Building on stiff clay requires special attention to the foundation due to the soil's tendency to shift with changes in moisture content. The primary method involves using substantial concrete strip foundations that extend deep into the ground.
Understanding the Challenge of Stiff Clay Soil
Clay soil is known for its reactive nature. As it dries, it shrinks, and as it absorbs moisture, it swells. This constant movement – known as heaving, subsidence, or cracking – can exert significant pressure on structures and cause severe problems for foundations.
Because clay soil has the greatest potential to cause these issues, a robust foundation is essential to provide stability and prevent structural damage over time.
The Recommended Foundation Solution
Based on the characteristics of clay sites, the standard approach for house construction is to use deep concrete strip foundations. These foundations are designed to be:
- Substantial: Built using a significant amount of concrete to provide weight and rigidity.
- Deep: Extending well below the surface layer that is most affected by moisture changes.
According to building practices for clay sites:
- Houses on clay sites are built on substantial concrete strips.
- These strips are at least 750mm deep.
- More usually, they are 100cm deep (which is equivalent to 1 metre).
This significant depth helps the foundation reach a more stable soil layer that is less prone to the moisture-induced movement characteristic of the surface clay.
Why Deep Foundations?
The extra depth ensures the foundation remains stable even when the upper layers of clay soil expand or contract. This helps protect the building from the potentially damaging effects of soil movement, such as:
- Walls cracking
- Floors becoming uneven
- Doors and windows sticking
By placing the foundation deeper, the structure is supported by soil that is less susceptible to seasonal moisture variations.
In summary: Building on stiff clay primarily involves constructing deep concrete strip foundations, typically extending to a depth of at least 750mm, and often up to 1 metre, to counteract the soil's reactive behaviour.
