1. Geotechnical Engineer, Atkins Ltd., Woodcote Grove, Ashley Rd., Epsom, Surrey KT18 5BW, U.K.formerly, Ph.D student, Univ. of Cambridge.
2. Professor of Civil Engineering, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge, CB2 1PZ, U.K. (corresponding author).
3. Ph.D. student, Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge, CB2 1PZ, U.K.