A landslide happens when soil, rock, or debris moves downhill under the pull of gravity. Landslides matter because they can damage homes, roads, rivers, forests, and entire communities in a matter of seconds or minutes. They are most common on steep slopes, especially where water, earthquakes, or human activity weaken the ground.
Understanding how landslides start helps people recognize danger signs and reduce risk.
Key Facts
- Gravity pulls slope material downhill, and a landslide begins when driving forces become greater than resisting forces.
- Steeper slopes have a larger downhill component of gravity, so they are generally less stable.
- Water pressure in saturated soil reduces friction and cohesion, making slope failure more likely.
- Factor of safety = resisting forces / driving forces; values less than 1 mean failure is likely.
- Common triggers include heavy rain, rapid snowmelt, earthquakes, deforestation, road cuts, and construction.
- Main landslide types include slide, flow, fall, and slump, each moving in a different way.
Vocabulary
- Landslide
- A landslide is the downhill movement of rock, soil, mud, or debris caused by gravity.
- Cohesion
- Cohesion is the sticking force between soil or sediment particles that helps a slope hold together.
- Saturation
- Saturation happens when the empty spaces between soil particles fill with water.
- Slump
- A slump is a landslide in which a block of earth moves downward and outward along a curved surface.
- Trigger
- A trigger is an event or change, such as heavy rain or an earthquake, that starts slope failure.
Common Mistakes to Avoid
- Thinking landslides only happen during earthquakes. This is wrong because heavy rain, erosion, deforestation, and construction can also trigger slope failure.
- Assuming dry and wet soil behave the same way. This is wrong because saturated soil often has less friction and cohesion, so it can slide more easily.
- Ignoring small warning signs like cracks, tilted trees, or sticking doors. These signs can show that the ground is slowly moving before a larger failure.
- Confusing all landslides with fast rockfalls. Landslides can be slides, flows, falls, or slumps, and some move slowly while others move very quickly.
Practice Questions
- 1 A slope has resisting forces of 9000 N and driving forces of 7500 N. Calculate the factor of safety and state whether the slope is likely stable.
- 2 After a storm, the resisting forces on a hillside drop from 12000 N to 8000 N while the driving forces stay at 9500 N. Calculate the factor of safety before and after the storm.
- 3 A forested slope is cleared for construction, then a week of heavy rain occurs. Explain how removing trees and adding water can work together to increase landslide risk.