Geological evaluation of subsurface suitability for shallow building foundations In Nairobi and the surrounding area

Abstract

This thesis work presents the results of geological and geotechrucal investigations carried out in the Nairobi area. Kenya. The study was undertaken with the aim of obtaining and providing information that could help in assessing the suitability, and capacity of the soils of the study area. to support shallow foundation engineering structures. This information could in turn assist in the design and construction 0;' projected shallow foundation structures. The study was centered on determining and, or establishing the various soil classes: and their lateral extent, thickness and possible genetic origin Other aspects of the study were aimed at establishing the position and nature of the bedrock, the general physiography of the study area and the geotechnical characteristics of the soils. The field investigations incorporated an integrated approach The methods used included photo - interpretation. geological reconnaissance and mapping. and auger - boring and disturbed soil samples collection. The western and north - western parts of the study area are predominated by the dark red friable clays and the red friable clays. Also found in this zone are the dark grey mottled clays and the shallow yellow-brown to yellow-red friable clays. Minor and isolated occurrences of shallow stony soils and peaty swamps are also found. The eastern and south - eastern sections of the study area are covered mainly by the black to dark grey clays and the dark greyish - brown calcareous clays. Also found but to a lesser extent are the shallow yellow-brown to yellow-red friable clay s. shallow stony soils and alluvium. The laboratory analysis of the collected soil samples involved soil classification testing (grain size analysis; natural moisture content; Atterberg limits: and free swell tests). More elaborate soil testing procedures involved CD shear strength tests and subsequent soil bearing capacity evaluation; and oedometer consolidation testing and subsequent evaluation of soil compressibility and settlement characteristics. The results show the mottled clays, the black clays and the calcareous cia; s to be of high plasticity and apparently more expansive. In addition the clays exhibit limited strength and stability and significant compressibility and settlement characteristics. Under normal conditions of occurrence these clays are generally not suited for shallow foundation engineering purposes. On the other hand the low to medium plasticity shallow yellow - brown to yellow - red friable clays exhibit some significant strength and stability and slightly limited compressibility and settlement characteristics. However. the soils are generally limited in depths and unsuitable for shallow foundation engineering purposes. The dark red clays and the red clays are generally inactive and exhibit very limited expansive behaviour. They represent a relatively more stable group of soils and are generally suited for shallow foundation engineering purposes. On the basis of the results obtained. it is recommended that the first 0.5 m thickness of the black clays, calcareous clays and grey mottled clays should be mass - excavated to allow for the partial or total compensation of projected shallow foundation structures. It is also recommended the total excavation of the shallow yellow-brown to yellow-red clays and the placement and construction of projected foundation structures right from the underlying bedrock.