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Underground excavation advancing on LHWP II diversion tunnels – International Water Power and Dam Construction

By Staff , in Hydropower , at October 22, 2020

22 October 2020

Underground tunnelling works inside the two Polihali Diversion Tunnels which form part of the Lesotho Highlands Water Project Phase II is progressing well with excavations advancing from the outlets at about six metres a day per tunnel, the Lesotho Highlands Development Authority has confirmed.

The advancements come despite a slow start in May due to extended COVID-19 restrictions which restricted shift work and reduced the number of workers, to enable social distancing. Continued border restrictions between Lesotho and South Africa also pose travel challenges for South African-based technicians and the movement of equipment and supplies. 

Excavation of the intake and outlet portals has been completed. Work on the tunnels’ concrete lining is underway with concrete casting and reinforcement having been started on tunnel one. Both tunnels intake and outlet portals drives have been supported with sprayed concrete and rockbolts. Preparatory works for the concrete inlet structure are also ongoing.

Diversion tunnels inlet portals with ventilation duct, October 2020

“As construction picks up on the diversion tunnels, the teams continue to observe LHDA’s strict health and safety protocols to ensure that the work environment is safe for crews,” explained Tente Tente, LHDA’s Chief Executive. “These protocols include airborne contaminant monitoring, monitoring of the excavation works, rock support and work area temperature and monitoring air flow among other measures. The project invests heavily in safety measures hence the installation of ventilation fans inside the tunnels, in August.” 

The tunnels, one 7m in diameter and almost a kilometre in length, and the second, 9m in diameter and also almost a kilometre long, run parallel to each other from the intake point to the outlet downstream of the dam. The tunnels are being excavated by drill and blast method and are supported by rockbolts and shotcrete as required.  They are being excavated in advance of the construction of the Polihali Dam to reduce the dam construction period. Impoundment of Polihali Reservoir is expected to commence in 2024 with water delivery planned to start in 2027.

Polihali Diversion tunnels with shotcrete support at the inlet portal, October 2020

SCLC Polihali Diversion Tunnel Joint Venture which comprises Salini Impregilo S.p.A (South African branch), Cooperativa Muratori Cementistri CMC di Ravenna (South African branch), LSP Construction (Pty) Ltd. (Lesotho) and CMI Infrastructure Ltd (South Africa) is constructing the tunnels while the Metsi a Senqu-Khubelu Consultants Joint Venture (MSKC) designed the diversion tunnels and is supervising construction. It includes South African and Lesotho-based firms i.e. Zutari (SA), Knight Piesold (SA), Hatch Goba (SA), SMEC (SA) and FM Associates (Lesotho).

The Lesotho Highlands Water Project (LHWP) is multi-phased, multi-billion Maloti/Rand project between the governments of the Kingdom of Lesotho and the Republic of South Africa. It comprises water transfer and hydropower generation components with associated ancillary developments.  The water transfer component entails the construction of dams and tunnels in Lesotho, enhancing the use of water from the Senqu (Orange) River and its tributaries by storing, regulating, diverting and controlling the flow to effect the delivery of specified quantities of water to South Africa, and utilizing the delivery system to generate hydro-electric power in Lesotho.

The major works of Phase I included the construction of the Katse Dam, the transfer and delivery tunnels, the Matsoku weir, the ‘Muela Hydropower Plant and the Mohale Dam. The Phase II water transfer component comprises a dam at Polihali and a gravity tunnel that will connect the reservoir at Polihali with the Katse reservoir. The further feasibility studies for the hydropower component of Phase II have concluded that conventional hydropower is the more feasible option to meet Lesotho’s energy needs.

The Lesotho Highlands Development Authority (LHDA) is the implementing and management authority of the Lesotho Highlands Water Project, on behalf of the government of Lesotho.