The contrast between the estuarine clays of the Pioneer River floodplain and the colluvial slopes of Mount Bassett is stark. On the flat, soft alluvium dominates — up to 12 metres of soft to firm silty clay with high plasticity. Up on the hills, residual soils from the Hodgkinson Formation are sandy and well-drained. A single geotextile specification will not work for both. We see this every week. In Mackay, selecting the right geotextile requires understanding the subgrade strength, the depth to groundwater, and whether the site sits on the old cane paddocks or the rocky ridges. That is why we tailor each geotextile specification to the actual ground conditions. For sites where differential settlement is a concern, we often pair the geotextile design with a densidad-cono-arena field test to verify compaction of the working platform before the geotextile goes down.
In the soft alluvium of the Pioneer floodplain, we often specify geotextiles with 50% elongation to accommodate the high strains that standard fabrics cannot handle.
Methodology and scope
A common mistake we see in Mackay is contractors ordering a woven geotextile based on road class alone, ignoring the actual CBR of the subgrade. That is a shortcut that can cost thousands. The right approach starts with a site investigation. We measure the in-situ moisture content and Atterberg limits, then run a soaked CBR on a recompacted sample. With those numbers, we calculate the required grab strength and elongation per AS 4678. The standard demands a minimum 5% elongation at peak for separation fabrics, but in the soft estuarine soils near the marina, we often specify 50% elongation to handle the high strains. The table below shows typical parameters we use for Mackay. For sites with high water tables, we also recommend a permeabilidad-campo test to confirm the drainage layer design before selecting the geotextile type.
Technical reference image — Mackay
Local considerations
Mackay grew fast in the 1960s and 70s, pushing suburban development onto the old swampy floodplains west of the city centre. Many of those subdivisions sit on soft estuarine deposits with high organic content. The risk is straightforward: inadequate geotextile specification can lead to intermixing of the subgrade with the granular fill, causing a soft, pumping pavement within two wet seasons. We have seen it happen on access roads in the Racecourse area. A properly specified non-woven geotextile with high permittivity and adequate puncture resistance prevents that mixing. The cost of re-specifying after failure is easily three times the upfront testing cost. In Mackay, where the water table sits at 1.5 m depth for much of the year, the geotextile specification must also account for filtration — preventing fine particle migration without clogging.
1.2 kN – 2.5 kN for separation; > 4.0 kN for unpaved roads
Elongation at peak (AS 1289)
≥ 50% for high-strain sites; ≥ 15% for standard fills
Apparent opening size (AOS)
0.30 mm – 0.60 mm for filter applications
UV resistance (AS 3706.9)
70% retention after 500 h exposure
Permittivity (AS 1289)
0.5 – 1.5 s⁻¹ depending on drainage requirement
Associated technical services
01
Geotextile selection report
Full laboratory testing of subgrade and fill samples, calculation of required tensile strength, elongation, and AOS per AS 4678, and a written specification with fabric grade recommendations. Suitable for subdivision roads, rail embankments, and canal levee works.
02
On-site verification testing
Field sampling of the installed geotextile and laboratory conformance testing to the specified AS 3706 parameters. Includes grab strength, seam strength, and permittivity checks. Essential for quality assurance on government-funded projects in Mackay.
Applicable standards
AS 4678 – Earth-retaining structures (general principles for geotextile selection), AS 3706 – Geotextiles – Methods of test (grab strength, puncture, permittivity), AS 1289 – Standard test method for grab breaking load and elongation of geotextiles, AS/NZS 1170.0 – Structural design actions (applied to fill surcharge loads)
Frequently asked questions
What is the typical cost range for a geotextile specification report in Mackay?
A standard geotextile specification report, including subgrade testing and fabric selection per AS 4678, ranges between AU$600 and AU$2,010 depending on the number of test locations and the complexity of the site geology. Costs increase if additional CBR or permeability testing is required.
How do I know if I need a woven or non-woven geotextile?
It depends on the primary function. For separation and filtration — typical in Mackay's soft alluvium — non-woven fabrics with high permittivity (0.5–1.5 s⁻¹) are standard. For reinforcement or stabilisation over very weak subgrades (CBR < 2%), woven geotextiles with high tensile strength (> 4 kN grab) are often specified.
Does AS 4678 cover geotextile specification for retaining walls?
AS 4678 provides general principles for earth-retaining structures, but it does not give specific geotextile selection criteria for wall drainage layers. For that, we refer to AS 3706 for fabric properties and to the project-specific wall design. We always check the drainage requirements separately.
How long does the geotextile specification process take?
A typical specification report takes 5–10 business days from sample receipt. The subgrade CBR and Atterberg limits testing takes 3–5 days, and the fabric selection analysis another 2–3 days. If on-site verification testing is needed, add 2 days for sampling and 5 days for conformance testing.
Do you test geotextiles from any manufacturer?
Yes. We test samples from all major manufacturers. Our laboratory is NATA-accredited for AS 3706 tests including grab strength, elongation, permittivity, and apparent opening size. We do not favour any brand — the specification is based purely on the required performance parameters for the site.
