1. Lecturer, Dept. of Materials, Imperial College London, Exhibition Rd., London SW7 2AZ, U.K.; formerly, Research Associate, Dept. of Engineering, Univ. of Cambridge, Cambridge CB2 1PZ, U.K.
2. Ph.D. Student, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K.
3. Reader in Geotechnical Engineering, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K. (corresponding author).