Construction World January 2018

ROADS & BRIDGES

INNOVATION cuts time and cost on N2 PROJECT In one of the largest South African National Roads Agency SOC Limited (SANRAL) highway projects currently underway, Concor Infrastructure’s innovative approach gave the client an option to reduce both the cost and the duration of the work – while ensuring safe driving conditions for motorists and minimal environmental impact.

A view of the new road and extension to one of the bridges.

T he contract being undertaken by Concor Infrastructure – formerly Murray & Roberts Infrastructure – is to build a new northbound carriageway for a 34 kilometre section of the N2 between Mtunzini and Empangeni in KwaZulu-Natal. It also includes substantial additional works including the building of 11 bridges and the rehabilitation of the existing road to form the future southbound carriageway. Switch-over saves time To facilitate traffic flow, the initial sequence of the project had envisaged the construction of the entire northbound carriageway – followed by the rehabilitation of the entire southbound route – which was expected to take about 40 months. “When preparing our proposal for this tender, we considered the overall structure density of the project, specifically looking at where most of the bridges and culverts were located,” says Concor Infrastructure contracts manager, Jonathan Pearce. “We also looked at the existing road that needs to be rehabilitated and saw that much of the work was to be done on the southern portion, while most of the structures – mainly the culverts – were located on the northern part of the contract area.” Concor Infrastructure therefore proposed a ‘switch-over’ roughly halfway through the contract, allowing the building of the

southern half of the new carriageway. And thereafter, the northbound traffic could be diverted onto that part of the road while work continues on the balance of the northbound carriageway. “That gave us early access to the southbound carriageway to start the rehabilitation works there,” says Pearce. “This alternative sequencing was able to achieve cost savings for the client, as well as reduce the project period by three months.” Many bridges The two largest bridges are a 240 metre, eight span structure over the uMhlathuze River, and a 120 metre, four span bridge over the uMlalazi River. The 11 bridges include two being built over rivers, two over railway lines and two over arterial roads, as well as four agricultural over-passes and one underpass. “The number of river crossings makes for ground conditions that are not always entirely predictable,” he says. “Being relatively close to the coast also introduces a tidal element to the levels of some rivers, which must be accommodated in our construction methodologies.” Apart from the 11 main bridges, there are 21 major in-situ culverts/underpass structures being built and over 130 smaller crossings constructed with precast portal

and pipe culverts. The two large bridges are among the engineering highlights of the project, requiring construction and placement of 30 metre precast, post- tensioned beams – constructed by a sub- contractor supplied with concrete from a batch plant near the work sites. The ends of each span rest on concrete piers rising up from bases underpinned with 900 mm diameter piles after the extension of causeways into the river’s path. Asphalt needs and stability The building of the new highway and the rehabilitation of the existing road will consume some 220 000 tonnes of asphalt over the duration of the project, making it cost effective for Concor Infrastructure to establish its own Comar asphalt batching plant near the headquarters of the project – and close to the edge of the new highway. Pearce highlights the large variance in the natural ground conditions, which range from old sand-dunes and shales to much more clayey ground. Substantial quantities of pioneer material are brought in from nearby quarries to ensure a well-drained foundation where necessary. To ensure the adequate stability of the new highway’s sub base, 179 000 bags of road stabilising cement are being used. Access to the work sites is one of the challenging aspects of the job, he says, as

Jonathan Pearce, Concor Infrastructure contracts manager.

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CONSTRUCTION WORLD JANUARY 2018