SPECIALIST CONTRACTORS OR SUPPLIERS
RELOCATION OF 30 M ℓ KHUTSONG RESERVOIR
T he community of Khutsong is located on the West Rand of South Africa near the mining town of Carletonville. Given the dolomitic geology, sinkhole formation is a common occurrence in Khutsong. When sinkholes form, the local municipality are often required to repair, rebuild or relocate infrastructure to ensure its residents are safe and that service delivery continues. The relocation of the 30 M ℓ Khutsong reservoir is one such project undertaken by the Merafong City Local Municipality. The existing reservoir servicing Khutsong Extension 3 was previously unable to retain water due to the formation of a large sinkhole under the reservoir floor. The relocation project involved, constructing a new thirty mega-litre reservoir and supporting infrastructure several hundred metres away from the existing, unused reservoir. The supporting infrastructure includes connecting pipelines for both the supply to and distribution from the reservoir, a pump station and general facilities for accessing, maintaining and securing the infrastructure. As a result of the dolomitic geology, one of the major design requirements for the reservoir floor was the capacity to span a sink hole five metres in diameter whilst fully loaded to overflow height. This requirement led to the first round, prestressed reservoir raft floor in Africa being designed and constructed at the Khutsong 30 M ℓ reservoir. The basic philosophy behind the prestressing option for the floor involves applying pre-compression to the raft floor to provide additional resistance to crack-inducing tensile stresses. The timing of the stressing, the profile of the cables and the concrete mix design were the three critical factors in ensuring the performance of the 450 mm thick raft floor slab. The concrete mix design for the raft floor required a compromise between an ideal water retaining concrete mix and PT slab mix. A very delicate balance needed to be achieved between reducing shrinkage, increasing the impermeability and ensuring sufficient early strength gain before shrinkage occurs. A further
complication was ensuring that the mix was still workable after being on the road for just over an hour given the vicinity of the nearest ready-mix batch plants. Several trial mix designs were conducted as finding the balance between acceptable shrinkage, permeability, workability and early strength gain proved a sensitive process. The optimum mix was however found by balancing a suitable water cement ratio for permeability with Chryso’s shrinkage compensating admixture, CHRYSO® Serenis. CHRYSO® Plast ZA 1559 also proved an effective plasticising admixture and ensured workability after 80 minutes whilst also providing adequate early strength gain. An Afrisam CEM II/A-M 42.5R type cement was selected for both product performance and reliability of delivery during what has been a tough period to secure cement deliveries post lockdown. To test the design’s performance, a sample beam measuring 59,25 m long, 0,210 m wide and 0,450 m deep was designed and cast on-site with a single mono strand cable to simulate the influence of one cable on the raft floor. In terms of concrete placement, the design engineer required the pour to be completed within a 24-hour window to ensure the homogeneity of the concrete under early stressing. At the work face itself, efficiency and adequate resourcing planning were crucial for successful execution.
PROJECT INFORMATION
• Company entering: CHRYSO • Client: Merafong City Local Municipality • Start date: 10 February 2021 • End date: 20 March 2022 • Main Contractor: Quantibuild • Consulting Engineer: Morad Consulting • Readymix: RMX WG Wearne Readymix
48 21 st Best Project Awards 2022
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