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 
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. Chase1 and James P. Uleryl 

ABSTRACT 

Clay veins found in coal mines have caused numerous injuries and fatalities. These structures plague all phases of mining, including entry 

development, pillar recovery, and panel extraction. Clay veins also increase production costs and may disrupt or halt mining. These detrimental 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 slickenside planes commonly parallel clay veins and disrupt the lateral continuity of the immediate and, sometimes, main roof. When clay veins 

parallel or subparallel the direction of face advance, the roof is segmented into cantilever beams, causing unstable conditions. Consequently, 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 

a major cause of injuries and fatalities in underground coal mines (26). - The Bureau of Mines, in keeping with 

its goal of promoting a safer work environment, has conducted several in-mine 

investigations to determine the geologic structures and conditions responsible for accidental roof falls. These 

studies have confirmed that many roof 

falls can be correlated with specific 

geologic features. Moreover, the Bureau's investigations have enabled limited prediction of occurrences of these 

hazardous geologic features in [lnlni i~ed 

portions of the coalbed. 

Previous studies have indicated that 

clay veins (also referred to as clay 

dikes, horsebacks, or mudslips) were 

formed by either compactional processes 

or tectonic stresses. Occurrence information collected during this investigation seems to confirm both theories. 

Furthermore, since clay veins are actually infilled fissures, the authors 

contend that any compactional process or 

ground stress capable of developing a 

fissure, or fracture that can later 

widen, can result in the Eormation of a 

clay vein. 

CLAY VEIN ORIGINS 

Clay veins are infilled f issllres. 

These fissures developed when tensile 

stresses ruptured the coa,l and adjacent 

sediments during or after the coalification process. Previous studies have indicated that the fissures responsible for 

clay vein formation can be propagated by 

compactional processes and/or tectonic 

(regional or mountain-building) stresses 

active during and subsequent to coalification. Theories advocating cornpaction 

suggest that the fissures resulted from 

the unequal shrinkage of peat (24) or 

from differential compaction (iy 30). 

Proponents for a tectonic origin inchl-de 

?kCulloch (14), Price (221, and Smith 

(25). other investigators , including 

Gresley (z), and Oldham (E), have 

attributed fissures to earthquake disturbances, which are often related to 

tectonic activity. 

Fissures may be infilled as a result of 

gravity, downward-percolating ground waters, or compactional pressures which 

cause unconsolidated clays or thixotropic 

sands to flow into the fissures. This 

latter (plastic-f low) nethod of infilling 

occurred in the Upper Freeport Coalbed in 

Garrett County, MD, where high compressive st rc?.;l;cs have pressure-injected unconsolidated underclays into fissures 

associated with floor heave. Some of 

the fissures intersect coal ribs, forming 

mining-induced clay veins. 

CLAY VEIN OCCURRENCES 

Clay veins occur in the United States, 

the United Kingdom, Czechoslovakia, New 

Zealand, and elsewhere. This investigation was limited to the eastern United 

States (fig. 1) and includes portions of 

the Arkoma, Illinois, Northern and Southern Appalachian, and Warrior Coal Basins. 

The information shown in figure 1 was 

compiled by Bureau personnel based on 

2~nderlined numbers in parentheses refer to items in the list of references at 

the end of this report. 

mine observations. Other clay vein occurrence information uas obtained from 

the Mine Safety and 'riealtl~ Administration 

(MSHA), State enforcement agencies, State 

3~lthough sedimentary dike or clas tic 

dike is the more exact geologic term for 

an approximately vertical, tabular, sedimentary infilled discontinuity, the term 

"clay vein" is used here because it is 

firmly es tahlished in the coalfields that 

have the greatest incidence of related 

injuries and fatalities. 
throughout the eastern and central United 

States. 

Distribution data indicate that clay 

veins commonly occur with varying Erequency in the Illinois and Northern 

Appalachian Coal Basins fig 1 The 

immediate and main roofs above co:ilheds 

various marine and nonmarine environments. Individual roof rock types containing clay veins incllide thin, massive, 

or interbedded limestnnes, mudstones, 

sandstones, shales, and siltstones; however, clay veins occur less frequently 

under massive sandstone units. 

DEPOSITIONAL SETTING AND INTEKPRE'rATIONS 

Depositional conditions in the investigated portions of the Illinois and Northern Appalachian Basins were similar in 

that both basins were deposited on more 

stable (less rapidly subsiding) platforms 

with correspondingly slower rates of sediment influx (9, 27). Clay veins are 

rarely encountered in the Arkoma, Southern Appalachian, and Warrior Basins. 1.1- 

vestigated areas in these basins were deposited on less stable (nore rapidly 

subsiding) platforms with correspondingly 

faster rates of sediment influx (2, 13, 

- 28). From a depositional point of view, 

these conditions are optimal for the 

widespread presccvat ion of fossilized 

trees 1cettl.ehottoms). This is consistnt with information the Bureau has collected in the Southern Appalachian and 

i-Jarrior Basins. 

Depositional conditions in less stable 

basins dere also responsible Eor similar 

roof rock types being stacked one above 

the other, as is the case in southern 

Vest Virginia where facies prograde slowly (5). Overall, less differential compaction or deformation occurs when the 

same type of roof rock is stacked vertically (2). Conversely, rock types prograde rapidly in northern Vest Virginia 
where the basin is more stable (8). 

These conditions are conducive to offset 

rather than vertical stacking, and consequently, more differential compaction 

occurs (3). 

The type of sediments involved also 

determines how much differential compaction occurs during lithification (10). 

In southern West Virginia, where clay 

veins are rare, sandstone is more abundant than shale (5). The opposite is 

true in northern West Virginia (21). 

During lithification, sands are not 

as readily sheared and/or distorted as 

are muds (29), - which cornpac~ Lnto shale. 

Increased soft sediment deformation, 

faulting, and fracturing associated with 

differential compaction during lithification may explain why clay vefns occur 

more frequently in a more stable basin, 

such as northern West Virginia. In the 

Illinois Basin, differential compact ion 

may also be responsible for clay veins 

tenrl i 115 to parallel boundaries between 

different roof rock types (17). - 

CLAY VEIN COMPOSITION 

Sediments that infill clay veins may be 

derived from above and/or below the coalbed. Sediments may enter the coalbed 

fissure via one conduit or feeder (fig. 

2) or through smaller, multiple, intersecting (branch-like) feeders (f ig. 3). 

Clay veins are predorni~lni~[:l.y cqmposed of 

claystone; however, structures inf illed 

with sandstorle, siltstone, and/or limestone do occur. Frequently, large (1 to 

8 in), angalar to sub-rounded fragments 

oE the wall rock (limestone, coal, sandstone, etc.) are encompassed in the clay 

vein :natrix. Pyrite nodules are common (c), and occasionally, secondary calcite 

and quartz mineralization occurs within 

clay veins. 

The type of sediments which infills a 

clay vein may affect mining conditions. 

Water from continuous miner spraying 

systems someE irnes softens and weathers 

the claystone matrix of clay veins, causing roof spalling, which continues until 

adequate support is employed. Mining 

through structures composed of hard sandstone or siltstone is more troublesome. 

Typically, roof vibration is severe, and 

miner bits shear constantly. Frictional heat and/or sparks generated while 

mining through these harder structtires 

have caused numerous face ignitions. Abnormally high methane emissions often 

occur when mining through clay veins 

because these stc:ictures act as natural barriers or dams to free gas flow 

(23) 

COALBED AND ROOF ROCK CHARACTERISTICS 

Most clay veins have a zigzag appearance, as if the coalb2d were pulled apart 

(fig. 2); others have V- or U-shaped configurations. A clay vein's geometry and 

cross-sect iorlal. width often change along 

its trend or strike. Marked variations 

in shape and size are commonly observed 

011 adjacent ribs. Clay veins observed 

underground ranged from less than 0.1 to 

16 Et in width where they entered the 

coalbed. The lengths of clay veins vary 

from tens of feet to over 1.5 miles (16). 

Approximately one-third of the exnmKe(1 

clay veins terminated in the co;il.i~t?cI, 

while the remainder penetrated throt~gh 

the coalbed into the floor. A convex upward bulge in the underclay is sometimes 

noted where clay veins penetrate the 

floor. 

Upward and/or downward warping of coalbed and roof rock bedding planes commonl-y 

occurs on either side of a clay vein. 

This warping is illustrated in Eigure 4, 

which shows normally horizontal bedding 

planes approaching 30" dips. Bedding 

plane warping das measured as far away as 

8 ft from the cJfly vein. Normally vertical coal cleat within this warped zone 

are also inclined. The degree and latc:r.al extent of the zone of inclination 

appeared to be governed by the severity 

of deformation associated with the clay 

vein. Bedding plane and cleat disturbances were readily observable at the 
FIGURE 3.-Multiple-feeder clay vein. 
FIGURE 4.

Coalbed bedding plane warping in proximity to a clay vein. 

distinguished from fracture planes, along 

which no detectable movement has occurred. Fault and fracture planes extending as high as 20 ft into the main 

roof were noted in the Springfield (also 

referred to as No. 5 or Harrisburg) Coalbed near Springfield, IL. 

Slickensides are highly polished and 

striated planes of weakness. Slickensides associated with clay veins may be 

oriented in parallel sets or randomly. 

Parallel sets of intersecting slickenside 

planes are oriented in one of two ways, 

"V up" (fig. 8) or "V down." The clay 

vein generally occurs in the middle of 

the intersecting sets, as shown in figure 8. Some major slickenside planes 

are actually warped and striated bedding 

planes. When clay veins become larger, 

the associated slickenside planes increase in frequency and magnitude (figures 7 and 9), expanding the distur5ed 

zone laterally. Figures 7 and 9 were 

taken 10 ft away From clay veins and only 

show the set of parallel slickenside 

planes on the left sides of two different 

"V-down" clay veins. In addition to the 

gajor planes mentioned above, minor or 

secondary sets of intersecting fracture 

and slickenside planes are associated 

with some clay veins (fig. 7). The cumulative effect of all fracture sets is a 

loose and fragmented roof. 

ROOF SUPPORT 

Clay veins with associated fault, frac- vein's strike is parallel or subparalture, and slickenside planes disrupt the lel to the face, the clay vein divides 

lateral continuity of the immediate and, the roof slab into parallel beams. 

sometimes, main roof. Where the clay However, where the stri-ke parallels or 
FtOURE 5.-Clay-vein.asrocfst& fracture plane. 
FIGURE 7.-Major and minor slickenside plane sets associatad with a large ~fay v&n. 
FIGURE 9.-Steel mats effectively controlling fragmented clay vein roof wlth slickensides. 
subparallels the direction of face advance, the roof slab is divided into one 

or two cantilever beams. One cantilever 

beam occurs if the clay vein coincides 

with or runs along the rib line, as shown 

in figure 10. Two cantilever beams are 

formed when the clay vein occurs within 

the entry or crosscut as in figures 5 and 

llA. To determine if there is a correlation between roof stability and a clay 

vein's strike with respect to the direction of face advance, 471 clay vein segments were analyzed in a mine operating 

in the Upper Kittanning Coalbed in southwestern Pennsylvania. Observations were 

categorized as follows: (1) clay veins 

striking parallel to subparallel to the 

direction of face advance (0" to 30°), 

(2) clay veins striking subperpendicular 

to perpendicular to face advance (61" to 

90°), and (3) clay veins striking intermediately (31" to 60"). An analysis of 

the data in table 1 indicates a direct 

correlation between roof stability and 

a clay vein's strike, corroborating the 

above mentioned beam theories. 

The cantilever effect often associated 

with clay veins can sometimes be corrected by bolting and strapping the roof 

on each side of the major fault or fracture plane together to construct a beam. 

Mine personnel should be aware of the 

plane's orientation (strike and dip) so 

they can determine the proper bolt length 

and angle of installation (fig. 11). The 

orientation of major fracture or fault 

planes may be hidden in the roof prior to 

spalling. However, in certain cases, examination of the clay vein material in 

the coalbed reveals one or more smaller 

parallel fault or shear planes (fig. 6). 

Observations indicate that these smaller 

coalbed fault or shear planes can be used 

to approximate the orientation of the 

major fault or fracture planes in the 

roof. 

Whether or not a beam can be built in 

highly fractured rock with multiple intersecting major and minor slickenside 

planes is debatable. Some operators and 

roof control specialists believe that the 

fragmented rock mass must be suspended 

or keyed to insure stability. Under controlled laboratory conditions, highly 

fractured rock can be stabilized, provided the fragments are compacted by vibration and confined laterally in boxes 

until bolted (12). Unfortunately, underground conditGns do not afford these 

amenities, and quite commonly, the fragmented roof sometimes associated with 

clay veins sags and unravels (spalls) 

prior to and subsequent to bolting. 

Rates of convergence beneath clay veins 

can sometimes be useful in predicting 

roof stability. For example, ground control personnel in a central Illinois mine 

monitor convergence beneath potentially 

hazardous clay veins located along the 

track, belt, and other critical areas. 

When sag rates exceed 0.2 in/d, past experience suggests an impending fall, and 

additional supplemental support is immediately installed. Rates equaling 1.1 

in/d have been recorded beneath clay 

veins prior to roof failure in this mine. 

Tensioned bolts help compress a loose 

fragmented roof with slickensides into 

a somewhat competent unit (6). Mechanical bolts can be used, provided they 

are anchored out of the disturbed clay 

vein zone in competent strata. However, 

delimiting the disturbed zone both vertically and horizontally immediately 

after mining is often difficult. Where 

mechanical bolts are anchored into clay 

veins composed of incompetent claystones 

or highly fractured shales with internal 

TABLE 1. - Roof instability associated with different clay vein strikes 

Strike of clay vein with respect to 

direction of face advance 

Parallel to subparallel (0"-30°).....,... 

Intermediate (31"-60") ...............bb.. 

Subperpendicular to perpendicular 

(61'-90') ........b..b...b....b..b..bbb.. 

Total 

observations 

141 

173 

157 

Roof falls requiring cleanup 

Number 

63 

15 

12 

Pct of total 

observations 

47 

9 

8 
FIGURE 10.-Rlb-llne clay veln. 
~oof Clay vein with 

I1 Entry associated fault 

or fracture plane 

FIGURE 11.-Clay.veln-associated fracture and fault plane bolting diagrams. 

planes of weakness, stress concentrations 

at the anchor may crush or break already 

weak or fractured rock (1). Under these 

conditions, excessive bleedoff or an inability to attain or maintain required 

torque values may be a signal that longer 

bolts or resin anchorage is needed. Conversely, resin-grouted bolts insure better stability in weak rock with a low anchorage capacity (2). Based on the 

above, tensioned point-anchor resin bolts 

should insure better stability in clayvein-disturbed rock. Underground observat ions confirm this. 

Full-column resin bolts are also effective in controlling some clay veins. 

Although nontensioned, fully grouted 

bolts eliminate problems associated with 

moisture along the length of the bolthole 

(1) and help to prevent slippage along 

planes by infilling fractures and voids, 

essentially gluing adjacent fragmented 

blocks. Therefore, in workings where 

clay veins frequently occur, the routine 

installation of resin grouted bolts 

should be considered. Since many northern West Virginia mines have converted to 

resin grouted bolts, the West Virginia 

State Department of Mines has reported 

fewer clay-vein-related ground control 

problems. 

Bolts should be installed in conjunction with crossbars or steel mats immediately after undermining to help stabilize slickensides and prevent spalling. 

Crossbars and mats should be installed 

perpendicular to the clay vein's strike 

for maximum effectiveness. Mats should 

be flexible enough to conform to irregular top. Fiber-reinforced concrete 
FIGURE 12.-Roof trusses effectively controlling clay-veln-disturbed roof. 
one (01). Therefore, clay vein distributions observed underground may be unidirectional, bidirectional, or multidirectional. To determine if the clay 

veins in a particular mine resulted from 

tectonic stresses, it is necessary to 

plot on a mine map the trend of every 

clay vein encountered. Clay veins are 

easily distinguished prior to rock dusting, and minimal time is required for the 

face boss to plot the structure's trend 

(orientation or strike) on a mine map. 

Analysis of clay vein distribution can be 

achieved by dividing the clay vein trends 

into straight-line segments and plotting 

the length of these segments versus their 

spatial orientation. 

Figure 13 represents the bidirectional 

clay vein distribution of a particular 

mine site. More complicated multidirectional distributions are more easily visualized and interpreted using a 360' rose 

diagram (fig. 14) rather than a histogram, although some distortion does result. The 6 miles of clay vein data 

shown on figure 15 were mapped along the 

northwestern flank of the Johnstown Syncline in Somerset County, PA. The average dip encountered was 11". Analysis of 

figure 14 indicates a clustering of clay 

veins approximately at right angles to 

the northeast-southwest-directed maximum 

compressive stress (18). This suggests 

that these clay veinsoriginaced as release fractures that were both formed and 

later widened during stress release or 

unloading. The clustering of clay veins 

at 45O to a1 may have originated as shear 

fractures which later widened during unloading. Although a certain amount of 

scatter about the idealized peaks is apparent in figure 14, this may be attributed to the fact that the rock mass was 

subjected to and fractured by at least 

two distinct stress fields (18). - Roof 

control problems associated with clay 

veins are compounded in tectonically disturbed areas. As figure 16 illustrates, 

up-dip lateral shifting due to flexural 

slip folding often masks zones of clayvein-dis turbed roof. 

roo 

0 

90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 

ORIENTATION, deg 

FIGURE 13.-Histogram of bimodal clay vein distribution. 

Knowledge of preferred clay vein orientations enables mine operators to plan 

main entries perpendicular to or at 

oblique angles to the maximum number of 

clay veins in order to achieve optimum 

roof conditions. Barring other mining 

factors, longwall panels should be oriented so that the face obliquely intersects preferred clay vein orientations. 

The longwall face should never parallel a 

preferred clay vein orientation. 

Clay veins associated with compactional 

processes may or may not be predictable. 

If clay veins are coincident with paleotopographic (ancient topographic) highs 

or lows as defined by coalbed structure 

contour maps, then clay veins can be 

anticipated under similar conditions in 

advance of mining. Patterns associated 

with topographic irregularities may be 

linear or radial. Some clay veins also 

appear to be byproducts of slumped and/or 

faulted sediments that have been differentially compacted adjacent to coarser 

paleochannel (ancient stream channel) deposits. The paleochannels and clay veins 

shown in figure 17 were mapped in southwestern Pennsylvania in the Pittsburgh 

Coalbed. Figure 17 indicates that all 16 

of the clay veins over 50 ft in length 

occur within 50 ft of the paleochannel 

system. Thirteen of the 16 clay veins 

occur along the margins of the two 

largest channels. Pesek (20) - has also 

noted the occurrence of clay veins in 

proximity to paleochannel deposits. 
- 

LEGEND 

Release fractures 

. ... .. Shear fractures 

4 fll - Direction of maxi mum 

principal stress 

FIGURE 14.-Rose diagram of multidirectional clay vein distribution. 
0 4 0 0 8 0 0 1,200 

Scale, ft 

FIGURE 15.-Dlstrlbutlon of clay velns on a synclinal flank. 

Scale, ft 

FIGURE 16.-Schematic drawing illustrating up-dip shifting of clay-vein-disturbed rock. 
pp/

stresses active during or subsequent to 

the coalification process. 

2. Distribution data indicate that 

clay veins frequently occur in more 

stable coal basins where more differential compaction took place. 

3. When clay-vein-related fault or 

fracture planes parallel or subparallel 

the direction of face advance, the roof 

is segmented into cantilever beams. The 

strata on either side of the clay vein 

should be bolted and strapped together 

to form a beam. Mine personnel should 

be aware of the fault or fracture 

plane's orientation so they can determine 

the proper bolt length and angle of 

installat ion. 

4. A fragmented, sagging, and spalling 

roof is sometimes characteristic of clay 

p p

spalling. Crossbars and mats should be 

installed perpendicular to the clay 

vein's strike. 

5. Preferred clay vein orientations 

can only be determined if clay veins 

are mapped and analyzed. Preferred clay 

vein orientations should be considered 

in the planning of main entries and 

longwalls. 

6. Clay veins generally have linear 

to curvilinear strikes. Therefore, once 

mapped, clay veins can be projected 

for varying distances in advance of 

mining. Anticipating clay veins in 

advance of mining sometimes enables 

slight mine plan modifications that 

can minimize or eliminate associated 

roof hazards. 
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