Construction World December 2021

CIVIL ENGINEERING CONTRACTORS

SELCOURT 30 Mℓ RESERVOIR IN-SITU CONCRETE RING BEAM AND FLOOR WITH PRECAST CONCRETE WALLS AND ROOF

PROJECT INFORMATION

• Company entering: Corestruc • Project start date: May 2020 • Project end date: July 2021 • Client: City of Ekuhuleni

• Main Contractor: Selby Construction • Principal Agent: Tango’s Consulting • Consulting Engineer: Tango’s Consulting • Subcontractor: Corestruc

A newly built 30 Mℓ reservoir in Selcourt has introduced a more efficient manner of constructing these technically complex water-retaining structures to Ekurhuleni Metropolitan Municipality. The municipality has embarked on a large water-augmentation programme to ensure water security in its jurisdiction and was, therefore, willing to test new technology that would enable it to deliver this vital service- delivery infrastructure quickly. The technology also enables the construction of final structure that is of a very high quality and provided notable savings in construction costs for the client. The reservoir was built by Selby Construction and the construction programme supervised by Tango’s Consulting. As the design engineer, Tango’s Consulting also reviewed the technology to ensure that it was structurally sound and adapted it to this project. This was undertaken together with the designer and developer of the technology, Corestruc. The technology was introduced to the professional team by Selby Construction which had worked with Corestruc on previous projects and was, therefore, aware of the quality of its precast-concrete systems and workmanship on site. Constructing reservoirs have traditionally been a very complex undertaking. Using conventional cast-in-place methods, the reservoir wall is constructed first. The task at hand entails significant steel-fixing and the onerous process of installing the side shutters. Once the wall and insitu columns have reached their final height, work commences on the roof of the structure. This entails installing many tons of scaffolding inside the reservoir to support the forms for the frame of the roof structure. Corestruc commenced manufacturing the roof and wall of the reservoir at one of its factories during the bulk earthworks and site terracing stages. The assembly of the inner-portion of the concrete roof was completed while the principal contractor constructed the ring foundation for the precast-concrete panels that make up the wall of the structure. The roof structure comprises precast-concrete columns

that are placed and grouted onto insitu stubs or bases that are constructed by the principal contractor while the precast- concrete system is being manufactured. These concrete columns support precast-concrete beams and hollowcore slabs. The hollowcore slabs are stitched together to create a single monolithic slab and a precast-concrete coping installed around the circumference of the reservoir as part of the final phase of the construction programme. Corestruc achieved tolerances of 20 mm on this aspect of the construction programme – and this was while working at heights. Construction then commenced on the wall with the placement of the first panel onto the ring-beam. It was supported temporarily and then with the other precast concrete elements to mitigate the need for extensive propping and, in so doing, freeing up space on the construction site. Unbonded cables were installed through the sleeves that were strategically positioned and cast into each precast- concrete panel at the factory. They are also installed between the joints between each precast-concrete element. These unbonded cables are tensioned via the four buttress panels, which are equally spaced around the circumference of the reservoir. The joints between the panels were then sealed with rubber gaskets before a low viscosity grout was pumped around the circumference of the reservoir in a controlled manner. Cooled to ensure its flowability while it was being pumped, the grout is self-healing and reaches compressive strengths of up to 100 MPa in a short period. Before the grout reached this compressive strength, the cables were stressed to their specified yield. The wall was then pinned by casting a 200 mm to 250 mm high reinforced kicker on the wall footing on both sides of the panel. While work proceeded with the construction of the reservoir, the principal contractor completed the ancillary works, including the inlet and outlet chambers, as well as all the interconnecting pipes. 

20 20 th Best Project Awards 2021