Bunbury sits on the Swan Coastal Plain, underlain by the Tamala Limestone formation and Quaternary dune sands that can liquefy under strong shaking. The city is classified as seismic zone A2 under AS/NZS 1170.4, with an annual probability of exceedance of 1/500. In practice, this means foundations must accommodate lateral spreading potential in the low-lying areas near Leschenault Estuary. Our team combines MASW surveys for Vs30 profiles with standard SPT borings to assign the correct site class. These two techniques give us the shear wave velocity needed for seismic foundation design, especially for medium-rise buildings in central Bunbury. Without that data, you risk underestimating settlement during an event.
We combine MASW surveys for Vs30 profiles with standard SPT borings to assign the correct site class for seismic foundation design in Bunbury.
Methodology and scope
Local considerations
During a seismic event, unconsolidated sands in central Bunbury can lose strength within seconds. The main risk is lateral spreading toward the estuary channels, which pulls foundations apart. We use a track-mounted CPT rig to map continuous soil behavior in 2 cm intervals, identifying thin loose layers that standard SPT might miss. For sites where liquefaction is confirmed, we run cyclic triaxial tests on undisturbed samples to get the CSR curve. That data feeds directly into the seismic foundation design, letting us size raft slabs or specify stone columns. We also install vibrating-wire piezometers to monitor excess pore pressure.
Applicable standards
AS/NZS 1170.4:2007 (Earthquake actions), AS 1726:2017 (Geotechnical site investigations), NCEER 1997 / Youd-Idriss 2001 (SPT-based liquefaction evaluation), AS 1289/D4428M-14 (MASW crosshole seismic testing)
Associated technical services
Site Class Assessment & Vs30 Profiling
Non-invasive MASW surveys to measure vs30/" data-interlink="1">shear wave velocity profiles down to 30 m depth. Combined with SPT borings to classify site class per AS/NZS 1170.4. Includes a written report with amplification factors and recommended design spectra.
Liquefaction Hazard Evaluation & Mitigation Design
SPT-based and CPT-based liquefaction triggering analysis using the NCEER 1997 method. Cyclic triaxial testing on undisturbed samples when required. Output includes factor of safety maps and specifications for stone columns or deep soil mixing.
Typical parameters
Frequently asked questions
What makes seismic foundation design in Bunbury different from other WA cities?
Bunbury combines limestone bedrock with loose Quaternary dune sands and estuarine deposits. The water table is shallow near Koombana Bay, raising liquefaction risk. Unlike Perth, Bunbury has a higher proportion of site class D soils, which amplify ground motion. Our designs must account for both the stiff limestone cap and the softer upper layers.
How much does a seismic foundation design study cost in Bunbury?
For a typical residential or low-rise commercial project, the cost ranges between AU$2.020 and AU$5.690. This includes MASW profiling, SPT borings, laboratory testing, and a final design report. Complex sites with deep sand layers or high water tables may push toward the upper end.
What standards apply to seismic foundation design in Bunbury?
The primary standard is AS/NZS 1170.4:2007 for earthquake actions, along with AS 1726:2017 for site investigation. For liquefaction evaluation, we follow the NCEER 1997 workshop guidelines (Youd-Idriss 2001). Local council may also reference the WA Building Regulations 2012 for site-specific hazard assessments.