Why Is Clay Bad For Foundations?


The nature of the soil is a very important factor. Depending on the composition of the subsoil, the structure may settle to varying degrees.

Consequently, the foundation must be carried out depending on the type of soil material. One of the most common soils is clay soil. In this guide, I have summarized how the foundation behaves in clay soil.

To cut to the chase: this guide provides only a rough guideline. In any case, a hydrogeological soil study is required for the foundation of a building.

For building projects requiring approval, a soil report has been a legal requirement for several years.

But even in the case of smaller foundations in clay soil, an expert should be consulted and possibly a soil survey should be performed.

First of all, the following soils are distinguished:

  • cohesive soil
  • non-cohesive soil

Clay would be cohesive soil, for example, while non-cohesive soil would be crushed stone or gravel. Foundations in non-cohesive soil undergo rapid but not too deep settlement.

In contrast, settlement may last for several decades in cohesive soil. This is the case with clay soil.

That is, in a non-technical explanation, clay soil tends to deform easily when it receives external pressures and forces, and that is why the settlement of a foundation of a structure could take up to years to be achieved in clay soil.

Why clay soil is problematic?

The problem with clay soil is, on the one hand, the high density of the grains, which are separated from each other by a lot of water. Even with high loading (or compression), it takes years to squeeze out the water.

In addition, corresponding soil distortions occur, e.g. due to spring thaw or very long-lasting rainfall. Thus, infiltration water can only be drained to a limited extent or not at all.

Of course, the problem or challenge that a foundation in clay soil can pose also depends on the size of the construction project.

There is a significant difference between whether you just want to build the foundation for a small, lightweight garden shed yourself, or whether you want to erect a large, multi-story building.

Adequate gravel backfill can be made for founding the garden shed on clay soil, for much larger structures, other measures might be necessary.

But this gravel backfill must also be adapted to the individual properties of the clay soil. Because the gravel fill does not only guarantee stability.

It also offers infiltration water, such as meltwater or rainwater, the opportunity to act as a kind of temporary storage.

In this case, however, the decisive factor is whether enough can be absorbed with average amounts of infiltration water. Therefore, the volume of the gravel backfill must be adapted to the local conditions.

Clay soils warrant a deep foundation or gravel embankment. The deep foundation under the foundation proper is placed at such a depth that it “sits” on solid soil. If this is not possible, gravel backfilling is performed as an alternative.

Essential Tips for Building in Clay Soil

Not all clays are expansive, It depends on the type of mineral that makes up the clay such as illite, montmorillonite, etc.

Look for local geographical reports or geotechnical reports on the clay in your area. In addition, a plasticity test should be considered, this is moisture dependent and indicates the point at which soil behaves plastically.

To actually determine the amount of clay in the soil, you have to perform a hydrometric test that determines the percentage of clay in the samples.

If you have expansive soils in your area:

Control the moisture:

Trees near your house can also affect the foundation.

In the case of foundations:

Essentially more reinforcement is required for structural elements and, in some cases, pile foundations may be a more practical option than footings. Soil expansivity tests are available to determine the amount of pressure that soil can exert on a surface.

In the case of slabs-on-grade or grade beams, a 4″ thick piece of Styrofoam can usually be added so that the Styrofoam is displaced in place of the concrete.

What is the minimum depth for a foundation in clay soil?

In my experience, for average-sized one or two-story house foundations, the minimum recommended depth for clay soils has been 80 centimeters (31.5 inches), this is in the case of strip footing foundations.

In the case of multilevel buildings, it is recommended to dig until firmer soil is found, otherwise, proceed with the advice to backfill with gravel and stones.

The strip footing is not my preferred type of foundation when working with clay soils, although they can work, the best type of foundation for this soil is the slab foundation and I will explain this shortly.

Drainage problems in clay soils

Construction on clay soil also suffers from the problem of poor drainage because clay soil has very poor infiltration properties.

In this case, it is necessary to order the installation of a drainage system. Decisive for this is the soil infiltration rate indicated in the soil report and the general condition of the soil. For this purpose, the groundwater situation must continually be assessed.

Drainage is basically used to evacuate rainwater and not groundwater. If the groundwater level is above the base of the foundation (the highest measured groundwater level is always used for this purpose), drainage should never be installed.

Slab foundations are my favorite for clay soils.

I have had to deal with constructions in very clayey soils, for me, one of the safest foundations for very clayey soils is plate foundations.

To give you a non-technical example, imagine that a slab foundation in clay soil is like putting a leaf of a tree in water, the leaf will not sink because all its weight will be evenly distributed on the surface of the water.

It is the same in clay soil, the point loads in foundations could generate differential settlements, that is to say, that the structure sinks more on one side than on the other.

While in a slab foundation, all the load is distributed uniformly on the surface that covers the house, it is a much safer foundation system for very heavy clay soil.

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