1. State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing Univ., Chongqing 400045, P.R. China (corresponding author).
2. School of Civil Engineering, Hunan Univ. of Science and Technology, Xiangtan 411201, P.R. China.
3. Key Laboratory of New Technology for Construction of Cities in Mountain Area, Chongqing Univ., Chongqing 400045, P.R. China.