1. Associate Professor, Dept. of Agricultural and Environmental Engineering, Faculty of Agriculture, Kobe Univ., 1-1 Rokkodai, Nada, Kobe 657-8501, Japan. E-mail: Kawabata@kobe-u.ac.jp
2. Associate Professor, Dept. of Civil Engineering and Engineering Mechanics, Columbia Univ., 500 West 120th St., New York, NY 10027, and Visiting Associate Professor, Division of Engineering and Applied Sciences, Harvard Univ. E-mail: ling@civil.columbia.edu
3. Research Manager, Soil Mechanics Laboratory, National Institute for Rural Engineering, 2-1-6, Kannondai, Tsukuba, Ibaragi, Japan. E-mail: mohri@nkk.affrc.go.jp
4. Graduate Student, School of Science and Technology, Kobe Univ., Japan. E-mail: 039d844n@y05.kobe-u.ac.jp