1. GeoDelft, P.O. Box 69, 2600 AB Delft, The Netherlands (corresponding author).
2. Dept. of Engineering, Univ. of Cambridge, Trumpington St., Cambridge CB2 1PZ, U.K.
3. Laboratoire Sols, Solides, Structures, Domaine Univ., BP 53 38041 Grenoble Cedex, France.
4. GeoDelft, P.O. Box 69, 2600 AB Delft, The Netherlands.