Atterberg Limits Testing for Wellington Soils

We see it constantly: a firm designs footings based on assumed clay behaviour, only to have the consent authority request Atterberg limits data mid-project, delaying the programme by weeks. Wellington's geology doesn't forgive assumptions. The city sits across greywacke bedrock, weathered zones, and pockets of alluvial silts that shift dramatically over short distances. A plasticity index that looks moderate on one side of Karori can swing high in a neighbouring terrace deposit. Getting accurate Atterberg limits isn't a checkbox exercise; it's the difference between a foundation that accommodates seasonal moisture change and one that cracks within the first three years. We run the full suite—liquid limit, plastic limit, and plasticity index—on samples prepped and tested the same day they arrive. No storage artefacts. No re-dried material. When you're working on Wellington's wind-scoured slopes, where clay reactivity can vary by 15% across a single cut face, that discipline matters. The grain-size analysis often runs alongside our Atterberg programme to establish the full fines fraction context, and we tie results back to the NZGS soil classification framework used by WCC consent engineers.

A plasticity index shift of just 12% can change a Wellington clay from low-expansion to high-expansion—and completely alter the required foundation depth.

Our approach and scope

Local ground factors

A three-storey apartment block on the old Hutt Road alluvium taught us a lesson we won't forget. The geotech brief assumed moderately plastic clay across all six boreholes. We ran Atterberg limits on every metre of core and found a 2-metre lens of highly plastic silt—PI of 38—directly under the proposed pile cap at the northern end. The original design had specified a bearing pressure that that material simply couldn't sustain under wet-season conditions. Had the limits not been tested at that exact depth, differential settlement would have shown up within two winters. Wellington's coastal and valley-floor sediments carry these lenses: old swamp deposits, estuarine silts, and reworked loess that standard logging misses. Without Atterberg data at close vertical spacing, you're guessing at the most critical parameter in cohesive soil design. That's also why we pair Atterberg limits with cpt-test profiling on soft-ground sites—the sleeve friction and pore pressure data from the CPT help us identify exactly where to sample for plasticity testing, so no high-risk layer goes uncharacterised.

Typical values

Regulatory framework

NZS 4402.2.2:1988 - Methods of testing soils for civil engineering purposes, NZGS Soil Classification Guidelines (current edition), NZS 3604:2011 - Timber-framed buildings (foundation soil categories reference)

Common questions