The intended site for the dam is located in a wide valley of the Gilgel Abbay River with an overall elevation difference between the upper plateau and the river level of about 85 m. The river has eroded a series of volcanic rock units of early Tertiary origin, with alternative levels of basaltic lava flows and ash or tuff deposits. The lava flows result in hard to medium rocks, jointed and fractured. The two volcanic units mentioned (lava flows and tuffs) are approximately laid in a horizontal manner. Tuffs and ash deposits, which have a very similar appearance, are white in colour and they are clearly identified when exposed. They constitute the substratum of a dominant proportion of the dam foundation. They tend to produce gentle slopes and, in the lower cultivated plots they are covered by alluvial clays. The exposed tuff is not cultivated. The exposed tuff is eroded by running waters and it shows erosion patterns similar to other soft clayey rocks. The resulting erosion forms tend to be rounded. However no firm evidence of piping was observed. The paper reports laboratory experiments on specimens of the intact tuff material. The suitability of the tuff, once compacted, as a core material for a zoned-earth dam was also investigated.

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