The Concor-Mota Engil Joint Venture has almost completed the four massive concrete blocks that will anchor the Msikaba bridge in place.

Concor project director Laurence Savage says the blocks represent the engineering on which everything else rests.

“Since they are largely buried structures, they tend to be overshadowed by other, more dramatic elements of the bridge, such as the 127-m-high concrete pylons, but they are just as critical, and their construction has been challenging in many aspects.

“The geology has been complex and each anchor block has faced its own unique demands, be it location, geology, the extent of rock fragmentation and water ingress.”

The Msikaba bridge, which forms part of the South African National Roads Agency’s N2 Wild Coast project, will span the 198-m-deep Msikaba River gorge.

The structure is set to cut travel times between the Eastern Cape and KwaZulu-Natal.

It features a main span of 580 m, which, on completion, will rank as the longest main span constructed using the cable-stay method in Africa.

The bridge is being built from the north and south banks of the gorge and comprises two identical halves, each spanning 290 m, which will meet mid-point over the gorge.

Each half is supported by 17 pairs of cables attached to the inverted Y-shaped pylons, one on each side of the gorge.

The pylons are back-stayed into the anchor blocks, two on either side of the gorge, by 34 pairs of cables – 17 for each half – positioned 130 m behind each pylon.

Each anchor block has a length of 49 m, a width of 10 m at the base (narrowing to 4 m on the spine of the structure), a depth of 17.2 m, and a mass of 21 500 t.

Their construction has been under way almost since work on the Msikaba project began in earnest in late 2019.

They will reach completion shortly, just weeks before the deck is launched over the gorge.

Precise blasting techniques were required for the bulk excavations for the four blocks, with the blasted rock material cleared out using 30 t excavators and articulated dump trucks supplied by local subcontractors.

Extensive lateral support was installed in the side walls.

Construction of the blocks has involved the placing of 4 100 m3 of structural concrete and 2 650 m3 of mass concrete, as well as the fixing of 200 t of reinforcement in each anchor block.

Each block was cast incrementally with 11 lifts, varying from 1.25 m to 1.75 m per lift.

Savage says challenges during construction included working in deep excavations and confined spaces, sometimes with heavy machines, and working in and over reinforcing, with much of the work taking place at night.