Geology

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Browsing Geology by Author "Ayenew, Dr. Tenalem"

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    Engineering Geological Study of Awassa Town and its Surroundings
    (2000-06) Geremew, Zemenu; Ayenew, Dr. Tenalem
    In the present work, engineering geological study has been carried Qlll (0 cha racterise and map the engineering geological conditions for the Awassa town and its surroundings, located 275 Km south of Addis Ababa in the central sector of the main Ethiopian rift. l-lJdrometeorological. hyd rological. hydrogeological. hydrochemical and geodynamic conditions have been assessed in view of tht;:ir relevance for future development of the town as related to Civil Engineering activities and to the rise of lake Awassa, which is becoming a threat for its future development . The engi neeri ng geological characteristics of the foundation soils and rocks has been assessed on the basis of field -obse rvations, simple in situ tests, and from the results of the laboratory analysis made on disturbed samples. Besides, borehole records from existing data have been studied in order to quantify and correlate the data from the test pits. " The main rock types in the area are volcano-lacustrine sediments and acid ic volcan ic rocks including ignimbrites, pyroclastic fall deposits, rhyolitic lava flows and associated obsidians . Basaltic lava flows, scorias and basaltic hayaloclasities are also locall y exposed . However. the town is wholly founded on the volcano-lacustrine depos its. The area generally gets moderate rainfall with moderate inter annual and seasonal variabilities. Lake Awassa has recently undergone lake level rise. The main causes of this phenomenon are believed to be high rainfall and reduced evaporation. The extreme value type 1 distribut ion is used for frequency analysis of annual maximum bike levels and the lake levels for di fferent years of recurrence interval is calculated. On the bases of these calculations. a flood hazard map of the lakeshore for 200 years retu rn period is established. Water quality analysis results indicate that the water in the area has no immedi ate danger on 'l!ngineering works. The waters in the area are of dominant ly Na - HCO) type lacking high concent ration of sulphate andlor chloride. However. the groundwater genera lly flu ctuates up to 5m following rainy seasons, and may be critical for ci vil enginee ring works close to the lake . The foundation soil is dominantly a fine-grained type though coarse-grai ned soils are also found at different stratigraphic positions. Three dominant so il types are identified in the field : clay soil s, sandy silt soils and silty sand/gravel soils. The clay soils occupy rel at ively the elevated central and southeastern part of the town and the poss ible expansion area while the other types occur in almost the whole area. According to the Unified Soil Class ification system, most of the soils belong to ML (inorganic silts, ve ry fine sands, rock flour, si lty or clayey fine sands. SM (silty sands. sand - silt mixtures) and CL ( inorganic clays of low to med ium plasticity, gravely clays. sa ndy clays. silly clays , lean clays). Relatively few soil samples are in groups MH (inorganic si lts, micaceous or diatomaceous fine sands or silts, elastic silts), CH (i norganic clays of high plast icity. fal clays) and GM (silty gravels, gravel-sand-clay mixtu res) . Al l the soil types show thickness variation both laterally and vertically ranging from about 0 .85 COl to about 3.5m. Moisture content at the time of sampling ranges from 25. 1 to 29.0% for clay soils, and from 18 to - 46 % for sandy si lt and silty sand/gravel soils. The specific gravity of the soils is low, in the range from 2.2 to 2.5. The liquid limit and plast icity index of the clay soils range from 28 10 53% and from 7 to 24% respectively . The sandy silt soils have LL% in the range from 26.7 to 56% and PI % in the range from 1.89 to 25% . The soils of the area are found out to be non expansive as revealed by the simple free swell test. However , some of the sandy sil! and silty sand/gravel soils show reduction in volume as large as IX 29% . thus suspected to have a collapsible structure . These soils may collapse both under load and when nooded by water and may also result in differential seulement. Transmitting the weighl of the structure to deeper soil or rock layer by means of piles or caissons. and if not. to make the foundation as deep as possible is believed to minimise the hazard . From engineering geological point of view, the rocks outcropping in the town area are grouped in 10 two: strong (basalts and rhyolites) and weak rocks (dominantly of basaltic hayalloclastites and tuffs). From field Schmidt hammer lest result, it is found out that the fanner ones have an unconfi ned strength in the range from 310 to greater than 700 Kgfcm1, while the laner less than 100 Kgfcm2 • The strong ones are found out to be good sources for aggregates while the weaker ones are not suitable fo r most of civil engineering works . Based on the fie ld and laboratory invest igation results, Engineering geological mapping at the scale of I: 10,000 was carried out for the town area including the possible expa nsion area
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    Legadembi Gold Open Pit Slope Stabiuty and Underground Mine Geotechnical Study
    (Addis Ababa University, 2004-06) Bekele, Yonas; Ayenew, Dr. Tenalem
    In Soulhern Elhiopia in Adola Precambrian rocks, the Legadembi primary gold occurrence have been under exploralion and feasibility study wilhin a period of 1975•- 1990 G.C .. In 1990 G.C the Legadembi Gold Mine has been established and officially started mining. In 1997 the mine has been privatized to Midrock Gold Private Limited Company