______________________________________________________________________ 

HIGH STRESS MINING UNDER SHALLOW OVERBURDEN IN 

UNDERGROUND U.S. STONE MINES 

Anthony T. Iannacchione, Dennis R. Dolinar, Thomas P. Mucho 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, PA USA

ABSTRACT:

The Chestnut Ridge Anticline is a prominent structure in southwestern Pennsylvania, 

USA. This structure has brought two economically valuable limestone formations to the 

surface, where they are easily accessible to room-and-pillar mining. Nine active crushed 

stone quarries, an underground research facility, and one industrial facility currently 

operate along the linear trend that represents the axis of the anticline. The National 

Institute for Occupational Safety and Health (NIOSH) has identified significant variation 

in levels of horizontal stress within relatively short distances at shallow overburdens 

along the anticline. At one quarry, the horizontal stress is high enough to shear strata with 

a modulus of approximately 77 Gpa. Conversely, zones of extension were observed in 

parts of another quarry, indicating very low horizontal stress conditions. This study found 

that high levels of horizontal stress are to be expected in this very stiff rock formation and 

that local geologic and topographic structures are capable of altering the magnitude and 

possibly the orientation of the horizontal stress field. 

INTRODUCTION 

While examples of ground control problems associated with mining at depth abound, high 

horizontal stresses at low overburdens also exist in some locations and can significantly 

impact local mine ground control. Factors responsible for these excessive levels of stress 

need to be understood and engineering controls must be developed and implemented in 

order to reduce the risk of falls of ground to miners. To evaluate these factors, a 

prominent stone mining district 90 km southeast of Pittsburgh, Pennsylvania, USA was 

selected. This mining district is within the Mid-North American Plate where high 

horizontal stresses have been recognized (Zoback, 1992) and measured in many of the 

area’s coal mines (Mark and Mucho, 1994; and Su and Hasenfus, 1995). 

In 1995 during some routine reconnaissance, the authors noted shear failures at an 

underground stone mine similar to the cutter roof failure so common in local underground 

coal mines. These shear failures were particularly revealing to the authors since they 

occurred in limestone roof rock with compressive strengths ranging from 138 to 240 Mpa. 

This is almost an order of magnitude greater than the brittle shale roof involved in many 

of the region’s coal mine cutter roof falls. It was apparent that stresses at this site must be 

high to fail such strong roof rock; however, the reasons for these excessive levels of stress 

were not readily apparent. This paper concentrates on explaining what factors might 

cause high stress conditions in this area. Discussions of the most effective techniques for 

minimizing the impact of excessive horizontal stress on miner safety have been discussed 

in another publication (Iannacchione et al., 2001). 
______________________________________________________________________ 

BACKGROUND 

Most of this region’s stone quarries occur in one of three geologic units. The youngest is 

the Vanport limestone, mainly mined north of Pittsburgh, Pennsylvania, and known for 

it’s high concentration of calcium carbonate. The other two limestones, the Greenbrier 

and Loyalhanna, are older geologic units contained within Middle-Mississippian strata. 

These two units are relatively close to one another, separated by non-carbonate rocks 

ranging in thickness from 0.3 m to approximately 10 m. Because of their close 

stratigraphic proximity, they often crop out in adjacent locations. In Pennsylvania, the 

Loyalhanna has been mined more extensively because it is thicker and contains a 

relatively high concentration of quartz, making it an excellent aggregate for highway 

construction. 

The Loyalhanna, typically well below the coal measure rocks that blanket much of this 

area, is brought close to the surface along prominent anticlinal structures. The largest of 

these structures, Chestnut Ridge, Laurel Hill, and Negro Mountain Anticlines, are located 

southeast of Pittsburgh in the Allegheny Mountain Section of the Appalachian Plateaus 

Province (Figure 1). These anticlines form topographic highs that extend for great 

distances, paralleling the trend of the Appalachian Mountains further to the southeast. 

Chestnut Ridge is the last major anticline in the northwest portion of the Allegheny 

Mountain Section with significant structural relief. The Loyalhanna rises to heights 

between 500 and 800 m along the crest of the Chestnut Ridge and falls to depths as low as 

150 m below sea level along adjacent synclines. It crops out at many locations along the 

length of Chestnut Ridge, supplying numerous locations for quarries. 

West Virginia

Pittsburgh

N

Pennsylvania

Allegheny Mountain Section

Maryland

Chestnut Ridge Anticline

Laurel Hill Anticline

Negro Mountain Anticline

Pennsylvania

Allegheny

KEY

Mountain

Section

(11)

(12)

(13)

(14)

Scale, km

0 5 10

(4)

Study area

(5 & 6) (7)

(8)

(9)

(10)

(1)

(2) (3)

Pittsburgh Low Plateau Section

location

Map

Figure 1 Three large anticlinal structures with more than 500 meters of 

elevation change occur within the Allegheny Mountain Section of the 

Appalachian Plateaus Province. Also shown are the locations of fourteen 

active and inactive quarries along Chestnut Ridge. 
______________________________________________________________________ 

Nine active and three abandoned quarries are located in the Loyalhanna along Chestnut 

Ridge (Table 1). Two other sites are currently being used for research or industrial 

activities. It is remarkable that within this relatively high concentration of quarries, 

significant variations in stress and associated ground conditions exist. To help illustrate 

this variation, a portion of Chestnut Ridge centered around the Youghiogheny River 

Gorge was examined (Figure 1). This area possesses many of the diverse characteristics 

of quarries within this mining district. One of the quarries has excessive levels of 

horizontal stress while another has low levels of horizontal stress. 

Table 1 Active and inactive quarries and other research and industrial sites within the 

Loyalhanna and Greenbrier Limestones along Chestnut Ridge. 

No. Name County, State Site Characteristic Strata Status 

1 Buckeye Monongalia, WV Surface quarry L1 & G2 Active 

2 Lake Lynn 

Laboratory 

Fayette, PA Experimental surface and 

underground quarry 

L & G Active 

3 Laurel Fayette, PA Surface quarry L & G Active 

4 Thompsons Fayette, PA Surface quarry L Inactive 

5 Coolsprings No.1 Fayette, PA Surface and underground quarry L Active 

6 Coolsprings No.2 Fayette, PA Surface and underground quarry L Active 

7 Dynaclad Fayette, PA Industrial site surface and 

underground quarry 

L Active 

8 Casparis Fayette, PA Surface and underground quarry L Inactive 

9 Springfield Pike Fayette, PA Surface and underground quarry L Active 

10 Rich Hill Fayette, PA Surface and underground quarry L Active 

11 Whitney Westmoreland, PA Surface and underground quarry L Active 

12 Latrobe Westmoreland, PA Underground quarry L Active 

13 Gray=s Westmoreland, PA Surface quarry L Inactive 

14 Torrance Westmoreland, PA Surface and Underground quarry L Active 

1

L = Loyalhanna Limestone 

2

G = Greenbrier Limestone 

STRESS CONDITIONS WITHIN QUARRIES ALONG CHESTNUT 

RIDGE

In the past, Loyalhanna quarries were not expected to have stress-related problems 

because of their relatively shallow overburdens. An area containing 150 m of overburden 

with rooms and pillars 12 m wide should have average vertical pillar stresses of less 14 

Mpa. Even with rooms 13.7 m and pillars 10.7 m wide, the average vertical pillar stress 

should be approximately 18 Mpa. If stress concentrations near the rib of the pillar double, 

these average values would reach 36 Mpa. Since the Loyalhanna’s unconfined 

compressive strengths are 4 to 6 times greater than these peak stress conditions, it is 

unlikely that vertical stress, generated from the thickness of rocks above the mining 

horizon, will be a significant problem for this mining district. 

If horizontal stresses were due solely to the Poisson effect, they would be even smaller 

that the vertical stresses. Such low stress concentrations could not possibly induce shear 

failures within the strong limestone roof rock, even in the thinnest of layers. However, 

this is not what is observed. Two quarries (9 and 12) have exhibited many of the 
______________________________________________________________________ 

characteristic signs of a mine with excessive horizontal stress conditions. At these 

quarries, extensive mapping of roof damage and roof falls confirms the presence of shear 

roof failure, a condition often found in Appalachian coal mines plagued with excessive 

horizontal stress ground control problems. Stress measurements at one quarry, using the 

hydraulic fracturing technique, found maximum principal stress values from 10 to 55 Mpa 

with the orientation of the principal stress direction from N 45E to 80E E (Iannacchione et 

al., 1998). 

Within the northern Appalachian coal mines, it is common to find a maximum principal 

horizontal stress from 9 to 23 Mpa at overburdens of 120 to 240 m (Table 2). Both the 

Columbus (37.9 Mpa) and the Loyalhanna (47.7 Mpa) Limestones are outside the coalbed 

stress magnitudes; however, it can be shown that much of the difference is due to strata 

stiffness effects. If stress magnitudes are divided by the rock’s Young’s Modulus, strata 

strain can be calculated. The strains from the maximum stresses across the region vary 

from 461 to 736 microstrains. For the Loyalhanna, the strain from the 47.7 Mpa stress 

value with a Young’s Modulus of 77.2 Gpa is just over 600 micro-strains. These results 

indicate that there is considerable uniformity in strains within the entire northern 

Appalachian region and the Loyalhanna seems to fit well into this regional pattern. 

Table 2 Average stress measurements from mines in the northern Appalachian region. 

Seam/formation 

Maximum 

horizontal 

stress, Mpa 

Minimum 

horizontal 

stress, Mpa 

Maximum 

horizontal stress 

direction 

Maximum 

horizontal 

strain, 

micro-strains 

Depth, m 

Lower Kittanning Coalbed 23.0 19.2 N 87E E 575 219 

Lower Kittanning Coalbed 20.8 15.6 N 75E E 721 168 

Pittsburgh Coalbed 9.1 7.1 N 32E E 736 122 

Pittsburgh Coalbed 16.3 15.6 N 78E E 557 213 

Pittsburgh Coalbed 21.2 15.0 N 70E E 461 244 

Columbus Limestone 37.9 27.6 E-W 611 701 

Loyalhanna Limestone 47.7 27.4 N 71E E 607 122 

The similarity of northern Appalachian strain magnitudes and stress orientations clearly 

points to the influence of plate tectonics on regional horizontal stress conditions. Most 

measurements show that the maximum horizontal stress direction ranges from N 70E to N 

80E E (Table 2). This direction correlates with the direction of movement for the North 

American plate as it is pushed from away from the Mid-Atlantic Ridge. The National 

Geodetic Survey computes the positions and velocities at numerous points around the 

world (Sella et al., 2002). These data have been used to determine the relative velocities 

of different tectonic plate motions (continental drift) and thereby map the movement of 

each plate. The North American Plate moves continuously away from the Mid-Atlantic 

Ridge at a rate of about 2.5 cm/yr. This knowledge is important since it helps us 

understand why some underground stone quarries can have such high horizontal stress 

conditions. 

It should be noted that not all ground control problems are stress related. For example, at 

quarries 6, 10, and 11, the roof rock is generally stable with the exception of areas where 

increased concentrations of low-angled fractures cut the roof rock into a series of wedge-
______________________________________________________________________ 

shaped structures. Quarries in this mining district seem to have a relatively high 

frequency of these kinds of roof rock conditions compared with other underground stone 

mines in the U.S. In general, the hazards they present can be controlled by appropriate 

scaling, bolting, or monitoring technique. Another ground control hazard that occurs at 

most Loyalhanna quarries is faulting. A series of faults extend across most mining 

properties at bearings of approximately N 38E E with dips of 15E to 32E in both the NW 

and SE directions (Iannacchione and Coyle, 2002). 

GEOLOGIC AND TOPOGRAPHIC STRUCTURES AND THEIR 

INFLUENCE ON HORIZONTAL STRESS 

Not all of the district’s quarries have excessive levels of horizontal stress. There are 

locations within several quarries where the horizontal stresses are so low that the roof 

appears to be extending. Explaining how such dramatically different horizontal stress 

conditions exist so close to one another is more than just an academic exercise, in that 

techniques used to control the roof and rib for these two extreme stress conditions can be 

very different. The first step in identifying the controlling factors is to examine the 

specific geologic and topographic structures present along Chestnut Ridge. The study 

area centered around the Youghiogheny Gorge because quarries there exhibit a wide 

range of stress conditions and there is sufficient data available to perform a geologic and 

engineering analysis. 

Chestnut Ridge is a large anticlinal structure with a broad flat crest and limbs that dip as 

much as 20E to the northwest and southeast (Figure 2). Along the crest of the anticline, 

the Loyalhanna experiences significant elevation changes. The highest elevations above 

sea level typically occur within domes, or elliptical areas along the crestline, where the 

rocks dip gently away in all directions. Quarries 5 and 6 are located along the 

northeastern end of the Summit Dome where the Loyalhanna rises some 700 m above sea 

level. Northeast from the Summit Dome the Loyalhanna falls along the crest of the 

Chestnut Ridge, entering the broad Youghiogheny Saddle. Structural saddles are low 

points along the crestline of a ridge. This saddle structure is coincident with the 

Youghiogheny River Gorge. To the northeast of this saddle structure, the Loyalhanna 

rises again to a minor unnamed dome. 

The northeast portion of Summit Dome has three caves and a large strike-thrust fault 

(Figure 2) that parallels the axis of the Chestnut Ridge Anticline (Iannacchione and 

Coyle, 2002). The Youghiogheny Saddle has only one cave, located along the 

Youghiogheny Gorge. The gorge may have formed in conjunction with a large crossstrike slip fault at depth. There is no direct evidence for this fault; however, in this area 

the anticlinal axis is significantly distorted, and the dips of the associated strike-thrust 

faults rotate from northwest to southeast on either side of the gorge. The existence of a 

major cross-strike structural feature is supported by the projection of the MarylandPennsylvania Lineament through this area (Pohn, 1995). This structure may form the 

lateral boundary between major sheets of strata thrusted away from the Appalachian 

Mountains to the southeast. 
______________________________________________________________________ 

High Stress Conditions 

Within the study area, Quarry No. 9 (Figure 2) is the one with excessive horizontal stress. 

It is located along the northeastern portion of the Youghiogheny Saddle. Horizontal 

stresses are great enough to cause sporadic roof shearing and, on occasion, have 

developed into massive directional roof falls. Roof instabilities typically begin with the 

development of compression zones consisting of low-angle shears oriented approximately 

N 30E W. Most roof falls are oval in shape with the long axis oriented in this same N 30E

W direction. Most of these failure patterns support the existence of a pervasive horizontal 

stress field oriented approximately N 60E E (Location A, Figure 3). 

At Quarry No. 9, the Loyalhanna outcrops in a relatively small area. During initial mine 

development, this quarry mined in a portion of the reserve that was surrounded by outcrop 

on three sides (Location B, Figure 3). Some portion of the original horizontal stress field 

was partially relieved through down-dip extension of the strata adjacent to the outcrop 

(see zone of extension, Figure 3). Here, no signs of excessive levels of horizontal stress 

were observed. The occurrence of shear failures and the associated directional roof falls 

only became a problem as the mine developed away from outcrop. Additionally, the 

character of joints from the quarry are unique, clustering around the N 65E E trend 

(Figure 4). Many of these joints have dips ranging from 25E to 70E. Weathered joints are 

rare, with some joints coated with a brown stain that implies surface water infiltration, but 

the vast majority of the joints show no signs of weathering. Faults are also found at this 

quarry and generally show signs of thrusting with minimal displacements (0 to 1 m). 

-150

0

150

300

Ligonier Syncline

N

0 2

Summit Dome

Youghiogheny

Saddle

450

450

300

150

7

8

9

10

Scale, km

750

600

5

6 Chestnut Ridge Anticline

Youghiogheny River

Uniontown Syncline

5 Quarries

(inferred)

Dip

Strike thrust fault

Cross-strike slip fault

Caves

30 m, interval

Structure contour

300

Loyalhanna Ls. outcrop

KEY

Figure 2 Chestnut Ridge’s structural setting in the Youghiogheny 

Gorge area, showing the locations of quarries, caves, faults, and the 

Loyalhanna Limestone outcrop. 
______________________________________________________________________ 

Figure 3 Structure contour elevations on the Loyalhanna Limestone, Loyalhanna 

outcrop, surface topographic contours, horizontal stress characteristics, and roof 

falls within and adjacent to quarry No. 9. Note: dashed lines are the Loyalhanna 

structure contour and solid lines are the topographic contours. 

W E

Number of measurements

Number of measurements

Quarry #9

W

N 65 W

20

Number = 70

10

N 65 E

20

N

10

Contour interval = 1,2,4,8,16%

Contour interval = 1,2,4,8,16%

45

E

W 0

N

90 E

N 65 E

Rose diagram

Number = 64

Quarry #5

N 45 W

N

N 35 E

Stereo net projection

W 0 45

N

90 E

Figure 4 Rose diagrams and stereonet projections of strike and dip datum 

for joints and faults within quarries No. 5 and No. 9. 
______________________________________________________________________ 

Low Stress Conditions 

Some 9 km from this site a significantly different condition exist. At quarry No. 5, the 

entire underground mine is surrounded on three sides by outcrop (Figure 5). During 

initial mine development, rooms were driven up-dip from the portal within a narrow strip 

of strata approximately 300 m wide. The first significant ground control problem 

occurred when mining encountered the regional strike-thrust fault (Location C, Figure 5). 

Once the mine advanced through this area, conditions improved until mining progressed 

under the nose of an oval-shaped knob at the top of Chestnut Ridge (Location D, Figure 

5), where significant ground control problems were encountered. In this area, the 

Loyalhanna gradually slopes towards the northwest. Here the N 35E E and N 65E E joint 

sets trend approximately parallel to both the knob and the anticlinal crest. Many of these 

joints are weathered with some of the them containing a residual fine-grained material. 

This material infilling was sometimes removed, probably by erosion, and voids several 

feet in diameter were observed. The joints in this area are sub-parallel with the mine’s 

headings and cross-cuts. The spacing of these joints is such that one to three joints were 

encountered during normal development of a room. On one visit to the mine, the authors 

observed several massive roof falls outlined by these weathered joint structures. Because 

of the extensive weathering and the occurrence of voids, pre-existing horizontal stress 

could have dissipated as these large blocks nearest the outcrop separated from the 

confined, intake rock mass under the influence of gravity (see zone of extension, Figure 

5). 

610

460

700

500

700

620

660

430

540

670

660

550

490

370

620

620

Chestnut Ridge Anticline

460

420

E

0 300

Scale, m

N 60 E

Zone of extension

Quarry No.5

Figure 5 Structural contour elevations on the Loyalhanna Limestone, 

Loyalhanna outcrop, surface topographic contours, roof falls, weathered joints, 

and faults within and adjacent to quarry No. 5. Solid lines are topographic 

contours and dashed lines are Loyalhanna contour elevations. 
______________________________________________________________________ 

DISCUSSION 

The quarry with the excessive horizontal stress conditions is located on the margins of the 

Loyalhanna Saddle structure. With the exception of the outcrops that developed along the 

Youghiogheny Gorge, this structural feature produced fewer outcrop exposures than the 

adjacent saddle structure. Additionally, the jointing within this area included of more 

dipping joints and fewer weathered joints than the average quarry within this district. 

Here it would seem the regional stress field has been unaffected by surface topography. 

There is also some evidence that this area is adjacent to a large cross-strike structure that 

extends from the Appalachian Mountains to the southeast through the Youghiogheny 

Gorge. Perhaps the distortion of the anticlinal crestline, the rotation of the strike-thrust 

fault dips, and the shape of the associated saddle structure are influenced by the inferred 

cross-strike slip fault. This kind of geologic structure could locally add additional strain 

to the substantial amount already present within the northern Appalachian region. In 

addition, the shape of the saddle structure may act to promote strata confinement, since 

the Loyalhanna dips both towards and away from its center. 

Reductions in horizontal stress due to geologic and topographic structures were observed 

and documented at both quarries No. 5 and No. 9. Within a dome structure, strata dip 

away from the center in all directions. Domes are characterized by large concentrations 

of caves and weathered joints. All of these conditions could act locally to lower 

horizontal stress. 

Certain topographic structural shapes can isolate strata, producing unconfined rock 

outliers. In the absence of confinement, water can readily attack the rock mass, dissolving 

limestone along its most vulnerable features–the rock joints. These joints isolate blocks 

of strata that, under the right conditions, can move away from the outcrop. The extension 

of blocks away from the solid, confined rock mass is influenced by the structural dip of 

the strata and the presence of a bedding plane that allows the rock to glide down-dip. 

At quarry No. 5, joints are strongly weathered, producing a fracture trace that contains 

clay infilling ranging from a thin film to several feet in width. The degree of joint 

weathering increases toward the outcrop and is especially prevalent along the nose of the 

outlier (surrounded on three sides by outcrop). In this area the strata dip to the northwest, 

promoting the movement of large blocks down-dip. The extension of the strata has a 

detrimental effect on strata stability. Joint orientation and spacing are such that large 

blocks of strata can occur within these large rooms that are unsupported by pillars. 

Horizontal forces, applied perpendicular to the joint surface, keep these blocks in place. 

The lower the stress, the less rock load the joint can support. Thus, when horizontal stress 

is reduced by strata extension, there is an increased risk of roof failure. 

SUMMARY AND CONCLUSIONS 

It has been established that levels of horizontal stress high enough to produce shear 

failures within the very stiff Loyalhanna limestone exist and are probably caused by 

strains within the middle of the North American Plate. Where these excessive stress 

conditions exist, free from the influences of significant geologic and topographic 

structures, they appear to fit the regional strain pattern. It has also been established that 

excessive horizontal stresses are not necessarily a condition found within all underground 
______________________________________________________________________ 

quarries of southwestern Pennsylvania. A detailed geologic and topographic examination 

was made of two relatively closely spaced quarries with very different stress conditions to 

help examine factors that may be responsible for these diverse conditions. 

Geologic and topographic structures can act to alter horizontal stress conditions. 

Geologic structures evaluated in this study were associated with the changing shape of 

Chestnut Ridge Anticline. The crestline of this anticline is composed of domes and 

saddles. Domes are enclosed structural contour elevation areas where the Loyalhanna 

dips rapidly away from the center of the dome. Domes are characterized by increases in 

caves and weathered joints. These geologic structures could be acting as stress reduction 

zones. 

Saddles are structural depressions along the Chestnut Ridge Anticline where the 

Loyalhanna strata dip both towards and away from its center. Saddles are characterized 

by a decrease in caves and weathered joints and an increase in cross-strike structural 

features. These structures do not reduce the stress and leave the regional stress regime 

intact. 

Topographic structures evaluated in this study were those shapes that produced laterally 

discontinuous Loyalhanna strata. These topographic structures were knobs and ridges 

where the Loyalhanna outcrops on several sides. In general, the closer the mining front 

comes to an outcrop, the greater the potential for stress reduction. If the outcrop partially 

surrounds the mining zone, then this zone becomes an outlier of the strata. If mining 

takes place in an outlier area and the dip of the strata is towards the outcrop, then there is 

an increased risk for strata extension. In some of these mining environments, the stress 

magnitudes are low enough to allow blocks of unsupported roof, outlined by weathered 

joints, to fail and produce large roof falls. These topographic structures reduce the stress, 

altering the regional stress regime. 
______________________________________________________________________ 

REFERENCES 

Iannacchione, A.T., Dolinar, D.R., Prosser, L.J., Marshall, T.E., Oyler, D.C., and C.S. 

Compton. 1998. Controlling Roof Beam Failures From High Horizontal Stresses in 

Underground Stone Mines. Proc. 17th Conference on Ground Control in Mining, 102-

109, Morgantown, WV. 

Iannacchione, A.T., Marshall, T.E., Burke, L., Melville, R., and J. Litsenberger. 2001. 

Safer Mine Layouts for Underground Stone Mines Subjected to Excessive Levels of 

Horizontal Stress, Society of Mining Engineers PrePrint #01-88, SME Annual Meeting, 

Denver, CO, 7 p. 

Iannacchione, A.T. and P.R. Coyle. 2002 An Examination of the Loyalhanna 

Limestone’s Structural Features and Their Impact on Mining and Ground Control 

Practices. Proc. 21st Conference on Ground Control in Mining, 218-227, Morgantown 

WV. 

Mark, C. and T.P. Mucho. 1994. Longwall Mine Design for Stress Control. Proc. New 

Technology for Longwall Ground Control. U.S. Bureau of Mines, SP 01-94, 53-76. 

Pohn, H.A. 1995. Lateral Ramps in the Folded Appalachians and in Overthrust Belts 

Worldwide - A Fundamental Element of Thrust Belt Architecture. U.S. Geological 

Bulletin 2163, 63 p. 

Sella, G. F., T.H. Dixon and A. Mao. 2002. REVEL: A Model for Recent Plate 

Velocities from Space Geology, Journal of Geophysical Research, Vol. 107, No. 94, 1-

30, April. 

Su, D. and G.J. Hasenfus. 1995. Regional Stress and Its Effect on Longwall Mining in 

the Northern Appalachian Coal Field. Proc. 14th Conference on Ground Control in 

Mining, 39-45, Morgantown, WV. 

Zoback, M.L. 1992. First- and Second-Order Patterns of Stress in the Lithosphere: The 

World Stress Map Project. Journal of Geophysical Research.