Project

Opening up a dangerous bottleneck

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“The new culvert will be located at the base of an existing steep sided gully, with Stringfellows and subcontractors complying with the strict environmental controls for working in and around the stream.”

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Constructing a $6.8 million culvert for the Whakaruatapu Stream near Dannevirke is one of the most important Transport Agency projects in the Tararua District for years. BY NEIL RITCHIE.

THE WHAKARUATAPU STREAM culvert for the Whakaruatapu Stream will bypass the old, almost dilapidated, Whakaruatapu Bridge known locally as “the skinny bridge”, as a welcome relief for traffic.

The supply and installation of the first Super-Cor Arch Culvert in New Zealand is a key part of this technically challenging project, which main contractor Stringfellows Civil Engineering Contractors started in late 2014. It is due to be completed by late 2016.

The new culvert will be located at the base of an existing steep sided gully, with Stringfellows and subcontractors complying with the strict environmental controls for working in and around the stream. A number of erosion and sediment control techniques have already been implemented to restrict run-off and prevent sediment build up in the stream.

IMG_2122Once preliminary earthworks have been completed, approximately 20 metres of fill will be placed on top and the existing meandering stream diverted in three places to enable the construction of the culvert foundations. This work will also include the construction of a flood wall between the diverted stream and the new concrete foundation for the
arch culvert.

The arch culvert will be six metres wide, 116 metres long and will have two 15 degree bends in it. It will be constructed from galvanised steel corrugated plates –in parts, two plates thick to provide extra strength. The foundation of the culvert will be 9.3 metres wide and 800mm thick and contain over 1000 cubic metres of reinforced concrete.

The majority of the steel components are being manufactured in South Korea and will be shipped to New Zealand early this year.

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“The design will widen the road and direct the stream through a culvert underneath, with the effect being that motorists will hardly know they are travelling across a stream”

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Once the concrete work is complete a series of weirs and groynes constructed from river stones will be cast into the invert of the culvert to duplicate a natural stream environment. The project is also located adjacent to a local cemetery, with NZTA and Stringfellows working closely with local iwi and having blessing ceremonies undertaken before site work started.

The narrow, aging Whakaruatapu Bridge and its approaches are key parts of State Highway 2 for most motorists travelling between Napier and Palmerston North, or between Napier and Wellington, as they nearly always use this particular piece of the state highway.

The bridge, built about 1927, is a level viaduct type structure with a reinforced concrete deck. However, the deck is only 5.79 metres wide between kerbs. It has plate girders supported on steel trestle piers, each resting on concrete pedestals, and mass concrete abutments. But all pedestals are now showing signs of extensive cracking, with some extending into the concrete foundations.

There is also concern about the bridge foundations relating to any movements during moderate to severe earthquakes.

So the sub-standard condition of the bridge itself and both bridge approaches pose significant safety concerns for motorists, particularly heavy vehicles.

This part of SH2 has a history of high crash statistics, including some fatalities. Some drivers slow down to give wider vehicles the right of way across the bridge knowing there is not enough width for both vehicles to pass each other. There is also particular concern regarding heavy vehicles approaching the bridge from opposite directions at the same time.

So the NZTA decided to replace the bridge with a culvert and secured funding for the Whakaruatapu Bridge and Approaches Reconstruction Project during the 2014-15 financial year.

NZTA’s highway and network operations project team manager, John Jones, says the bridge will be demolished on completion of the culvert and new road alignment.

SAM_1520The project involves over 150,000 cubic metres of earthworks and 22,000 square metres of new pavement. The majority of work over the past summer involved constructing the new culvert, which did not affect the old traffic lanes. The most noticeable changes to road users will occur during the 2015-16 summer as traffic is shifted onto the new alignment.

Jones says the initial stages of this project have gone very well.

“The design will widen the road and direct the stream through a culvert underneath, with the effect being that motorists will hardly know they are travelling across a stream.

“It’s more than just replacing a bridge. It’s a 1700 metre project that will also improve the safety of the approaches.”

The primary objectives for the project are: to remove the existing seismically deficient Whakaruatapu Stream Bridge and install a suitable culvert solution; construct a new realignment of SH2 surrounding the Whakaruatapu Stream Bridge to incorporate a 100 kilometre per hour speed environment; improve safety and route efficiency for all road corridor users in the area; and widen this section of SH2 to the Standard Regional Link width.

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