A study on the engineering behaviour of Nairobi subsoil

Abstract

Nairobi City is underlain by volcanic materials that resulted from the formation of the rift valley. Some of the challenges that face design of structures in the city include: the need to develop structures in areas with poor site conditions; distress in structures that could be related to foundation conditions; encounter of unexpected subsoil conditions even after carrying out some geotechnical investigations, and; development of defects related to adjacent deep excavation. The objectives of this study were: to investigate the qualitative and quantitative properties of subsurface materials, to establish the engineering property variations and to provide properties that represent the best estimates. Information available for this study included; Atterberg limits, grading, consolidation, swell and collapse, triaxial shear and direct shear, point load and unconfined compressive strength. Failure investigation and resistivity survey were carried out to fill the gaps in the available information. The results indicate that the engineering properties of the materials are very variable. Material property variations and the expected performance of the various subsoil materials are provided. The study concluded that before construction, it is necessary to carry out geotechnical investigations to delineate sensitive soils, determine weak and strong spots and evaluate the relationship between total and differential settlements.