1. Professor, School of Civil, Urban, and Geosystem Engineering, Seoul National Univ., San 56-1, Shinlim-dong, Kwanak-Gu, Seoul, 151-742, Korea.
2. Graduate Student, School of Civil, Urban, and Geosystem Engineering, Seoul National Univ., San 56-1, Shinlim-dong, Kwanak-Gu, Seoul, 151-742, Korea.