Replacing old bridges

The old narrow concrete bridges over the Manawatu River and surrounding floodplain just south of Foxton on SH1 are both being replaced at a cost of $75 million. Richard Silcock makes a site visit.

The Manawatu River rises in the nearby Ruahine Range and flows 180-kilometres westward through the Manawatu Gorge and out across the Manawatu plain to the Tasman Sea.

It is fast flowing and prior to remedial work with the construction of high stopbanks and a floodway passage (the Moutoa floodway), it regularly flooded the low-lying surrounding farmland and the tidal estuary near Foxton.¹

The existing short concrete bridge over the river itself was constructed in 1942 and the one-kilometre long, narrow, concrete bridge over the floodplain was built in 1943.²

Replacement was a priority, as both structures show structural deterioration and are very near the end of their economic life. The narrowness is also regarded as a safety issue given the increased amount of traffic now using the highway.

“The primary purpose for replacing these bridges is to improve safety and efficiency,” says Ross I’Anson, NZTA’s regional transport systems manager for the Manawatu area.

“These new bridges will be stronger, wider and require far less maintenance.

“Also, there is a restriction on heavy load vehicles as neither of the existing bridges is able to accommodate high productivity motor vehicles (HPMV) or some oversize loads and these operators currently have to make a detour via SH57 to avoid crossing them.”

The contract to design and construct the new bridges was awarded to Fletcher’s Civil Division, Brian Perry Civil (BPC) in December 2016 with Novare Design as design consultants, Arup as geotech engineers and BBO as the client’s principal advisor.

Enabling works and construction commenced in May 2017 along with the repositioning of the main gas pipe and other utilities which were attached to the old bridges.³

A temporary ‘staging bridge’ across the river was also constructed to provide a separate river crossing for heavy project vehicles so that traffic using SH1 was not impacted.

When Contractor visited the site in late January, construction was well advanced, with the piling complete, all the piers in position and work on the super structure ahead of schedule. 

Some 800-metres in total length, 12.76-metres wide and eight-metres above the riverbed, the two bridges and the embankment between are considerably higher than the existing structures.

“For the 17-span bridge over the floodplain there are 18 two-column reinforced concrete piers in total siting on 1.2 to 1.5 diameter bored piles sunk to a depth of between 20-30 metres into compacted sand,” says Steve Beddow, BPC’s project manager for the construction.

“For the six span river bridge there are 25 piles, 2.1-metres in diameter and seven single column reinforced concrete piers. All of the piers are topped by cast in situ concrete crossheads.”

Steve says that to ensure absolute accuracy the crossheads were levelled by jacks mounted on brackets attached to the pier columns.

“Where we were working above the river, the pile casings were held vertically in place by a guide-frame braced to the staging while we installed the reinforcing steel cages and pumped in the concrete, with some 240-tonnes used.”

A 250-tonne crane was used to lift and place the super-tee beams which are pre-cast concrete, 37-metres long and weighing some 65 tonnes each. The decking will be 200mm in situ reinforced concrete.

By comparison to the old bridges, the new bridges have fewer piers.

“This allows for a better unhindered flow of the river and being further apart the piers are less likely to snag old trees and other debris washed down in a flood situation,” says Steve.

All of the pre-cast was manufactured at Fletcher’s pre-cast plant near Otaihanga, which also supplied components for the construction of the Kapiti Expressway.

The surrounding low-lying floodplain is of sandy/peat composition, crisscrossed by wetlands and the remnants of dykes from which, during the course of excavation work, hundreds of eels were caught and returned to the river.

Some 350,000 cubic-metres of material has been excavated from a nearby site south of the river by sub-contractors, Goodman Contracting for use in constructing the embankment, abutments and for pre-loading compaction purposes.

Stan Goodman, of Goodman Contractors says they had four dozers and a number of track excavators carrying out this work, with a fleet of 10 Moxys transporting the sand to site.  

Due to the nature of the ground, ground strengthening was required in areas under the abutments. This was done using a continuous flight auger, with some 1000 holes drilled over a period of six months and each filled with concrete. Structural geotextiles were used beneath the embankment.

“Using this method allowed us to pump concrete into the drilled hole while still drilling,” says Steve.

“When the required depth is reached the auger is withdrawn, leaving the concrete to solidify. With the holes all close together we have in-effect created a continuous underground concrete wall.”

Each bridge will provide for two lanes of traffic (one lane each way), wide shoulders to allow for cyclists and a separating wire rope central barrier.

NZTA’s principal project manager, Glen Prince says planning and construction was preceded by significant consultation with the Horowhenua District Council, Regional Council and other community stakeholders.

“As much of the land is historically significant to Maori, with a number of culturally sensitive sites, we worked closely with the local iwi to identify known sites and plan accordingly,” adds Glen.

“During excavation work we also have Maori observers (kaitiaki) on site in case unknown sites or middens are discovered.

“In further recognition of the association Maori have with the region, we have cast a patiki (fish) design on the bridge’s concrete outer side barriers, signifying the connection to the river as an important food resource.”

A concrete underpass has been constructed to allow cattle to pass between grazing pasture on either side of the central embankment and several farm access roads have also been upgraded.

Construction cost is around $60 million, with feasibility, planning, consenting, design and property requisition costs adding another $15 million. No private dwellings have been directly affected by the construction.

Paving of the highway tie-ins at either end of the bridges will be carried out by Higgins to NZTA highway specification and the bridges are expected to be largely completed ahead of schedule and ready for traffic by October this year, despite three floods delaying work during the early stages of construction.

The old bridge structures will be demolished following the opening of the new bridges.⁴ Local reaction to the new bridges has been applauded by both local authorities and the public, with the Horowhenua District Council mayor, Michael Feyen saying in a local paper that the council welcomed the bridge replacements as it would be far safer.

Jonathan Bhana-Thomson, chief executive of the NZ Heavy Haulage Association says their industry also looks forward to the bridges being completed.

“We had discussions with the project design team and have been assured that overweight loads will be able to travel within a traffic lane over the bridges and not impede traffic flow,” says Jonathan.

“Over-dimension loads will still require management of traffic when crossing, but we welcome the improvement these new bridges will provide as it allows us to use this section of SH1 and negate any need to travel the longer and restricted SH57 route.”

Should traffic capacity increase significantly in the future, provision has been made for the construction of a replica second bridge on the site of the old structures, with the two split between north and south-bound traffic.

Footnotes

¹ The town of Foxton, some five-kilometres to the north lies on the banks of a shallow largely stagnant ‘loop’ waterway which was the original course of the Manawatu River. In the late 1800s through until the 1950s Foxton was a thriving river port for coastal scows loading flax from the nearby flax plantations and mills, however following the 1942 Masterton earthquake, severe flooding and the construction of the ‘Whirokino Cut’ by the Works Dept, when 370,000 cubic-metres of soil was removed to create a floodway, the river altered course to its present path, cutting the township off from the river. (Ref. Foxton Historical Society)

² During the 1800s the river was crossed by ferry. The original wooden bridge over the river, which had a removable middle section to allow scows to pass through, was constructed in 1900. It was replaced in 1942 by the existing concrete structure, following the central section collapsing as a result of flooding undermining the piers. The original wooden ‘Whirokino trestle’ over the floodplain was opened in April 1939 and replaced by the existing concrete structure in 1943 at a cost of £50,000. It has 90, 12.2-metre spans set on concrete piles. In 1961 the Moutoa flood control gates were constructed upstream to channel water down the 10-kilometre, 600-metre wide Moutoa floodway.

³ The gas pipe has now been buried underground and was accomplished using a thrust boring machine.

⁴ Consideration was given to using the old bridges as a separate cycleway, however it was decided to incorporate a wider shoulder on the new bridges to provide for cyclists and upgrade the existing cycleway as a part of this project.