Coffs Harbour sits on a mix of Quaternary alluvium and residual soils derived from metamorphic rock, with groundwater levels often sitting just 2–3 metres below the surface in low-lying areas near the coast. These conditions demand a flexible pavement design that accounts for variable subgrade support and the risk of moisture-induced weakening. Before we set the layer thicknesses, we run a full subgrade assessment using the densidad cono de arena method to verify compaction in the field and complement it with CBR testing in the lab. The result is a pavement structure that distributes traffic loads effectively across the local soil profile.
In Coffs Harbour, subgrade CBR values can drop below 3% after heavy rain, making drainage design as important as layer thickness.
Method and coverage
We design to Austroads Guide to Pavement Technology Part 2 (2017) and AS 1726 for geotechnical site investigations, which is critical in Coffs Harbour because the seasonal rainfall exceeds 1,600 mm annually and can saturate the subgrade quickly. Our process includes:
Traffic load analysis (ESA and design traffic)
Subgrade CBR determination from field and lab tests
Selection of asphalt and granular base materials
Drainage design to prevent water entrapment
For projects on steep blocks we also incorporate estabilidad de taludes into the alignment review, ensuring the pavement sits on a stable formation.
Technical reference image — Coffs Harbour
Regional considerations
Using a falling weight deflectometer or a dynamic cone penetrometer in the field lets us map subgrade stiffness along the proposed alignment in Coffs Harbour. The biggest risk we see locally is pavement failure from inadequate subsurface drainage — many subdivisions sit on clay-rich soils that swell when wet and shrink during dry spells. Without a proper flexible pavement design that includes a drainage layer and sealed shoulders, the surface cracks and the base loses support within a few years. We mitigate this by specifying a minimum 300 mm granular subbase with a permeability above 10⁻⁴ m/s.
In-situ CBR, DCP, and laboratory soaked CBR tests to classify subgrade strength and determine design values. We correlate results with local soil maps from Coffs Harbour City Council.
02
Pavement Structural Design
Layer thickness design using Austroads mechanistic-empirical method. We model traffic loads, material modulus, and fatigue life to produce a pavement that lasts the design period without premature failure.
03
Drainage & Moisture Control Strategy
Design of subsoil drains, edge drains, and impermeable capping layers to keep the subgrade dry. In Coffs Harbour we also check the seasonal water table using shallow piezometers installed during the investigation phase.
Standards that apply
Austroads Guide to Pavement Technology Part 2 (2017), AS 1726:2017 Geotechnical Site Investigations, AS/NZS 1170.0:2002 Structural Design Actions, AASHTO T-307: Resilient Modulus of Soils
Top questions
What is the typical cost range for a flexible pavement design in Coffs Harbour?
For a standard subdivision or access road, the design and investigation package ranges from AU$2,580 to AU$7,360 depending on the length, traffic class, and number of test pits or boreholes required. We provide a fixed-price quote after reviewing the site constraints.
Why does Coffs Harbour need a different pavement design than inland areas?
The coastal rainfall and shallow water table create a wet subgrade environment. CBR values can drop below 3% after prolonged rain, so we design thicker granular layers and include positive drainage. Inland areas with sandy soils often require thinner pavements.
What tests do you run on the subgrade before designing the pavement?
We perform in-situ CBR, dynamic cone penetrometer (DCP), and moisture content profiling. In the lab we run soaked CBR, particle size distribution, and Atterberg limits to classify the soil. All tests follow AS 1726 and Austroads procedures.
How long does a flexible pavement design take from start to finish?
Typically 3 to 5 weeks for a standard project. The field investigation takes 1–2 weeks depending on access and weather, followed by lab testing and the design report. We can fast-track the process for urgent projects by running parallel field and lab work.
