After seven months of tunnelling on the Polihali transfer tunnel construction site, the Lesotho Highlands Development Authority (LHDA) has achieved the successful breakthrough of the upper intake tunnel into the lower intake tunnel.

The successful completion of the intake tunnels brings the LHDA one step closer to realising the full potential of the Polihali dam: increasing the volume of water transferred to South Africa, increasing the volume of hydropower generated in Lesotho, enhancing the livelihoods of the Basotho and contributing to the economies of Lesotho and South Africa.

The intake tunnels will allow water from the 5 042-ha Polihali reservoir to enter the transfer tunnel for delivery to the Katse reservoir, the centre point of the Lesotho Highlands Water Project.

The authority will now embark on the next two critical phases in the project, one of which is the safe start of the impoundment of the Polihali reservoir, made possible by the collapse of the upper tunnel into the lower one.

The 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, the LHDA said in a statement sent to Engineering News on Thursday.

“This efficient operation is crucial for maintaining project momentum while ensuring the structural integrity of the tunnels.”

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 and is essential for ensuring the uninterrupted flow of water.

Key milestones in the Polihali transfer tunnel works include the excavation of the 450-m-long tunnel, which started on January 16 and was completed on August 19; the completion of the 370-m-long lower intake tunnel on August 5 and the ongoing excavation of the connecting tunnel.

The 1 322-m-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 m 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, designers of the Polihali transfer tunnel, and the Kopano Ke Matla joint venture, which is constructing the tunnel,” the LHDA concluded.