1. Associate Professor, Dept. of Engineering Science, Univ. of Oxford, 15 Parks Rd., Oxford OX1 3PJ, UK (corresponding author).
2. Research Fellow, ETSI Caminos, Universidad Politécnica de Madrid, Calle del Prof. Aranguren 3, Madrid 28040, Spain.
3. Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of California, 760 Davis Hall, Berkeley, CA 94720-1710.
4. Professor, Head of Civil Engineering Div., Cambridge Univ., Trumpington St., Cambridge CB2 1PZ, UK.