Infinite Engineers Projects

Infinite Consulting Engineers’ unique precast, prestressed concrete reservoir design helped Vhembe District Municipality to deliver critical basic service delivery infrastructure quickly and efficiently. Considering the absolute control provided by prefabricated concrete, Infinite Consulting Engineers’ unique design also facilitated a higher quality final build.

Precast, prestressed concrete technology was successfully deployed on components of the upgrade of the Vondo Regional Water Scheme, which will provide a reliable and secure source of drinking water to about 500,000 people who reside in Phiphidi.

Located along the road to Sibasa to Nzhelele, the scheme consists of the Vondo Dam and water-treatment works (WTW). From here, water is pumped to two command reservoirs that supply specific areas within the scheme. Water is also pumped and gravitated to the WTW from two other reservoirs where it is distributed to various areas.

Infraburo Civil and Structural Consulting Engineers, a leading consulting engineering firm, commenced with the preliminary design of the upgraded scheme in 2012. In 2017, the construction work went out to tender and was awarded to Morawa Building and Civils, the principal contractor. The upgrade of the scheme is being financed by the Municipal Infrastructure Grant.

This project entails upgrading the WTW and associated infrastructure. It includes the gravity-feed pipeline from Vondo Dam, as well as a pipeline from the WTW to a command reservoir. Moreover, a new command reservoir was constructed and work on the second water retaining structure was recently completed. This is to supply the 10ML daily water demand with an additional 10ML of water held in reserve. The construction of the two reservoirs was undertaken in phases. One of the reservoirs had to be commissioned before an existing water-retaining structure could be demolished to make space for the construction of a new reservoir on the restricted site.

This is where the municipality is successfully exploiting the many inherent benefits of prefabricated concrete technology. Vhembe District Municipality is now one of many South African municipalities to use Infinite Consulting Engineers’ unique reservoir design to construct these technically complex structures.

Infinite Consulting Engineers designed the reservoirs while working closely with Infraburo Civil and Structural Consulting Engineers. Corestruc, a precast-concrete specialist, was tasked with erecting the system while working alongside principal contractor, Morawa Building and Civils. The various precast-concrete elements that make up the system were manufactured by Coreslab, one of Corestruc’s approved manufacturers. Infinite Consulting Engineers provided technical support to all members of the professional team throughout the project lifecycle.

Erected in record time, the first reservoir passed all relevant tests and has now been operating successfully for more than two years. The stellar performance of the reservoir to date further motivated the use of the technology to also build the second water-retaining structure.

Using this precast-concrete system, a 10 ML reservoir can be completed in only four months, therefore, providing significant savings in construction costs for the client. Infinite Consulting Engineers’ system enables the various trades to work simultaneously. The wall and roof are manufactured while earthworks and site terracing, as well as the construction of the roof column bases are under way. This work is undertaken by the principal contractor.

Notably, the system facilitates greater participation of emerging contractors in these projects. Bear in mind their highly specialised and technically complex nature with only a few contractors having the skills, experience and capacity to build cast-in-place reservoirs. Morawa Building and Civils introduced Infraburo Civil and Structural Engineers to the system, noting the role it could potentially play in keeping this aspect of the work scope on track, while also providing two reservoirs of an unrivalled quality.

“We initially designed conventional pre-stressed reservoirs for this project. However, the principal contractor had undertaken extensive research into Infinite Consulting Engineers’ system and put forward the idea to use it to construct the two reservoirs. Although initially sceptical, I personally visited the factory of one of the approved manufacturers of the system and spent time with representatives of the company to gain a greater understanding of the system. Needless to say, I was very impressed especially with the quality control processes deployed in the factory to ensure prefabricated concrete elements of the highest possible quality are used to build these structures. I, thus, motivated the use of the system to the client. Vhembe District Municipality was also already aware of Infinite Consulting Engineers’ design considering that it had previously been successfully deployed in several other municipal jurisdictions. The client, therefore, accepted my proposal,” Rudolph Dippenaar, Infraburo Civil and Structural Consulting Managing Director, says.

Infinite Consulting Engineers’ design consists of a prefabricated reservoir roof and wall system. Dispatched to site once the cast-in-situ bases have been completed, the roof columns are the first precast-concrete elements to be erected. They are connected to the hold-down bolts in the column bases. Suspended precast-concrete beams are then connected to the dowels that protrude from the precast-concrete columns. Thereafter, hollow-core slabs are connected to the stirrups protruding from the precast-concrete beams. Steel reinforcing is placed into the cores of the hollow-core slabs and these voids are then filled with in-situ concrete. By forming a composite mechanism with the infill concrete, the stirrups act as mechanical interlocks.

Once the inner portion of the roof has been erected, the principal contractor is able to commence constructing the in-situ floor slab. The cover provided by the roof structure also facilitates optimal curing conditions. At this stage, the individual wall panels are ready to be dispatched to site where they are placed on the ring beam that was constructed by the principal contractor. The first panel is supported by a push and pull prop. To provide temporary stability, the wall is braced back to the roof structure. The steel brackets assist in holding the panels together and, thereby, eliminating the need for extensive propping to free up space. Once all of the panels have been placed, unbonded cables are pushed through the polyvinyl sleeves in the panels. They are then grouted monolithically with the joints of the panels. Hereafter, a grout is poured continuously in between the wall panels and horizontal cable sleeves. It is a high strength and flow grout with an extended pot life so that it does not segregate and set too early. These characteristics are achieved by manipulating the water-to-cement ratio of 0:37 with the use of admixtures. The water temperature is also reduced and controlled to act as a chiller in the mix. In addition, only cement, including an un-hydrated type that reacts with water to seal possible leaks, is used in the concrete mix. The cables are stressed to 75% when the grout has cured to a strength of 80MPa. This is undertaken via four precast concrete buttress panels that have been spaced along the perimeter of the reservoir.

The wall is then pinned by casting a 200mm to 250mm-high reinforced kicker on the wall footing on both sides of each panel. Joints between the panels are grouted with a high-flow and strength grout and post-tensioning renders them in compression to achieve water tightness. Infinite Consulting Engineers uses a “slide-and-pinned” system. Post-tensioning is undertaken 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 designed to maintain a residual compression of a minimum of 1MPa 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.

As part of the final aspects of the build, the concrete floor slab is completed, and the hollow-core slabs that make up the outer portion of the roof structure are connected to the precast-concrete beams. A grout topping is then placed over the hollow-core slabs to form a single monolithic structure, and a precast concrete coping installed around the perimeter of the roof as an aesthetic finish. Dippenaar says that he was also very impressed with the ease at which the system was erected. “I do believe that Infinite Consulting Engineers is at the cutting edge of precast, prestressed concrete technology. Based on my experience working with this technology, I will not hesitate to recommend it again. I do believe that it is the future of reservoir construction,” he says.