Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
The alluvium material will constitute more than 70 per cent of the aggregate sources. The above photo 3.1
shows quarry 2a and 2b on the left bank of the Omo river at the beginning of the rainy season.
The alluvium can be identified as GP (poorly graded gravel) or sometimes SM (low plasticity silt)
particularly in the river bed terraces.
The petrographic analysis showed the basaltic, sometimes trachytic, origin of the alluvium. The chemical
tests confirmed its suitability for concrete and particularly for the RCC dam construction
3.15.3 Basalt
A secondary source of aggregates for concrete is the basalt (columnar, veshicular and unweathered) along
the Omo river Valley.
Both chemical and petrographical analysis have been performed, including mortar bar tests for alkaliaggregate
reaction, confirming its suitability for the aggregate production. Basalts will be also used for the
cofferdam embankment construction.
The following photo 3.2 shows the Tsida I quarry where the columnar basalts are found.
3.15.4 Trachyte
Photo 3.2: Tsida I basalt quarry
The trachyte materials (i.e. tout venant) coming from the excavation of the diversion tunnels will be used for
the cofferdam construction.
The material is recognized as trachyte of light grey type with occasional presence of dark grey and slightly
weathered type.
The following photo 3.3 shows the excavated front just after blasting operations, with excavated trachyte.
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