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:
- 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;
- 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
- 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 guideline for Geotechnical Investigation Requirements for Buildings in the Far North.