ACTIVITY EXERCISE #3
PSCI 183
Angle of Repose
Introduction
Rocks are deposited on Earth over millions of years. Sometimes they are deposited rather quickly. Other times it
may take millions of years. Landslides occur when a rock is not strong enough to resist the forces of gravity.
Background
I. Determining the Slope
The slope equals the rise divided by the run. The rise is the height difference from the top of the hill to the
bottom. The run is the length of the slope.
Slope = rise/run
II. Determining Angle of Repose
The angle between the edge of the pile and the horizontal surface is known as the angle of repose. To measure the
angle of repose, use a protractor to measure the angle from horizontal to the slope angle.
Purpose
The purpose of this exercise is to investigate both slope stability and landslides. You will create sand “mountains”,
measure slopes, and test the effects of water and rock layering on slope stability.
Materials Needed
 400 ml of sand
 400 ml of gravel
 400 ml of clay
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1 400-ml beaker
1 100-ml graduated
cylinder
1 pie plate
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Newspaper
Paper towels
Compass
Ruler
PART I. Building a DRY mountain
Procedure
Hypothesis Testing:
When building a mountain, would you expect to see a difference in the slope (angle of repose) of a mountain with
different grain sizes? If yes, which material would produce the largest slope – clay, sand, or pebbles?
Step 1: Create a dry mountain by pouring clay, sand, and pebbles.
1. Measure 400-ml of dry clay in a beaker.
2. Place the plate over the beaker and flip the beaker and plate so that the beaker is resting upside down on
top of the plate.
3. Carefully lift the beaker and allow the clay to rest. Be careful. You are trying to build a sand mountain
with a MAXIMUM slope.
4. Measure the slope of the angle as a ratio (Slope = Rise/Run). Repeat the procedure three times to average
out errors in measurement. Record you measurements in the table below.
5. Measure the Angle of Repose of the clay mountain (Angle of Repose = Degrees from horizontal). Repeat
the procedure three times to average out errors in measurement. Record you measurements in the table
below.
6. Repeat steps 1-5 using sand and then pebbles.
Material
Clay
Sand
Pebbles
Material
Clay
Sand
Pebbles
Slope
Trial 2
Trial 1
Angle of Repose
Trial 1
Trial 2
Discussion
1. Graph your findings.
Trial 3
Average
Trial 3
Average
50°
40°
Angle of Repose (°)
30°
20°
10°
Clay
Sand
Particle size
Pebbles
2. Which particle had the steepest slope (largest angle of repose)?
3. Were your findings consistent with your hypothesis? Explain why you were or were not correct.
PART II. Building a WET sand mountain
Hypothesis Builder:
How do you think the sand will behave if it were slightly moist? Will the slopes be steeper or shallower? Why?
Will there be any other differences?
Step 2: Creating a wet sand mountain
1. Pour 400 ml of sand into a beaker.
2. Pour the sand into a pie plate, add 50 ml of water, and mix the sand and water so that the grains appear
wet.
3. Put the wet-sand back into the beaker and pack it down tightly. Record the total volume of sand and water
below.
400 ml of sand + 50 ml of water = _________________ ml of sand and water mixture
4. Carefully pour the sand in the center of a pie plate. You are trying to achieve a wet sand mountain with a
MAXIMUM slope.
5. Measure the slope of the mountain as a ratio.
6. Measure the angle of repose
7. Repeat two more times.
Wet Sand
No Packing Trial
Slope
Angle of Repose
Trial 1
Trial 2
Trial 3
Average
Discussion
1.
If you add 400 ml of sand to 50 ml of water, what total would you expect? _______ml.
a. What was the actual total you got? ________ ml.
b. Why was the actual total different from the expected total?
2. By comparing the “wet” mountain with the “dry” mountain from step 1, was there a difference in the
average of slope and angle of repose? If so, in what way were they different?
3. If one mountain had a higher slope (angle of repose), explain your reason.
PART III. Compressed Wet Sand Mountain
Step 3:
1. Using your hands on your wet sand mountain from PART II, compress the sand and form the steepest
symmetrical cone-shaped sand mountain with the maximum slope that you can achieve.
2. Measure the slope of the mountain as a ratio (rise/run).
Slope = ____________________________
3. Measure the angle of repose of the compressed wet sand mountain.
Angle of Repose = _________________________
Discussion
Many times when homes are built on a hillside, it is required that the ground that it is being built on be
compressed. Draw how the grains would look before compaction and after compaction.
Open packing
Cubic packing
Part IV. Mass movement and the effects of the addition of water
Step 5:
1. Using the 100 ml graduated cylinder slowly pour water onto the top of your wet-sand mountain. Pour
enough water until you first see a settling, and then you should see mass movements.
2. Sketch the shape of the mass movement. Label the “scarp” (crescent-shaped lobe at the head of the
slide) and “toe” (hummocky or bumpy texture at the base of the slide). This texture is the same texture is
the same property you should look for prior to purchasing property at the base of the hill!
3. Keep adding water until the sand mountain collapses. Record the total amount of water added to the wet
sand. Don’t forget the initial 50 ml.
Total amount of water added to sand = ______________________________
Discussion
1. List 3 reasons why the wet compacted mountain finally collapsed completely.
a.
b.
c.
Variations
You may have groups of students work in stations. Run through the 1st step as a class and then break out into
stations. Each station may test different variations, i.e. dry vs. wet, compacted vs. not compacted, angular vs.
round, etc.