Feature

Core benefits Ashley River bridge

A new North Canterbury bridge features what are thought to be the longest hollow-core beams ever used in this country. HUGH DE LACY takes a look.

WHEN LAST YEAR’S June 18 floodwaters swept away one of the concrete piers of the Ashley River Bridge on the northern outskirts of Rangiora, cancelling 10,000 traffic movements a day across it, the Waimakariri District Council had to work fast to organise a replacement.

Within a fortnight local firm Daniel Smith Industries had installed a temporary steel pier, and the old bridge was re-opened to all traffic.

Though traffic growth was not a major driver of the project, populations on both sides of the river are booming as a result of the north-west flight of Christchurch residents after the 2010- 2011 earthquakes, adding urgency to the need for a new bridge.

Hollow core beams are craned into place on the third iteration of the 300 metre bridge to cross the Ashley River, on the northern outskirts of Rangiora. Construction is expected to be finished by April 2015.
Hollow core beams are craned into place on the third iteration of the 300 metre bridge to cross the Ashley River, on the northern outskirts of Rangiora. Construction is expected to be finished by April 2015.

And by April next year it should be a reality.

The old Ashley bridge remains in service while the new one is being built along its upstream side by Rotorua-based Concrete Structures as the lead contractor, and with Wanaka-based Thomson Earthmoving in charge of the approach roading, and of shifting the flow of the river as needed across its braided bed.

At 300 metres long, this will be the third bridge over the Ashley River at that point: the first, a wooden one for single-lane dray traffic, was built in 1897, and the present one in 1912.

With a carriageway of just 5.8 metres, the second bridge is scarily narrow for two-way traffic and has no provision for pedestrians or cyclists.

The new bridge, already half built, will cater to both, with 1.8 metre cycleways on either shoulder of the two 3.5 metre carriageways, and a 1.5 metre footpath outside the cycleway on the downstream or eastern side.

Scouring under the existing bridge was first detected in 2010, requiring the immediate replacement of one pier, and thereafter it had to be closed every time the river flow rose above 150 cumecs at the Ashley Gorge – reduced to 100 cumecs after the other pier was washed away in the June 2013 incident.

It takes about an hour and a half for the river flow at the gorge to reach the bridge site.

Before the existing bridge can be reopened to traffic, the river flow has to drop far enough to allow the measuring of the depth of the scouring around the piles, as well as an inspection of the bridge itself, and surveys of both its and the riverbed’s levels.

If the scour depth is too great, the holes have to be filled in to stabilise the piles, and all of this can keep the bridge closed for days, much to the frustration of commuters who have hiked the population of Rangiora by 25 percent to well over 15,000 since the Canterbury quakes.

Closures force traffic to take a long eastward detour out to SH1 which crosses the same river with a similarly narrow but unthreatened concrete bridge near Waikuku.

Between them, Concrete Structures and Thomson Earthmoving have some pretty impressive machinery on the site to impress the thousands of commuters eyeballing it daily from the old bridge.
Between them, Concrete Structures and Thomson Earthmoving have some pretty impressive machinery on the site to impress the thousands of commuters eyeballing it daily from the old bridge.

The existing 102-year-old bridge at Rangiora comprises 24 continuous 12.5 metre spans, and features gently curved tee-beams and piers each with three hexagonal openings.

It was built at a cost of $26,000.

The initial design of the new bridge was prepared for the Waimakariri District Council by Opus International, envisaging the use of Super T beams across nine piers 28 metres apart between the two abutments, with each pier having two piles driven to a depth of about 16 metres.

The failing piles on the existing bridge are only about three metres deep, of which only about two metres is below the existing level of the riverbed – which is why, after a century of use, scouring is occurring underneath.

Concrete Structures tendered for the job on the basis of the Opus design, but also submitted an alternative using hollow-core pre-stressed concrete beams rather than Super T beams.

Slightly cheaper than the conforming design, the variation designed by Holmes Consulting for Concrete Structures won the day, and the resultant 110, 40-tonne 28 metre beams, 1.2 metres in diameter and cast at Concrete Structures’ Rolleston factory in Christchurch, are thought to be the longest ever used in bridge construction in the country.

“We’re not aware of a longer hollow-core beam being used in New Zealand, and no one we’ve spoken to has ever come up with one,” Concrete Structures project manager Dean Quickenden tells Contractor.

Opus remains involved in the project as the council’s project manager, and overseer of the construction of the $10.6 million job.

Work on the new bridge began in mid-January this year, and quickly ran into trouble when the river decided it wasn’t going to co-operate.

Floods in March, April and June this year meant that the project had burned up all three of the scheduled flood event allowances in the contract within the first six months.

Since then the Ashley has relented, indeed behaving itself so well that by the beginning of October the project had passed the halfway stage.

Between them, Concrete Structures and Thomson Earthmoving have some pretty impressive machinery on the site to impress the thousands of commuters eyeballing it daily from the old bridge.

Thomson has assembled no fewer than three big white Terex motorscrapers, not just for the road works on the bridge approaches, but also to shift the river flow away from working areas, a process which is quicker than using excavators and dump trucks.

The actual earthmoving volumes involved are not great – about 15,000 cubic metres, mostly building up the new carriageway approaches, plus a bit of cut-to-fill and stockpiling.

Concrete Structures has two of its own 85 tonne crawler cranes lifting the hollow-core beams into position, and they’ll also be called into service when it comes to demolishing the existing bridge after the new one is opened.

Demolition is not exactly a core activity for Concrete Structures, but the cranes will make it easy to cut the existing bridge up and recycle the concrete, a job that will probably take around three weeks.

The Rolleston pre-cast concrete factory is just one that Concrete Structures operates around the country – the others, besides the Rotorua headquarters, being in Auckland and Hastings.

Though a North Island outfit, Concrete Structures is well established in the South, with previous Canterbury jobs including the underpinning of the piers on the old two-lane road bridge, over the Waimakariri River near Kaiapoi, that were damaged in the earthquakes.

Project manager Quickenden is delighted with progress since the flooding abated in early winter, and provided the river continues to co-operate, Rangiora’s frustrated commuters will have nothing to complain about from next April onwards.

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