Geology_Lab1
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University of Colorado, Denver *
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Course
1011
Subject
Geography
Date
Dec 6, 2023
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docx
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4
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EXERCISE 4
GEOGRAPHY
1011
HILLSLOPES AND
MASS
WASTING
Turtle Mountain Landslide
•
On the map found on the following page, identify and
label the failure plane, and the slide debris. Sketch a
topographic profile·from
the
head
of
the
failure
plane
(point
A)
to
the
toe
of the
debris
(point
C).
Your
sketch
need
not
be
exactly
to
scale,
but
it
should
show
the
relative
change
in
elevation
and
the
important
topographic
differences
between
the
failure
plane
and
slide
debris.
•
Now, find the slope of the failure plane
between the
points labeled A and B. Remember, to find the slope, in
degrees, we calculate:
slope= tan
-1
(rise/run)
rise H =
-2800
ft.
run
L=
3600
ft.
tan¢=
1.2857
slope
: - 0.6610
degrees
•
Compare your answer with the
angle of internal friction
for typical
soil masses, given on page 3 of the lab handout.
What did you find?
On the handout, in typical noncohesive soils, the angle of internal friction tends to
be 0.6. The measurements in this lab have a value of 1.2857, which is about
twice as large.
•
Obviously, this slide didn't just stop at the base of
the slope; it appears to have traveled well beyond.
This
is
a
unique
characteristic
of
large
landslides:
they
seem
to
travel
much
farther
than our
simple
analysis
above
would
indicate
(by
analogy
to
the
block
on
the
inclined
plane:
it
is
as though the
block kept sliding once it reached the end of the
plane; how can it keep sliding on a flat surface?).
Let's determine the
angle of internal friction
for
the entire slide. To do this,
H
will
be
the
height
over
which
the
debris
fell
(the
same
number
you
used
above
for
the
rise).
Find
L,
the
total
distance
traveled,
from
the
head
of
the
failure
(pt.
A)
to
the
end
of
the
debris
(pt.
C).
H= 2800
ft.
L= 7100 ft.
Then use the equation to find
tan ø =
2.5357
slope=
37.56 degrees
•
No one really knows why large landslides travel
so far. Some have postulated that the slide rides
on a cushion of air, but we've seen them on the
moon, and the moon has no air. Others have
postulated that the slide moves more like a fluid,
but evidence from these slides suggests the
debris does not get very
mixed up
like it would in
a fluid.
In
other words, once the slide comes to
rest, blocks in the debris tend to be found in the
same relative position as where they started.
Whatever the case, you found here that, in
comparison to simple landslides, the Turtle
Mountain landslide had a
higher
/ lower
(circle your answer)
coefficient of internal
friction.
Pickle Prairie Earthflow
Earthflows are very common in the Coast Ranges of
California. The features originate in highly sheared
sedimentary rocks and can move at rates of several
meters
per year.
On
the next page is a figure that
shows the downslope displacement of a surveyed stake-
line across the earthflow. At the time of the initial survey
on April, 1989, the stake-line
was straight
(along a line
corresponding to the x-axis. Assume all measurements
thereafter were made the first of each month.
•
What
is
the
approximate
maximum
total
distance
of
movement
between
April
1,
1989
and June 1,
1990?
10.8 meters
•
Based
on
the
value
above
(#5),
calculate
the
average
downslope
rate
of
movement between
the
first
survey
and
most
recent
survey
(show
your
calculations).
_14
total number of months
- from April 1989 to June 1990
0.77
meters/month
- 10.8/14
8.65 meters/year
- 6.5+10.8/2 = 8.65
•
The
rate
calculated
in
#6
is
for
the
movement
of
the
surface
of
the
earthflow.
The rate
of movement
in
earth flows
actually
decreases
with
depth
because
of
resistant
frictional forces. Calculate the
average
velocity of the earthflow assuming it
is 80% of the surface rate.
0.77(avg velocity from #5)*0.8(80 percent of the rate) = 0.616
m/month
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0.616*12 = 7.392 meters/year
•
Based on the Classification for Rates of
Movement chart from page 1 of the handout,
how would you classify the average velocity
of the Pickle Prairie Earthflow?
- Extremely slow, in both, the m/year is much larger than 1.5
m/year or 0.3 m/5 years.
•
High
rates of earthflow movement tend to
correlate well with times of greatest
precipitation. Based on this information, which
consecutive 4-month period do you
think
was
the
wettest?(
note that each measurement is a
2-month period
)
February 1990- June 1990