1. Associate Professor, Key Laboratory of Mechanics on Disaster and Environment in Western China, the Ministry of Education of China, School of Civil Engineering and Mechanics, Lanzhou Univ., Lanzhou 730000, P. R. China (corresponding author).
2. Associate Professor, Key Laboratory of Mechanics on Disaster and Environment in Western China, the Ministry of Education of China, School of Civil Engineering and Mechanics, Lanzhou Univ., Lanzhou 730000, P. R. China.