THE design and construction of the Polihali Dam, focusing on structural integrity, materials and seismic considerations
Nestled in the rugged highlands of Lesotho in Mokhotlong, 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 cubic metres of water is poised to become a feat of engineering.
Standing at an impressive height of 166 meters, this Concrete Face Rockfill Dam (CFRD) is the centerpiece of Phase II of the Lesotho Highlands Water Project (LHWP).
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 – the basalt rockfill CFRD constructed during Phase I of the Lesotho Highlands Water project, solutions are adopted to mitigate risks.
These include 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 15 meters to 7.5 meters 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 the suitability of the dam’s slopes 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.
The Polihali and Katse reservoirs connection – ensuring consistent water flow for hydropower and water delivery.
The Polihali reservoir plays a pivotal role in ensuring consistent water flow between the Polihali and Katse reservoirs, enabling reliable hydropower generation and water delivery to South Africa.
This strategic infrastructure not only supports regional economic growth but also reinforces bilateral cooperation between Lesotho and South Africa.
Water stored in the 5,053-hectare Polihali reservoir will be transferred via a 38-kilometre gravity tunnel to the Katse reservoir.
From there, the water flows through existing infrastructure, including tunnels and the Muela Hydropower Station, before reaching South Africa’s Gauteng region—an industrial hub that accounts for nearly 60% of South Africa’s GDP.
This seamless connection ensures an incremental increase in water supply from the current 780 million cubic metres per year to over 1,270 million cubic metres annually, meeting growing demands for domestic, agricultural and industrial use.
For Lesotho, the interconnected system bolsters hydropower capability.
The Muela Hydropower Station, which was constructed during Phase I, will see the energy generated increase as the additional water provided by the Polihali infrastructure passes through its turbines.
In addition, this increase in energy production will be complemented by the planned Oxbow Hydropower Scheme. Working as a peak plant for approximately seven hours per day, Oxbow’s 80MW capacity will add to ‘Muela’s installed capacity of 72MW.
This integration highlights the benefits of the LHWP: revenue generation from water royalties and reduced reliance on imported energy for Lesotho, and enhanced water security for South Africa.
The LHWP II like Phase I integrates roads, bridges, power, telecommunications and lodges infrastructure through which it aims to deliver long-term socio-economic benefits that extend far beyond water transfer and hydropower generation.
As Phase II progresses, it is clear that, the project is as much about empowering communities as it is about engineering excellence.
As construction progresses toward its expected completion in 2029, the Polihali Dam stands as a testament to what can be achieved through innovative engineering, international cooperation, and a vision for sustainable development. It is not just a dam; it is a catalyst for regional transformation.
