Ruakanakana Overpass

摘要

The final project of the Kaikoura coast reconstruction, following the 14 November 2016 earthquake's extensive damage to the Main North Line (MNL), was the construction of the Ruakanakana Overpass. This overpass bypasses Tunnel 21 (Tar Barrel Tunnel) on the MNL, and has improved the alignment and resilience of key rail and road links. Based on the relationships built up between KiwiRail and Waka Kotahi/New Zealand Transport Agency during the Kaikoura Coast reconstruction, this has been a collaborative project led by KiwRrail. Tunnel 21 was built through hard mudstone. This rock has not been as troublesome here as in other locations, though overlying ground based on it can suffer from the same instability problems when water is present. For example, a short distance further north, in the Mirza Valley, the Kaikoura earthquake set off a landslide that disrupted the line. The tunnel had been a cause for concern for some time. It had the most restrictive clearances on the MNL. Curvature on the climb from the south limited train speeds to 55kph and there was a permanent speed restriction through the tunnel of 15kph for some time prior to the 2016 earthquake. Cracking of the concrete block soffit (roof) lining had been noticed in the 1970s and was being monitored.These cracks were increased by the Seddon earthquakes of 2013 and later, and again by the Kaikoura earthquake of 2016. Initial repairs consisted of applying mesh to the soffit of the arch to contain any fragments of the lining that might fall out. Track work done in early 2019 allowed the speed restriction to be increased to 25kph. The tunnel was not unsafe, but it was an ongoing maintenance concern and a significant restriction on train operations.