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The giant tunnel boring machine activated in April in Lesotho marks a new phase of the Polihali water megaproject, which is now advancing with two machines simultaneously in the 38.5 km tunnel, while the main dam exceeds 53% of the total fill volume and is heading towards operation in 2029
The giant tunnel boring machine used in the Polihali transfer tunnel came into prominence in April 2026 in Lesotho and accelerated the pace of one of the region’s most ambitious water projects. With approximately 423 meters in length and a 5.38-meter cutting head, the machine joined the first excavator already in operation to simultaneously open, from both sides, the 38.5-kilometer tunnel that will connect the Polihali and Katse reservoirs in the Maluti mountains.
The progress is notable for the size of the structure and the expected impact when the work is completed. The gravity tunnel will transport water from the Polihali reservoir to Katse, enhancing regional water security and strengthening hydroelectric generation in Lesotho. At the same time, the Polihali dam has already reached 7,756,980 cubic meters of rockfill, equivalent to 53% of the total fill volume, and continues to rise towards its final height of 165 meters above the Senqu riverbed.
What happened in April with Polihali’s giant tunnel boring machine
In April, the second giant tunnel boring machine came to symbolize the project’s entry into a more intense excavation phase. The new machine will be officially presented by authorities from South Africa and Lesotho, in a move considered a milestone for Phase 2 of the Lesotho Highlands Water Project, the LHWP.
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As a result, the Polihali transfer tunnel is now being excavated simultaneously from both ends. This change accelerates a decisive stage of the work and reinforces the project’s transition from preparation to large-scale execution.
The numbers that explain the size of the giant tunnel boring machine and the tunnel
The new giant tunnel boring machine used in Polihali measures approximately 423 meters in length and operates with a 5.38-meter cutting head. According to the Department of Water and Sanitation, the machine was designed to work continuously even in the harshest underground conditions.
The Polihali transfer tunnel will be 38.5 kilometers long. It was designed as a gravity tunnel, meaning that water from the Polihali reservoir will flow to the Katse reservoir without relying on constant pumping. During excavation, the machine also installs pre-cast concrete lining segments, transforming raw rock into the final structure in a continuous process.
How much each machine has already advanced in the work
The Polihali tunnel boring machine arrived on site in July 2025 and began excavation in January 2026. By mid-February 2026, it had already advanced 235.8 meters, according to the Lesotho Highland Development Authority.
The first tunnel boring machine, launched on the Katse dam side in February 2025, had been opening the tunnel for longer. By mid-January 2026, this machine had reached 614.5 meters. Now, with both in operation and with the new milestone recorded in April, the work gains speed in a critical stage.
What changes in practice when the tunnel is completed
When the Polihali tunnel is ready, it will allow the transfer of significantly larger volumes of water. This is the central point of the work. The project was designed to reinforce regional water security and, at the same time, boost hydroelectric power generation in Lesotho.
In practice, water will flow from the Polihali reservoir through the tunnel to the Katse reservoir and then continue towards South Africa. This expands water delivery capacity and consolidates the system’s strategic role within regional water integration.
Polihali dam has already exceeded 7.7 million m³
While the tunnel advances under the mountain, the Polihali dam continues to grow at a strong pace. The work has already reached 7,756,980 cubic meters of rockfill placed, which represents 53% of the total volume planned for the embankment.
At the current pace, with over 20 thousand cubic meters per day, the structure continues to rise towards its final height of 165 meters above the Senqu River bed. The dam crest will be 921 meters long, and the reservoir created by it will have a total storage capacity of 2,325 million cubic meters, more than double the capacity of the Mohale Dam, built in Phase 1 of the project.
Why this project became one of the most ambitious in the region
The Polihali Dam is one of the main components of Phase 2 of the LHWP. In addition to it, the package includes the transfer tunnel and the Senqu bridge, an essential structure to maintain access to the region when the reservoir is filled.
The dam project was implemented downstream of the confluence of the Khubelu and Senqu-Orange rivers. Its role goes beyond water storage. The structure is the physical basis that enables the new water transfer arrangement and supports the expansion of the system’s regional capacity.
The next steps after the progress recorded in April
Other fronts of the project are also advancing. The construction of the main dam’s outer plinth was 95% complete by the end of February. The concrete for the base of the intake tower was completed on February 21, and the spillway excavation reached 97%.
The work remains on schedule for commissioning in 2029. Another milestone expected this week is the joint inauguration of the Senqu bridge in Mokhotlong, by the President of South Africa, Cyril Ramaphosa, King Letsie III, and the Prime Minister of Lesotho, Samuel Matekane.
The Senqu bridge shows that the project goes beyond the tunnel and the dam
The new Senqu bridge was built to replace the current structure, which will be submerged when the Polihali reservoir is filled. This ensures continuous access to Mokhotlong, Sani Pass, and neighboring areas, preventing the new dam from isolating the region.
Built for approximately R2.4 billion, the bridge is the largest of three major bridges planned to cross the Polihali reservoir. It measures about 825 meters long and 90 meters high, which demonstrates the scale of the ongoing infrastructure package.
Why the giant tunnel boring machine became a symbol of the project’s progress in April
The entry of the second machine was described by the Department of Water and Sanitation as a powerful symbol of the project’s progress. In April, this movement gained even more political and technical significance, as it marks the effective acceleration of the excavation of the tunnel that will connect two large reservoirs.
Overall, the project brings together a 423-meter giant tunnel boring machine, a 38.5-kilometer tunnel, a 165-meter high dam, a reservoir with a capacity of 2,325 million cubic meters, and an 825-meter bridge. It is this sum of numbers that explains why Lesotho considers the project strategic for strengthening water and energy in Southern Africa.
Do you believe that projects of this scale, already accelerated in April with a giant tunnel boring machine and a high-speed dam, should become a priority in regions that need to ensure more water and energy in the future?
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