Too many foundation designs in Christchurch rely on assumed soil behaviour, and then the first wet winter exposes the problem. Silty soils across the city—particularly the loess-derived deposits on the Port Hills and the fine alluvium on the flats—change volume dramatically with moisture. We run Atterberg limits testing to give you the liquid limit, plastic limit, and plasticity index before a single pile goes in. Without these numbers, you are guessing at shrink-swell potential and bearing capacity changes. The 2010–2011 earthquake sequence remoulded a lot of ground here, and disturbed soils do not behave like intact ones. A CPT test can map the stratigraphy, but it cannot tell you the plasticity of a clay layer—that is where the Atterberg limits come in. We follow NZGS guidelines and ASTM D4318-17e1 in our IANZ-accredited lab, turning samples around quickly because Canterbury’s construction pace does not tolerate delays.
Plasticity index is the single fastest indicator of whether a Christchurch clay will heave or shrink under a lightly loaded slab.
