The Lower Kittanning Coallbed, 

Cambria County, Pa. 

UNITED STATES DEPARTMENT OF THE INTERIOR 
U N I T E D S T A T E S DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Elburt F. Osborn, Director 
This publication has been cataloged as follows: 

McCulloch, Charles M 

Geologic factors causing roof instability and methane 

emission problems: the Lower Kittanning coalbed, Cambria 

County, Pa., by C. M. McCulloch and Maurice Ueul. [washington] U.S. Bureau of Mines [I9731 

25 p,, illus,, tables, (U,S, Bureau of Mines. Report of investigations 7769) 

Includes bibliography. 

1. Coal mines and mining-Pennsylvania-Cambria County, I. 

Deul, Maurice, jt. auth. 11. U.S. Bureau of Mines. 111. Title. 

(Series) 

TN23.U7 no. 7769 622.06173 

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

Page 

Abstract ................................................................ Introduction ............................................................ Acknowledpents ......................................................... Previous and concurrent work ...........................................a 

Stratigraphy ............................................................ Regional and local structure ............................................ SLAR imagery ....................................................... Rock joints and coal cleats ........................................ Methane emission in the Cambria mines.^.......,,................... 

Roof joints and roof falls ......................................... Lithologic variability ....................................e............. 

Lithologic frequency ............................................... Lithologic frequency between Lower Kittanning coal and the 

next higher coal ................................................. Relative distribution of sandstone and shale ....................... Shale plus limestone to sandstone ratio ............................ Vertical thickness of interval between Lower Kittanning coal 

and the next higher coal ......................................... Discussion and engineering applications ma..............................e 

Conclusions and recommendations ......................................... References ...e.......................................................... 

ILLUSTRATIONS 

Generalized section of the Allegheny group of the southern part 

of Somerset County. Pa ........................................... Generalized section of the Kittanning formation of central 

Cambria County. Pa ............................................... Generalized structural contour map of area around Cambria 

Nos . 32 and 33 mines .................................a.a......... 

Index map ofsurface joint sampling locations ....................... Strike of coal cleats and rock joints .............................. Sketch map showing where unstable roof is likely to occur .......... Panel diagram of interval between Lower and Upper Kittanning 

coalbeds ..................................me....mmm.mmm.a.em..... 

Lithologic frequency map of interval between Lower and Upper 

Kittanning coalbeds .............................................. Lithologic frequency map of interval between Lower Kittanning 

coalbed and next higher coalbed a.O............................... 

Percentage of sandstone 50 feet immediately above the Lower 

Kittanning coalbed ............................................... Percentage of shale 50 feet immediately above the Lower Kittanning 

coalbed .......................................................mm. 

Map of shale plus limestone to sandstone ratio ..................... Inferred sandstone-filled channel .................................. Isopach map of interval between Lower Kittanning coalbed and 

next higher coalbed .............................................. Present mine working outline showing known areas of unstable roof .. 
TABLES 

Page 

1. Average strike of coal face cleat and systematic rock joints ........ 6 

2. Average strike of coal butt cleat and nonsystematic rock joints..... 10 
GEOLOGIC FACTORS CAUSING ROOF INSTABILITY 

AND METHANE EMISSION PROBLEMS 

The Lower Kittanning Coalbed, Cambr ia County, Pa. 

by 

C. M. McCulloch and Maurice Deul 

ABSTRACT 

A geologic study of Mines 32 and 33 in Cambria County, Pa. , was conducted 

as part of a methane control research program. Areas of high methane emission 

and roof instability are encountered which are related to the structural and 

stratigraphic features that characterize the geologic setting of these mines. 

Locally the folds trend N 20' E +2O, the direction of the butt cleat in the 

Lower Kittanning coal. The face cleat strikes at N 69" W +2O and the systematic rock joint in the surface bedrock strikes N 67" W +Zoo This suggests 

that a directional stress oriented N 70' W produced the folds and influenced 

the directions of coal cleat and systematic joint sets. Analysis of SLAR indicates three directions of prominent regional lineation: N 39" E, N 31" W, and 

N 70° W. 

Little gas was encountered during development mining; large volumes of 

gas are encountered when roof falls. Most of the gas comes from strata above 

the mined coal; therefore, to control methane, roof instability must be 

controlled. 

Roof falls and high rates of methane emission are most likely where less 

than 30 feet of many thin layers of limestone, shale, and sandstone occurs 

between the Lower Kittanning coal and the next coal and in zones along the 

margins of a sandstone channel. 

INTRODUCTION 

The Cambria Nos. 32 and 33 mines are located in western Pennsylvania near 

Ebensburg, about 60 miles east of Pittsburgh. The No. 33 is a slope mine, 

whereas the No. 32 is a shaft-type mine; both are presently working beneath 

300 to 1,000 feet of cover in the Lower Kittanning coalbed. Data on the compass directions of joint sets and coal cleats, together with regional structure and stratigraphic observations, were analyzed for indicators of geologic 

control of the emission of methane from the coalbed into the mine workings, 

and of relationships to roof falls in the mines. 

Supervisory geologist . 
This project was conducted as part of the methane control research program to determine the cause of the large amount of methane being emitted into 

the mine workings. Areas of large methane emissions were found to be areas of 

roof instability. Therefore, for these mines, studies of roof instability 

were a natural consequence, although not the original objective. 

ACKNOWLEDGMENTS 

We acknowledge the cooperation and assistance of the management of 

Bethlehem Mines Corp. with whom we have held cooperative agreements for many 

years. Special thanks are due to personnel of the Cambria division for their 

personal interest and support, notably A. T. Sossong, the division engineer, 

We acknowledge the assistance of Lou Benedict from the Bethlehem Research 

Center for the time and effort he devoted in the field and for his critical 

comments. We also thank Bion Kent of the U.S. Geological Survey for his 

critical review and many thoughtful suggestions, 

PREVIOUS AND CONCURRENT WORK 

The first geologic work in the Cambria County area was done by 

H. D. Rogers in 1858 (15) ,3 who conducted the first geologic survey of the 

entire State of Pennsylvania. In 1877, F. Platt and W. G. Platt (14) did a 

geologic study of the Cambria and Somerset districts. Both these reports 

were excellent and have been updated recently. 

In 1905, Charles Butts (3) completed the next major work on the Cambria 

County area with his "Ebensburg Portfolio," a geologic study of the Ebensburg 

15-minute quadrangle that included a structure map drawn on the Upper Freeport 

coalbed. The stratigraphy of the area was described in that report but the 

group, formatiozl, and member names of some of the units have been changed. 

Ashley, Sisler, and Reese (L), 1925-1928, of the Pennsylvania State Geological Survey wrote a report on the "Bituminous Coal Fields of Pennsylvania." 

This was a description of individual coalbeds, coalfields, and a summary of 

resources of Pennsylvania. 

Dutcher (5) produced a field trip guide to this general area for the 

Pittsburgh meeting of the Geological Society of America. 

Concurrent work in the Cambria County area is being done by Lou Benedict 

of Bethlehem Steel Corp. on some of the same problems dealt with in this paper. 

His results to date agree generally with the findings of this report even 

though his techniques are somewhat different. 

STRATIGRAPHY 

Only the Allegheny group of the Pennsylvanian system will be discussed 

since all the coalbeds that are important to this study are included within it, 

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

the end of this report. 
The Allegheny group extends from the base of the Brookville coal to the top of 

the Upper Freeport coal (8). The average thickness in Cambria County is 

approximately 250 feet. This group is composed of shale, siltstone, sandstone, 

coal, and some fresh-water limestone lenses found in the upper third of this 

group. Flint (8) states that the Johnstown limestone is persistent and is 

used as a stratigraphic guide in Somerset County to the south. In the surface 

area around Ebensburg, where this study was conducted, and from core hole data, 

this limestone, although less persistent, is still found in 10 of the core 

holes. 

The Allegheny group is subdivided into three formations following 

Shaffner (16) and Williams (17). These are Clarion, Kittanning, and Freeport. 

The boundary between the Clarion and Kittanning formations falls within the 

Lower Kittanning coal group. The Kittanning formation (l8) is made up of 

A Upper Freeport coal 

per Freeport limestone and 

Butler sandstone 

-Lower Freeport coal group 

Freeport sandstone 

Upper Kittanning coal group 

nstown limestone 

pper Worthington sandstone 

'Middle Kittanning coal group 

,Lower Kittonning coal group 

ubgrass coal 

-Clarion coal 

-Clarion clay (Mt.Savage) 

C lor ion sandstone 

Brookvi l le coal 

clay 

FIGURE 1. - Generalized section of the Allegheny group of the southern part of Somerset 

County, Pa. 
structures are the Ebensburg anticline and the Wilmore synciine (fig. 3). 

These features are part of the 

Upper Kittanning coal Allegheny Mountain section of the 

Appalachian Plateaus province and 

Johnstown limestone trend northeastward. 

The structural contours in 

figure 3 were drawn on the bottom of 

the Lower Kittanning coalbed from 

mine and core hole data. 

SLAR Imagery 

Upper Worthington sandstone 

The SLAR (side look airborne 

radar) imagery was used to supplement 

the fieldwork and computer treatment 

of field data. The main utility of 

Middle Kittanning coal 

60 SLAR imagery is that it permits sys- - -- - - -- - tematic examination of large areas 

making it possible to note subtle , . . , - - - - >::.:<..> - - - ;-:z:<$>-~~.:: topographic differences (lineations, , ,;f.2.r;..1?, -- 

ULower Ki ttanning coal 

faults, etc.) over long distances 

that would have been missed by normal 

geologic methods (5, 10). FIGURE 2. - Generalized section of the Kittanning formation of central The interpretation of the SLAR 

Cambria County, Pa. imagery is similar to that used in 
FIGURE 3. - Generalized structural contour map of area around Cambria Nos. 32 and 33 mines. 
standard photogeologic interpretations. A Ronchi grating (a type of diffraction grating) can be used between the eye and the imagery; when rotated, 

strong lineations will become easily visible. The grating is to SLAR what 

stereographic glasses are to paired aerial photographs. 

SLAR imagery was adjusted for true distances and directions from geological reconnaissance studies. According to MacDonald (12) , "This computation 

can be accomplished most easily where a known distance between two points on 

the ground can be compared with the same two points on the radar imagery." 

SLAR imagery of Cambria County, Pa., was studied for this report. Three 

prominent directions of lineations were observed on the SLAR imagery; these 

are (1) N 39" E, (2) N 31" W, and (3) N 70" W. These directions are indicated 

on figure 4. 

Rock Joints and Coal Cleats 

The strike and dip of the coal cleats and the surface rock joints were 

measured. The orientations of the various cleat and fundamental joint system 

directions are shown in figure 5. Nickelson (13) divided the fundamental 

joint system into two parts: (1) systematic joints and (2) nonsystematic 

joints. 

According to Nickelson (g), "The complex pattern of jointing results 

from overprinting of two or more fundamental systems" with the systematic 

joint having well-defined axes whereas the nonsystematic joints are poorly 

de fined. 

From table 1 the average strike for the face cleats of the coal is 

N 69" W, whereas the average strike for the systematic surface rock joint is 

N 67" W, including the measurements of rock joints at location 4 in figure 4. 

The strikes of the two features are about the same, suggesting that there is 

a relationship between the strike of the systematic rock joints and the coal 

face cleats. 

TABLE 1. - Average strike of coal face cleat and systematic 

rock joints 

Coal face cleat................. 

Systematic rock joints.......... 

DO......................... 

Total.. ............... l~umber in parentheses corresponds to position on index 

map. All are +2" compass directions. 

The general locations of surface joint measurements are shown in figure 4. 
FIGURE 4. - Index map of surface ioint sampling locations. 
I:I ratio Scale, IOmeawrements 1:2 ratio Scale. IOmeasuremenrs 

ROCK-JOINT MEASUREMENTS 

FIGURE 5. - Strike of coal cleats and rock ioints. 

Measurements at location 4 were made at sites where we found later there 

had been major surficial dislocation either by slumping due to erosion of 

adjacent stream valleys or to caving of underground workings. Comparing 

figure 4 with figure 6, it can be observed that "individual surface measurement locations" 16 and 17 are directly over areas where roof falls have 

occurred, supporting the idea that caving may be a factor in the deviation of 

these measurements from the norm. The computer program used to treat these 

data selected the center of a trimodal distribution as the strong direction 

rather than one of the extremes. Figure 5 illustrates the trimodal distribution at location 4. It appears that the lower of the three northwest lobes 

is the stronger but the difference is small. Field checks show that the most 

prominent direction is the one designated at location 4 as being the "systematic rock joint ," N 68' W, very close in direction to the other four areas of 

surface measurement, but the other two lobes indicate some change or rotation 

and contribute to making the average direction N 58O W. 
FIGURE 6. - Sketch map showing where unstable roof is likely to occur. 
From table 2 the average strike for the butt cleat is N 19" E, and the 

average strike for the nonsystematic rock joint is N 04O W. As can be seen 

from the data, the variations in the nonsystematic joint measurements are too 

great to have any significance; this was also supported by the computer program printout. 

TABLE 2. - Average strike of coal butt cleat and nonsystematic 

rock ioints 

Nonsystematic rock joints.......... 

DO............................ 

DO............................ 

DO............................ 

Coal butt cleat.................... 

All are +2" compass directions. 

Measurements 

1,725 

DO............................ 

Total.................... 

There is a relationship between the axial direction of the local folds 

and the strike of the butt cleats. The local fold axes range from N 35" E 

to N 12" E but mostly between N 17" E and N 22" E, nearly the same as the 

average butt cleat direction of N 19" E. Figure 5 shows a relationship 

between the coal face cleat and systematic rock joint direction but shows no 

relationship between the coal butt cleat and nonsystematic rock joint 

direction. 

strike1 

N lgO E 

This is a summary of the strikes of the coal cleats and rock joints: 

 umber in parentheses corresponds to location on index map. 

170 

1,045 

1. Strike of face coal cleat--N 69" W. 

N 62" E (5) 

2. Strike of systematic surface rock joint--N 67" W. 

3. St~ike of butt coal cleat--N 19" E. 

4. Strike of nonsystematic surface rock joint--N 04" We 

From this it is evident that there is a parallelism between the N 70" W major 

regional lineation, the face coal cleat, and the systematic rock joint. No 

relationship is evident between any of the three major regional lineations 

determined from SLAR imagery and the butt cleat or nonsystematic rock joint 

strikes. 

Methane Emission in the Cambria Mines 

Underground, in the Cambria Nos. 32 and 33 mines, water seepage and 

methane emission from the roof is observed along prominent fractures that 

trend N 70" W, a direction that parallels the systematic joint sets, the coal 

face cleats, and the major regional lineations. 
The amount of gas emitted directly from the Lower Kittanning coal into 

the mine working is known to be small. Upon development of mine workings and 

in preparation for installation of longwall mining equipment, methane emission 

rates are very low. In one methane emission study in a development section, 

only 23 cfm of methane was measured. In contrast, when longwall mining is 

being conducted, methane emission rates rise to more than 800 cfm from a 

single section (ll). Normally, in these mines, most of the methane emission 

occurs when retreat longwall mining induces roof falls with subsequent methane 

accumulation in the gob. Methods of removing this methane have been described 

by Elder (6) - and by Ferguson (L). 

Methane emission rates from all the principal coal mines of the United 

States are listed in a report by Irani (9). These reported data do not 

include the amounts of methane removed from gobs by vertical boreholes. 

Because methane occurs in the strata immediately above the Lower 

Kittanning coalbed in these mines, unexpected roof falls are the main cause of 

high rates of methane emission in development sections. Consequently, methane 

control during development is best accomplished by controlling roof. 

Bureau of Mines studies demonstrated that within any mine directional 

permeability does exist. The face cleat is the dominant cleat direction; 

because it has greater horizontal length and is usually better developed, one 

would expect to encounter more gas in mining perpendicular to the face cleat. 

This is a demonstration of directional permeability. 

Two holes were drilled horizontally into the coalbed at N 65" W and 

N 25" E. The gas pressure along the N 65" W direction was 12.0 psi, whereas 

the gas pressure measured along the N 25" E direction was 30 psi. The direction of these measurements approximate the face and butt cleat directions, 

which average N 68" W and N 19" E. Because the face cleat is more continuous, 

it would be expected to yield more gas where transected; this is supported by 

the fact that the horizontal borehole drilled at N 25" E (almost parallel to 

the butt cleat) yields 2.5 times as much gas as the hole drilled parallel to 

the face cleat. 

Roof Joints and Roof Falls 

We have observed roof instability (1) where differential compaction and 

folding have occurred along the side of channel sandstone deposits, as along 

the east side of the Cambria 32 mine, and (2) where the sandstone has formed a 

sheetlike structure and numerous (greater than nine), thin lithologic units 

occur directly above the Lower Kittanning coal within an interval of 30 feet. 

Where the next coal above the Lower Kittanning occurs less than 30 feet above, 

and/or where the space between joints is less than 4 feet, roof instability is 

even more likely. Most of the roof falls observed showed coal in the upper 

part of the fall and the vertical interval was less than 30 feet. 

Roof joints were measured underground where possible. Most (26 outof35) 

ranged from between N 65O W to 73" W, roughly parallel to regional lineation 

of N 70" W (SLAR). The other nine readings were scattered, but at least two 

each mirrored the other regional lineations (N 31" W, N 39" E) as determined 

from the SLAR. 
LEGEND 0493 Core no. 

0 Shole 0500 Limestone 

Sondstone and shale 

503 interbeded 

Coal, base is Lower 

Kittonning (B) cool 

4 

491 A 483. 0494 Horizontal scale, feet 

FIGURE 7. - Panel diagram of interval between Lower and Upper 

Kittanning coalbeds. 

nel fill deposit 

changing abruptly 

to shale or as 

sheetlike deposits 

which interfinger 

with shales. 

To display 

the lithologic variability of this 

area, a panel diagram (fig. 7) was 

prepared from core 

logs using the 

interval between 

the Lower and 

Upper Kit tanning 

coalbeds. In the 

north, sandstone 

is the predominant 

rock; southward, 

the shale becomes 

dominant. The 

panel diagram 

shows a channel 

area along the 
east margin of the mine that eventually broadens to form a sheetlike structure 

to the south and west. 

As an aid to analysis of the sedimentation and to insure complete objectivity, the following maps were drawn by computer to indicate the following: 

lithologic frequency of the interval between the Lower Kittanning and Upper 

Kittanning coalbeds, lithologic frequency of the interval between the Lower 

Kittanning and the next higher coal, percentage of sandstone in coalbed area, 

percentage of shale in coalbed area, shale plus limestone to sandstone ratio, 

and the interval between the Lower Kittanning and the next higher coalbed. 

This was done by laying a grid over a base map of the core holes; these could 

be located by the computer program along X (distance in feet east of the starting point or zero point) and Y (distance in feet north of the zero point) 

coordinates. The X and Y values are used in the computer program only to 

locate the core holes on the map. The data to be plotted (the Z value) are 

selected from the core logs as required; for example, the percent sandstone 

for a given interval as in the maps indicating the percentage of sandstone, or 

the number of lithologic units as in the maps on lithologic frequency. The 

Z value is established for each core hole, and this value plus the X and Y 

coordinates are used in the computer program to plot a map using an appropriate contour interval. The X and Y coordinates are constant for all computer 

output maps, and only the Z value and the contour interval vary. 

Lithologic Frequency 

A lithologic frequency map (fig. 8) shows the number of separate lithologic units for a given interval. The interval is arbitrarily selected depending on the information to be illustrated. This lithologic frequency map was 

drawn using as contours the number of lithologic units occurring in the approximately 100-foot interval between the Upper and Lower Kittanning coalbeds. 

The number ranges from four to 24 lithologic units. A lithologic unit is 

defined here as any thickness greater than 0.5 foot in thickness. 

One area showing an extremely large number of lithologic units, more than 

20, and two other areas showing more than 15, were found. These areas coincide with known areas of roof instability in the mine. When conducting underground surveys in these areas of roof instability, it was noted that the 

joints in the roof were closely spaced and that the distance to the next coal 

above the Lower Kittanning was less than 32 feet wherever the roof had fallen. 

In contrast, a smaller number of thicker lithologic units would indicate relatively stable roof conditions. 

The close proximity of zone of a very high and low number of lithologic 

units is an indication of rapid sedimentational changes and a site of potentially unstable roof. 

Lithologic Frequency Between Lower Kittanning Coal 

and the Next Higher Coal 

A low lithologic frequency indicates areas where relatively stable roof 

rock can be expected. Areas of intermediate lithologic frequency may or may 
the Lower Kittanning coal appears. 

There is a marked 

similarity between 

these two maps, 

especially in the 

northwestern part 

where there is 

rapid transition 

from less than five 

lithologic units to 

more than nine 

lithologic units in 

a horizontal distance of 2,000 feet. 

The interval 

between the Lower 

Kittanning and the 

next higher coal 

(whether the Middle 

FIGURE 8. - Lithologic frequency map of interval between Lower and Kittanning or a 

Upper Kittanning coalbeds. wild coal) was 

selected because, 

as was stated earlier, it was observed underground that the upper extent of 

most roof falls was the bottom of the coal directly above the Lower Kittanning 

coalbed. 
FIGURE 9. - Lithologic frequency map of interval between Lower 

Kittanning coalbed and next higher coalbed. 
Relative Distribution of Sandstone and Shale 

The maps showing the percentage of sandstone and the percentage of shale 

(figs. 10-11) outline a channel. In preparing these maps, the interval used 

was the 50 feet immediately above the Lower Kittanning coalbed. These maps 

show a high percentage of sandstone and a low percentage of shale, which 

define the channel area. Outside the channel there are, conversely, high 

shale and low sandstone percentages. 

Shale Plus Limestone to Sandstone Ratio 

Study of figure 12 substantiates the existence ofa channel. A relatively 

large ratio number indicates a small percentage of sandstone in relation to 

shale and limestone. The sv~ll ratio numbers, representing sandstone-rich 

zones, can be followed through the middle of the map, indicating channel and 

sheetlike sandstone deposits. At the southern end, no definite sandstone 

pattern can be detected. The trend of the channel as inferred from this study 

is shown in figure 13. This shows that the inferred boundary between the 

channel sandstone deposit and shale follows the trend of known roof instability in the mines. 

Vertical Thickness of Interval Between Lower Kittanning Coal 

and the Next Higher Coal 

The interval between the Lower Kittanning coal and the next higher coal 

is critical because the uppermost plane along which the roof tends to separate 

is the bottom of the coalbed immediately above (whether a wild coal or the 

Middle Kittanning coalbed). From falls observed underground and related to 

this map (fig. 14) we find a correlation between roof instability and a thin 

zone, less than 30 feet (vertically) between the upper coal and the Lower 

Kittanning coalbed, especially where the roof strata are well jointed. Where 

this interval is more than 50 feet, the probability of roof instability is 

greatly decreased. The areas of highest occurrence of roof falls correlate 

with the thin areas of this interval on the isopach map and the areas of high 

frequency of lithologic units. 

DISCUSSION AND ENGINEERING APPLICATIONS 

On June 23, 1972, after the main theme of this report had been developed, 

a plan view of the two mines was received from the Bethlehem Mines Corp. showing areas of unstable roof. This map (fig. 15) was then reduced in size to 

overlai it on the panel diagram (or fence diagram) and the other maps used in 

this study. By comparing the map of unstable roof areas with the maps drawn 

by us, it became apparent that falls most commonly occur along the roofmargins of the channel fill sandstones which parallel the eastern boundary of 

both the Cambria 32 and 33 present mine workings. From these data we compiled 

a map (fig. 6) that shows where unstable roof is likely to occur in the future. 
Scale, feet 

FIGURE 10. - Percentage of sandstone 50 feet immediately above the 

Lower Kittanning coa lbed. 
FIGURE 11. - Percentage of shale 50 feet immediately above the Lower 

Kittanning coalbed. 

LEGEND 

0487 Core no. . . . . . . . . 

.:.:.;. : :.: Sandstone . . . . . . . Shale 

- 

-- 

- 

0-00 

- -Stole, feet 

FIGURE 13. - Inferred sandstone-filled channel. 
FIGURE 14. - lsopach map of interval between Lower Kittanning coa lbed 

and next higher coalbed. 
Scale, feet 

FIGURE 15. - Present mine working outline showing known areas of unstable roof. 
CONCLUSIONS AND RECOMMENDATIONS 

This study shows that methane emission in these mines is more closely 

related to roof falls and emission from strata above the Lower Kittanning 

coalbed than to any other single factor. 

A channel sandstone deposit occurs immediately above the Lower Kittanning 

coalbed along the eastern part of the present No. 32 workings and extends over 

much of the No. 33 mine (fig. 12); this sedimentational feature is the cause 

of some of the mining problems encountered. 

There are four geologic reasons for inherent roof instability in these 

mines: (1) Differential compaction of the sediments along the boundaries of 

the channel sandstone deposit, (2) a large number (generally more than nine) 

of lithologic units in the 30-foot interval above the Lower Kittanning coalbed, 

(3) the occurrence of another coalbed about 30 feet above the Lower Kittanning 

coalbed, and (4) a spacing of less than 4 feet between the systematic joints 

in the rock immediately above the Lower Kittanning coalbed. Examinations of 

roof falls in the mine indicate that falls usually are due to at least two of 

these reasons and that the first two are the major causes. 

There is an alinement between the N 70" W regional lineation, the strike 

of the face cleat of the coal and systematic joints of surface rocks, and at 

most of the roof joints observed underground. 

Consideration of the criteria for roof instability as developed in this 

report should aid in the planning for further mine development so that 

unexpected roof falls can be minimized. Furthermore, when development must 

proceed in areas where there is a high probability of roof instability, the 

mine operator will be able to anticipate and prepare for the consequences so 

as to arrive at an alternate plan for mining this area and providing supplementary ventilation to cope with the expected influx of methane. 

Valuable data for such interpretation can be acquired from core-drilled 

vertical holes where detailed geological information is systematically 

collected. We recommend that more core holes be drilled at a distance no 

greater than 3,000 feet from the nearest existing hole. This would be especially helpful in determining if the criteria for roof instability are evident 

to the east and southeast of the present workings of the Cambria No. 33 mine. 
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