Auckland sits on the Auckland Volcanic Field, underlain by Waitemata Group sandstones and siltstones with interbedded volcanic ash layers. The shallow soil profile often includes soft clays and peats from reclaimed harbour areas, especially around the CBD and waterfront. These low-shear-wave-velocity materials amplify long-period ground motions during earthquakes. A proper site response analysis quantifies this amplification using site-specific shear wave velocity profiles. We apply NZS 4203:1992 seismic loading provisions and NZGS guidelines to classify each site. The method combines downhole MASW surveys with masw-vs30 measurements to define the soil column's dynamic properties. This data drives equivalent-linear or fully nonlinear 1D/2D ground response computations. The output includes acceleration response spectra and amplification factors for the site's specific soil profile.
Soft Holocene clays in central Auckland can amplify peak ground acceleration by more than 1.8 times the rock outcrop motion at 1-second period.
Methodology and scope
Our analysis uses code-compliant software like DEEPSOIL and STRATA for equivalent-linear and nonlinear solutions. The input motion suite is selected from New Zealand strong motion databases matching Auckland's seismotectonic context. We calibrate modulus reduction and damping curves using resonant column tests on undisturbed samples when available. The shear wave velocity profile comes from in-situ MASW arrays deployed across the site. Typical parameters measured include Vs30, natural period, and peak ground acceleration at surface. We also compute spectral accelerations at 0.2 s and 1.0 s for foundation design. The work follows NZS 4402 for MASW acquisition and NEHRP site classification criteria. Each report presents hazard-consistent response spectra for 10% and 2% probability of exceedance in 50 years.
Technical reference image — Auckland
Local considerations
Soils in the Viaduct Basin and Wynyard Quarter differ sharply from those in Remuera or Mt Eden. The waterfront areas have up to 15 m of soft estuarine clay over volcanic basalt. This soft layer generates strong long-period amplification. On the volcanic cones, shallow basalt rock over weathered tuff produces short-period amplification instead. A site response analysis for an Auckland apartment tower in the Viaduct Basin must account for the thick soft layer. Ignoring the basin-edge effect could underestimate spectral accelerations by 40% at periods critical for tall buildings. The contrast between the CBD's reclaimed land and the elevated suburbs demands site-specific evaluation for every project.
Uses the SHAKE-type approach with strain-compatible modulus and damping curves. Suitable for sites with relatively simple stratigraphy and moderate seismic hazard. Output includes acceleration time histories and response spectra at the ground surface. We apply the method for most Auckland residential and commercial projects where soil layering is sub-horizontal.
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2D/3D Nonlinear Finite Element Analysis
Captures basin effects, topographic amplification, and soil-structure interaction. Required for critical facilities like hospitals, bridges, and high-rise towers in Auckland's reclaimed zones. We use PLAXIS 2D and OpenSees for fully nonlinear time-domain analysis. The model incorporates cyclic degradation of soft clays and pore pressure buildup in liquefiable layers.
Applicable standards
NZS 4203:1992 General Structural Design and Design Loadings, NZS 1170.5:2004 Earthquake Actions – New Zealand, NZS 4402/D4428M-14 Standard Test Methods for Crosshole Seismic Testing, NEHRP Recommended Seismic Provisions (FEMA P-1050) – Site Class Definitions
Frequently asked questions
When is a site response analysis required for an Auckland project?
It is required under NZS 1170.5 for all Importance Level 3 and 4 structures on Soil Class C, D, or E sites. For low-rise buildings on rock (Class A/B) it is optional but recommended near the waterfront. Auckland Council may request it for sites with more than 5 m of soft soil or near volcanic craters.
How much does a site response analysis cost in Auckland?
The typical cost ranges between NZ$1.890 and NZ$6.600 depending on the complexity. A 1D equivalent-linear analysis for a simple site costs around the lower end. A full 2D nonlinear study with site-specific testing falls at the upper end. The price includes a full report with response spectra and amplification factors.
What is the difference between equivalent-linear and nonlinear analysis?
Equivalent-linear analysis iterates on strain-compatible properties but assumes linear behavior within each iteration. It works well for moderate shaking and stiff soils. Nonlinear analysis tracks stress-strain paths in real time, capturing permanent deformations and pore pressure changes. It is essential for soft soils under strong ground motions, such as those expected in the Auckland Volcanic Field.
How is the shear wave velocity profile obtained for the analysis?
We use the MASW method (Multichannel Analysis of Surface Waves) following NZS 4402. This provides a 1D Vs profile down to 30 m depth. For deeper profiles we combine MASW with passive microtremor arrays (HVSR). The Vs30 value derived from this profile determines the NEHRP site class and drives the response analysis.
Location and service area
We serve projects across Auckland and its metropolitan area.
