1. School of Civil Engineering, Wuhan University, Wuhan 430072, P. R. China. E-mail: jqh1972@yahoo.com.cn
2. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, P. R. China
3. Key Laboratory of Geological Hazards on Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, P. R. China