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SLOPE STABILITY AND SAFETY FACTOR ANALYSIS

FOR AN AREA ALONG THE CASCADES TRAIL OF JEFFERSON NATIONAL FOREST

GILES COUNTY, VIRGINIA

By

Paul Bartholomew Engineering Geology

Dr. Watts Radford University

Fall 1999

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Table of Contents

Introduction:...................................................................................................................... 2

Geological Setting.............................................................................................................. 2

Parameters of the Study Site ............................................................................................ 3

Analysis .............................................................................................................................. 3

Stereonet Analysis ............................................................................................................. 5

Safety Factor Calculations ............................................................................................... 8

Recommendations ............................................................................................................. 8

Bibliography .................................................................................................................... 12

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Introduction: The Cascades trail is a popular well traveled hiking trail located in the Jefferson

National Forest about 15 miles northwest of Blacksburg, Virginia. The two-mile trail

parallels Stony Creek up to a large circular cliff formation of shale and interbedded

limestone. In the middle of the cliff structure is a beautiful waterfall called the Cascades

with a vertical drop of approximately 80 feet into a wide shallow pool.

During the summer of 1999, a large piece of rock broke loose from a high position of the

cliff on the north side of the falls. The car-tire-sized boulder bounced roughly 125 feet

down-slope breaking through a wooden post guardrail and damaging a lookout platform

(see figures 1, 2, and 3). Around the same time, and less then a half mile down stream of

the falls, a rockslide covered almost 20 feet of the hiking trail. In neither instance was

anyone near to witness the actual events. As a result, the National Forest Service sought

the services of Dr. (Skip) Watts of Radford University to do a safety survey of the sites

mentioned above and a third site of concern farther downstream along the trail. It is this

third site, located about one half mile downstream of the falls, that is the subject of the

slope stability analysis discussed here.

Geological Setting

Far up stream of the Cascades, Stony Creek spills over a series of pool-drop rapids

formed by outcrops of Tuscarora sandstone and then into gentler rapids of the Rose Hill

sandstone and shale formation (See figure 4 for the geologic map of the area). Just above

the falls, the creek again crosses a thin (< mile) formation of hard Tuscarora sandstone

that forms the erosion resistant rock forming the Cascades. The Tuscarora is a fine to

medium grained pure quartz Silurian sandstone that is highly resistant to weathering and

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forms many of the mountain ridges in the Valley and Ridge Province (Summer Field

School, 1997). In places one can find conglomerates forming stratum at the base of thick

sandstone beds. The pureness of the sand grains and frequent cross bedding are evidence

of a near shore environment that sat quietly in the Silurian period as the Iapetus ocean

closed. The cliffs surrounding the falls cataract seem to be those of the Juniata

formation. This formation is composed of thin red sandstone, gray sandstone, and some

minor limestone, all of which are interbedded within the dominant red shales. Farther

downstream, the Stony crosses into a large formation of the Ordovician Martinsburg

formation. Downstream, along the trail of the site being studied, one can see large

boulders and rock debris of the upper Martinsburgs olive green to gray sandstone, shale,

and siltstone (see figure 5).

Parameters of the Study Site

The study site is a massive outcrop of Sandstone with thick beds dipping 160 (SE) and

cliffs breaking sharply off at 90 or more from the horizontal (See Figure 6). The outcrop

exhibits fine cross-bedded sandstone as well as layers of white pebble conglomerate at

the bottom of some of the thicker beds. Most of the beds seem to be of a fine to medium

grain sandstone and is probably part of the Tuscarora formation. The bottom part of the

outcrop seems to have a lot more structural integrity then the thinner beds that form the

upper parts of the cliff faces (See figure 7).

Analysis The first step in the analysis, after a general study of the areas geology, is to map the

discontinuities that cause the structural weakness within the rock mass. In this case, the

most obvious contact is the large bedding planes daylighting out of the cliff at a dip angle

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of around 20-25 degrees. Many of these contacts have a low wave pattern due to ripple

marks that have a wavelength of approximately 3 feet or more. Other beds have rough

pebble surfaces as mentioned above. These rough or undulating surfaces increase the

friction angle of the contacts which increases the overall integrity of the bedding planes.

There are large fracture joints that dip almost 90 (see fig. 8) facing roughly west and a

set of relatively medium sized joints facing south and dipping an average of about 80.

Harder to pick out are small repeating fracture joints that face roughly south-southeast

with an average dip angle of about 75.

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Friction Angle

Topple Zone

Critical Zone

Slope Dip and Direction

Contact Intersections

Figure 9: Stereonet Analysis for Wedge

Dip and dip-directions of contacts.

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Stereonet Analysis Dip and dip direction data was collected from a part of the outcrop that roughly parallels

the trail for about 100 feet. The data was input into the Rock Pack 2 program (Watts,

1999) to produce stereonets (see figure 9 & 10).

Figure 9 shows that a plane failure of the main bedding is unlikely. The bedding planes

are less steep then the friction angle. The lower the angle of the beds, the larger the

normal force and the greater the coefficient of friction. Theoretically, an angle of 32 will

produce just enough friction to equal the sliding force due to gravity. Contacts dipping

steeper then the friction angle do not have enough frictional force to keep the slope from

sliding and the rock mass will give way. It should be noted that this preliminary analysis

assumes a friction angle of 32 based on the rock type. This is a fairly safe assumption

since most competent rocks have a friction angle of 30 35 degrees, although some rock

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Figure 10: Stereonet Topple Analysis

Topple Zone

Bedding Planes Large Joints Medium Joints Small Joints

types, such as shales, can have a friction angle of 15 or less (West, 1995). To be sure, a

shear test should be done in the lab to account for variations in this particular rock mass

as well as the idiosyncrasies of the contacts.

Also in figure 9 are three sets of great circles showing the average dip and dip directions

of the various types of contacts. Note that there are three areas where the circles

intersect. If these intersections were to occur within the Marklands critical area (i.e. if

the conjugate joint sets form a V-wedge that daylights out of the slope at an angle greater

then the friction angle) then there would be a danger of wedge failure. The stereonet

analysis shows that there is no such danger for this outcrop (again assuming a 32 friction

angle).

Figure 10 is the same stereonet as in figure 9 but without the great circles used in the

wedge analysis or labels. For rock toppling to occur (i.e. rocks falling vertically out of

the rock slope), the ratio between the height of the joint face perpendicular to the bedding

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plane and the bedding plane itself (or other weak contact) has to be greater or equal to the

tangent of the bedding plane dip angle (West, 1995). If this condition is present, the

separating block of rock will act as a moment arm being torqued, or rotated, by gravity

out of the slope. This condition can be seen in the pie wedge at the upper part of the

stereonet called the topple zone (See Figure 10). The small joints clustered in the

toppling zone represent fracture planes that have a dip roughly perpendicular to the

bedding planes and which are steeper then the friction angle. Based on the stereonet

analysis there seems to be a significant risk of toppling. It seems quite evident by the

looking at the rocks in figure 11 that toppling is part of the erosional cycle of this

outcrop.

R ota tion al force

R ough ly 90

Figure 11

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Safety Factor Calculations A true factor of safety (FS) calculation is beyond the scope of this preliminary study, however, a reasonable factor of safety can be calculated by: FS = tan / tan (Watts, 1999) Where is the friction angle (assumed to be 32) and is the dip angle of the joint. If the dip of the joint averages approximately 75 then the factor of safety is:

FS = tan (32) / tan (75) = 0.167 Which is very low but seems to match the physical evidence of past and almost present toppling from the outcrop. Recommendations There are many places along the outcrop where it is doubtless that a high probability

exists for topple failures in the near future. Fortunately, most of the topple areas are off

the trail and not easily accessible. There are some beds high above the trail (see figure 7)

which need further analysis and may need to be cleaned; i.e. determine which rocks pose

a safety hazard and bring them down in a controlled, safe manner. Also of concern are

areas on the northwest side of the outcrop (i.e. the downstream side). Along area tall

trees have rooted into the contact crevices and may cause serious toppling failures if

blown down by wind or snow accumulation on branches. Thickening root systems may

also exacerbate the problem. As a precaution, some of these beautiful trees may have to

be trimmed or felled completely.

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Figure 4 Geologic m

ap of the C

ascades area

Appendix

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The Rock Fall Analysis -

Gathering the data w

ith the Brunton Com

pass

Bedding Planes

Large Joints

Medium

JointsSm

all Joints

Notice the

repeating natureof the jointpatterns

i

Figure 7

Rotational force

Roughly 90

Figure 11

Rock Fall A

nalysis

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Figure 1 The rock that toppled out of

TheC

ascadesW

ater Fall

The rock fall came

from this direction

and damaged the

platform and

railingFigure2

The rock fall came

from this area of

The outcrop...

...And broke through

The wood railing

here.

Figure

Boulders R

olling Dow

n The Hill

Figure 5

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Bibliography

Tso, Dr. Jonathan L; Field Notes, 1998 Summer Field School; Radford University,

Radford, VA. Johnson, Stanley S; Geologic Map Of Virginia Expanded Version: Virginia Division

Of Mineral Resources, 1993. Unpublished Map; Geologic Map of Virginia; Virginia Division Of Mineral Resources. Watts, Dr. Chester F; Class Notes, 1999 Engineering Geology; Radford University,

Radford, VA. West, Terry R; 1995, Geology Applied to Engineering; Prentice Hall, Englewood Cliffs,

NJ; p. 295.