[4] Investigation of the dam site in the "Highlands of Lesotho" has revealed "rounded, steep sided valleys" where the soil cover is thin and basaltic lava is the dominant formation.

[5] The dam site has been identified with two geological features of faults which needed treatment measures.

Subsequently, Director of Public Works, requested an engineer to examine the potential of the project as a possible means to supplement the water supply needs in South African gold mines.

Phase I covered some of the key strategic components of the project, while Phase IB's financing covered not only engineering design and supervision of the main works but an institutionalized framework with a panel of experts in the field of engineering and the environmental and social aspects of the project.

The water supply meets the needs of the Gauteng region which encompasses the mining and industrial cities of Johannesburg and Pretoria.

Both the "socle" and the toe-slab incorporate movement joints consisting of 20 millimetres (0.79 in) compressible materials and a PVC water stop.

A layer of earth fill is also laid on the upstream part of the toe slab over a width of 70 metres (230 ft), and on the downstream side protection includes a geo-textile and "reverse filters of 2D and 2E material".

[7][16] The intake for water transfer is located 5 kilometres (3.1 mi) upstream of the dam face on the left bank.

Two quarries were used; one to supply the bulk of the rock for the rock-fill and the other for the higher quality doleritic basalt used in the concrete aggregate and filter layers in the dam.