1. College of Civil Engineering and Mechanics Lanzhou University Lanzhou 730000 P. R. China
2. Key Laboratory of Mechanics on Disaster and Environment in Western China, the Ministry of Education of China (Lanzhou University) Lanzhou 730000 P. R. China
3. State Key Laboratory for Geomechanics & Deep Underground Engineering School of Mechanics and Civil Engineering China University of Mining and Technology Xuzhou 221116 P. R. China