1. Dept. of Civ. and Envir. Engrg., Imperial Coll. of Sci., Technol. and Medicine, London SW7 2BU, U.K. (corresponding author). E-mail: j.stegemann@ic.ac.uk
2. Dept. of Civ. Engrg., Univ. of Moratuwa, Katubedda, Sri Lanka.
3. Dept. of Civ. and Envir. Engrg., Imperial Coll. of Sci., Technol. and Medicine, London SW7 2BU, U.K.