Engineering geomorphological investigation of the Kasavu landslide, Viti Levu, Fiji

Abstract

Rainfall-triggered landslides are prevalent along Fiji’s main roads, but detailed studies of factors pre-conditioning slopes to fail and slope failure mechanisms are sparse. This study examines the factors leading to the Kasavu landslide, a small, shallow rotational landslide that transitioned into an earthflow along the Kings Road in Viti Levu, Fiji. The Kasavu landslide was triggered on 17–18 December 2016 by prolonged rainfall from a tropical depression. The extensive damage from the landslide resulted in closure of a section of the Kings Road for over a year incurring significant financial costs to the Fijian government and public. Geotechnical tests revealed soils sampled from the Kasavu landslide headscarp to be very stiff, sensitive, and close to the liquid limit. Although having a low clay content, the soils are highly active due to their smectite content. Geomorphic investigation revealed the landslide is located on a > 21° slope near a ridgeline affected by piping and soil erosion processes, above a major floodplain. The primary trigger for the Kasavu landslide was daily rainfall of 176 mm followed by 3-day antecedent rainfall of 361 mm. Pre-conditioning factors included the steep slope, expansive clays, and piping erosion. Dynamic loading from heavy goods vehicles (HGVs) likely also have played a role in slope failure. Considering the current geomorphic setting and soil properties, further landslides in the area are possible. Thus, future management of this section of the Kings Road should consider drainage of the ridgeline and groundwater monitoring, together with provision of load support to existing slopes.