1. School of Geology and Environment, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, P. R. China
2. School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, P. R. China
3. Geological Research Institute for Coal Green Mining, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, P. R. China
4. Department of Civil, Environmental and Architectural Engineering, University of Padua, Padova 35129, Italy
5. School of Sciences, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, P. R. China