Abstract

The Republic of Ghana in West Africa has experienced several strong earthquakes within historical times, and this record, combined with relatively slight erosion-modification of linear fault scarps, influenced the designers of Akosombo Dam to use a seismic allowance of 0.15 g for this important structure. The Volta River Project supplies hydroelectric power for the entire country and is the largest hydroelectric development in West Africa. The reservoir created by the rockfill dam is one of the world's greatest, with storage capacity approaching 120 million acre-ft, a water surface area of 3000 sq mi, and power production capability of over 700 megawatts with present generating facilities.

As a result of disclosures during design and in the actual construction of the project, the need for use of above-normal earthquake allowances was developed.

Movements on the Accra fault have amounted to more than 32 ft of vertical displacement and have taken place within the past 3000 yrs. The time at which offsetting occurred has been dated accurately as a result of discovery of an inscription carved in the hard Togo orthoquartzite at the Volta Dam site. The carving was at a depth of 32 ft below elevation of the normal river surface. It was disclosed when the area isolated by cofferdams was unwatered to enable construction of the dam's impervious core. Post-Pleistocene offsetting on the Accra fault is proven to exceed 200 ft as established by Fathometer surveys of the river bottom upstream and downstream from the location of the dam's axis. The downstream end of the river's “lake” section is the trace of the Accra fault.

First Page Preview

First page PDF preview
You do not currently have access to this article.