We were called to a site off Robertson Drive last year, where a three-storey commercial building was showing early signs of differential settlement. The original geotechnical report was thin on actual penetration data. We ran three SPT (Standard Penetration Test) boreholes to 15 metres depth and found a 4-metre layer of loose silty sand over stiff clay — exactly the sort of profile that needs careful footing design. That project convinced the developer to always request SPT (Standard Penetration Test) data before pouring concrete. In Bunbury, where the coastal sands can hide variable bearing strata, this test gives you the blow-count numbers you can trust for foundation decisions. We combine it with plate load testing when the design requires direct modulus verification.
In coastal sands like Bunbury’s, a single SPT blow-count can mean the difference between a shallow footing and a deep pile foundation.
Methodology and scope
Local considerations
Compare the sandy ground in Koombana Bay with the alluvial clays near the Preston River. In the dunes, a loose layer can give N-values of 3 to 6 — high liquefaction risk during a magnitude 6.5 earthquake scenario. Over in the river flats, stiff clays with N60 above 20 can still suffer from shrinkage and swelling. The SPT (Standard Penetration Test) catches both extremes. We recently tested a site in Carey Park where the blow-count jumped from 5 to 35 within two metres, revealing a dense cemented layer that changed the foundation concept entirely. Relying on nearby boreholes without local SPT (Standard Penetration Test) data is a gamble we see too often in Bunbury developments.
Applicable standards
AS 1726-2017: Geotechnical Site Investigations, AS 1289.6.3.1: Standard Test Method for SPT, AS 4678-2002: Earth Retaining Structures
Associated technical services
SPT Drilling & Sampling
Track-mounted or truck-mounted rigs for depths up to 30 metres. Continuous sampling with split-spoon at 1.5 m intervals or as specified. Real-time blow-count recording on site.
N-Value Interpretation & Correction
Raw blow-counts corrected for energy ratio, overburden pressure, and rod length. Delivered as N60 and N1,60 values for design use. Correlated with relative density and friction angle using recognised empirical relationships.
Integrated Geotechnical Reporting
SPT data combined with laboratory classification (grading, Atterberg limits, moisture content). Stratigraphic logs and bearing capacity recommendations. Certified by NATA-accredited laboratory.
Typical parameters
Frequently asked questions
How deep should SPT boreholes be for a residential slab in Bunbury?
For a standard residential slab on ground, we recommend boreholes to at least 6 metres or until refusal in dense material. This depth covers the active zone of moisture change (typically 2–4 m in Bunbury's clay sands) and verifies bearing strata below any loose surface layers. Deeper boreholes may be needed if filling or soft pockets are suspected.
What is the typical cost range for an SPT test in Bunbury?
The typical cost for a single SPT borehole to 6 metres depth, including mobilisation within Bunbury urban area and a basic report, ranges between AU$720 and AU$1,080. This varies with number of boreholes, depth, and whether laboratory testing is included. We provide fixed-price quotes after a brief site review.
How does SPT data help with liquefaction assessment in coastal Bunbury?
SPT blow-counts are the primary input for simplified liquefaction triggering analyses (e.g., Youd et al. 2001, NCEER method). In Bunbury's coastal sands, N60 values below 10 indicate high susceptibility during seismic events. We correct for fines content and overburden stress to produce a factor-of-safety profile against liquefaction, which directly informs foundation type and ground improvement design.