Rainfall-induced failures of natural slopes in tropical regions

[Truncated] Rainfall-induced slope failures are one of the most damaging natural hazards in the world, with slope failures occurring every rainy season throughout the world. The occurrences often cause tremendous losses and show an increasing frequency in the last decade. These slope failures are likely to occur on natural slopes, which are heterogeneous due to the processes of natural soil formation, unlike constructed slopes that are designed and mechanically placed and compacted to withstand specified loads. Simplifications are commonly applied in slope stability analyses, such as the use of deterministic analysis methods, to make analyses of stability problems tractable. Such simplifications ignore the real nature of many controlling factors, such as rainfall intensity fluctuation and patterns as well as spatial variability of soil properties, potentially leading to inaccurate results. In this thesis, numerical modelling was used for coupled seepage and slope stability analyses to consider more realistic representations of the controlling factors in evaluating rainfall-induced slope failures. The majority of occurrences are shallow slope failures, so only shallow failure mechanisms were considered throughout this thesis.

The Jabiru landslide, which occurred during extreme rainfall in February and March 2007 in the region of Arnhem Land, Northern Territory of Australia, was selected as a case study for this thesis. High resolution (hourly) rainfall data was applied in slope stability analyses to take account of fluctuating intensities of rainfall. In-situ hydraulic conductivity obtained from field tests using a tension infiltrometer/disc permeameter was also considered in these analyses. Parametric studies were carried out to investigate the effect of variation of the controlling factors, both rainfall events and soil properties, on rainfall-induced slope failures. Results indicated that the Jabiru landslide could have been predicted to occur due to the extreme rainfall. This study also highlighted the role of various aspects of the controlling factors, including rainfall intensity, duration, resolution and pattern as well as hydraulic and shear strength properties of soils, in the slope failures.