An Investigation Of Seasonal Effects On In Situ Test Parameters In Perth Sand

Barry Lehane and Martin Fahey

This paper presents the results of a study into the effect of the seasonal nature of rainfall in Perth on the measurements obtained from CPT tests in unsaturated sands at a site in Perth. The site is located in the Spearwood dune system, which extends in a North-South trend through the central part of the Perth metropolitan region. Rainfall in Perth is concentrated in a wet winter season (June to September), with very little rainfall occurring between about the end of November and the middle of May. The study involved carrying out CPT qc measurements and seismic CPT tests (to determine the initial tangent shear stiffness Go) at the end of the wet season and the end of the dry season at the site. Tests were carried out in ‘open areas’ and in areas with native trees. The results show that there is a very significant seasonal effect on qc and Go values, but that this occurs only the treed areas; in ‘open areas’, there is no seasonal effect. This effect is therefore most likely to be due to the suctions induced by the water uptake by trees.