Brisbane’s subtropical climate means intense summer storms and prolonged wet seasons. The city’s dominant soils — mostly sedimentary and alluvial — tend to have moderate to low permeability. That makes an infiltration test a critical step for any drainage, detention basin, or soakwell design. Without a measured infiltration rate, you risk over-designed systems or, worse, under-sized infrastructure that fails during a 1-in-100-year event. Our team runs both Porchet and double-ring infiltrometer tests on site, following AS 1726 and AS 4678. We also cross-check results with a permeability field test in the same borehole to validate consistency. Every test is logged with exact time, depth, and head conditions.
A single infiltration test can save a project from over-sizing soakwells by 30% or more just by using actual field data instead of assumed tables.
Scope of work
A recent residential subdivision in the outer suburb of Springfield Lakes required infiltration tests for 28 detention basins. We deployed double-ring infiltrometers at every second borehole to capture variability across the lot. The Porchet method was used in shallow silty-clay layers where ponding was expected. Typical procedure: excavate a 150 mm diameter hole to target depth, saturate for 24 hours, then measure the falling head over 60-minute cycles. We record data at 1, 2, 5, 10, 20, 30, 45, and 60 minutes. The infiltration rate is calculated using the Porchet formula corrected for side-wall area. For projects requiring a complete geotechnical profile, we combine the test with a study of soil mechanics that includes classification and compaction characteristics. This dual approach gives council engineers the confidence they need for stormwater approvals.
Technical reference image — Brisbane
Area-specific notes
Brisbane sits on a mix of weathered basalt, sandstone, and river alluvium. The city’s average annual rainfall is around 1 150 mm, with most of it falling between October and March. If you skip a proper infiltration test and rely on literature values for urban fill, you risk surface ponding, slope saturation, and even structural damage to adjacent footings. In dense residential areas like Paddington or New Farm, water mounding under a soakwell can saturate reactive clay foundations and trigger differential movement. That’s a liability no builder wants.
150 mm (Porchet) / 300 mm inner ring (double-ring)
Saturation period
24 hours minimum (pre-wetting)
Recording intervals
1, 2, 5, 10, 20, 30, 45, 60 min
Reported value
Infiltration rate (mm/h) at field moisture
Applicable norm
AS 4678:2002, AS/NZS 1170.2:2011
Linked services
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Porchet Infiltration Test (Falling Head)
Ideal for shallow excavations and small-scale stormwater designs. A single 150 mm hole is saturated overnight, then the water-level drop is measured over 60 minutes. Suitable for sandy loams and clayey sands common in Brisbane’s western suburbs.
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Double-Ring Infiltrometer Test (Constant Head)
Used for larger detention basins and council-compliant reports. Two concentric rings maintain one-dimensional vertical flow. Test runs for 2–4 hours until steady-state is reached. Recommended for alluvial plains and reclaimed industrial sites.
Standards used
AS 4678:2002 Earth-retaining structures (referenced for drainage design), AS/NZS 1170.2:2011 Structural design actions — wind actions (storm recurrence), AS 1726:2017 Geotechnical site investigations (field testing procedures)
Frequently asked questions
How much does an infiltration test cost in Brisbane?
A standard Porchet test is between AU$500 and AU$590, including field setup, 24-hour saturation, and reporting. Double-ring tests start at the upper end of that range due to longer monitoring time. Volume discounts apply for multiple boreholes on the same site.
Which Brisbane projects require an infiltration test?
Any development that proposes on-site stormwater detention — residential subdivisions, commercial car parks, and industrial lots — needs one. Brisbane City Council’s stormwater code requires measured infiltration rates for soakwell sizing. If your site has fill or reactive clay, the test is even more important.
Can I use a Porchet test result for double-ring design?
Not directly. The Porchet method measures falling head in a single hole, which includes side-wall infiltration. Double-ring gives a more conservative vertical rate used by engineers for detention design. If your report requires a double-ring value, run the double-ring test — don't calculate it from Porchet data.
