1. Ph.D. Candidate, Institute of Geotechnical Engineering, Southeast Univ., No. 2 Sipailou, Nanjing 210096, China (corresponding author).
2. Research and Development Manager, David Ball Group, Huntingdon Rd., Bar Hill, Cambridge CB23 2TQ, U.K.
3. Reader, Dept. of Engineering, Univ. of Cambridge, Trumpington Rd., Cambridge CB2 1PZ, U.K.