1. Engineer, Parsons Brinckerhoff, 6 Devonshire Sq., London EC2M 4YE, U.K.; formerly, Doctoral Student, Dept. of Engineering, Univ. of Cambridge, Cambridge, CB2 1PZ, U.K.
2. Lecturer in Civil Engineering, Dept. of Civil Engineering, Univ. of Bristol, Bristol BS8 1TR, U.K. (corresponding author).
3. Laing O’Rourke Professor of Construction Engineering, Dept. of Engineering, Univ. of Cambridge, Cambridge CB2 1PZ, U.K.