The engineering geological controls on the geotechnical behaviour of Waitemata Group residual soils

Abstract

Residual soils are developed from the weathering of in-situ geologic materials. Although they have been extensively researched globally, the focus has been placed primarily on tropical soils and on clean soils which do not occur naturally in the soil profile. This research has neglected the importance of the geologic materials from which the soils are sourced. Furthermore, there is a lack of details pertaining to New Zealand residual soils. Hence this thesis is aimed at providing details of the geological and geotechnical properties of Auckland residual soils. In-situ block samples soils were collected from two locations in Auckland to reduce soil disturbance. The techniques used to characterise these soils were X-ray diffraction, scanning electron microscopy, particle size analysis, atterberg limits, phase relationships and direct shear testing. The results obtained show that soils are highly variable at a microscale. Clays adhere to sand grains and also cluster together within pore space as small groups of platelets or in large agglomerations. The soils were composed of quartz and kaolinite, the latter having a greater proportion in West Auckland samples. The presence of more clay, which in general is also associated with greater moisture contents, is very influential on the behaviour of the soil. These properties are associated with lower shear strengths, with a notable decrease occurring above 25% to 30% clay content. Cohesion and friction angle are both influenced by clay and moisture content, with cohesion increasing with both parameters due to the attraction of clay minerals that occur below 20% moisture content, and friction angle decreasing with moisture content and increasing with clay content, reflecting the washing of fines from between sand grains at lower clay contents and higher moisture contents. These findings are significant as they provide relevant geological and geotechnical properties of Auckland residual soils, which have not been adequately studied previously. They also highlight the significance that clay has on soil behaviour, and show the importance of carrying out detailed field studies in order to fully understand the behaviour of the subsurface soil to ensure the success of future engineering projects.