Integrated Geophysical and Geotechnical Investigations for Building Site Characterization of Wachamo University, North-east of Hosanna Town, Southern Ethiopia
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Date
2011-12
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Addis Ababa Universty
Abstract
The present study was carried at Wachimo University which is located northeast of Hossana
town in Southern Ethiopia, 230 km from Addis Ababa city. The main objective of the present
study was to assess the suitability of the foundation condition for the new building site at
Wachemo University campus. In order to achieve the proposed objective systematic
methodology was followed. The methodology adopted for this purpose includes an integrated
approach where geophysical and geotechnical investigation was carried out. For geophysical
investigation Electrical Resistivity Tomography (ERT) and magnetic methods were
employed. For geotechnical investigation the physical properties of the foundation soils were
characterized and classified based on their visual interpretation and different laboratory tests.
The recently developed technique of Electrical Resistivity Tomography (ERT) gives 2-D
electrical resistivity variation of the subsurface geological conditions. During the present
study four profiles of ERT were conducted to know the different electrical properties of subsurface
conditions at the proposed building site. The ERT results showed presence of thick
top soil, weathered and fresh bedrocks, continuous moisture content and geological weak
zone below the proposed building site. Further, the stacked apparent resistivity pseudo
section map presented the depth-wise electrical variation at the proposed building site. The
VES and well log results indicated that the site has deep groundwater table. Magnetic
anomaly map prepared during the present study exhibits good anomaly contrast. From this it
may be deduced that high magnetic anomaly responses are the results of fresh igneous rocks
whereas, low magnetic anomaly responses are resulted due to weak zones and high degree of
weathering in rocks. Analytical signal map clearly presents a contrast between lithological
contacts and geologic structures. The correlation of the magnetic anomaly and the inverse
model resistivity section has enabled identification of geological weak zones where specific
design considerations are essential.
During the present study in total soil samples from 9 test pits were tested for specific gravity,
natural moisture content, grain size distribution, Atterburg limits and swelling potential. All
soil samples from the study area falls in low plasticity silts to intermediate clays, except for
two soil samples which showed high plasticity silt soils. Low to intermediate plasticity soils
of the study area shows good to fair engineering property but high plasticity silt show poor
engineering property. In general, the soils of the study area may be classified as CI, ML and
MH. In terms of suitability for building foundation 33.3% of the soils possess good to fair
bearing capacity, 44.4% as fair and remaining 22.2% of soils possess poor bearing capacity.
Shallow foundation material in the study area is thick residual soils which possess moderate
to good bearing capacity and may be suitable for low rise buildings. Further, the geophysical
investigation has revealed potential hazardous sub-surface conditions which may have
adverse effect on the proposed buildings therefore, it is strongly recommended that the
building foundation sites should be selected away from these geologically active/weak zones.
The study area is situated in seismically active zone therefore designers should consider the
seismic code in addition to the building code before designing the multi-story buildings.
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Wachimo