Laboratory CBR Testing in Wagga Wagga – Reliable Subgrade Assessment for Regional Roads

Wagga Wagga’s road network has expanded steadily since the first Murrumbidgee River crossing was formalised, and with each new subdivision pushing into the former grazing land north of Estella or south of Lake Albert, the need for precise subgrade characterisation grows. The city sits on a mix of alluvial silts and weathered granitic clays that can change stiffness dramatically over a short distance—something we have verified on projects along the Sturt Highway corridor. A laboratory CBR test, run according to AS 1289.6.1.1 with a controlled surcharge and four-day soak, gives us the soaked strength value that RMS and local council engineers require to design the pavement depth. Without that number, you risk either over-excavating good material or placing asphalt on a subgrade that will rut after two wet winters.

A four-day soaked CBR on Riverina clay is not an academic exercise—it is the most reliable predictor of whether your pavement will survive the next flood cycle without expensive patching.

Technical details of the service in Wagga Wagga

There is a marked difference between the residual granitic profiles on the southern slopes around Bourkelands and the transported clay deposits closer to the river at North Wagga. The granite-derived material often yields CBR values above 15% when compacted dry of optimum, but it is sensitive to overworking and can turn to slurry with too much water. The North Wagga clays are plastic, with liquid limits frequently exceeding 50%, and their soaked CBR rarely climbs above 5% even with careful moisture conditioning. Our sampling teams take bag samples from the upper 500 mm of the formation, seal them immediately to preserve field moisture, and remould them in the laboratory to replicate the post-construction density. This comparative approach—testing both ends of the moisture-density curve—has saved several RMS-funded intersection upgrades from premature deformation because the designer could select a realistic CBR rather than a generic table value. We also run companion Atterberg limit tests on the same material to confirm the plasticity index, which helps decide whether lime stabilisation is viable before the pavement structure is finalised.

Laboratory CBR Testing in Wagga Wagga – Reliable Subgrade Assessment for Regional Roads

Parameter	Typical value
Minimum sample mass required	20 kg (passing 19 mm sieve)
Mould diameter	152 mm (CBR mould to AS 1289)
Surcharge mass during soak	4.5 kg annular surcharge per mould
Soak period	96 hours (4 days) immersed in water
Penetration rate	1.0 mm/min ± 0.2 mm/min
Reporting values	CBR at 2.5 mm and 5.0 mm penetration
Compactive effort reference	Modified Proctor (AS 1289.5.2.1)
Typical testing temperature	23°C ± 2°C (controlled curing room)

Critical ground factors in Wagga Wagga

The standard CBR mould is a 152 mm-diameter steel cylinder with a detachable collar and base plate, packed with material compacted at optimum moisture content from a modified Proctor effort. In our Wagga Wagga lab we run the penetration piston at 1 mm/min, reading the load at 2.5 and 5.0 mm against a calibrated proving ring. The most common failure we detect is not outright collapse but gradual strength loss when samples are soaked for 96 hours; many of the red-brown clayey silts from the Glenfield Park area hold up well at placement moisture but drop below 3% CBR after saturation. Local contractors who skip the soaked CBR and rely on field density alone find cracking along the edge line within two years—the pavement base pumps fines upward and the seal debonds. For subdivisions near the floodplain, we often combine the CBR with a site-specific liquefaction screening to check whether the deeper sand lenses could amplify seasonal movement under the formation level.

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Applicable standards: AS 1289.6.1.1 – Determination of the California Bearing Ratio of a soil – Standard laboratory method, AS 1289.5.2.1 – Soil compaction and density tests – Determination of the dry density/moisture content relation of a soil using modified compactive effort, AS 1289.2.1.1 – Determination of the moisture content of a soil – Oven drying method (standard method), RMS QA Specification R71 – Construction of unbound and modified pavements (subgrade acceptance), Austroads Guide to Pavement Technology Part 2: Pavement Structural Design

Our services

Our Wagga Wagga laboratory processes CBR samples within 48 hours of field collection, and we tailor the compactive effort and moisture points to match the earthworks specification. The three service packages below reflect what local contractors and design consultants most often request.

Standard Soaked CBR (Single Point)

One mould compacted at the target moisture and density, soaked for four days under a 4.5 kg surcharge, then penetrated. Includes moisture content before and after soak plus a brief interpretive note on the CBR value relative to typical RMS subgrade categories.

Three-Point CBR with Moisture-Density Relationship

Three moulds compacted at different moisture contents spanning optimum, each soaked and tested. Paired with a full modified Proctor curve, this package lets the pavement designer select the design CBR for a realistic field moisture range rather than a single idealised point.

CBR with Preliminary Lime Demand

For plastic subgrades where stabilisation is under consideration, we run the soaked CBR series alongside an Eades and Grim pH test to estimate the lime addition rate. This package gives the geotechnical engineer enough data to decide between a deeper granular layer and a chemically stabilised working platform.