The mitigation of geo-hazards caused by heavy rainfall is a common issue in Asian community of geotechnical engineers. Such geo-hazards are often highlighted by the catastrophic failure of natural slopes and manmade fills comprising initially unsaturated soil. Despite that we can learn a lot from such case history, it is regrettable that the cause of failure is rarely pursued or disclosed. Moreover, the mechanical behavior of unsaturated soil subjected to cycles of saturation/desaturation should be better understood so that much better idea for preventive works may be hinted. In this keynote paper, the small-strain stiffness and strength characteristics of unsaturated soils are, first, described by showing the results of laboratory tests recently performed in Geotechnical Engineering Laboratory at Kobe University. Second, engineering practices for evaluating the mechanical behavior of largely-deformed (or failed) natural slopes and reinforced earth walls are described by portraying a series of well-documented case studies in Japan. Finally, lessons learnt from each of these case studies are exposed.