What is NZS3604:2011 'Good Ground'? A Guide for Building in New Zealand

NZS3604:2011 is the New Zealand Standard for Timber-framed buildings and is a key document used to design most homes and low-rise buildings. This building standard provides a set of guidelines to meet the New Zealand Building Code without requiring a specific engineering design (SED) for every detail.

For the design of a building’s foundations, the standard only applies to properties on ‘good ground’. The standard defines ‘good ground’ as: "any soil or rock capable of permanently withstanding an ultimate bearing capacity of 300kPa (an allowable bearing pressure of 100kPa)".

This minimum geotechnical ultimate bearing is required to ensure buildings are adequately supported and do not undergo differential settlement (when one part of a building sinks more than another, causing cracks).

However, 'good ground' clearly excludes several types of ground that are unsuitable for standard foundations:

• Compressible Ground: This includes topsoil, soft clays, and uncompacted loose gravel or fill.

• Expansive Soils: These soils can swell and shrink with changes in moisture, which can cause movement under your house.

• Unstable Ground: This includes any ground that could foreseeably experience movement of 25 mm or greater due to land instability, ground creep, subsidence, or erosion.

For the soil to be assumed to be ‘good ground’, the standard states that several conditions must be met:

• A reasonable inquiry and site observation show no evidence of buried services, landslips, surface creep, or earth fill.

• Excavations for footings do not reveal organic topsoil, soft peat, or very soft clay.

• A specific geotechnical investigation or geotechnical completion reports identify the area as providing ‘good ground’.

For building projects in the Far North or Northland, it is especially important to check the local Geotechnical Investigation Requirements for Buildings.

If you are unsure whether your property has ‘good ground’, you can undertake an NZS 3604 Good Ground Investigation. However, to make the process easier and more reliable, you can hire a Chartered Professional Engineer experienced in geotechnical investigations to do it for you. This proactive approach can avoid costly surprises and delays in the building consent process.

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In-Depth Technical Details for NZS3604:2011 'Good Ground'

For readers who are keen to understand the full technical details, here is an excerpt of official information from the standard.

NZS3604:2011 is the New Zealand Standard for Timber-framed buildings and is generally used to design most homes and low-rise timber framed buildings in New Zealand. NZS3604:2011 provides standard details to meet the requirements of the New Zealand Building Code without the need for specific engineering design (SED).

For the design of foundations, the standard only applies to buildings founded on ‘good ground’.

The standard defines ‘good ground’ as: “any soil or rock capable of permanently withstanding an ultimate bearing capacity of 300kPa (i.e. an allowable bearing pressure of 100kPa using a factor of safety of 3.0.), but excludes:

1. Potentially compressible ground such as top soil, soft soils such as clay which can be moulded easily in the fingers, and uncompacted loose gravel which contains obvious voids;

2. 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

3. 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, seasonal swelling and shrinking, frost heave, changing groundwater level, erosion, dissolution of soil in water, and effects of tree roots.

The minimum geotechnical ultimate bearing is required so that buildings are supported adequately by the ground and do not undergo differential settlement during their lifespan.  Differential settlement is when part of a building sinks more than another which can lead to uneven or cracked floors/walls.

Further, the standard states that the soil supporting the footings shall be assumed to be ‘good ground’ when all the following conditions are met:

“(a) Reasonable inquiry, through project information memorandum (PIM) and site observation show no evidence of buried services and none is revealed by excavation for footings;

(b) Reasonable inquiry, of PIM and site observation shows no indication or record of land slips or surface creep having occurred in the immediate locality;

(c) Reasonable inquiry shows no evidence of earth fill on the building site, and no fill material is revealed by the excavation for footings. This shall not apply where a certificate of suitability of earth fill for residential development has been issued in accordance with NZS 4431 for the building site, and any special limitations noted on that certificate are complied with; and

(d) Excavation for footings does not reveal buried organic topsoil, soft peat, very soft clay, soft clay, or expansive clay (see 3.2.1 and 3.3.8);

And any of the following:

(e) Where indicated by specific site investigation, using the test method for soil bearing capacity contained in 3.3;

(f) Where inspection of existing structures on this or neighbouring sites and reasonable enquiry, including territorial authority records, local history of the site, and published geological data such as structural geology where appropriate, shows no evidence of erosion (including coastal erosion, bank erosion, and sheet erosion), surface creep, land slippage, or other falling debris (including soil, rock, snow and ice), uncertified fill, fill over original water course, or subsidence having occurred in the immediate locality;

(g) When geotechnical completion reports in accordance with NZS 4404 identify subsoil class and areas that provide good ground.

If you are up for it, you can undertake an NZS 3604 Good Ground Investigation following the methods described in that document. Or keep life easy by hiring a Chartered Professional Engineer experienced in geotechnical investigations to do it for you. Don’t forget to check out the guidelines for Geotechnical Investigation Requirements for Buildings in the Far North.

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