Engineering Geological Investigation of Jimma Town:-Implication to Engineering Practice

Abstract

Jimma is one of the major and densely populated town in Ethiopia. It is located 356 Km southwest of Addis Ababa and, covers an area of 50.52 square Km. The topography is predominantly flat and rolling terrain. As in many towns of Ethiopia, very little is known about the soil and rock engineering properties and sub surface conditions of the town. The present research work is, therefore, aimed at assessing and evaluating the geological and engineering geological condition of the town. Detailed field survey, in-situ strength test sampling, geological mapping and laboratory testing of samples were conducted in order to achieve these objectives. A total of 12 disturbed soil samples were collected from 12 test pits, which were dug manually up to a depth of 3m. Depending on field observation, in-situ rock strength and laboratory analysis, the engineering geological properties of rocks and soils were determined. To give a picture of the location and extent of coverage of the different geological materials and their engineering properties, geological and engineering geological maps were prepared at a scale of 1:25,000. From the laboratory test results, the natural moisture content, liquid limit, plastic limit, plasticity index, specific gravity and free swell ranges from 42.18 - 62.45%, 66-101%, 31-53%, 19- 53%, 2.36-2.75, and 33-130% respectively. The undrained shear strength of the studied soils ranges from 24.57 KPa to 173.66 KPa. According to the Unified Soil Classification System, the soils are classified as medium to high plastic silt and clay soils. As per AASHTO classification system, the soils are classified as clayey soils (A 7-6) which is unfavorable for subgrade material. The rocks of the study area were classified into very high strength, high strength and low (soft) strength rock units based on the strength (Unconfined Compressive Strength) of intact rock. The ignimbrite unit shows variable strength from very high rock strength (169.82MPa) to high rock strength (50.118MPa). Basalt unit revealed extremely high rock strength ranging from 89.12MPa to 194.98MPa. Tuff unit revealed extremely low rock strength (<1MPa). The soils of the area exhibits high swelling-shrinkage behavior that can cause problems on engineering structures. Hence, proper counter measures need to be considered during the design and construction of engineering structures in these areas.