Construction World December 2021

CIVIL ENGINEERING CONTRACTORS

RELOCATION OF 30 Mℓ 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 Mℓ Khutsong reservoir is one such project undertaken by the Merafong City Local Municipality. The existing reservoir servicing Khutsong Extension 3 is currently unable to retain water due to formation of a large sinkhole under the reservoir floor. The relocation project involves constructing a new thirty mega-litre reservoir and supporting infrastructure several hundred meters 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. Following the termination of the previous Contractor in July 2020, Morad Consulting were appointed on a turn-key basis to complete the works from no-fines levels upward. Quantibuild was appointed on a design and build basis to execute a proposed alternative in December 2020. The construction physically commenced in February 2021. 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 Mℓ reservoir. The basic philosophy behind the prestressing option for the floor involves applying pre- compression to the raft floor to provide additional resistance

PROJECT INFORMATION

• Company entering: Quantibuild • Project start date: 1 February 2021 • Project end date: 30 March 2021 • Client: Merafong City Local Municipality • Main Contractor: Quantibuild • Principal Agent: Morad Consulting • Project Manager: Morad Consulting • Consulting Engineer: Res-Spec • Subcontractor: Amsteele Systems

to crack inducing tensile stresses. The timing of the stressing, profile of the cables and the concrete mix design were the three critical factors to ensuring the performance of the 450 mm thick raft floor slab. The Khutsong reservoir is located on hazardous ground underlain by dolomitic rock with long-term risk of sinkhole formation. Risk mitigation measures included ground improvement, in the form of a 3-4 m thick engineered soil raft, and the specialist design of a concrete raft foundation/floor slab. The raft needed to be capable of spanning a 5 m diameter sinkhole, located anywhere under the 59,25 m diameter floor, under 12 m water load (12 ton/m 2 ). Quantibuild Construction partnered with Res-Spec to offer a superior and cost-efficient PT Slab-on-grade raft solution, which ensured continued water-tightness and long-term serviceability under emergency sinkhole formation state. The solution, which was cheaper than a conventionally

18 20 th Best Project Awards 2021