Gefersa-III Embankment Dam Heightening
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Date
2021-02
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Addis Ababa University
Abstract
The sediment problem is a major problem that highly reduces the live storage capacity of the reservoir and its life span. The Gefersa-I/II dam is one of the major gravity dams in our country Ethiopia which faces a series of sediment deposition problems for a long period. To trap the incoming sediment and to reserve additional live storage, AAWSA tried to build an embankment dam called Gefersa-III the upstream of Gefersa-I/II dam. Even if this dam was built for sediment trap, the reservoir of this embankment was filled with the coming sediment and then the sediment starts flowing to the former dam (Gefersa-I/II gravity dam). Initially, different alternatives were seen as a possible solution to overcome the sediment problem, but in the end, AAWSA decided to heighten the existed embankment dam with a 7m additional height. Hence this case study mainly focused on determining the suitable and stable heightening type which is compatible with the existing dam condition. Among different heightening types, this case study was considered two heightening alternatives (homogeneous and concrete face rockfill heightening) to come with one best heightening type by compare and contrast the considered alternatives from a different point of view. In both heightening alternatives, this case study was tried to keep the operation of the reservoir in the existing embankment during the time of heightening. Due to this reason in both alternatives, this case study was decided to heighten the embankment starting from the crest of the existed embankment by performing local modification only. When the design for both heightening alternatives done, a berm was created on the crest of the existed embankment which was used as a working space for the heightening of the embankment. In both cases, the fill was decided to be done on the downstream face of the embankment to keep the reservoir operation during heightening. In case of homogeneous heightening, the heightening was designed to be raised with 1:3 side slopes at the upstream side and the downstream side slope was kept as 1:3. Whereas the concrete face rockfill heightening was raised with 1:1.3 and 1:1.75 side slope for upstream and downstream side respectively. Steady-state Seepage analysis was conducted for existing embankment and the considered heightening alternatives using seep/w of Geostudio software and as the result shows, the seep water to the body of the embankment was 1.3713*10-6, 3.05*10-7, 8.255*10-8m3/sec for existed, homogeneously heightened and vertical face rockfill heightened embankment respectively. The second section of the modeling part was all about slope stability analysis for different loading conditions of existing and considered heightening alternatives. As the slope stability analysis section shows, the existing and considered heightening alternatives have become safe against slope failure. Deformation analysis was conducted for the existing embankment and both heightening alternatives. In this section upstream and vertical settlement was determined for existing and heightening alternatives. Not only settlement, but stress distribution was also conducted for existing and heightening options. While the modeling section was accomplished, the heightening alternatives were compared and contrasted concerning different points of view, and finally, the concrete face rockfill heightening type was decided as the best heightening type for the existing condition of the Gefersa-III embankment dam.
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Keywords
Gefersa III, Embankement, Heighteng, Dam