The falling weight deflectometer truck rolls into Launceston early, its hydraulic loading plate ready to measure pavement deflection on the Tamar River floodplain. Our field team deploys dynamic cone penetrometers and nuclear density gauges across the site, collecting data on the existing subgrade and base layers. For flexible pavement design in Launceston, we rely on these in-situ tests to determine the structural number needed. The city sits on Quaternary alluvial deposits and Tertiary basalts, which produce highly variable subgrade moduli. We combine DCP results with laboratory CBR tests and resilient modulus testing from repeated load triaxial equipment. This data feeds directly into the AASHTO 1993 design equation for flexible pavements. Before finalizing, we also run a subrasante vial assessment to confirm the formation level stiffness, and cross-check with ensayo Proctor compaction curves to set target density.
A single CBR test on Launceston floodplain can mislead the entire pavement design. We sample every soil unit encountered.
Methodology and scope
A common mistake in Launceston road projects is assuming uniform subgrade strength across the site. The city spans from river flats to hilly basalt ridges near the Cataract Gorge. A contractor once designed a 150 mm asphalt overlay based on a single CBR test at the lowest point. The pavement failed within two winters on the higher basalt sections where the subgrade was much stiffer. Our flexible pavement design approach avoids this by requiring a minimum of one CBR test per 500 m per lane, plus a permeabilidad-campo survey to check drainage potential in the saturated zones. We also run limites-atterberg on every soil type encountered. Plasticity index data helps us predict swelling and shrinkage behavior that can cause longitudinal cracking in asphalt surfaces. These tests are not optional for Launceston's mixed geology.
Technical reference image — Launceston
Local considerations
A developer built a residential subdivision near Prospect Vale without subgrade drainage consideration. Launceston receives about 665 mm of rain annually, concentrated in winter. Water infiltrated the pavement base through a cracked asphalt surface. Within three winters, the base course lost 40% of its modulus due to moisture weakening. Pumping of fines through the joints created depressions at the cul-de-sac turnaround. This could have been avoided with a proper flexible pavement design that included a subgrade drain layer and a minimum 2% crossfall. We now mandate a drainage check before any pavement design in Launceston.
AASHTO 1993 Guide for Design of Pavement Structures
Subgrade CBR Range (Launceston)
2% (floodplain clay) to 15% (basalt residual)
Traffic Loading (ESA)
0.1 to 10 million equivalent single-axle loads
Asphalt Layer Modulus (EAC)
2500–4500 MPa at 25°C
Base Course Material
Unbound crushed rock or cement-treated, 200–300 mm thick
Subbase Thickness
150–300 mm, adjusted for frost protection in winter
Design Life
20 years for urban roads, 40 years for highways
Associated technical services
01
Pavement Structural Design Report
Full AASHTO 1993 analysis with traffic load projection, subgrade CBR mapping, layer thickness optimization, and material specifications. Includes a sensitivity analysis for floodplain clay zones.
02
Field & Laboratory Testing Package
DCP, FWD, and nuclear density gauge surveys on site. Laboratory CBR, resilient modulus, and compaction curves. All testing is NATA-accredited under ISO/IEC 17025.
Applicable standards
Austroads Guide to Pavement Technology Part 2: Pavement Structural Design (AGPT02-17), AS 1726:2017 Geotechnical Site Investigations, AASHTO T 307: Determining the Resilient Modulus of Soils and Aggregate Materials, AS 1289.6.1.1 Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils
Frequently asked questions
What is the difference between flexible and rigid pavement design?
Flexible pavement uses asphalt layers that distribute load through the structure, relying on the subgrade for support. Rigid pavement uses a concrete slab with high flexural strength. For Launceston's variable subgrade, flexible design handles differential settlement better and costs less to maintain.
How much does a flexible pavement design study cost in Launceston?
A full design study including field testing, laboratory work, and report ranges from AU$2,210 to AU$6,970 depending on site size, number of CBR tests, and traffic load complexity. Contact us for a quote tailored to your project.
Why is Launceston's subgrade so challenging for pavement design?
Launceston sits on the Tamar River floodplain with deep Quaternary clays that have low strength when wet, plus basalt residuals on the hills that are highly permeable. The water table can rise to within 1 m of the surface in winter, reducing subgrade modulus by 50% or more. Flexible pavement design must account for this seasonal variation.
Do you use the Austroads or AASHTO method for pavement design?
We use the AASHTO 1993 method as the primary design approach, calibrated to Austroads traffic loading categories. We also cross-check with the empirical DCP-based method from AGPT02-17 for local validation. Both methods produce compatible layer thicknesses for Launceston conditions.
