Make Up Lab 9 Unstable Slopes
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Dec 6, 2023
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Lab 9 Make Up- Unstable Slopes
Slopes can become unstable for a number of reasons.
When slopes fail, they can produce violent
torrents of mud and debris and massive earthen slumps.
Between 2002 and 2011, catastrophic slope
failures were responsible for the loss of over 83,000
lives worldwide (Petley, 2011) and they also caused
extensive property damage.
What are the factors that contribute to slope failure?
Does slope failure
happen relatively quickly or over a long period of time?
Goal:
In this lab exercise, you will measure the slope angles of different types of sediments to determine
what critical angles cause them to fail.
You will also examine the role water plays in slope failure.
Key Terms:
angle of repose, landslide
I.
The Angle of Repose
The maximum angle that a slope is stable is called the angle of repose.
This angle varies for different
sediments depending on properties such as grain size, angularity, and amount of water saturation.
One
way to measure the angle of repose is to use a protractor to measure the acute angle between the slope
and the horizontal base (Figure 1).
Figure 1: Assume your sediment pile is a triangle.
Angle of repose (A) is the acute angle between the slope and the base of
the pile.
A more accurate way of measuring the angle of repose is to use trigonometry: the sine (sin (A) =
opposite/hypotenuse), cosine (cos (A) = adjacent/hypotenuse), and tangent (tan (A) =
opposite/adjacent) relationships between angles and distances of a triangle.
In this case, we will use
the tangent of angle A because we are able to measure both the height (opposite) and base (adjacent)
lengths of the sediment pile.
To find the angle of repose you will use the tangent relationship and solve for A:
Solving for A:
Tangent:
Note: All measurements should be in centimeters.
Record (H/R) to 3 decimal places.
Record angle of
repose to 1 decimal place. Your calculator should be set to degrees (not radians). Record your answers
on the Lab 9 student answer sheet.
Part 1: Dry Sand
In Figure 2, we’ve s
lowly poured dry sand into a pile
–
making sure that the pile overlaps a ruler.
We’ve
also indicated the measurements
for height and radius that you’ll
need to complete the calculations.
Figure 2: Dry sand with measurements of height and radius.
1.
Record the radius and height of the dry sand pile.
Show the equation you would use to calculate the
angle of repose.
(6 points)
the height is 6cm and the radius is 9cm, the equation is Tan ( A ) =
Height / Radius , A = Tan^ - 1 * ( height / radius )
2.
Calculate the angle of repose (steepest slope at which the sediment is stable) using the tangent
relationship.
(2 points)
Tan ( A ) = Height / Radius , A = Tan^ - 1 * ( height / radius )
the steepest
slope at which the sediment is still stable.
Adding a pinch (just a few grains) of dry sand to the top of the pile will cause some the grains to move.
To see this for yourself, log into Bb Learn and click the
“dry sand video
clip
”
located in the assignment
folder for Lab 9 Alternate- Unstable Slopes.
H = 6 cm
R = 9 cm
3.
Describe the downward movement of the dry sand.
Note whether the sand grains move individually
or in large groups.
Record these observations on your data sheet.
(2 points)
Dry sand is not able to
stay stable past 35degrees and the sand grains more in large groups made by induvidal sand
grains falling.
Part 2: Damp Sand
Now we’ll repeat the process
using damp sand (see Figure 3).
We’ve created
a pile of damp sand
–
again
taking care that the pile overlaps the ruler.
Once again, use the measurements that are included to
complete the calculations.
Figure 3: Damp sand with measurements of height and radius.
4.
Record the radius and height of the damp sand pile.
Show the equation you would use to calculate
the angle of repose.
(6 points)
the damp pile has a height of 6.5 cm and a radius of 9cm the
equation for this t
an^-1*(height/radius).
5.
Calculate the angle of repose (steepest slope at which the sediment is stable) using the tangent
relationship.
(2 points)
The sand is still able to remain stable at 35.8 degrees. t
an^-1* (6.5/9)=35.8
degrees
H = 6.5 cm
R = 9 cm
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Adding a pinch of damp sand to the top of the pile will cause some the grains to move. Once again, log
into Bb Learn and this time click the
“wet sand video
clip
”
located in the assignment folder for Lab 9
Alternate- Unstable Slopes.
6.
Describe the downward movement of the damp sand.
Do the sand grains move individually or in
large groups/clumps?
(2 points)
The sands downward movement is not very much because the
sand is in large clumps and doesn’t move too much.
7.
Think about how the angle of repose for wet sand compares to the angle of repose for the dry sand.
Which one has a steeper/higher angle of repose?
Why do you think that is?
(4 points)
The wet sand
has the higher angle of repose because the sand clumps together and this allows for the sand to
stay together and not fall due to the higher steepness.
Now we will slowly pour water onto the pile of damp sand and observe what happens to the sand pile as
the sand becomes saturated.
Go into Bb Learn to watch the
“saturated sand video clip”
and see what
happens when the wet sand pile becomes saturated with water.
8.
Describe what happens when your sediment pile becomes saturated.
How does it compare to the
movement of dry sand?
(4 points)
When the sand is wet it causes the spaces in between to
become filled and this gives the consistency and effect of mud. This causes it to move in clumps as
opposed to the dry sand moving more individually.
Part 3: Gravel
Now let’s look at
gravel (Figure 4).
Complete the same calculations using the measurements included in
this figure to see how the gravel compares to dry and wet sand.
H =6.5 cm
R = 10 cm
Figure 4: Gravel with measurements of height and radius.
9.
Record the radius and height of the gravel pile.
Show the equation you would use to calculate the
angle of repose.
(6 points)
The height of the gravel was 6.5cm and it had a radius of 10 cm the
equation was tan^-1*(height/radius)
10.
Calculate the angle of repose (steepest slope at which the sediment is stable) using the tangent
relationship.
(2 points)
the angle where the gravel can still remain stable is 33 degrees.
11.
Describe the size, shape, and angularity of the gravel (individual grains):
(4 points)
The grains are all varying in size some big and round and some of them are small. They have lots of
different sizes and shapes and is not as small as the sand. The gravel is not very stable because of this
and has a lower angle that the other types.
Part 4: Cinders
Finally, let’s look at
cinders (Figure 5).
Complete the calculations one more time!
H = 6.5 cm
R = 9 cm
Figure 5: Cinders with measurements of height and radius.
12.
Record the radius and height of the gravel pile.
Show the equation you would use to calculate the
angle of repose.
(6 points)
The height of the cinders was 6.5 cm and it had a radius of 9cm. The
equation I woul use is tan^-1*(height/radius)
13.
Calculate the angle of repose (steepest slope at which the sediment is stable) using the tangent
relationship.
(2 points)
The angle that they can stya stable at is 35.8 degrees t
an^-1* (6.5/9)=35.8
degrees
14.
Describe the size, shape, and angularity of the cinders (individual grains):
(4 points)
These rocks are
much bigger than the previous ones and because of this they have a awkward stability and cant
stack very high without tumbling down. They also have a arraw of shapes with some of the rocks
having jagged edges.
15.
Which type of sediment has a higher angle of repose: gravel or cinders? Why do you think that is?
(4 points)
The cinders are able to have the higher angle and this is because they stack together
than the gravel does. The gravel is smaller and tumbles down quicker than the cinders.
II. Applying your knowledge:
16.
Would a house built on top of loose sediment that is at or slightly higher than its angle of repose be
at risk for mass movement?
Why or why not?
(6 points)
A house that is built on this type of
sediment would be at risk for mass movement. This is because the sediment is at its max angle
and because at its max degree it is not very stable an this addational weight would cause the
loose sediment to fall.
a.
What evidence from your experiment supports your conclusion?
(6 points)
The evidence
behind this would be erosion which is the process when allows for lots of loose sediment
movement. It is a endless cycle of the loose sediment falling and exposing more loose
sediment that over time will become unpacked and separate aswell.
17.
Based on your experimental results, list some factors that contribute to slope stabilization.
(6 points)
Based on my results some factors that would add to a stable location would be a flat area that is
also very compact. Both of these would result in a stable location that is not at risk to erosion or
weathering.
18.
How does water act to both stabilize and destabilize slopes?
(6 points)
The water can act as a stabilizer if the perfect amount is added and this would result in a clay like
interaction that adds firmness to the material. Water can also destabilize by seeping into the
sediment causing erosion where the gravel disconnects from each other with aid from the water.
III. Seattle Landslide Study: Investigating annual precipitation and
landslide occurrence
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Figure 6: Average precipitation and number of reported landslides in Seattle from 1890-1999. (City of Seattle, 1999)
19.
In what four-month time frame does the most precipitation fall in Seattle?
What about
landslides?
(5 points)
The four month timeframe of November to February results in the most
precipitation. Landslide occur from December to march and January has the most amount of
landslides.
20.
What is the amount of time between the onset of increased monthly precipitation and the incidence
of landslides (i.e. lag time)?
(5 points)
The amount of time onset is around 2 months since january
where the most landslides occur is 2 months after the start of the rain.
21.
What is the relationship between the timing of storms in the Seattle area and the timing of
landslides?
(5 points)
The relationship between the storms and the landslides has to deal with the
water taking its time to seep into the hills and ground. The water slowly weaves its way between
the sediment and eventually reaches the point where the stability is at its lowest causing a
landslide.
22.
Using your knowledge of how water may cause slopes to fail, why do you think landslide activity
increases after average precipitation increases?
(5 points)
I think the landslide activity increases
because of the water reducing friction between the sediment and this makes it less stable
eventually resulting in a collapse. As I explained in the previous question it takes a bit of time
before the water can take effect into a landslide.
Exercise Modified from:
Whittington, C. and Baer, E., Angle of Repose. Retrieved August 1, 2011 from
http://serc.carleton.edu/quantskills/activities/Angleofrepose.html
References:
City of Seattle, Department of Planning and Development, 1999, “Seattle Landslide Study.”
Accessed February, 2011.
http://www.seattle.gov/DPD/Landslide/Study/default.asp
Petley, David, “Global Deaths from Landslides in 2010.” Weblog entry. The Landslide Blog. Posted February 5, 2011. Accessed F
ebruary 7, 2011.
http://blogs.agu.org/landslideblog/2011/02/05/global-deaths-from-landslides-in-2010/