an engineering geological characterisation of tropical clays - GBV

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The following section includes a summary of the major volcanic rocks underlying the present
study area.
3.3.1 Kapiti phonolite
Borehole records from Nairobi city and the current study area show the Kapiti phonolites as
underlying associated volcanic rocks. They are also observed to overly a sub-volcanic floor of
erosion surface previously cut into Precambriam metamorphic rocks. These rocks therefore
represent the oldest unit of a succession of lavas present in the study area (Gregory, 1921).
Exposures of Kapiti phonolite are observable in the south-eastern part of the study area,
generally in the form of small outcrops confined along some of the stream courses found; and
in the southern part of the area in the tributary valleys of the Athi river. Farther south, outside
the present area, small outcrops also occur in the valley of the Athi river and in the tributary
valleys of Kitengela river; while extensive outcrops are in the valley of the Stony Athi River,
which forms the north-western border of the Kapiti plains. The rock unit descends with a
slope nearly identical to that of the sub-volcanic floor, from 1585 m above sea-level in the
south-eastern part, to1280 m above sea-level in the northern part of the study area and the
Nairobi City centre (Saggerson, 1991).
The lava originated from an area to the south of the present study area and was most probably
derived from vents located on the flanks of the Rift Valley and/ or from the site of the former
Olorgasailie central volcano (Baker, 1958). It was a part of the first Miocene flood eruptions
and flowed generally northwards onto the eroded surface, which was covered in places by
Tertiary conglomerates and grits (Fairburn, 1963). The Kapiti lava also extends westwards up
to Lion Drift in the Nairobi National Park by thinning against the higher ground. The Mua
hills and the Kanzalu range located to the east of the study area acted as a barrier to the
northerly lava flow, and instead, encouraged a north-westerly flow across the area. A further
contribution originated from areas to the north of the study area, where the flows had also
given rise to the formation of the Yatta plateau phonolite.
Borehole records from Nairobi metropolitan area, the study area and around Athi River
township (south of the study area) show the presence of sedimentary strata in the form of
clays and other sediments occurring intercalated between and separating several lava flows
(Gevaerts, 1964).
Exposures show parts of the phonolite to be vesicular in nature, commonly giving rise to
formation of small patches of amygdales filled with calcite and zeolite (mainly natrolite). The
elongated feldspar phenocrysts ( up to 76 mm long) and vesicles frequently display a
preferred orientation (Gregory, 1921; Saggerson, 1991).
Hand specimens of Kapiti phonolite are distinct, and show large crystals of feldspar and
waxy-looking nephelines occurring as phenocrysts set in a fine-grained dark green to black or
dark bluish-grey groundmass. The presence of phenocrysts of these two minerals is
diagnostic, and serves to identify and distinguish the Kapiti phonolite from other lavas across
Kenya (Gregory, 1921).
Thin sections of phonolite show phenocrysts of anorthoclase and nepheline set in an
interstitial groundmass (Häkli, 1960). Serpentinised pseudomorphs and patches of chloritic
material found in the groundmass are probably secondary after olivine, while opaque iron ore