Problematic Sub-Grade Soil Reinforcement Using Local Natural Fibers on Selected Road Section From Wolaita, South Western Ethiopia

No Thumbnail Available



Journal Title

Journal ISSN

Volume Title


Addis Ababa University


A road is very important for the community, because it is base for the socio-economic interaction among people. Based on road life and its economic considerations, road pavement designs mainly need good sub-grade materials or reinforcement of existing problematic soil. Natural fibers have capability to reinforce problematic soils such ranging from clay to silty sand. Wolaita (project area) is one of the highly populated area in Ethiopia and its land is almost completely covered by farm and house. The soil in wolaita area is lateritic clay. However, road construction site’s soil does not fulfill the sub-grade soil requirements economically. Hence hundreds of millions metric cube soil is needed for sub-grade layer and the same volume of soil is cut to drop site in one road project. This research work come up with mechanism, mechanical conjugation of random distributed local natural fibers and soil, in order to reinforce the problematic sub-grade soil. In general the research work comprises the tasks such as characterization and selection of fibers, representative problematic soils and soil-fiber mixtures. Method of characterization is done as per AASHTO and ASTM test methods which are recommended in Ethiopian Road Authority. Accordingly the characterization of the worst problematic soil and selected fibers (Enset ventricosum and Teff straw fibers) is done before conjugating them. The effect of fibers percentage by weight and of fiber length in the soil on CBR, surcharged swell, dry density, moisture content and their optimum improvements are discussed in this paper. The percentage of individual fiber in the soil is taken as 0%, 0.25%, 0.5%, 0.75%, 1%, and 1.25% by air dry weight. The result show that MDD is decreased and OMC is increased with increased fibers percent while CBR value is increased until 1% for Enset fiber and 0.75% for Teff straw fiber with fiber length of 6 cm. However, the degree of improvement of fiber mixed soil is 157 and 133 times greater than the soil without the fiber respectively. One point unsoaked CBR test shows that the value increased from 57% to 62%. 4.54kg surcharged swell value of teff straw fiber conjugated soil is decreased from 5 to 3 as percent of fiber increased from 0% to 0.75%, respectively and decreased from 5 to 3.5 as fiber increased from 0% to 1% respectively for Enset fiber. According to improvement, change in road pavement design as per ERA manual and its subsequent benefits are also discussed in this paper. Finally, appropriate recommendations are written according to the finding.



Reinforce, Problematic Soil, CBR, Natural Fibers, Enset Fiber, Teff Straw, Sub-Grade