Suction measurements and Resonant Column-Torsional Shear (RCTS) testing of expansive clays from Africa

The work described in this thesis was carried out at the University of Cambridge in the wider context of the WindAfrica research project, a collaborative effort involving the Universities of Durham, Cambridge, Pretoria, Khartoum and Dar Es Salaam. This project aims to develop a set of design guidelines for piled wind turbine foundations in expansive clay supporting the growth of a sustainable energy market in Africa. In particular, this thesis focuses on the characterisation of two tropical expansive clays from two locations in Africa: Black Cotton Clay from Al Fao, Sudan (SUD) and Atta Clay from Steelpoort, South Africa (SA). Firstly, a review of the technical literature on the mechanical and hydro-mechanical behaviour of unsaturated soils is presented, together with a description of the distribution of tested tropical clay soils (geology, climate, localization and sampling procedures) . The initial characterisation results include soil particle size distribution (PSD) by sedimentation hydrometer analysis, Atterberg liquid limit by using fall cone method and plastic limit. According to the experimental program the hydraulic behaviour of tested materials is investigated under unsaturated conditions, by measuring sample suction through a WP4C device, in order to plot Soil-Water Retention Curves (SWRCs) of tested clays and to estimate parameters of the best fitting curves according to Van Genuchten’s model. On the other hand, small strain mechanical behaviour was investigated only under saturated condition, using a suction-controlled Resonant Column Torsional Shear apparatus (RCTS), designed specifically to test partly saturated soils by measuring the shear stiffness (G) and the damping ratio (D) in isotropic stress conditions. A detailed description of each part of the RCTS apparatus is presented, together with all preliminary steps of saturation of both water circuits and the High Air-Entry Value (HAEV) porous stone. After the initial set up of the apparatus, consolidation and loading phases are described and, in particular, a resonant column test, a torsional shear test and a decay test were carried out on Black Cotton Clay. Finally, results are presented in terms of obtained normalized shear modulus and damping ratio variation curves and comparisons with empirical correlations and Italian case studies.