1. Researcher, Tokuyama College of Technology, 3538 Takajo, Kume, Shunan City, Yamaguchi Prefecture 745-8585, Japan.
2. Professor, Dept. of Civil Engineering, Yamaguchi Univ., Tokiwadai 2-16-1, Ube 755-8611, Japan.
3. Professor of Geotechnical Engineering, Dept. of Civil and Structural Engineering, Univ. of Sheffield, Mappin St., Sheffield S1 3JD, U.K. (corresponding author).