Landslide Lab: When Does a Slope Give Way?
Understand slope stability as a contest between gravity pulling soil downhill and friction plus cohesion holding it, measured by a factor of safety that rain, tree loss and toe cutting drive below one.
Before you try
What if a hill had no trees at all? How much rain would it take to bring it down?
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What happened?
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The infinite-slope formula with one soil layer and typical values; real slopes have varied layers and drainage.
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Try these ideas
- Rain for 12 hours on the default slope and note how the factor of safety changes.
- Remove the trees and repeat. How many hours until it fails?
- Find the steepest dry slope that stays above a factor of safety of 1.
Why it works
Gravity pulls the soil on every hillside downhill, and it pulls harder the steeper the slope. Friction between soil and rock, plus cohesion from clay and the grip of tree roots, holds it back. Engineers compare the two as a factor of safety; above about 1.3 a slope is considered safe, and below 1 it fails. Most of the year the holding side wins.
Rain changes the balance: water filling the pores pushes grains apart so friction drops, and it adds weight. Cutting a road into the foot of a slope removes the support of the soil below, and felling trees removes root cohesion. That is why monsoon landslides strike the Himalaya and the Western Ghats after roads and deforestation, as at Wayanad in 2024. In this lab you set the slope, rain, trees and road cut, run the hours of rain and read the gauge.
Check your understanding
Question 1 of 4
A slope fails when
Rain makes slopes weaker mainly because
Tree roots help a slope by
Cutting a road into the bottom of a slope
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Pile dry sand and then wet sand and compare how steep each will stand.
- Ask: why are hill roads often blocked after heavy rain?
- Look at slope-protection measures near a hill road: nets, walls, drains.