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The highways circumnavigating Viti Levu play a pivotal role in Fiji's socioeconomic development, and are a crucial link to transport aid during disasters. Slope failures triggered by heavy rainfall and tropical cyclones have led to considerable damage and fatalities along Viti Levu's main roads in the past. Knowledge of slope instability mechanisms is key for the successful identification and management of slope failure hazards, however, there is a paucity of such information for Fiji's main roads. Severe Tropical Cyclone (STC) Winston was the strongest cyclone on record to make landfall in Fiji and affected the Fiji Islands from the early hours of 20 February until 3 am, 21 February, 2016. We present a field reconnaissance examining the characteristics and mechanisms of slope failures induced by STC Winston along a 35 km stretch of the northern part of the Viti Levu's ring road, “Kings Road”. Approximately 61 distinct shallow, small‐scale slope failures were identified most of which were of a complex type and included earth and debris slides (planar and rotational) with a minor flow component. The trigger for the slope failures was the ~258 mm of rainfall in 24 hours after 30‐days of antecedent rainfall totalling 482 mm. This caused rapid increases in porewater pressures, especially at the contact of residual soils and the underlying weathered basement rock. Evidence of recurring slips along Kings Road indicates that contemporary slope instability is likely caused by elevated porewater pressure during high intensity rainfall events in clay‐dominated soils.

Fiji's infrastructure is regularly affected by rainfall-induced slope failures, but the engineering properties of failed soils are rarely described. We report mineralogical, geotechnical and index properties of soils from headscarp exposures of 18 slope failures from tropical residual soils of differing parent rocks in Viti Levu, Fiji. Scanning electron microscopy and X-ray diffraction revealed that kaolinite and smectite are the dominant clay minerals in the soils. Index properties included in situ moisture content (28-114%), dry bulk density (0.7-1.5 g cm −3 ), Atterberg limits (25-56% plastic limit; 38-79% liquid limit), effective particle size (0.4-12.6 μm) and clay fraction (0.6-19%). Geotechnical measurements included field compressive strength (127-461 kPa), hydraulic conductivity (c. 10 −7 m s −1 ), shear vane (16-128 kPa), ring shear (9.3-17.4°) and Emerson dispersion. Collectively, results indicated that most of the soils were cohesive, stiff, sensitive and in a plastic state in the field. Soils plotted below the A-line on the plasticity chart as fine silts of intermediate to high plasticity, and can theoretically sustain >50°slopes. Failure of these soils following high rainfall events is influenced by low permeability and the presence of expanding clays (e.g. smectite), causing temporary porewater pressure increases. No explicit relationships between soil properties and parent lithology were evident.

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.

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