1. Ph.D. Candidate, Dept. of Civil and Environmental Engineering, Imperial College, London SW7 2AZ, U.K. (corresponding author).
2. Reader in Particulate Soil Mechanics, Dept. of Civil and Environmental Engineering, Imperial College, London SW7 2AZ, U.K.
3. Research Associate, Dept. of Civil and Environmental Engineering, Imperial College, London SW7 2AZ, U.K.
4. Professor, Dept. of Civil Engineering, Univ. of British Columbia, Vancouver, BC, Canada V6T 1Z4.