• PROJECTS
& DEVELOPMENTS
TUNNEL 21 (TAR BARREL TUNNEL BYPASS) – KIWIRAIL MNL RESILIENCE PROJECT
Lindy Andrews, Senior Content Advisor & Rob Hunter, Engineering Geologist & Project Manager, Tonkin+Taylor
T
unnel 21 is a KiwiRail tunnel situated on the Main North Line (MNL) between Ward and
the Ure (or Waima) River, about 45 minutes south of Blenheim. The tunnel was constructed during the ‘Golden Age’ of the railways (1870s – 1945), with the tunnelling works completed in 1913. At this time, the MNL was segmented along the east coast of the upper South Island, with this section of the line starting in Blenheim and extending towards Kaikōura. It reached Seddon in 1902, Ward in 1911 and then Wharanui (13 km south of Ward) in 1913, before the onset of World War 1 saw progress halted and 7529 railway workers released for war service. Thousands were again released during World War 2. The MNL was finally completed in 1945, the feat marking a significant point in history as New Zealand’s longest railway construction project. The line now spans from Addington in Christchurch to Picton in Marlborough, along a total length of 348 km. In its heyday, there were 65 bustling stations along the route.
A deteriorating tunnel
works had done and with the recent Seddon and Kaikōura earthquakes,
Tunnel 21 is also known as the ‘Tar
the tunnel’s condition had deteriorat-
Barrel’ and extends 167 m through a
ed. Whilst local repairs and ongoing
weak and fractured mudstone rock,
maintenance kept the tunnel
adjacent to the London Hill Fault.
operating safely, KiwiRail would like
There have been several stories as to the origin of the name ‘Tar Barrel’. The most likely explanation is attributed to a shipwreck near to the mouth of the Ure River, where barrels of recovered tar were carted up the hill and stored near to tunnel portal. For some reason, a barrel reportedly tumbled down the hill, exploded and spilled tar near to one of the
to improve the long-term resilience of the MNL, while also increasing the line speed, and therefore reducing travel time and reducing operating costs. The tunnel has now been infilled with site-won mudstone fill to eliminate the liability, and to ensure resilience to SH1 above.
tunnel portals. The tunnel was lined with in-situ concrete and concrete blocks which had seen better days. Records dating back to the 1970s showed there was cracking in the concrete lining and recommendations for repair works were proposed. Since then, only minor
Tunnel 21 is also known as the ‘Tar Barrel’ and extends 167m through a weak and fractured
mudstone rock, adjacent to the London Hill Fault. SURVEYING+SPATIAL
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Issue 108 June 2022 17
