Assessment Of Causes For Partial Settlement Of Gidabo Dam Southern Ethiopia

The present study was carried out at Gidabo Dam, which is proposed on Gidabo River inrnOromia Regional State, about 375 km from Addis Ababa, the capital city of Ethiopia. GidabornDam has faced settlement at the conduit outlet foundation during the construction time whichrnwas measured to be about 0.4 m. The main objectives of this study were to assess the possiblerncauses of partial settlement and to estimate the amount of potential future settlement of therndam. The general methodology followed for the present study was based on thoroughrnliterature review, field investigations and data collection, analysis and evaluation of variousrnsoil parameters of settlement. For the present study immediate and primary settlementrnanalysis was carried out. Elastic theory for cohesive soils, Janbu’s approach and onerndimensional settlement analysis were applied to estimate the settlement amount of the upperrnpart of backfill foundation unit and compressible silty clay layer of the dam foundation. Forrnthe granular soil foundation at the bottom immediate settlement was estimated from in-siturnstandard penetration test (SPT) results.rnThe present study results showed excessive settlement. The estimated settlement is more thanrnthe expected settlement as anticipated in the design of the dam. The differential settlement isrnalso expected at the contact of the backfill material, at outlet conduit and in between thernintake tower and the outlet conduit. As investigated in the present study, the primary causesrnof the settlement are related to unsuitable backfill material comprising alluvium backfill andrnclay cutoff, compressible silty clay layer (organic) below the excavation and due to inappropriaternexcavation method (dewatering process) followed during the construction stage.rnBesides, granular type of soil in the foundation has also contributed for the settlement of therndam in general, and of conduit section in particular. The study also showed that thisrnsettlement also continue in future. Therefore, it is strongly recommended to adopt appropriaternmeasures, as suggested through the present study, so that possible safety and stability of therndam can be ensured during the performance stage.rnKey words: Gidabo dam; Settlement analysis; Janbu Settlement analysis; Consolidation

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