Mackay sits on a coastal floodplain with deep alluvial deposits overlying bedrock. Many times we see that the top 20 to 30 meters consist of soft silty clays and loose sands, which can significantly amplify earthquake ground motions. A thorough site response analysis is the only way to quantify how these soil layers will affect the seismic demand on a structure. Without this study, you risk under-designing foundations or over-specifying reinforcement. The analysis combines shear wave velocity profiling, typically via MASW or HVSR, with local ground motion records to produce site-specific response spectra. This is not a generic code check; it is a tailored evaluation that respects Mackay's unique geological setting. We follow the framework of AS 1726 for geotechnical site investigation and AS/NZS 1170.4 for seismic actions, ensuring compliance with Australian standards.
In Mackay's deep alluvial soils, ignoring site-specific amplification can lead to spectral accelerations up to 2.5 times the code-mapped values.
Methodology and scope
Mackay lies within Seismic Zone 2 per AS/NZS 1170.4, with an annual probability of exceedance of 1/500 for ultimate limit state. The 2018 magnitude 5.7 earthquake near Bowen reminded the region that moderate seismic events are not hypothetical. A proper site response analysis in Mackay must account for the deep soft soil column that can amplify long-period motions. This is especially critical for tall buildings, bridges, and liquid storage tanks.
Key parameters we evaluate include:
Average shear wave velocity in the top 30 m (Vs30) — used for NEHRP site class determination
Fundamental site period (T0) from HVSR microtremor measurements
Peak ground acceleration (PGA) and spectral acceleration (Sa) at 0.2 s and 1.0 s
Strain-dependent modulus reduction and damping curves (Darendeli 2001 model)
A five-story commercial building we analyzed near the Mackay Marina had a design based on the default Site Class D spectrum from AS/NZS 1170.4. The one-dimensional site response analysis showed that the actual spectral acceleration at 1.0 seconds was 80% higher than the code spectrum predicted. The original design would have been unsafe for a 1-in-500-year event. The client had to revise the lateral system, adding two shear walls and increasing foundation stiffness. That extra cost could have been avoided if the site response analysis had been done early in the design. In Mackay, the combination of soft estuarine clays and moderate seismic hazard makes this study a necessity for any structure taller than three stories or founded on deep piles.
Non-invasive surface wave surveys to measure Vs30 and identify the depth to bedrock. Ideal for Mackay's urban areas where boreholes are restricted. Results are directly entered into the site response model.
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Equivalent Linear and Nonlinear Site Response
One-dimensional wave propagation analysis using DeepSoil and DEEPSOIL. We calibrate modulus reduction curves to local soil types and generate site-specific acceleration time histories for structural design.
Applicable standards
AS/NZS 1170.4:2007 (Structural design actions — Earthquake actions), AS 1726:2017 (Geotechnical site investigations), NEHRP Recommended Seismic Provisions (FEMA P-1050), Guidelines for Seismic Site Characterization (NZS 1170.5:2004)
Frequently asked questions
What is the difference between a site response analysis and a standard geotechnical report?
A standard geotechnical report provides soil classification, bearing capacity, and settlement estimates. A site response analysis specifically evaluates how the soil column modifies earthquake ground motions as they propagate from bedrock to the surface. It produces design spectra and acceleration time histories that are unique to the site. In Mackay, where soft alluvium overlies bedrock, the analysis often reveals amplification factors of 2 or more at periods between 0.6 and 1.2 seconds.
Does Mackay require a site-specific seismic study for low-rise residential construction?
For single-family homes on shallow foundations, the AS/NZS 1170.4 default spectra with Site Class D or E are usually sufficient. However, for houses on steep slopes, near the Pioneer River floodplain, or with irregular geometry, a simplified site response analysis can help avoid differential settlement and structural damage.
How much does a full site response analysis cost in Mackay?
The cost typically ranges from AU$2,220 to AU$6,710 depending on the number of input ground motions, the depth of the model, and whether a MASW survey is included. A basic desktop study with existing borehole data is at the lower end; a full investigation with field testing and nonlinear analysis is at the higher end. We recommend budgeting for at least AU$4,500 for a medium-rise building project.
