The Msikaba Bridge project notched up an engineering milestone last month, when the south pylon legs were hydraulically jacked apart.

The 580-m-long, stay cable bridge – which will span the 198-m-deep Msikaba Gorge – is located near Lusikisiki, in the Eastern Cape, and forms part of the South African National Roads Agency’s N2 Wild Coast project.

The bridge is being constructed by the CME joint venture (JV), a partnership between Concor Construction and MECSA Construction.

According to CME JV project director Laurence Savage, the jacking apart of the legs countered bending at the bases of the cantilever legs, and was conducted using two 150 t hydraulic jacks.

These two legs make up the first 20 m of the inverted Y-shaped pylon, says Savage, and are built without any lateral support as free cantilevers.

Once completed, four lanes of vehicles and a pedestrian walkway on either side will pass beneath these legs at the start and end of the bridge deck.

“In layman’s terms, the jacking eradicated the effect of the legs bending towards each other,” explains Savage. “As engineers would see this, the legs are effectively vertical due to the jacking process.”

The bridge includes two pylons that will stand 128 m high on each side of the gorge.

The pylons support the bridge deck using a network of 34 cable tendons strung through their upper reaches.

“These cables then run from the top of the pylons, back into anchor blocks located 100 m to the rear of the respective pylons,” notes Savage.

“Each of the four anchor blocks is made up of over 1 600 t of structural reinforced and mass concrete and extends 17 m – the equivalent of six storeys of a building – into the ground.”

Savage explains that the lateral support was installed on the sixth lift of the pylon structure, after 520 m3 of concrete had been poured to reach a height of 20 m.

“Two sets of hydraulic jacks were installed in parallel to each other and a jacking force of 1 750 kN applied to the two pylon legs to counter the bending moment for the freestanding cantilever legs.

“To achieve the required force, the two 150 t hydraulic jacks were loaded to 90 t, developing a pressure of 41 MPa. The jacking was done on 5 MPa intervals, and deflections of the structure were monitored using dial gauges and surveying.”

Savage notes that a key consideration was the punching force on the flat face of the pylon legs, owing to the jack load.

This also dictated the size of the bearing plates affixed to the inside of the legs, enabling a dissipation of the force across the appropriate surface area. This avoided any damage to the structure, owing to the loading of the concrete surface.

“The jacks only have a 50 mm stroke, which required the installations to be exact – as the 41 MPa pressure had to be achieved before the jack ran out of stroke length,” explains Savage.

“The base plates were installed on the pylon leg structure with 29 mm nonshrink grout, minimising the use of the stroke length.”

After the lateral support was aligned and seated, a grout biscuit was cast, which served two key functions. Firstly, it absorbed any tolerances in the installation after the initial base plate installation and, secondly, it assisted with the removal of the lateral support.

Breaking out the grout biscuit released the pressure in the lateral support, once the seventh lift was cast and the legs permanently locked together.

“The temperature of the pylon structure and lateral support was measured to ensure an average of 22 ºC, to limit any unforeseen changes in force due to changes in the temperature during the construction cycle casting lift seven,” notes Savage.

“The lateral support was wrapped in a 25-mm-thick thermal blanket to limit temperature change movements and any resulting changes in prop forces.”

The lateral support was locked into place using a two-part system: the locking ring on the hydraulic jack, and the locking ring on the super-shore jack housing.

The preparation for the jacking took three months of planning and analysis, while the setup was conducted over nine days. The actual jacking process was completed in less than eight hours from its start.