Geotechnics for Sustainable Infrastructure Development - Geotec Hanoi 2016, Phung (edt). ISBN 978-604-82-0013-8 1029 Keywords: shallow landslide, infiltration, slope stability, pore pressure, TiVaSS ABSTRACT: This study identifies the problem related to shallow landslides triggered by rainstorms infiltration in mountainous areas. On doing so, a new physical grid-based Time-Variant Slope Stability Model (TiVaSS) is developed. TiVaSS numerically solves the three-dimensional (3D) Richards’ equation for the temporal built-up pore pressure induced by rainfall in the first step. An infinite slope stability model is then applied using the simulated pore pressure for stability assessment. To verify the performance of TiVaSS, the severe rainfall-induced landslide event in July 2011 in Umyeon. Mt, Seoul, Korea is considered. The   index which compares the real unstable sites observed during the rainstorm with those predicted by TiVaSS is used for verifying. Despite some limitations on the lack of input data, the predicted landslides by TiVaSS show a consistent agreement with the reality. Therefore, it can be said TiVaSS is a potential tool for the prediction of landslides those induced by heavy rainstorms. 1. INTRODUCTION Landslides and slope instability are a common natural hazard in mountainous areas, causing severe damage and casualties worldwide each year. In the case of Korea, a country with almost 70 % of the land is in mountainous areas, and 60 % of them have steep slopes (30 0 – 60 0 tilt angles), the potential damage of landslides is significant. According to a statistic, the landslide damage areas in this country increased largely from the 1990s to the 2000s, and this due to the increases in both rainfall intensity and rainy days in addition to haphazard land development. For example, even though the annual average landslide areas in the urban areas of Seoul and Gyeonggi province was merely 0.061 km 2 from 2001 to 2010, it was recorded at 1.52 km 2 in 2011 (Lee et al., 2012). In particular, from July 26 th to July 28 th , 2011, heavy rains assailed Korea, triggering mass flooding and landslides in Seoul, Korea. Following this severe event, as much as 110 mm of rain fell in just one hour in the capital. This is the third-highest hourly rate recorded in the country since rainfall data collection began in 1907. More than 400 mm of precipitation was recorded from Tuesday (July 26th) until early Wednesday (July 27th). As the consequence, tons of mud from Mt. Umyeon swept through nearby villages in the city’s southern ward of Seocho, killing 16 people and destroying many houses (more details of the Umyeon landslide can be found in section 3). Therefore, an accurate landslide susceptibility assessment is essential and effective to prevent such disasters and minimize the damages. In recent years, improvements in computer technology as well as geographical information systems (GIS), Regional mapping of rainfall -induced shallow landslide using a new time-variant slope stability model Viet Tran The Kyungpook National University, Daegu, Korea. E-mail: trantheviet@tlu.edu.vn Giha Lee Kyungpook National University, Daegu, Korea. E-mail: leegiha@knu.ac.kr Hyunuk An Chungnam National University, Daejeon, Korea. E-mail: hanlight20@hotmail.com Thu Trinh Minh Thuyloi University, Hanoi, Vietnam. E-mail: tmthu@tlu.edu.vn