1. Professor, Civil and Environmental Engineering Dept., Massachusetts Institute of Technology, Cambridge, MA 02139 (corresponding author).
2. Senior Geotechnical Engineer, ITASCA S.A., Dr. José Luis Aguilar 1178, Providencia, Santiago, Chile.
3. Lecturer, Civil and Environmental Engineering Dept., Massachusetts Institute of Technology, Cambridge, MA 02139.
4. Formerly Parsons Brinckerhoff, 75 Arlington St., Boston, MA 02116.