an engineering geological characterisation of tropical clays - GBV

9
slopes. The northern boundary between the two physiographic units of the Kikuyu highlands and
the plains is roughly along the east - west line across the Nairobi city centre (Sikes, 1926). The
plains are made up of two parts, i.e. the Athi plains and the northern section of the Kapiti plains,
both of which extend further southwards and eastwards, respectively, beyond the study area. These
plains are underlain by a succession of lava flows which usually alternate and/ or intermingle with
lake beds, stream deposits, tuffs and volcanic ash (Saggerson, 1991).
Plate 1.8 (a) Flat plains around Embakasi station. Plate 1.8 (b) Flat plains around J.K.I.A. Airport.
The Athi plains rise from an altitude of about 1500 m at Athi River, located about 8 km south-east
of the present area; to about 1800 m above sea level farther west in the faulted region near Ngong.
The plains form gently rolling grasslands with a fairly even surface. They end in an abrupt
escarpment just west of the Athi river. The abrupt escarpment represents the end of a lava flow. In
addition, some smaller and low east-facing scarps also occur within, and represent the margins of
partly eroded lava flows (Gregory, 1921; Saggerson, 1991). The plains are being dissected by
down-cutting rivers (over 30 m below the general land surface in some places) currently flowing
into the main Athi river. Some steep-sided valleys are also found in the form of gullies and canyonlike
gorges cutting into lavas such as along the Nairobi, Mathare and Gitathuru river valleys in the
northern part, as well as the tributaries of the Athi river which include Mokoyeti, Sosian and
Mbagathi river valleys in the southern part of the study area, respectively. On the other hand, areas
with soft material are characterised by relatively wide valleys, and this is noticeable in lower
sections of the Nairobi river valley, as well as along other stream courses in the southern and southwestern
parts of the study area. The plains are mainly underlain by phonolitic volcanic rocks and
tuff (Saggerson, 1991).
The Kapiti plains occur to the east of the Athi river, where the topography is more variable, from
gently undulating to flat with a number of small hills protruding above the general land surface.
One large hill mass, the Lukenya, is found located in a metamorphic region of the plains to the east
of the present area. Underlying the plains are volcanic rocks, tuff and metamorphic rocks. The
plains are located in a drier region having no permanent rivers; and the seasonal stream-beds found
contain water for only short periods of time in a year. The dissection of these plains is therefore
relatively less severe. Gully erosion is, however, common and characteristic.
The Athi and Kapiti plains extend farther south-eastwards and eastwards beyond the present study
area, where they are bounded by the central hill masses of Machakos District (Machakos hills).
These consist of the Ithanga, Mua and Iveti hills as well as the Kanzalu range and Donyo Sabuk.
The hills rise abruptly above the surrounding plains. They are also steep-sided and being actively
dissected, partly as a result of existing poor land management.
There is also a general rise in relief from the south-east towards the north-west, across the study
area. Elevations of 1540 - 1600 m above sea level occur in the south-eastern parts; 1620 - 1680 m