Sophisticated civil-engineering teams bring water to Katlehong

Pre-cast system saves six months on reservoir build
February 25, 2017
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Sophisticated civil-engineering teams bring water to Katlehong

Water has been placed right on top of the service-delivery agenda with custodians of this important resource under constant pressure to build high quality infrastructure quickly and efficiently.

This includes at municipal level where increased urbanisation is placing significant strain on existing infrastructure and intervention is urgently needed.

Ekurhuleni Metropolitan Municipality is taking such action in Katlehong which, together with Thokoza and Vosloorus, is the second-largest township in Gauteng after Soweto.

Together with the efforts of consulting engineer, TLS Engineers & Project Managers, as well as civil-engineering contractors, Civcon and QC4 Civils, the municipality is delivering a new reservoir that will bring significant relief to this high-density area.

QC4 Civils’ contracts manager, Ettiene Coetzee, tells IMIESA that this is the largest reinforced, post-tensioned ground reservoir to be built by the company – a testament to its rapid growth in the South African civil-engineering construction industry, while adding to Civcon’s already impressive project portfolio.

Constructed with six 16,15 metre (m) by 4,2 m buttresses for post-tension anchors, the 35 mega-litre reservoir has an internal diameter of 54,4 m, as well as wall height of 16,15 m and wall thickness of 450 mm. The structure features a 700 mm diameter inlet pipe, two 800 mm diameter outlet pipes, a 600 mm diameter scour pipe and 800 mm diameter reservoir overflow pipe.

Importantly, it is also the first time that the company has used a pre-cast roof system on any of its reservoir construction projects.

Right from the outset in the design stages of the project, it was decided that the roof of the structure would be constructed with pre-cast concrete elements to save significant time on the construction programme. The motivation was based on a string of successful projects undertaken by Corestruc since completing its first 50 mega-litre project in Krugersburg in 2010.

The project shaved as much as eight months off the construction programme, allowing the reservoir to be delivered in as little as 11 months, and similar successes have been realised by the professional team working in Katlehong.

Coetzee says the team allowed for 26 weeks to construct the columns and roof deck, however, this period could have easily be halved using Corestruc’s system.

“One of its biggest advantages is that it can be installed without the need to erect, dismantle and remove tons of supporting scaffolding, while the pre-cast system can be programmed concurrently with other activities and, therefore, does not restrict multiple activities on the site,” says Coetzee.

More than 2 300 square metres (m2) of slabs, each up to 250 millimetres (mm) deep, make up the roof of the reservoir. They are supported by 16 columns and 18 beams, weighing a total of 12,3 tons and 7,5 tons, respectively.

These were manufactured by Corestruc and stored, before being delivered to site ahead of installation in February.

Installation of the pre-cast items started when 80% of the walls had been cast. An opening was left just wide enough for Corestruc’s team to access the inside of the structure with a mobile crane.

The construction of the reservoir wall, alone, was an impressive achievement by Civcon and QC4 Civils, considering the shorter timeframe the contractors allowed themselves for this aspect of the works. Using a new climbing system designed and supplied by Form-Scaff, the civil engineering contractors were able to erect 747 m2 per section in as little as two weeks, as opposed to the initial plan of 200 m2 per section.

It took time to train the construction workers to use the new system, but the programme picked up quickly once they were familiar with the formwork.

Construction of the wall and installation of the post-tensioning system followed the installation of the internal pipes, pouring of the reinforced concrete foundation, installation of load-bearing pads for the wall and completion of the sub-surface drainage system. This programme was divided into three sections and four lifts.

The floor slab had also been cast, while the bases and plinths were ready to receive the columns.

Led by a seasoned surveyor equipped with a state-of-the-art theodolite, Corestruc’s team lifted and fastened the components into place, achieving tolerances of between about 20 mm at heights of 15 metres.

A total of 2 000 square metres of precast items were installed in as little as eight working days – a feat Corestruc’s Willie de Jager attributes to the extensive upfront planning implemented on all of the company’s projects.

“Nothing is left to chance,” says De Jager. “Each component has been manufactured in a closely monitored production cycle with extensive pre-checks and post-checks in place to ensure absolute accuracy. We also undertake rigorous planning before we arrive on site to ensure a seamless operation.”

The choice of connections was also key to the swift production rate achieved on site.

“Selection is already done in the design stages where Corestruc’s advice was essential in helping engineers strike a delicate balance between the price and practicality of the connections,” he says, adding that the cost of constructing reservoir roofs in this manner compares favourably with in-situ methods.

Once Corestruc had completed its operations, the civil-engineering contractor’s team started installing water proofing and covering it with a stone layer. They installed the tendons and commenced tensioning, followed by grouting. A lightning-protection system was installed; the reservoir cleaned and the inside surfaces chlorinated, before filling the structure and undertaking water-proof testing.

An added benefit of Corestruc’s technology is the high durability of the roof with all the elements manufactured from a 55 MPa concrete – far exceeding the 40 MPa specification of the in-situ material. This contributes to the overall performance of the asset, reducing maintenance requirements of the structure and costly rebuilds.

The high quality of Corestruc’s pre-cast concrete elements can be attributed to the exacting standards adhered to at its batching operation.

Only washed aggregates and sand from reputable suppliers enter the production cycle, while silica fume supplements some of the cement content to create a denser micro-structure of the concrete.

The batching plant is extremely sophisticated with sensors constantly monitoring the temperature and moisture content of the concrete mix.

Concrete is poured into specialised forms and moulds that guarantee accurate casting, a prerequisite for these projects. A separate steel-fixing yard facilitates accuracy and therefore a high quality reinforcing in the concrete casing, enhancing the quality of the final product.

With a steady pipeline of reservoir projects in various stages of implementation, one can expect to see more Civcon and QC4 Civils teams on sites throughout the country as they help bring water to where it is needed. It is very likely that Corestruc will accompany them, considering the significant praise its activities have received from these respected contractors!

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