1. Lecturer, Dept. of Civil Engineering, Kasetsart Univ., Jatujak, Bangkok, Thailand, 10900; formerly, Imperial College London, U.K. (corresponding author). E-mail: fengatj@ku.ac.th
2. Company Director, Geotechnical Observations Ltd, Egham, Surrey, U.K.; formerly, Imperial College London, U.K.
3. Reader, Dept. of Civil and Environmental Engineering, Imperial College London, SW7 2AZ, U.K.