A foundation design in the CBD hit a snag last winter when the borelogs showed 4 metres of Christchurch Formation silt sitting right above the Riccarton Gravel. The structural team needed drained and undrained strength parameters fast, because the groundwater was barely a metre down. That’s exactly the scenario where a routine triaxial test turns into a critical path item. We see these layered profiles every week—post-quake alluvium, estuarine clays, and clean gravels all stacked within 10 metres. If you’re working near the Avon or Heathcote corridors, the undrained behaviour of those near-surface silts dictates whether you’ll need Improvement or can rely on a shallow footing. For deeper strata, we often pair the triaxial program with CPT testing to cross-check the undrained shear strength profile before selecting consolidation stages.
Effective stress paths from CU triaxial testing on Christchurch silts often reveal contractive behaviour that peak friction angles alone hide.
