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 Water Project and are constructed in the basalts of the Drakensberg Formation. The project will supply water to the main industrial area of South Africa around Pretoria, Johannesburg and Vereeniging. The project commenced in 1987. Many of these basalts have been subjected to deuteric alteration and consequently contain expansive clay minerals. The presence of expansive minerals can mean that the basalts disintegrate rapidly on exposure. A series of tests were carried out to assess the durability of these basalts. The olivine basalts and those types which contained disseminated clay spots were the least durable. They were unsuitable for use as concrete aggregate. The texture of the basalt, however, can restrict access of water to swelling minerals and therefore affect breakdown and durability. Breakdown of basalt led to slabbing and spalling from the perimeter of the tunnel, and the complexity of weathering 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 completion of the Transfer Tunnel as it now has to be lined completely instead of partially.