined numbers in parentheses re- fer to items in the list of references preceding the appendix at the end of this report. types of roof-fall patterns are sche- matic. Often the cause of roof failure is obscure and cannot be determined with any degree of certainty, or only through prolonged and sophisticated research. The following categories are intended only to provide mine operators and/or TABLE 1. - Tabulation of roof-fall occurrences At inter- Between At minor >1 Principal cause of failure Mine sections inter- sections structures pillar length Total and support required 1: Number 163 23 4 10 200 Low-strength roof rock, Pet. .. 82 11 2 5 100 requiring bolted headers, 2: Number straps, and trusses. 4 10 115 129 High lateral stresses (cut- Pet. .. 3 8 89 100 ter roof) , requiring posts 3: Number and crossbars. 21 12 350 383 High lateral stresses (cut- Pet... 5 3 92 100 ter roof) , requiring posts, crossbars, and 4: Number steel sets. 10 34 44 Minor structures, chiefly Pet.. . 23 77 100 clay veins, requiring straps or header block. mine safety personnel with the means to make a preliminary and rapid assessment of roof problems using observable pat- terns of roof failure and geologic infot- mation. While underground options always are limited, some early remedial measures might be attempted once the probable cause of failure has been established. Some of these measures are suggested. All roof-fall patterns have been divided into either of two categories — Type S or stress related, and Type G or geology related — and a schematic illustration of each subtype is provided for quick refer- ence. Type G includes both lithology and structure-related failure to facilitate classification. Caution must be exer- cised in using only the illustrations since the accompanying description of re- lated surface or subsurface features may be equally diagnostic. TYPE S— STRESS RELATED Subtype S^, In Situ Stress In the northern Appalachian coal re- gion, one of the most common and easily recognized types of roof failures occurs beneath narrow stream valleys in areas of high relief (fig. 1). It is referred to by various miners' terms such as "pressure falls," "snap top," and "cutter roof." These falls can be recognized by comparing their occurrence with a map showing surface stream valleys where top- ographic relief is at least 100 ft. It has been estimated by several operators that when mining beneath or near such stream valleys, severe roof-fall prob- lems will develop 90 pet of the time. ^WWWXi!W>.VAV-«"'-ll.i.lWW- Topographic "notch" or ~,, valley at surface" .„,„,(Not to scale) . Scole, ft FIGURE 1. - Subtype $1 roof failure associated with topographic "notch." Examination of these falls shows little if any evidence that jointing contributed to the failure, and even the most compe- tent roof rock is subject to this type of failure. The falls frequently result from a concentration of high lateral com- pressive stresses, a phenomenon described by several authors (4-^) . They are rec- ognized underground sometimes by an audi- ble snapping sound immediately after min- ing, or by the development of a steeply dipping shear or "cutter" at the inter- section of rib and roof within a few hours to a week or two after mining. This type of failure usually develops be- tween intersections but may progress along cutters across one or more inter- sections for perhaps several hundred feet. In some instances of S^ roof failure, further falls have been prevented by the installation of supplementary angle bolts to intersect the cutter plane and anchor above the pillar (fig. 1). Currently, angle bolting is being tested in at least three mines for this purpose. The imme- diate installation of support to prevent any yielding of roof is commonly recom- mended. More severe S] failure will re- quire posts and crossbars or possibly roof trusses. It may be virtually uncon- trollable with crushing of posts or cribs leading to massive high roof falls. Roof failure that clearly is caused by high lateral compressive stress, but that is not limited to occurrences be- neath valleys and occurs somewhat random- ly, is included here (fig. 2). It is characterized by the development of a cutter along the rib line within a few weeks of mining, roof cracks, and a typi- cally sudden roof fall if not well sup- ported. Kripakov (9^) describes cutter roof failure in detail, discusses current control methods , and suggests some new alternatives. Competent shale roof fails under these pressures as readily as does softer laminated roof rock. The pattern of these falls commonly shows a preferred north-south trend in several of the U.S. coal regions. Blevins (4^) reported a similar north-south failure condition in the Illinois coal basin. The falls .Absence of pronounced topographic -'//-, "notch "or valley at surface Z - — Underclay-^_j^^^~~~~^^;^-^~__~:j-_^ 5 3 Scale, ft FIGURE 2. - Subtype S 1 roof failure attributed to high lateral regional stress. generally begin between intersections and may zigzag around pillars to follow a north-south trend, or may show no clear directional preference. Subtype S^ falls can be distinguished from those attrib- uted to pillar punching by the absence of accompanying floor heave and pillar spalling, and by the evidence of a direc- tional trend. S] roof failure includes roof falls that tend to occur chiefly at the bound- aries of multiple-entry mains. These falls typically begin as a cutter along the rib line and commonly have been at- tributed to "abutment pressures." The exact cause, however, remains disputable. When adequate pillar support is provided and there is no evidence of roof deflec- tion in the central zone, the pressure arch theory does not seem to offer an adequate explanation of failure, and in situ stresses are suspected. A change in entry orientation, where possible, has proved to be more beneficial than a modi- fication of pillar design, but experience in controlling this problem is very lim- ited. Destresslng of the rock surround- ing an entry by means of roof or rib slotting or induced caving of an adjacent entry has been attempted with varying de- grees of success. Destresslng, however, while sound in theory, is risky, and requires equipment not always available in a mine. Normally, supplementary support as described in S^ failure is suggested. Subtype S2> Induced Stress Induced-stress roof failure (S2 — fig- ure 3) nearly always can be related to superjacent or subjacent second mining or to a squeeze or pressure override from a panel that has not fully caved. The map overlay technique, such as that described by Ellenberger (10) , will detect the ef- fects of multiseam mining on entry sta- bility, while pressure overrides nearly always occur within a few hundred feet of adjacent pillar extraction or pillar stumping. Geologic and mining correla- tions are useful in resolving the problem of induced stresses, especially with re- spect to the massive character of strata that transfer overburden weight onto pil- lars or abutments. Induced stresses from multiseam mining or pressure overrides lead to various combinations of floor heave, pillar spelling or deformation, pillar punching into floor and roof, and cutter roof, with the actual failure of roof sometimes occurring late in the se- quence of events. The remedy for S2 roof failure due to pressure overrides probably lies with improved pillar extraction and caving or with oversized pillars. Ground con- trol problems resulting from multiseam mining may be alleviated by following extraction sequencing guidelines as described by Britton (11). An increase in conventional bolting, strapping, or posting seldom is of much value in pre- venting further failure, therefore steel sets generally are needed. TYPE G — GEOLOGIC DEFECTS Categories of roof failure attributed to geologic defects or geologic character of rock are divided into five subtypes, as follows: G] - Low rock strength. G2 - Water sensitivity. G3 - Bedding-plane spacing, G4 - Minor structures, G5 - Major structures. Subtype G^, Low Rock Strength This category of roof includes all roof rock that is relatively soft, usually poorly laminated, and low in RQD (Rock Quality Designation), point load, and compressive strength. The physical prop- erties for subtype G^ (fig. 4) commonly would fall below the following values: Point load index 0.3 MN/m^ Shore hardness 20 Compressive strength.... 2,500 psi 3 I I Scole, ft Floor heave FIGURE 3. - Subtype $2 roof failure attributed to mining-induced stress. Limestone Scale, ft FIGURE 4. - Subtype G^ roof failure attributed to low rock strength. Low-Strength rocks include most clay- stone (especially the "drawslate" that overlies the coalbed) and underclay or seat earth. These rocks generally are not self-supporting for normal entry widths, tend to fall from the roof soon after the supporting coal has been re- moved, and therefore must be supported as quickly as possible after exposure. Bolted headers or straps usually are needed, and trusses are useful in severe cases, where the deadweight of collapsing roof is not excessive; otherwise, posts and crossbars become necessary. Full- column resin-grouted and tensioned resin- anchored bolts have been used success- fully at some localities to support this type of roof, but further assessment is needed. With most mechanical bolts, there is a problem of tension bleedoff in soft rock due to anchor slippage; in addition, segments of soft rock tend to fall from between bolts, and the re- inforced beam effect of the roof is disrupted. Systematic drilling, core logging, and point-load testing of drill core are useful in delineating areas where low- strength roof rock can be anticipated. This type of rock commonly occurs between coal splits where the roof of the lower bed consists of the underclay of the up- per bed; it is widespread over the Pitts- burgh coalbed in the Upper Ohio River Valley. Subtype G2 , Moisture Sensitivity Moisture sensitivity is used here to indicate a significant reduction in the strength of roof rock from exposure to high humidity or water, such as is com- monly found in the mine environment. (See figure 5.) The effects of moisture sensitivity consist of a progressive softening or slaking, whereby rock grad- ually disintegrates and eventually re- verts back to a mudlike unconsolidated condition. The slaking process is one of moisture absorption, expansion, and softening. In a humid mine atmosphere, slaking may be a gradual process, the effect of which often is measured in months or years, resulting in a slow failure of roof by attrition as the Limestone —^ Roof bolts— "^ Clay shale or ^ clays' One " Y ^ '- T - ~ n — ^ _ — — --> — V, _ -_ _ ^ X _/ ^-x^ --^ ^ ^y-i i \ __ ^^^^^ I Entry -Underclayr. Scale, ft FIGURE 5. - Subtype Q>2 roof failure attributed to moisture-sensitive roof rock. dislodging of small fragments leads to larger falls of roof. The integrity of an entire mass of roof can be destroyed by this process. In mechanically sup- ported areas, roof-bolt tension bleedoff occurs as the rock immediately against the bolt plate becomes softened, or when the anchor slips. Moisture-induced roof failure generally is most pronounced and severe near shaft bottoms and along in- take air courses. The rate of roof slak- ing is greatest during the humid summer months when roof rock commonly is wet with condensation. Where moisture-sensitive roof rock is thinly interbedded with moisture-stable rock, the bond between the two types of strata is weakened, leading to strata separation. Sandy roof rock rarely is affected by moisture except where the sand grains are poorly cemented and the rock reverts to a loose sand. The principal type of moisture-sensi- tive roof rock in the Appalachian coal region consists of a poorly laminated lumpy claystone containing numerous slickensides , which sometimes is known as clod in miners' terminology. Generally a simple water- immersion test or exposure outdoors will determine the relative moisture sensitivity of roof rock samples and the extent to which this may become a problem underground. The prevention or control of moisture-induced roof failure can be accomplished by any of the follow- ing four principal methods: 1. Head coal . — The uppermost 4 to 6 in of coalbed, if left unmined, serves as a moisture barrier and may prevent slaking of shale roof. 2. Sealing . — Sealing entails the coat- ing of exposed roof with an impervious layer of material to exclude moisture. The sealant can consist of an asphalt- or latex-base material with little physi- cal strength, or it may consist of a cement-base gunite with fiber additive sprayed over a bolted wire mesh for added strength and reinforcement. The effec- tiveness of these measures depends large- ly on the quality of the sealant and the care with which it is applied. Sealing of large areas of roof invariably is a costly procedure. 3. Artificial support . — Some form of supplementary support almost always is required to reduce the failure of mois- ture-sensitive roof. The options are nu- merous and need not be discussed here. Limited experience with full-column resin bolts indicates their superior ability to hold soft, slickensided, claystone-type roof, as opposed to mechanical bolts, which lose tension as moisture attacks the rock at the bolt head and anchor. As disintegration due to moisture pro- gresses, a larger area of roof than that immediately above the bolt plates re- quires support, and this usually is pro- vided by bolted headers, straps, or mesh. Long-term disintegration usually necessi- tates trusses or posts and crossbars to support an increasinly large amount of deadweight from sloughing roof. 4. Air tempering . — The term "temper- ing" as used here refers to a modifying, adjusting, or stabilizing of mine air moisture and temperature, usually with a resulting decrease in humidity levels and humidity fluctuations. Air tempering has been accomplished through the use of water sprays, heaters, and cooling units, depending on the season, but at high cost and with limited success. A passive and more cost-effective method of temper- ing mine air is through the use of air- tempering rooms or entries. Here, fresh air is passed through a set of rooms or multiple entries where it is cooled and loses moisture in the humid season and, to a lesser degree, is warmed and absorbs moisture in the winter months. This eli- minates large fluctuations in humidity and temperature before the air enters haulageways and other active sections of a mine. Provision must be made for some roof deterioration in the tempering rooms or entries, which should be included in the original mine design. This method of air tempering was assessed recently in both field and laboratory investigations ( 12 ) . It was concluded that the use of air tempering entries may be cost effec- tive in controlling roof disintegration, depending on conditions at a particular mine. Subtype G5 , Bedding-Plane Spacing A bedding plane in laminated roof strata constitutes a potential plane of separation (G3 — figure 6). The weaker the bonding along the bedding plane, the more likely a roof separation will occur as the coal underneath is removed. Weak bonding usually results from an abundance of mica flakes, clay, or coal material along the bedding plane; the more closely spaced the bedding planes or thin lamina- tions, the more difficult it will become to form a beam of the immediate roof un- less it is strongly reinforced with roof bolts. Thinly laminated roof strata of both low strength and closely spaced bed- ding planes, such as a "rash" of coal, claystone, and shale, are certain to be troublesome roof to support, Stackrock, Massive sandstone * ' Lominated:' bondsfone:: -* 't <, * «. « Wi 1 m [p:^ rUnderclay: Scale, ft FIGURE 6. - Subtype G3 roof failure attributed to thinly laminated strata. a miner's term for very thinly laminated sandstone, does not respond well to conventional bolting and is prone to fall on exposure. Roof falls attributed to closely spaced and poorly bonded lamina- tions usually occur at intersections, where the greatest span of roof is ex- posed, but sometimes occur randomly wherever the roof support or installation is inadequate or defective. Falls of immediate roof due to a high density of poorly bonded bedding planes tend to de- velop first as roof sag because the roof bolts are not anchored into overlying competent thick-bedded strata. As the strata in the immediate roof separate along bedding planes and sag, a slip- page along the planes also occurs. This alone, however, is unlikely to prevent eventual roof failure unless some support is provided by longer bolts anchored in overlying competent strata. Severe sag of thinly laminated strata that does not respond to fully grouted, tensioned resin-anchored, or longer bolt calls for the use of roof trusses, posts and cross- bars, or entry narrowing where feasible. The sagging of laminated strata often results in a tension fracture along the center of the entry roof caused by the bending moment. As sagging progresses. I I fracturing of the eventually destroys leads to a general collapse. roof occurs , which roof integrity and disintegration and 'I I Limestone T J FIGURE 7. - Subtype G4 roof failure attributed to s lickensides. Scale, ft FIGURE 8. - Subtype G4 roof failure attributed to kettlebottoms. Subtype G4 , Minor Structures Falls of roof attributed to minor geo- logic structures generally are recognized by a minor structure that is exposed in the roof or fall or lies adjacent to the fall. Minor structures include virtually any geologic feature other than a normal parallel layering of roof strata. These include slickensides (fig. 7), kettlebot- toms (fig. 8), clay dikes (fig. 9), pa- leochannels (fig. 10), joints (fig. 11), pinchouts (fig. 12) , concretions (fig. 13), and faults (fig. 14). Most minor structures constitute a discontinuity in the normal beamlike structure of mine roof and thereby have a weakening effect. J _± H T Limestone Scale, ft FIGURE 9. - Subtype G4 roof failure attributed to clay vein (clay dike). 10 Sandstone FIGURE 10. - Subtype G4 roof failure attributed to paleochannel (roof roll). 3 1 I Scale, ft ^ _ Underclay- r ~:_~ T ^^~ ~ :C ~ - ^ Z ZJr FIGURE 13. - Subtype G4 roof failure attributed to concretions. Joints Shale - Scale, ft FIGURE 11. - Subtype G4 roof failure attributed to joints. ■■•■ Sandstone . . =::7v"^T^."vT^^^7Tr7rv," !^Roof.'--';.''\ ■bolts ■'" '^ig;— - ■,/.■;.■';■. 'Sandstone' .-,.' .fflW* ,, J- p— /^ 3 I I Scole, ft -;j~_;^~ ~-Underclay-x_-^~-_~^~-^'XX-::r-c-c c ::: ~i FIGURE T2. - Subtype G4 roof failure attributed to pinchouts. The roof rock, around minor structures tends to fall soon after the supporting coal is removed and before a permanent support can be installed. Underclay Scale, ft FIGURE 14. - Subtype G4 roof failure attributed to faults. A multitude of minor structures have been encountered in Appalachian coal mines. Few have been fully described as to identify or effect on mine roof. Virtually all are either syngenetic or diagenetic in origin; that is, they are nontectonic, having been formed con- temporaneously with deposition or short- ly thereafter during compaction and consolidation. The actual character or identity of many minor structures can only be estab- lished through careful examination by an experienced geologist. The correlation between structure and roof falls, how- ever, can be fairly readily established by a systematic mapping of roof falls and minor structures, even though the iden- tity or trend of the structure is not al- ways apparent. Many minor structures such as paleo- channels , clay dikes, slickensides , slumps, rolls, and horsebacks, tend to- ward linearity, so that directional 11 trends of falls soon can be established and projected. Kettlebottoms and con- cretioas tend to occur sporadically and are particularly common in southern West Virginia. Intraformational joints are found in virtually every mine where thick massive strata occur. They commonly will form a boundary of a roof fall but do not con- stitute major causative factor. However, in eastern Kentucky, the so-called hill- seam, a weathered, valley stress-relief form of joint, has been the cause of nu- merous roof falls in drift mines. Joints are reported to play a much greater role in roof failure in the Western United States than in the Appalachian region. It would be impractical to attempt to describe all the minor structures and their variants. However, a knowledge of the nature of each structure above the exposed roof can be vital in preventing failure by tailoring the supplementary support to the local conditions. For ex- ample, neither kettlebottoms, concre- tions, nor jointing in mine roof are nec- essarily better supported by increasing the bolt length, while pinchouts may ben- efit from this procedure. The support of several minor structures, such as slick- ensides (slips), paleochannels , and clay dikes, seem to be improved when angle bolting is employed. Resin injection and dowelling have proved effective in many instances of consolidating clay dikes in the roof. Bolted straps and headers are widely used with virtually any type of a minor structure that constitutes a dis- continuity in roof strata. The severity of failure due to minor structures can be reduced when the gen- eral directional trend of these struc- tures can be established and entries can be turned to intersect them at a large angle as opposed to driving parallel to the structures. Every effort should be made to identify correctly the character and trend of troublesome structures on exposure, as they are not usually detect- able by exploratory drilling, occur er- ratically, and tend to fail without warn- ing when unexpectedly encountered during mine development. Subtype G3, Major Structures This category is intended to cover the large tectonic structures such as the faults and folds that occur along the eastern limits of the Appalachian coal region, the anthracite region of east- ern Pennsylvania, the Coosa and Warrior Basins of Alabama, the Illinois Basin, and some Western U.S. coal regions. Major tectonic structures, while recog- nized, are outside the scope of this paper and therefore are omitted from de- tailed discussion. However, major struc- tures in Illinois and their effect on mine roof have been described by Nelson (13) , and similar structures in Western U.S. coal regions were studied by Laird and Amundson (14). DISCUSSION The authors have presented a scheme for categorizing roof falls in mines based on causative factors and have indicated pos- sible means of upgrading roof support practices to prevent their occurrence. This scheme requires some data collection regarding the pattern and character of roof falls. Each setting has its own ground control problems , which can be de- scribed as stress effects and geologic defects. These two salient conditions can occur together, but they require a somewhat different approach in terms of improved roof support. Although improper extraction or support methods can con- tribute to roof failure, these factors usually can be identified by the absence of geologic defects or stress effects and the close examination of operating procedures . The proposed scheme for the diagnosis of roof falls and improvement of support clearly is only a framework in which the roof specialist of a mining company can begin to sort out his or her troubles. It is not always possible for a mine op- erator to allocate technical staff mem- bers full time for this kind of study. 12 But, if pursued conscientiously along with some sort of experimentation within the constraints of the approved roof support plan, it could contribute to accident prevention and reductions in cleanup and repair costs. REFERENCES 1. Weir, C. E. Factors Affecting Coal Mine Roof Rocks in Sullivan County, Indi- ana. Proc. IN Acad. Sci. 1969, v. 79, 1970, pp. 263-269. 2. Hylbert, D. K. The Classification, Evaluation, and Projection of Coal Mine Roof Rocks in Advance of Mining. Min. Eng. (NY), V. 30, No. 12, 1978, pp. 1667- 1676. 3. Patrick, W. C, and N. B. Aughen- baugh. Classification of Roof Falls in Coal Mines. Min. Eng. (NY), v. 31, No. 3, 1979, pp. 279-283. 4. Blevins, C. T. Coping With High Lateral Stresses in an Underground Coal Mine. Pres. at Soc. Min. Eng. AIME Annu. Meeting, Dallas, TX, Feb. 14-18, 1982. Soc. Min. Eng. AIME preprint 82-156, 1982, 7 pp. 5. Roley, R, W. "Pressure-Cutting:" A Phenomenon of Coal-Mine Roof Failures. Mechanization, v. 12, Dec. 1948, pp. 69- 73. 6. Agapito, J. F. T., J. R. Aggson, S. J. Mitchell, M. P. Hardy, and W. N. Hoskins. Study of Ground Control Prob- lems in Coal Mines With High Horizontal Stresses. Paper in Proceedings of the 21st Rock Mechanics Symposium (Univ. MO- Rolla, May 28-30, 1980). Univ. MO-Rolla, 1980, pp. 820-828. 7. Aggson, J. R. How To Plan Ground Control. Coal Min. and Process., v. 16, Dec. 1979, pp. 70-73. 8. Lang, T. A. Theory and Practices of Rock Bolting. Trans. AIME, v. 220, 1961, p. 335. 9. Kripakov, N. P. Alternatives for Controlling Cutter Roof in Coal Mines. Paper in Proceedings of the Second Con- ferences on Ground Control in Mining, Morgantown, WV, (July 19-21, 1982). WV Univ., Morgantown, WV , 1982, pp. 142-151. 10. Ellenberger, J. L. Hazard Predic- tion Model Development: The Multiple Overlay Technique. Pres. at Soc. Min. Eng. AIME Annu. Meeting, Chicago, IL, Feb. 22-26, 1981. Soc. Min. Eng. AIME preprint 81-16, 6 pp. 11. Britton, S. G. Mining Multiple Seams. Coal Min. and Process., v. 17, No. 12, 1980, pp. 64-70. 12. Cummings , R. A., M. M. Singh, and N. N. Moebs. Effect of Atmospheric Mois- ture on the Deterioration of Coal Mine Roof Shales. Min. Eng. (Littleton, CO), V. 35, No. 3, 1983, pp. 243-245. 13. Nelson, W. J. Faults and Their Effect on Coal Mining in Illinois. IL State Geol. Surv. Circ. 523, 1981, 40 pp. 14. Laird, R. B., and A. A. Amundson. Geologic Conditions Affecting Coal Mine Ground Control in the Western United States (contract J0145032, Goodson & As- sociates, Inc.). BuMines OFR 14-86, 1985, 62 pp. 13 APPENDIX. — GLOSSARY 1. Abutment pressure - In underground mining, the weight of rock above an exca- vation which has been adjoining walls. transferred to the 2. Angle bolts - Bolts installed in the rock over an underground opening at an angle of less than 90° from vertical (usually 45°). They usually are in- stalled so as to penetrate a slip or shear plane and anchor over the adjacent rib (sidewall of the opening). 3. Beam effect - The result of bolting of the mine roof whereby the bolted strata behave as a single beam, stabiliz- ing the overlying rock. 4. Bedding plane - The surface that sep- arates each successive layer in a strati- fied body of rock. 5. Clay dike (clastic dike, clay vein) - Many sedimentary formations contain transecting tabular bodies of clastic ma- terial. These intruding bodies, usually called clastic dikes, are composed of ex- traneous materials that have invaded the containing formation along fissures ei- ther from below or above. When the in- vading material is composed of clay, the dikes are frequently called clay dikes or clay veins. In the Western United States, the fill material is often called "spar;" in the East, "spar" refers to a narrow clay vein occurring only near the top of the coalbed. 6. Clay stone - An indurated (hardened) clay. 7. Cleat - A system of joints in a coalbed. 8. Coal split - A coalbed that is sepa- rated by rock partings into two or more layers that may or may not rejoin some distance away. The layer of rock that separates the coal. 9. Concretion - In this report, concre- tions are defined as aggregates of min- eral material in other sediments such as coal balls , Frequently , they have a nu- cleus and concentric internal structure. 10. Crib - A structure composed of frames of timber laid horizontally upon one another, as in the walls of a log cabin (used to support the roof in under- ground mines) . 11. Crossbar (collar, cap, bridge board, roof bar) - The horizontal roof member of a timber set in mine entries. A horizon- tal bar supported by roof bolts. 12. Cutter - A stress-induced, steeply dipping fracture that initiates at the rib line and propagates upward into the roof rock. 13. Cutter roof - A coal mine roof that is prone to cutter-type failure. 14. Destressing - The process of reliev- ing the pressure or load on rock around underground openings . 15. Draw slate - A weak shale in the immediate mine roof that falls when the supporting coal is removed, or soon thereafter, 16. Fault - A rock fracture of natu- ral origin along which there has been displacement. 17. Full-column resin bolts - Roof bolts that are grouted in place in the rock and have a column of grout that extends along the entire length of the bolt. The terra "resin bolt" is a misnomer, and resin re- fers to the type of grout; the bolt it- self is type 40 steel (or better) . 18. Head coal (top coal) - Coal that is left on the roof of a coal mine for the purpose of shielding the roof from the effects of exposure to mine air humidity. 19. Header - A block of wood used under the roof bolt plate to increase the effective bearing area for installed roof bolts. In some minir^g areas, the term refers to the block of wood placed 14 between the top of a post and the mine roof. 30. Paleochannel - An stream channel. ancient buried 20. Horseback - In this report, it re- fers to rolls at the top or bottom of a coal seam. The term is sometimes applied to clastic dikes in coal, large inter- secting slickensides in the roof, or fos- silized tree trunks. 21. Induced stress - Rock pressure around the mine opening that has been caused by excavation of the mine it- self or by other mine excavations in the vicinity. 22. In situ stress (far field stress, remnant stress) - Rock pressure that was present prior to the creation of the mine opening. 23. Joint - A fracture of natural origin that is not attended by displacement. 24. Kettlebottom (pot, bell) - Columnar masses of rock in mine roof consisting usually of the casts of ancient tree stumps. These may drop out of the roof without warning. The surface is usually highly slickensided and striated. 31. Pillar punching - When the load on a mine pillar exceeds the bearing strength of the underlying floor (without causing the pillar to fail) , and the pillar is pushed into the floor. 32. Pillar spalling (pillar sloughing) - The breaking off of pieces of coal from the rib or pillar; the term can include minor rib failures. 33. Pinchout - The wedging out (by lat- eral thinning) of one layer of rock be- tween two other layers. 34. Point-load test - A test designated to measure crushing strength of a mate- rial by using a force applied through two opposed, pointed platens (hence point load) . Mathematical procedures then are used to estimate compressive strengths using point-load data. 35. Posts - Timber placed upright that are used to support the mine roof. They may be used alone with cap blocks or with headers or crossbars. 25. Laminations - To rock bedding layers less than 1 in thick. in 26. Lateral stress - In situ stress that is horizontal or near horizontal in orientation. 27. Layer - Any stratum of rock sepa- rated from superjacent and subjacent rock by a poorly bonded bedding plane. 28. Mesh (road mesh, wire mesh, weld mesh, chain link fence) - Interlaced or woven heavy steel wire used to help sta- bilize the roof and ribs of mine openings or to catch and hold rock that breaks away from the roof and ribs. 29. Multiseam mining - The mining of two or more coal seams underlying the same surface area. 36. Pressure arch theory - The pressure arch theory states that when an opening is driven in a coalbed, the vertical load once supported by the extracted mate- rial is transferred to the sides of the opening. 37. Override (squeeze, ride over, pres- sure override, ride) - Downward and lat- eral movement of mine roof accompanied by pillar crushing, pillar punching, and roof failure, resulting from an exces- sive load of overburden. This condition usually develops from improper pillar extraction. 38. Parting - A thin sedimentary layer, sometimes organic, separating thicker rock or coal strata. 15 39. Rash - Thinly interlaminated layers of shale and coal that sometimes occur between the coalbed and the overlying rock. 40. Resin-grouted bolt - A steel bolt that is installed by using a resin to anchor the bolt in the rock prior to tensioning. 41. Roof truss - An arrangement whereby opposite-placed angle bolts are connected by a turnbuckle and thereby placed in tension, thus exerting an upward compres- sive force against the exposed roof. 45. Seat earth - Stratum underlying a coal seam. Commonly a rooted clay-stone. 46. Slickenside - A polished, striated surface caused by differential compaction of coal-bearing strata. 47. Slump - The mass of sediment that has slid down from a stream bank into an open stream channel. 48. Snap top - Highly stressed coal mine roof, which under some conditions, emits audible snapping sounds soon after mining. 42. Roll - A minor protrusion of rock into the top or bottom of a coalbed. Term can include small flow and compac- tion structures and paleochannels . 43. RQD (Rock Quality Designation) - A quantitative index, expressed as percent- age, that is based on a recovery proce- dure for drill core. It reflects the fracturing and softening in a rock mass. 44. Sealant - Any material that is painted or sprayed onto mine roof or rib to prevent slaking or spalling. Also used on stoppings to seal off air leakage. 49. Stackrock - Thinly interlaminated shale and sandstone occurring in coal mine roofs. Individual layers in stack- rock may lack lateral continuity. 50. Strap - A corrugated steel sheet (4 to 15 in wide) against the roof to assist in maintaining the stability of the roof. 51. Tension bleedoff - A decrease in the tensile prestress of point anchored, ten- sioned bolts that results from creep of the anchor. 52. Underclay - A bed of claystone un- derlying a coal seam. See "Seat earth." •.-,- U.S. GOVERNMENT PRINTING OFFICE: 1986-605-017/40,028 INT.-BU.OF MIN ES,PGH.,P A. 28248 H 20 1 U.S. Department of the Interior Bureau of Mines— Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh, Pa. 15236 OFFICIAUBUSINESS PENALTY FOR PRIVATE USE, S300 I I Do not wi sh to receive thi s material, please remove from your mailing list. I I Address change* Please correct as indicated* AN EQUAL OPPORTUNITY EMPLOYER •f* ^•^^ . <^ * » » o ^ .'„v* -^m^i^^\ ^^^^ =^ "oV" ,\ -n^o^ o, .^ .< b^'^f^ 0^ ^^,^>^- / 0.^^*0 V>^.;/ -o^^^^.oO \'>^-' ^<>- ^'=>' ^-^.^^^ yA%iA^. % .<.'^* '^^IK'- '^^ y ^'^Va\ •^<>. ..'?^'' » vO' .r^% '^ u o •^..^^' » ^^v^^C,*^' ^ ™,^ ■* ■ay o» • ^^" * , V ^ •»^^^^r^ o^ o . . » A ^ .* J''^^^ ^: •^ iO^ . * * <* ^^ ^^ • ^ \ -. - ^^-^^"^^^ 1%!^: A^^\. cC^°.^ « 5°^ vv 'J>^r 1 » «<* « •••'• .^ %,♦ |T> * '\. ^'']^^'> .^<'^iX /.c:^^"°o ^^ .,_^,_ ......-,0^^ ^0.-'o<^^A^ ^<.-?^\0^^ \. •'f.^i^^ A^ V" • <^ M • ■ i