1. Associate Professor, School of Highway, Chang’an Univ., Xi’an 710064, China.
2. Graduate Researcher, School of Highway, Chang’an Univ., Xi’an 710064, China.
3. Professor, School of Highway, Chang’an Univ., Xi’an 710064, China; Emeritus Professor, Dept. of Civil and Environmental Engineering, National Univ. of Singapore, Singapore 117576 (corresponding author).