1. Lecturer, Dept. of Civil Engineering, Aristotle Univ. of Thessaloniki, Thessaloniki 541 24, Greece (formerly Imperial College London).
2. Professor of Analytical Soil Mechanics, Imperial College London, London SW7 2AZ, U.K.
3. Senior Lecturer, Dept. of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, U.K. (corresponding author).