1. Graduate Student, State Key Laboratory of High-Efficient Mining and Safety of Metal Mines of Ministry of Education, Univ. of Science and Technology Beijing, Beijing 100083, China.
2. Graduate Student, Dept. of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109-2125.
3. Professor, Dept. of Civil and Environmental Engineering, Univ. of Michigan, Ann Arbor, MI 48109-2125 (corresponding author).