SPT (Standard Penetration Test) in Melbourne

Melbourne’s geology is a study in contrasts. The city sits on Quaternary basalt flows in the west and deep alluvial silts and clays toward the Yarra River. That Yarra silt – the infamous Coode Island Silt – can extend 30 metres deep and behaves nothing like the dense basalt you find in the north-west suburbs. Running a Standard Penetration Test in Melbourne means you get a continuous blow-count profile that tells you exactly where the refusal layer sits. Before we mobilise a rig, we always cross-reference the local borehole database to choose the right hammer energy and rod length. For shallow investigations, pairing the SPT with calicatas exploratorias gives a direct visual of the soil fabric, while the SPT itself quantifies the resistance at every 1.5-metre interval.

A single SPT blow count at one corner won't catch the silt lens shift that can cost you a foundation redesign.

Our service areas

Ground improvement

Unsaturated soil analysis ›Stone column design ›Dynamic compaction design ›Deep Soil Mixing (DSM) design ›Geotechnical drainage design ›

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Slopes & Walls

Soil erosion analysis ›Slope stability analysis ›Slope failure analysis ›Debris flow analysis ›Factor of safety (FS) calculation ›

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Foundations

Differential settlement analysis ›Settlement analysis ›Bearing capacity analysis ›Foundations on fill (analysis) ›Shallow foundation design ›

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Methodology and scope

A common mistake we see with builders in Melbourne is relying on a single SPT at one corner of the site. The Coode Island Silt can vary from soft to firm within 10 metres horizontally – that silt lens shifts without warning. Our team runs the Standard Penetration Test at every change in stratum and at maximum 1.5-metre vertical spacing, per AS 1726. We record three numbers: the seating drive (150 mm), then the test drive for N-value, and we note any rod-free fall or water inflow. When we encounter gravel layers near the Maribyrnong River, we switch to a solid cone tip to avoid over-counting. For projects on the basalt plains around Werribee, we complement the SPT with ensayo CPT to get a continuous cone resistance log that picks up thin dense layers the SPT might skip.

Technical reference — Melbourne

Local considerations

The Coode Island Silt that underlies much of central Melbourne is soft to very soft, with N-values often below 5 in the upper 10 metres. That means high settlement risk under relatively low loads. We have seen cases where a single SPT at 6 metres depth showed N=3, but a second hole 20 metres away hit N=12 at the same depth – a sand seam that had not been detected. If you do not run enough SPTs, you can easily overestimate bearing capacity in the silt and underestimate it in the dense basalt. The Standard Penetration Test is the only in-situ method that gives you a direct disturbed sample for identification, so you can confirm that the material is truly silt, not a clay with silt. In Melbourne’s seismic zone (AS 1170.4), a low N-value also flags potential liquefaction in loose saturated sands near the Yarra estuary.

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Explanatory video

Applicable standards

AS 1726-2017 – Geotechnical site investigations, AS 4678-2002 – Earth retaining structures (references SPT for soil strength), AS 1289.6.3.1 – Standard Test Method for SPT, AS/NZS 1170.4:2007 – Structural design actions (seismic, uses N-values for site class)

Technical parameters

Parameter	Typical value
Hammer energy ratio (ERi)	60-80% (safety hammer, rope-cathead or automatic)
Rod length correction (CR)	1.0 (>10 m) to 0.85 (3-4 m)
Borehole diameter correction (CB)	1.05 (100 mm hole) to 1.0 (150 mm)
Sampler type	Standard split-spoon (35 mm ID, 50.8 mm OD)
Blow count recording	Three 150 mm increments; N = blows for last 300 mm

Frequently asked questions

How much does a Standard Penetration Test cost in Melbourne?

For a standard SPT run to 20 metres with borehole logging and a laboratory report, the typical range is AU$840 to AU$1,200 per test point. This includes mobilisation within the Melbourne metro area, the field test, and a certified summary of N-values and soil classification. Volume discounts apply when you book five or more points on the same site.

What is the difference between N-SPT and bearing capacity?

N-SPT is a blow count that measures relative density and consistency of the soil. Bearing capacity is the calculated allowable pressure that the soil can support without excessive settlement. We use the N-value as an input to empirical correlations (Terzaghi, Meyerhof) to estimate ultimate bearing capacity, but for soft clays like the Coode Island Silt, laboratory triaxial tests are needed to refine the value. The SPT alone gives you a preliminary capacity that is conservative for sand and gravel, but not for saturated clays.

When should I use SPT instead of CPT in Melbourne?

Use SPT when you need a disturbed soil sample for visual classification and laboratory index tests (Atterberg limits, moisture content, particle size). The CPT gives a continuous profile but no sample. In Melbourne’s heterogeneous geology – where a sand seam can be missed between CPT readings – the SPT is preferred for foundation design in alluvial soils. We also recommend SPT for seismic site classification because AS 1170.4 explicitly references N-values for site class determination.

Location and service area

We serve projects across Melbourne.

Location and service area