ANALYSIS OF THE YATTA-CANAL INTAKE VALLEY ALONG THIKA RIVER IN
KENYA FOR A STORAGE RESERVOIR BASED ON GEOPHYSICAL AND
GEOTECHNICAL SURVEY DATA
ABSTRACT
The aim of this work was to carry out an assessment on the suitability of the surface and
subsurface geological conditions of a valley occurring within the precincts of the Yatta-canal
intake area along River Thika for a storage reservoir. The Yatta District is one of the arid and
semi-arid Districts in Kenya. Over 70% of the population in Yatta depends on agriculture for
their livelihoods. Because rains are locally erratic and unevenly distributed, the poverty levels
are high in the area at 67.5%.The reservoir will ensure a hitherto perennial flow of water in the
canal. The Yatta area is situated within the Mozambique Belt, a major structural/metamorphic
unit which extends along the African east coast from Mozambique and Malagasy into the Sudan
and possibly as far north as Egypt and Arabia; it represents one of the fundamental geological
features of Africa.
The old geological map of the region depicts that the study area consist only of boitite gneiss but
the
facts
on
the
ground
reveal
the
existence
of
more
than
one
geological
formation though biotite gneiss is predominant. Both Geophysical and Geotechnical techniques
were employed in this work. The logging of the trial pits revealed that there exists three soil
types within the study area. Geological profiles inferred from the interpreted geophysical data,
indicate that the subsurface conditions at the study area mainly consists of two geologic
formations namely soil overburden overlying either weathered biotite gneiss or fresh basement
rock consisting of banded migmatites and granitoid gneisses occasionally containing quartzo
feldspathic
veins.
All the soils samples tested fall within the semi-permeable category. Although the calculated
Optimum Moisture Content (OMC) for the trial pits is within recommended percentage
according to the US Classification and identification of soils for general engineering purposes for
fine grained Soils, it should always be less than the shrinkage limit. There are no faults or
significant fractures observed at the area near HEP 1, which is recommended for the dam axis
area. Subject to confirmation of the subsurface foundation conditions by core drilling and testing
of the bedrock geotechnical characteristics, geophysical and geotechnical investigations done
imply that the study area is feasible for dam site and is founded on a sound rock foundation. The
valley has an approximate water holding capacity of 24 million cubic litres.