The Lesotho Highlands Development Authority (LHDA) has reached a milestone in the construction of Phase 2 of the Lesotho Highlands Water Project (LHWP) with the breakthrough of two diversion tunnels on August 16.
This marked the completion of the 1 870 m of tunnel excavation that the authority has been working on since June last year.
The Polihali diversion tunnels, which are 7 m and 9 m in diameter, respectively, and both almost a kilometre long, were excavated by drill and blast method.
The progress on the tunnels was achieved despite challenges around Covid-19, harsh weather conditions, flooding of the Senqu river in January and other social challenges.
Completed works on the contract include excavation of the portals and the in-situ reinforced concrete lining of the inside walls of the two tunnels for about 35 m from the entrance of both tunnels.
Ongoing works on the project include the finalisation of construction of the concrete intake structure on the 9-m-diameter tunnel, while construction of the intake structure on the ancillary 7-m-diameter tunnel is almost complete.
The remaining works include invert concrete lining for the overall lengths of the tunnels and some minor surface works.
The diversion tunnels will convey water that is diverted away from its natural path (the river) to ensure that, during construction of the Polihali dam, work can take place in a dry area, uninterrupted by the river flow.
The LHDA advanced the construction of these diversion tunnels to enable the dam contractor, once appointed, to immediately focus on diverting the river through the tunnels instead of the traditional approach of incorporating the diversion tunnel works as part of the dam construction scope.
As a result, the dam contractor will also have earlier access to the river section than normal.
The contractor for the diversion tunnels, the SCLC-JV, comprises South Africa-based Salini Impregilo, Cooperativa Muratori Cementistri CMC di Ravenna (South African branch), Lesotho-based LSP Construction and South Africa-based CMI Infrastructure. The JV is building the tunnels under the supervision of the Metsi a Senqu-Khubelu Consultants Joint Venture (MSKC-JV), which also designed the diversion tunnels and oversaw construction.
The supervising engineer, MSKC, also includes a number of South African firms such as Zutari, Hatch Goba, Knight Piesold and SMEC, as well as Lesotho-based firms FM Associates, S5 Construction Consultants and White Life Consultants.
The diversion tunnels construction contract was awarded in February 2019 and its completion is planned for the end of this year.
The LHWP Phase 2 builds on the successful completion of Phase 1 in 2003. It delivers water to the Gauteng region of South Africa and uses the water delivery system to generate electricity for Lesotho.
Phase 2 will increase the current water transfer rate of 780-million cubic metres a year incrementally to more than 1 270-million cubic metres a year.
Simultaneously, it will increase the quantity of electricity generated at the Muela hydropower station from about 500 GWh/y to 800 GWh/y and is a further step in the process of securing an independent electricity source to meet Lesotho’s domestic requirements.