	Teacher Sheet

Activity #41: Slope Stability

OBJECTIVES

Students will apply the concept of slope to observe and measure the stability angle of fine gravel, sand, and topsoil. Selected follow-up activities will allow them to test the validity of their measurements in the first part of this exercise.

TIME

One 45 minute period, with students working in groups of three.

BACKGROUND

In examining erosion and runoff problems, two factors normally taken into account are the type of soil and the slope stability. In this exercise students will measure slope stability using three different substrate materials. Under natural conditions, all materials will tend to slump until they reach a specific angle, often called the angle of repose, at which the slope stabilizes. Movement of the materials composing the slope ceases. Many factors affect this angle and may be examined in the follow-up activities. As you ride along roads in your area where new construction has taken place, look for evidence of slope instability, such as earthflows, and the practices used to help eliminate this problem.

MATERIALS

Each student team will need some sheets of newspaper, a protractor, a stiff piece of cardboard box approximately 14x20x3 inches in size and open on one end, 400 ml of fine gravel, sand (beach sand works well), and topsoil or loam. All students will need pens and notebooks.

PROCEDURE

Before starting, have students place newspaper on their desks or lab tables. Place the section of cardboard box on the table with the one open end near the center of the newspaper. Deposit all 400 ml of one of the samples in the center of the cardboard box and spread it out into a flat .5 inch layer. At this point one of the students holds the protractor vertically with the origin at the end of the cardboard on the newspaper (see student sheet). One of the other students will be an observer/recorder and the third student will be the tilter. The tilter slowly lifts the end of the cardboard continuously. The observer will note when the first particles begin to slide off and will record the angle. Students should switch assignments and repeat steps 1 - 8 for three trials. Repeat steps 1 - 9 for the other three samples. See student data tables. Gravel should should slump at the lowest angle, sand next, and topsoil last if all materials have relatively the same moisture content.

SPECIAL SAFETY PRECAUTIONS

None

FOLLOW-UP

The angle at which the sample slides down the face may be used as an index of slope stability. The angle at which particles begin to slide indicates the approaching loss of slope stability, and the final slide indicates total loss of stability. Exceeding these angles, even in nature, will normally result in the same collapse of the slope unless other factors are at work in the environment.

Have students discuss and describe the impact of any variables, such as moisture content, which may alter the outcome of the experiment.

Indicate what effect wet samples would have on the outcome of the experiment.

What would be the results of testing mixed samples? This can be quickly and easily tested using the same procedure.

Have students check with a construction company to find out if the instability angles would have any effect on construction standards and practices.

Predict the effect of ground cover on stability and describe what actions a person might use to prevent problems in their own local environment.

See diagram of typical failed slope (Figure 1).

SOURCE

Activity developed by James H. Barden, in conjunction with the 1991 CREST intern program.

	Student Sheet

Activity #41: Slope Stability

PURPOSE

To apply the concept of slope and slope stability to different types of soil and ground material.

MATERIALS

You will work in groups of three, each group should have the following - some sheets of newspaper, a protractor, a piece of stiff cardboard box, open on one end, that measures 14x20x3 inches, 400 ml each of fine gravel, sand, and topsoil. Each student will need their pens and notebooks.

PROCEDURE

1. Cover your desk or work area with newspaper. This will make cleaning up after each trial a lot easier.

2. Place a 14x20x3 inch piece of cardboard box so that the open end is more or less in the center of the newspaper.

3. Carefully deposit one of the 400 ml samples in the center of the cardboard, and flatten the pile into a flat layer about .5 inch thick.

4. Assign one of your group to be the observer/recorder, another member to be the holder of the protractor, and the third member to be the tilter of the cardboard.

5. Have the holder of the protractor hold the protractor vertically with the origin (zero point) at the end of the cardboard sheet on the newspaper (Figure 2).

6. Have the tilter SLOWLY lift the end of the cardboard in a smooth but continuous motion.

7. The observer will note when particles first begin to slide off the pile and will observe and record this angle.

8. The lifter will continue lifting until all the remaining particles slide off in a mass. The recorder will observe and record this second angle.

9. Switch assignments and repeat steps 1-8 for two more trials with the same material.

10. Repeat steps 1 - 9 with the other two samples.

DATA TABLES
GRAVEL
OBSERVER	ANGLE OF FIRST FALL	ANGLE OF TOTAL FALL
1
2
3
AVERAGE
SAND
OBSERVER	ANGLE OF FIRST FALL	ANGLE OF TOTAL FALL
1
2
3
AVERAGE
TOPSOIL
OBSERVER	ANGLE OF FIRST FALL	ANGLE OF TOTAL FALL
1
2
3
AVERAGE

Answer the following questions and perform the activities indicated by your teacher.

QUESTIONS

1. Describe and discuss any variables which have affected the outcome of this experiment.

__________________________________________________________________________________

2. What do you think would happen if the experiment were tried with damp or wet materials?

Predict the results.

__________________________________________________________________________________

3. What would be the results of doing this experiment with a surface that was coarser or smoother? Predict the results and explain the reasoning behind your predictions.

__________________________________________________________________________________

Last updated on October 6, 2005

Teacher Sheet

Activity #41: Slope Stability

OBJECTIVES

TIME

BACKGROUND

MATERIALS

PROCEDURE

SPECIAL SAFETY PRECAUTIONS

FOLLOW-UP

SOURCE

Student Sheet

Activity #41: Slope Stability

PURPOSE

MATERIALS

PROCEDURE

QUESTIONS