an engineering geological characterisation of tropical clays - GBV

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1.4 Summary of case problems
The Nairobi city has a current population of about 3 million people (Nairobi City Council, 1999). It
is also characterised by a number of social - economic activities which involve construction of
residential housing and industrial facilities. Other activities involve construction of roads, rail lines,
bridges, airports and sports facilities; power and telephone lines; water and oil pipelines; as well as
sewerage lines. Tunnelling and dam projects are also found. The population of the city is expected
to grow in the coming years. It is also expected that the above - mentioned activities will expand
and/ or multiply, while totally new ones may also be introduced. It is therefore anticipated that there
will be increasing pressure on currently available space and facilities in the city, prompting the need
by the Nairobi City Council and City planners to consider expanding into adjacent areas.
In the past, residential construction on the outskirts of Nairobi city, which include the present study
area, were predominated by structures constructed out of timber and iron sheets, with strip/ pad
footings. However, the recent ten years have witnessed strict regulations put in place by the Nairobi
City Council aimed at encouraging construction of more durable low-rise buildings and bungalows
(Plates 1.1 (a) & (b)) on slab foundations using dimension stones and / or bricks to cater for the
increasing urban population. This shifting trend towards masonry construction justifies a close
study, examination, engineering properties characterisation and assessment of the potential
capabilities and/ or limitations of the soils found, especially the expansive black cotton soils.
Plates 1.1 (a) & (b) New stone structures coming up in the study area.
The current project area is therefore also a potential area of expansion of the Nairobi metropolitan
due to its proximity to the city. It is especially favoured by a number of major communication lines
already traversing it and include the north - south running Nairobi - Mombasa road and the Kenya -
Uganda railway; while the Jomo Kenyatta International Airport and Wilson Aiport are also found
within. However, the ambitious plans by the Nairobi City Council to locate new and light
constructed facilities in the study area may be complicated by the fact that the area is covered by
black cotton soils and also receives heavy rainfall during the wet season. These soils are highly
expansive and reactive, and would usually exhibit very strong shrink-swell capabilities during
alternating dry and wet months of the year, with accompanying general lowering or rise of the
ground surface, respectively (Mitchell,1993; Nelson and Miller, 1992).