A Slippery Slope. A Mudslide in Sierra Leone. What Happened?

Satellite image of mudslide area in Freetown, Sierra Leone, from DigitalGlobe.

Last week, there was a mudslide in Sierra Leone. Subject to heavy rains, large masses of soil came down the hill and into town, crushing houses, suffocating lives… 500 by last count, but they are still counting.
In the tragic history of landslides, of which mudslides are one type, the Sierra Leone disaster is only the latest chapter. Measured by human deaths, the worst was in China in 1920. The Haiyuan landslides, caused by an earthquake of the same name, had an estimated death toll of 100,000. Whole villages were swallowed.

National Geographic, which every month depresses with tales of us destroying our earth, seized upon deforestation as a likely cause. Indeed, the explanation has merit. Tree roots act to keep the soil together. An area of Mount Sugarloaf, up hill from Freetown (Sierra Leon’s capital, population 1.1 million) had been denuded of trees. Also a possible cause: climate change causing more intense storms. Third world countries, like Sierra Leone, struggle to cope with effects as they lack resources and infrastructure.

Sierra Leone, a small country in equatorial Africa.

A mudslide is one of many types of landslides, soil is wet and act more like a liquid flow than solid particles. The USGS lists 24 specific causes, including deforestation by humans, vegetation removal (by fire), mining, earthquakes, etc.

An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide.

The basic cause of soil moving downhill is a fairly simple concept. A trigger can be a seismic event, explosion, removal of material below (mining), addition of material above, or just a steady buildup of human-made or natural forces. For example, heavy rains wet the soil, making it heavier, and with nothing holding it together (like tree roots), the soil breaks apart, sliding over itself and/or solid material underneath (bedrock).

Hills are usually soil on top of harder material. The soil sits on top of rock, and would sit there on its own if left undisturbed. Even fine sand can sit on smooth rock if the angle of the slope is low enough because the particles of sand will interfere with each other and prevent movement. Add roots, chunky matter like large stones or clay-like soil, or if the bedrock is not smooth, and the slope can be increased. The whole of it can be kept in an uneasy equilibrium even on steep slopes. There are many slopes that are accidents waiting to happen.

From ESYS150 Lecture on Mass Movement, by Marion Mosley, SlidePlayer

At a 30-degree slope, each pound of weight exerts a 0.5 lb of force in a direction downward parallel to the slope. This is the shear force felt in the soil. If that is less than the shear strength keeping the soil on the bedrock, the soil is going down.

Types of landslides, courtesy of British Columbia Geological Survey

While basic mechanics can explain soil movements in the most basic way, as seen in the image above, civil engineers must do a more careful study to predict collapse of existing geological features or to mitigate effects of a landslide, especially when they have the potential to affects large public infrastructure projects like highways and dams.