The Lesotho Highlands Water Project: A Geotechnical and Engineering Geological Review of the Initial Phase

摘要

The purpose of Lesotho Highlands Water Project isprimarily to supply water from Lesotho to Gauteng Province, theindustrial region of South Africa. In addition, the hydroelectricscheme at Muela will supply enough electrical power to meet theneeds of Lesotho. The project consists of a number of dams,reservoirs and tunnels, and a hydroelectric scheme which will beconstructed in four phases. Construction operations, if the projectruns to completion, will last until around 2020. Phase IAcommenced in 1987 and consists of Katse Dam, a Transfer Tunnelfrom the impounded reservoir to the Muela Hydroelectric PowerStation, the Muela Dam, and a Delivery Tunnel from the latter damto the outfall on the Ash River, which is part of the Vaal Riversystem. The tunnels total some 81 km in length. Except for theadits which were drilled and blasted, the tunnels were constructedby five tunnel boring machines.The Katse Dam and Transfer Tunnel were constructed in thebasalts of the Drakensberg Formation. Many of these basalts havebeen subjected to deuteric alteration and consequently containexpansive clay minerals. Their presence can mean that the basaltsdisintegrate rapidly on exposure, which led to increased excavationat the dam site, and slab-bing and spalling from the perimeter ofthe tunnel. The Muela Hydroelectric Power Station and DeliveryTunnel South were constructed primarily in sandstones of theClarens Formation. These sand. stones presented few problemsduring excavation and this tunnel was bored 20 months ahead ofschedule. It was anticipated that the Delivery Tunnel North wouldbe the most difficult to excavate as it runs through a succession ofmudstones and sandstones. Hence, this tunnel was excavated by adouble-shielded tunnel boring machine which could bore throughvaried ground and provide protection against rock fall, whilesimultaneously installing precast lining segments.