Proof in the pudding - Multi-plate™ Arch - a good option for seismic zones

The 7.8 magnitude earthquake that struck Kaikoura in November 2016 - one of the most complex quakes in New Zealand’s recorded history due to more than 20 fault ruptures - resulted in 18 months of frantic effort to repair the widespread damage to property, including key infrastructure such as the Waima Rail Overpass in southern Marlborough. With the overpass - which features a Steel Multi-plate™ Arch structure supplied by CSP around 35 years ago - deemed unsafe at the time to remain open to rail, CSP were asked to assist with repairs.

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 in southern Marlborough. With the huge forces exerted by the earthquake the Multi-plate™ was pushed out of shape affecting the rail-loading envelope - a contributing factor to the closure of the main line from Picton to Kaikoura.

The tunnel is a long corrugated steel arch first built on concrete footings in about 1980. The earthquake resulted in the deformation of this structure, where approximately half its length was distorted – with the arch shifting laterally but staying on the footings.

“In simple terms, during the earthquake the earth on either side of the structure rolled it backward and forward affecting the shape of the Multi-plate™, 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 was uneven at certain points. Where the backfill was 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 Multi-plate™. Once about 4000 m³ of soil was removed from the top it allowed the Multi-plate™ to almost return to its original shape. This allowed repairs to be made and meant the main rail line could reopen from Picton to Kaikoura.