1. School of Mathematics, China University of Mining and Technology, Xuzhou 221116, Jiangsu, P. R. China
2. State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, P. R. China
3. School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, P. R. China