Stress-strain and strength properties of an Auckland residual soil

Abstract

The stress-strain and strength properties of a residual soil sampled from the North Shore, Auckland, were investigated through stress and strain-controlled triaxial tests. Emphasis was placed on determining the behavioural characteristics of the soil under conditions of very low effective stress. The soil sampled was a silty clay, derived from the Waitemata Series, with the following average properties: natural water content 45.5%; initial bulk density l707 kg/m3; density of soil particles 2.63 t/m3; plastic limit 32; and liquid limit 60. The peak shearing resistance of the soil was observed to be accurately defined using the Mohr-Coulomb failure criterion, even at very low confining pressures. In addition, the Waitemata clay exhibited a measurable tensile strength of between 7.7 and 12.0 kPa. These results lead to the conclusion that the observed cohesion intercept for the soil could be relied upon for design purposes. The natural variation in void ratio of the Waitemata clay led to the use of total volumetric strain for improved stress-strain correlations. A modified critical state relationship for the soil was subsequently presented, with a unified soil model being used to predict the behaviour of the Waitemata clay. This model demonstrated the ability to replicate the general stress-strain and peak characteristics of the soil. The Waitemata clay did not display the yielding characteristics which are common to residual soils, rather the soil demonstrated continuous yielding behaviour. Anisotropy of the Waitemata clay was also found to be negligible. The use of volumetric strain in the calculation of consolidation properties required only simple modifications to existing consolidation formulae. Bender element tests enabled the small strain shear modulus of the soil to be evaluated. Comparisons of Gmax with the undrained shear strength produced a linear correlation (Gmax =284su) which was significantly lower than expected.