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Steel Multiplate - out of shape, but not out of action
 
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Principal: NZ Transport Agency
Customer: NCTIR (North Canterbury Transport and Infrastructure Recovery)
Consultant: Opus Consultants
Location: Waima Overpass, Marlborough
Product: Steel Multiplate Arch
Date: August 2017
 
 
 

The 7.8 magnitude earthquake that struck Kaikoura in November 2016 - one of the most complicated quakes in New Zealand’s recorded history thanks to more than 20 fault ruptures - resulted in widespread damage to property, including key infrastructure such as the Waima Rail Overpass in Southern Marlborough. With the railoverpass - which features a Steel Multiplate Arch structure supplied by CSP Pacific around 35 years ago - deemed unsafe, NCTIR asked the team at CSP Pacific to assist with repairs to get it back up to speed.

The 115m long arch allows State Highway 1 to cross the main Picton-Christchurch rail line and is about 1km south of the Waima River (also known as Ure River) in Marlborough. With the huge forces exerted by the earthquake the Multiplate was ‘pushed’ out of shape making it encroach the rail-loading envelope.

“In simple terms, during the earthquake the earth on either side of the structure rolled it backward and forward affecting the shape of the Multiplate, but not to the point it couldn't be repaired,” explains Frank Westergard, Senior Civil Structures Engineer for Opus International Consultants. “The angle of the rail line in relation to the road is only 22 degrees, which means backfill around the structure is uneven at certain points on the structure.  Where the backfill is more even there has been little or no change to its shape, despite the shaking.” 

To undertake repairs to the structure the NCTIR team removed the backfill covering the Multiplate. “Once we removed about 4000 m³ of soil from the top it allowed the Multiplate to almost return to its original shape,” says Kevin Lambe, Project Manager from NCTIR. “This has allowed repairs to be made and for trains to operate safely through the overpass and meant the main rail line could reopened from Picton to Kaikoura.”

“We worked with the design team at CSP Pacific to work out how we could successfully strengthen the bolted seams that had been damaged and it was decided that patching plates would be fitted to the existing structure to help return it to its original shape,” adds Frank.

"We were asked by Opus to help with the designs of the plates,” says Mo Yang, Product Support Engineer for CSP Pacific. “Multiplate structures are an effective bridging alternative due to their strength and versatility. As the Multiplate structure is flexible it can withstand huge seismic loads during an earthquake. More rigid structures, such as a concrete culvert or bridge, would probably not have been repairable.”

Repairs to the structure have been hampered by the now open rail line with Kevin and the team having to work around the train timetable. “We should have all the plates in place in the next month which will allow us to evenly backfill the culvert and reinstate the road which is currently diverted onto the old SH1 via a temporary level crossing.”

“The Multiplate held up very well considering the loads which land movement inflicted on it during the earthquake,” explains Frank. “The 35-year old galvanized Multiplate is in very good condition and once the repairs are complete it should easily meet or exceed the original design life.”

 

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