Replacing a rail bridge’s piers without stopping the trains has set a unique challenge for KiwiRail engineers and contractors in the South Island.

Bridge 72 on the Main South Line crosses the Opihi River around 2 km south of Temuka. The 512 m bridge was built in 1932 to replace the original, constructed in the late 1800s. Since then, the Opihi River has lost bed material, and the section under the bridge is now up to 3 m deeper in the active channel than when the bridge was built.

“As the depth of the river has increased, the 7 m long concrete piles which the bridge’s piers are founded on have been scoured out,” says KiwiRail project manager Robert Talarkiewicz. “Storm events and flooding in 2000 and 2013 further exacerbated this problem, and the combination of the scouring and flood damage has led to a loss of support for the bridge spans and ultimately the track.” 

 Following the storms of 2000 and 2013, KiwiRail had to close the line while emergency repairs were carried out on some of the piers. “KiwiRail identified the bridge as needing work to minimise the risk of any future disruption to its network,” says Mr Talarkiewicz.

The bridge structure itself is safe and in good condition. It is only the 15 piers located in the main waterway, out of the total of 41 for the whole bridge, that need replacing.

MEETING THE CHALLENGE

The new concrete piles will be bored approximately 19 m below the river bed level, and the existing piers will be demolished and replaced. Two piles per pier will be constructed by sinking steel casings to the designed depth, excavating the casings, installing the reinforcement cages and pouring the concrete.

“This provides the simplest, most efficient, and most robust long-term solution with least disruption to rail operations during construction,” says Mr Talarkiewicz. “However, it does present us with a unique challenge. The bridge is important to KiwiRail’s network, and with the project expected to take around 10 months, we needed to find a way to replace these piers while still allowing our trains to continue travelling across the bridge. To meet the challenge, the bridge will effectively be ‘jacked up’ onto temporary supports while the old piers are demolished and new pier caps constructed.”

The temporary supports have been designed by Fulton Hogan with design support from Holmes Consulting. They’re strong enough to carry the dead load of the bridge, as well as the loads of trains passing overhead, and will be welded to the new pile casings.

“Once we’ve assembled the temporary works, the load will be transferred from the old pier onto the temporary support by raising the adjustor struts,” says Mr Talarkiewicz. “We’ll then grout the strut adjustors, keeping a close eye on the temporary supports and the track to make sure there’s no movement that exceeds the allowable limits.”

ACHIEVING A QUALITY RESULT

The old piers will be demolished by cutting off the top using a wire saw, and then removing the concrete blocks individually using an excavator. This provides enough space to build the new pier cap.

“The new pier caps will be cast in-situ in reinforced concrete,” explains Mr Talarkiewicz. “One key issue is to limit the vibrations caused by passing trains, which are transferred from the bridge through the temporary support into the formwork for the new pier caps, while the concrete is fresh. Coordinating the concreting of the pier caps with the timing of the trains operating over the bridge is key to achieving a quality result.”

 When the new pier caps have reached their design strength, the load will be transferred from the temporary supports onto the new caps, the temporary supports removed and the remains of the old piers demolished.

ENVIRONMENTAL CONSTRAINTS

As well as the challenges of coordinating work and train schedules, Mr Talarkiewicz says there are environmental constraints around working in the river, including water quality and protecting fish and bird life.

“We’re ensuring we comply with resource consent conditions to keep the impact on the environment to the absolute minimum. We’re monitoring water quality, treating wastewater, and have organised independent surveys of bird life,” he notes.

‘We’ve used river gravels to build temporary gravel platforms to access the piers, and we’re doing it in a way that means there’s always enough water flow to allow fish to travel freely. Regular environmental audits are being carried out, and we’ll work closely with Environment Canterbury and Te Runanga o Arowhenua throughout the project.”

Work started in January this year and is scheduled to be completed in October, depending on river levels and any high-water events. “So far we’ve completed 24 of the 30 piles and the first pier cap has been concreted. We’re pleased with the overall progress so far,” Mr Talarkiewicz concludes.

Source: New Zealand Construction News