The contrast between Bendigo's gold-bearing quartz reefs and the alluvial flats along Bendigo Creek creates a tricky subgrade puzzle. Around Eaglehawk, you often hit stiff clay over weathered bedrock, while out near Kangaroo Flat the profile shifts to deeper sandy loams with occasional gravel lenses. That variability means a one-size-fits-all approach to road subgrade design fails fast. We combine laboratory CBR testing with field plate load tests to assign realistic design values. Before setting the pavement thickness, we check the clasificación de suelos to anticipate shrink-swell behaviour and the CBR vial to confirm the bearing capacity. This dual assessment prevents under-design on soft spots and over-design on competent ground.
A subgrade CBR below 2% in Bendigo's wetter months means the pavement section must be thickened or the soil improved — there is no shortcut.
Methodology and scope
In Bendigo, we often see that the top 300–500 mm of the subgrade has been disturbed by historical mining activity or reworked topsoil. That layer needs to be stripped and re-compacted to at least 95% standard Proctor density before any pavement goes down. The key parameters we measure include the soaked CBR at 2.5 mm penetration, the plasticity index of the fines, and the linear shrinkage. A subgrade with PI > 25 and shrinkage > 6% is treated with lime or cement. We also verify the moisture-density relationship using AS 1289.5.1.1. For deep cuts or fills, we integrate terraplenes viales to guarantee uniform support and drenaje vial to control water ingress. Every test is logged with GPS coordinates so the design team can map subgrade zones across the alignment.
Technical reference image — Bendigo
Local considerations
Bendigo grew rapidly during the 1850s gold rush, and many of the current road corridors were carved through shallow alluvial gullies and mullock heaps. That legacy means the subgrade is rarely uniform. Old mine shafts, buried timber, and uncompacted fill exist beneath asphalt that looks fine from the surface. If those zones go undetected, differential settlement cracks the pavement within two or three wet-dry cycles. A thorough subgrade investigation with test pits and dynamic cone penetrometer profiles cuts that risk. We also flag areas where the seasonal water table rises to within one metre of the formation level — that is where the road subgrade design must include a capillary break or drainage blanket to prevent strength loss.
Field sampling at planned pavement depth, laboratory soaked and unsoaked CBR tests, Proctor compaction curves, and Atterberg limits. Report includes recommended design CBR values with statistical confidence intervals for each subgrade zone.
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Pavement Subgrade Improvement Design
When natural subgrade falls below minimum CBR, we evaluate lime or cement stabilisation, geotextile separation layers, and granular capping. We provide mix designs, expected CBR gain curves, and construction specifications tailored to Bendigo's local materials.
Applicable standards
AS 1726:2017 Geotechnical site investigations, AS 1289.6.1.1:2014 Methods for soil CBR determination, Austroads Guide to Pavement Technology Part 2: Pavement Structural Design (AGPT02-17)
Frequently asked questions
What is the typical subgrade CBR for roads in Bendigo?
Soaked CBR values in Bendigo range from 2% in the clay-rich areas of Long Gully to 8% in the sandy profiles near Strathfieldsaye. The design CBR is selected at the 80th or 85th percentile of all test results along the alignment, not the average.
How deep should the subgrade investigation be for a residential street?
We recommend test pits or boreholes to at least 1.5 metres below the proposed formation level. That depth captures the active zone for moisture changes and any relic mining fill. A shallower investigation misses the weak layers that cause long-term rutting.
Do you use the Austroads method or the AASHTO method for design?
We follow the Austroads Guide to Pavement Technology (AGPT02-17) for all Bendigo projects because it accounts for Australian climatic zones and local traffic spectra. The AASHTO method is used as a cross-check only when specified by the client.
What is the cost range for a subgrade design study in Bendigo?
A typical study covering 500 m of road alignment costs between AU$1.760 and AU$5.310 depending on the number of test locations and the complexity of the soil profile. This includes field sampling, laboratory testing, and a design report with CBR recommendations.
