HIGH in the Maluti Mountains of Lesotho, an engineering landmark stands proud.
The Senqu Bridge, the first extradosed bridge to be built in Lesotho, will be more than just a crossing over the Polihali reservoir. It will be a lifeline, reconnecting communities on either side of the vast new reservoir, and a showcase of innovative bridge engineering in one of Africa’s most challenging environments.
When the Polihali Dam is complete, its reservoir will flood 5,042 hectares of the Senqu and Khubelu river valleys, submerging roads, farmland, and even sections of Lesotho’s vital A1 national route. Without intervention, Mokhotlong would lose its primary road link to the rest of the country.
Three existing river crossings on the primary A1 route will vanish beneath the water, including the deepest at the Senqu River, where the valley will be inundated to a depth of around 80 metres.
The solution: a new bridge that could soar over the future lake, restore connectivity, and withstand the extremes of mountain weather.
From conventional to cutting-edge
The original concept for the Senqu Bridge was an 830-metre incrementally launched structure with 16 piers and 50-metre spans. But this plan posed a major problem: three piers would have to be built in or near the river itself, requiring massive coffer dams to hold back unpredictable floodwaters.
The breakthrough came with the decision to adopt an extradosed bridge design — a hybrid between a box-girder bridge and a cable-stayed bridge.
By doubling the central span from 50 to 100 metres, engineers eliminated the need for a central pier in the river. This not only reduced construction risk and the environmental impacts but also simplified work in the steep, oblique valley crossing.
The role of the cables
The extradosed design’s signature cable-stayed section plays a dual role. During construction, as 25-metre deck segments were launched from each abutment, the cables supported the leading edge of the deck as it extended over the void.
Initially, only the topmost cables are installed but once the 100-metre central span is reached, the full array of cables is engaged, permanently supporting the deck against traffic loads, wind, and seismic forces.
Further, the cables have enhanced the aesthetic appeal of the bridge, consequently creating spin off tourism opportunities from the looks of the bridge, which will consequently create benefit for communities down the tourism value chain.
Engineering at 2,100 metres
Building in the Lesotho Highlands is not for the faint-hearted. The site sits at 2,100 metres above sea level, accessible only via steep mountain passes topping 3,000 metres. Most materials must be trucked in from South Africa, adding border logistics to the list of challenges.
Winter temperatures can plunge below –15°C, while summer highs can exceed 35°C. Violent thunderstorms, strong winds, and snow complicate work schedules. To keep crews safe and productive, the project team deployed freezer suits, infrared heaters, and low-temperature accommodation units — measures rarely seen on African construction sites.
The terrain itself is equally unforgiving. On the west bank, access roads had to be blasted into sheer cliffs, and foundations for the first five piers were excavated sequentially from the top down.
Designing for a flooded future
One of the most unusual engineering challenges was designing tall, slender reinforced concrete piers that will eventually stand submerged up to 80 metres underwater. The team had to account for hydrodynamic forces, potential seismic events, and even the potential impact of a landslide into the reservoir. This required significant reinforcement in both piers and foundations.
The extradosed deck also demanded dense reinforcement, particularly around the cable pylons, leading to complex steel fixing operations. Close collaboration between contractor and engineer was essential to keep progress on track.
Milestones and momentum
Construction of the bridge began in earnest in June 2023 following the completion of the site establishment work.
By July 2024 eight of the 15 piers had been completed, and work was underway on the leading deck segments on both banks.
By December 2024, the contractor had completed all 15 piers and both abutments — a major milestone.
By the end of February 2025, the two extradosed cable-stay sections had been cast and launched, the deck was more than 25% complete and the overall project was past the 75% mark.
In early August 2025 excitement was building on the Senqu Bridge construction site: the leading edge of both decks had reached the final piers leaving only the final 100m left to launch.
On 15 August, the final deck segment on the western abutment was cast with the final deck segment on the eastern abutment cast only a week later.
By the end of August, the final stages of the deck construction were well underway. Inside the deck, construction of the stiffening diaphragms that sit above each of the piers were well underway.
On top the installation of the ten remaining cable stays (five on each side) had begun. Installation of the final cable stays on the two extradosed sections of the bridge decks was completed in early October.
On the leading edges removal of the temporary steel “noses” was nearly complete in August. While on the abutments the dismantling of the temporary construction sheds was also close to completion.
With crews working from both ends of the bridge the deck steadily extended toward the centre. In September the two halves left a 1.5-metre gap which was stitched together to complete the deck on 21 October 2025. This involved winching the bottom shutter and incorporated platform into place below the stich, to enable the ‘stitching’ to be completed to form continuous deck.
After the stitching work continues the final stressing of the deck’s internal cables, the balustrades and handrails. Work on the approach roads continues in earnest and finally the bridge deck will be surfaced.
This stage saw the careful winching of the bottom shutter and its platform into place beneath the stitch, making it possible to complete the ‘stitching’ that joins the two sides of the bridge deck into one continuous structure.
Following the stitching, work proceeds with the final stressing of the internal cables and the installation of balustrades and handrails. Work on the approach roads is progressing steadily, and afterwards the bridge deck will be surfaced, marking one of the final steps toward completion.
The Senqu Bridge is planned to be open to traffic in the first quarter of 2026 — restoring a vital artery across Lesotho’s highlands and standing as a testament to ingenuity under extreme conditions.
“This bridge is not just about engineering,” says Ntšoli Maiketso, Divisional Manager for Phase II of the project. “It’s about ensuring that communities remain connected, that children can still get to school, and that trade and livelihoods can continue despite the changes the dam will bring.”
When the ribbon is finally cut, the Senqu Bridge will not only span a deep valley — it will bridge the gap between isolation and opportunity for thousands of people in Mokhotlong’s rugged mountains.
The Senqu Bridge construction tender was awarded to the WRES Joint Venture, which includes South African, Lesotho and international companies as per the requirements of the Phase II Agreement. The primary partners are: Webuild S.p.A. (Italy); Raubex Construction (Pty) Ltd (South Africa); Enza Construction (Pty) Ltd (South Africa) and Sigma Construction (Pty) Ltd (Lesotho).
Sub-contractors include: EXR Construction (Pty) Ltd (South Africa; Gleitbau-Geselschaft (Austria); Post Tensioning and Structural Solutions (Pty) Ltd (South Africa) and Freyssinet International et Cie (France).
Work on the bridge design started in 2018, led by Zutari, formerly Aurecon Lesotho, which also designed the Mabunyaneng and Khubelu bridges, the other two major bridges constructed under Phase II. Zutari is also supervising the construction of the three bridges.
