Giant stitch makes history as Mt Edgecombe gap is closed

The north and south decks of the Mt Edgecombe Interchange have been 'stitched' together.

Concrete is poured to close the gap between the two ramps. Picture: Berea Mail

There were cheers all around on Friday, August 4, when the north and south decks of the one-kilometre ramp of the Mt Edgecombe Interchange in Durban, KwaZulu-Natal, were joined, just two-and-a-half years after they first started moving progressively towards each other, reports the Berea Mail.

While motorists went on 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 Mt Edgecombe Interchange in the north of Durban, connecting Phoenix and uMhlanga with Durban and the North Coast of KwaZulu-Natal.

Incremental launching is a relatively recent method of building a complete bridge. It involves casting 12-30m 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. 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.

“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.

Gert van Schalkwyk, resident engineer (structures) 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.

“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.