1. College of Transportation Science & Engineering, Nanjing Tech Univ., 200 Zhongshan North Road, Nanjing 210009, Jiangsu, CN.
2. School of Civil Engineering, Nanjing Forestry Univ., 159 Longpan Road, Nanjing 210037, Jiangsu, CN (Corresponding author), e-mail: seuxt@hotmail.com
3. School of Civil Engineering, Nanjing Forestry Univ., 159 Longpan Road, Nanjing 210037, Jiangsu, CN.