Construction of sustainable civil engineering structures, whether simple or complex ones, requires profound knowledge about the characteristics of subsurface earth materials, particularly physical peroprties of the underlying rocks/soils, distribution of tectonic elements, contents of moisture / fluid within them, etc. Discontinuities in the form of bedding planes, joints, faults and folds highly determine the physical strength (deformation characteristic) of rocks. Similarly, properties of materials filling voids (openings), such as pure/mineralized water, air or both in unconsolidated soils or fractured rocks influence their physical characteristics. Therefore, the stability of civil engineering constructions depends on the correct assessment of the various physical and geotechnical properties of the underlying earth materials where the structures are intended to be erected (Johnson, 1991).
However, constructions undertaken over formation with insufficient bearing capacities often result in unexpected failures, manifested by cracks, settlements, displacements or total collapses. Particularly, those structures erected over areas where expansive soils (such as clays) are widely distributed demand special attention as their shrinking and swelling characteristics can easily cause damages due to their property variations as a result of moisture / fluid content fluctuations associated with seasonal changes.
Therefore, geotechnical investigation of any construction sites is essential to obtain reliable inputs that enable to develop economically and technically feasible structural designs incorporating mitigation measures to anticipated geo-hazard events.
Like elsewhere in the world, also in Ethiopia public officials require geotechnical investigation data acquired in accordance with the Ethiopian Building construction Code with accompanying recommendations prior to issuing a building permit in order to protect the safety of the public the surrounding environment (Dagnachew, D., 2011).
Unlike drilling, pitting and trenching, geophysical methods are environmentally safe and also do not cause any significant damages/ concerns to the communities. Geophysical measurement responds to change in the physical, chemical, mechanical, elastic, radioactivity or thermal properties of the underlying earth materials. Because of such divers characteristics usually one or more of the properties correspond to certain features of earth materials, i.e., contact, discontinuity (fracture/fault zones). Unlike direct sampling, such as drilling or pitting and sending samples to laboratories for analyses, geophysical methods respond to different parameter in different ways and deliver information in a short time with minimum expenses.
For engineering applications seismic refraction, electrical resistivity and magnetic methods are widely used to map the subsurface structures. These methods depend on the acoustic impedance, ground resistivity and magnetic susceptibility contrast of the subsurface materials respectively.
The unique tectonic setting of Ethiopia results in complex geological and geo-morphological setups where along with these and continuously deteriorating environmental conditions, the country is very vulnerable for such geo-hazard risks, as volcanic, seismic, land slide and alike. Every year Ethiopia allocates quite a substantial amount of budget to the expansion of infrastructures: roads, bridges, dams and building complexes. Particularly, to expand access to education the construction of universities is taking place in different parts of the country and among these is the Injibara University.
To study the foundation conditions at site and evaluate its suitability for erecting a four story building to be used as dormitory for student, subsurface investigations were carried out employing electrical resistivity, magnetic and seismic refraction methods with an ultimate objective of generating inputs for civil applications.
1.2 Description of the Study Area
Abaya is one of the six campuses of Arba Minch University, which is one of the long- established universities of Ethiopia. It is located in Secha Kifle Ketema of Arba Minch town, a town located 500km south of Addis Ababa in the Southern Nations, Nationalities, and People's Region (SNNPR). It is located in the UTM zone of 37N, and it is geographically bounded by 0664000m to 0665000m north and 0339200m to 0340200m east.
Methods and materials
Four traverses magnetic data which is established approximately in the east west direction with a profile spacing of ten meter and one traverse perpendicular to the three traverse and randomly collected magnetic data were taken in the study area. Magnetic measurement where taken at ten meter interval using ENVI-MAG instrument (Scintrex) along the traverses. Three magnetic measurements were taken at each station and mean was adopted as the raw data for each observed station point for further processing. With these data the diurnal and geomagnetic corrections were carried out. Then with the corrected data oasis montaji software was used for magnetic interpretation by displaying magnetic anomaly map and magnetic profile.
The materials used during the field work are:
Proton precision magnetometer (ENI-MAG)
Geographic positioning system (GPS)
Satellite imagery and aerial photos