1. Department of Geotechnical Engineering, College of Civil Engineering, Tongji University, Shanghai 200092, P. R. China
2. Key Laboratory of Geotechnical and Underground Engineering of the Ministry of Education, Tongji University, Shanghai 200092, P. R. China
3. Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, 2851 Woodside Dr., Cincinnati, OH 45221, USA
4. Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA