Bowen Island retaining wall supporting a man-made plateau

Smart Material Choices | Bowen Island

Abstract: Bowen Island Home Inspector notes on service life of inter-dependent systems, in this case the retaining wall with the septic tank and field.

Steep Lots Need Terraforming

Retaining walls are often overlooked or ignored, even when there are signs of distress. Here’s why it’s important to consider the life-span of systems and their inter-dependencies.

On a recent Bowen Island home inspection, the issue of the integrity of retaining walls was a consideration. Let me explain. A lot of our coastal view properties are built on bedrock and slope. This is especially true of waterfront lots not only on Bowen but also on Passage, Keats and Gambier Islands. Sometimes the slope is really more of a cliff. Nevertheless the view ‘wow factor’ makes it a worthy to build upon.

Creating Flat Land

If it’s semi-rural or cottage country it is likely to have no municipal sewer to tie-into. Then the homeowner has to construct a private septic system on the property. This system will need some flat land, which is created with a retaining wall and fill material. Any retaining wall above 4ft needs to be engineered so most homeowners will only build to that height. The more slope, the more terraces needed to reach the appropriate height and area required by the septic tank and leach field.

Minimizing Impact of Interdependent Systems

Timber, stone, interlocking blocks and concrete are all used to build retaining walls. If build quality is equal, timber will still have the shortest material life expectancy due to rot from wood-soil contact. On the other hand, septic systems are of plastic or concrete, both being longer lasting materials than wood.

Ideally we want to have both the retaining walls and septic systems age equally. This is the ‘gold standard’ of build quality. Coupling mid-life wood (retaining walls) with long-life plastic/concrete (septic system) is not ideal. However it is manageable. It just means that after 15-20 years the owner must do preventative maintenance on the timbers to ensure they do not fail pre-maturely and compromise the septic system.

Choose materials in any one given system wisely, so that all components have a similar effective service life.