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) .