Engineering Geological Study Of Awassa Town And Its Surroundings

In the present work, engineering geological study has been carried Qlll (0 cha racterise and map thernengineering geological conditions for the Awassa town and its surroundings, located 275 Km southrnof Addis Ababa in the central sector of the main Ethiopian rift. l-lJdrometeorological. hyd rological.rnhydrogeological. hydrochemical and geodynamic conditions have been assessed in view of tht;:irrnrelevance for future development of the town as related to Civil Engineering activities and to thernrise of lake Awassa, which is becoming a threat for its future development . The engi neeri ngrngeological characteristics of the foundation soils and rocks has been assessed on the basis of fieldrn-obse rvations, simple in situ tests, and from the results of the laboratory analysis made on disturbedrnsamples. Besides, borehole records from existing data have been studied in order to quantify andrncorrelate the data from the test pits.rn" The main rock types in the area are volcano-lacustrine sediments and acid ic volcan ic rocksrnincluding ignimbrites, pyroclastic fall deposits, rhyolitic lava flows and associated obsidians .rnBasaltic lava flows, scorias and basaltic hayaloclasities are also locall y exposed . However. the townrnis wholly founded on the volcano-lacustrine depos its.rnThe area generally gets moderate rainfall with moderate inter annual and seasonal variabilities.rnLake Awassa has recently undergone lake level rise. The main causes of this phenomenon arernbelieved to be high rainfall and reduced evaporation. The extreme value type 1 distribut ion is usedrnfor frequency analysis of annual maximum bike levels and the lake levels for di fferent years ofrnrecurrence interval is calculated. On the bases of these calculations. a flood hazard map of thernlakeshore for 200 years retu rn period is established.rnWater quality analysis results indicate that the water in the area has no immedi ate danger onrn'l!ngineering works. The waters in the area are of dominant ly Na - HCO) type lacking highrnconcent ration of sulphate andlor chloride. However. the groundwater genera lly flu ctuates up to 5mrnfollowing rainy seasons, and may be critical for ci vil enginee ring works close to the lake .rnThe foundation soil is dominantly a fine-grained type though coarse-grai ned soils are also found atrndifferent stratigraphic positions. Three dominant so il types are identified in the field : clay soil s,rnsandy silt soils and silty sand/gravel soils. The clay soils occupy rel at ively the elevated central andrnsoutheastern part of the town and the poss ible expansion area while the other types occur in almostrnthe whole area. According to the Unified Soil Class ification system, most of the soils belong to MLrn(inorganic silts, ve ry fine sands, rock flour, si lty or clayey fine sands. SM (silty sands. sand - siltrnmixtures) and CL ( inorganic clays of low to med ium plasticity, gravely clays. sa ndy clays. sillyrnclays , lean clays). Relatively few soil samples are in groups MH (inorganic si lts, micaceous orrndiatomaceous fine sands or silts, elastic silts), CH (i norganic clays of high plast icity. fal clays) andrnGM (silty gravels, gravel-sand-clay mixtu res) . Al l the soil types show thickness variation bothrnlaterally and vertically ranging from about 0 .85 COl to about 3.5m.rnMoisture content at the time of sampling ranges from 25. 1 to 29.0% for clay soils, and from 18 torn- 46 % for sandy si lt and silty sand/gravel soils. The specific gravity of the soils is low, in the rangernfrom 2.2 to 2.5. The liquid limit and plast icity index of the clay soils range from 28 10 53% andrnfrom 7 to 24% respectively . The sandy silt soils have LL% in the range from 26.7 to 56% and PI %rnin the range from 1.89 to 25% .rnThe soils of the area are found out to be non expansive as revealed by the simple free swell test.rnHowever , some of the sandy sil! and silty sand/gravel soils show reduction in volume as large asrnIXrn29% . thus suspected to have a collapsible structure . These soils may collapse both under load andrnwhen nooded by water and may also result in differential seulement. Transmitting the weighl ofrnthe structure to deeper soil or rock layer by means of piles or caissons. and if not. to make thernfoundation as deep as possible is believed to minimise the hazard .rnFrom engineering geological point of view, the rocks outcropping in the town area are grouped in 10rntwo: strong (basalts and rhyolites) and weak rocks (dominantly of basaltic hayalloclastites andrntuffs). From field Schmidt hammer lest result, it is found out that the fanner ones have anrnunconfi ned strength in the range from 310 to greater than 700 Kgfcm1, while the laner less than 100rnKgfcm2rn• The strong ones are found out to be good sources for aggregates while the weaker ones arernnot suitable fo r most of civil engineering works .rnBased on the fie ld and laboratory invest igation results, Engineering geological mapping at the scalernof I: 10,000 was carried out for the town area including the possible expa nsion area

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