Our field crew runs a 6-tonne plate load test rig behind a Hilux on Hobart's western suburbs. That machine seats a 450 mm diameter plate against the subgrade while four dial gauges record every 0.01 mm of deflection. For rigid pavement design we need the modulus of subgrade reaction — the k-value — not just CBR. We apply the load in increments, hold each for one minute, and plot the stress-settlement curve on site. That data feeds directly into the concrete slab thickness calculation. Before that test we dig a calicata exploratoria to log the soil profile and check for fill or bedrock near the surface.
The k-value from a plate load test at the site is the single most critical input for rigid pavement thickness design in Hobart.
Methodology and scope
- Subgrade investigation — CBR, k-value, moisture content
- Concrete flexural strength specification (4.2 MPa minimum)
- Joint layout and dowel bar sizing
- Fatigue analysis for traffic load spectra
Local considerations
Hobart sits on a mix of dolerite bedrock, alluvial silts, and colluvial clays. The city receives about 620 mm of rain per year, with winter saturation dropping subgrade bearing capacity by 40 % in low-lying areas like Glenorchy. A rigid pavement design that ignores the wet-season k-value risks pumping at joints. We have seen edge curling and corner cracking on industrial slabs designed only for dry conditions. The solution is to run the plate load test at field moisture content in winter, and apply a 0.75 reduction factor to the k-value if the soil is moisture-sensitive.
Explanatory video
Applicable standards
Austroads Guide to Pavement Technology Part 2, AS 1379:2007 Specification and supply of concrete, AG:PT04:2022 Pavement evaluation
Associated technical services
Subgrade investigation and k-value testing
Plate load tests per AS 1289.6.4.1, CBR determination on undisturbed samples, and moisture profiling. We deliver the k-value report within 48 hours of fieldwork.
Concrete mix and joint design review
We review the flexural strength specification, aggregate grading, and joint layout against the actual traffic load spectra. The review includes a fatigue analysis and a joint sealant recommendation.
Typical parameters
Frequently asked questions
What is the difference between CBR and k-value for rigid pavement design?
CBR measures the bearing ratio of a soaked soil sample and is used for flexible pavement. The k-value is the modulus of subgrade reaction measured on site with a plate load test. Rigid pavement design requires the k-value because the concrete slab distributes loads differently than asphalt.
How does Hobart's winter climate affect the subgrade modulus?
Saturated subgrades in Hobart can lose up to 40 % of their stiffness during winter. We recommend running the plate load test at field moisture content in July or August, or applying a reduction factor of 0.75 to the dry-season k-value to account for seasonal softening.
What thickness of concrete slab is typical for a Hobart industrial pavement?
For a standard industrial pavement with subgrade k between 40 and 60 MPa/m, we typically design a 180 to 220 mm unreinforced concrete slab with dowelled contraction joints at 5.0 m spacing. Heavier loads or weaker subgrades push the thickness to 250 mm.
How much does a rigid pavement design study cost in Hobart?
A full design study including subgrade investigation, plate load test, mix review, and joint layout typically costs between AU$3.140 and AU$9.100 depending on the number of tests and the complexity of the traffic load spectra.