1. Assessment of the Buckhorn Canyon Landslide, Beaver Head Mountains, Idaho
Sara Ramos, Barry Miller, Julie Willis Ph. D.
Map
Map
45
24
27
Re-Activated
Area
Slide A
Slide B
Slide C
Head Wall
Structure
0 m 150 m
Figure 4. This graph by Wells and Coppersmith (1994) show the relationship between falt rupture
lenght and magnitude of earthquake. The magnitude of the ground acceleration is shown by the
red line. This indicates a rupture length of 30 km.
Contour lines indicate Slide B is the oldest slide. Areas where Slide B
should continue are cut-off by Lobe C. Slide B supports more re-activa-
tion areas than the other Lobes. This supports the hypothesis that Slide
B is older. If all slides failed due to the same event Slide B would have
flowed first followed closely by Slide A and C.
Lobe Relative Age
The strike and dip datum (map) indicates folding in the headwall. This is
likely associated with the Sevier Orogeny (approximately 140 Ma - 50 Ma).
The folding may have fractured the rocks creating a failure point. Further
research is needed to confirm fractures and folding in the headwall.
Head Wall Structure
The factor of safety calculates the structural capacity of the landslide.
This calculation is used to determine cause of failure. The study area is
composed of three main slides. I calculated the factor of safety for each
slide. (“Rock Slope Stability Analysis”2014).
Table 1 shows the amount of water required to cause failure is
impossible. The determining factor for failure was ground acceleration.
This ground acceleration converts to a 6-7 moment magnitude
earthquake.
Wells and Coppersmith (1994) indicate that a moment magnitude of
6-7 is caused by a 30 km rupture length for a normal fault (Figure 4). The
Beaverhead fault is approximately 28 km in length (location indicated on
index map). The fault is located 5.5 km from the landslide. Age estimates
for the Beaverhead fault are Holocene to 30 ka (Haller, et. Al 2010).
Further study is needed for more precise dating.
Figure 3. Factor of Safety calculates the
structural capacity of a landslide.
c = cohesive strenght of failure surface
A = area of failure surface
W = weight of sliding block
a = horizontal acceleration
= inclination of failure plane
U = uplift water force
V = driving water force
[W(sin + a cos ) + V cos ]
FS =
{cA+[W(cos - a sin ) - U - V sin ]tan }
Factor of Safety
Fault Failure
Factor of Safety Ground Acceleration Water Level (ft)
Slide A 1.55 0.147 8770
Slide B 1.45 0.129 18085
Slide C 1.65 0.165 12338
Table 1. This table shows the values for calculated Factor of Safety, Ground Acceleration, and Water Level. Because
the landslide was less than 200 ft deep this water level is impossible to reach.
References
“Rock Slope Stability Analysis.”2014. Scribd. Accessed December 8. https://www.scribd.com/doc/37466746/Rock-Slope-Stability-Analysis.
Haller, K.M., and Wheeler, R.L., and Adema, G.W., compilers, 2010, Fault number 603e, Beaverhead fault, Nicholia section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults,
accessed 12/07/2014 09:27 PM.
Wells, Donald L., and Kevin J. Coppersmith. 1994.“New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement.”Bulletin of the Seismological Society of America 84 (4): 974–1002.
Gutenberg, B., and Richter, C.F. Seismicity of the Earth and Associated Phenomena (Princeton, New Jersey: Princeton University Press, 1954), p. 18; and U.S. Department of Defense. The Effects of Nuclear Weapons (Washington, D.C.: S. Glasstone ed., Govern
ment Printing Office, 1962); pp. 14.
Conclusion
Folding in the headwall fractured the rock creating a point of failure. Slope
failure was caused by the rupture of The Beaverhead fault approximately 1
ka – 30 ka. This rupture produced an earthquake of moment magnitude
6.5-7. Contour data suggests Slide B was the first to fail followed closely by
Slide A and Slide C. All slides could have been activated by a single seismic
event or multiple events.
Further Research:
Further Research is needed to determine relative or absolute age of Slides.
Dating material collected from each slide would provide an absolute date for
each slide. Trenching the Beaverhead fault would provide a relative date for
seismic event.
Additional strike and dip data is needed for a better interpretation of Head-
wall Structure.
Abstract
Landslides can cause property damage, injury, and loss of life. It is
important to understand development of slides because of the associated
hazards. Slide progression is best observed in areas that have not had
human development. The Buckhorn Canyon slide is not affected by
human development. A study of the cause, movement, and reactivation
of the slide is feasible. Studying movement and reactivation leads to
better slide prediction. An assessment of uninhabited, pre-historic slides
provides an opportunity to understand new slides affecting human
developed areas. Slide assessments help produce effective preventative
action plans.
Map
The base map is a DEM made from the data collected through USGS,
The National Map Viewer. The resolution of the DEM is 1/3 arc-second.
This provides a basic representation of the study area. A slope raster and
detailed contours were derived from the DEM in ArcGIS. These were used
to estimate different lobe locations.
Headwall Structure
Data taken at the site included strike and dip information (map) of
headwall and rock samples. Headwall structure was inferred from strike
and dip data.
Lobe Estimates
Lobes were determined by a change in elevation. Each lobe is
classified by a steep headwall, foot, and side. A contour map showed
three main slides located within this study area. Each slide has been
re-activated multiple times. Relative ages are difficult to interpret due to
weathering and re-activation of the different lobes.
Factor of Safety
Factor of Safety indicates structural integrity of the slope. This
equation (Figure 3) is used to determine main cause of failure. The two
main causes of failure are ground acceleration and water pressure. To
calculate the factor of safety the follow variables must be estimated:
slope angle, density of landslide, cohesive strength of failure material,
friction angle of failure surface, and area of failure surface.
• Slope angle was determined by projecting the headwall topography
beneath the surface profile. I estimated a linear slope which
represented the failure surface.
• Density of the landslide material was determined using three samples
collected at the site. I measured the density of each sample by
calculating the weight and volume. The average density of the
samples was 160.68 lbs/ft2.
• Cohesive strength and friction angle of failure material was calculated
using a direct shear device (figure 1). Cohesive strength is
represented by the y-intercept. Friction angle is calculated by
using the slope angle. The resulting failure envelope graph shows
the estimated limits of sigma 1 and sigma 3.
• Area of failure surface was estimated by measuring the area of each
lobe in ArcGIS.
I estimated water pressure by setting the factor of safety to the value
of failure and omitting ground acceleration. The level of water required
for slope failure was greater than the depth of the landslide.
I estimated the ground acceleration by setting the factor of safety to
the value of failure and omitting water pressure. The calculated ground
acceleration indicates an 6-7 magnitude earthquake.
Fault Estimations
Rupture length was estimated from the correlation between magni-
tude and rupture length. According to Wells and Coppersmith (1994) the
type of fault and magnitude of earthquake determines length of fault
rupture (figure 4).
Estimated rupture length and fault type limit possible sources for
earthquake.
Methods and Results
Figure 1. This is the Direct Shear Device used to test a
sample of the layer responsible for failure. We would like
to acknowledge BYU-Idaho Engineering Department for
the use of this machine.
Figure 2. This graph is the result
of a Direct Shear Test. The slope
indicates friction angle of failure
surface. The y-intercept indicates
cohesive strength of failure
surface. A Mohr’s circle is drawn
on the graph as an estimate. This
indicates maximum values of
sigma 1 and sigma 3 prior to
failure.
y = 0.4741x + 6.0683
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ShearStress
Normal Stress
Failure Envelope
Figures