A zig-zag geomembrane for a rockfill dam in Ethiopia

A zig-zag geomembrane for a rockfill dam in Ethiopia

The third phase of the Gibe cascade, known as Gibe III HPP, with an installed power of about1870 MW will be one of the largest hydropower plants in Africa. The plant inlcudes a 240 m high RCC dam and a 50 m high rockfill cofferdam on the Omo river. Studio Ing. G. Pietrangeli S.r.l. is the designer ( www.pietrangeli.it ).

The cofferdam body is made with river gravel, basalt and trachyte. The impervious core of the cofferdam is made with a flexible 3.5 mm thick PVC geomembrane sandwiched between two layers of 1200 g/m2 geotextile that protect it against puncturing by the fill materials.

The geomembrane core solution was adopted in light of the necessity to finalize the approximately 500,000 m3 embankment during the short, six-month span of the dry season when the average river flow is 200 m3/s, since during the rainy season the average flow is between 1000 and 1500 m3/s with peak floods that can reach 5200 m3/s, in a return period of Tr=30 years. The geomembrane waterproofing system, which has been designed to follow step by step the construction ofthe rockfill cofferdam, will create a continuous impervious barrier running all along the longitudinal axis of the dam from the bottom cut-off up to the crest.

The geomembrane and its protection geotextiles have been placed in a zigzag pattern. To achieve this result the body of the dam has been constructed in alternated sections which start from the centre line defined by the longitudinal axis and are upstream and downstream directed. The face of each section toward the centre of the dam is finished with a 1V:1H slope on which the waterproofing system has been placed. The first section of the dam body has a height of 6 m, the next sections have a height of 12 m.

The technical paper enclosed at http://www.pietrangeli.it/sppubblications.htm , presented recently at the several International congresses, describes the rationale and details of the design, and the various steps of construction and geomembrane installation.

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