The geology of the Lesotho Highlands Water Project with special reference to the durability of construction materials

The Lesotho Highlands Water Project is situated in the Kingdom of Lesotho and the adjoining north-easthern part of the Orange Free State Province of the RSA in an areaunderlain by Triassic and Jurassic basalts of the Lesotho Formation and Triassic sandstones and mudrocks of the Karoo Sequence. The Project will consist of a series of dams and tunnels to convey water from the Lesotho Highlands to the industrial centre of the Republic of South Africa.

The geological setting of the project area and some engineering geological properties of the underlying stratigraphic horizons is discussed.

The geotechnical properties of the different rock types are discussed with special reference to their use as concrete aggregate. The durability of basalt is primarily determined by the amount of secondary smectite clays present in the rock. These clays occur as discrete grains or as intergrowths with other secondary minerals disseminated throughout the rock. The clays originate from the deuteric alteration of primary glass, pyroxene, olivine and rarely plagioclase and also occur as infillings of amygdales. The durability of the sedimentary rocks is a function of their dimensional change with variability in moisture content and also the degree of cementation between grains.