Engineering Geological Characterization of Soils and Rocks For Foundation Suitability of Engineering Works in Ambo Town, West Showa, Ethiopoia.

Abstract

For engineering constructions and earth works to be successfully and economically designed, site characterization is an essential. It is also required to gather enough information on the kind, qualities, and distributions of rock and soil beneath a site of proposed structures. The geotechnical and engineering geological qualities of the soils and rocks of Ambo town are not well understood, and the town does not have an engineering geology map which is the best approach to represent the surrounding environment for engineering purposes. The present research work is therefore, aimed to assessing and evaluating the geological and engineering geological condition of the town and to produce engineering geological map. To accomplish these goals, thorough field survey, an in-situ strength test, sampling, and sample laboratory testing were carried out. Eight disturbed soil samples were taken from seven test pits that were manually excavated at maximum depth of three meters, and five representative rock samples were taken by breaking them with geological hammer in order to achieve these objectives. Depending on field observation, in-situ rock strength and laboratory analysis, the engineering geological properties of rocks and soils were determined. Geological and engineering geological maps were created at a scale of 1:50,000 to show the location, coverage area, and engineering qualities of the various geological materials. Based on laboratory test results the natural moisture content, liquid limit, plastic limit, plasticity index, specific gravity, and free swell vary from 8.24 to 16.1%, 24.06 to 86.81%, 10.34 to 47.6%, 13.65 to 40.24%, 2.49 to 2.65, and 0 to 80%, respectively. The tested soils’ undrained shear strengths range from 64.97 kPa to 97.2 kPa. The Unified Soil Classification System categorizes soils into sandy clays and silty clays of low plasticity (SC), inorganic silt clays of medium plasticity (ML), inorganic silts, fine sands and clayey silts of high plasticity (MH), gravelly sands (SW), inorganic silt clays of high plasticity (MH), and inorganic clays of high plasticity (CH). The soils are categorized as clayey soils (A-7-5) and silt / gravel sand (A-2-7 and A-2-6), which are unfavorable and favorable for subgrade material, respectively, according to the AASHTO classification system. According to those soil classification systems the problematic soil areas mostly identying in the eastern, north eastern and northern parts of the town. The rocks of the study area were classified into very high strength, high strength, low strength and very low strength rock units based on the strength (Unconfined Compressive Strength) of intact rock. The strength of the basaltic unit varies from very high strength rock to high strength rock (242.88 MPa) to (155.48 MPa). The strength of sandstone varies from very low (25.5Mpa) to low strength (37.8Mpa). Very low rock strength, measured with an ignimbrite unit, is (24.31MPa). The main geotechnical problems that affect the design and development of civil structures in the town were identified as the existence of different rock falls and soil sliding which were formed due to quarry site and river cutting of slope in the townrespectively and presence cohesive soils of high plasticity character areas.