STABLE ANGLES OF SLOPES
Department of Geological Sciences
State University of New York
Binghamton, NY 13902-6000
Level: Grades 5-7
Estimated Time Required: 45 minutes collecting data, 45 minutes discussion
Anticipated Learning Outcomes
Slopes composed of loose debris can be seen all around us. The angle at which such a mass of debris rests under given conditions is called the angle of repose. It has generally been accepted that such slopes tend to have an angle varying from 25 to 40 degrees. The exact angle depends upon conditions such as size, shape, and density of the grains, roughness of the grain surfaces, sorting or mixture of sizes, and height of fall of the grains. Studies of the effects of these various characteristics determining the angle of repose of loose material have produced diverse results. In general, an increasing fragment size results in lowering the angle of repose.
Procedures to Prepare the Boxes
These should be prepared in advance, by the teacher. However, once made, they can be used repeatedly. Allow one box per student team (of 2 - 4 students).
Figure 1. Set-up for the stable slope measurement. a) Plastic box for the experiment. b) Box filled with sand before removing the cardboard at the end. c) Box and slope formed on the sand after the cardboard at the end is removed.
Results and Discussion
Discuss the results among the group using the following questions as a guide.
What is the variation in angle of repose for any one sample?
Not much, I hope.
How do the different materials differ in angle of repose?
Can you deduce any relations between characteristics of the material and angle of slope from your results? (e.g., what is the relation between size of material and angle of repose? Do your observations indicate that the slope angle decreases with an increase in grain size?)
Why does the angle of repose of the soil material differ from that of the sands? I hope it does.
Are there any landslides in your area? Along roadcuts or other slopes? Of what practical use it it to know the angle of repose of a given material?
Take the class out to look at some unstable slopes. Discuss
why the slopes are unstable.
Source of Activity
Adapted from MORISAWA, M., 1976, Geomorphology Laboratory
Manual: John Wiley and Sons.
Recommended Reading for Teachers
RITTER, D., 1986, Process Geomorphology: Wm. C. Brown.,
COSTA, P. and BAKER, V.C., 1981, Surficial Geology: John Wiley and Sons, Ch. 9, p. 242-281.
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