A GEOTECHNICAL OVERVIEW OF KATSE DAM AND TRANSFER TUNNEL, LESOTHO, WITH A NOTE ON BASALT DURABILITY

The Katse Dam and Transfer Tunnel are part of the Lesotho Highlands Wa ter Project and are constructed in the basalts of the Drakensberg Form ation. The project will supply water to the main industrial area of So uth Africa around Pretoria, Johannesburg and Vereeniging. The project commenced in 1987. Many of these basalts have been subjected to deuter ic alteration and consequently contain expansive clay minerals. The pr esence of expansive minerals can mean that the basalts disintegrate ra pidly on exposure. A series of tests were carried out to assess the du rability of these basalts. The olivine basalts and those types which c ontained disseminated clay spots were the least durable. They were uns uitable for use as concrete aggregate. The texture of the basalt, howe ver, can restrict access of water to swelling minerals and therefore a ffect breakdown and durability Breakdown of basalt led to slabbing and spalling from the perimeter of the tunnel, and the complexity of weat hering and jointing in the basalt meant that the foundation depth was increased at the dam site. It also has led to a delay in the completio n of the Transfer Tunnel as it now has to be lined completely instead of partially.