The Mohale Dam on the Senqunyane River, the largest of the three main contracts of Phase IB, started in March 1998. It is a concrete-faced, rock-fill embankment. In contrast to Katse's high-tech structure, Mohale works on a simple principle: a mass of rock compacted across a valley to form a reservoir.
The dam wall is large by world standards - a volume of 7,8 million m3 of rock, 145m high, 620m long at the crest, 600m thick at the base, and l0rn thick at the crest. The reservoir the dam will create is equally large -947 million m3 when full.
A huge spillway, with a weir and flip bucket to slow water flow, will, when complete, be able to divert about 2 500m3/sec of water in case of a flood. The dam is being built by a joint venture between an Italian, a German and a South African company.
Early work included the drill-and-blast construction of two 650m long river- diversion tunnels. The larger of these will be plugged later in the construction process, while the other tunnel (concrete lined) will maintain the 300 litre/second minimum compensation flow into the Senqunyane River.
To facilitate construction, a 300 000m3 cofferdam, with a pre-cofferdam, were built in mid-1999 .
The rock being used to build the dam is basalt taken from quarries on the upstream side of the dam, below the full supply level. Approximately 13 000m3 of rock is placed every day.
Two quarries are 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.
A plinth, built around the edge of the upstream face, seals the dam 's concrete barrier as well as the rock of the riverbed and valley walls. By February 2001 76% of the dam construction was completed. The spillway is presently under construction and Mohale 's constructors are confident of completing the project ahead of the April 2002 date set out in the contract.