Tar Barrel Tunnel, Marlborough

If you’ve ever travelled on the Coastal Pacific – the long-distance passenger train that runs between Picton and Christchurch in New Zealand’s South Island – you’ll know that it’s well deserving of its place as one of the most picturesque train journeys in the world. You might also have noticed a drop in speed, while going through a tunnel just south of the small town of Ward, and not known there was a 100-year-old story behind it. With the help of Contech, we can now reveal some of the history, as well as offering a glimpse into a fascinating project.

Project history

The tunnel was called KiwiRail Tunnel 21, and for unknown reasons sometimes also the Tar Barrel. It was built in 1913 during the 1870s-1945 ‘Golden Age’ of railways and extended for 167m through mudstone rock.

By the 1970s, Tunnel 21 was the oldest tunnel on the Main North Line and its concrete lining was starting to crack – a situation made worse in 2016 with the 7.8-magnitude Kaikoura earthquake. While repairs kept the tunnel operating safely, its long-term future was looking bleak – especially as its small size meant trains had to reduce speed to 25km per hour to travel through it. As they say, time is money and this flaw was affecting journey times on – and the efficiency of – a key route for freight movements between Auckland and Christchurch.

Seismically strong solution

In 2018, KiwiRail, the owner and operator of New Zealand’s rail network, decided it was time to decommission Tunnel 21 to improve rail operations and strengthen the railway’s seismic performance. In a NZ$35m project, the railway was to be re-routed away from Tunnel 21 and a new 100m-long road overpass – to carry traffic on State Highway 1 – would be built across the tracks. Trains would be able to travel under the overpass at up to 80km per hour and the risks posed by the aging tunnel to the road and rail networks would be eliminated.

PT tendon installation

So, where is Contech in all this? HEB Construction sub-contracted Contech to execute the post-tensioning work. The latter involves the installation and stressing of 15 BBR VT CONA CMI 406 internal multi-strand tendons – each 34m long – which stretch through the overpass’s 82 double-hollow-core precast beams in three stages. Project completion is due in mid-2021.

Interestingly, the 500,000m3 of excavated soil has not been taken off site – it will be used as part of a landscaping program that includes two hectares of native plants. This project offers a great result for all stakeholders!