Project

Widening the Waimakariri bridges

Capacity for extra lanes on a pair of Christchurch motorway bridges was built into the original structures, but the passage of time has complicated the job. Hugh de Lacy reports.

The necessary headstocks jutting out from the piers are there from when the twin motorway bridges were first built over the Waimakariri River north of Christchurch in the early 1970s, but tacking a third lane onto each of them posed challenges the original designers could not have anticipated.

The bridges lie side-by-side across the flood-prone Waimakariri, and the eventual need for two more lanes was recognised by the designers who had the foresight to construct headstocks on the inside of each of the 16 piers.

In the interim however, both the increased loads from heavier trucks and advances in reinforced concrete building standards meant the headstocks as they were built were not suitable.

The answer was to sandwich the existing concrete headstocks between two new steel units to create an enhanced headstock, and to bolt them in place.

Only then could the 25-tonne beams be laid across them along the 420 metre length of the bridges to support the carriageways.

The Christchurch Northern Corridor (CNC) alliance is the head contractor on the $240 million project which sees the Main North Road and the Western Belfast By-pass converge with the new Northern Corridor just south of the Waimakariri River.

It’s this concentration of traffic flows onto and off the motorway that has created the need for the extra lane northbound in the space between the twin bridges, and intricate sequencing of the work was required to minimise the impact on traffic flows.

The New Zealand Transport Agency, Christchurch City Council, Fulton Hogan, Jacobs and Aurecon form the alliance which hopes to have the 16-months Waimakariri Bridges widening job completed by the end of this year.

“The technology had changed over time along with design requirements,” CNC manager James Harrison tells Contractor.

“Trucks are heavier today, and earthquake seismic loadings are more complex, requiring higher design standards.”

The steel units encasing and strengthening each headstock had to be precisely designed so that the holes for the 80mm diameter bolts could be cored through without touching any of the critical reinforcing steel, which would otherwise weaken the structure.

Finding the exact points at which to drill the holes required the original drawings to be consulted, and then confirmed by surveying the reinforcement within the existing structure.

These core holes – five per headstock – had to be incorporated into the design of each pair of enhanced steel headstock units supplied by Pegasus Engineering of Rolleston, which provided all the structural steel members for the bridges.

Each set of steel headstock units weighed between five and eight tonnes and were lifted into place by a 30-tonne all-terrain crane operating on the flood plain below the bridges, and by March this year all those for the northern half of the bridges had been fitted.

By that time too the first of the 25-tonne main beams had been lifted into place on the northern end of the northbound bridge by a 250-tonne Kobelco crane.

Reinforced concrete planks for the deck, pre-cast at the local Fulton Hogan yard, and each weighing about 1.5 tonne, are then lifted into place with the smaller crane, but before the deck can be poured no fewer than 11,000 holes, 300mm deep and 20mm wide have to be drilled into edge of the existing deck – again avoiding the original reinforcing steel – to take the 16mm reinforcing bars that, packed with resin, serve as the stitches holding the old and the new deck sections together.

 

Rain on the plain is no challenge, but within 12 hours of 100mm falling in the alpine catchment, the river level at the motorway bridges can come up two metres.

 

A team of two or three has been working full-time on drilling these holes for several months.

As work on that side of the river winds up, the bridge site office and base of operations will move to the south side to repeat the process there.

Each deck will require 460 cubic metres of placed in situ concrete, and each will be poured in two lots of 230 cubic metres.

The river itself poses its own problems, with the builders keeping a constant monitoring eye on the weather in the Waimak’s vast catchment in the Southern Alps.

Rain on the plain is no challenge, but within 12 hours of 100mm falling in the alpine catchment, the river level at the motorway bridges can come up two metres.

“You can see the river rising,” Harrison says.

The bridge project requires no new construction on the river bed or plain itself.

“The trickiest part of the project will be working over the river channel because we can’t easily get adjacent to it and we would prefer to not move the channel around.

“Instead we will build a platform out as far as we safely can, and lift everything from the eastern downstream side of the bridges.

“Overhead power lines on the upstream side stops us using the crane from that side.

“We have to lift the headstocks over the southbound bridge and down between the two bridges, swing them into the correct orientation below the bridge, then bring them up to the underside and stitch them in place – all over a flowing river,” Harrison says.

And then there’s the traffic.

To allow for the drilling work underneath the bridges while keeping all traffic lanes open, steel safety barriers were installed on both, and a temporary speed limit of 80kph is in place during the entire construction period.

When the first of the big girders was lifted over the southbound bridge and into place on the northbound one in late February, the southbound bridge was closed, and both north and southbound traffic were diverted onto the northbound bridge as a contraflow for one night.

“We managed to install the three sets of two beams in just six hours – faster than we expected – and we were done by 3am.”

There are some on-going challenges with the traffic: On the one hand some people are not sticking to the 60kph speed limit that applies during the contraflow, and on the other hand rubber-neckers have been slowing down to take in the big crane that’s been looming over the site since early February

“We want people to keep their eyes on the road and not be distracted,” Harrison says.

As the new lane is completed on the southbound bridge, a cycleway will be clipped onto its other (downstream) side.

Harrison expects the work on the bridges and cycleway to be completed by spring, after which the alliance will take on the task of knitting the two new lanes of the bridges into the existing four to the north and south.

The alliance expects this to be completed by June next year.

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