Project Roading

A long time in planning

The big Waioeka Gorge landslip of 2012 highlighted State Highway 2’s vulnerability and finally led to the replacement of its last one-lane bridge. HUGH DE LACY reports.

THERE ARE NO FEWER than 52 bridges within 142 kilometres of State Highway 2 between Opotiki and Gisborne, and the relatively flat and open site seemed to suggest that the replacement of the last remaining one-laner, across the Motu River 75 kilometres north-west of Gisborne and 7.5 kilometres from the settlement of Matawai, would be a bit of a doddle.

It wasn’t – the soft alluvial soil made sure of that.

The existing bridge was built in 1930 over a 12 metre-wide stream that you could normally walk through in gumboots without getting your feet wet, which is why a ford was considered adequate for the Motu decades after every other creek and river on SH2 had been bridged.

The bridge’s replacement has been on the books for over 75 years.

In the 1950s a big pile of aggregate was imported for the construction of a new two-lane bridge but the funding to complete it never came through from the government of the day.

Three years ago the NZ Transport Agency spent $100,000 on temporary strengthening to get it to meet the 50-max truck-weight requirements, before it was decided to go the whole hog and $6.5 million was made available for a new two-lane bridge in the Government’s $212 million Accelerated Regional Roading Programme of 2014.

Fulton Hogan won the tender and work started on the replacement bridge in October of last year, and it is expected to be completed by May 2018.

From the start the project was so hammered by rain that it might have been on the West Coast of the South Island rather than the east coast of the North: in the first full year the site received 2.35 metres of rain – an average of 6.5mm a day.

The key to the bridge’s construction was the abutments, which required the removal of 13,000 cubic metres of solid cut-to-waste that included large Kahikatea logs.

The cut had to go three metres below the normal river level and be filled by 8300 cubic metres of aggregate supported by sheet piles on both sides for erosion control and containment.

It was further backed up by bulk earthworks and ground improvements to prevent liquefaction of the thin soils, and built up to eight metres above river level.

As well as the abutments, a pier had to be built in the middle of the river to take piles, of which there are two on either bank and two in the middle.

A staging post had accordingly to be built in the middle of the river to meet environmental concerns, and this was done by cutting and milling poplars into 350mm by 50mm boards and laying them lengthwise in the stream to support the deck without interrupting the water flow.

 

Project manager Rick Gardner told Contractor the untreated timber met the stringent environmental controls placed on the project.

“We had consent to have construction equipment in the river, but we had to ensure that the passage of dozens of truckloads of building materials did not impact on the riverbed.”

The formwork for the pier required 40 tonnes of concrete and this weight was initially borne by hanging supports within the piles until the pier crosshead was constructed, at which point, with the weight reduced to 10 tonnes, temporary pile-clamps took the load.

The six piles, each 1200mm in diameter, had a total length of 105 metres, and they had to be socketed into the greywacke bedrock.

This work was done by Auckland company CLL, and was followed by the placement of the 25 metre long main beams which had been prefabricated in Te Puke by HEB.

The beams were transported and placed one at a time by Bay of Plenty company McLeod’s Cranes, using a 350 tonne mobile crane.

This job was expected to take just two days, but the weather delivered 108 kilometre winds to go with the rain, and it took twice as long.

Once the beams were in place, preparations could go ahead for the placement of the concrete deck, and the start of the approach roadworks.

These cover about 450 metres and have the effect of realigning the carriageway about 50 metres west of the existing bridge.

The silty soil offered challenges to the road-builders, no less than to the bridge-builders, with a retaining wall having to be installed on the southern approaches.

Right beside the old bridge a side-road, Te Wera Road, meets SH2 in a Y-intersection, and this will be reconfigured into a T-intersection by the time the project is complete.

Fulton Hogan had a commercial edge in tendering for the job in that it has a quarry just a kilometre up Matawai Road that was able to supply all the aggregates needed.

By late October this year the road approaches were nearing completion, and the company hopes to have the bridge bearing traffic by Christmas.

There will still remain the dismantling of the old bridge, and this is expected to be completed, along with the accompanying roadworks, by the end of May.

Being so far from any town, workers often had to stay out on the Motu Bridge job, living in a rented farmhouse with up to nine people at a time.

The farmhouse, which complements the array of service buildings that were installed at the start of the job as temporary office accommodation, gave Rick Gardner the opportunity to keep his boys on-site happy with a succession of monthly team dinners featuring venison, crayfish and fresh local lamb.

The slips in the Waioeka Gorge five years ago highlighted the vulnerability of SH2 to weather-driven disasters, with the mainly heavy traffic having to be diverted along SH35 around the East Cape, which adds 185 kilometres and two and a half hours to the trip.

While not shaving much time off the trip between Opotiki and Gisborne by way of the Waioeka, the new Motu Bridge will improve SH2’s resilience under the heavier legal truckloads and, as such, will reinforce the government’s regional development initiatives.

This article first appeared in Contractor November 2017.

 

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