New bridge over the Awatere River
It’s a quaint old curiosity that will continue to serve the South Island rail network, but the Awatere road-rail bridge is relinquishing its role as an annoying bottleneck to road traffic on State Highway 1, 25 kilometres south of Blenheim. By HUGH DE LACY.
About the middle of this month the wooden decking of the single road-lane on the 105-year-old Awatere Bridge will be pulled up, and road traffic will instead sweep across the magnificent new bridge constructed 30 metres to the west.
There will then be only two combined road-rail bridges left on the state highway network nationwide – the Arahura and the Taramakau, both on the West Coast – though several others remain on roads administered by district councils.
The original Awatere bridge was designed by Peter Seton Hay of the Public Works Department and completed in 1902 for about $50,000 by the Christchurch company Scott Brothers, with the railway running over the top deck and the road underneath. At 325 metres, it was the longest of its kind in the country at the time.
Traffic over the road level has for years been controlled by give-way signs and, latterly, traffic lights. As well as the delays, the narrow single lane has been a nightmare for truck-drivers trying to manoeuvre B-trains onto and off it without losing paint. Occasionally trucks got jammed, blocking the main road between Blenheim and Christchurch for hours.
The residents of the small farming town of Seddon, a couple of kilometres to the south, had been agitating for years for the old bridge’s replacement, and Transit New Zealand was finally able to give the go-ahead a little over two years ago.
The project makes the budgeted $15.2 million seem to go a long way because it actually involves two new bridges: The 274 metre-long by 10 metre-wide road bridge across the Awatere River, and a new rail overbridge on the southern approaches to the old bridge. Also included in the budget were an upgrade of the Awatere Valley/Redwood Pass Road intersection and the installation of a passing line on the northern side of the road bridge, plus the realignment, including another passing lane, of the road leading under the overbridge and into Seddon on the south side. The work involves the reconstruction of three kilometres of highway.
The project was designed for Transit by Bloxham Burnett and Olliver, a consulting firm wholly owned by its 30-odd mostly professionally-qualified staff. Their assignments since the firm’s formation in 1993 have included the Mercer to Langswamp and Rangiriri to south-of-Ohinewai four-laning of State Highway 1 in the northern Waikato, Auckland’s Spaghetti Junction intersection, and the raising of the Tangiwai Bridge so that it comfortably survived the lahar flow off nearby Mount Ruapehu early this year.
The main contract was let to North Island firm HEB Smithbridge which has established a beach-head office in the South at Blenheim, as well as a new office in Christchurch, consolidating its position with a Transit road maintenance contract as well as the Awatere Bridge job. HEB’s contract manager for Awatere was David Wyeth, who arrived in New Zealand from England in 2000 sporting a CV that included working on London’s Gatwick Airport extensions.
Work on the Awatere project began in January last year with the upgrade of the valley road intersection, long identified as a traffic hazard but not initially intended as part of the bridge project. Bowing to the logic of doing it as the same time as the bridge, Transit brought it forward.
South of the intersection a 200 metre northbound passing lane now complements a straight-line approach to the new bridge, instead of the sharp downhill S-bend that used to challenge traffic approaching the old one.
These parts of the project were relatively straight-forward. The real difficulties Wyeth and his HEB team encountered were on the south side of the river with the rail overbridge. It has the dual function of carrying rail traffic and allowing road traffic to pass underneath onto the road bridge approaches round a slight curve, instead of the old level-crossing on a tight right-hand bend.
A key decision made early on in the work was to fabricate the structural components of both new bridges on the building site. In the case of the overbridge there were two main girders, one weighing 700 tonnes and the other 400 tonnes, and they were pre-cast just a few metres from and more-or-less alongside the railway. Trying to get the larger of the two into place within the 24-hour closure window allowed by rail network owner OnTrack – which also owns the old road-rail bridge and rented access across it to Transit – proved a problem, and the first attempt had to be aborted.
The girders lay at an angle of just under 24 degrees from the railway.
“They had to be pulled in at that angle which meant we had to move them about 21 metres rather than just the six to seven metre distance that they were from the track,” Wyeth told Contractor. “We then had a lifting frame that went over both ends and we jacked the whole segment up, put our skates underneath and then ran it on the 24 degree angle on our bearing plates.”
Unfortunately the first time they tried to get the 700 tonne unit onto the abutment it caused a subsidence of about 10mm that in turn led to the collapse of some of the skates. That set the project back about four weeks.
There were no such problems with the second attempt, and both girders were placed and the railway reopened with 20 minutes of the 24 hours to spare.
Then it was on to the construction of the road bridge, comprised of 10 equal spans, with pre-cast concrete deck slabs and U-beam stringer units supported by cast-in-situ concrete reinforced crossheads, supported in turn by twin 1.5 metre diameter reinforced concrete piles. The piles were sunk through the gravel of the riverbed and six metres into the papa clay base below.
It was the on-site pre-casting of the main bridge units that not only allowed the project to regain the time lost on getting the overbridge girders into place, but also ensured that the entire project will be completed in November rather than January as scheduled.
“We really picked up time on the pre-cast,” Wyeth says. “There was an allowance for a certain amount of time for each U-beam and deck slab pour, but because we had heated beds and thermal blankets on-site we were able to get them out of the mould in approximately two days, and still have 45MPa concrete.”
Without the beds and blankets, the units would have taken 28 days to reach the required degree of strength. The pre-casting began in November of last year, with the last U-beam being poured on March 22 and the last deck-span unit on June 22. Thereafter the work was relatively straight-forward.
“The deck-slabs just basically fit over reinforcing bars sticking up out of the top of the U-beams and were then grouted in, so the only real in-situ concrete pour was the pier heads and the in-fill sections of the deck slabs,” Wyeth says.
As well as being finished two months ahead of schedule, the Awatere bridges project is coming in under budget, according to Transit’s Marlborough projects manager Andrew Adams. How much under budget won’t be known till the job’s completed with the removal of the timber decking on the road part of the old road-rail bridge.
For David Wyeth a particularly gratifying part of the Awatere job was the enthusiasm with which it was greeted by the people of Seddon. He gave talks at the local school where the children built models of the new road bridge, and he’s under pressure from them to ensure they’re the first people to walk across it when it’s opened this month.
The response of the locals “made us feel rather proud because it made us realise what an impact the project will have on their lives,” Wyeth says.
Contractor Vol.31 No.9 October 2007
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