1. Research Assistant, Dept. of Civil Engineering, Laboratory of Geotechnics, Ghent Univ., Technolgiepark 905, B-9052 Ghent-Zwijnaarde, Belgium.
2. Professor, Dept. of Civil Engineering, Laboratory of Geotechnics, Ghent Univ., Technolgiepark 905, B-9052 Ghent-Zwijnaarde, Belgium (corresponding author).
3. Professor, Dept. of Civil Engineering, K.U. Leuven, Kasteelpark Arenberg 40, B-3001 Leuven, Belgium.
4. Research Assistant, Dept. of Civil Engineering, K.U. Leuven, Kasteelpark Arenberg 40, B-3001 Leuven, Belgium.