BALANCED CANTILEVER VIADUCTS

Posted on Mar 5, 2009 in Media Releases

An innovative construction technique is being used for the construction of the Gautrain viaducts crossing over the busy N1 and N14 freeways on either side of Centurion. The so-called Balanced Cantilever construction method is used for building long-span bridges over areas where construction from the ground upwards is not possible.

The method is therefore ideal for the viaducts being constructed over the N14/Jean Avenue and N1/John Vorster Drive interchanges in Centurion where traffic cannot be interrupted.

Construction Methodology

Following extensive geological investigation work to determine the depth and quality of bedrock, construction starts by constructing reinforced concrete shafts or piled foundations in the ground. These are socketed into hard dolomite bedrock. Each shaft foundation is 7m in diameter and can reach a depth in excess of 30m deep. Then follows the construction of the viaduct pier:

Once a pier is securely in place, construction of the viaduct segments can start. The first cast-in-place section will be constructed on top of the pier (segment on pier). From here concrete sections are completed one-by-one, moving outwards in both directions simultaneously.  The method employs a sophisticated hydraulically controlled concrete shutter.

This process continues symmetrically in segments until the centre of the span is reached. Segments are continuously in balance and a closing segment will be cast when the centre of a span is reached. When all the sections are constructed, the shape of the viaduct is completed.

The viaducts will be constructed at an acute angle where the rail alignment runs diagonally over the interchanges. This requires a unique engineering solution. The piers are shaped elliptically so that they are sufficiently slender to fit into confined areas between carriageways and ramps in the interchanges, but strong enough to support the viaduct superstructure. The pier heads, which are also shaped elliptically are rotated relative to the piers itself where required so that the bridge bearings can be positioned at right angles to the viaduct alignment.

To ensure a ready supply of high strength concrete, a batching plant will be erected at each construction site. The sections of the viaduct spans will be post tenstioned. During stressing, steel cables, running through the concrete sections, are put under tension and anchored at the ends.

Safety and traffic flow

Cantilevering allows construction to continue while the traffic flow continues on the roads below. Safety nets are securely fitted around the formwork traveller. Safety barriers are placed on the ground, especially around the construction of the piers.

Several temporary steel pedestrian bridges have been erected over the N14 highway at the Jean Avenue interchange and across the N1 at John Vorster interchange to provide construction workers safe access across these busy highways.

VIADUCT SPECIFICATIONS

Due to the arched design of the viaducts, the mid-spans will be shallower than the segments resting on the piers resulting in a graceful curve.

Viaduct over N14/Jean Avenue

• Length: 571.5m
• Number of spans: 6. The longest span is 121m.
• Mid-spans: 3.5m deep.
• Spans on top of the piers: 7.5m deep.

Viaduct over N1/John Vorster Drive

• Length: 502.75m.
• Number of spans: 6. The longest span is 109.8m.
• Mid-spans: 3.5m deep.
• Spans on top of the piers: 6.2m deep.