A research project was carried out in 2001–02 to investigate subgrade soil water conditions of road pavements in New Zealand, in particular the applicability of soaked or unsoaked test specimens, and seasonal influences on subgrade stiffness.
Test sections were established on three roads in the Auckland region. The test sections were subjected to four rounds of field tests over a period of two years to determine subgrade water content and strength/stiffness properties. Standpipes were installed to measure ground-water levels and subgrade samples were taken for laboratory (soaked) CBR tests.
The results showed very few correlations between the various subgrade test parameters measured. No clear relationship was found between rainfall records and subgrade water content or in situ CBR. A reasonable correlation was found between the ground-water level and the rainfall record at one test site, and the in situ CBR and dynamic cone penetrometer-inferred CBR showed reasonable correlation.
The laboratory CBR tests show that soaked soil conditions would be appropriate for two of the sites but overly conservative for the third site. The observations are considered to be consistent with the topographical features of the various sites.
A simple method for determining the applicability of soaked or unsoaked parameters is presented, based on an assessment of the potential for the ground-water level to be within 1 m of the top of the subgrade. Factors that influence the ground-water level are discussed.
Keywords: drainage, moisture, New Zealand, pavements, pavement design, roads, soaked CBR, subgrade SBR, subgrade drainage, subgrade moisture