Bureau of Mines Report of Investigations/l975 

Geologic Investigations 

of Underground Coal Mining Problems 
Report of Investigations 8022 

Geologic Investigations 

of Underground Coal Mining Problems 

By C. M. McCulloch, P. W. Jeran, and C. D. Sullivan 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

Jack W. Carlson, Assistant Secretary-Energy and Minerals 

BUREAU OF MINES 

Thomas V. Falkie, Director 
This publication has been cataloged as follows: 

f 

McCulloch, Charles M 

Geologic investigations of underground coa 1 mining pfob 

lems, by C. M. McCulloch, P. W. Jeran, and C. D. Sullivan. 

[Washington] U.S. Bureau of Mines [1975] 

30 p., illus. (US. Bureau of Mines. Report of investigations 8022) 

Includes bibliography. 

1. Coal mines and mining. I. Jeran, Paul W., jt. auth. D[. 

Sullivan, C. D., jt. auth. LII. U.S. Bureau of Mines. IV. Title. 

(Series) 

TN23 .U7 no. 8022 622.06173 

U.S. Dept. of the Int. Library. 

k 
CONTENTS 

Abstract ................................................................ Introduction ............................................................ Acknowledgments ......................................................... Geologic mine survey techniques ......................................... Maps of mine problem areas and their preparation ................... Underground surveys ................................................ Surface fieldwork .................................................. Documentation ...................................................... Surveys of six mines and their problems ................................. Marianna No . 58 mine ................................................ Lucerne No . 6 mine ................................................. Somerset No . 60 mine ............................................... Federal No . 2 mine ................................................. Howe mine .......................................................... Beatrice-Pocahontas mine ........................................... S-ry ................................................................. References .............................................................. Glossary of terms ....................................................... ILLUSTRATIONS 

Page 

1 

1 

2 

2 

2 

3 

3 

3 

5 

5 

7 

10 

14 

16 

18 

25 

27 

28 

Fence diagram of strata above the Pittsburgh coalbed. Marianna 

NO . 58 mine ...................................................... 4 

Structure drawn on base of the Pittsburgh coalbed. Marianna No . 58 

mine ............................................................. 5 

Overlay of mine streams on surface related to underground falls .... 6 

Isopach of the wild coal interval over the Pittsburgh coalbed in 

the Marianna No . 58 mine ......................................... 6 

Location of clay veins in the Marianna No . 58 mine ................. 7 

Surface joint data above the Marianna No . 58 mine .................. 8 

Fault exposure at Lucerne No . 6 mine ............................... 9 

Generalized sketch of typical thrust fault exposure at the Lucerne 

NO . 6 mine ....................................................... 9 

Structure map drawn on the base of the Upper Freeport coalbed. 

Lucerne No . 6 mine ............................................... 10 

Fence diagram of strata above the Upper Freeport coalbed. Lucerne 

NO . 6 mine ....................................................... 11 

Structure map drawn on the base of the Pittsburgh coalbed at 

Somerset No . 60 mine ............................................. 12 

Fence diagram of strata above the Pittsburgh coalbed. Somerset 

NO . 60 mine ...................................................... 13 

Lithologic change between drill holes TH1 and TH2 .................. 14 

Location of possible sand channel washout. Somerset No . 60 mine .... 14 

Isopach of the draw slate unit above the Pittsburgh coalbed. 

Somerset No . 60 mine ............................................. 14 

Outline of the Federal No . 2 mine .............. .................... 15 

Structure map drawn on the base of the Pittsburgh coalbed. Federal 

NO . 2 mine ....................................................... 16 
ILLUSTRATIONS - -Continued 

Structure map drawn on the base of the Lower Hartshorne coalbed, 

Howe mine........................................................ 

Fence diagram of the strata above the Lower Hartshorne coalbed, 

Howe mine.... .................................................... Map of Keen Mountain fault in the Pocahontas No. 3 coalbed, 

Beatrice-Pocahontas mine......................................... 

Cleat rotation in relation to shear fault, Beatrice-Pocahontas mine 

Structure map drawn on the base of the Pocahontas No. 3 coalbed, 

Beatrice-Pocahontas mine.... ..................................... Fence diagram of the strata above the Pocahontas No. 3 coalbed, 

Beatrice-Pocahontas mine.. ....................................... Nature of sandstone above Pocahontas No. 3 coalbed, BeatricePocahontas mine...... ............................................ Isopach of Pocahontas No. 3 coalbed (inches), Beatrice-Pocahontas 

mine............................................................. 

Isopach of interval between Pocahontas No. 3 coalbed and sandstone 

above Beatrice-Pocahontas mine................................... 

Overburden isopach map of interval from Pocahontas No. 3 coalbed 

to surface, Beatrice-Pocahontas mine...........................,. 
GEOLOGIC INVESTIGATIONS OF UNDERGROUND 

COAL MINING PROBLEMS 

by 

C. M. McCul loch, 1 P. W. Jeran, 1 and C. D. Sul l ivan2 

ABSTRACT 

A series of geological surveys were completed in six coal mines in 

connection with a Bureau of Mines research program on methane control. The 

aim of these surveys was twofold: First, to study the overall geology of the 

coalbed, and second to use the resulting data to evaluate the influence of the 

geological characteristics of the coalbed on safe and efficient coal extraction. 

This report explains the advantages of geologic maps and fence diagrams 

in coal mining, and how the knowledge gained from such geologic investigations 

of mines can help control many underground problems. 

INTRODUCTION 

In connection with a Bureau of Mines research program on methane control 

in coal mines, a series of geological surveys were completed in six mines: 

Three in the Pittsburgh coalbed and one each in the Upper Freeport, 

Pocahontas No. 3, and Lower Hartshorne coalbeds. The aim of these surveys 

was twofold: First, to study the overall geology of the coalbed area and 

second to use the resulting data to evaluate the influence of the geological 

characteristics of the coalbed on safe and efficient coal extraction. 

Such geological investigations can help to alleviate many underground 

mining problems (3).3 They can locate sand channels that may intersect and 

cut out the coal Tn advance of mining. They can determine trends of channels, 

making it possible to predict their probable course and influence on future 

mining operations. 

Clay veins can be located and their trends sometimes established. Thus, 

the high gas concentrations that frequently occur with clay veins can be 

anticipated. 

1Geologist. 

'physical science aid. 

sunderlined numbers in parentheses refer to items in the list of references 

preceding the glossary of terms. 
A detailed map of the rock above the coalbed can be used to determine the 

length of bolts needed to support the roof and to locate competent strata into 

which the bolts can be anchored. Because certain rocks deteriorate rapidly 

when exposed to air, such a map can also help to determine areas where rock is 

of the type in which experimental resin bolts can be used. 

An abnormal accumulation of water can be evaluated, and in many cases 

related to fracturing of the roof rock, or to a fault zone which can also be 

responsible for high gas concentrations. 

A detailed joint pattern in the area of the mine can be established and 

examined for a possible relation with the coal cleat directions andwith 

joints present in the roof. Local deviations in joint and cleat directions can 

serve as a basis for modifying mining conditions. The cleat and joints can 

also supply information on roof stability. Excessive spalling of ribs when 

mining with the cleat direction can sometimes be corrected by a 45' rotation 

of this direction. 

ACKNOWLEDGMENTS 

The authors wish to thank the staffs of Bethlehem Mines Corp., Island 

Creek Coal Co., Eastern Associated Coal Corp., Helvetia Coal Co., and Howe 

Coal Co., without whose assistance a report of this type could not have been 

prepared. These companies all supplied data for the preparation of maps and 

met with Bureau personnel to discuss the company mines. Bureau personnel 

worked underground in all mines and were escorted by company officials to any 

of the sections of the mine requested. 

GEOLOGIC MINE SURVEY TECHNIQUES 

Maps of Mine Problem Areas and Their Preparation 

Maps of problem areas within mines are a necessary preliminary to an 

underground survey. Such maps are sometimes supplied by the company. They 

may show (1) zones of roof fall, (2) zones of excessive spalling of ribs, 

(3) zones of channel washouts, (4) zones of abnormal concentrations of clay 

veins, and (5) zones of excessive accumulations of water and .gas. Very few 

maps show clay veins, sandstone channels, and so forth, but many companies do 

have maps of zones of bad roof or areas where a specific feature occurs with 

an abnormally high frequency . 

When company maps were not available or were inadequate, Bureau personnel 

constructed their own maps. The majority of all maps were prepared from data 

taken from the corehole logs supplied by the company. The accuracy of the 

resulting data relates to the accuracy of the driller's log. Coreholes should 

be spaced at not more than 3,000 feet and when possible should be drilled on 

a suitable grid system. Interpretation of the driller's logs can be a problem, 

and it is helpful if the same driller has logged most of the holes, although 

when a number of holes have been drilled over a period of years, this may not 

be possible. Corehole logs can be used to prepare a variety of maps, including isopach maps of given intervals, structure maps, percent maps of rock 
units, lithologic frequency maps, lithofacies maps, and fence diagrams. The 

analysis of inherent problems is dependent upon such maps. 

Underground Surveys 

The geologists planning an underground survey first consult with company 

personnel as to the specific problems that may exist in the mine. Once underground, the geologic features of particular interest to the geologist himself 

are (1) orientation of coal cleats, (2) occurrence of clay veins, (3) occurrence of faults, (4) sandstone-or claystone-filled channels, (5) abnormally 

high concentrations of gas and water, and (6) roof falls. 

Coal cleat measurements are used to determine the face and butt orientations of individual mines. The cumulative information is used for correlation 

with local and regional structure (4). - 

Clay veins or sedimentary dikes are a particular problem in mining 

because they may intersect so as to form isolated blocks of coal that may act 

as gas cells. When a continuous miner cuts through such a clay vein, the 

sudden rush of methane into the working area can greatly increase the fire 

and explosion hazard. Clay veins are also areas of mine roof instability. 

In the Pittsburgh coalbed, at sites where there are large numbers of clay 

veins, they appear to be controlled by structure, but additional evidence is 

needed to confirm this. 

Where sandstone channels are encountered in mining, they must be penetrated by driving rock tunnels by drilling and blasting. This is a timeconsuming operation that interrupts coal product ion. In particular , the 

plough-type longwall miner requires an area relatively free of sand channels 

and clay veins, while the shearer type may be able to mine through these areas 

without too much trouble. 

Faults may displace a coalbed and also create areas of abnormally high 

gas and water accumulation. In many mines faults are hard to detect because 

they are of the strike-slip type, with no verticaloffset. Moreover, thecleat 

readings near a fault can differ greatly from the overall pattern of the area. 

A rotation of local cleat direction has been detected to 2,000 feet from the 

fault itself. 

Surface Fieldwork 

Generally a quick check is also made of the surface geologic features 

around the mine. Joints in the rocks are measured following guidelines of 

Lahee (2). Surface areas directly above underground problem areas are of 

specialinterest. When faults are encountered underground, the trend is 

plotted and its expression on the surface is sought. 

Documentation 

All available aerial photographs, IR photographs, SLAR mosaics, and photo 

index sheets of the area were examined for linears. The State geological 
LEGEND 

Shaly lime Sandy shale 

0 shale Wild cool interval 

Limestone Cool 

Sandstone 

" 

Scale, ft .- 

d 

1 r 

3,000 0 

Horizontal 

FIGURE 1. - Fence diagram of strata above the Pittsburgh coalbed, Marianna 

No. 58 mine. 
surveys were contacted for any additional information on the study area, and 

the literature was checked for pertinent information. 

SURVEYS OF SIX MINES AND THEIR PROBLEMS 

Surveys were conducted in the following mines: Marianna No. 58 and 

Somerset No. 60, Bethlehem Mines Corp.; Lucerne No. 6, Helvetia Coal Mining 

Co.; Beatrice, IslandCreek Coal Co.; Federal No. 2,Eastern AssociatedCoal Corp; 

and Howe, Howe Coal Co. Each mine presented different structural problems. 

Marianna No. 58 Mine 

The mine is located near Ten Mile, Pa. off Interstate Highway 79. It 

operates in the Pittsburgh coalbed. The survey, conducted in September 1972, 

showed the main 

directions of the 

cleat to be ~68W 

(face) and N28"~ 

(butt) . Mining is 

by the room-andpillar method, and 

aver age methane 

emission per day 

is 2.3 MM ft3. 

They are now 

mining at 45' to 

the cleat direction because of 

excessive spalling 

of the pillars and 

some roof control 

problems when 

mining parallel to 

the cleat directions, This 

change in direction has greatly 

reduced the amount 

of spalling and 

has caused no slowdown in mining. 

A panel (see 

glossary) or fence 

diagram (fig. 1) 

and a structure 

map (fig. 2) of 

the property were 

prepared. A 

topographic map 

was overlayed on FIGURE 2. - Structure drawn on base of the Pittsburgh coalbed, the mine map to 

Marianna No. 58 mine. correlate the 
Key Mop 

LEGEND 

0 Underground fall 

- 

Scale. f t 

FIGURE 3. - Overlay of mine streams on 

surface related to underground falls. 

Scale, ft 

FIGURE 4. - lsopach of the wild coal interval over the Pittsburgh 

coalbed in the Marianna 

No. 58 mine. 
Key Mop \ 

Scale, f t 

FIGURE 5. - Location of clay veins in the Marianna N* 58 mine. 

already passed through the axis and were now encountering few clay veins, it 

was anticipated that when mining approached the axis of this syncline the 

occurrence of clay veins would increase. 

Joints were measured on the surface (fig. 6) to evaluate correlations 

between the principal fracture directions underground and those on the 

surface. Correlation was excellent. 

Lucerne No. 6 Mine 

The mine is located approximately 2 miles southwest of Homer City, Pa., 

and operates in the Upper Preeport coalbed. A geological survey was conducted 
1 in March 1972. Cleat measjoint direction 

urements showed a uniform 

pattern with the face cleat 

striking N56"W and the butt 

cleat N32"E. The dip was 

essentially vertical; The 

spacing of the face cleat, 

on the order of an inch, 

appeared to be uniform 

throughout the mine, except 

near the fault exposures, 

where the cleat was more 

closely spaced. The most 

FIGURE 6. - Surface ioint data above the Marianna significant geologic 

No. 58 mine. features of the mine were 

the fault exposures (fig. 7). 

They are low-angle thrust faults which offset the coal only a few feet along 

the fault plane (fig. 8). They affected both coal and roof rock, but did not 

appear to disturb the floor rock. These faults are not the result of sedimentary slumping. They occur at regular intervals and can be traced to the 

surface. When encountered by mining, the fault zones emitted large quantities 

of gas and water. There is insufficient evidence to conclude that all the 

faults observed were laterally connected. It is more probable that they were 

a group of small thrust faults occurring within a narrow thrust zone. 

Aerial photographs were analyzed for linears. Two groups were identified, 

N47"E to N65OE and N25"W to N56OW, and interpreted as the regional joint 

pattern. In addition, poorly developed groups of linears striking N13"W to 

NOSOEwerepresent locally in 500-foot-wide bands at 6,000-to 7,000-foot 

intervals. The first of these lies about 2,000 feet west of the fault 

encountered in the West Mains. This may be the surface expression of the 

underground fault. If so, then additional faults or water zones could be 

expected as mine workings progress further to the west. 

A structure map (fig. 9) of the base of the Upper Freeport seam, drawn 

from corehole data, shows a synclinal structure with axis bearing approximately N30°E and passing through the easternmost section of the mine. The 

largest part of the mine workings and reserves lies in the western limb of 

the syncline. The data points were too widely spaced to determine more than 

the gross structure of this area. 

The core logs were studied with particular attention to the transition 

zone between the thick and normal Upper Freeport coal. The thick coal is east 

of the transition zone. This local thickening, limited in extent, is due to 

deposition. The thick Freeport coal is split from the main coal by a wedge of 

shale that thickens to the southwest. The general strike of the transition 
FIGURE 7. - Fault exposure at Lucerne No. &mine. 

Foultzorte . 

oug h, irregular roof 

FIGURE 8. - Generalized sketch of typical thrust fault exposure at the Lucerne No. 6 mine. 
FIGURE 9. - Structure map drawn on the base of the Upper Freeport coalbed, Lucerne 

No. 6 mine. 

zone is N30°W. This is illustrated by the fence diagram (fig. 10) constructed 

from the core log descriptions of the first 40 feet of rock above the coal. 

Within this interval, sandstone is present throughout almost the entire area. 

This sandstone is separated from the coal locally by shales and sandy shales. 

Its large lateral extension makes it a probable water and gas reservoir. 

Thick sandstone directly overlying coal often exhibits deformations due to 

compaction which appears locally to be thrust faults, but this was not the 

case here. The core logs were examined for records of broken rock, but they 

yielded no pattern indicative of possible fault zones. 

Somerset No. 60 Mine 

The mine is located several miles west of Ellsworth, Pa. It is working 

in the Pittsburgh coalbed and produces 1.5 MM ft3 of gas per day. A geological survey was conducted in February 1973. 
-- - 

0 10.000 

Horizontal 

e easued ocjot 

orientations were be tween 

N57"W and N7S0W, corresponding roughly to face cleat 

orientation. 

FIGURE 10. - Fence diagram of strata above the Upper The structural map Freeport coal bed, Lucerne No. 6 'mine. of the area, based on the 

Pittsburgh coalbed, shows 

that mining is preceding down the northwest' flank of the Amity anticline 

towards a syncline (fig. 11). Water is reportedly accumulating at the face 

of entries being driven to the northwest. This condition may be expected as 

long as the entries dip into the working face. Once they advance on the axis 

of the syncline, the floor will dip away from the face and water should no 

longer accumulate there. 

A fence diagram of the 40 feet of strata immediately above the main bench 

of the Pittsburgh coalbed (fig. 12) in the 84 field directly to the west of 

the present mining was constructed, because of the occurrence of sand channels 

cutting out coal in the western part of the active mine workings. The data 

were taken from the company core logs. There was an abrupt change in this 

interval from north to south, with shale predominating in the north and 

sandstone in the south. The abruptness of this change is shown by drill 
thickness composed of 

alternating slickensided - 

FIGURE 11. - Structure map drawn on the base of the claystones and coals 

(fig. 15). The interval Pittsburgh coalbed at Somerset No. 60 has very little strength mine. and can fall, causing gas 

emissions as well as haulage 

problems. Most mines prefer to anchor roof bolts in the more competent rock 

above this interval. Figure 15 can be used in a general way to determine the 

minimum length of bolt that would be necessary to bolt through the interval 

in any given area. 

The geological data from core holes spaced more than 3,000 feet apart 

are difficult to correlate. More closely spaced drilling is necessary to 

delineate the northern limit of the sandstone. This zone is of particular 

importance in mining because it represents the northern limit of stream 

erosion of the strata overlying the Pittsburgh coalbed. Zones of abrupt 

lithology change can also coincide with areas of unstable roof, high gas 

emission, and related mining problems. 
Wild coal interval 

0 Shale 

Coa I 

Scale, f t 

Horizontal 

FIGURE 12. - Fence diagram of strata above the Pittsburgh coalbed, 

Somerset No. 60 mine. 
LEGEND 

Key Mop 

FIGURE 14. - Location of possible sand FIGURE 15. - lsopach of the draw slate 

channel washout, Somerset unit above the Pittsburgh 

No. 60 mine. coal bed, Somerset No. 60 

mine. 
Key Map central portion of the mine 

property, there is a local 

FIGURE 16. - Outline of the Federal No. 2 mine. upwarping which interrupts 

the gentle westerly dip. 

Analysis of the cleat data showed uniformity within the present mine 

development. The face cleat strikes N68"W, the butt cleat strikes N22"E, and 

the dip is essentially vertical. A series of slump fractures were observed 

in the East Bleeder, and one clay vein (fig. 16) approximately 150 feet long 

extends from the roof 2 feet into the coalbed near the mouth of the East 

Mains. The slump f~ature may be due to some local channel erosion or to 

differential compaction. Geologically related mining problems are methane 

emissions and roof instability, which go hand-in-hand in many instances. 

Driving north-south tends to develop high gas by intersecting face cleats. 

Roof control problems are probably due to bolting into the incompetent alternating coals and shales (draw slate interval). 

4The drillers' location descriptions were general, and with the high relief 

in the area, it is possible for elevations to be in error by as much as 

50 feet. 
Key Map 

FIGURE 17. - Structure map drawn on the base of the Pittsburgh coalbed, Federal No. 2 mine. 

Howe Mine 

The mine is located 6 miles south of Poteau, La Flore County, Okla. It 

is on the south side of the Arkoma Basin, which is described as a synclinorium. 

Figure 18 shows the generalized structure of the Lower Hartshorne coalbed and 

the two major structures present in the area--the Howe anticline and Poteau 

syncline--with the cleat orientations. The Hartshorne coalbed consists of an 

upper bench averaging about 32 inches in thickness, below which there is a 

shale parting up to 24 inches thick, followed by several thin layers of 

usually impure coal laminated with black, carbonaceous shale. 

A panel diagram (fig. 19) constructed from core logs of the test holes 

drilled by the company shows that the immediate roof over most of the property 

is sandstone. According to Bureau research, the absence of shale between the 

coal and sandstone greatly increases the probability of sandstone channels. 
N2S0W, and a single butt 

FIGURE 18. - Structure map drawn on the base of the cleat approximately perpenLower Hartshorne coal bed, Howe mine. dicular to these at N74"E. 

There was only one primary 

joint direction in the roof and floor rocks in the mine, N23"W. This is 

parallel to one mode of the bimodal face cleat. The surface rocks have two 

joint directions approximately 90" apart, N15"W and N72"E. These directions 

correspond closely with face and butt cleats in the Hartshorne coalbed. 

The inclination of the coalbed makes it very difficult to work. Where 

the coal is overlain by sandstone, the bed is undulatory so that the continuous miner must be backed up and floor rock taken to obtain a proper angle 

of attack. The presence of shale roof to the west indicates that the undulation of the coalbed may disappear in this direction. 

Water is a nuisance but does not affect production significantly. It 

appears to enter the sands from older mine workings and the outcrop, and 

migrate down dip to the present .mine-workings. This has been alleviated by 

pmping out the abandoned workings up dip. Possibly where the immediate 

roof is solid shale, the water influx will be less, but it is believed that 

anything short of sealing the roof will not eliminate the water. 
Province and less than 15 miles northwest of the Pine Mountain overthrust 

block. It is operating in the Pocahontas No. 3 coalbed and produces an 

average of 13.2 MM ft3 of methane per day. A geologic survey was conducted 

in April 1973. 

A shear fault (fig. 20) of 2 to 18 feet vertical displacement, but 

unknown horizontal displacement, was encountered withinthe mine, trending 

northwest-southeast at 145" and 155" alinement and dipping 60" to the southwest. Slickensides on the faulted rock surfaces were parallel to bedding, 

indicating that the major movement was horizontal rather than vertical. 

Further evidence from surface exposures, together with data from mine operations in seams above the Pocahontas No. 3, permitted the delineation of a 

strike-slip fault at least 9 miles long. A more detailed study of the area 

using side-looking airborne radar (SLAR) was done by Elder (1). 

Cleat measurements showed face cleat trending N18"W and butt cleat 

trending N67"E. On the northeast side of the fault, a rotation of the cleat 

(fig. 21) indicated that this side had acted as a buttress against which 

the southwest block moved. 

=This survey was made in March 1971. This mine has since closed down, but 

there is a strong possibility that it will reopen in 1975. 

FIGURE 21. - Cleat rotation in relation to shear fault, Beatrice-Pocahontas mine. 
- 

Scale, ft 

Key Mop 

FIGURE 22. - Structure map drawn on the base of the 

Pocahontas No. 3 coal bed, BeatricePocahontas mine. 
0 8.000 

Horizontal 

FIGURE 23. - Fence diagram of the strata above the Pocahontas No. 3 coalbed, 

Eea trice-Pocahon tas mine. 
During the survey, it was noted that washouts occurred with massive sands, 

and a majority of the falls observed had micaceous sands composing the fall 

unit. A map of the type of sandstone (massive or micaceous) found above the 

coal (fig. 24) should anticipate such problems and make it possible to modify 

mining techniques before problem zones are encountered. 

FIGURE 24. - Nature of sandstone above Pocahontas No. 3 coalbed, Beatrice-Pocahontas 

rnl ne. 
An isopach pap (fig. 25) of the coal thickness was prepared to determine 

if any washouts could be located. A second isopach from the coal to the 

sandstone above (fig. 26) was constructed to determine if the roof falls and 

gas content correlated with proximity to the sands and as an additional aid 

in locating washouts and channels. 

Finally an overburden isopach (fig. 27) was drawn to verify a Bureau 

of Mines theory that relates the amount of gas to the thickness of the 

overburden. 

FIGURE 25. - lsopach of Pocahontas No. 3 coal bed (inches), Beatrice-Pocahontas mine. 
-no* Contour intervals / 

0 - 10.000 0 10,000 

Scale, f t 

FIGURE 26. - lsopach of interval between FIGURE 27. - Overburden isopach map of 

Pocahontas No. 3 coalbed interval from Pocahontas 

and sandstone above No. 3 coal bed to surface, 

Beatrice-Pocahon tas mine. Beatrice-Pocahontas mi ne. 

S UMMARY 

The knowledge gained from geologic investigations of mines can help 

control many underground problems. It is of primary importance for the 

coal companies to have accurate core logs from closely spaced drill holes. 

A grid system with not more than 3,000-foot centers, and preferably with 

2,000-foot centers, is recommended for obtaining useful geologic information. 

Basic to the investigation of the various types of mining problems 

encountered is the construction of geologic maps and fence diagrams. They 

can help predict areas of probable roof instability and trends of sandstone 

channels that may cut through the coalbed. 

Clay veins can be mapped in underground surveys and their trends 

predicted into future areas of mining. Predicting trends of clay veins has 

been somewhat less successful than predicting sandstone channels. 

The sources of water accumulations can be evaluated for possible 

geologic influence. Fault zones and roof ftacturing have been shown to 

induce the production of water, as well as gas, into a mine. 
A detailed joint pattern of the area above a mine can be established 

and its relationship to coal cleat direction and roof fractures underground 

evaluated. Excessive spalling of ribs when mining is with the cleat direction can sometimes be corrected by a 45' rotation in the mining direction. 

Joint information can also be of benefit when degasification wells are 

planned for virgin coal areas. Local deviations in the joint and cleat 

directions can be an indicator of geologic irregularities that might adversely 

affect mining conditions underground. 
REFERENCES 

1. Elder, C. He, P. We Jeran, and D. A. Keck. Geologic Structure Analysis 

Using Radar Imagery of the Coal Mining Area of Buchanan County, Va. 

BuMines RI 7869, 1974, 29 pp. 

2. Lahee, F. He Field Geology. McGraw-Hill Book Co., Inc., 6th ed., 1961, 

926 pp. 

3. McCulloch, C. M., and M. Deul. Geologic Factors Causing Roof Instability 

and Methane Emission Problems: The Lower Kittanning Coalbed, Cambria 

County, Pa. BuMines RI 7769, 1973, 25 pp. 

4. McCulloch, C. M., M. Deul, and P. We Jeran. Cleat in Bituminous Coalbeds. 

BuMines RI 7910, 1974, 25 pp. 
GLOSSARY OF TERMS 

Anticline: A fold that is convex upward or had such an attitude at some stage 

of development. 

Axis: A straight line, real or imaginary, passing through a body or system 

around which the parts are syrmnetrically arranged. 

Bimodal: A frequency distribution characterized by two localized modes, each 

having a higher frequency of occurrence than other immediately adjacent 

individuals or classes. 

Butt cleat: A short, poorly defined cleavage plane in a coalbed, usually 

oriented at right angles to the face cleat and sometimes terminating 

against it. 

Buttress: A part of the rock near a fault along which no movement has 

occurred. The movement has taken place against this wall. 

Clay vein: A body of clay, usually roughly tabular in form like an ore vein, 

which fills a crevice in a coal seam. It is believed to originatel where 

pressure has been sufficient to forceclay from the roof or floor into small 

fissures and in many cases to alter and enlarge them. 

Coal cleat: A joint or system of joints along which the coal fractures. 

There are usually two cleat systems developed perpendicular to each other. 

Draw slate: Soft shale that occurs above a coalbed and collapses after the 

removal of the coal. 

Differential compaction: The relative change in thickness of mud and sand 

(or limestone) after burial due to reduction in pore space. 

Dip: The angle at which a stratum or any planar feature is inclined from 

the horizontal. The dip is at a right angle to the strike. 

Face cleat: A well-defined joint or cleavage plane in a coal seam. The 

major joint in a coal seam. 

Fault: A fracture or fracture zone along which there has been displacement 

of the sides relative to one another parallel to the fracture. 

Fence diagram: A diagram that is a drawing in perspective of three or more 

geologic sections with their relationship to one another (also called 

panel diagram). 

Isopach map: A map that shows the varying of the interval (thickness) between 

two designated stratigraphic units or horizons; for example, a map prepared 

directly from two structure-contour maps by subtracting the elevations of 

the lower surface from those of the upper surface at each control point and 

drawing contours of equal interval between the two surface. 
Joints: Fracture in rock, generally more or less vertical or transverse to 

bedding, along which no appreciable movement has occurred. 

Linears: A straight or gently curved physiographic feature on the earth's 

surf ace. 

Lithofacie map: A map that is based on lithologic attributes, showing 

variation in the overall lithologic character of a given stratigraphic 

unit. The map may emphasize the dominant, average, or specific lithologic 

aspect of the unit, and it gives information on the changing composition 

of the unit throughout its geographic extent. 

Lithologic frequency map: A map that shows the number of rock types or the 

frequency of times that different rock types occur in a given vertical 

sequence over a given horizontal distance. 

Monocline: Strata that dip for an indefinite or unknown length in one 

direction, and which do not apparently extend from sides of ascertained 

anticlines or synclines. 

Orthogonal: A combining form meaning straight, at right angles. 

Percent map: A map that depicts the relative amount (thickness) of a single 

rock type in a given stratigraphic unit as a percent of the total rocks in 

that vertical interval. 

Rider coal: A thin unminable coal found closely above a thicker minable 

coalbed. Normally only a few inches thick. 

Sandstone channel: A sandstone body that ranges in thickness from several 

inches to many feet and in length up to several miles and that cuts across 

structure and bedding of the enclosing rocks. Also called a clastic dike. 

Shear fault: A fracture that results from stresses which tend to shear one 

part of a specimen past the adjacent part. 

Slickenside: Polished and striated (scratched) surface that results from 

friction along a plane. The movement can be very slight. 

Spalling of ribs: Relatively thin, commonly curved and sharp-edged, piece 

of rock (coal) produced by weathering. 

Strike: The course or bearing of the outcrop of an inclined bed or structure 

on a level surface. It is perpendicular to the direction of the dip. 

(Also, the bearing of a horizontal trace on the bedding plane.) 

Structure contour map: A map that portrays the subsurface configuration of 

structure of a unit by means of contour lines. 

Syncline: A fold in rocks in which the strata dip inwafd from both sides 

toward the axis. 
Synclinorium: A broad regional syncline on which are superimposed minor 

folds . Thrust fault: A reverse fault that is characterized by a low angle of 

inclination with reference to a horizontal plane. 

Washout: A channel cut into or through a coal seam at some time during or 

after the formation of the seam, and generally filled with sandstone. 

* U.S. GOVERNMENT PRINTING OFFICE: 1975-603-752/4 INT .-BU.0 F MINES,.PGH.,PA. 20077