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February 28

Are Your House Foundations On Expansive Soils?

New Zealand has an interesting and mixed geological history, which has resulted in a wide variety of soil types throughout the country, with different engineering characteristics.  Some of our soils contain “active” clay minerals, which means that they are capable of swelling and shrinking, by taking in water or losing water.  In New Zealand these types of soils are defined as “expansive” soils.

Expansive soils are not considered to be “good ground, as defined by NZS 3604:2011, New Zealand, Timber Framed Buildings (which is the New Zealand Standard Code of Practice for the design of simple residential structures).  Therefore, foundations founded on expansive soils require specific foundation design.

Shallow foundations, supported on expansive soils, which are not appropriately designed, can be adversely affected by the seasonal swelling and shrinkage of the supporting soils.  This can result in foundation movement, which can distort the superstructure. If this movement occurs it typically manifests as cracking damage to foundations, rigid cladding systems and to the internal linings (ceilings and walls).  Due to the nature of the damage mechanism, i.e. wetting and drying of the supporting soils, this damage tends to occur seasonally.  Typically, during very dry summers, the soils dry out and the supporting soils lose their water and shrink, resulting in foundation settlement.  Conversely, in wet winters, the soils gain water and swell, which can result in foundation heave.

A catalyst for more severe differential foundation movement, due to expansive soils, is normally the proximity of large trees to some parts of the house footprint.  Large trees have root systems that can extend beneath foundation footprints.  The tree roots have the effect of removing water from the soils in localised areas, which can result in localised soils settlement and subsequently more pronounced differential foundation movement.

It is important to understand, however, that not all soils are expansive and therefore the expansive soil problem is not an issue for some parts of the country.  Speaking from my experience, there are areas in the Auckland region which have highly expansive soils. The soils overlying the Canterbury area, however, typically do not contain the “active” clay minerals, and therefore these soils are generally not considered to be expansive. 

It should also be noted, that minor seasonal movement of foundations is normally tolerated.  Foundation/cladding cracking that may occur in dry seasons, typically closes up again in the wet season.  I can remember a particularly dry summer in Auckland, some 10 years ago, when a lot of house cracking was occurring (due to soil shrinkage). We were run off our feet with calls from concerned home-owners wanting us to investigate the cause of the damage, and to provide remedial measures.  In a lot of cases the first option we provided was to wait until the rain comes, and the cracks will likely disappear (which proved to be the case).  That being said new foundations should be designed to mitigate the adverse effects of all potential geotechnical hazards (including soil swell/shrink), which is why, if you are building  a new structure, that you should engage a suitably qualified and experienced geotechnical engineer to undertake the necessary investigation and appraisal works, to determine the nature of the supporting soils, as it pertains to soil expansivity.  The geotechnical engineer can provide foundation design recommendations, which mitigate the risk of the foundations being adversely affected by soil swell/shrink (if required).