an engineering geological characterisation of tropical clays - GBV

2
In most developing countries including Kenya, there are difficulties of undertaking and / or
executing an exhaustive and comprehensive soils investigation programme time and again, due to
economic constraints related to limited finance as well as lack of adequate expertise and/ or
equipment. In such cases, the use of soil index properties to infer for soil reactivity and stability
characteristics could be partly a solution to the problem. This study attempts to correlate soil index
parameters with those related to shrink-swell potential, swelling pressures, shear strength and
consolidation-settlement (compressibility); in an effort to find closely approximating and/ or fitting,
and therefore predictive relationships between them.
Engineering geological investigations and mapping of soils carried out in this study was in
accordance with known standard practices and/ or procedures (IAEG, 1981a/ b; IAEG, 1976 ).
1.2 Location of the study area
Nairobi city is currently bounded by latitudes 1°15´S and 1°20´S to the north and south ; and by
longitudes 36°45´E and 36°55´E to the west and east; respectively (Fig. 1.1). It is the largest town
in Kenya and is located about 30 km to the east of the north - south running Great Rift Valley
(Saggerson, 1991; and Fig. 3.1). It extends for about 18,7 km east - west and about 9,3 km north -
south, covering an area of about 174 km² (Nairobi City Council, 1999). The western and northwestern
sections of the city are a part of the Kikuyu highlands that rise from an altitude of about
1660 m at the City centre to over 1800 m above sea level on the immediate eastern flanks of the Rift
Valley. These sections are covered by thick deposits of red soils that increase in their depth
westwards and north-westwards from about 4 m at the City centre to over 10 m in the vicinity of the
Rift Valley, outside the present area. The soils are underlain by a series of Tertiary volcanic rocks, a
result of several episodes of volcanic activity that accompanied the formation of the Great Rift
Valley (Fairburn, 1963; Gregory, 1921; Saggerson, 1991; Sikes, 1939).
The southern and eastern sections of the city are a part of the generally flat to gently rolling Athi
and Kapiti plains. The plains are covered by thick surficial deposits of black cotton soils that are
underlain mainly by volcanic rocks; while isolated exposures of Basement system Precambrian
metamorphic rocks also occur (Saggerson, 1991).
The present study area is largely located to the immediate south of Nairobi metropolitan which is a
part of the Athi and Kapiti plains. The remaining part of the area to the north forms the southern
half of Nairobi city. The whole project area is bounded by latitudes 1°15´S and 1°24´S to the north
and south; and longitudes 36°45´E and 36°58´E to the west and east ; respectively. It occupies an
area of about 367,38 km² in a zone of mainly black cotton soils; and to a lesser extent, red soils
which are confined in the north-western section of the area (Scott, 1963; Surv. of Kenya, 1990).
1.3 Definition of study problem
The undertaking of the present research study has been prompted by a number of factors, and
include:-
- changing residential construction practice
- widespread damage to roads/ pavements, residential and low rise buildings as a result of
expansive movements in areas of black clays
- lack of sufficient data to facilitate an engineering characterisation of the soils found
- predicted urban population growth of the Nairobi city
- predicted growth of services and industrial activities in the city
- predicted growth and expansion of the Nairobi city into its surrounding areas