Integrated Geological, Engineering Geological andGeophysical Investigation of Adama and Its SurroundingAreas: Implication to Engineering Practice

Abstract

Adama was founded hundred years ago and is one of the major and densely populatedtowns in Ethiopia. It is located 100 km south–east of Addis Ababa and at the heart ofboth Ethiopia and the National Regional State of Oromiya as well. It is the hub of a vastregion of immense economic potentials accessible by means of rail ways andexpressways. Due to the mentioned reasons, the urbanization is rapid. However as inmany towns of Ethiopia, very little is known about the soil and rock engineeringproperties and subsurface condition of the town. The physical and mechanical propertiesof earth materials have major influences on the success, economy, and safety ofengineering structures. Moreover, the town is located in the southern extreme of theactive volcano-tectonic segment of the northern Main Ethiopian Rift (MER). The presentresearch work is, therefore, aimed at assessing and evaluating the geological,engineering geological and subsurface condition of the town.Detailed field investigation,in-situ geotechnical test and laboratory analysis were carried out to studygeological,engineering geological and geotechnical condition of the rocks and soils.During currentresearch work, 30 soil samples were taken from 16 test pits, which were dug manually to3 m depth.In addition, geophysical technique involving Vertical Electrical Sounding(VES) was also implemented and the data were processed, inverted and interpreted. Thenatural moisture content of the soil of the study area ranges from 6.34 - 25.2%, liquidlimit 26.06 - 54%, plastic limit 7.5 - 40.83%, plasticity index 2.22 - 26.83%, free swell 0 -40%, specific gravity 2.12 - 2.67, and pH 4.48 - 6.7.Low to medium plastic silty sand andsand are the dominant soil type in the study area.The rocks of the study area wereclassified into Low Strength, Medium Strength and High Strength rock massbased on thestrength (Unconfined Compressive Strength) of intact rock and discontinuitycharacteristics. Using interpolation methods, spatial distribution maps of electricalresistivity were produced from which the overburden material distribution in the town isshown. The overburden thickness is relatively thin and very thick on horest and graben,respectively and the thickness increases to wards south and south western part of thetown. Gully erosion, ground fissure, seismic activity and flooding are the main geohazardthat need attention.