In this section:
- What is a landslide?
- Why do landslides happen?
- What can increase the chance of a landslide?
- Landslides in the UK and around the world
- How to classify a landslide
What is a landslide?
A landslide is a mass movement of material, such as rock, earth or debris, down a slope. They can happen suddenly or more slowly over long periods of time. If the force of gravity acting on a slope exceeds the resisting forces of a slope then the slope will fail and a landslide occurs. External factors, such as heavy rainfall leading to saturation of the ground, erosion of the base of a slope or changes to the materials strength through weathering can lead to landslides happening.
Landslides can be classified as just one of these movements or, more commonly, can be a mixture of several.
Why do landslides happen?
A landslide may occur because the strength of the material is weakened. This reduces the power of the ‘glue’ that cements the rock or soil grains together. Located on a slope, the rock is then no longer strong enough to resist the forces of gravity acting upon it.
What can increase the chance of a landslide?
Several factors can increase a slope’s susceptibility to a landslide event:
- water — adding water to the material on a slope, makes a landslide more likely to happen. This is because water adds weight, lowers the strength of the material and reduces friction, making it easier for material to move downslope
- erosion processes — such as coastal erosion and river erosion — if the bottom of a slope is continually eroded by the sea or a river, the slope will eventually become too steep to hold itself up
- steepness of slope — the slope angle is a key factor as far as landslides are concerned. Any change to this that makes it steeper (such as coastal erosion) increases the likelihood of a landslide
- type of ‘rocks’ — soft rock such as mudstone or hard rock such as limestone — the type of rocks in the slope, and their combination
- shape of the rock ‘grains’
- jointing and orientation of bedding planes
- arrangement of the rock layers
- weathering processes — for example freeze-thaw reduces the stickiness (cohesion) between the rock grains
- lack of vegetation which would help bind material together
- volcanoes and earthquake activity nearby
- human activity — mining, traffic vibrations or urbanisation which changes surface water drainage patterns