Experimental Performance Studies on the Improved Expansive Subgrade Soil Formation by Using Mechanical Stabilization with Natural Gravel Around Jimma Quarry Sites

Expansive soil refers to a particular type of soils – almost clayey soils in property - that expand as they absorb water and shrink when they dry out. The process of swelling generates pressures that can cause lifting, or heaving of structures whilst shrinkage process can cause differential settlement on civil structure. A large number of buildings and roads are placed on such type of soil and exposed to the danger from this swelling and shrinkage processes. In order to avoid the previous mentioned risk from the expansive soil, a technique of soil stabilization should be made for this type of soil to enhance some of its properties. In this study, the experimental investigations have been made to study the suitability of natural gravel from jiren quarry site to improve properties of expansive soil. The important parameters to be determine the geo-technical properties such as Specific gravity, Liquid limit, Plastic limit, Plasticity index, dry density and California Bearing (CBR) ratio values. According to ERA 2002 this type of soils is poor for subgrade for road construction. In order to realize the desired objective, a purposive sampling technique which is non –probability method was adopted in order to collect disturbed soil samples at depth of 1.5m and natural gravel from jiren quarry site used for the preparation of different laboratory tests. The laboratory work involved sieve analysis along with consistence test to classify the soil sample. The preliminary investigation of the soil shows that it belongs to A-7-5 class of soil in AASHTO and CH in USCS soil classification system. Soils under this class are generally of poor for subgrade road construction. Atterberg limits, compaction and CBR tests were used to evaluate properties of stabilized soil. The soil was stabilized with natural gravel in stepped concentration of 5%, 10%, 15%, 20% and 25% by dry weight of the soil. Analysis of the results shows the addition of natural gravel improve the geotechnical properties of the expansive black cotton soil. The addition of natural gravel reduces plasticity index, swelling and OMC with an increase in MDD and CBR with all increased natural gravel contents. The addition of 20% and 25% of natural gravel established an increased CBR value by 670% and 958% respectively, indicating the subgrade class falling under S4 and S5 respectively. From this study it was found out that natural gravel stabilized with soil do meet the minimum requirement of ERA pavement manual specification for use as a sub-grade material in road construction.