Underground tunnelling works inside the two Polihali diversion tunnels are progressing well, with excavations advancing from the outlets at about 6 m a day per tunnel at the Lesotho Highlands Water Project (LHWP) site.
So far, the excavation of the intake and outlet portals have been completed and work on the tunnels’ concrete lining is under way, with concrete casting and reinforcement having been started on tunnel one. Both tunnels’ intake and outlet portal drives have been supported with sprayed concrete and rock bolts and preparatory works for the concrete inlet structure also ongoing.
This is despite a slow start in May owing to extended Covid-19 restrictions, which restricted shift work and reduced the number of workers on site to enable physical distancing.
Continued border restrictions between Lesotho and South Africa also posed travel challenges for technicians, also affecting the movement of equipment and supplies, from South Africa.
The tunnels (one 7 m in diameter and almost 1 km long; and the second, 9 m in diameter and also almost 1 km long) run parallel to each other from the intake point to the outlet downstream of the dam.
The tunnels are being excavated in advance of the construction of the Polihali dam to reduce the dam construction period. Impoundment of the Polihali reservoir is expected to start in 2024, with water delivery planned to start in 2027.
“As construction picks up on the diversion tunnels, the teams continue to observe the Lesotho Highlands Development Authority's (LHDA’s) strict health and safety protocols to ensure that the work environment is safe for crews.
"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,” says LHDA CE Tente Tente.