Soil Liquefaction Analysis in Melbourne

The first step is a site investigation to determine soil stratigraphy, groundwater depth, and in-situ density. We typically drill boreholes with SPT at 1.5 m intervals and collect disturbed samples for fines content and Atterberg limits. If the water table is within 10 m of the surface and the soil is loose sand or silty sand — common in Melbourne's alluvial zones — we proceed to cyclic resistance evaluation.

CSR is computed using the simplified Seed-Idriss method: CSR = 0.65 × (amax/g) × (σv/σv') × rd, where amax is the peak ground acceleration from AS 1170.4 (typically 0.08g to 0.12g for Melbourne), σv and σv' are total and effective vertical stresses, and rd is a depth reduction factor. We apply depth increments of 1.5 m from the surface down to 20 m.

A factor of safety below 1.0 indicates that liquefaction is expected to trigger during the design earthquake. This means the soil loses most of its shear strength and behaves like a viscous fluid. Foundation options then include deep piles extending through the liquefiable layer to competent bearing strata, ground improvement such as stone columns or deep soil mixing to densify the soil, or ground improvement such as drainage measures to dissipate excess pore pressure.

For a standard mid-rise building on a 1000 m² site, the cost ranges between AU$4,380 and AU$5,750. This includes drilling two boreholes to 15 m depth, SPT at 1.5 m intervals, fines content testing, cyclic triaxial testing on two samples, and a full analysis report with depth profiles and LPI. Larger sites or advanced cyclic testing can increase the cost.