1. Professor, Key Laboratory of Road and Traffic Engineering of the Ministry of Education, Tongji Univ., Shanghai 201804, P.R. China.
2. Assistant Engineer, Key Laboratory of Highway Construction and Maintenance Technology in Loess Region of the Ministry of Transport, Shanxi Transportation Research Institute, Taiyuan, Shanxi 030006, P.R. China; mailing address: Room 414, 36 Xutan West St., Xiaodian District, Taiyuan 030006, P.R. China (corresponding author).