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Expansive Soils

June 27, 2018


Expansive soils can often cause great loss to residential and commercial property. Numerous Canadian cities and towns are located in geographic areas with soils that are classified as clays having medium to high plasticity. This means that these soils are capable of absorbing water and will expand and contract as the moisture content changes. Activation of highly plastic clay (also called heaving or swelling) can cause damage to a building structure, a concrete slab on grade and/or surrounding landscaping.

To prevent damage from expansive soils, it is imperative that the moisture content within these soils stays consistent. If moisture fluctuations do not occur, the soil remains in a consistent state and can provide adequate bearing capacity. However, as natural moisture changes do occur and prevention of all mechanical failures that would allow moisture to enter a soil is impractical, damage as a result of soil expansion and contraction is impossible to prevent in all cases.

If a building is constructed on a soil with a high swelling potential, movement of the slab on grade can occur when moisture is added to the soil due to mechanical leaks, such as a water main break or plumbing failure, or natural events including heavy precipitation or rapid snow melt. The influx of water in these instances can change the moisture content in the soil and allow swelling of the material beneath the building and its foundation.

The forces resulting from the addition of water into plastic clays will typically affect landscaping or concrete floor slabs to a much greater degree than the foundation elements themselves. For example, a heaved and cracked basement floor slab or displaced patio pavers are much more common than damage to a footing, foundation wall or piled foundation. Foundation elements are more robust than a concrete slab on grade, and typically have a greater load applied to them, which increases their resistance to heaving forces. Expansive clays can result in cracking above the basement level if partition walls attached to a basement floor slab exert force on a floor structure. This can lead to movement and cracking of the finishes and even a wood framed floor structure.

As moisture migrates through a soil of high heaving potential, the moisture content is reduced and the soil shrinks. It is important to understand that this soil may not return to its exact original state. As well, a concrete slab or footing above the expansive soil will likely not settle at the same rate as the soil, and will likely not reach its original position. Due to different rates of settlement of the slab, and the soil beneath, voids are often left in the soil directly underlying the concrete. If not repaired properly, these voids can result in movement of the concrete element over time. As such, it is important to address the issues after the moisture has completely dissipated and the soil has returned to its optimal moisture content. If repairs (such as mudjacking) are undertaken before the moisture content in the soil returns to its original level, additional movement of the soil can render the repairs ineffective.    

Prior to construction of any building, a geotechnical report should be conducted on the property to identify the subgrade material and provide foundation construction recommendations. However, in residential construction, the advice of a geotechnical engineer is rarely sought and instead, typical construction practices are utilized. The result is the construction of residential structures that are susceptible to heaving and settlement during periods of groundwater fluctuation. Construction methods can reduce the impact of swelling soils. Methods to be considered include:

  • stripping expansive clay soils and installing compacted engineered fill materials;
  • adding swelling-control admixtures to expansive soil;
  • using only non-plastic fill materials (not original clay material);
  • installing deeper foundations and/or installing a structural floor slab instead of a grade supported slab.

However, many of these methods are impractical or cost prohibitive for a residential or lighter commercial structure.

If a building has been constructed on expansive soils, there are some things an owner can do to prevent or reduce the potential for swelling, and subsequent structural issues:

  • Monitor sump pump for adequate function;
  • Ensure adequate grading away from foundation;
  • Ensure downspouts direct water away from foundation;
  • Avoid having trees/vegetation close to foundation;
  • Minimize or monitor built in sprinklers adjacent to foundations and;
  • Maintain plumbing and drainage lines to reduce the possibility of leaks.

In periods of dry weather, the addition of water (using a garden hose for example) to a foundation constructed on soils with high swelling potential is not recommended, as the water will not distribute evenly beneath the foundation. Instead, some pockets of soil will be wetter than others. The result will be an uneven expansion in some areas while in other areas, the soil will remain dry.

If cracks do appear on concrete slabs or in concrete basement floors, experts should be consulted to assess and determine the cause of the cracks and to consult on what should be done to mitigate further damages.

Written by Carla Ladner


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