New Zealand Geotechnical Society (NZGS) Symposium 2021
Expansive soils are present across the wider Auckland region and in many other parts of New Zealand, where residential buildings are increasingly supported on shallow slabs and footings in clay-rich soils subject to shrinking and swelling behaviour. Currently NZS3604 provides first-line guidance on the identification of these so-called expansive soils, based on the use of Atterberg Limits and Linear Shrinkage. Since 1999, NZS3604 has codified the use of Australian Standard AS2870 to guide residential foundation design which requires the classification of a site based on the expected ground surface movement. Show more…There are several limitations in the application of AS2870 to New Zealand conditions, which have been known about for some time, yet still remain unresolved today. Of even greater concern, however, is the unreliability of the shrink swell test for New Zealand soils. If the test results are unreliable, then so too must be the site soil classification from which it is derived. Apart from New Zealand, no other country (besides Australia) has adopted the shrink swell test to classify expansive soils.
Internationally there are alternative methods available to systematically identify and classify expansive soils for the purposes of foundation design, including the Volume Change Potential approach used in Britain. Without the shrink swell test, AS2870 cannot be practically applied in New Zealand. This paper reviews the known problems associated with NZ’s current approach to identifying and classifying expansive soil, comparing it with some international alternatives and poses the question: Is it time to ditch AS2870 from the New Zealand Building code. Show less…
