1. Distinguished Professor and Holder of the Spencer J. Buchanan Chair, Zachry Dept. of Civil Engineering, Texas A&M Univ., College Station, TX 77843 (corresponding author).
2. Principal Engineer, Geocomp Corporation, 125 Nagog Park Acton, MA 01720.
3. Ph.D. Student, Zachry Dept. of Civil Engineering, Texas A&M Univ., College Station, TX 77843.