The Lesotho Highlands Water Project: A geotechnical and engineering geological review of the initial phase

The purpose of Lesotho Highlands Water Project is primarily to supply water from Lesotho to Gauteng Province, the industrial region of South Africa. I n addition, the hydroelectric scheme at Muela will supply enough electrical power to meet the needs of Lesotho, The project consists of a number of dams, reservoirs and tunnels, and a hyd roelectric scheme which will be constructed in four phases. Construction op erations, if the project runs to completion, will last until around 2020, P hase IA commenced in 1987 and consists of Katse Dam, a Transfer Tunnel from the impounded reservoir to the Muela Hydroelectric Power Station, the Muel a Dam, and a Delivery Tunnel from the latter dam to the outfall on the Ash River, which is part of the Vaal River system. The tunnels total some 81 km in length. Except for the adits which were drilled and blasted, the tunnel s were constructed by five tunnel boring machines. The Katse Dam and Transfer Tunnel were constructed in the basalts of the Dr akensberg Formation. Many of these basalts have been subjected to deuteric alteration and consequently contain expansive clay minerals. Their presence can mean that the basalts disintegrate rapidly on exposure, which led to i ncreased excavation at the dam site, and slabbing and spalling from the per imeter of the tunnel. The Muela Hydroelectric Power Station and Delivery Tu nnel South were constructed primarily in sandstones of the Clarens Formatio n. These sandstones presented few problems during excavation and this tunne l was bored 20 months ahead of schedule. It was anticipated that the Delive ry Tunnel North would be the most difficult to excavate as it runs through a succession of mudstones and sandstones. Hence, this tunnel was excavated by a double-shielded tunnel boring machine which could bore through varied ground and provide protection against rock fall, while simultaneously insta lling precast lining segments.