Geotechnical and Engineering Geoloqical Investigation of Sibilu Dam Site, Reservoir and Catchment Area

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Addis Ababa University


In Sibilu dam catchment geomorphological, geological hydrological and engineering geological studies were conducted 10 characterize the lim itat ion and importance of the area for the new d:un construction. Using main ly Ihe, aerial photos and lopo-maps of the area relief and landform, s lope and geomorphological maps of the catchment afC produced at the sca le of I :50,000. Topographic features clearly show that all s urfncc water converges to the reservoir. Hydrological eva luati ons also indi cate that there is no out now of groundwater from Ihe catchment through other rou les, whi ch is in fa vor of the reservoir condition Gcologlcal 11l3pS of Sibi lu catchment and reservoir area arc produced al the scale of 1.50,000 and 1 12,500 lespecti veiy. Basalt. trachyte, pyrocl astic depOS its, rhyolit e, ignimbrite and recent Quatern ary deposits arc exposed III the area The sln/ctmal featu res of the catchment arc mapped from aerial photograph in terpretations. Th ~ N\V-SE alld NE-SW ll11e:unents arc found to he the d0l111113nt lineaments in the catchment Bul the meas urcments and int erpretation of the Jomt sets orient atIons in the reservoir area, conformed that the N-S and E-W arc the dominant jOint sets over NW-SE and NE-SW. Bes ide their concentrat ion th ese Joint sets, especial[y the E-W, nrc op..:n and cont inuolls -n le rock and so ils IIni ts of the reservOIr were described usi ng the ir lithology and genetic type a.<; the main classes for engineering geologlca[ purpose But det.:ul engineering geological descn ptlons lelay on Ihe assemblage of basic engineenng behaviors of these l11a1l1 groups So eight engmccflnf; geologlca[ subgroups of the roc~ IIl11tS arc Identified by their textural, joint and compaction nalures aphanallc, scaraCIOUS, ves icular and halocrystalhne basalis; masS ive and columnar jOlilted tf:lchyte; unwcldcd friab le and welded pyroclastiC deposl1s Rod units III the area are fractured and JOlllted by different sets The strength of basaltic rock ranges from strong to extremely strong With weathcflllg grades or I to IV. Columnar trachyte show very strong nature and weathering grade II Welded VIII pyroclastic depos its and massive trachytc show medium strong nature wilh weathering grades of II and IV respect ive ly. Friable pyroclastic deposit is the only rock un it show weak mass strength. Soil units wcre described into fi ve engineering geological subgroups us ing their grain size distributions and index limi ts: clay, s ilty clay, sandy clay, gravelly clay and clayey gravels. Clays have 1·3% gravels, 3-9% sands, 25-32% silts and 57-68% clay fract ions; with 65- 106% liqu id lim it, 30-85% pl as tic lim its and shrinkage lim it 8- 15%. Silty clays have 4-5% grave ls, 6- 11% sands, 37·41% sillS and 46·5 1 % clay frac tions; with 28-34% liquid limit, 5-6% plas tic limit and 5 I ·57% shri nkage limit. S;Uldy clays h3ve 2-4% grave ls, 38·39% sands, 22·25% s ilts and 34-36% clay fra ctions; with 43-48% li quid, limit 12- 18% pl astic limit and 20-25% shrinkage limi t. Gravelly clays have 22-3 1% gravels, 13· 17% sands, 23·24% silts and 3 I -39% clay fractions. with 3 I -38% liquid limll, 6· [1% pl astic limit and 15- 19% shrinkage limit. Clayey gravels have 54-61% gr.wels, 8- 18% sands, 16% s ilts and 12- 15% clay fractions. With these deSCri ptions, It was possible to classify rock uni ts as IlIgh, Illedllllll and low roc!. mass s trength ; and so ils class ified using the UCS as lIlorganic clays of lugh to extremely high plasticity (CY, CE, ClI), organic d ay of high plasti city (all), inorganic s ilts and clays of low plasticity (CL, ML), inorgan ic sil ts of intermediate plasti city (M I) and clayey gravels, poorl y graded gra vel-sandcl ay mi xtures (GC) All these are presented oil lhe engineering geological map of the reselvoi r area at the sca le of I: 12,500 Black clay soi ls are found in flat terra ins with poor drainage conditions, while red clay soils were developed on s loppy and gentle areas with good dram age conditions. These SOils have formed over the volcanic rocks m the area. Chemi cal analYSIS shows th at the black clays have an average of 41 % sihca, 14 1% of Iota I Iron and 25.7% ofalulllma Red clays havegol an average of34% Silica. 24 5% of tot al iron and 24 9% of alumina. The average COll1pos ltion of basalts of the Ethiopia plateau (Barlh. 1962) is 44% silica. 13% of total iron and CJOIo of alumina Through this, It IS clear that the loss of si lica III black clays is s light, but more evident III red cl ays. Their phYSIcal geotechlllcal analysl"; exhibits that black clay soils show moisture conlent of 38. 75-57.2%, liqUid Ilillit of 89- I 06% and 69- 83 plas ticity index and have 57-68% clay fractions While the red clay soils with 28.6-47% moisture IX corllent, 28·70% liquid limit and 5·32 plasticity index have 34·65% clay fractions. In the pl asticity chart tIlost of the black clay soils plot near U· hne and the red clays at nnd below A·line. Comp3rison of the acti vity of these soils indicates that black clay soils arc more active than the red clay soi ls. Most of the black clay soils arc also potentially expansive while the red clays do not. The bl ack clay soils conta in montimorillonite and red clay soils contain kaolini te as the prin cipal and dominant clay minerals in the reservoir. The main concern of deal ing the clay minerals in the dam engineering is to deline th cir dispers ivity and the black clay soils, contain high Illontirnori ll onite, arc found to be pot entially dispers ive. Sibilu reservoir intersects ridges and hills, which shows small ·scal ed mass movements The tectonic and weathering reatures on these terrarns seem not to create considcrable problems for the reservOir and dam condrt ions. But the one on the Ascre rrdge hab le to cause stability and storage problems duri ng the establishment of the dam From the borehole daw it was ab le 10 IdentLfy three gcotcchn ical layers III thc dam site. sod matenals~ highl y weathered and fragment ed basalIs; and joi nted sound basalt The rock and soi l materials for the construction of the dam arc almost avail able in the vicinity of the area For thc clay core the reserVOlT soils arc cxami ned and the red clay soils arc pre ferred than the black clay saris by their geotechnical propertIes. The Buba hills basalIs fullill the requirements as the construction material for the dam In the construc tion matenals only sands for li lter material are not ava ilable III the surroundlllg area



Geotechnical and Engineering Geoloqical Investigation