Monitoring Slope Instability During Reinstatement of State Highway 11 at Lemon’s Hill(35˚S) Northland, New Zealand

Slope failures within weathered rock are characteristic of road cuttings in the humid subtropics, where weathering profiles can extend 10s of meters into the subsurface. Typically, the high rainfall intensities provided by the passing of tropical cyclones, can generate widespread slopes failures along road transport corridors, requiring engineering responses such as hazard assessment and road reinstatement.

Here, we report on some engineering geological aspects of the 2018 slope failure and subsequent slope monitoring response along State Highway 11 (SH11) on the southern side of Lemon’s Hill (35°S), in Northland, New Zealand. In much of New Zealand, landslides along transport corridors are often initiated by seismic activity, but in the subtropical north, >500 km west of the active plate boundary, rainfall is the key landslide trigger. The 13 February 2018 landslide at Lemon’s Hill adjacent to SH11 followed prolonged rainfall from the passing of Tropical Cyclone Fehi. SH11 is a popular tourist route to the Bay of Islands region of Northland, and the landslide occurred as an ‘overslip’, shallow (2 m deep) translational failure, within completely (CW) to highly weathered (HW) greywacke.

The response included the construction of engineered batters, resulting in six months of traffic disruptions. The engineering geological investigation of the site including characterisation of the materials, and identification of probable failure modes. Monitoring via multi-temporal Unmanned Aerial Vehicle (UAV) photogrammetry and AccuMM GPS nodes installed on the slopes was also part of the engineering response, while recently available LiDAR provides future opportunities for an enhanced engineering geological understanding of slopes instability in the area.