The new Polihali Dam and its reservoir will augment the water supply to the existing Katse Dam. This enhanced flow will boost power output in Lesotho, addressing domestic energy demands while lowering dependency on electrical imports. As part of the Metsi a Senqu-Khubelu Consultants (MSKC) Joint Venture (JV), SMEC is overseeing the Polihali Transfer Tunnel project, a crucial element of the works.

The MSKC JV, consisting of SMEC, Knight Piésold, Zutari, Hatch and FM Associates, was appointed in December 2017 for the design and construction supervision of the Polihali Transfer and Diversion Tunnels. The appointment encompasses the preliminary design, tender design, tender documentation (Stage I); detailed design and construction monitoring (Stage II) and assessment and close-out (Stage III).

Solution

In collaboration with other members from the MSKC JV, SMEC has undertaken construction monitoring and construction administration in Stage ll for the Polihali Transfer Tunnel. The multidisciplinary SMEC team is made up of several specialists in the areas of tunnel design, hydro-mechanical, hydraulics, geotechnical and geology. The MSKC JV oversaw the tendering process of the Polihali Transfer Tunnel construction contract and the selection of the preferred bidder, Kopano Ke Matla JV.

Polihali Diversion Tunnel, site tour – Nov 2024

SMEC, as part of the MSKC JV, has actively participated in the design and construction supervision of the diversion tunnels at the Polihali Dam site. The design contained two parallel tunnels, one with a diameter of 7m and a length of approximately 1km, and the other with a diameter of 9m and a comparable length. Both tunnels connected the intake point to the exit downstream of the dam. Drilling and blasting were used for excavation. The diversion tunnels played an important role in temporarily redirecting the flow of the Senqu River, generating the dry conditions required for the construction of the Polihali Dam.

The Polihali Transfer Tunnel will be excavated using two hard rock-shielded tunnel boring machines (TBMs), operating simultaneously from Polihali and Katse. Key works include intake structures and a gate shaft at the Polihali Reservoir, as well as an underground outlet structure, gate shaft, and gate shaft gallery at the existing Katse Reservoir. A notable technical feature of the project is the Katse Lake Tap, a specialised underwater connection where the transfer tunnel is excavated directly into the Katse Dam basin below water level. This method allows for the controlled integration of the new tunnel with the existing reservoir without the need for extensive dewatering. SMEC has also led design for access adits to the waterway tunnel, and associated construction infrastructure.

Katse Lake tap structure

Mechanised (TBM with pre-cast concrete segmental lining) and conventional (drill and blast with cast-in-situ concrete lining) methods are being used to construct the 4,5m finished diameter water transfer tunnel. The Lake Tap structure (as part of outlet works) will be excavated by drill and blast method and will be lined completely before blasting the rock plug, the last part of the waterway tunnel to the existing Katse Reservoir. SMEC played a key role in the preliminary and tender design of the underground structures and overseeing the detailed design and construction stage.

Impact

Once completed, the dam will allow water transfer through the 38km long Polihali Transfer Tunnel, which connects to the current Katse Dam, advancing the project’s goal of improving water supply to South Africa and generating hydropower for Lesotho.

The ZAR9.2-billion Polihali Transfer Tunnel will transfer water by gravity from the Polihali Reservoir to the Katse Reservoir. From Katse, water is transferred via the delivery tunnel to the ‘Muela Hydropower Station, then on to the Ash River outfall in the Free State, and finally to Gauteng.

SMEC has been involved with the design and construction of major works for the LHWP since 1988 when designs for the Phase lA works commenced. The Polihali Transfer Tunnel appointment serves as the MSKC JV’s second appointment on Phase II after being selected for the design and construction supervision of the Polihali Diversion Tunnels in September 2016.

The project will mitigate water shortages in South Africa’s Gauteng region and increase energy capacity, boosting Lesotho’s socio-economic development. In addition to the skills and technology transfer elements of the project, the newly awarded Polihali Dam and Polihali Transfer Tunnel contracts are estimated to create 5,000 job opportunities for a period of five years. Commissioning of both the Polihali Transfer Tunnel and Polihali Dam is expected to take place in 2028/9.