Pennsylvania

Allegheny

Allegheny Front Section

Appalachian Mountain Section

West Virginia Maryland

Loyalhanna Limestone

Laurel

(Major Decollement Surface)

Chestnut

Oriskany Sandstone

A

Ridge

Negro Mt. Upper Salina Salt

Ridge

Chestnut Ridge Anticline

Blairsville

Allegheny Mountain Section

Pittsburgh Low Plateau Section Connellsville

Pennsylvania

Uniontown

A Pleasant

Mount

Pittsburgh

Latrobe Laurel Hill Anticline

Negro Mountain Anticline

B

Front

B

Scale, miles

0 5

N

Pennsylvania

Study Area

KEY

Figure 1. Location of major anticlinal structures and a generalized

cross-section through the Allegheny Mountain Section of the

Appalachian Plateaus Province.

AN EXAMINATION OF THE LOYALHANNA LIMESTONE’S STRUCTURAL FEATURES AND

THEIR IMPACT ON MINING AND GROUND CONTROL PRACTICES

Anthony T. Iannacchione, Senior Scientist

Paul R. Coyle, Geologist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

A close look at the Loyalhanna Limestone of southwestern

Pennsylvania reveals a complex structural environment. Most

exposures of the Loyalhanna occur along Chestnut Ridge and Laurel

Hill within the Allegheny Mountain Section of the Appalachian

Plateaus Province. Because the Loyalhanna is rated as a superpavement aggregate by the Pennsylvania Department of

Transportation and because of its proximity to the population centers

of southwestern Pennsylvania, the Loyalhanna has been extensively

mined along these prominent anticlinal structures.

Geologic and engineering analyses were performed using gas well

and core logs, outcrop examinations, underground observations, and

mine maps. Strata exposures from both outcrops and quarries were

used to construct geologic maps, stereonet, and rose diagram

projections so that the Loyalhanna’s structural environment could be

understood better. Many of these structural conditions cause ground

control problems at local quarries. These problems ranged from

small-to-moderate sized rock falls associated with roof jointing to

pillar failures associated with dipping discontinuities. Stresses ranged

from tensional on structural domes where weathering dominates, to

high levels of compression within structural saddles. A greater

understanding of these characteristics is a prerequisite to develop

engineering controls, such as improved mine layouts, pillar sizes, etc.,

that lessen miner exposure to these hazards.

INTRODUCTION

The Loyalhanna Limestone varies in composition from a

quartzose limestone to a calcareous quartz arenite (1). It underlies

17,000 square miles of West Virginia, Maryland, Pennsylvania, and

Ohio and averages 60 ft in thickness with a maximum thickness of

103 ft (1). Wells (2) has since expanded the aerial extent of the

Loyalhanna to Sullivan Co. in the northwestern portion of

Pennsylvania. While this limestone is found under an extensive area

in four states and is a major source of crushed stone for southwestern

Pennsylvania, it has a limited area accessible to mining. The

Loyalhanna, which is typically greater than a thousand feet below the

surface in the Pittsburgh Low Plateau Section of the Appalachian

Plateaus Province, crops out at numerous sites along the crest of

Chestnut Ridge, Laurel Hill, and Negro Mountain Anticlines in the

Allegheny Mountain Section of the Appalachian Plateaus Province

(figure 1). These outcrops have historically been the sites for

commercial aggregate mining, especially those closest to the

population centers in and around Pittsburgh, Pennsylvania. Currently

nine quarries mine the Loyalhanna through underground and surface

operations in Monongalia Co., West Virginia, and Fayette and

Westmoreland Counties, Pennsylvania, (table 1) along the linear trace

of Chestnut Ridge.

The Chestnut Ridge Anticline is a fold with limbs dipping away

from each other and, with erosion, exposes older geologic beds

nearest its central core. As this fold was produced in conjunction

with the formation of the Appalachian Mountains, the rocks fractured

and broke to form joints and faults. Understanding the characteristics,

such as orientation, dip, and spacing, of these structural features is

necessary for mine design and development.
x, y & z

o

(A)

f e

d

b (B) a c

g

k

h

 (F)

 (G)

l & m

n

Pittsburgh

r

(H)

 (K)

ee & ff

u

 (I)

p & q

s

t v (J) aa

cc

bb

dd

w

gg

Scale, miles

0

N

5

x, y & z = Rattlesnake, Ken's & Bone

u = Con

w = Bear

v = Evac

aa = Coon

dd = Tannery Hole

cc = Sawmill

bb = Lemon

gg = Strangford

ee & ff = Torrance & Vex

g = Laurel

f = Spider

c = Barton

e = Parva

t = Spider

n = Freeman Falls

b = Smith's Ladder

k = Brooks Spring

d = Maybe-Nasty

o = Lycippus

r = Loyalhanna

s = Copperhead

a = Roades

j = Jumonville

h = Franks

i = Trout Hollow

p & q = Flatfoot & Stone Knife

l & m = Casparis & Daniels

Caves

Outcrop

Location of sites visited

Loyalhanna Limestone

Chestnut Ridge

Anticlinal axis

(A)

 (C)

 (D & E)

i

j

Uniontown

Connellsville

Mount Pleasant

Latrobe

Blairsville

Figure 2. Location of Loyalhanna Limestone outcrop, caves, and

sites visited during this study. Capital letters A, B, D, E, F, G, H, I,

and J are quarries; C is a highway outcrop; and K is an abandoned

coal strip mine.

Figure 3. Massive roof fall measuring 45 ft wide by 20 to 30

ft high and extending for approximately 200 hundred feet.

Table 1. Quarries, strip mines, and road cuts visited during study

along Chestnut Ridge.

Letter Name County Site Characteristic

A Buckeye Monongalia Surface quarry

B Laurel Fayette Surface quarry

C Rt. 40 Fayette Road outcrop

D Coolsprings No. 1 Fayette Surface and

underground quarry

E Coolsprings No. 2 Fayette Surface and

underground quarry

F Springfield Pike Fayette Surface and

underground quarry

G Rich Hill Fayette Surface and

underground quarry

H Whitney Westmoreland Surface and

underground quarry

I Latrobe Westmoreland Underground

quarry

J Torrance Westmoreland

Surface and

Underground

quarry

K Bolivar Westmoreland Abandoned coal

strip mine

These characteristics are also found in the 33 Loyalhanna caves

along Chestnut Ridge (3). In general, these caves are comprised of a

single passage extending several hundred yards although a few are

made up of extensive interconnecting passages, such as Laurel

Caverns and Bear Cave (Sites G and W, figure 2). Most of the caves

are found within two structural highs or domes along Chestnut Ridge

which are named after the gas fields they encompass, Summit and

Griffin. In these areas the Loyalhanna has the greatest elevations and

outcrop exposures, producing joints that are deeply weathered and, in

many places, caves.

Mines along Chestnut Ridge have experienced roof and rib falls

with some falls resulting in injuries to mine workers. One quarry with

a workforce of approximately 30-miners experienced 7-injuries from

ground falls over a 30-month period from March 1993, until

September 1995 (table 2). A second mine had vehicles struck by

falling rocks approximately a dozen times during a 12-month period.

A third mine reported 9 roof falls to the Mine Safety and Health

Administration (MSHA) during an 18-month period from April 1999,

until November 2000 (4). Ground falls range in volume from boulder

size rocks, falling from the roof or rib, to massive roof falls measuring

45 ft wide by 20 to 30 ft high and extending as far as 500 ft (figure 3).

Many of the ground falls were, in part, due to the unique and complex

structural environment of the Loyalhanna. The primary reason for

initiating this National Institute for Occupational Safety and Health

(NIOSH) study is to increase knowledge of the root causes of these

hazards and promote the use of timely ground control measures so

that miner safety can be improved. 

Table 2 - Roof fall injury data for one quarry from

March, 1993, to September, 1995.

Date Injury Job

March 3, 1993 1 fatal injury Pumping

September 19, 1994 2 injured Drilling

October 25, 1994 1 injured Scaling

December 28, 1994 1 injured Blasting

May 25, 1995 1 injured Blasting

August 23, 1995 1 injured Scaling
Chestnut Ridge 2500

500

500

Uniontown

Ligonier

Ligonier

Chestnut Ridge

Uniontown

Ligonier

1500

2000

0

1000

1500

1500

-500

0

2000

0

500

1000

1000

Youghiogheny River

Jacobs Creek

Loyalhanna River

Conemaugh River

Blacklick Creek

Allegheny River

Monongahela River

N

3

1

2

6

5

0 5

Scale, miles

4

Summit Dome

Loyalhanna Saddle

Griffin Dome

Youghiogheny

Saddle

Jacobs Creek

Saddle

Anticline axis

500 ft, interval

Structure contour 1000

Syncline axis

3 Fault number and location

(inferred)

Cross-strike slip fault

Strike thrust fault

Dip

Lineament (Gwinn, 1964)

Figure 4. Structure contour map on the top of the Loyalhanna with major fold structures, surface

faults, lineaments, and rivers and streams.

THE STRUCTURE OF THE CHESTNUT RIDGE

ANTICLINE

Chestnut Ridge is a large anticlinal fold which was formed as part

of the Appalachian Mountain system. This topographic feature is a

subdued structure in comparison to the intensely folded and faulted

strata in the Valley and Ridge Province some 30 miles to the east.

However, upon closer inspection, significant structures exist in the

form of steeply dipping strata, thrust and slip faults, lineaments, and

a complex array of joints. All of these structures can present

localized hazards to miners. This factor is important since Chestnut

Ridge is southwestern Pennsylvania’s major source of high-quality

aggregate and miners will inevitably be exposed to these conditions

as they do their work.

Major anticlinal folds within the Appalachian Plateaus Province

of Pennsylvania are generally arcuate convex to the northwest and

parallel to the arc of the Central Appalachians. Structural relief on

anticlines decreases in step-like fashion northwestward in all parts of

the plateau (5). Chestnut Ridge is the last anticline in the northwest

portion of the Allegheny Mountain Section with significant structural

relief. The shape and character of the Chestnut Ridge Anticline was

analyzed in past geologic reports by McElroy (6), Hickok and Moyer

(7), and Shaffner (8, 9). In this study, a structure contour map on the

top of the Loyalhanna was prepared from oil and gas wells and

limestone exploration drilling data (figure 4). The Loyalhanna was

found to rise from depths between sea level and 1,000 ft along the

Ligonier Syncline to heights between 1,500 and 2,700 ft along the

crest of the Chestnut Ridge. Along the western flanks of Chestnut

Ridge, the Loyalhanna rapidly drops into the Uniontown Syncline,

where it can be found as much as 500 ft below sea level. The highest
?

4th Elk

3rd Elk

E Salt

F Salt E Salt

F Salt

Wills Creek

-2000 -2000

-4000

-6000

-8000

-4000

-6000

-8000

Gwinn (1964) Shumaker (2002)

0 1000 2000 4000

Horizontal Scale, ft

Elevation, ftElevation, ft

Faults

Heiderberg Fm.

Oriskany Sandstone

Huntersville Chert

Marcellus Shale

Mahantanga Fm.

Tully Limestone

Upper Salina Salt

Lower Helderberg shale

Devonian Onondaga-Oriskany-Helderberg sequrence

Figure 5. Complex structure under Chestnut Ridge at depths of -2,000 to -8,000 ft below sea level (5, 11).

areas form enclosed contour elevations called domes. The Summit

Dome (figure 4) lies midway between the Cheat River Gorge to the

southwest and the Youghiogheny River Gorge to the northeast. The

Griffin Dome (figure 4) lies further to the northeast between the

Loyalhanna and Blacklick Creeks.

The crest of the Chestnut Ridge Anticline experiences significant

elevation reductions, dropping to approximately 1,500 ft along three

prominent saddle structures. These three structural depressions or

saddles along the anticlinal axis have rivers or streams coincident

with them. The saddle northeast of the Summit Dome is coincident

with the Youghiogheny River, northeast of it lies the Jacobs Creek

Saddle, and further to the northeast lies the Loyalhanna Saddle. In

one case, an inferred cross-strike slip fault (Fault No. 4) is coincident

with the Jacobs Creek Saddle structure (9). In another case, the

Loyalhanna Saddle structure coincides with a major lineament

structure discussed by Gwinn (5). A lineament is “a mappable simple

or composite linear feature of the earth’s surface whose parts are

aligned in a straight or gently curved relationship and presumably

reflects a subsurface phenomenon” (10). The relationship between

the saddle structures and other structural features will be discussed

later in the paper.

DEEP-SEATED STRUCTURES AND DISCONTINUITIES

AT THE SURFACE

With accelerated oil and gas exploration in the 1950's and 60's

along Chestnut Ridge, unsuspected structures were discovered at

depth. Repeated geologic sections were noted along with other

evidence that suggested significant faulting near the base of the

Devonian. Deeper wells below the Upper Silurian Salina Group

found the geologic section to be regular with no evidence of complex

structure (5). The Salina Salts act as the principal basal detachment

horizon or decollement beneath much of the Appalachian Plateaus

Province.

Apparently the Loyalhanna and surrounding strata in the

Allegheny Mountain Section have moved some distance along the

Salina detachment horizon (figure 1). In addition, the overlying

Middle Devonian rocks are intensely faulted. The faults originate

within the Salina and quickly rise into a series of branching splay

thrust faults under the limbs of the anticline. The complicated nature

of the stiffer rock units between the Salina and the Tully Limestone

is shown in figure 5 where these units are either stacked in an

imbricate fashion by ramping thrust faults (5) or flexurally bent by an

overturned fold with imbricate faulting (11). Gwinn explained the

stacking at depth as caused by shortening of the detached strata above

the Salina Salt. This produced a depressed zone along the anticlinal

crest which has helped to create Chestnut Ridge’s broad fold at the

surface. Shumaker indicates that the highly deformed Salina has

thickened into a salt core under some parts of Chestnut Ridge. The

overlying Middle Devonian rocks are overturned in relatively tight

folds with large splay faults emanating from the base of the

overturned limb.

Both Gwinn and Shumaker state that the strata above the Tully

shows far less deformation than the rocks below this unit. Wiltschko

and Chappel (12) suggested that most faults were absorbed by the

Upper Devonian shales and did not extend above the Tully. While

the displacements and frequency of faults are far less at the surface

than at depth there is little doubt that many of the Middle Devonian

faults have made it to the Loyalhanna. It is important that this

connection be made so that the trends established at depth can be
looked for in the near surface rocks.

The intensity of movement at depth is not uniform along Chestnut

Ridge as evidenced by differences in deformation patterns.

Movements have been concentrated within lateral ramp zones that are

bounded by cross-strike slip faults. Pohn (13) noted that sometimes

these faults reach the surface and are observed as: 1) an abrupt change

in wavelength or a termination of folds along strike, 2) a conspicuous

change in frequency of mapped faults at the surface, and 3) long,

straight river trends. The Conemaugh and Jacobs Creek slip faults,

the Loyalhanna lineament, and the Youghiogheny saddle all have

some of the features listed by Pohn (figure 4). To date, none of the

quarries are actively mining within one of these structures, but several

are mining in proximity and in time must decide if mining there is

prudent. Thus the potential for exposing unstable strata associated

with prominent discontinuities in these areas is very high.

EFFECT OF FOLDING ON MINING

The Chestnut Ridge Anticline provides access to the Loyalhanna

but also limits the extent to which room-and-pillar mining can occur.

Along the length of this anticline, the fold rises and falls in a fairly

gentle fashion into domes and saddles. Here the strata typically dips

from 1 to 5° and rarely exceeds 10°. The anticlinal crest is

surprisingly broad so that many quarries have the opportunity to mine

across these relatively flat areas for 0.5 to as much as 1.5 miles.

Along the flanks of Chestnut Ridge, the strata quickly drop off into

the adjacent synclines. This is especially true of the western flank,

where the distance from anticline to syncline ranges from 4 to 6 miles

with elevation changes of 1,800 to 2,800 ft. Several traverses were

measured where strata dips ranged from 5 to 10°, with the steepest

sections approaching 20°. On the eastern flank of the anticline the

dips are slightly less severe, where the distance from anticline to

syncline ranges from 4 to 5 miles with elevation changes of 900 to

2,100 ft. Here the average strata dips ranged from 3 to 6°. The

degree of difficulty in mining with the room-and-pillar techniques

grows significantly with dips exceeding 10°. 

AFFECTS OF FAULTING ON MINING

Six major faults have been previously recognized within the study

area as shown in figure 4. Three of the faults were reported by

Hickok and Moyer (7) and three were reported by Shaffner (8, 9).

Two were observed on the western flank of Chestnut Ridge (Fault

No. 1 and No. 2), a third along the Monongahela River (Fault No. 3),

a fourth and fifth inferred slip faults transecting Chestnut Ridge

(Fault No. 4 and No. 5), and a sixth was observed along the eastern

flank of Chestnut Ridge (Fault No. 6). These faults can be classified

as either strike thrust faults or cross-strike slip faults. The strike

thrust faults, in general, have bearings along the strike of the

anticlinal structure and dip at a relatively low angle. The cross-strike

slip faults (Faults No. 4 and No. 5) were never actually seen by

Shaffner although he speculated that they must exist in order to

explain the topographic and structural anomalies in this area. The

inferred strike of these faults is perpendicular to the anticlinal

structure with dips of approximately 90°. These trends are consistent

with those expected in response to the formation of the Appalachian

Mountain System. A summary of the characteristics of each fault

shown on figure 4 follows:

1. The first fault is exposed along Rt. 40 (Site C, figure 2) where it

climbs the western flank of Chestnut Ridge at an altitude of about

1,600 ft. It appears to be a thrust fault striking nearly parallel to

the Chestnut Ridge Anticline (N 30° E) and the dipping about

30° SE.

2. The second fault is along the Western Maryland Railroad track in

the Youghiogheny gorge about 0.7 mile south of the water

pumping station across the river from South Connellsville. Here

the fault itself was not seen, but the contact of the Greenbrier

limestone with the overlying Mauch Chunk red shales has been

moved down on the southeast side of a gully with respect to the

same beds on the northwest side of the gully. According to

Hickok and Moyer (7), a fault occurs here, but what its dip and

strike are could not be determined. The displacement seems to be

about 40 to 60 ft. The authors believe this to be a strike-thrust

fault.

3. The third fault was found to parallel an adjacent kimberlite dike

(14) which cuts the same strata and is recognized as a cross-strike

slip fault. Its strike is N 45° W and the dip is nearly vertical. The

horizontal displacement could not be determined but there was a

3.5 ft vertical displacement (7).

4. The fourth fault is inferred to occur along Jacobs Creek where the

axis of the anticline has significant flexures, a local bulge in the

structure, and a mild local reversal of the dip on the east limb of

the anticline in the vicinity of the Pennsylvania Turnpike (9). A

somewhat similar structural condition exists along adjacent

portions of Laurel Hill Anticline, where a relatively milder flexure

of the anticline axis is accompanied by a structural bulge on the

east limb of the anticline. This inferred feature is considered a

cross-strike slip fault.

5. The fifth fault is again inferred by Shaffner (9) on the basis of the

difficulty in matching structure contour lines on opposite sides of

the Conemaugh River. He estimated that this slip fault displaced

strata by as much as 1,000 ft and probably influenced the course

of the Conemaugh River in this area. This inferred feature is

considered a cross-strike slip fault.

6. The sixth fault is exposed along the east flank of Chestnut Ridge,

south of the Conemaugh River where the Connoquenessing

(Pottsville) sandstone crops out, striking generally N 35° E and

dipping from 40 and 85° southeast, and forming a conspicuous

hogback running parallel to the ridge (8). This strike-thrust fault

was also observed by Puglio and Iannacchione (15).

In addition to these known and inferred faults, numerous other

faults were observed during visits to several quarries. Figure 6 shows

the location, bearing, and dip of these faults in various formats. First,

the location, strike, and dip of each fault are displayed in conjunction

with the structure contour map on the Loyalhanna. Second, a

stereonet projection is provided which locates the poles of the fault

planes. Two important facts are revealed in the stereonet: (1) the

faults align along a consistent orientation of N 40° E, and (2) the dips

of these faults vary widely from approximately 60° to the northwest

through 80° to the southeast. The consistent strike is further

illustrated with the associated rose diagram. The strike of N 40° E,

observed at all quarries with the exception of Site F (figure 6), aligns

closely with the general bearing of Chestnut Ridge. While the dips

vary widely, some consistencies can be observed. For example, all

the faults from sites G, H, and K dip to the southeast, whereas the

majority of the faults from site D dip to the northwest. Site F shows

both northwest and southeast dipping faults.
N21E, 20SE

0

N

3

1

2

6

5

Strike

N

0 5

Scale, miles

Dip

3

4

 (J)

(A)

 (I)

 (C)

 (K)

 (B)

 (F)

 (G)

(H)

 (D & E) (A)

of observed faults

 Strike and dip

1000 ft, interval

Structure contour

Anticline axis

Chestnut Ridge

Fault number and location

Location of sites visited

Cross-strike slip fault

(inferred)

Strike thrust fault

(Dip direction)

Lineament (Gwinn, 1964)

K

K K

D

D

D

F

F F

F

S

W 45 90E

N

H

C H

G

H

D

F

K

F

D

0

Poles of faults

Stereonet

Rose Diagram

E

N

W

N30E, 30SE

N42E, 15SE

N41E, 32NW

N34E, 25NW

N51E, 30SE

N21E, 24NW

N62E, 9NW

N6E, 27E

N40E, 35SE

N32W, 11NE

N46E, 50NW

N46E, 23SE

N36E, 72SE

N48E, 74SE

N41E, 55SE

N38E, 85SE

N38E, 75SE

N39E, 63SE

N38E, 45SE

0

2000

1000

1000

0 1000 2000

Figure 6. Location and strike and dip of faults observed during this study.

Figure 7. Unstable roof associated with typical

low-angle thrust faults.

Most mines should expect to encounter strike faults with dips

ranging from 10 to 60° either to the northwest or southeast. These

strike faults are more likely to occur on the southeastern side of the

Chestnut Ridge Anticline from the Loyalhanna Creek northward and

on the northwestern side south of Loyalhanna Creek. It is also likely

that Loyalhanna dips will increase significantly on the flank side of

these faults. The immediate roof associated with typical low-angle

thrust faults can be very unstable and in many locations has failed

entirely (figure 7).

In addition to the strike thrust faults, significant deformation must

have taken place along bedding in the less-stiff units above and below

the Loyalhanna and along bedding planes and low-angle joints within

the stiffer Loyalhanna. While the lateral deformations on these

discontinuities may be slight, they can significantly weaken the roof

strata, producing thinner roof beams which are not conducive to

stable mine roofs in rooms averaging 45 ft wide.
40 30 20 10 0 10 20 30 40

N

W E

Number of locations per sector interval

n = 371

o

Sector = 10

N48 E

o

N47 W

o

Faults High Angle Joints Low Angle Joints Cave Joints

Figure 8. Rose diagram of fault and joint data from study area. 

371 measurements were grouped into two clusters. Northwest

quadrant data had a mean resultant direction of N 47° W (left

arrow). Northeast quadrant data had a mean resultant direction of

N 48° E (right arrow). 

CI=1,2,4,8,16%

CI=3,6,12,24%

W

W

S N=33

0 45 90 E

H

N=28

CI=3,6,12%

J N

S

0

N

45 90 E

W

W

F

S N=59

0

N

S

45 90 E

CI=2,4,8%

N=39

0

N

45 90 E

D

N=41

CI=2,4,8%

N=Number of readings

CI=Contour interval

W

S

0

N I

45 90 E

N=16

CI=6,12%

N=32

CI=3,6,12%

W

5/1/02

S

0

N

S

G

W 0

N E

45 90 E

45 90 E

Figure 9. Stereonet projections for seven of the sites visited

during this study.

While faults in highwalls create significant planes of weakness,

which can produce unstable strata, they seem to present a less

formidable hazard to surface miners than underground miners. The

exception to this is when the dip of a fault plane is directly into the

excavation. In this case, large-scale highwall instabilities are possible

if the hanging wall rock moves into the surface pit.

On the positive side, a consistent overall orientation of the strike

thrust faults allows mine planners to easily project their trends into

unmined areas. None of the quarries intercepted cross-strike slip

faults. This is fortunate because cross-strike faults are thought to

have considerable slip or displacement associated with them and may

have an increased concentration of jointing. On the negative side,

locally the strike thrust faults are very complicated, generally

consisting of a number of related fault planes that can dip in different

directions. Clearly, Chestnut Ridge has a relatively high degree of

faulting for an area within the Appalachian Plateaus Province and

presents a potential hazard for underground mining.

JOINTING IN THE LOYALHANNA

Jointing was observed throughout the study area and was found

to have two distinct trends. A total of 371 joint bearings and dips

were measured and plotted on a rose diagram shown in figure 8.

These measurements were grouped into two clusters: 1) northwest

quadrant data with a mean resultant direction of N 47°W of mostly

vertical joints; and 2) northeast quadrant data with a mean resultant

direction of N 48° E with varying dips. The N 47° W joint trend

strikes across Chestnut Ridge and the N 48° E trend roughly parallel

the strike of Chestnut Ridge. These trends are coincident with the

orientation of local folding and faulting and are thought to be caused

by the same processes.

The joint bearings and dips become more significant when the

measurements from individual sites within the study area are analyzed

(figure 9). For example, at site D (figure 2) most of the joints are

vertical with a modest concentration of joints dipping from 10 to 45°

to the northwest and lesser concentration of joints dipping from 0 to

45° to the southwest. Site E and J are composed almost exclusively

of vertical joints. Sites F and G are dominated by joints dipping from

0 to 90° to the northwest. Site H has a strong concentration of joints

dipping from 0 to 45° to the southeast. Sites F and H have a

significant number of joints dipping from 0 to 45° oriented over a

wide range of bearings. The spacing of most joint sets ranges from a

few feet to tens-of-feet. Displacements along vertical joints were not

observed, however, lateral displacements along joints dipping from 0

to 45° were observed at Sites F and I.

It should be noted that the authors believe the dipping joints are

structural in nature. They should not be confused with the strikes and

dips of the very distinctive cross-bedding structures described by

Adams (1) and Ahlbrandt (16). Adams (1) measured a N 72° E mean

strike with a standard deviation of 76° for Loyalhanna cross-bedding.

The mean dip was measured at 20° with a range of 5 to 40°. Crossbedding can often provide a plane of weakness in roof and rib rocks
Figure 10. Changes in average vertical peak stress as the dips of discontinuities are varied from 0 to 90° for two different width-to-height

ratio pillars.

which can result in small-scale hazards. Structural discontinuities

such as faults and joints are much more likely to produce significant

ground fall hazards. The authors were careful to measure only joints

as opposed to cross-beds during this study.

EFFECT OF DISCONTINUITIES ON GROUND CONTROL

Discontinuities in the form of faults, joints, and bedding planes

have a significant impact on Loyalhanna ground control. They

control immediate roof beam stability, roof line character, and pillar

strength (17). Bedding plane structures within the Loyalhanna are

sometime produced by the same processes that formed faults and

joints. In general, these bedding planes lack persistence and have

very shallow dips that produce roof beams of variable thicknesses.

This becomes a ground control problem when the beds thin to less

than one foot. At these thicknesses the magnitude of roof beam sag

can reach critical levels. Additionally, these shallow dipping bedding

planes make for poor roof lines because they lack persistence and

inevitably lead the mining horizon too-close-to or too-far-away from

the overlying shales in the Mauch Chunk Formation. Of course

joints, especially those oriented parallel to the entries, further

diminish beam strength.

Perhaps of even greater concern for miner safety is the effect that

discontinuities have on the stability of Loyalhanna pillars. Important

discontinuity properties which in some way affect pillar strength

include: lengths, spacings, orientations, material properties, and dip.

For example, the length of the discontinuity must be on the same

scale as the pillar to impact its strength. For a single discontinuity, it

will be of most concern if this structure passes entirely through the

pillar. Sometimes the discontinuity spacing can be close enough so

as to produce a strength reduction by allowing the edges of the pillar

to progressively fail. The orientation of the discontinuity is important

when the pillars are rectangular, in that strength will be most affected

when the orientation of the discontinuity is aligned with the long axis

of the pillar and the dips range from 40 to 90°. The material

properties of the discontinuity can be used to assess the magnitude of

strength reduction. The character of these discontinuities can range

from sharp with a rough surface and no in-filling, to smooth or even

polished, to in-filling by a residual fine-grained material with very

low strength. Lastly, the dip of the discontinuity can dramatically

affect pillar strength.

To illustrate some of these concepts, a model with very closely

spaced joints, relatively low joint strength, and variable joint dip was

presented by Iannacchione (18). The highest pillar strength occurred

with a discontinuity dip of 0° and gradually decreased as discontinuity

dip angle increased (figure 10). As the discontinuity dip angle

increased above 60°, pillar strength began to increase again, although

the original starting strength was never achieved. This result could be

expected when near vertical joints are closely spaced, allowing

buckling at the edges of the pillar and producing a strain-softening

material behavior. Conversely, a near vertical through-going

discontinuities with a dip approaching 90° could allow a pillar to

regain almost of its strength (figure 10).

Of the total number of failed pillars observed by Iannacchione

(18), approximately half were from mines in the Loyalhanna. These
pillars took on the appearance of the “real problem” pillars shown in

figure 10. Because of the significant percentage of joints within the

critical dip range of 30 to 75°, pillar stability is expected to be an

important safety issue, especially when pillar width-to-height ratios

fall below 1.0 (18, 19). Width-to-height ratios above one are more

desirable because there is less chance for the discontinuity to pass

entirely through the pillar.

THE EFFECT OF STRUCTURES ON STRESS LEVELS

The Loyalhanna generally has been observed to have excessive

levels of horizontal stress because of its stiffness and position within

the middle portion of the North America Plate (20). Stresses in this

part of the Appalachian Plateaus Province are generally considered

high, even for the less stiff coal-measure rocks in the Pennsylvanian

strata above the Loyalhanna. It should be noted that current stresses

are due to plate tectonics, not the ancient Appalachian Mountain

building event. Horizontal stresses within the Loyalhanna have been

measured from 2,200 to 8,000 psi with an orientation that clusters

between the N 60° E and N 75° E (21). These excessive horizontal

stresses were shown to be responsible for the mining induced

seismicity and associated ground falls at one mine site.

It must be noted that high levels of horizontal stresses are not

present at all Loyalhanna quarries. In fact, several quarries located

within structural domes are thought to have very low compressive

stresses within their immediate roof rock. These domes have brought

the Loyalhanna to its highest elevations where it has been exposed to

considerable weathering. Figure 2 shows that the most extensive

areas of Loyalhanna outcrop occur within the Summit and Griffin

Domes.

The Summit and Griffin Domes have extensive caves developed

in the limestone. Caves occur when existing joints open as the

confining stresses are relieved by erosion, uncovering the limestone.

These unconfined joints are then widened as flowing water dissolves

the limestone over extended periods of time. The residual finegrained material that is left in place can be easily carried away by the

small streams of flowing water. Under these conditions, whatever

locked-in stresses that once existed are released. Quarries that clearly

have low stress conditions (Site D and J, figure 2) are located within

the Summit and Griffin Domes.

Despite the low stresses discussed above at two quarries, the level

of horizontal stress is, in general, considered to be excessive. Mines

with excessive levels of horizontal stresses (Sites F and I, figure 2)

are located in saddle structures and are close to either an inferred

cross-strike slip fault or a projected lineament that may form the

boundary of a large ramp structure. At both of these mines a

significant number of joints with low dip angles were observed.

Additional research is required to fully understand the important

structural factors that either diminish or concentrate stresses in this

area.

SUMMARY AND CONCLUSIONS

Using data gathered from gas wells, core logs, mine visits, and

geologic references, the geology of the Loyalhanna was investigated

and its influence on mining was determined. The important structural

characteristics of the limestone follow:

1. Underground mining has been concentrated along the crest of the

Chestnut Ridge Anticline where dips range from 0 to 10°. These

relatively flat crest zones range in width from 0.5 to 1.5 miles.

The flanks of Chestnut Ridge have dips that range from 5 to 20°

and are generally steeper with more overall relief to the

northwest. Because mining becomes more difficult with

increasing dips and corresponding overburdens rise quickly,

future mining will continue to occur along the crest of Chestnut

Ridge Anticline.

2. Two types of faults were recognized: strike thrust faults and

cross-strike slip faults. The strike thrust faults parallel the trend

of the Chestnut Ridge, whereas the cross-strike slip faults are

perpendicular to the trend of Chestnut Ridge. Six faults have

been noted in the literature. Several additional faults were

observed as part of this study in six underground and surface

mines.

3. Two general clusters of jointing were observed: vertical joints

trending across the strike of Chestnut Ridge (N 47° W) and

variable dipping joints roughly parallel to this same strike

(N 48° E). These joints and the previously discussed faults have

the potential to significantly reduce pillar strength. Whenever

discontinuities dip more than 30° the potential for pillar failure

increases. The most hazardous dips are from 45 to 70°.

4. Two major domes, Summit and Griffin, bring the Loyalhanna to

elevations in excess of 2,500 ft. These domes have the highest

amount of limestone exposed along outcrop and are sites where

caves have developed. These areas have high concentrations of

weathered, vertical joints. They are sites of relatively low

concentrations of horizontal stress. The principal ground control

concern in these areas should be with roof falls developing

between blocks of rock outlined by weathered, vertical joints.

5. Three major saddles, Youghiogheny, Jacobs Creek, and

Loyalhanna, are locations where significant elevation reductions

occur along the crest of Chestnut Ridge. These structural

depressions are coincident with major rivers and streams

transecting Chestnut Ridge. The Jacobs Creek Saddle is also

coincident with an inferred cross-strike slip fault. The

Loyalhanna Saddle is coincident with the lineament identified by

Gwinn (5). These areas have high concentrations of joints with

low dip angles. They are sites of relatively high concentrations of

horizontal stress. The principal ground control concern in these

areas should be roof falls developing perpendicular to the major

stress direction of N 60 to 70° E.

The shape and character of Chestnut Ridge Anticline has been

influenced by the same tectonic forces that formed the Appalachian

Mountains further to the east. These forces have created a complex

structural setting that contains variably dipping strata, prominent joint

structures, and significant fault trends. These structures have had

considerable impact on mining in the area. Prominent discontinuities

are evident in all of the underground mines and have been responsible

for significant ground control issues. A case for relating ground

control problems to the structural geologic environment of all these

mines can be established. The future challenge is to apply this

knowledge to proscribe operational remedies to mitigate the implied

hazards to miners and facilities. Remember that over time, these

hazards will become more prevalent as mining progresses into less

accessible, higher stressed, and greater overburden areas.
EFERENCES

1. Adams, R.W. Loyalhanna Limestone - Cross-bedding and

Provenance, in Studies in Appalachian Geology - Central and

Southern, ed: Fisher, C.W., Pettijohn, F.J., Reed, J.C., and

Weaver, K.N., John Wiley and Sons, New York, 1970, pp. 83-

100.

2. Wells, R.B. Loyalhanna Sandstone Extended into North-Central

Pennsylvania (abs). Geological Society of America Programs

with Abstracts, Vol. 6, 1974, pp.84-85.

3. White, W.B. Caves of Western Pennsylvania. Pennsylvania

Topographic and Geologic Survey, General Geology Report 67,

1978.

4. MSHA Data Retrieval System, www.msha.gov, 2002.

5. Gwinn, V.E. Thin-Skinned Tectonics in the Plateau and

Northwestern Valley and Ridge Provinces of the Central

Appalachian. Geological Society of America Bulletin, Vol. 75,

Sept. 1964, pp. 863-900.

6. McElroy, T.A. Ground Water Resources of Fayette County,

Pennsylvania. Pennsylvania Topographic and Geologic Survey,

Water Resource Report 60, 1988, 57 pp.

7. Hickok, W.O. and Moyer, F.T. Geology and Mineral Resources

of Fayette County, Pennsylvania. Pennsylvania Geological

Survey, County Report 26, 1940, 530 pp.

8. Shaffner, M. N. Geology and Mineral Resources of the New

Florence Quadrangle, Pennsylvania. Pennsylvania Topographic

and Geologic Survey, Atlas No. 57, 1958.

9. Shaffner, M. N. Geology and Mineral Resources of the Donegal

Quadrangle, Pennsylvania. Pennsylvania Topographic and

Geologic Survey, Atlas No. 48, 1963.

10. Altamura, R.J. Geologic Mapping Using Radar Imagery in the

Ridge and Valley Province. Pennsylvania Geology, Vol. 30,

No. 3/4, Fall/Winter, Pennsylvania Topographic and Geologic

Survey, 1999, pp. 2-13.

11. Shumaker, R.C. Reinterpreted Oriskany Structure at the North

Summit Field, Chestnut Ridge Anticline, Pennsylvania. Am.

Assoc. of Petro. Geol. Bull., Vol. 86, No. 4, April, 2002, pp. 653-

670.

12. Wiltschko, D.V. and Chapple, W.M. Flow of Weak Rocks in

Appalachian Plateau Folds, Am. Assoc. of Petrol. Geol. Bull.,

Vol. 61, No. 5, May 1977, pp. 653-670.

13. Pohn, H.A. Lateral Ramps in the Folded Appalachians and in

Overthrust Belts Worldwide – A Fundamental Element of ThrustBelt Architecture. U.S. Geological Bulletin 2163, 1995, 63 pp.

14. Shultz, C.H. Chapter 13: Jurassic Kimberlite Dikes. The

Geology of Pennsylvania, Pennsylvania Geological Survey, 4th

Ser., Special Publication 1, 1999, pp. 210-217.

15. Puglio, D.G. and Iannacchione, A.T. Steeply Dipping Upper

Freeport Coals. Geol. of the Northern Appal. Coal Field,

Guidebook for Field Trip No. 2, Stop No. 21, 9th Int. Cong. of

Carb. Strat. and Geol., May 10-16, 1979, pp. 28-30.

16. Ahlbrandt, T.S. The Mississippian Loyalhanna Limestone

(Formation) - A Paleozoic Eolianite in the Appalachian Basin.

Predictive Stratigraphic Analyses Guidebook Series No.1 - The

Mississippian and Pennsylvania Systems in West Virginia, ed.

Cecil, C.B., U.S. Geological Survey Open-File Report 95-240,

1995, 25 pp.

17. Iannacchione, A.T. and Prosser, L.J.. Roof and Rib Hazard

Assessment for Underground Stone Mines. Mining Engineering,

February 1998, pp. 76-80.

18. Iannacchione, A.T. An Analysis of Pillar Design Practices and

Techniques for U.S. Limestone Mines. Transactions of the

Institution of Mining and Metallurgy, Vol. 108, Sept.-Dec. 1999,

pp. A152-A160.

19. Parker, J. Everybody Goes Underground Eventually. Aggregate

Manager, June 1996, pp. 30-35.

20. Zoback, M.L. First- and Second-Order Patterns of Stress in the

Lithosphere: The World Stress Map Project. Journal of

Geophysical Research, 1992.

21. Iannacchione, A.T., Marshall, T.E. and Prosser, L.J. Failure

Characteristics of Roof Falls at an Underground Stone Mine in

Southwestern Pennsylvania. Proceedings, 20th International

Conference on Ground Control in Mining, Morgantown, WV,

2001, pp. 119-125.