CHARACTERIZATION ON GEOTECHNICAL BEHAVIOR OF SUBGRADE SOILS LOCATED IN HOMBBO-HOBICHA GRAVEL ROAD

Abstract

Road transport has a vital contribution for economic, social and also political development. Roads have to be designed in the way that it can function in all season of the year. In general roads are classified as paved and unpaved roads. Unpaved roads are roads with surfacing material of gravel or other sub base materials. Gravel roads account for a large portion of the rural roads in Ethiopia. The problems of meeting standard requirements are not well identified to all parties involved in the construction process, due to the scarcity of studies that address the impact of this practice on social and direct costs. The aim of this study is to characterize sub grade soils for gravel road located along Hombbo-Hobbicha road project. To achieve this objective a total of 10 subgrade soil samples have been taken at 1km regular interval and tests were conducted at project geotechnical laboratory for determination of Atterberg limit, Specific gravity, Natural Moisture Content, Maximum Dry Density and Optimum Moisture Content, CBR and CBR Swell values and Sieve Analysis test is conducted at Arbaminch University geotechnical laboratory. Finally, interpretations have been made with the data obtained from laboratory investigation and standard manual of the subgrade characteristics. From the laboratory investigation the results showed that NMC ranges from 21% to 32.85%, LL ranges from 40.11% to 59.794%, PI ranges from 7.34% to 19.08%, Gs ranges from 2.35 to 2.59, Grain size analysis fine soils ranges from 2.35% to 77.45%, sand fraction 22.15% to 90.38% and gravel content 0% to 7.4%, GI ranges 0% to 17%, OMC ranges from 13.43% to 24.22%, MDD ranges from 1.52 g/cm3 to 1.772 g/cm3, CBR ranges from 0.87% to 18.85% and CBR swell 0.25% to 8.22%. Based on this result the study area was characterized as silt and clay soils and for classification under A-7-5 and A-2-5 according to AASHTO M 145 soil classification system. And from the laboratory investigation it has been concluded that about 50% of subgrade soil is found to be unsuitable on the bases of bearing capacity. Hence, remedial measures such as removal and replacement, proper drainage design and blending with locally available materials are proposed and mechanical stabilization methods have been proposed for unsuitable sections.