Civil Engineering March 2022 | Vol 30 No 2

46 March 2022 Civil Engineering grout and post-tensioning renders them in compression to achieve water tightness. Notably, unlike most conventional post-tensioned reservoir walls which are designed to slide, Infinite Consulting Engineers and Corestruc use a “slide-and- pinned” system. Post-tensioning is under- taken when the wall is not yet fixed to the ring footing and it is, therefore, allowed to slide on a steel bearing or locating plates. The coated post-tensioned cables are not bonded to the grout with the reservoir de- signed to maintain a residual compression of a minimum of 1 MPa in all directions. Horizontal reactions to the wall base are transferred to the ring foundation through the second phase cast-in-situ kicker. This is where the ring tension in the base is also activated to resist the reaction. Additional post-tensioning of the lower part of the wall reduces the amount of rebar required in the cast-in-situ ring footing. OVERCOMING CHALLENGES The project was not without its share of challenges. Initially, the municipality intended to build this reservoir using conventional methods, but based on the success that Infinite Consulting Engineers and Corestruc achieved on the other three reservoirs, the client body decided to build this water-retaining structure in the same manner. Tango’s Consultants, therefore, initially designed an integrated column base-to-floor connection. Infinite Consulting Engineers and Corestruc decided to adopt the system because the principal contractor had already procured the floor and base rebar. The original grid spacings were therefore modified to suit Corestruc’s optimised grid proposal and the rebar cages were relocated without the need to alter the rebar details. An advantage of this method is that it eliminates the need for waterproofing bandage along the perimeter of the base. However, it does not allow for the floor to be cast below the completed precast con- crete roof structure. As such, the curing of the floor concrete on this project required meticulous attention to detail. Conventionally, the column founda- tions are independent of the floor slab and feature waterproofed expansion joints around the base. This allows the bases to be constructed and roof structure to be erected before the floor slab is cast. It creates the best curing conditions, eliminating any risk of shrinkage cracks. The foundation design requires a minimum safe bearing capacity of 200 kPa at founding level. Moreover, the cast-in-situ ring footing had to be modified to accommodate the larger 750 mm diameter outlet pipe sections that were also ordered by Anita Building before Corestruc was appointed to construct the reservoir using the precast concrete system. This meant that the pipe would clash with the underside of the ring foundation. Infinite Consulting Engineers had to widen the ring founda- tion over the pipe to maintain a constant concrete and rebar area along the perimeter. The length of the widening was calculated to maintain sufficient transfer stress with the longitudinal rebar without creating concentrated stresses. There are also other important advan- tages offered by the system that have been noted by the municipality on these proj- ects. This includes the precast concrete reservoir’s smaller carbon footprint. The vertically stressed precast concrete wall panels are thinner, reducing the amount of aggregate and concrete used to build the reservoir. The hollowcore slabs alone reduce the carbon footprint of the struc- ture by up to 40%, which is augmented by the use of prestressed 665 mm × 460 mm I-shaped beams. The self-compacting concrete used to manufacture the various precast concrete elements also reduces the volume of the construction material required and conserves energy by elimi- nating the need for vibration. It is also provides a safer means of constructing reservoirs. This is con- sidering that the various elements are manufactured at ground level and erected on site by a skilled and experienced team. Certainly, one of the project mile- stones was witnessing all the precast concrete elements fitting into place and integrating without any discrepancies to complete a structure that is now receiving and retaining water.  The roof structure consists of hollowcore slabs that are placed on precast concrete beams, supported by prefabricated columns