A near miss

A truck crashing through the side of a Desert Road bridge in 2017 threatened to shut the entire highway for months. It didn’t, but it was a near thing. Hugh de Lacy explains.

The fastest route up through the central North Island was down to one lane for nine months during 2017-2018 after a truck’s trailer unit jack-knifed on the Waihohonu Bridge, wiped out the railings on the western side and severely damaging the bridge’s structure.

The 32-wheeled truck-and-trailer unit ended up on its side draped right across the northern end of the bridge, with the trailer hanging precariously over the side and its back end virtually in the water but still connected to the truck.

The accident happened about 1.30am on a mid-winter July night, and a large crane had to be called in to remove the wreckage.

Happily, neither the driver nor anyone else was hurt.

The Waihohonu Stream crosses SH1 about 40 kilometres north of Waiouru, and the accident forced traffic to be re-routed round the western side of Mount Ruapehu for 36 hours at a time while the bridge’s new decking was poured in two sections.

It wasn’t until April last year that repairs were completed and the bridge was open to two-way traffic again.

The bridge was not a long one but it sat at the bottom of a dip in the road, and at the time of the accident black ice was reported in the Three Sisters area that the Waihohonu flows through, and the approaches were covered by snow in some places.

The toppled trailer-unit demolishing the safety barrier along the eastern side left the entire bridge structurally unsound.

The barrier posts were smashed, severely damaging the pre-cast concrete deck panels.

The net effect was to render the western side of the bridge dangerously unusable.

Beca NZ, as the NZ Transport Agency’s structures management consultant, advised on the temporary management of the damaged bridge, and designed a repair strategy which involved the full removal of the existing concrete deck slab panels, and their replacement with a cast-in-situ composite concrete deck.

The resultant $1.3 million tender was won by Fulton Hogan, operating through its New Plymouth office, with Julian Bowles as project manager.

The bridge deck sat about six metres above the stream, was about 30 metres long with a carriageway eight metres wide.

“Our job involved closing one lane at a time and removing the existing pre-cast decking and beams – in effect removing half the carriageway,” Julian tells Contractor.

“The barrier was demolished in the impact and the pre-cast concrete deck panels were severely damaged by the truck.

“All that concrete had to be demolished and got rid of before the actual repairs could start,” Julian  says.

The damaged parts were removed from the top of the structure by a small crane working on one side of the bridge at a time.

There were 23 of these damaged sections, each 1.2 metres wide by 150-180mm thick, and once they’d been removed it left only the western side of the bridge with anything between the stream and the sky.

The next step was to build scaffolding under the entire structure to support the eventual re-pouring of the decking after the installation of replacement beams.

“This,” says Julian of the scaffolding, “was a complex, major job” – a challenge to the designers no less than the builders.

And the project was unexpectedly set back weeks when quality assurance checks just before the first concrete pour revealed the scaffolding had problems.

Once these had been sorted out, new formwork was put up between the main bridge beams and also outside of them to form a cantilever, allowing the placement of new reinforcing across the whole deck.

Taranaki Reinforcing of New Plymouth supplied and fixed the reinforcing steel.

The new deck was then able to be poured one side at a time, requiring 36-hour closures of the highway to let the concrete mature enough to withstand the vibrations of the traffic crossing it.

During both closures traffic had to be re-routed up SH49, SH4, SH47 and SH46, adding at least 40 minutes to the time usually taken to traverse the Desert Road.

That diversion was not suitable for vehicles over 44 tonnes, apart from the 50MAX units, so high productivity motor vehicles had to use SH3A and SH3 between Bulls and Hamilton.

The construction work along one side of the bridge at a time meant that vehicles with loads over three metres wide could not get across, and had to use the detours.

With the deck poured, the carriageway was sealed one side at a time, and the bridge was ready to be re-opened again.

On such a key highway, traffic management was a constant challenge: a 30 kilometres per hour speed limit had to be imposed and the approaches to the bridge controlled by traffic lights which had to be monitored constantly to ensure there was no overly long build-up of motor vehicles at either end.

It didn’t help either that, during the three and a half months autumn construction period, which ended in April last year, the highway showed its winter teeth with some unseasonal snow, as it had in setting the scene for the accident in the first place.

There were, however, upsides to the project: the bridge was able to be kept open to traffic except for the couple of short closures while the concrete cured; the new deck is stronger and provides more resilient edge barriers.

Also, there was hardly any excavation work required, other than that needed to reform and seal 20 metres of the approaches to the bridge at either end, and the site needed no restorative landscaping.

Even before the accident that temporarily closed the bridge, the NZ TransportAgency was working its way through a programme of upgrading other parts of the Desert Road to the north and the south.

These works had further disrupted traffic between Waiouru and Rangipo over the summer of 2017-2018, so the completion of all the projects at about the same time last year was a relief to travellers.

However the crash and its aftermath served to highlight the vulnerability of both the road and the traffic on it to the vagaries of the weather at the highest part of the main route between Wellington and Auckland.

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