an engineering geological characterisation of tropical clays - GBV

8
and soil behaviour, based on results of field and laboratory investigations. The investigations would
be specific in seeking to establish and/ or model moisture variation with depth, as well as providing
details on depth of cracking, soil reactivity in terms of shrink-swell potential, active depth; and
other essential variables necessary in the assessment and/ or computation of potential ground
surface movement.
In conclusion, this study sought to investigate and present a background on the nature of expansive
and/ or reactive soils of the study area, by following suitable field procedures and sampling criteria
as well as laboratory chemical/ mineralogical and engineering soils testing methods. It also sought
to present results of soil classifications and correlations between soil index and/ or engineering
properties. Results of statistical analyses involving hypothesis testing seeking to determine the
degree of homogeneity or uniformity of the soils found, have also been presented. The undertaking
of this project by the author was also an initiative aimed at providing an approach of study that
would serve as a basis and/ or framework for comprehensive and detailed engineering geological
investigation and assessment of expansive and reactive soils in lithologically similar areas around
Nairobi, and Kenya at large.
1.5 Physiography
The geological history of Nairobi, including the current study area, has been dominated by volcanic
activity since Miocene times. These areas are currently underlain by a series of volcanic rocks as a
result of successive lava flows that originated from centres and fissures on the high eastern flank of
the Rift region to the west (Gregory, 1921; Saggerson, 1991; Fig. 3.1; Plates 1.7 (a) & (b)).
Plates 1.7 (a) & (b) View of the Great Rift Valley from its eastern flank, to the west of Nairobi area.
The flow of lava was generally eastwards, onto a warped and partly dissected pre-Miocene erosion
surface below which were older crystalline metamorphic rocks. A few isolated occurrences of rocks
of Precambrian and Quaternary age are also found.
The geomorphological evolution of Nairobi and the study area has therefore been mainly controlled
by the volcanic activity that accompanied the Rift Valley formation. The physiography of these
areas is consequent upon the volcanic rocks found, and the tectonic movements which have affected
them (Saggerson, 1991; Sikes, 1926).
The study area is a part of the lava plains (Plate 1.8 (a) & (b))that are bordered to the north-west and
west by the Kikuyu highlands, an extension of the high ground of the eastern flank of the Rift
Valley; and to the south-west by the Ngong hills. The Kikuyu highlands are characterised by a steep
downward slope in an easterly direction. The drainage is consequent, follows the slope and
comprises of a number of streams giving rise to parallel ridges, some of which get broader as the
streams converge. The streams are still down-cutting giving rise to ridges with convex to uniform