The Lesotho Highlands Development Authority (LHDA) this week celebrated a tunnelling milestone on the Polihali Transfer Tunnel construction site.
This, the LHDA said, is a successful breakthrough of the upper intake tunnel into the lower intake tunnel after seven months of tunnelling.
It was referred to as a milestone in Polihali Transfer Tunnel works and a significant step in the journey towards the impoundment of the Polihali reservoir.
According to LHDA public relations officer Mpho Brown, key milestones in the Polihali Transfer Tunnel works include the excavation of the 450-metre long tunnel which began on January 16 and was completed on August 19; the completion of the 370-metre long lower intake tunnel on 5 August and the ongoing excavation of the connecting tunnel.
“The 1322-metre long connecting tunnel will connect the tunnel boring machine (TBM) access adit at the chamber where the TBM will be assembled, marking the start of the TBM-bored transfer tunnel to Katse. To date, 26 metres of the connecting tunnel have been completed.
“These achievements are testament to the dedication and expertise of the LHDA and its consultants and contractors: Metsi a Senqu Khubelu Consultants (MSKC), designers of the Polihali Transfer Tunnel and the Kopano Ke Matla Joint Venture, which is constructing the tunnel,” he noted.
The intake tunnels will allow water from the 5,042-hectare Polihali reservoir to enter the transfer tunnel for delivery to the Katse reservoir, the centre point of the Lesotho Highlands Water Project.
Brown said the breakthrough of the intake tunnels is more than just a technical accomplishment; it signifies the start of two critical phases in the project.
He explained that the first phase involves the safe commencement of impoundment of the Polihali reservoir, made possible by the collapse of the upper tunnel into the lower one.
“This process allows for a controlled and secure increase in water levels up to the lower tunnel, which can then be temporarily sealed.
“The second phase enables continuous debris removal from the remaining tunnel works. As construction progresses towards its connection with the access adit, debris can be efficiently cleared through the upper intake tunnel without interrupting the impoundment process. This efficient operation is crucial for maintaining project momentum while ensuring the structural integrity of the tunnels,” he explained.
Brown also pointed out that the lower intake tunnel, which plays a vital role in the Polihali Dam’s operational future, will facilitate the intake of live water even at low reservoir levels. This feature is essential for ensuring the uninterrupted flow of water, underscoring the importance of this milestone.
