Rainfall-induced slope failures are commonly observed in residual soils during rainy seasons. Numerous residual soil slopes are generally unsaturated because of the commonly deep groundwater table. The unsaturated soil zone is greatly affected by the environmental changes. Therefore, rainfall-induced slope failure mechanism and its preventive measures can be better understood using unsaturated soil mechanics and unsaturated soil properties that will become the emphasis of this lecture.

Incorporating unsaturated soil properties in the development of slope susceptibility map will result in more realistic and sustainable assessments of regional stability of slopes under rainfall. The development of such a susceptibility map is made possible through regional seepage analysis and three-dimensional slope stability assessment. Several selected residual soil slopes with past failure history were analysed to evaluate the slope susceptibility map. The results were found to be comparable with the results from two-dimensional seepage and slope stability analyses. It is equally important to monitor suction values in the field to assess the stability of the slope. The role of soil suction can be significant in maintaining the stability of the slope during rainfall. The conventional water-filled tensiometer and the osmotic tensiometer have been found to be effective in measuring low to high values of suction, respectively.

Several slope improvement methods using principles of unsaturated soil mechanics such as the Capillary Barrier System (CBS), GeoBarrier System (GBS) and vegetation cover will be illustrated with some case studies.

Professor Rahardjo has conducted extensive research on unsaturated soil mechanics to solve geotechnical problems associated with tropical residual soils. His research focus has been on rainfall-induced landslides; one of the major natural disasters occurring in many parts of the world.He has applied unsaturated soil mechanics principles to better understand the mechanisms of rainfall-induced slope failures especially in tropical residual soils. These research activities have led to the development of an advanced unsaturated soil mechanics laboratory at Nanyang Technological University and numerous comprehensive instrumented slopes.Professor Rahardjo and his team have developed the Capillary Barrier System (CBS) for slope stabilization, GeoBarrier System (GBS) for a cover system and a retaining structure, Slope Management and Susceptibility Geographical Information System for Singapore. In addition, Professor Rahardjo has also applied unsaturated soil mechanics to soil improvement for tree stability, understanding the effects of rainfall on tree stability and developed instruments for tree inclinometer with the associated analytics. Professor Rahardjo is currently developing Advanced Moisture Sensing Technology for Urban Greenery and Monitoring of Slope Stability.

Professor Rahardjo is the co-author of the first textbook on unsaturated soils “Soil Mechanics for Unsaturated Soils“, by D.G. Fredlund and H. Rahardjo, published by John Wiley in 1993, the second textbook “Unsaturated Soil Mechanics in Engineering Practice”, by D.G. Fredlund, H. Rahardjo and M.D. Fredlund, published by John Wiley in 2012 and over 400 technical publications. He has also presented his research works in numerous keynote /invited lectures and short courses in various countries.

He has received several awards : Singapore International Chamber of Commerce Awards (2021) on Most Sustainable Collaboration on GBS, Korean Geotechnical Society (KGS) Awards Lecture (2019), Minister for National Development’s R&D Awards (2017) on Slope Stabilisation using Mangrove based on Unsaturated Soil Mechanics Principles, ISSMGE TC106 Award (2015) for the “Inaugural Asia-Pacific Distinguished Lecture on Unsaturated Soils”, ASTM International Award (2006) for “Outstanding Article on the Practice of Geotechnical Testing”. He holds a patent on GeoBarrier System (GBS).

He is a registered Professional Engineer in Saskatchewan, Canada and Singapore. He has also served as a consultant on various projects for several engineering firms and government institutions in Southeast Asia, Hong Kong, China, Canada, and Australia.