Mitigating landslide risk to the puketeraki embankment on New Zealand’s Main South Line

The Main South Line (MSL) is the only railway line that links Dunedin and Southland to the rest of New Zealand’s railway network. The Puketeraki Embankment is between 337.9 km and 338.5 km along the MSL and traverses the Puketeraki Landslide Complex. The Puketeraki Landslide Complex is made up of a series of deep-seated movements and shallower earthflows. Based on the current monitoring data and considering the effects of climate change, the predicted movement of the landslide over the next 50 years is in the order of 7 m horizontally and 2 m vertically along the landslide complex beneath the embankment.

Several historic and recent ground investigations as well as detailed geological mapping throughout the site indicate the embankment is underlain by earthflow materials followed by Otakou and Onekakara Group Sediments. The east-west trending, steeply north-dipping, active Puketeraki Fault is inferred to cross the southern section of the existing rail embankment. Survey monitoring data suggests the average displacement rate of the embankment is 123 mm per year since 1992. Recent inclinometer readings suggest the majority of displacement occurs at approximately 21 m depth, corresponding to the transition from completely weathered to slightly weathered Burnside Mudstone.

Design of measures to arrest the ongoing movement of the landslide is impractical due to its scale, and therefore a practical approach allowing for widening of the existing rail corridor to accommodate the predicted movements coupled with improved site drainage has been adopted. This paper presents a technical overview of the instability mechanisms at the site and conceptual mitigation measures to accommodate the forecasted displacement.