The eight-month shutdown of the Lesotho Highlands Water Project tunnels was one of southern Africa’s most complex maintenance projects. Corrocoat SA’s corrosion protection expertise and experience ensured the tunnels were restored to secure Gauteng’s water supply and sustain Lesotho’s hydroelectric power for decades to come.

The Lesotho Highlands Water Project (LHWP) ranks among the continent’s greatest engineering achievements. Established through the 1986 Treaty between Lesotho and South Africa, it delivers water to South Africa’s economic heartland, Gauteng, while generating hydroelectricity for Lesotho. Phase I, completed in 2003, comprised Katse Dam, Mohale Dam, Muela hydropower station, and 83 km of tunnels, transferring 780 million cubic metres per year into the Integrated Vaal River System.

Phase II is now underway, adding the Polihali Dam and a further 38 km gravity tunnel ultimately adding an additional 480 million m3 of water per year into the current South African water system.

Need for maintenance

A 2019 inspection revealed significant deterioration of the steel liners that protect critical sections of the 82 km tunnel system. If corrosion was left unchecked, the risks were stark: Reduced hydraulic efficiency, potential collapse and loss of supply.

Given that LHWP delivers around 60% of Gauteng’s water, and underpins Lesotho’s electricity grid, the stakes could not have been higher.

Officials from the Lesotho Highlands Development Authority (LHDA) and South Africa’s Trans Caledon Tunnel Authority (TCTA) stressed that preventive maintenance was a necessity, not an afterthought.

For the first time in 25 years, the ageing tunnel system of Phase I was refurbished. The original liners had been coated in South African workshops prior to installation in the 1990s. The Transfer Tunnel, Delivery Tunnel North and South, and associated structures were closed for grit-blasting, steel repairs, and full relining with modern corrosion protection.

Grit blasting

Grit blasting is a surface preparation technique where small, hard particles (grit) are propelled at high speed using compressed air to clean a surface. As part of the tunnel refurbishment, grit blasting played a central role in preparing the ageing steel liners for recoating. Decades of biofilm, rust, and worn epoxy had to be stripped back to bare metal to ensure a clean surface and guarantee that the new glass-flake epoxy system would bond effectively.

“To carry out corrosion protection works across nearly 20 000 m² surface area, we imported more than 1 850 tonnes of abrasive blast-cleaning grit from Rustenburg in South Africa to Lesotho (630 km trip). This equates to around 50 interlink truckloads, each carrying 32 tonnes of blast-cleaning grit,” explains Louis Pretorius, MD, Corrocoat.

At each project site, the vast majority of all blast cleaning grit (in 25 kg bags) was loaded, carried and off-loaded by hand. That is 74 000 bags. Once used, all abrasive blast cleaning material had to be collected and loaded back into bags before being removed from the project site, mostly by hand. “This was undertaken with a smile and without too many complaints; a testament to the tenacity of the Basotho site teams,” adds Pretorius.

All blast cleaned surfaces were completed to a surface cleanliness of Sa 3 with a minimum of a 50 μm blast profile. Sa 3 blast cleanliness is the highest cleanliness standard possible with the definition of an Sa 3 blast cleaned substrate being that the substrate is cleaned to “white-metal”, a visually clean substrate with zero shadowing, misses, residual surface contamination or deleterious material remaining. Sa 3 cleanliness represents the highest achievable standard.

Coupled with the interlink trucks for the blast cleaning grit, approximately 70 truck-loads of plant and equipment was shipped to Lesotho from Benoni in South Africa. Corrocoat employed a fleet of approximately 25 off 1-ton pick-up trucks as well as numerous other trucks, including tipper and crane trucks, fork-lift trucks, tele-handlers and other plant & equipment.

An additional 24 x 12 m long site accommodation and office modules were transported to both Muela as well as Ngoajane project sites, as well as an additional 18 site steel containers.

“Approximately 120 000 litres of diesel was consumed throughout the corrosion protection refurbishment project. It must be noted that the entire project was completed without any major vehicle accidents or incidents; a testament to the maturity and self-control of all staff during the project,” states Pretorius.

Epoxy coating

To complete the works, Corrocoat imported 43 000 ℓ of solvent-free, glass-flake epoxy from the United Kingdom – 9 000 ℓ more than budgeted. Corrocoat absorbed the additional cost, ensuring thicker, longer-lasting protection. “With solvent-free coating materials, a thicker coating is directly proportional to a longer total life-cycle from a corrosion protection perspective,” states Pretorius.

They used Corrocoat Zip-E Coating; a two-component solvent-free epoxy glassflake coating designed for cost-effective, single-coat application to provide durable protection in harsh environments, including aggressive atmospheric conditions and aquatic immersion. It offers excellent corrosion resistance for assets such as structural steel, bridges, marine structures, and pipelines. Key features include good cosmetic appearance, good edge coverage, and a higher temperature limit for non-immersed applications compared to immersed ones.

Once the coating had cured, dry film thickness was measured across all sections to confirm compliance before subjecting 100% of the surfaces to high-voltage spark testing. Any areas found below specification were reworked to meet the required standard.

The upgraded coating system is expected to last 30 to 50 years, compared to the original lining’s lifespan of around 25 years.

Engineering and environmental challenges

Water ingress proved the greatest obstacle. Decades of seepage through saturated mountain rock meant the tunnels never stayed dry. At Muela Ridge alone, Corrocoat pumped water 24 hours per day, every second of every minute of every day from November 2024 until April 2025.

“We pumped more than 320 000 m³ of water between November 2024 and April 2025, installing 1.2 km of transfer piping and deploying more than 20 pumps. Across six sites, 15 coffer dams were constructed to isolate wet sections,” adds Pretorius.

Low temperatures (12–15 °C) and nearly 100% humidity further complicated epoxy coating application. To achieve the required curing, Corrocoat installed powerful heaters and dehumidifiers, and controlled airflow with over 20 axial and centrifugal fan systems. Even then, shifts of wind direction inside the tunnels could disrupt the environment.

Six project sites ran simultaneously, with some sites more than a two hour drive from each other.

“A significant unknown at project inception was the degree of deterioration within the steel liners of the tunnel system. While external inspections suggested general wear, the true condition could only be assessed once the tunnels were dewatered and made accessible. Detailed visual surveys, ultrasonic thickness testing, and magnetic particle inspection revealed varying levels of corrosion, from superficial surface rust to severe pitting and through-wall perforations in localised sections. In areas where metal loss had compromised structural integrity, coded welders installed steel patch plates or carried out full-section weld repairs.

“Approximately 75 such interventions were executed, each subjected to stringent non-destructive testing (NDT), including ultrasonic and radiographic methods, to confirm repair quality and alignment with design tolerances. Only once the mechanical integrity of the liners had been restored were the surfaces grit-blasted to Sa 3 white-metal standard and prepared for epoxy coating application. This approach ensured that the refurbished tunnels not only received long-term corrosion protection but also regained their original hydraulic and structural performance characteristics,” explains Pretorius.

Social impact, quality control, safety

Approximately 300 people were employed throughout the project execution, with Basotho staff comprising more than 70% of the workforce. Furthermore, Corrocoat went beyond its contractual obligation to train 10 local staff, expanding the opportunity to 19 top-performing Basotho employees. These workers completed the SAQCC Paint Applicator’s Course (PA1), funded entirely by Corrocoat at no additional cost to LHDA.

“This course is the only industry-recognised and certified Corrosion Protection Application Qualification available in South Africa and will without doubt allow the attendees to find work in the corrosion protection industry throughout Southern Africa, such is the esteem and recognition of the qualification,” says Pretorius.

The project operated around the clock, seven days a week — even through Christmas, New Year, Easter, and other public holidays — to sustain momentum and limit delays. Close to 652 000 man-hours were committed over the course of the project

Significantly, there were no major injuries or fatalities, despite the hazardous conditions of confined spaces, water ingress, blasting operations and heavy equipment. All personnel underwent confined-space training, medical checks, and site-specific inductions and were equipped with full PPE.

Corrocoat also refurbished community infrastructure, including the Likileng Lodge clubhouse, Muela Village accommodation, and water reticulation for the Muela police station and information centre.

Quality control was uncompromising:

• 100% third-party inspection at every stage
• Dry film thickness (DFT) checks across all surfaces
• High-voltage spark testing to detect pinholes, with all defects repaired before final sign-off
• Non-destructive testing (NDT) on all 75 steel weld repairs.

“LHDA’s exceptional support, alongside the 3rd Party inspectorate, ensured seamless collaboration and a unified focus on project success. Despite unforeseen challenges, Corrocoat embraced each one, delivering the refurbishment to full specification. We are proud to have played a role in such an important project.”