1. Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong Univ., Chengdu 610031, China (corresponding author).
2. Professor, Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong Univ., Chengdu 610031, China.
3. Dept. of Civil and Environmental Engineering, Univ. of Pittsburgh, Pittsburgh, PA 15260.