NZS 3604-type foundations

The majority of residential building foundations in New Zealand are constructed to comply with Building Code Acceptable Solution B1/AS1, which cites NZS 3604:2011 Timber-framed buildings.

The standard provides prescriptive details for constructing seismically resilient slab-on-ground foundations and pile foundations based on:

  • seismicity of the region
  • characteristics of the site
  • strength of the underlying soil
  • materials used to build the foundation.

Good ground

In May 2011, following the Canterbury earthquakes, the Ministry of Business, Innovation and Employment refined the existing definition of ‘good ground’ in Acceptable Solution B1/AS1 to exclude ground in the Canterbury earthquake region that is subject to liquefaction and/or lateral spread.

The changes also required the design of stronger foundations in that region. In August 2011, the requirement for stronger foundations was extended to include the rest of New Zealand. Acceptable Solution B1/AS1 modified its referencing of NZS 3604:2011 Timber-framed buildings to exclude unreinforced slabs. All concrete floor slabs, even those on ‘good ground’, are required to have reinforcing steel mesh and all perimeter foundations tied to the concrete slab with reinforcing steel.

Acceptable Solution B1/AS1 defines good ground as follows:

Any soil or rock capable of permanently withstanding an ultimate bearing capacity of 300 kPa … but excludes:

(a)   Potentially compressible ground such as topsoil, soft soils such as clay which can be moulded easily in the fingers, and uncompacted loose gravel which contains obvious voids;

(b)   Expansive soils being those that have a liquid limit of more than 50% when tested in accordance with NZS 4402 Test 2.2, and a linear shrinkage of more than 15% when tested from the liquid limit in accordance with NZS 4402 Test 2.6; and

(c)   Any ground which could foreseeably experience movement of 25 mm or greater for any reason including one or a combination of land instability, ground creep, subsidence (liquefaction, lateral spread – currently for the Canterbury earthquake region only), seasonal swelling and shrinking, frost heave, changing groundwater level, erosion, dissolution of soil in water, and effects of tree roots.

In other words, good ground is soil with sufficient bearing strength that it is not only able to carry its own weight but will not settle significantly when also carrying the additional weight of a building.

Testing for good ground

As a simple guide, signs that may indicate the presence of good ground include:

  • foundations of adjacent buildings show no signs of settling or inadequate bearing strength
  • no evidence of landslides in the vicinity
  • no evidence of buried services
  • no organic soil, peat or soft clay.

The procedure to carry out investigations to determine the suitability and bearing capacity of the soil are outlined in NZS 3604:2011. The basic procedure involves testing the site using a dynamic cone penetrometer, commonly known as a Scala penetrometer. Soil is tested to a depth of 2 m for strip or pile foundations or 600 mm below driven timber piles.

If testing determines the ground does not meet bearing-pressure requirements or the building or foundation is outside the scope of NZS 3604, it may still be possible to build on the site. However, the foundation is subject to specific engineering design and must demonstrate compliance with the Building Code in another way, such as Verification Method B1/VM4. This method defines two types of engineered foundations – shallow and deep foundations.