The 'sand castles' activity will help students understand slope
stability and how adding water to earth materials affects slope
Make your own clinometer
by gluing a protractor to a piece of cardboard, then threading a
weighted string through the center. Your clinometer will help you
measure slope angles as you build sand castles and models in these
Each group of students will need:
- One 1/2-gallon plastic bottle filled with dry sand,
- One plastic gallon-size tub,
- One foam cup with holes punched in the bottom (use a paper clip as a hole-punch),
- One graduated cylinder or measuring cup (something to measure the volume of water used during the activity),
- One ruler or yardstick, and
protractor with a piece of string and a small weight (or contact
goniometer if available - used to measure the angle between mineral
each group of students to build a sand castle in the plastic tub by
using all the sand in the bottle. How tall can they make their castles?
Encourage them to try different options with the dry sand, such as
building the castle in the middle or against the side of the tub. Have
them discuss the problems they encounter trying to build the tallest
- Once they have experimented for several minutes, have the
students measure the height of their castles with the ruler. Make sure
students measure from the base of the castle to its tallest point, not
the length of its slope. Have students report their measurements to the
- Have students brainstorm about how they might measure the
slope angle. To measure the slope angle, students can construct a
simple clinometer by using a protractor, a piece of string and some
small weight, such as a nut or small bolt. Thread a weighted string
(the string should be at least the same length as the straight edge of
the protractor) through the pencil hole in the protractor. Glue the
protractor to a piece of cardboard as shown in the illustration. Place
the straight edge of the cardboard on the slope to be measured and read
the angle of slope on the protractor scale. Have students record the
- Challenge students to suggest ways they can make their
castles higher. Invite them to think of any means possible, even using
materials other than sand. Students may suggest such methods as
building walls, bracing the sand, adding water to the sand, and adding
other materials to the sand. Record these suggestions on chart paper so
students can refer to them later.
- Have the students add water to the sand and, again, try to
build a sand castle. Let them experiment until they are satisfied they
have the highest possible castle. Collect height measurements from each
group. Discuss the results of this second attempt to build the castle,
then compare the wet sand castle heights to the dry sand castle
heights. Have students calculate the average height of the dry sand
castles and the average height of the wet sand castles.
- Ask, "Why do you think the wet sand castles are higher?"
Students will realize that wet substances have different properties
than dry ones. Ask students to think of other instances in which adding
moisture changed the property of a dry substance, such as adding milk
to flour when baking.
- Ask students what they think will happen if they add more
water to their sand castles. Suggest that the groups 'rain' on their
castles by filling the perforated Styrofoam cups with water and then
letting the water rain on the castle. They should fill the perforated
cups with 100 mL of water at a time so they can easily record the
amount of water that caused the castle to slump.
- Have the groups observe how their castle changes as it
becomes wetter. Students should describe these changes in their lab
notebooks, as well as the amount of water they rained on the castle.
- Initially, the changes in the sand castles will be subtle.
At some point the sand castle will slump, causing a landslide. When all
of the sand castles have slumped, gather the class together to discuss
their observations. Ask students how much water they had poured on the
castle when it began to look like a liquid instead of a solid. Ask them
where the castle failed first-the top, bottom, or middle?
This activity is an excellent way to introduce concepts, such as
erosion, geologic and hydrologic hazards, mass movements of earth
materials (landslides) and water-earth interactions. Gravity provides
the energy for landslides, but water also plays a number of roles.
Small amounts of water added to the sand increase the cohesion of the
sand grains - surface tension - as the water begins to fill pore spaces
between the sand grains. As more water is added to the sand castle
during the student-generated rain storm, the pore spaces fill with
water and the force of the water actually pushes the sand grains apart,
causing a landslide. Dry materials, such as sand, have a threshold of
slope stability related to gravity and the cohesion of the material.
Known as the angle of repose, this threshold limits the height of the
castle for a given volume of sand.
how the sand castles could be designed to prevent them from slumping.
Ask students what methods they have observed of preventing walls from
slumping. They might recall seeing retaining walls or terraces along
rivers, streams, or highways.
- Build castles using other earth materials, then compare the
height of these castles to the sand castles. Students can bring samples
of soil from home.
- Explore how the rate of water flow affects erosion.