1. Faculty of Civil Engineering and Mechanics, Jiangsu University, Zhenjiang 212013, P. R. China
2. School of Mathematics and Statistics, Yancheng Teachers University, Yancheng 224002, P. R. China
3. College of Engineering, Mathematics and Physical Sciences, University of Exeter, Harrison Building, North Park Road, Exeter, EX4 4QF, UK
4. School of Mathematics and Physics, China University of Geosciences, Wuhan 430074, P. R. China