1. Department of Mechanics and Engineering Science, College of Civil Engineering and Mechanics, Lanzhou University, No. 222, South Tianshui Road, Lanzhou, Gansu 730000, P. R. China and Key Laboratory of Mechanics on Environment and Disaster in Western China, The Ministry of Education of China, Lanzhou University, No. 222, South Tianshui Road, Lanzhou, Gansu 730000, P. R. China