NZ Geotechnical Society (NZGS) 22nd Symposium 2025 Auckland
The Tar Barrel Tunnel Bypass project was undertaken to redirect the railway around Tunnel 21, located on the Main North Line (MNL) connecting Christchurch and Picton. This bypass was situated between Ward and the Waima River, roughly 45 minutes south of Blenheim. The existing tunnel is 167 m long and takes the railway under State Highway 1 (SH1).
The condition of Tunnel 21 had declined over the years, especially after the seismic events from the Seddon and Kaikōura earthquakes. In an effort to improve the long-term resilience of the MNL while enhancing the line speed, reducing travel times, and improving operational performance, a decision was made to bypass the tunnel and realign the railway beneath a different section of SH1.
This undertaking involved a rail realignment stretching 1.5 km and a road realignment covering 0.7 km, necessitating the excavation of roughly 400,000 m3 of mudstone to adjacent fill locations, and creating cut slopes of up to 36 m high. Furthermore, a new 100 m rail underpass structure with supporting retaining walls was constructed to enable passage beneath SH1.
The bypass has encountered various challenges, including seismic hazards, slope instability concerns, the presence of compressible soils, and potential settlement issues surrounding the rail underpass and SH1 embankments. Additionally, ongoing road and rail operations presented logistical constraints.
