Pavement Design

Pavement design is the process of determining the appropriate thickness and composition of pavement layers to safely distribute traffic loads to the underlying subgrade. In Australia, pavement design follows the Austroads Guide to Pavement Technology, using the California Bearing Ratio (CBR) method for structural design.

What Is Pavement Design?

Pavement design involves selecting the right combination of materials and layer thicknesses to create a durable, safe, and cost-effective pavement that can withstand predicted traffic loads and environmental conditions over its design life.

Types of Pavements

Flexible Pavement (Asphalt)

A flexible pavement consists of multiple layers that distribute loads from the surface down to the subgrade. The top layer (wearing course) is typically asphalt, with granular base and sub-base layers underneath.

• Most common in Australia for roads and highways
• Allows for staged construction
• Easier to repair and maintain
• Lower initial cost than rigid pavement

Rigid Pavement (Concrete)

Rigid pavement uses a Portland cement concrete slab as the surface layer, which provides high flexural strength.

• Used for high-traffic roads, airports, and industrial pavements
• Longer design life (30–40 years)
• Higher initial cost but lower maintenance
• Requires load transfer at joints

Interlocking Concrete Block Pavement

Used for low-speed areas, footpaths, and heavy-duty industrial areas.

The Austroads Pavement Design Method

The Austroads method is the standard pavement design procedure across Australia. It is a mechanistic-empirical method that considers:

1. Design traffic — number and type of heavy vehicle axle loads over the design life
2. Subgrade strength — design CBR of the prepared subgrade
3. Pavement materials — modulus and fatigue characteristics of each layer
4. Climate — rainfall, temperature, drainage conditions
5. Reliability — design confidence level

Design Traffic

Traffic is expressed in terms of Equivalent Standard Axles (ESA) — the number of 80 kN single axle loads that would cause equivalent pavement damage.

Subgrade Design CBR

The subgrade CBR is the most critical parameter in pavement design:

Pavement Layer Components

Pavement Design Process

Step 1: Traffic Assessment

Determine the expected traffic volume and heavy vehicle loading over the design life (typically 20–40 years).

Step 2: Subgrade Investigation

• Conduct CBR testing of the subgrade
• Determine Design CBR (80th–85th percentile)
• Assess drainage conditions

Step 3: Material Selection

Select appropriate pavement materials based on availability, cost, and performance requirements.

Step 4: Structural Design

Using Austroads software (such as CIRCLY or equivalent) or design charts, determine the required layer thicknesses.

Step 5: Pavement Verification

After construction, verify that:

• Achieved compaction meets specifications
• Material properties match design assumptions
• Subgrade CBR meets design values

Australian Standards and References

Common Pavement Design Issues