Slope Stability (I)

Landslides can have devastating impacts. They can move and destroy roads, rail, cars and even topple buildings. Having a good understanding behind the causes of slope instability is critical for engineering and can be a matter of life or death. 
Collapse of a hillside causing roads to break up and risking lives. (By photographer: Liz Roll, uploaded by User Storm05 on en.wikipedia - This image is from the FEMA Photo Library., Public Domain,, accessed 27/05/2020)
What pulls you towards the ground when you are at the top of a slide? The answer is gravity. You have probably noticed that when you wear different clothes you might go down the slide faster or slower. This is because of friction.  Reducing friction will make you move faster down a slide. Land can move for the same reasons. When land moves down slope it is known as a landslide.
Rough surfaces cannot glide past each other as easily as smooth surfaces.  (, accessed 27/05/2020)
Landslides can be triggered by earthquakes, volcanoes, heavy rain and even large vibrations from machinery or building work. 
This railway clings to the mountainside. It is known as the Death Railway. (, accessed 27/05/2020)
When engineers design roads and railway lines, they must consider the terrain that they are passing through. Going up steep hills is avoided and so paths are often cut out of the hillside to make routes that are easier and safer for transport.  It is vital that engineers have a good understanding of the material which is being removed from the hillside to ensure maximum slope stability and avoid landslides from happening.
There are a few factors that can affect the maximum slope a material can be stabilised at. How compact the material is will greatly alter slope stability. A well compacted material, like rock, will generally be stable and can be cut nearly vertical.  However, if a sediment is very loosely packed it will be unstable. The shape and size of sediments can also greatly affect slope stability. Rough, uneven sediments will have more frictional force between them, this holds them together more.  Smaller particles are also likely to fit together more tightly, as the smaller pore spaces allow in less fluid which can act as a lubricant.
Investigate these concepts for yourself, try the angle of repose activity in the Landslides STEM project, which has a corresponding video.
This retaining wall was built to prevent a landslide over this former railway line. (, accessed 27/05/2020)
Engineers will conduct investigations to calculate the safest angle to cut into a hillside. Reinforcements and structures may also be added to the slope to improve safety, such as rock bolts and retaining walls.