Investigation of Index Properties and Swelling Potential of Expansive Soils in Lege Tafo Area

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Addis Ababa University


This Study is conducted on Lege Tafo area which is located on flat plain just east of Addis Ababa City. The area is covered with thick clay soil and new infrastructures are under construction in the region as part of the fast growing urbanization and industrial development around Addis Ababa and this is one of the reasons why the area is selected for the study. The objective of this research is to understand the nature of the soil and obtain sufficient information on type and characteristics of the soil in the area to provide general data for future construction projects. To achieve these objectives the surrounding of Lege Tafo is studied in the field, and representative clay soil samples are collected and analyzed in the laboratory. Black soils in Ethiopian are derived from weathering of basalt volcanic rocks and are invariably clays or silty clays. Natural deposits of black cotton soils in the field are characterized by a general pattern of surface cracks during the dry season of the year. During wet seasons, the soil first expands horizontally, filling up the shrinkage cracks. Further volumetric expansion causes vertical heaving of the soil which may cause damage to an overlying structure. Deformations occurring in the soil affect the stability of structure negatively, and the consequence could be tremendous loss. A total of 15 disturbed and undisturbed soil samples were collected from 9 test pits. Three undisturbed samples at 2 meter depth are tested for investigation of swelling pressure. Expansive soils of Lege Tafo are generally black in color and underlain by gray soils that have significant swelling potential. From soil classifications, soil samples taken from 2 meter and 3 meter depths have shown the same capacity for expansion/ swelling potential. As a result of this swell consolidation test was conducted on three samples collected at 3 meter. The soil in the study area is mostly clay ranging from 53.6-75.3%, silt 22.9-33.6%. According to consistency test results, liquid limit ranges from 91 -116%, plastic limit from 27.8-46.6% and plastic index from 52.37-81.6%.Within the depth of investigation, the specific gravity of the study area ranges from 2.60 to 2.85. According to the Unified Soil Classification System, the soil in the study area is categorized as highly plastic inorganic clays. AASHTO classification system also shows that the soil is highly plastic with a potential for a considerable volume change. Swelling pressure test carried out on undisturbed samples from the study yield results ranging from 76 kPa to 165 kPa. The results are compared with results of previous studies on similar soils around Addis Ababa.



swelling, expansive soils, Index properties, Lege Tafo