The Lesotho Highlands Development Authority (LHDA), consultants and contractors on the Polihali dam have marked an early construction milestone – the diversion of the Senqu river by the pre-cofferdam, and the subsequent diversion of the river into and through the diversion tunnels ahead of the construction of the cofferdam upstream of the Polihali dam wall.
This forms part of the Lesotho Highlands Water Project Phase 2.
"This is quite an achievement to the engineering fraternity. It is a key moment of the construction programme because unless this happens, no other elements of dam construction can happen. It had to happen now because the timing of it during the winter season without rain was critical. If you miss it by a week or two, you may miss the entire window by another 12 months.
"We are proud of everyone who has had a hand in this work. It is indicative that Phase 2, just a couple of months after being officially opened by our principals, is indeed now going full steam ahead. Our consultants and contractors have hit the ground running and we are all excited for what this means for the project, and for Lesotho and South Africa,” LHDA CE Tente Tente comments.
The Polihali dam requires extensive excavation of the foundations before work begins in bringing it up in height. The works in the foundations require dry access to enable the rock to be excavated down to sound rock, to allow grouting of the rock to be undertaken where required beneath the dam and to allow the construction of a plinth beam which will form the water-tight seat for the upstream impermeable, concrete facing of the main dam wall.
To provide this dry environment, a cofferdam is to be built upstream of the main dam to protect the main dam works from floods until the main dam concrete face is high enough to be free of such floods. In turn, to allow the coffer dam to be built there is a pre-cofferdam upstream of the cofferdam. This has the task of diverting the river through the diversion tunnels and thus around the cofferdam and the main dam foundations.
Chris Hall, project manager of the Matla a Metsi Joint Venture, which designed the Polihali dam and is overseeing dam construction, explains what will happen after the closure of the pre-coffer dam.
“Now, the Senqu river level will rise and the water will then enter the diversion tunnels to bypass the dam construction site. Whilst the riverbed is drained of water the coffer dam foundations will be excavated within the river and the flanks of the valley. The plan is to complete the cofferdam before the end of October this year, according to the current master programme, and then to start on the main dam in earnest.”
The diversion tunnels were built as part of the advance infrastructure contracts.
Construction of the Polihali dam started in November 2022, with the awarding of the M7.68-billion construction contract to the SUN Joint Venture. The joint venture includes main partners: Sinohydro Bureau 8 (China); Sinohydro Bureau 14 (China); Unik Civil Engineering (South Africa) and Nthane Brothers (Lesotho). Subcontractors include Melki Civils and Plant Hire (South Africa); MECSA Construction (South Africa); SIGMA Construction (Lesotho) and Kunming Engineering (China).
The Polihali dam is a concrete-faced rockfill dam, like the majestic Mohale Dam which was constructed in Phase I of the LHWP. It will create a reservoir on the Senqu and Khubelu rivers with a surface area of 5 053 ha. The infrastructure also includes a spillway, an intake tower and bottom outlet works, a compensation outlet structure, bridges, roads and a mini-hydropower station.
The Polihali dam adds 2.33-billion cubic metres in storage capacity to the LHWP and will increase the current annual supply rate capacity from 780-million cubic metres to 1.27-billion cubic metres, contributing towards meeting South Africa’s increasing water needs. The additional flow of water from Polihali will simultaneously increase power generation within Lesotho towards meeting Lesotho’s domestic needs and reducing the country’s dependence on electricity imports.