The raising of the Tzaneen Dam wall ogee crest and tongue wall and replacing its existing ogee spillway with a combination of labyrinth and ogee-spillway crests, will entail placing 16 400m³ of concrete, which is about 3 280 concrete-mixer truck loads. This project again demonstrates the fundamental role that the cement and concrete industry plays in ensuring water security.

So said Wessel Janse van Rensburg, Technical Advisor to the Construction North Infrastructure Management Directorate of the Department of Water & Sanitation, at Cement & Concrete SA’s (CCSA) Concrete Working for Water Roadshow. The raising of the Tzaneen Dam wall by 3m was just one example of excellence in the design and application of concrete technology that was showcased at this prestigious CCSA event. It brought together representatives of client bodies; engineering firms; principal and specialist sub-contractors; and participants in the South African water-infrastructure supply chain.

The construction of Tzaneen Dam was completed in 1977. Comprising a mass concrete-gravity spillway section that is flanked by earth-fill embankments that are paved with bricks, the dam has a current capacity of 157,3-million m³. This will be increased to 193-million m³ when the project is completed.

“The more than R600-million upgrade needs to be undertaken to ensure an acceptable level of supply based on an anticipated increase in water demand over a 20-year planning cycle. In addition to supplying municipalities for domestic use, Tzaneen Dam is a source of important irrigation water for farmers in the Mopani district of Limpopo, many of whom are small-scale producers. This project will, therefore, ensure their sustainability,” Janse van Rensburg said.

Construction North Infrastructure Management Directorate of the Department of Water & Sanitation is the principal contractor and the Department of Water & Sanitation Chief Directorate of Engineering Services and ARQ Consulting Engineers serve as employer’s agents. The implementing agent is the Development Bank of Southern Africa.

Construction North Infrastructure Management Directorate started working on the 18-month project in June 2023 and sound progress has been made by the seasoned dam builder thus far.

The river has already been diverted by 125m and construction of the access road completed. By May 2024, the contractor will have erected the state-of-the-art on-site concrete batching plant that will supply the large quantities of concrete that will be placed for this project.

In preparation of the large works to come, Construction North Infrastructure Management Directorate is also busy erecting the tower cranes; clearing areas for material stockpiling; and excavating, loading and transporting construction material from the stilling basin. These activities have not been without their challenges, not least of which was the flooding of the stilling basin during December wet season. To continue with the works, Construction North Infrastructure Management Directorate has had to first redirect large volumes of water from the toe drain.

The upgrade is an extensive undertaking that will also entail excavating 18 500m³ – equivalent to 3 170 truckloads – for the tail pond and placing 140 000m³ of soil fill – translating into 24 000 truckloads for the construction of the embankment.

Tzaneen Dam’s left embankment will be raised to 175m and right embankment to 780m and the new labyrinth ogee concrete spillway will be 91m in length.

A total of 3 770m³, 2 950m³ and 2 250m³ of concrete will be used to construct the bason-apron slab, ogee and still-base toe, alone. This is in addition to the placement of 1 630m³ for the construction of the retaining wall and 1 450m³ of concrete for the labyrinth spillway. The construction of the tongue wall will entail placing 505m³ of concrete and the diversion pipe 310m³ of concrete. Moreover, 150m³, 123m³, 80m³ and 20m³ of concrete will be used to build the side/tongue wall and intake tower floor slab and its wall, respectively. An additional 50m³ of the construction material will also be used for general concrete repairs to the existing infrastructure as part of the upgrade.

Janse van Rensburg told delegates during his session of the CCSA Concrete Working for Water Roadshow that a high-strength, water-tight concrete mix that will reach 30MPa within 25 days had been designed for this project.

“The water-to-cement ratio does not exceed 0:48 and to improve workability, we will use water-reducing admixtures that comply with the SANS 1200G standard that have been sourced from reputable manufacturers. 30% of some of the concrete mixes will also contain fly ash to further improve workability of the concrete and increase its durability. The cementitious content of these mixes will be about 450kg/m³ and about 250kg/m³ for those without fly ash,” he said.

He concluded by noting that, when the project is completed, the dam will be classified in the 53,3m-high Category III. This is the highest hazard rating by the Dam Safety Officer considering the size of damage and potential for loss of life in the unlikely event of a catastrophic failure. “Therefore, the highest possible standards are being adhered to throughout the construction project.”