Analysis and prediction of rainfall-induced shallow landslides along the road corridors in northern New South Wales, Australia

Abstract

Globally, landslides have occurred regularly and caused massive destruction with numerous loss of life. Although they may result from earthquakes, geological factors and human activities, landslides are mostly caused by heavy or prolonged rainfall. Furthermore, research indicates that rainfall-induced landslides are primarily responsible for landslides that occur on mountainous terrains. Some of the worst rainfall-induced landslides across the world which caused loss of life and destruction of property and infrastructure, occurred in Nepal in 1926, 1988 and 2009; in Thailand in 2008; in India in 2009 and in Japan in 2012. Thredbo landslide occurred in southern region of New South Wales (NSW), Australia in 1997, killed 18 people, destroyed two lodges and moved over 1000 tonnes of liquefied earth and debris over slope. The cause of this landslide was heavy rainfall, melting snow and leaking water pipeline. In northern NSW, many roads were closed for weeks and months due to rainfall-induced landslides from 2009 till 2019. Waterfall Way leading to Dorrigo Mountain from Raleigh was closed in 2009 (3 times), 2011, 2013, 2015, 2017 & 2019 and the road users have to travel more than three hours on alternative routes to reach the other side of the Dorrigo Mountain. Gwydir Highway at Gibraltar Range was closed in 2011 and 2013; Oxley Highway at Mt Seaview was closed in 2011 and 2013. Summerland Way at Mt Lindesay was closed in 2011 and 2017 and Bruxner Highway at Mallanganee Range was closed in 2011, 2013 and 2015. Road closures make inconvenience to the road users impacting access, work travel, school travel, medical travel and long-distance travel and there are economic consequences as well. There were over 100 landslides occurred along the state road corridors in northern region of NSW, Australia, since 2009 till 2019. Rainfall events triggered about 80% of landslides in northern NSW. Rainfall-induced landslides occur in this region in every two years according to historical records from 2009. Mountain passes such as Mt Seaview, Dorrigo Mountain, Gibraltar Range, Ramornie – Cangai Bluff, Mallanganee Range and Mt Lindesay are adversely affected by rainfall-induced landslides. Therefore, there are economic and social needs to address root causes of rainfall-induced landslides along the road corridors. Despite the common occurrences of rainfall-induced landslides in the Australian region of northern New South Wales (NSW), there has been no study conducted on this phenomenon for this region. This thesis conducts a comprehensive analysis of shallow landslide events that occurred along the road corridors in northern New South Wales (NSW) throughout the period of 2009 - 2019. Furthermore, this thesis employs the analysis to prognosticate future patterns of rainfall-induced shallow landslides for this region. The scope of this thesis includes analysis of rainfall-induced shallow landslides occurred in coarse-grained soils, prediction of shear strength of coarse-grained soils using soil suction and prediction of rainfall-induced shallow landslides using different approaches such as rainfall threshold, rainfall index and limit equilibrium method in northern NSW. It involves developing a relationship among rainfall characteristics, soil characteristics and slope characteristics. The study found out that when moisture content of soil is increased, there is a reduction in matric suction and thereby a decrease in shear strength. It also revealed that slopes consist of coarse-grained soils are vulnerable to rainfall-induced landslides. The slopes consist of sedimentary rocks and igneous rocks at mountain passes are vulnerable to landslides. The slopes are susceptible to rainfall-induced shallow landsides when the slope angle is greater than 25 degrees. The key benefit of this thesis is that government, road authorities and industry can use the simple tools that can predict rainfall-induced shallow landslides in northern NSW. These simple tools include rainfall threshold, rainfall index and the SLIP model that are specifically developed for northern NSW. The findings are also useful for management of road corridor and slopes in the mountain passes. This study presents the major factors that lead to shallow rainfall-induced landslides in northern NSW and proposes simple tools that can be used to predict this natural disaster. It is believed that these findings will be useful for the relevant industry including decision-makers who manage the slope assets and landslide hazard along the road corridors in northern NSW, Australia. These findings are also applicable to other parts of the world.