A giant stitch closes the Mount Edgecombe gap

Incremental launching is a relatively recent method of building a complete bridge and can save time, money, space and disruption while easing access and delivering a high-quality finish.

Champagne was popped on 4 August 2017 when, after two-and-half years since they first began extending progressively towards each other, the north and south decks of the one-kilometre ramp of the Mount Edgecombe Interchange were joined.

While motorists went about their way negotiating the labyrinth of deviations below the multidirectional bridges being constructed, history was unfolding more than 20 metres above them when the longest incrementally launched bridge in South Africa neared completion.

The one-kilometre long bridge forms part of the improvements being undertaken by the South African National Roads Agency (SOC) Limited (SANRAL) at the N2/M41 Mount Edgecombe Interchange north of Durban, connecting Phoenix and uMhlanga with Durban and the north coast of KwaZulu-Natal.

Innovative methods

Incremental launching is a relatively recent method of building a complete bridge and can save time, money, space and disruption while easing access and delivering a high-quality finish.

It involves casting 12-30 metre long sections of the bridge superstructure in a stationary formwork behind an abutment and pushing a completed section forward along the bridge axis. The sections are cast contiguously and then stressed together.

Corne’ Roux, project manager for SANRAL Eastern Region, said due to the expansion of the uMhlanga and La Lucia Ridge areas, the existing interchange had been operating at capacity with vehicles backing up on the M41 and onto the N2 in peak hours.

“An additional 40 000 vehicles enter or leave the N2 from the M41 daily resulting in substantial queuing of vehicles during the day,” said Roux. “This, together with expected future expansions and anticipated development of the Cornubia area, required the existing interchange to be upgraded in order to improve the flow to and from the N2 and M41 to the supporting road network.”

He added that the Mount Edgecombe interchange upgrade has changed the landscape forever and is sure to become a well-known landmark in years to come.

“Constructing one of the longest structures ever built in South Africa over one of the busiest intersections in KwaZulu-Natal successfully, without ever closing any of the roads permanently, bears testimony to the success of the selected construction methods and materials,” he said.

The intricacies

Gert van Schalkwyk, resident engineer for the main consulting engineers SMEC South Africa, said Bridge B0215 has a deck length of 947 metres, which not only makes it the longest incrementally launched bridge in the Southern hemisphere, but also one of the longest structures in South Africa.

It has a total number of 23 piers of which the highest is 26 metres with typical spans of 42 metres and the longest span of 50.5 metres.

What sets B0215 apart from anything else previously or currently being done in South Africa is not only its sheer size and length, but also the fact the bridge is constructed in two decks which are both incrementally launched from opposite sides with the intention to meet in the centre.

Van Schalkwyk said: “Ensuring that the two decks would meet up after each individual deck was launched was a very challenging exercise and took some innovative engineering and design.

“The deck was modelled on AutoCAD in 3D to accurately plot the launching path of the deck during construction. This model was used by the surveyors to set out and track the bridge within 1mm of its design trajectory, alignment and level.

“A total of three surveyors were present during each of the launching operations to ensure live tracking of the front and back of the deck as well as provide constant verification of their surveys.”

Both decks were successfully launched into their final position and completed in July 2016. The decks were launched to within five metres from each other: the last five metres of deck was cast insitu and is referred to as the “stitch” which neatly and monolithically joins the decks together.

Final piece of the puzzle

Like the final piece of a giant Lego puzzle, it was the stitch that was placed in position on 4 August.

“The final position of the decks, transversely, was 7mm from the design position and nought millimetres difference in level from the design level,” said Van Schalkwyk.

“Given the size of the decks and the fact these decks were launched from the two opposite ends of the site, this accurate final position is a testimony of the workmanship and world class engineering capabilities in South Africa,”

“The positioning of the stitch between the north and south ramps to close the gap may well be considered as the crowning achievement and is testament to not only design excellence but also implementation,” he said.