Across much of the St. Lawrence Lowland, including the Montréal region, the soil that matters most for a foundation is the clay left behind by the Champlain Sea. This clay can settle under a load, shrink when a dry summer dries it out and, in its “sensitive” form, turn fluid when it is disturbed. Other soils behave differently: only a soil study or an assessment of your lot will tell you what is under your feet.

The facts below come from The Canadian Encyclopedia (article “Champlain Sea”), the fact sheet Racines d’arbres et fondations d’immeubles from Espace pour la vie (in French), a Polytechnique Montréal thesis on Champlain Sea clay (in French), the engineers at Onesta Bâtiment Conseil (in French), the APCHQ Performance Guide (edition consulted: 2006, in French) and CAA-Québec.

What Is Champlain Sea Clay?

The Champlain Sea was a 55,000 km² body of saline to brackish water that occupied the St. Lawrence Lowland between Quebec City and Brockville, Ontario, and extended up the Ottawa River Valley, 12,000 to 10,000 years ago. The most abundant sediments it left are marine clays, mainly rock “flour” from glacial abrasion.

These clays are classed as “sensitive soils”: the sea salt they contained has dissolved and left voids, so the sediment can hold more water than its usual liquid limit. When disturbed after heavy snowmelt or rainfall, including by earthquakes, heavy traffic or construction, the clay becomes fluid and landslides result. A Polytechnique Montréal thesis adds that these clays, deposited 8,500 to 12,000 years ago, produce failures and settlement when they are under load.

How Can Clay Move a Foundation?

By drying out in summer. According to Espace pour la vie, living next to trees is normally not a problem for residential footings. When a summer drought hits, the clay and the roots are no longer hydrated and settlement can follow. Many other things dry out clay too: paved surfaces, drainage pipes, shrubs and small plants, excavation trenches and poorly insulated basements. The main cause remains the lack of rain during summer. Our article Can Tree Roots Damage a Foundation? looks at the role of trees.

By lateral thrust. The engineers at Onesta Bâtiment Conseil explain that horizontal cracks come from the lateral thrust of the soil, which increases considerably in frost-susceptible soil such as clay, especially without proper drainage behind the walls. See our article on hydrostatic pressure.

By settling under load or during work. This is the behaviour described above for sensitive clays: disturbed or loaded soil can give way. It matters most before an addition, a pool or an excavation near the house.

What Is Required When Building on Clay?

Espace pour la vie notes that clay soils call for stricter standards: footings set deeper, widened and reinforced to improve their monolithic behaviour. Those requirements may not have existed when your house was built, or may not have been followed by the contractor. The fact sheet also points to the brochures Votre maison est fissurée 1 and 2 from the Association des Consommateurs pour la Qualité dans la Construction (in French).

Which Signs Point to a Soil-Related Problem?

An open crack that widens at one end, a stair-step crack in a block wall, a horizontal crack with a bowing wall, sticking doors. Each type is detailed in How Do You Recognize Each Type of Foundation Crack in Quebec?, and the effects on the rest of the house in beyond the basement. To judge whether a crack is worrying, read When Is a Foundation Crack Serious in Quebec?

How Do You Protect a House Built on Clay?

Keep surface water away. The APCHQ sets a minimum 2% slope around the building and notes that disturbed soil naturally compacts. CAA-Québec recommends bringing downspout water 2 m (about 6.5 ft) away from the house. See our articles on gutter systems and the 4 kinds of water damage that threaten a foundation.
Keep a working drain. See interior or exterior French drain.
Choose trees and their location carefully, keeping at least one times their mature height between the tree and the house, according to Espace pour la vie.
Have the soil assessed before an addition or a new build, by an engineer, rather than assuming how the lot behaves.

When Should You Call a Specialist?

As soon as a crack widens, a wall bows or water comes back after rain. Marco Réparation de Fondation visits in person for every estimate and every job site inspection. The company holds a valid license from the Régie du bâtiment du Québec (RBQ No. 5775-3246-01) and serves Montreal, Laval, the South Shore (including Longueuil and Brossard), the West Island and Salaberry-de-Valleyfield. See our crack repair page or what to expect during an inspection.

Suspect a soil-related problem?Contact Marco Réparation de Fondation for a free inspection and an honest opinion on the cause.

Frequently Asked Questions

How can I tell what soil is under my house?

Only a soil study carried out on your lot, by an engineer, tells you for certain. The St. Lawrence Lowland is marked by Champlain Sea clay, but the soil can vary from one lot to the next.

Does clay crack every foundation?

No. According to Espace pour la vie, living with clay is normally not a problem for residential footings. Trouble comes mostly in dry summers, and depends on how the footings were built.