an engineering geological characterisation of tropical clays - GBV

7
Available literature in the form of geological reports (Fairburn, 1963; Gregory, 1921; Saggerson,
1991; Sikes, 1939) is also regional in approach and dwell mainly on rock formations, while the
soils are merely described as recent to present surficial deposits. Two reports are available on soils
(Gethin - Jones, 1952; Scott, 1963) but the aspects contained are discussed and treated from an
agricultural perspective and therefore almost entirely irrelevant and of little value to the needs of
geo-engineers and city planners. Site investigation reports from isolated areas over the city are also
available (Anyumba, 1991; Manga, 1988; Mowlem, 1985). They report mainly on the state and
condition of the underlying bedrock, the investigations having originally been meant for more
versatile engineering projects involving heavy foundation loads (Plates 1.6 (a) & (b)). As a result,
little useful information, if any, on geotechnical characteristics of soils is included.
Plates 1.6 (a) & (b) Heavy structures/ buildings in a zone of black clays in the study area.
The present study and research therefore aimed to investigate, assess and establish the engineering
geological characteristics of soils, and especially as regards the potentially expansive and reactive
black cotton soils in the present area, for geotechnical purposes. Attempts were made to present the
results of the investigations in geotechnical criteria and/ or such a similar form that would be readily
interpreted and applied by geo-engineers and city planners.
Best fitting and/ or closely approximating models to describe, present and predict distribution and
spatial variation of important ground information and engineering soil properties across the study
area have been developed. Use was made of systematically determined soil parameters, both
laterally as well as with depth, across the area.
Indirect characterisation of engineering soil behaviour in situ (shear strength, compressibility,
shrink-swell potential, swelling pressures) has also been made through correlation between results
of index tests performed on disturbed and part samples on one hand, and those of the above
mentioned engineering properties performed on core and/ or undisturbed samples, on the other
hand. The modelled results may be usefully applied by geo-engineers and city planners in readily
anticipating probable geotechnical soil behaviour with respect to projected structures at chosen
sites, based on results of soil index tests. Possible forms of interrelationship between soil index
properties and Atterberg limits were also investigated.
In short therefore, this study embarked on accumulating relevant site specific information across the
study area. The information involved soil properties as well as environmental characteristics such as
climate, groundwater conditions and vegetation. The approach followed in site characterisation
included among others the examination, assessment and interpretation of the performance of
existing constructed structures and buildings; examination, description and classification of soil
profiles; as well as computation and prediction of potential ground movements