Meaningful progress is being made on the Oxbow hydropower scheme in Lesotho with the award of two geotechnical investigations contracts by the Lesotho Highlands Development Authority (LHDA).

The new power station will supplement Lesotho’s only hydropower plant at ‘Muela, constructed in Phase I of the Lesotho Highlands Water Project (LHWP), which has an installed capacity of 72 MW.

The preferred scheme within LHWP Phase II currently under development, the Oxbow hydropower scheme will increase security of supply for Lesotho and reduce the country’s dependence on electricity imports.

Oxbow, a fully underground facility with an installed capacity of 120 MW, will be operated as a peaking plant running for six hours a day, seven days a week.

The scope of the geotechnical investigations is split into two phases.

The R6-million Phase 1 contract, which comprises the completion of geophysical investigations, field mapping and other related surveys over a five-month period, was awarded to Open Ground Resources.

The R100-million second phase contract was awarded to GEOMECH AFRICA, a South African entity, in early August.

This phase involves more intrusive sub- surface work, including borehole drilling, in-situ testing and laboratory testing. The investigation involves nine field activities, expected to be completed in about 11 months.

Subcontractors include South African firms Skyhorse Aviation and Open Ground Resources, as well as Germany-based Sol Experts.

“For the main drilling contract, there is a total of 18 boreholes combining vertical, inclined and sub-horizontal drilling with a total combined drill length of about 2 648 m,” the LHDA said in a statement sent to Engineering News & Mining Weekly.

The onsite testing is expected to comprise hydraulic conductivity, groundwater piezometric level determination, lugeon testing, stress hydraulic fracture logging and standard penetration tests, besides others.

The laboratory testing will include acid soluble chloride tests for aggregates, acid soluble sulphates, potential alkali aggregate reactivity, water absorption, methylene blue, X-ray diffraction and petrographic analysis.

“On projects such as this, geotechnical investigations are designed to achieve a reasonable mitigation of the geotechnical risks, which primarily address the unknown sub-surface conditions which could be encountered during actual construction.”

Geotechnical conditions are a potential risk for the construction phase of such projects.

The start of the geophysical investigations marks a notable milestone in the development and preliminary engineering of the Oxbow scheme.

The engineering process started in early 2025 following the appointment in late 2024 of the engineering design and construction supervision consultant ASG joint venture (JV), which comprises France-based Artelia, Australia-based SMEC International, South Africa’s SMEC South Africa and Lesotho-based GWC Consulting Engineers.

The services for environmental and social impact assessment (ESIA) stated on January 6, 2025, following the appointment of the ELC-Consultant JV, comprising Italy’s ELC, Lesotho’s Greenway Consultancy and South Africa’s Green Gold Group.

The ESIA contract is expected to be completed in 2027.