1. Formerly, Ph.D. student, Dept. of Civil Engineering, Univ. of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.
2. Professor, Dept. of Civil Engineering, Univ. of Tokyo, Bunkyo-ku, Tokyo 113-8654, Japan (corresponding author).
3. Professional General Manager, Integrated Geotechnology Institute Ltd., 6-9, Iidabashi 4-chome, 1-23-6 Yotsuya, Shinjuku-ku, Tokyo 160-0004, Japan.
4. Lecturer, Dept. of Civil and Natural Resources Engineering, Univ. of Canterbury, Christchurch 8041, New Zealand.