Watch Out for Landslides

Association of American State Geologists

Activity Source: 

Adapted with permission by the Association of American State Geologists,
from AGI’s Earth System Science in Your Community.

Background

Landslides not only are dangerous — causing on average more than 25 deaths and over $1 billion in damages a year — but are also widespread, occurring in all 50 states. Compounding the hazards, these natural disasters often occur along with other similar natural phenomena, such as floods or earthquakes.

To minimize risk, the slope of land and materials underground must be considered when planning how to build in a community. Altering the slope of land, or even the amount of vegetation on a slope, can have dangerous consequences.

Materials

  • 500 mL of fine sand
  • a dry container such as a can or jar
  • Funnel
  • Protractor
  • Newspaper to cover flat surface
  • Calculator
  • Paper and pen to record findings
  • 500 mL of dry materials such as mud, gravel, soil, table salt or granulated sugar, along with a can or jar large enough to hold this material, and a piece of cardboard large enough to cover it

Procedure

    1. Cover a flat surface, such as a lab table, with newspaper. Slowly pour 500 mL of dry sand through a funnel onto the flat surface to makes a pile. Describe what happens to the sides of the pile as you pour.

    2. Hold a protractor upright (with the bottom edge against the flat surface) and carefully slide it behind the pile as shown in the diagram.
      Model of protractor against pile of sand.
      AGI
    3. Where the curved upper edge of the protractor intersects the surface of the pile of sand, read the angle in degrees. This is the angle of the side (slope) of the pile, the “angle of repose.” It is the steepest slope that can be formed in the material without the material slumping or sliding down the slope.
    4. Repeat step 3 several times. Record the angle measurement each time. Do you get the same angle each time? Why or why not? Why is it important to make this measurement several times? What do you think will happen to the angle with a greater or lesser amount of sand? How might the addition of water, as in heavy rains or flooding, affect the risk of sliding?
    5. Repeat steps 1, 2, and 3 using different amounts of sand. Record the angle measurement each time. Does the angle change? If so, how much?
    6. Pour extra sand onto a pile of sand several times. Record the angle measurement each time. Does the angle of the pile change?
    7. Gather materials such as coarse sand, clay or mud, gravel, silt, soil, table salt, and granulated sugar. For each of these materials:
        • Place a handful of the material in a dry container such as a can or jar.
        • Cover the container with a piece of cardboard and turn the container upside-down onto a flat surface.
        • Lift the container slowly so the material forms a cone-shaped pile.
        • Measure the angle of the slope. Make three measurements for each material.Record measurements on a chart like this one. How does particle size and shape relate to the maximum slope angle the particles will maintain?
  1. Record measurements on a chart like this one. How does particle size and shape relate to the maximum slope angle the particles will maintain?
  2. Material

    Angle Measure of Slope

    Average Angle

    Fine sand

     

     

     

     

    Coarse Sand

     

     

     

     

    Clay

     

     

     

     

    Gravel

     

     

     

     

    Silt

     

     

     

     

    Soil