Clay soils are marine deposits that followed the ice age, where the Champlain Sea. In the Greater Montreal area, there are several sectors with clay soil. There is an important relationship between the clay soil and the movement of the foundations.
Foundations, including walls and footing, distribute building loads on the floor. For this reason, the movements of the foundations (walls and footings) affect the structure of the building as well as the elements of the envelope and the interior finish. In case of movement of foundations, it is important to check and solve the problem of stability of foundations, before proceeding to repair the structure and components affected by this movement. Indeed, if the movement continues, the damage in the interior elements and in the envelope will worsen.
Here is a non-exhaustive list of probable causes of movement of foundations:
- Inadequate foundation drainage - Unsuitable drainage of building foundations, such as the absence of a French drain, a non-functional drain, or exterior landscaping that requires drainage of foundations (negative slope, gutter downspouts that drain foundations, lack of gutters, etc.).
- Weakness of foundation walls - Foundation walls that do not have the capacity to withstand the lateral force of the soil and / or frost. In general, concrete block foundations are less resistant to lateral forces than concrete foundation walls.
- Inadequate foundation soles - Insoles too small that do not have the rigidity to support the loads.
- Insufficient Frost Depth - Foundations are not deep enough and not insulated to resist freezing and thawing.
- Concrete Weakness and Inappropriate Concrete Resistance - Foundation concrete is weak and lacks adequate strength to withstand imposed loads. In buildings of the 1930 years and earlier, there is often concrete with a capacity less than 15 MPA.
- Low soil bearing capacity - Very low soil with inadequate bearing capacity and undisturbed soil.
- Clay soil dewatering - The presence of clay soil beneath the building that inflates and contracts greatly depending on the amount of water in the soil.
Clay soils include clays and silts. "The word clay usually means soils containing a quantity of clay particles sufficient to confer cohesion and plasticity" (Soil mechanics, Vincent Robitaille and Denis Tremblay).
The clay particles are very fine and "when the soil is dry, the cohesion between the clay minerals is very strong: the clay then has a high shear strength and can withstand heavy loads without deformation. If however the water content is increased, the cohesion decreases, as well as the shear strength: the clay then becomes plastic and malleable. If the water content continues to increase, the clay becomes very soft and, at the limit, it can flow like a thick liquid; in such a case, the cohesion between the clay minerals as well as the shear strength are practically zero "(Mechanical Soil Book).
So, we can say that the main problem of clay soils is the variation of soil volume during changes in the amount of water in the soil.
Signs of subsidence of a building
When a building is affected by a significant subsidence problem, the following deficiencies generally occur:
- Several cracks on the walls of foundations;
- Shearing on the exterior cladding;
- Structural cracks in the walls inside the building;
- Deformations in structural elements (beams, joists, roof trusses).
With respect to remedial work, the following remedial work must be completed, but not limited to:
- Drive piles to stabilize the foundations.
- Repair cracks in foundation walls using tie plates to secure both sides of the foundation wall together. Use flexible materials such as polyurethane. to seal these cracks from the outside to the sole.
- Properly repair shear in the brick siding, to prevent water ingress into the envelope and to ensure the integrity of this envelope.
- Replace broken or damaged stone lintels over openings (doors and windows).
- Check and replace any structural or other elements (beams, door columns, windows, seals, etc.) that are deformed or damaged due to subsidence of the building.