1. Assistant Senior Researcher, Foundation and Geotechnical Engineering Laboratory, Structures Technology Division, Railway Technical Research Institute, Tokyo 185-8540, Japan (corresponding author).
2. Professor, Dept. of Engineering, Geotechnical and Environmental Research Group, Univ. of Cambridge, Cambridge CB3 1PZ, U.K.
3. Postdoctoral Research Fellow, Soil Mechanics Laboratory, Swiss Federal Inst. of Technology–EPFL, CH-1015 Laussanne, Switzerland.