Large dams | engineering insights Polihali Dam:
Nts’oli Maiketso, Divisional Manager for Phase II of the Lesotho Highlands Water Project, shares insights into the design innovations, engineering challenges, and community benefits of the land-mark Polihali Dam
NESTLED IN THE RUGGED highlands of Lesotho near Mokhotlong and approximately one kilometre downstream of the confluence of the Khubelu and Senqu (Orange) rivers, the Polihali Dam – which will create a reservoir capable of storing 2,325 million m3 of water – is poised to become a feat of engineering. Standing at an impressive height of 166m, this Concrete Face Rockfill Dam (CFRD) is the centerpiece of Phase II of the Lesotho Highlands Water Project (LHWP).
Design and construction of the Polihali Dam Engineering challenges and innovations Concrete Face Rockfill Dams are renowned for their adaptability to varying terrains and cost-effectiveness. However, they are not without challenges, such as face slab cracking, water leakage and material degradation. Learning from these experiences, including from the Mohale Dam – another basalt rockfill CFRD constructed during Phase I of the Lesotho Highlands Water project – solutions were adopted to mitigate risks. These included the use of advanced 3D numerical modelling. Back- analyses of the Mohale Dam, for example, informed the understanding of deformation patterns and stress concentrations. This approach allowed engineers to refine Polihali’s design, incorporating measures to reduce rockfill compressibility and enhance the concrete face’s deformation capacity. For instance, vertical joints in the face slabs were
reduced from 15m to 7.5m near the steeper right abutment to alleviate tensile stresses. Additionally, a three-barrier joint system was introduced to ensure watertightness and accommodate potential deformations. These innovations represent a significant step in CFRD design.
Harnessing local materials Basalt, the predominant rock type in the region, is the primary source of both rockfill and concrete aggregates. Stringent construction specifications have been implemented to address concerns about basalt durability, particularly in exposed areas. By targeting optimal porosity levels and conducting trial embankment tests, the designs have ensured that the rockfill will perform reliably under various conditions.
Stability and safety first
Stability analyses for the Polihali Dam considered a range of loading scenarios, from usual operations to extreme events. Using large-scale triaxial tests and size- scale corrections, engineers defined shear strength parameters for the basalt rockfill. The results confirmed
46 | September 2025 |
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the suitability of the dam’s slopes – 1V:1.4H upstream and 1V:1.5H downstream –for maintaining stability. Long-term settlement projections were informed by data from similar projects, such as Brazil’s Foz do Areia Dam. These insights guided the design of the dam crest and other critical components, ensuring resilience over decades of operation.
Modelling the future At the heart of Polihali’s design process was the Modified Mohr-Coulomb constitutive model, an advanced tool for simulating soil and rockfill behaviour. By validating this model against real-world data from the Mohale Dam, a high degree of confidence in the predictions for Polihali was achieved. The 3D Finite Element Model (FEM) provided invaluable insights into expected deformations, stresses, and joint behaviours during construction and impounding phases. For example, the FEM revealed that maximum settlements at the end of construction would be around 1.1m, while displacements during impounding would peak at 0.4m –well within acceptable limits. These findings informed the design of face slabs, joints, and other elements, ensuring robust performance under all conditions.
Broader infrastructure While the Polihali Dam is the centerpiece of Phase II
of the LHWP, the broader infrastructure supporting its construction promises lasting benefits for local communities. Roads, housing, telecommunications, and other essential services have been developed not only to facilitate the dam’s construction but also to leave a legacy of improved connectivity and quality of life. Key among these developments is the construction of access roads, including a 54.3km permanent road (Polihali Western Access Road or PWAR) that will connect the dam site to national infrastructure. The upgraded road network will improve transportation for residents and open up economic opportunities by enhancing access to markets and services. Additionally, major bridges like the Senqu Bridge - spanning 880m and rising 110m above the riverbed – will provide critical links for both project logistics and long-term community use. The bridge enhances access between communities and critical services like education and health and will be a tourist attraction for regional and international visitors, improving livelihoods throughout the tourism value chain. Housing and associated infrastructure are also central to the project. A permanent staff village (Polihali Village), the Polihali Operations Centre which houses
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