Map of the Champlain Sea according to S. Occhietti, Quaternary geology of the sub-region of the St. Lawrence Valley and the Appalachians; in The Quaternary of Canada and Greenland, Geological Survey of Canada, 1989.
It is common practice to build a house on clay soil. However, since clay soils can be highly compressible, it is important to take the necessary precautions to limit the risk of foundation subsidence.
If you must build in an area where the soil is clay, take every precaution to reduce the risk of foundation subsidence.
Clay soils, by their properties, can have a low-bearing capacity. Correctly assessing the pressures they can withstand to ensure the efficient transfer of loads from a building to said soils and prevent possible settlement effects.
It is normal for the foundation of a house to stabilize over time. Even new homes experience movement that can cause small cracks to form in the concrete. But as homes age, this phenomenon can become critical if it turns into sagging—a much bigger problem.
There are several easily detectable clues that can indicate the presence of a foundation subsidence problem:
- The presence of cracks in the foundation or on the exterior cladding;
- Doors and windows that are difficult to close properly;
- The appearance of unevenness in the floor;
- Deformations in the structure of the house;
- The appearance of cracks on the interior walls.
While not an absolute guarantee that your home is sagging, the appearance of these signs is nevertheless a serious warning. From the moment you notice them, you will need to keep an eye out and be vigilant. If you are concerned, you can also ask a specialist to do a foundation inspection.
Foundations and clay soils.
Since 2006, there have been few summer drought problems in Quebec. But from 1983 to 1995, as well as in 1998, 2001 and 2005, Quebec experienced several dry summers which caused the drying out and subsidence of many clay soils in the greater Montréal area and the St-Lawrence Valley. The drying of the clay below the level of the foundations can cause foundation subsidence ranging from 0.5 to 30 cm in some places causing worrying damage to structures and exterior siding.
It is advisable to provide land improvements that do not influence the water content of the soil. The planting of certain tree species influences the water content of the soil and can cause the clay to rise or contract. The situation could worsen if there were a longer-than-normal drought.
It is sometimes recommended to create a sidewalk or install a 1.5 meters wide anti-evaporation geomembrane around the periphery of the house.
Go for Simple Measures
It is better to prevent the risk of foundation subsidence if you have to build in a place where the soil is clay, so as not to face stabilization problems. Among these measures:
- a little deeper and thicker foundations;
- the use of reinforcement;
- control of surface water a little further from the foundation;
- the installation of a peripheral geomembrane to limit evaporation;
- respecting a minimum distance between the house and trees.
Since 1994, a new technique for stabilizing foundations on clay soils by deep irrigation has been tested in Quebec, but the results are sometimes very problematic, especially in the presence of trees. Spot watering and the steel pile stabilization method remains the best choice.
In the case of foundation subsidence, a solution that is often recommended is the installation of piles at the base of the foundations in order to level them and stabilize them. This intervention is quite complex, as it requires excavating the entire perimeter of the building.
For this reason, it is essential to have an expert in crack repairing perform the operation.
In other cases, the expert may carry out short-term reinforcement work, involving waterproofing the soil with a membrane and adding support posts at strategic locations.
In a real estate transaction, the building inspector must observe all signs of trouble cracks, including cracks and sloping floors. If they do not know the soil of the place, they will have to consult a “geological map of unconsolidated deposits.” For their part, the buyer and his broker would do well to question the seller and his neighbors about possible soil problems. Beware of buildings that have only been staked on one or two sides. Partial piling cannot completely stop foundation movements. Staking a building can cost anywhere from $30,000 to $50,000.
