Investigation of construction practices and test procedures for road pavements on expansive subgrades

Abstract

[Abstract]: Expansive subgrades in the Toowoomba District have been contributing to pavement failures and subsequently causing increased annual maintenance expenditure. The Department of Main Roads Toowoomba have been unsuccessfully constructing roads on expansive soils, which meet the full life expectancy. Inaccurate testing procedures and pavement designs are believed to be the primary causes of these failures. The Department of Main Roads and other road agencies commonly use standard four (4) day soaked CBR tests to measure the swell potential of an expansive soil sample. Main Roads specifications state the use of a four (4) day test four granular material, and ten (10) day soaked CBR testing for more expansive materials. Many soil laboratories do not carry out ten days tests because of time restraints, funding restraints and the conception that ten day tests do not provide additional information. Forensic investigation into the comparison between the four day and ten day test was carried out. Recommendations on testing procedures and subsequent accuracy of the four day testing methods are discussed. Compilation of the Pavement Performance Profile required analysis of yearly data relating to structural, functional and other attributes. The functional attributes of Rutting and Roughness were considered as they correlate well with road user expectations. Structural performance assessments were based on CBR strength, including insitu CBR and Compressive Cylinder tests which indicated the pavements structural capacity to withstand current and future loadings. Traffic Loadings and Maintenance costs, both historical and projected, were also integrated into the Profile. Forensic Investigation into the current failures included conventional methods and the modern technologies of Loaded Swell Testing (LST) and Hydrometer Analysis. Investigations into construction practices such as compaction, moisture addition and application of overburden was carried out through varied CBR test methods. The Pavement Performance Profile and Forensic Investigation are reconciled to produce an interrelationship between performance and failure. This interrelationship formed the basis for construction guidelines and design recommendations for road construction on expansive soil. The methodology developed for this study is reviewed and considered for its application to other construction projects completed by the Department of Main Roads.