1. Postdoctoral Research Associate, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, P. R. China.
2. Professor, State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, P. R. China (corresponding author).
3. Professor, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, P. R. China.
4. Assistant Professor, State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, Hubei 430071, P. R. China.