COMMON CAUSES OF SLOPE FAILURE

some common causes of slope failure or landslides

In order to determine where to begin the slope failure mitigation in your property, you must first take the time to understand the factors that lead to slope failure in the first place.  After all, slope repair is not a one-size-fits-all solution.  Each slope is different when it comes to geology, soil composition, vegetation and a myriad other factor.  Consequently, for any slope repair strategy to effectively mitigate the threat of landslides and mudslides, it should be tailor-fit to the slope on which it is to be applied and the causes underlying the instability of such slope.

Here are some of the common causes of slope failure:

Steepness of the Slope

It goes without saying that the steeper a slope is, the more unstable it will be.  It’s true for making sand castles and it’s true for making hillside homes.  The natural tendency of steep slopes is to move some of its materials downwards until the natural angle of repose is found.  Any form of slope modification, whether it be through natural means such as a stream undercutting the banks of a river or by workers removing a section of the slope’s base to build roads, will impact the stability of a slope.

Water and Drainage

Water is several times heavier than air.  During heavy rains when the soil becomes saturated and water takes the place of air between the grains of soil, the earth in slopes becomes a lot heavier.

This becomes a problem when the earth is being held back by a retaining wall at its base.  Specifically, if the weight of the earth behind the retaining wall exceeds the retaining wall’s structural capacity, the retaining wall will buckle and collapse releasing the earth behind it in a catastrophic deluge.

Water also reduces grain-to-grain contact which, in turn, reduces cohesiveness and the soil’s angle of repose. Along with changes in the groundwater fluid pressure in slope rocks during the rainy season, water saturation by itself already increases the probability of downslope mass movement.

Soil Composition

The composition of the slope’s soil is a very important consideration when it comes to mitigating slope failure. Different types of soils will have very different characteristics when it comes to frictional resistance to erosion and cohesion among the grains.  Loose soil or sand, for example, has very low cohesion and will easily erode when saturated with water.  Soils that have a large amount of clay, on the other hand, tend to expand when exposed to water; this makes them heavier and more prone to movement.

Vegetation

The amount and type of vegetation found in a slope is also proportional to the strength of that slope.  Vegetation, specifically its roots, holds the soil in place and makes it more resistant to erosion.  The bigger the size of vegetation, the more widespread its roots are and the more it is able to hold the soil in place. The more vegetation there is, moreover, the more stable the slope is likely to be. This is the reason why slopes that have had their vegetation removed or razed by bush fires are prime candidates for slope failures during the rainy season.

Bedding Planes

A bedding plane is basically a surface that separates a layer of stratified rock or bed from another. Think of it as butter spread between two slices of bread. Because of their nature, exposed beds in a slope are also at a high risk of slope failure. This is risk is exacerbated if there is a weak layer of rock sandwiched in the bed.

To illustrate, imagine placing a panel of glass on a slide and a block of wood on top of it.  The contact surfaces between the slide, the glass and the wood are bedding planes angled downwards.  Even though the frictional force keeping the block of wood on the glass is strong, the glass-slide connection is weak, and this causes the whole structure to erode downwards.

Joints & Fractures

Joints and fractures are natural cracks in the rocks forming a slope. These are caused by the natural expansion of rocks due to cooling or the removal of overlying rocks due to erosion.  Because of these cracks, the cohesion between the rocks that make up the slope is greatly reduced, increasing the likelihood of a landslide in the slope.

Sudden Shocks

Lastly, sudden shocks like earthquakes, hurricanes, volcanic eruptions, the passage of heavy trucks, blasting, and others may trigger the sudden mass movement of the soil in slopes.

Based on the above discussion on the common causes of slope failure, you should realize that repairing a slope and preventing a landslide is not a simple task. If you’re going to dop it, then you should do it right. Band-aid solutions will only be a waste of money; they could also give you a false sense of security, which could ultimately cost you your house and out your family at risk.

If your house is on a slope or located near one, get the slope inspected to understand if you are at risk and, if you are, learn about slope failure solutions that you can apply to mitigate such risk.