Seismic Foundation Design in Sydney

According to AS 1170.4, any Importance Level 2 or higher building — which covers most commercial, multi-residential, and institutional structures — must consider seismic actions. A formal seismic foundation design is required when the site class is De or Ee, when the building period exceeds 0.5 s, or when the design PGA at the site exceeds 0.10 g. For a typical three-storey office on Class Be rock, the code may not require a full dynamic analysis, but we still recommend a simplified check for overturning and sliding.

The standard AS 1170.4 spectrum uses generic shape factors that are conservative for most sites. A site-specific spectrum, derived from Vs30 measurements and a local seismic hazard model, can reduce the design base shear by 15–35% on stiff rock sites and increase it on soft soil sites where the code underestimates amplification.

We determine site class by measuring the average shear-wave velocity in the top 30 metres (Vs30) using a combination of MASW and down-hole seismic testing. The result is compared against the AS 1170.4 Table 4.1 boundaries: Class Ae (Vs30 > 1.500 m/s), Be (760–1.500 m/s), Ce (360–760 m/s), De (180–360 m/s), and Ee (Vs30 < 180 m/s). In the Botany Sands area, we typically measure Vs30 between 200 and 280 m/s, which classifies the site as De and triggers the need for a detailed liquefaction assessment.

For a standard commercial project in Sydney, the cost of a seismic foundation design study — including Vs30 profiling, site classification, and a liquefaction assessment — ranges between AU$1,920 and AU$7,120. The variation depends on the number of boreholes needed, the depth of the seismic array, and whether the site requires additional cross-hole tomography. A simple two-borehole MASW on Class Be rock will be at the lower end, while a six-borehole programme on Class De fill with a deep water table will approach the upper bound.