There is a high prominence of retaining walls fail far before they should. Generally these problems go unnoticed and are accounted for as the aging of the landscape project or an ‘act of god.’   One of the greatest contributors to these common problems happens to be water. Water is an extraordinary element that has immense power when pushed to its limits.

Retaining walls are meant to retain, so why do they fall over? Well we all know about erosion, the fact that gravity and wind naturally push soils and sediments from their source and deposit it somewhere else, typically downhill. One might think erosion is what causes failure; oddly enough erosion is only a minor factor in the failure of retaining walls. The main player in failure of retaining walls happens to be water. Water has a property called ‘Hydrostatic pressure.’ This is the force exerted by water while it is at rest. If you can imagine a huge tank with paper thin walls filled with water. It would extremely hard to press your fist into the paper walls even though they were paper thin because of the pressure the water exerts when it is at rest. In this situation the paper thin walls must be able to overcome the hydrostatic pressure of the water.

How does this apply to retaining walls?  Well the purpose of retaining walls is to retain dirt and earth but in the process they also end up retaining water.  When it rains water flows into the ground and the earth retains the water, this is called groundwater. Groundwater like dirt flows with gravity, therefore when reaching a retaining wall water tends to backup behind the wall. Just like in the example above, water exerts hydrostatic pressure on the retaining wall, just as it did on the paper thin walls of the tank.  Sometimes when a retaining wall is not engineered and built correctly hydrostatic pressure can overcome the strength of the wall and that is often when retaining walls fail. If built correctly a retaining wall should be able to direct the water down through drainage gravel behind the wall and into a drainage pipe under the wall and dispose of the water.  This functionally reduces the hydrostatic pressure on the wall.

When diving into a retaining wall project there are a lot more things to think about than retaining the soil and building a wall. Factors such as soil type also play into a strong retaining wall; if the soil drains slower then more drainage is needed to reduce the hydrostatic pressure. Most of the functional part of a retaining wall is actually contained behind the wall out of sight. Always build to industry standards or consult the manufacturers recommended installation procedure unless the wall needs engineering in which case you will build to the engineer’s specifications.

The article was brought to you by Merlin Construction. Merlin Construction is a Toronto Landscape Contractor.


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