Bureau of Mines Report of Investigations/ 1987 Clay Veins: Their Occurrence, Characteristics, and Support By Frank E. Chase and James P. Ulery UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 9060 Clay Veins: Their Occurrence, Characteristics, and Support By Frank E. Chase and James P. Ulery UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES Robert C. Horton, Director Library of Congress Cataloging in Publication Data : Chase, Frank E. Clay veins : their occurrence, characteristics, and support. (Report of investigations/United States Department of the Interior, Bureau of Mines ; 9060) Bibliography: p. 18-19. Supt. of Docs. no.: I 28.23: 9060. 1. Ground control (Mining) 2. Clay veins. 3. Coal mines and mining-Safety measures. I. Ulery, J. P. (James P.) 11. Title. 111. Series: Report of investigations (United States. Bureau of Mines) ; 9060. TN23.U43 86-600245 CONTENTS Page Abstract ....................................................................... Introduction................................................................... Clay vein origins .............................................................. Clay vein occurrences.......................................................... Depositional setting and interpretations ....................................... Clay vein composition.......................................................... Coalbed and roof rock characteristics .......................................... Roof support................................................................... Prediction and mine plan modifications ......................................... Conclusions and recommendations................................................ References..................................................................... ILLUSTRATIONS Clay vein occurrence map for the Eastern United States .................... Single-feeder clay vein ................................................... Multiple-feeder clay vein ................................................. Coalbed bedding plane warping in proximity to a clay vein ................. Clay-vein-associated fracture plane ....................................... Clay-vein-related fault plane ............................................. Major and minor slickenside plane sets associated with a large clay vein .. Clay-vein-associated slickenside plane sets displaying a "V-up" pattern ... Steel mats effectively controlling fragmented clay vein roof with slickensides............................................................. Rib-line clay vein ........................................................ Clay-vein-associated fracture and fault plane bolting diagrams ............ Roof trusses effectively controlling clay-vein-disturbed roof ............. Histogram of bimodal clay vein distribution............................... Rose diagram of multidirectional clay vein distribution ................... Distribution of clay veins on a synclinal flank ........................... Schematic drawing illustrating up-dip shifting of clay-vein-disturbed rock ..................................................................... Distribution of clay veins and paleochannels in mine workings ............. TABLE 1 . Roof instability associated with different clay vein strikes.............. UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT feet m meter inch Pet percent inch per day CLAY VEINS: THEIR OCCURRENCE, CHARACTERISTICS, AND SUPPORT By Frank E. Chase1and James P. Uleryl ABSTRACT Clay veins found in coal mines have caused numerous injuries and fa- talities. These structures plague all phases of mining, including entry development, pillar recovery, and panel extraction. Clay veins also in- crease production costs and may disrupt or halt mining. These detri- mental aspects have prompted the Bureau of Mines to investigate the physical characteristics of and roof instability problems associated with clay veins. This was accomplished by observing and mapping clay veins in surface and underground mi-nes. The occurrence and origins of clay veins were also investigated to determine predictive capabilities. The investigators found that clay veins normally occur in more stable, less rapidly subsiding coal basins. Clay veins result when tensile stresses develop fissures which are later infilled. These fissures can be propagated by compactional processes and/or tectonic stresses active during and subsequent to coalification. The Bureau also found that associated faults, fractures, and slicken- side planes commonly parallel clay veins and disrupt the lateral con- tinuity of the immediate and, sometimes, main roof. When clay veins parallel or subparallel the direction of face advance, the roof is seg- mented into cantilever beams, causing unstable conditions. Consequent- ly, the strata on either side of the clay veins should be bolted and strapped together to form a beam. '~eolo~i4t,Pittsh~~rgh Research Center, Rurenu of Mines, Pittshrrrqh, PA. INTRODUCTION Accidental roof falls continue to be Previous studies have indicated that a major cause of injuries and fatal- clay veins (also referred to as clay ities in underground coal mines (26).- dikes, horsebacks, or mudslips) were The Bureau of Mines, in keeping with formed by either compactional processes its goal of promoting a safer work envi- or tectonic stresses. Occurrence infor- ronment, has conducted several in-mine mation collected during this investi- investigations to determine the geo- gation seems to confirm both theories. logic structures and conditions respon- Furthermore, since clay veins are ac- sible for accidental roof falls. These tually infilled fissures, the authors studies have confirmed that many roof contend that any compactional process or falls can be correlated with specific ground stress capable of developing a geologic features. Moreover, the Bu- fissure, or fracture that can later reau's investigations have enabled lim- widen, can result in the Eormation of a ited prediction of occurrences of these clay vein. hazardous geologic features in [lnlni i~ed portions of the coalbed. CLAY VEIN ORIGINS Clay veins are infilled f issllres. Gresley (z), and Oldham (E), have These fissures developed when tensile attributed fissures to earthquake dis- stresses ruptured the coa,l and adjacent turbances, which are often related to sediments during or after the coalifica- tectonic activity. tion process. Previous studies have in- Fissures may be infilled as a result of dicated that the fissures responsible for gravity, downward-percolating ground wa- clay vein formation can be propagated by ters, or compactional pressures which compactional processes and/or tectonic cause unconsolidated clays or thixotropic (regional or mountain-building) stresses sands to flow into the fissures. This active during and subsequent to coalifi- latter (plastic-f low) nethod of infilling cation. Theories advocating cornpaction occurred in the Upper Freeport Coalbed in suggest that the fissures resulted from Garrett County, MD, where high compres- the unequal shrinkage of peat (24) or sive st rc?.;l;cs have pressure-injected un- from differential compaction (iy 30). consolidated underclays into fissures Proponents for a tectonic origin inchl-de associated with floor heave. Some of ?kCulloch (14), Price (221, and Smith the fissures intersect coal ribs, forming (25). other investigators , including mining-induced clay veins. CLAY VEIN OCCURRENCES Clay veins occur in the United States, mine observations. Other clay vein oc- the United Kingdom, Czechoslovakia, New currence information uas obtained from Zealand, and elsewhere. This investi- the Mine Safety and 'riealtl~Administration gation was limited to the eastern United (MSHA), State enforcement agencies, State States (fig. 1) and includes portions of the Arkoma, Illinois, Northern and South- 3~lthough sedimentary dike or clastic ern Appalachian, and Warrior Coal Basins. dike is the more exact geologic term for The information shown in figure 1 was an approximately vertical, tabular, sed- compiled by Bureau personnel based on imentary infilled discontinuity, the term "clay vein" is used here because it is 2~nderlinednumbers in parentheses re- firmly es tahlished in the coalfields that fer to items in the list of references at have the greatest incidence of related the end of this report. injuries and fatalities. FIGURE 1.-Clay vein occurrence map for the Eastern United States, after McNeal (75). geological surveys, and mine personnel containing clay veins were deposited in throughout the eastern and central United various marine and nonmarine environ- States. ments. Individual roof rock types con- Distribution data indicate that clay taining clay veins incllide thin, massive, veins commonly occur with varying Ere- or interbedded limestnnes, mudstones, quency in the Illinois and Northern sandstones, shales, and siltstones; how- Appalachian Coal Basins fig 1 The ever, clay veins occur less frequently immediate and main roofs above co:ilheds under massive sandstone units. DEPOSITIONAL SETTING AND INTEKPRE'rATIONS Depositional conditions in the investi- widespread presccvat ion of fossilized gated portions of the Illinois and North- trees 1cettl.ehottoms). This is consis- ern Appalachian Basins were similar in tnt with information the Bureau has col- that both basins were deposited on more lected in the Southern Appalachian and stable (less rapidly subsiding) platforms i-Jarrior Basins. with correspondingly slower rates of sed- Depositional conditions