Project Status: Completed


Continuing 35 years of research, WARRIP is investigating the field performance of thin asphalt surfacings on granular pavements constructed along parts of the Tonkin and Reid Highways. Previous trial sections have performed exceptionally well such that the challenge is to improve value-for-money outcomes using a recently developed revised Austroads design method.


Review of the design procedures for unbound granular pavements with thin asphalt surfacings

WARRIP investigated the better-than-expected performance exhibited by the trial sections, which were originally designed to fail within two years. However, these are still performing up to 35 years later without any major overlays or rehabilitation


With the continuing performance not only of these trial pavements, but also of the majority of metropolitan
granular pavements, Main Roads proposed further investigation of the better-than-expected performance.
The aim is to reduce conservatism within the current pavement design procedure for granular pavements
with thin asphalt surfacings, and in-turn, improve value-for-money outcomes.


  • Review of design pavement profile and assumptions
  • Review of subsurface and climatic conditions
  • Review of construction data
  • Review post-construction monitoring
  • Traffic history
  • Review of performance data, including historic Benkelman beam (BB), falling weight deflectometer (FWD), and roughness and rutting data.

Further to the data review, field investigations of Tonkin and Reid Highway was undertaken to confirm the
pavement profile, collect samples for laboratory testing and conduct FWD testing.

Following the data analysis, a review of the current design procedures outlined by Main Roads Engineering
Road Note 9 (Main Roads 2013) and Austroads Guide to Pavement Technology Part 2: Pavement Structural
Design (Austroads 2012) was undertaken.


For each trial section, the cumulative traffic loading was compared to the allowable traffic loading predicted
by the current design method. It was determined that for all trial sections the cumulative traffic loadings were
greater than the predicted fatigue lives.

This disparity between the measured and allowable traffic suggests that all sections should be showing signs
of fatigue cracking. However, this has not been observed at either the Tonkin Highway, Reid Highway or
Kwinana Freeway trial sites.


In situ base, subbase and subgrade moduli were back-calculated from measured surface deflections. The
moduli of the sand subgrade exceeded 150 MPa which is larger than the allowed maximum of 120 MPa,
stipulated by ERN9. In addition, the moduli of the limestone subbase was greater than the currently assumed
250 MPa.

These findings indicate that presumptive moduli and the current method of elastic characterisation is needed to more appropriately reflect the structural contribution of the sand subgrade and limestone subbase to the fatigue performance of thin asphalt surfacings.

Such a change to the elastic characterisation may enable the use of these cost-effective granular pavements in place of more expensive full depth asphalt pavements


Austroads 2012, Guide to pavement technology:
part 2: pavement structural design, 3rd, edn,
AGPT02-12, prepared by G Jameson, Austroads,
Sydney, NSW.

Main Roads 2013, Engineering road note 9:
procedure for the design of road pavements, Main
Roads, Perth, WA

Source: ARRB 2016.
  • Observations were based on three separate trial locations with varying designs, traffic levels, conditions, age and specification conformance.
  • All have shown similar performance trends throughout their service life.
  • Conduct further investigation through analysis of other sections of road to verify observations.

Future considerations