IMPROVING SUBGRADE AND SUBBASE SOILS USING MARBLE DUST OF KAWO KOYSHA WOREDA, WOLAITA ZONE.

Abstract

Infrastructure is a major sector that propels overall development of Ethiopian economy. Construction of engineering structures on a weak or soft soil is considered as unsafe. The subgrade and subbase of the soil has to be strong enough to support the entire load. But the most common type of the road soil in Kawo koysha woreda is expansive soil and subgrade and subbase fail to carry the load on it. There are visible damages on the road because of the properties of the soil that the characterized by volume change due to variation in moisture content. It causes horizontal and vertical movement in expansive soils and leading to failure on the road. The purpose of this study is improving expansive subgrade and subbase soil by stabilizing with more available and low cost material such as marble dust. Nine natural soil sample was taken from nine pits. The expansive subgrade and subbase soil was mixed with marble dust by increasing percentage of marble dust from 0 to 20% at increment of 5% and improvements in engineering properties are studied. From the analysis of the test results it was found that liquid limit, plastic limit, plasticity index, optimum moisture content, and free swell decreased, and maximum dry density and California bearing ratio (CBR) increased with an increase in marble dust content in nine disturbed sample pits taken. From the economic analysis it was found that Marble dust up to 20% can be used in strengthening the subgrade and subbase of flexible pavements to save the cost of construction. According the investigation is carried out on natural subgrade and subbase of road of study area, the LL(68.9%), PL(28.99%) and PI(39.91). The particle passed through 75µm 99.58%, California bearing ratio value is 2.09% and free swell value is 74%. Then the soil is characterized highly expansive soil with low bearing capacity, high swell and also characterize as A-7-5/A-7-6 according to the AASHTO classification and rated as poor subgrade and subbase material. With the 20% of marble dust stabilization, substantial decreases in Atterberg’s limit, LL, PL, and PI were observed. LL reduced from 68.9% to 37.5%, PI reduced from 39.91% to 11.1%, OMC reduced from 24% to 13.5%, MDD increased from 1.63% to 1.75%, CBR increased from 2.09% to 10.65% and free swell is from 74% to 21%. Also from the economic analysis it was found that marble dust up to 20% can be used in strengthening the subgrade and subbase soils to save the cost of construction.