1. School of Civil Engineering, Xi ’an University of Architecture and Technology, Xi ’an, P. R. China
2. Shaanxi Key Laboratory of Geotechnical and Underground, Space Engineering, Xi’an, P. R. China
3. School of Science, Xi ’an University of Architecture and Technology, Xi’an, P. R. China
4. Department of Mathematics, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia