Best Soil for Foundations – How to Know if Yours is Suitable

Clay, sandy, silty, till, and organic soils—foundation excavation conditions are particularly diverse in Quebec. However, clay soils cover most of the inhabited territory of the province. Therefore, there’s a high chance you’ll be digging your house’s foundations in clay. Here’s everything you need to know to build a solid foundation wall in this capricious soil.

How to recognize good soil for a foundation's excavation?

Source : RenoQuotes

One might say that, for excavation, good soil is stable soil. In reality, while some soil types are more stable than others, all present the same challenges: identifying the forces the soil imposes on a structure and implementing a method to contain them.

Thus, before undertaking an excavation, it’s advisable to consult an expert who will define these constraints. This analysis helps determine if the 10 conditions of good soil are met:

1. Good plasticity (the soil sticks and can be molded)

2. Particles smaller than 0.002 mm, with content more affected by ionic bonds than gravity

3. Ability to support vertical trench walls

4. Resistance evolving with vibrations and heavy equipment

5. Consistency varying between soft and hard soil

6. Does not easily allow water passage

7. Can be drained from within the excavation

8. Is cohesive, meaning it has a low erosion rate

9. Can withstand uplift from the excavation bottom

10. Cracks due to shear and settlement

Of course, the ideal soil rarely exists, especially in Quebec. As mentioned earlier, the inhabited areas of Quebec are clayey. In practice, this poses serious problems.

What foundation to create for clay soil?

Source : DEMCO Canada

Look around you. You walk down the street and see that one house has a cracked facade, while the one next to it is intact. Yet, both are near large trees, built on the same clay soil, and subjected to the same traffic density.

The difference often comes down to one thing: the foundation wall. One has good foundations, the other does not. So, how deep should you go, and what should these foundations look like?

Basics for Building Solid Foundations

For clay soil, your foundation walls should:

• Extend to a depth of at least 3 ft 11 in (1.2 m)

• Rest on footings with a resistance of at least 10 MPa (megapascals)

• Not backfill the excavations

• Be insulated

• Be dug away from a slope

• Be surrounded by an anti-evaporation geomembrane around the house

• Be at a distance from trees

We’ll revisit the issue of trees in the section on houses that crack, but note that the anti-evaporation geomembrane is precisely there to prevent trees from cracking the house. To do this, you should install it at a distance of 3 ft 28 in (1.5 m) from the foundation walls.

Regarding the slope, we’ll discuss this later in the article, but know that erosion, digging a pool, or backfilling can cause slope slippage and thus change the forces the foundations must combat.

If you don’t follow these tips, the weight of your house and the natural movements of clay shrinkage and swelling will deform the soil and crack your house. It’s now time to understand why.

At what depth should you dig to avoid freezing during a foundation?

It’s essential to distinguish between two situations:

• The basement or crawl space is heated

• The foundations do not adjoin any heated room

In the first case, and in clay soil, a depth of at least 3 ft 11 in (1.2 m) must be respected. With everything we’ve said about clay so far, you’ve surely understood the reason: it easily becomes waterlogged.

In the second case, if you live south of Lac-Saint-Jean, you will need to dig up to 5 ft (1.5 m) deep.

In any case, since clay is particularly sensitive to freezing, you must add rigid insulation and place it outside your foundation walls. This way, they won’t freeze.

Why is clay soil fragile?

Source : Gestion Léonard et Fils Inc.

What is clay soil?

Clay soil is sedimentary in origin. It is mainly found around lakes and rivers. However, there isn’t just one type of clay soil. Not only do the properties of clay evolve and are thus directly related to its geological history, but its composition also differs from one soil to another.

Indeed, clay can be composed, in varying proportions, of minerals such as:

• Kaolinite

• Illite

• Halloysite

• Chlorite

• Montmorillonite

To these elements are added cementing agents, like calcite, present in varying amounts. The more cementing agents there are, the more solid the clay is, and vice versa. Clays dating from the post-glacial period, like those in Quebec, are particularly young and thus poorly consolidated.

Problems Related to Clay Soil

Clay presents two problems, which become apparent:

During the excavation, we already observed the instability of the clay. It’s important to know that many landslides have occurred in the past. The cause? The leaching of salt left in the sediments of the Champlain Sea, and the incision created by the St. Lawrence River and its tributaries. This intrinsic fragility of clay also becomes evident when digging a home’s foundation wall. In fact, clay soil is made up of layers of silt and sand that can be affected by pore water pressure. When that pressure is released during excavation, it can cause instability in the loose soil layer.

During the Life of the Construction

A second problem arises once the house is built. It’s important to know that clay soil can be, geologically, more or less young. The younger the soil, the more precarious its balance. What does this mean? Clay will struggle to support its own weight. The weight of infrastructures built on this type of soil will thus cause deformations. The problems don’t stop there, as clay is particularly sensitive to water variations. When it’s hot and the clay dries out, it shrinks. This is called shrinkage. Conversely, when it rains and the ground becomes waterlogged again, it swells. This is called swelling.

These are the reasons why a house’s foundations must be able to handle all these constraints.

Why is my house cracking despite having good foundations?

Source : RenoQuotes

Most likely because it was built on clay soil. If you’re confident your foundations are solid, the issue could be related to the presence of large trees near your home.

Large trees can cause two main problems. The first appears during periods of drought. Thirsty trees draw moisture from the ground, drying out the soil and causing nearby buildings to crack due to the clay shrinkage effect. This is why a geomembrane, which we’ve already discussed, is so important.

The second issue arises when the roots of large trees extend beneath the house. In this case, when strong winds shake the tree’s canopy and risk toppling it, the roots can lift the footing, damaging the structure above.

That said, large trees aren’t the only cause of cracks in your home. In cities like Montreal, where clay soils are common, other factors include:

• The overuse of asphalt and concrete, which makes the soil impermeable

• Drainage systems that prevent water from surrounding the foundation footings

• Stormwater being directed straight into sewers rather than into the ground

If you're experiencing this kind of issue, a geotechnical engineer can recommend how to limit the damage. In some cases, simply watering your yard yourself can be enough to correct the problem.

Frequently Asked Questions (FAQ)

What are the best soils for foundations?

The best soil for a foundation is a stable, cohesive soil with low water permeability and high load-bearing capacity. In Quebec, soils are often clay-based, which can be tricky but manageable with proper foundation design. Ideal soils include well-compacted clay, till, or dense sand that resists shifting and erosion.

What makes a good soil for foundation work?

A good soil for foundation work should:

• Stick together and be moldable (plasticity)

• Support vertical trench walls

• Resist water penetration

• Remain stable under heavy machinery and vibrations

• Be slow to erode and crack

• Withstand freezing and thawing without significant movement

These properties help ensure that your foundation won’t shift, sink, or crack over time.

How do you build on sandy soil?

Building on sandy soil requires special considerations:

• Compact the soil thoroughly before construction

• Use wider or deeper footings to distribute weight

• Add geotextiles or gravel to improve stability

• Ensure proper drainage to prevent washouts

• Consult a geotechnical expert for site-specific recommendations

While sandy soils drain well, they lack cohesion and can shift, so proper design is critical.

What is soil and foundation engineering?

Soil and foundation engineering is a branch of geotechnical engineering that studies soil behavior and how it interacts with building foundations. It includes:

• Soil testing and classification

• Load-bearing analysis

• Foundation design (e.g., shallow, deep, piles)

• Managing risks like soil movement, erosion, or groundwater impact

It ensures that structures are safe, stable, and durable.

What is the impact factor of Soils and Foundations?

Soils and Foundations is a peer-reviewed journal in geotechnical engineering. As of the latest data, its impact factor varies by year but typically ranges between 2.5 and 4.0. For the exact current impact factor, consult the publisher or indexing services like Clarivate or Scopus.

Why is soil important in foundation construction?

Soil is the very base upon which structures rest. Its properties affect:

• How deep and wide a foundation must be

• The type of foundation used (slab, crawlspace, piles, etc.)

• Risks of settling, heaving, or shifting over time

• How the structure withstands moisture, freeze-thaw cycles, and vibrations

Understanding the soil type ensures safe, cost-effective, and long-lasting construction.