Earth and Environmental Science 11–12 · Year 12
Slope stability: angle of repose of dry, damp and saturated sediment
Module 6: Hazards
The idea
Loose material stands only up to its angle of repose, and a little water raises that angle while more water can lower it again, which is part of why heavy rain triggers landslides.
What you need
- dry sand, a coarser sand and fine gravel, 1 kg each; a clear jar with a screw-top lid as in the Earthlearningidea Sandcastles method, a tray, protractor and ruler
- a spray bottle and a jug for the damp and saturated runs
- a hinged board to tilt a loaded sand layer to failure
How to do it
- Half-fill the jar with a dry material, lay it on its side and roll it until the material is packed evenly with its top surface horizontal; then roll it gently until that surface just begins to slide and read its angle from the horizontal with the protractor. Repeat five or six times for each material and average (Earthlearningidea Sandcastles).
- Repeat with the material dampened with the spray bottle, rolling the jar to pack it as before.
- On the hinged board, tilt a layer of dry sand until it slides and read the angle; repeat with saturated sand.
- Tabulate material, moisture and failure angle.
- Relate the result to cut slopes beside roads and to the syllabus question of how human activities can add to the frequency and magnitude of landslides, for example clearing vegetation, undercutting a slope or loading its crest.
What you should see
Each dry material gives a repeatable angle of rest, whose spread over five or six runs the learner reports; rough, angular grains stand steeper than smooth ones, and damp material stands steeper than dry because a little water binds the grains (Earthlearningidea Sandcastles, which gives about 34 degrees as the maximum angle usually quoted). Saturated sand on the board may fail at a lower angle than damp sand, though the same sheet notes that dry and fully submerged material often differ surprisingly little; the learner reports what the runs show and the ranking of dry, damp and saturated.
What changes
- What you change
- moisture content (dry, damp, saturated)
- What you measure
- angle of repose or failure angle (degrees)
- What you keep the same
- material and grain size
- packing the jar the same way for every run
- board surface
- measurement method
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Water always makes a slope weaker (a little makes sand stronger).
- Landslides need an earthquake to start.
Safety card
Hazards
- sand in eyes
- wet floor
Controls
- safety glasses when pouring
- tray under the board
Note
No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.