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A unique set of challenges faced the builders of a new bridge on the West Coast. HUGH DE LACY examines how they did it.
But there were others: they had to build the new bridge around the historic existing one while keeping the road open, and without interrupting the rail traffic from the Westland Dairy Products factory in Hokitika. And they had to work together as lead contractors for two different clients while respecting local iwi’s concerns for the river as a traditional pounamu (greenstone) source, and retaining part of the old bridge as the centrepiece of a nearby heritage park. The bridge comprises two road lanes side by side, with the railway on the upstream side, plus pedestrian and cycle paths, all sharing the same set of foundations. The contractors – HEB Structures for the bridge and Downer EDI for the roading approaches – not only managed to meet all these intertwined requirements, but completed the job at least three months ahead of schedule. The client for the bridge was Ontrack, now part of KiwiRail, while the NZ Transport Agency was in charge of the roadworks.
A classic of its kind, though with cast-iron cylinders instead of the usual hardwood piles of the day, the old bridge comprised seven Australian hardwood Howe trusses – one of which is being kept for the heritage park – and seven smaller timber spans giving it a total length of just over 200 metres. It had served for over 120 years with only scheduled maintenance, most recently carrying 3500 road vehicles and four trains a day. The new bridge, which is 220 metres long, boasts 10 sets of twin-bored steel-reinforced concrete piles, with supporting abutments and pier caps of post-tensioned concrete. The superstructure of concrete pedestals carries nine spans of hollow-core pre-tensioned beams forming the road, cycle and pedestrian paths, while the rail bridge comprises steel through-plate girders with a steel floor and rail beam system. The rails themselves sit on 730 timber sleepers.
“It was all part of the resource consent that we do the project in consultation with local iwi,” project engineer Richard Keenan told Contractor. “The other big constraint was keeping the road open at all times, either on the old bridge on the new one.” Keenan’s Novare Design was the consultant engineer to KiwiRail, and a colleague, Dr Rob Park, designed the bridge. The first step in the project, begun in June 2008, was the driving of the new foundations 20 metres into the riverbed on either side of the old bridge. This was achieved by lowering a concrete plug to the bottom of each steel-cased pile and then driving the plug down with an 18-tonne piling hammer. The reinforcing cage would then be lowered into the pile and the concrete pumped in. After the fitting of the pier caps, which ran under the old bridge, enough road beams were lifted into place to create the first road carriageway. There are 99 of these 24-metre road beams in total, each weighing 30 tonnes, and they were lifted into place by two 80-tonne cranes. The beams were prefabricated in Rangiora by Daniel Smith Industries as subcontractor to HEB, and railed three at a time across the island.
The construction of the second road lane was achieved by using a Bailey bridge to take traffic across each successive span while post-tensioning and concreting were carried on underneath. While HEB was building the bridge, Downer EDI Works was handling the road approaches, which included a roundabout on the south end with the railway passing through the middle of it. This created two new level crossings, protected by four sets of automatic barrier arms. It also created a few teething problems for local drivers unused to roundabouts: there were several minor accidents on the project’s completion. The level crossings feature an innovation. For the first time in the Southern Hemisphere, German-made rubber panels have been fitted hard up against either side of the rails so that vehicles passing across feel virtually no bump. Trains going through depress the panels which then spring back to road level after the train has passed.
The Arahura road/rail bridge was formally opened on December 11 last year, and Keenan attributes its early completion to good communication between both clients and contractors, and HEB’s ensuring the beams for both road carriageways were on the site before train services temporarily ceased in mid-2009. And to the best of his knowledge, he says, the life-cycle of not a single whitebait was disrupted by the project.
Contractor Vol.34 No.2 March 2010 |
Making sure they didn’t disturb the whitebait was just one of the complications encountered by the teams – two of them – constructing the new road/rail bridge over the Arahura River on State Highway 6 north of Hokitika.
At $25 million, jointly funded by the two clients, the Arahura road/rail bridge was the biggest built on the country’s rail network in years. It replaced an iconic single-lane combined road/rail bridge built between 1891 and 1893 for the Public Works Department, originally just for trains but quickly adopted by horse and pedestrian traffic too.
Timing was crucial to the execution of the project, revolving round the dairy production season which allowed a window between June and August when the trains weren’t running. Of equal importance to local Coasters was the whitebait season from the beginning of September to the end of November when no piling could be done, nor construction equipment allowed in the riverbed for fear they disturbed the fish.
The completion of the first carriageway in May of last year allowed road traffic to be diverted from the old bridge and, after the last train crossed it the following month, it was demolished. The new one was in place before the trains began running again 13 weeks later. The railway approaches were realigned 3.5 metres upstream from the old bridge, and the first train crossed it, heading south with empty containers to be filled at the dairy factory in Hokitika, on September 4.
HEB was responsible for building a complex set of protective guidebanks to protect the bridge from the river’s regular savage flooding. Besides gravel from the river itself, 60,000 tonnes of rock was carted from a nearby quarry to form long arcs of stopbank at either end of the bridge.