KiwiRail North Auckland Line recovery project: a case study on design and construction at NAL 147.145km

The KiwiRail North Auckland Line (NAL) recovery project was initiated in response to land damage across Auckland and Northland caused by two large storm events during January and February 2023. These events resulted in damage across the NAL line including scour, erosion and landslides. Tonkin & Taylor (T+T) was engaged by KiwiRail to undertake the investigation, design and construction monitoring at seven sites along the NAL. This paper focuses on the damage encountered at chainage NAL 147.145 km, near Maungaturoto, Northland, New Zealand. The damage manifested as a 60 m long rotational landslide along the rail embankment positioned above a 15 m deep fill embankment transecting a broad valley feature. T+T produced a geotechnical risk assessment heat map to highlight over steepened embankment slopes and identify areas at risk of potential future instability beyond the visible damage, which aided discussions with KiwiRail to determine an appropriate design solution and extent. The design solution adopted by KiwiRail was a 10 m deep, 110 m long partial rail embankment replacement. Challenges were encountered when unexpected fill materials were found within the embankment during excavation. The final design remediated the landslide damage, ensured safe temporary working conditions during construction and re-used all site-won material within design, which provided the client cost and sustainability benefits.