Acknowledgments
We sincerely thank the Executive Editor of the Journal, Prof. J.-P. Anselme (University of Massachusetts Boston), for his careful revision of this paper. We appreciate the National Natural Science Foundation of China (No. 21372034), the Science and Technology Bureau of Sichuan (No. 2011HH0016), the Incubation Program for Excellent Innovation Team of Chengdu University of Technology and the Opening Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (No. SKLGP2012K005) for financial support.