Seismic site investigation in Auckland addresses the critical need to understand ground behaviour during earthquake events, a requirement driven by the region's complex geological setting. Much of the city is underlain by the East Coast Bays Formation, a soft and highly variable sedimentary rock, alongside volcanic deposits from the Auckland Volcanic Field and areas of reclaimed land. This work is governed by NZS 1170.5:2004 (Structural design actions – Earthquake actions) and the MBIE/NZGS Module 4 guidelines for earthquake geotechnical engineering, which define how to classify a site based on its soil profile. The initial phase of any investigation often requires an exploratory test pit to visually assess shallow soils, or an investigation program designed to map subsurface variability and identify potential hazards like liquefaction and lateral spreading.
The methodology for seismic assessment in New Zealand relies on a combination of intrusive testing to measure soil strength and stiffness parameters directly. The SPT (Standard Penetration Test) remains the fundamental tool for liquefaction assessment, with its N-values corrected for energy and overburden to calculate the cyclic resistance ratio in accordance with NZGS Module 4. For critical structures or softer ground, advanced In-Situ is deployed to capture low-strain stiffness. The Ménard pressuremeter test (PMT) provides a direct measurement of the modulus of deformation and limit pressure, essential for modelling soil-structure interaction under seismic loads, providing data that surpasses correlations from simpler penetration tests.
Typical seismic investigation projects in Auckland range from assessing the vulnerability of existing building stock on the soft alluvial soils of the CBD fringe to new infrastructure developments on the city's volcanic scoria cones. The assessment of tailings dams, major cut-and-cover tunnels, and port structures requires high-quality parameters that depend on pristine samples. This is achieved through undisturbed sampling (Shelby tube) in cohesive soils, allowing for dynamic laboratory testing like cyclic triaxial or resonant column tests. For critical groundwater assessments related to seismic dewatering or the evaluation of hydraulic barriers, a field permeability test (Lefranc/Lugeon) is integrated into the investigation to determine the mass hydraulic conductivity of the ground, a parameter that influences pore-water pressure generation during shaking.
The process begins with a targeted investigation plan, mobilizing drilling and testing equipment to the Auckland site. Our team executes the fieldwork, culminating in a comprehensive geotechnical interpretative report that explicitly addresses NZS 1170.5 site classification and provides design parameters for seismic analysis. The core value lies in technical certainty: we deliver defensible ground models, accurate liquefaction triggering curves, and soil-structure interaction springs derived from direct measurement, not just empirical correlation. This rigorous approach enables structural engineers to optimize foundation designs, confidently satisfy the consenting requirements of the Auckland Council, and ensure the seismic resilience of the final asset.