A common mistake we see in Bunbury is treating base isolation as a purely structural solution, skipping the geotechnical characterisation needed to size the isolators properly. In the sandy soils of the Swan Coastal Plain, the design acceleration at the base of the building depends heavily on the vs30/" data-interlink="1">shear wave velocity profile of the top 30 metres. Without a measured Vs30 from a MASW survey or a down-hole test, the code-based site class can underestimate the seismic demand by a factor of two. That mismatch leads to undersized bearings and unacceptable displacements during a design earthquake. Getting the ground response right is the first step.
A Vs30 mismatch of 100 m/s can double the design displacement of an isolation system in Bunbury's coastal sands.
Methodology and scope
Local considerations
Compare two Bunbury suburbs: the eastern side of Withers sits on deeper sand deposits with a natural period around 0.6 s, while central Bunbury near the Koombana Bay marina is built over reclaimed land with soft estuarine silts. The same base isolation system designed for Withers would not work near the marina because the soft soil shifts the predominant period of the ground motion. If the isolation period coincides with the site period, the bearings will resonate rather than decouple. That is why we always couple the seismic design with a site-specific response spectrum analysis, avoiding the code-prescribed generic spectra that mask these local differences.
Applicable standards
AS 1170.4:2007 (Earthquake actions), AS 1726:2017 (Geotechnical site investigations), NZS 1170.5:2004 (Seismic actions – supplementary guidance), ASCE/SEI 7-16 (Chapter 17 – Seismic isolation systems)
Associated technical services
Site-Specific Response Spectrum
One-dimensional equivalent-linear ground response analysis using DEEPSOIL or equivalent software. We model the shear modulus reduction and damping curves from resonant column tests to generate a firm-ground spectrum that reflects the real Bunbury soil column.
Vs30 Profiling & Site Classification
MASW arrays and HVSR microtremor measurements to define the time-averaged vs30/" data-interlink="1">shear wave velocity of the top 30 metres. The result is a site class per AS 1170.4 that directly controls the design acceleration spectrum for the isolation system.
Isolation Bearing Load & Settlement Check
Plate load tests on the sub-isolation raft to verify bearing capacity and immediate settlement under vertical loads. We also run cyclic triaxial tests to evaluate stiffness degradation under the repeated load cycles expected during a design earthquake.
Typical parameters
Frequently asked questions
How does the Bunbury soil profile affect the design of a base isolation system?
The combination of Tamala Limestone and Bassendean Sand creates a velocity inversion that can shift the site period. If the isolation period (typically 2.0–3.5 s) matches the predominant site period, the bearings may amplify instead of decouple motion. We use site-specific response spectra to avoid this resonance.
What seismic hazard factor applies to base isolation design in Bunbury?
Per AS 1170.4:2007, the hazard factor Z for Bunbury is 0.09. That corresponds to a peak ground acceleration of approximately 0.09 g for a 500-year return period. The isolation system must be designed for a displacement that accounts for the site class amplification, not just the rock-outcrop value.
How much does a geotechnical investigation for base isolation cost in Bunbury?
A full programme including MASW, HVSR, resonant column tests, and site-specific response analysis typically ranges between AU$5.550 and AU$13.120 depending on the number of boreholes and the depth of profiling required.
Can I use the AS 1170.4 generic site class instead of a site-specific Vs30 measurement?
Not recommended. The generic site class for Bunbury can be Class C or D depending on local geology. A Vs30 mismatch of 100 m/s can change the design displacement by 40–60%. We always recommend a measured Vs30 from MASW or down-hole testing to size the isolators correctly.