Site characteristics

The concept of good ground, as defined by NZS 3604 Timber-framed buildings for residential construction, does not exist for commercial buildings (unless the building is designed within the scope of NZS 3604).

The New Zealand loading standard NZS 1170.5:2004 Structural design actions – Earthquake actions – New Zealand defines five classes of ground (Class A to Class E). These use a combination of ground strength, wave propagation velocity and subsoil depth.

A geotechnical investigation must be undertaken to establish these ground characteristics. It examines factors such as geological history, soil composition, bearing strength and stratification and builds a model of the site and its surroundings.

The investigation should extend through all soil strata considered able to affect the behaviour of the site and the building foundations. It should then continue to a sufficient additional depth to ensure all potential problem soils have been identified. If deep pile foundations or ground-improvement techniques are being considered, which is often the case for larger commercial buildings, the investigation will also need to analyse into the bearing stratum.

Site investigations for commercial developments almost always extend to the performance of surrounding sites as well. This is especially important when considering the risk of widespread liquefiable soil layers and potential lateral ground movements.

Although a commercial site investigation may consider a wider area, delve into deeper soil strata and conduct a more thorough analysis than a smaller-scale residential investigation, it has the same objective. This is to reliably predict the future behaviour of the ground at and around the site so that the designer can design a suitable foundation. The investigation should be sufficiently detailed that the designer has confidence that the foundation will perform satisfactorily given the building’s performance requirements.