| dc.description.abstract | The adaptation of geophysical methods for engineering geology purposes represents a
significant contribution to the development of geotechnical site investigation methodology.
Prior to construction of any engineering structure, proper geotechnical site investigations
should be carried out. This helps refine a conceptual geological model as well as to provide a
model of geotechnical design parameters. This dissertation focuses on both geophysical and
geotechnical investigations for a dam site in Nambale area, Busia County. The principal goal
of this exercise was to characterize the geological formation by using geophysical and
geotechnical properties, so as to evaluate and determine their competence for dam
construction.
The bedrock mainly consists of stratigraphic rock units of the Nambale formation which is
part of Yala group belonging to the Nyanzian super-group. The geophysical results revealed
three distinct layers. These are basically top soil, fractured rocks and bedrock. The top soils
generally consist of arable loam soils, mixed with stiff clays, silts and sands. The second
layer is composed of fractured shale, mudstone and sandstone which are mixed with sands
and stiff clays. This is underlain by the third layer comprising of dense"-, hard and compact
shale which forms part of the bedrock in some sections. However, the igneous granitic rocks
form the main bedrock of the area.
The depth to the bedrock ranges from 13 metres to 30 metres with an irregular morphology
and has resistivity ranging from 120 Ohm.m to 500 Ohm.m. Seismic velocities of this layer
range from 2500 m/sec to 7000 m/sec. The area has a shallow water table with the aquifer
forming at the depths between 10 metres to 15 metres, with a resistivity range 54 Ohm.m to
100 Ohm.m. The Multi-Channel Analysis of Surface Wave (MASW) gave clear variations in
the velocities even those occurring within short ranges: therefore it came handy in delineating
and mapping of the subsurface soil formation. The geotechnical results show that the
consistency limits of the soils within the area had moderate to high plasticity; hence, the soils
are expected to exhibit moderate to high swelling potential.
The overall assessments show that the subsurface formation is competent and appropriate for
a dam. structure. It is therefore recommend.ed based on the results of this study that the
foundation of the embankment set up at depth of about 4 metres, where the rock are strong
and that grouting should be done on weak zones. | en_US |
