Effect of Fine Fractions on Engineering Properties of Sandy Soils in Gamo Zone, Southern Ethiopia

The current study examines the effect of fine content (passing a 75µm sieve) on the engineering properties of sandy soil. Sand samples were collected from the Kulfo riverside(site one), Hamasa(site two), Mirab Abaya(site three), and the sand samples were derived in laboratory as medium sand and fine sand by sieving them through a 2mm - 0.425mm and 0.425mm – 0.075mm sieve sizes respectively and fine fractions of the samples through a 75 m sieve. Laboratory tests were conducted by varying the proportions of the sand and fine fractions to form reconstituted sandy soil samples with fine contents of 0%, 5%, 12%, 20%, and 30% by weight. Direct shear, permeability, consolidation, compaction, CBR, and plasticity tests were performed on the formed reconstituted samples to determine their engineering properties. The plasticity of the reconstituted sandy soil for both medium and fine sand from 0% to 12% fine content was zero (non-plastic) for all sites, and that plasticity starts at 20% and increases up to 30% fine content. The average plasticity index for medium and fine sand is 5.88% and 8.46% respectively, as fine content increases. As the fine fraction increases, the average MDD for medium and fine sand increases from 1.79g/cm3 to 1.95g/cm3 and the OMC climbs from 10.64% to 13.9%. The increase in fine fractions owing to particle interlocking has a major impact on MDD. The average CBR value for medium sand decreases from 32.60% to 13.14% and fine sand decreases from 24.785 to 11.80% as fine content increases. The result showed that when the fines content grows, compression index rises to a 0.03368 of fine sand and 0.03059 of medium sand for site three linked with a fines content of 30 percent. The maximum permeability value for medium sand at site three was 2.18* cm/sec, while the values for medium sand and fine sand at site three were 1.57 cm/sec and 7.22* cm/sec, respectively. The direct shear test results for medium sand from Site one range from 16.6° (for 0 percent fines) to 0° (For 100 percent fines). The results for sample fine sand range from 10.4° (for 0 percent particles) to 0° (for 100 percent fines). The fine fraction of sandy soil affects the CBR, Permeability, Shear Strength, Plasticity, Compressibility, and MDD values of sandy soils, according to the study. Keywords:- CBR, Fine Fraction Content, Maximum Dry Density (MDD), Optimum Moisture Content(OMC), Reconstituted Sandy Soil.