INVESTIGATION OF THE ENGINEERING PROPERTIES OF SOILS IN SOUTHERN PART OF ETHIOPIA (THE CASE OF BONKE-GAZESSO TOWN)

Abstract

Structures such as buildings, bridges, highways, etc., rest directly on, in or against soil. Hence for structures to be durable, cost effective, remain safe and free undue settlement and collapse, proper analysis of soil and design of foundations are necessary. The main objective of this research is to investigate soils of Bonke Gazesso town in order to get reliable results of the engineering properties of soils and to understand about the nature of soils so that the suitable foundation can be recommended for better design and construction of civil engineering works in the town. To succeed in reaching this goal, visual site investigation (field observations), disturbed and undisturbed (carefully chunked) samples from seven sampling areas (depth of nearly three to five meters) of the town were collected and laboratory tests were done on the collected samples. For one-dimensional consolidation test, rings were pushed into the ground (without disturbing soil, gently) and the soils were taken. For all tests the aparatus and the procedures used were done according to ASTM standard. The natural water content of the soils of the study area ranged from 41.35 to 53.0%. The specific gravity of oven dried samples ranged from 2.69 to 2.74 and air dried from 2.70 to 2.75. From grain size analysis results, the clay content samples ranged from 26.12 – 57.47%; the silt content from 34.09 – 61.65%; the sand fraction from 0.72- 15.52% ; and finally the gravel fraction ranged from 0- 7.11%. The liquid limit samples ranged from 62 – 81%. The plastic limit of samples ranged from 32- 45%. And plasticity index ranged from 21- 44%. The textural classification system of soils indicated that soils of the study area were clay, silty clay and silty clay loam soils. According to USCS most soils of the study area fell under MH region, which shows that the soils were inorganic silts of high plasticity. But, some soils fell on the boundary line, A-Line. According to BS classification system the soils of the study area were in between MH (high plasticity) and MV (very high plasticity). The consistency of soils of study area ranged from very stiff to hard. The compression index, Cc, ranged from 0.08 to 0.110 and recompression index, Cr, ranged from 0.011 to 0.0201. Coefficient of compression (av) from 3.13*10-5 to 5.35*10-4 kPa -1 ; the coefficient of volume compression (mv) ranged from (1.63*10-5 to 2.67*10-4 ) kPa -1 ; and coefficient of consolidation (Cv) ranged from (1.69*10-3 to 7.40*10-3 ) cm2 /min. The coefficient of permeability indicated that the soils of the study areas were impervious or impermeable.