What are Expansive Clay Soils?

by Andrew Lees, on september 30, 2021

Expansive clay soils, sometimes referred to as ‘shrink-swell’ clays, cause billions of dollars of damage every year. In the UK, insurance claims for shrink-swell damage have been as high £0.5billion in a single year, while in the US problems related to expansive clay cost an estimated $15 billion annually, more than all other geotechnical hazards (such as earthquakes, landslides, etc.) combined. 

Expansive clays and the problems they cause are particularly expensive because this type of soil covers large areas of land in countries across the globe.

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What are Expansive Clay Soils?

Expansive clay soil is liable to shrink and swell when its water content changes. When wet, it becomes soft and expands; in dry conditions, it becomes hard and shrinks, resulting in cracks. This shrink-swell behaviour is due to the soil’s mineral content and particle shape.

Expansive clay soil characteristics

Expansive clay soils have very small, plate-like particles which can absorb large quantities of water, expanding to become plastic and sticky. When they are dry, however, these same soils become hard; they shrink, and cracks appear in the ground (shrink-swell behaviour). Clay soils containing certain mineral types, such as montmorillonite, smectite or vermiculite, are particularly prone to shrink-swell behaviour and volume changes of up to 10% are not uncommon.

What influences this shrink-swell behaviour?

The degree of ground shrinkage will depend upon the shrink-swell characteristics of the clay and the water content near the surface. In the ground, moisture content will vary based upon seasonal rainfall and vegetation growth to a depth of possibly 3m and the depth or variability can be increased by the presence of tree roots, or by localised changes to surface drainage or paved areas. 

Changes in rainfall intensity, or extended periods of drought, will also have a major impact on ground shrinkage. Clearly changes in climate will have consequences for structures built over areas of expansive clay soils. Designing for climate change resilience will need to account for this in the future by building to mitigate against shrink-swell effects.

In this episode of "Ask Andrew", Andrew Lees tells us more about expansive clay soils

The problems caused by expansive clay soils

There are several key areas where expansive clay soils will cause significant issues. These include buildings, road pavements and large paved areas such as school playgrounds and car parks.

The damage to buildings

Building damage due to heave caused by expanding foundation soils can result in cracking of walls or floor slabs, but the most dominant form of building damage is from subsidence caused by shrinkage of clays soils below the foundations. Extended periods of dry weather often result in major structural damage, fractured drains and services.

The damage to road pavements and similar structures

In many regions across the globe, expansive clays give rise to major pavement problems requiring regular and expensive maintenance. The problems are caused by variability in moisture content across the road pavement: the pavement surface protects the subgrade from the ingress of moisture, but rainfall on the shoulders does seep down, and penetrates below the edges of the pavement. The subgrade soil below the central section of the road is then drier than the subgrade below the edges and the result is non-uniform shrink-swell behaviour across the road width. The variable volume change across the pavement results in the development of longitudinal cracks, usually located close to the road edge. These cracks can be wide: up to 50mm is not uncommon. For many low-volume roads in expansive clay soil regions, this is the primary cause of pavement failure.

How do you check if clay soil is expansive?

There are a wide range of different tests you can use to identify expansive clay soil, the most common of which is to use a potential volume chance (PVC) meter to carry out a swell test. Other possible methods include:

  • Coefficient of linear extensibility (COLE)

  • Standard absorption moisture content (SAMC)

  • Chemical methods such as Cation Exchange Capacity (CEC)

  • Mineralogical methods like X-ray diffraction.

Building on expansive clay soil

It certainly isn’t ideal to build on expansive clay soil due to its potential for shrink-swell behaviour and cracking. However, there are circumstances in which doing so is unavoidable because of the prevalence of this type of soil (for example, it is present in abundance throughout large parts of South Dakota).

To prevent buildings from cracking and subsiding, it is vital to use appropriate building methods for construction projects on expansive clay soil. These methods largely focus on mitigating the damage of shrink-swell through specialised foundations.

The best foundation for expansive clay soil

Pier and beam foundations are typically used for buildings that sit on expansive clay soils – with this type of foundation, strong wooden supports called piers are driven into the ground, and then beams sit across these piers to support the structure above the soil.

Pavement construction on expansive clay soil

As we discussed above, the shrink-swell behaviour of expansive clay soil can also spell trouble for pavements, resulting in extensive cracking. To help prevent this, engineers will usually rely on a form of geotechnical stabilisation such as geogrids.

Geogrid stabilisation - a proven solution

Incorporating a stabilisation geogrid - such as Tensar TriAx - within the unbound layers of a pavement, has been proven to be an effective way to control surface cracking, due to shrink-swell behaviour. The aggregate particles interlock with the geogrid and the resulting confinement controls particle movements, distributing strain to prevent surface cracking. Research and trials in Texas have led to geogrid stabilisation becoming the primary solution adopted by the Texas DoT, and further successful examples from the US, Australia, France, and elsewhere have proven the value of this approach.

Next steps

This guide has explained the fundamentals of expansive clay soil, including its characteristics, the mechanics of shrink-swell behaviour, and the damage this causes. We’ve also touched on how to identify expansive clay soil and account for it in construction designs.

Visit our page on geogrids for road and pavement stabilisation to learn more about how Tensar products can prevent damage to roads built on expansive clay soil, and discover how Tensar+ can aid your road and pavement construction projects.

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