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Clay Soils - How Can They Affect Foundations?

Updated: Jun 20, 2023

In our last article, we looked at how shrinkable clay soils change their volume depending on their moisture content and how this can affect structures built on them. In this article we want to look at how we avoid these issues in the houses and extensions that we build today.

Foundation concrete

Foundation Design

Put simply, one of the key ways of avoiding ground movement issues for your new project is to ensure the foundations are designed correctly.

For good foundation support you need ground that is capable of bearing the load you intend to place upon it through the foundations. The deeper you dig, the denser the soil tends to get and generally speaking, the more capable of taking a greater load. However, this bearing capacity varies dramatically depending on the location and type of ground. For larger projects borehole samples are taken and tested in a laboratory to establish the exact nature of the ground so a foundation can be designed. But for low level domestic construction this bearing capacity is considered to start at about 1.0m deep, but this would need to be checked during excavation.

This is much more important with modern construction where our structural elements, including foundations, are designed to be quite rigid. Older & more traditionally constructed buildings (properties up to the 1930’s) used more flexible materials, such as lime mortar and can tolerate more movement. Without the need for a rigid foundation, these types of older buildings are generally quite happy sitting on shallow foundations.

The Role of Moisture

In our previous article we discussed that moisture levels in the ground vary mainly due to seasons and the presence of trees. These can combine to dehydrate the soil more dramatically in warm dry summers, shrinking the clay soils.

Fortunately, even in clay soils you find that a moisture equilibrium is found at slightly deeper levels. This is due to several reasons. Firstly, at about 1 metre down the fluctuating effects or rain and sun are felt less due to the mass of soil in between. Also, the deeper you go, the closer you are to the water table – the fluctuating level of water saturation in the ground. This keeps the moisture level in the clay soil high and less prone to dehydration and shrinkage.

Tree Roots

Lastly, trees and their root systems can have a profound affect on moisture levels within the ground, some trees drawing hundreds of litre of water per day. The worst of these effects are felt closer to the tree, especially within its canopy where the roots are larger and deeper. Outside of the canopy the root systems tend to be higher in the ground, within 600-700mm of the surface. Therefore, placing our foundations outside of the influence of tree root systems will help to ensure a good moisture equilibrium is maintained.

Typically, in clay soils the minimum foundation depth for a mass fill or strip foundation will start at 1 metre for the above mentioned reasons. However, Building Control Officers will look into the trench to see the type of ground and whether any tree roots are present. If they are, you will probably be made to dig down deeper.

Clearly, there are many variables that must be considered when a structural engineer determines a foundation depth. These would include the type of soil, type & size of tree, proximity of the tree and climate. The NHBC (National House Building Council) produced a set of tables many years ago to help determine minimum depths for foundations near trees. These tables can be freely accesses online ( and via a free smartphone app.


If a standard trench foundation is deemed to be unsuitable due to the presence of clay soils or trees, alternative foundation types are quite commonly available now. Micro piling systems that are suitable for the domestic market can often provide a good solution and overcome many of the issues. As always, our advice would be to involve a qualified structural engineer to guide you through the design process, so you can consider what options would work best for your project.

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