Rebuilding the Waiho River bridge and reconnecting communities

The 2nd Engineer Regiment of the New Zealand Army worked with Downer New Zealand to rebuild the Waiho River bridge on the West Coast in early April

With torrential rain battering the West Coast of the South Island, the Waiho River – notorious for its dynamically fluctuating levels – became a raging torrent. The single-lane bridge carrying State Highway 6 never stood a chance.

Fed by the meltwaters of the Franz Josef Glacier, the Waiho River skirts the main township of Franz Josef to its south. The main road, SH6, which connects the West Coast communities and is the principal route for tourists and visitors to the West Coast, crosses the river via a long singlelane bridge.

Following a severe rain storm on 26 March, in which it is estimated some 400 mm of rain fell in just a few hours, the river rose rapidly, carrying massive amounts of storm debris, boulders and mud. The waters battered the bridge’s support piers relentlessly all afternoon – and late in the day the bridge gave way.

“State Highway 6 is currently closed south of Hokitika. Given the bridge is now gone on one side, this closure will continue until the bridge and its abutment can be safely replaced,” announced NZ Transport Agency network manager Colin Hey shortly after 5pm.

Why did the bridge fail?

The Waiho River bridge was a Bailey bridge that had been installed over the river in 1990. The Transport Agency says that due to the unstable nature of the riverbed, the bridge had been raised and lifted several times because of the continual buildup of gravel. The riverbed level fluctuates so much due to the large volumes of sediment that are washed down the river from the Franz Josef Glacier.

Developed by the British during the Second World War, a Bailey bridge is a prefabricated truss-style bridge that uses lightweight, easily portable components (usually wood and/or steel) that require no special tools or heavy lifting equipment to assemble. They are highly versatile and relatively quick and easy to build and dismantle.With a basic component of a 3 m long truss panel, they can be configured to provide much longer spans and cater for a wide range of loads. They are used extensively in civil engineering construction projects to provide temporary crossings for foot and vehicle traffic.

It was suggested the Waiho River bridge failed due to a lack of maintenance or earlier work upstream, but Transport Agency system manager Pete Connors says there is no evidence to support this. “Given the massive force of the swollen river on the north bank and the huge amount of rock pounding the bridge’s piers, on top of debris buildup over the course of the day, it is unlikely anything could have been done to save the bridge.”

The river had been building up in the centre in recent months, pushing its main channel towards the north bank. “That was beyond anyone’s ability to alter,” Mr Connors says. “Combined with the significant flood event, this resulted in extremely high river velocities in the main channel which aggressively attacked the rock groynes that protect the bridge and the rock embankment protection at the northern abutment [where the bridge joins the bank].

“The scour and displacement of rock resulted in the shallow pad foundation at the northern abutment being undermined. The northernmost pier [which stands in the river] appears to have been hit by a large boulder, causing the pier to buckle and fail and resulting in the collapse of the main northern span. Once that entered the river floodwater, subsequent bridge spans were pulled downstream off their pier foundations.”

Assembling the team

The original Waiho River bridge was swept away in floodwaters following severe rain on 26 March

Transport Agency roading crews and engineers were on the scene the following day to inspect the piers and abutments as the river level dropped, and announced that they expected a replacement Bailey bridge to be installed within 7–10 days to reconnect the communities. This timeframe was later amended to 14–17 days for the bridge to be replaced and operational, with the extra benefit of being able to carry heavy trucks, not just cars and buses.

The Transport Agency quickly mobilised plant, crews and materials from around the country, with Bailey bridge contractor Downer arriving onsite to manage the assembly and rollout job. Several truckloads of componentry were transported through the Haast Pass to the south side of the Waiho River where a construction site was established.

Complex pier and foundation design work was quickly completed, with a method for demobilising and erecting the new seven-span bridge being finalised. Truckloads of rock and earth were moved, while work crews constructed a rock-lined guidebank to allow pile-driving at the northern abutment.

Collaborative effort

Second Lieutenant Laura Bayfield: “The Army is used to assembling Bailey bridges manually – it’s hard work, but very rewarding”

In a remarkable collaborative effort, the NZ Defence Force provided 17 personnel from the 2nd Engineer Regiment of the New Zealand Army to assist Downer with the project. “It’s great to contribute our professional expertise to support government agencies responding to communities that need our assistance,” says Lieutenant Colonel Terry McDonald, the regiment’s commanding officer.

“Having Army engineers working with the Downer team means we have highly experienced Bailey bridge construction people on this project to ensure there will be no delays when we are ready to launch the bridge from the south side to the north,” said Mr Connors in the first week of April.

The team of Army engineers was led by Second Lieutenant Laura Bayfield. “The new bridge will have 52 bays over piers, whereas most of the bridges we do are 10–12 bays without piers, so it’s epic to be involved in this one,” she says. “The Army is used to assembling Bailey bridges manually. It’s hard work, but very rewarding – it’s like putting together a giant puzzle.”

Supporting the Bailey bridge team on both sides of the river were numerous subcontractors who ensured the piers were repaired (over two overnight shifts) and the whole northern abutment was fully rebuilt. Fulton Hogan, the Transport Agency’s highway maintenance team for the West Coast, mobilised a big support crew to ensure the abutments on each side were ready for the new bridge, with designers from WSP Opus working hard to have designs completed on time.

Launching the bridge

Several truckloads of componentry were transported through the Haast Pass to the south side of the Waiho River where a construction site was established

Just a week after the bridge was washed away and taking full advantage of a period of fine weather, contractors had dismantled the old bridge sections, and new piles had been driven on the northern side of the river. The 3 m long panels that would be connected to form the new bridge – just like a giant Meccano set – were assembled on the southern side of the river.

A useful feature of a Bailey bridge is its ability to be launched from one side of a gap – the frontmost panel is angled up to form a ‘launching nose’

A useful feature of a Bailey bridge is its ability to be launched from one side of a gap. The frontmost panel is angled up to form a ‘launching nose’ while the rest of the bridge is connected from the safety of the riverbank. The bridge is placed on rollers and simply pushed across the gap, using manpower or a truck or tracked vehicle.

As the bridge progresses across the gap, another panel is craned into place at the back and fastened on. Once completely launched across the gap, the rollers are removed (with the help of jacks) and the ribands and roadbed installed, along with any additional panels and transoms that are needed.

The Waiho River bridge structure was successfully rolled across the bridge piers over a period of two days to the north bank. However, another period of heavy rain caused delays, with the river levels rising again, forcing work crews out of the riverbed.

Whilst the bridge structure was being rolled across the bridge piers, another period of heavy rain caused delays, with rising river levels forcing work crews out of the riverbed

Open to traffic

With the sun shining again and the river level once more abating, work crews were able to complete the job of securing the new 170 m long bridge, and on Saturday 13 April, just 18 days after the original had been swept away, the new bridge was opened to traffic, with Westland Mayor Bruce Smith amongst around 1000 people who turned up to drive across.

“It was incredible to watch the first volume of people and cars pass over the bridge following a blessing conducted by Te Runanga o Makaawhio,” says Mr Smith.

“People have worked extremely hard to get the bridge back over the river and reconnect the communities in South Westland,” says Moira Whinham, Transport Agency maintenance contract manager for the West Coast. “The Transport Agency is proud of the work done and the positive attitude everyone has brought to this urgent highway project. Our thanks go to everyone, especially the South Westland community.”

And while the bridge is now open, there is still work to do: sealing the approaches, reinstating the guardrails and pedestrian walkway, installing lighting and other work which may involve temporary short-term closures.

This story was first published in the April/May’19 edition of NZ Construction News and is reproduced here with the permission of Aston Publishing.