Kateena St/South Rd Intersection looking south. Image Source: Infrastructure SA
As the largest ever road building project in the state, the South Road Superway in South Australia was always going to be challenging, in large part because it involves 2.8 kilometres of elevated road which is being built over a pre-existing thoroughfare which has remained open to traffic throughout construction.
Thus far, however, Wayne Buckerfield and his team at Infrastructure SA are making great progress. In late February, the team lifted the largest span of the raised section – one measuring 83 metres – into position across Grand Junction Road. At the time, 40 per cent of the elevated Superway and 60 per cent of the supporting piers were in place. Federal and state ministers boldly proclaimed construction was on track and the new road would come in on time and within budget before the end of the year.
Buckerfield, Infrastructure SA’s project director of planning, transport and infrastructure, described six key areas of challenge and outlined a number of strategies his team has undertaken in order to overcome them.
1) Building above a road still open to traffic
Because the existing road has remained open throughout the construction period, Buckerfield and his team have had to build an enormous section of elevated road while traffic still flows through on a major arterial road below.
To get around this, the team first upgraded surrounding roads to provide detours around the site. They have also performed significant volumes of work at night when the road below could be closed – particularly the erection of overhead segments.
“There was a key challenge building something so significant and still having traffic flow through adjacent to the work site,” Buckerfield explains.
“The way that issue has been addressed is through some upgrades to adjacent road networks to cater for traffic which can detour around the site. We upgraded a few key roads which would become natural detours for traffic which didn’t need to travel along that section of road. So that involved some early works which were quite significant in their own right.”
2) Ground water/acid sulphate soils
In terms of piling, issues associated with acid sulphate soils, which attack the steel reinforcing in the concrete, and groundwater were amplified by the large number of piles (740 piles, roughly 30 metres in depth).
With regard to groundwater, Buckerfield says the contractor used continuous flight auger (CFA) piles, which are formed by drilling a continuous flight auger into the ground. In addition to solving the water issue, these delivered further advantages in terms of avoiding the need for temporary casing or bentonite slurry (the holes are supported at all times by soil filled auger) and involving minimal vibration.
The acid sulphate issue, meanwhile, was addressed by using a highly durable mix of concrete, which included additives.
3) Large pile caps/close to traffic.
Third, there were the pile caps, which were quite large and close to traffic. Because of this, the team used sheet piling to retain the excavation for the pile cap and keep it as small as possible. This had added advantages in terms of preventing egress of water during the excavation and setup for the concrete pour as well as allowing for a vertical face below ground level as opposed to an angled embankment which would have impinged on traffic.
4) Complex piers.
The 68 piers involved are complex in shape and, largely because of load bearing requirements, feature intensive reinforcing, creating challenges associated with fabrication of the reinforcement cages.
To get around this, Buckerfield says the cages were fabricated off-site in accordance with tight tolerances and quality standards. This has allowed them to be effectively placed on site with minimum difficulty, saving time, delivering better quality fabrication and minimising on-site materials and debris.
5) Large number of skirter segments.
An astounding 2,203 skirter segments created issues in terms of minimising impacts on traffic caused by transporting them from a remote location and ensuring adequate supplies in tight production time frames.
Like the piers, these were pre-fabricated but this was done at a purpose built facility adjacent to the site, saving on transport and creating productivity benefits associated with having storage on the spot.
6) Fitting the segments together.
One aspect of the project Buckerfield stresses in particular is that each and every segment was designed and manufactured to fit in one specific location.
This has been achieved through match casting, a process where once a segment is cast and slid out of the mold, its face becomes the formwork for the next one, which goes into the mold and joins onto it. Buckerfield says this process is not new – it was invented in the 1950s by a French engineer – but is useful in overhead construction.
“Using match casting means when you erect the segments and get them up in the air, you can be assured that they are going to fit together smoothly,” he says.
“That’s a particularly useful technique where you’ve got individual segments or individual locations that you want to be sure match up with the one that you are joining them to once they are up in the air, because it’s a bit late then to find that things don’t join properly. It’s a pretty common technique for these types of constructions, but it’s a good method to make sure everything goes together properly.”