Engineering geological investigation of slope failures along roads in Viti Levu, Fiji

Abstract

Landslides are a common geological hazard causing immense damage to life and structures around the world. In the nation of Fiji, especially on the main island of Viti Levu, landslides repeatedly damage key pieces of infrastructure such as arterial roads. This study addresses the issue of understanding the properties and mechanism of landslides in tropical residual soils triggered mostly by tropical cyclones and long-duration rainstorms. The study firstly began by understanding landslides along Kings Road in Viti Levu, Fiji in the aftermath of a Tropical Cyclone. This road of national importance is repeatedly affected by landslides during major rainfall events. The second phase of the study investigated the engineering properties of tropical residual soils of varying parent lithologies prone to landslides along roads in Viti Levu. The landslides were found to be numerous, but small, shallow and of complex types whereby rotational and translational earth and debris landslides transformed into flows. Residual soils most prone to slope failure along Kings Road, Lololo Road and Namosi Roads in Viti Levu are cohesive, plastic, sensitive, have low permeability and low residual strength. Failures are mostly driven by the formation of a perched water table during heavy rainfall, leading to excess porewater pressures to trigger the landslide. The third phase of study examined a specific landslide (the Kasavu Landslide). Computer modelling of this showed how the Kasavu Landslide materials were highly sensitive to increasing groundwater levels, with failure mechanisms varying with location in different parts of the landslide. No relationship was apparent between the engineering properties of failure-prone residual soils and underlying lithologies, but this requires further study. Tropical cyclones and floods affect Fiji annually, hence a proactive approach is needed to managing landslides hazards along Fiji’s trunk routes by relevant authorities. Several recommendations with regards to slope monitoring and stabilization have been made in this thesis which relevant authorities can consider. Monitoring slopes via Unmanned Aerial Vehicles, groundwater and precipitation are considered most useful for Fiji’s environment while relevant stabilization techniques not yet tried include use of soil nails and counterfeit drains.