MULTIPURPOSE ENGINEERING GEOLOGICAL MAPPING OF NORTHERN PART OF ARBA MINCH TOWN, ETHIOPIA

Abstract

This research work aims to address the common geological problems of Sikala town by providing geological and engineering geological data and respective maps and provide possible mitigation measures for the problems that are widely observed in the town. The objectives of the research were, to characterize engineering properties of soils and rocks prominent in the town, to conduct engineering geological classification of soils and rocks ,to map the lithological and geological structures, to produce engineering geological map at a scale of 1:33, 000 using ArcGIS software. The ground water level of surrounding area of Sikala town was estimated from 12 boreholes. The ground water gradient follows the surface gradient and flows towards the East(Lake Abaya).The engineering geological map of Sikala town was prepared after characterizing the geotechnical properties of soils and rocks. The engineering geological and geotechnical condition of soils were evaluated from ten test pits of 1 to 6 m depth. Based on field description and laboratory analysis, soils of Sikala town were classified by consistency limit and Unified Soil Classification System (USCS) . The average grain size distribution of gravel, sand, silt and clay soils of the town was 18.76, 26.36, 39.38 and 16.6% respectively. Soil laboratory tests results of liquid limit, plastic limit and shrinkage limit range from 24.1 to 56%, 0 to 38.9% and 1.4 to 28.6% respectively. The internal friction angle (ɸ°) and cohesion (C kN/m2 ) of soils were determined and varies from 14.06° to36.04°and 10.81 to 54.46 respectively. Based on USCS, soils of Sikala town fall in to six classes. They are inorganic clay of high plasticity (CH), inorganic silts and very fine sands, clayey silts with slight plasticity (ML), inorganic clay of low to medium plasticity (CL), well graded gravels, gravel sand mixture (GW), poorly graded gravel-sand mixtures with no fines(GP) and clayey gravel, gravel-sand -clay mixtures (GC). To characterize the engineering geological condition of rocks,13 sampling points of rock discontinuity data and 30 sampling points of Schmidt hammer rebound tests were carried out in addition to visual description and the use of geological hammer. Based on the in situ rock strength tests, rock masses of Sikala town were grouped into three major classes as rocks with medium, high and very high mass strength. Schmidt and geological hammer were most widely used field equipments to estimate in situ rock strength. Scanline method was applied to collect rock discontinuity data. Based on Rock Mass Rating (RMR) index rocks of Sikala town fall into three major classes I (very good), II (good) and III (fair).