Client: GIBB – GOLDER (UK)
The Drakensberg PSS is a major underground pumped storage development (1000 MW) traversing the Drakensberg escarpment N E of Lesotho in weak sedimentary rock. The scheme has a total head in excess of 500m and an overall alignment length of about 4km. The underground caverns are extensively supported by rock reinforcement and permanently lined with reinforced shotcrete. The high head pressure tunnels incorporate reinforced concrete prestressed linings and downstream pressure relief / cut-off measures.
Dr Sharp as the technical project director for Golder Associates, was responsible for all major rock engineering aspects including the investigation, design and commissioning phases.
The project included innovative design approaches to large caverns in weak rock masses subjected to shear overstress. Particular emphasis was given to feasible excavation and support techniques (using incremental, staged construction and composite rock reinforcement – shotcrete lining systems) validated by comprehensive construction monitoring.
Innovative studies also included the design of high head, prestressed concrete linings (in association with Gibb) and engineered cut-offs to inhibit water flow into the cavern zone.
The Drakensberg Project won several awards for innovative cavern design and support methodology and was unprecedented in terms of the cavern span in relation to the weak, horizontally bedded, sedimentary rock mass conditions. The cavern complex was fully integrated with the prevailing geological environment and utilised trapezoidal cavern profiles. The innovative, permanent cavern support, provided by composite rock reinforcement and reinforced shotcrete linings, has proved both appropriate and reliable over a 25 year operating period.