Ottawa Soil Conditions Can Present Major Challenges for Home Builders

18 July 2023by Sandy Hill

Ottawa residents who are not involved in the construction industry may not be aware of this region’s reputation for extremely variable soil conditions.

On a new project, builders spend hundreds of thousands of dollars taking soil samples on a very dense grid pattern, having them tested and designing appropriate footing systems to support new homes but very often even this isn’t enough.

If you have driven past the site of a home being built recently, you may have seen big white blocks of polystyrene foam, and wondered what they’re for.

The reason for the blocks goes back some 12,000 years, to the end of the last Ice Age, when the Champlain Sea covered much of what are now the St. Lawrence and Ottawa valleys. Some of the former area of this now-vanished sea became covered with what is called Leda clay.

Leda clay has a high-water content, which makes it highly compressible. Putting extra weight on it – such as the weight of a new home – has the potential to squeeze out some of the water, so that the clay compresses. While exposure to the surface has dried out and hardened the upper layers of this clay, the un-weathered soil below still compresses easily.

The limited ability of Leda clay to support a load is what causes problems for builders.

Without proper steps being taken, a house constructed on this type of clay might develop foundation cracks, subside, or even collapse. Thorough analysis of the soil underneath the building site helps guide the builders regarding what construction methods are needed.

It is important to learn the maximum weight the soil will support, and to work within those limitations. This weight includes the house itself, as well as the weight of any soil fill that has been added to bring the house up to a level that will allow adequate drainage.

Sometimes, the calculations in planning a new home show that the expected weight of a house, plus the weight of the necessary fill, might be too much for the underlying soil to support.

Due to the compressible nature of Leda clay, this is not uncommon in the St. Lawrence and Ottawa valleys. So, the builder must find a way to reduce the planned weight of the house or the weight of the fill – and this is where those big white blocks come in.

By inserting some light manufactured materials underground, such as polystyrene foam blocks, the house grade can be raised to the required height so the property will drain properly, without exceeding the load-bearing capacity of the soil.

These blocks are used only in areas where it is safe to do so, in that they do not support the house itself. This means that the blocks sometimes get inserted around the outside of the basement walls or below the garage floor. The result may be a slight increase in cost – but it also means that the home can be built safely.

Lightweight construction materials have been used for similar purposes in construction projects such as bridge rams and embankments in the Ottawa area for many decades.

Anyone driving between Kanata and Ottawa over the past several months will probably have noticed massive quantities of white polystyrene blocks being used in construction of the highway 417 widening since this type of construction also suffers from the compressibility of Leda clay.

These types of problems and solutions are representative of the construction challenges constantly faced by the home building industry, but with sufficient professional analysis, reasonable solutions can usually be found. We only wish that this approach could also work to solve the higher-level housing problems being created by the city of Ottawa.

Unfortunately, it doesn’t seem to matter how much study and professional advice is offered to staff and members of city council.

They continue to labour under the illusion that they can control and direct the minutest details of the housing system when in fact, all they are doing is adding needless cost and delay to every new home.

As with most other industries, physical problems can usually be overcome, it’s the problems created by human nature that remain unsolvable.

Sandy Hill Construction

Phone: 613-832-1717

Address: Sandy Hill Construction Ltd.
35 G Robinson Ave.
Ottawa ON, K1N 8N8

Sandy Hill Construction