The design of shallow foundations in Bunbury must account for the city's unique coastal geology, where Quaternary sands overlie the Tamala Limestone formation. Australian Standard AS 1726 sets the framework for site investigation, but the local variability in soil density and the presence of shallow groundwater demand a more granular approach. In our experience, a standard borehole program rarely captures the full lateral variability of the dune sands, so we routinely complement the investigation with a placa de carga test to verify modulus of subgrade reaction at footing level. Only by combining systematic SPT profiling with in-situ plate loading can the design engineer assign reliable bearing capacities that are neither overly conservative nor unconservative for the specific lot.
The real challenge in Bunbury is not the bearing capacity of the sand itself, but the lateral variability of the soil profile within a single building footprint.
Methodology and scope
Local considerations
Bunbury sits in a region of moderate seismicity, with the nearest significant event being the 1979 M5.6 earthquake near Meckering, roughly 150 km east. While the probability of strong ground shaking is lower than in Perth, the city's sandy soils are prone to amplification of longer-period waves. For shallow foundation design, the main risk is not liquefaction — clean sands here are generally dense enough — but rather the loss of bearing support due to cyclic softening in loose fill zones. We address this by requiring a minimum of three SPT borings per residential lot where fill is suspected, and by specifying cimentaciones-sismicas design checks for structures over two storeys. The cost of this precaution is modest compared to the potential for differential settlement after a moderate event.
Explanatory video
Applicable standards
AS 1726 – Geotechnical site investigations, AS 2870 – Residential slabs and footings, AS/NZS 1170.4 – Earthquake actions, AS 4678 – Earth retaining structures (adjacent foundations)
Associated technical services
Residential footing design (AS 2870)
Classification of site reactivity, design of stiffened rafts, waffle slabs, and strip footings for single-family homes on Bunbury sands. Includes SPT-based bearing capacity and differential settlement check.
Industrial pad and footing analysis
Bearing capacity and settlement analysis for warehouse slabs, crane bases, and machine foundations. We use plate load tests and finite-element modelling to limit total and differential settlements to 15 mm.
Shallow foundation peer review
Independent geotechnical review of existing design reports prepared by others. We verify assumptions on soil strength, water table, and load distribution against AS 1726 and local experience.
Typical parameters
Frequently asked questions
What bearing capacity is typical for shallow foundations on Bunbury's sandy soils?
For clean, well-graded dune sands with SPT N-values between 10 and 15, we commonly assign an allowable bearing capacity of 100 to 200 kPa. In areas with loose fill or organic lenses, the value drops to 50–80 kPa. A plate load test on site is the most reliable way to confirm the actual capacity before footing construction.
How does the shallow groundwater in Bunbury affect footing design?
Seasonal groundwater at depths of 2.5 to 5.0 metres can reduce the effective stress in the sand, lowering its bearing capacity. For footings near the water table, we recommend a drained analysis using the saturated unit weight and a factor of safety of 3.0. In extreme cases, we specify a gravel drainage blanket beneath the slab to prevent capillary rise.
What is the typical cost range for a shallow foundation design report in Bunbury?
The cost for a full geotechnical design report including site investigation, laboratory testing, and footing recommendations typically falls between AU$2.560 and AU$4.460. This covers SPT borings, Atterberg limits, and a settlement analysis. Larger or more complex sites with multiple footings may be at the upper end of the range.