Geotechnical and Engineering Geological Investigation of Iyandafaro Dam Site, Konso, Southern Ethiopia
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Date
2001-07
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Addis Ababa Universty
Abstract
The Iyanda catchment covering an area of 658km2 is studied for the selected dam site. Land
use/ Land cover, soil and slope of the area were mapped, so that erodability for vegitation soil
complex is computed. Geology and hydrometheorology of the catchment was studied and
engineering propeliies of the mapped rock and soil units are discussed. The geology and
engineering geology of the catchment was mapped at the scale of 1:50000 and the engineering
geology of the reservoir at 1 :2000. The area is dominantly covered by volcanics(basalts and
metamorphic (gneiss) at relatively elevated areas and QuaternaIY deposits at flat plain areas.
Areal mean annual rain fall was analyzed using Arithmetic mean and Thiessen weighted
average and a value of 830.6mm and 830.5mm was obtained. Piche evaporimeter installed at
Konso was used for the evaluation of the storage site(1680.3mm) using a pan coefficient of
0.85 due to the high evaporation rate at the site. The potential evapotranspiration calculated by
the Thornthwaite method is 1534.3mm, that is almost twice of precipitation and is typical
properties of arid to semi arid climate. The soils of the study area never reaches to field
capacity in general, due to this, no surplus is generated. The possible method of ground water
recharge and slllface IUn off is rain fall above average and storm IUn off. The peak storm 11m
off for 100 year return period was 86mm. The selected reservoir site has storage capacity of
more than 16Mm3 Water samples taken from the study area and its environs show decreasing
tendency from the Lake to the study area with respect to EC, TDS and dominating cations like
sodium.
The engineering geological mapping was carried out based on material strength, degree of
weathering and fi'acturing for rock units and gradation and Atterberg limit for soil material.
Four geological units are mapped at the dam site: Gravel and sand at the river, clay and silt at
the flat plain area, well-graded 'soil at the right abutment and slightly fractured basaltic rocks at
the left abutment. The geology of the side channel spill way, selected at the left side is a
continuation of the left abutment, which is stable, slightly weathered and fractured basalt. The
main canal lithology is fine grained clastics (clay and silt) which is impermeable lithological unit. The consolidation test of clayey silt soil analysis shows that, the soil is stiff with maximum
seteliment 7mm for the aplied stress of3200kN/m2
The permeability coefficient computed fi·om the consolidation test is 0.7 x 10.7 cm/s. Both the
index and engineering properties of the dam site shows that, the clayey silt soil and well graded
soil at the right abutment are stable and watertight. Hence the foundation depth should be on
this layer. But at the main river and its levees, no watertight unit was mapped to 7m depth,
though the geophysical investigations show the bed rock symptom at 13.5m depth.
The study area catchment rock units can be classified in to five strength zones:
Zone 1 - Moderately fi·actured, slightly weathered to fresh basalt
Zone2 - Moderately to highly fractured, slightly to moderately weathered basalt
Zone 3 - Moderately fractured, slightly weathered to fresh gneiss
Zone 4 - Moderately to highly fractured and weathered gneiss
Zone 5 - Moderately to highly fractured and weathered gabbros
Based on strength classification zone 1 and zone 2 are very strong to extremely strong rocks
and zone 3, 4 and 5 are strong to velY strong rocks. The approximate range of unconfined
compressive strength of extremely strong, very strong and strong rocks used for tllis
classification is: > 250, 100 - 250, and 50 - 100Mpa respectively.
The results of the electrical sounding survey presented in the form of geo-electric section
which shows the distribution of layer resistivities and thickness. This is believed to be a closer
approximation to the actual geologic setting in the sub surface. The YES and profiling
investigation show that the over burden (soil) is thick at the flat plain areas and thin at the
abutments
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Keywords
Iyandafaro Dam Site