EXPERIMENTAL STUDY ON VACUUM PRELOADING WITH COMBINATION OF FILL SURCHARGE Wan-Huan Zhou 1 and Guo-Xiong Mei 2 1 Department of Civil and Environmental Engineering, University of Macau, Macao, China. E-mail: hannahzhou@umac.mo 2 School of Civil Engineering and Architecture, Nanchang Hangkong University, Nanchang, Jiangxi, China. E-mail: meiguox@163.com Vacuum preloading with combination of fill surcharge has been widely applied in the infrastructure development projects as a common and effective method for soft soil improvement. Complete procedure of soft soil improvement by vacuum preloading method includes not only the application of vacuum pressure but also the removal of vacuum pressure. In this paper, the mechanisms of vacuum preload- ing and vacuum removal are discussed with the consideration of anisotropy of soils. The analysis shows that the stress-induced anisotropy due to the removal of vacuum pressure may cause soil settlement. A series of triaxial stress path exper- iments on undisturbed soil samples have been conducted. The experiments were designed to simulate the soil behavior during (a) vacuum preloading and vacuum removal and (b) combined vacuum and surcharge preloading and vacuum removal. Typical test results of axial displacement variations with time are presented and discussed. It is found that the soil deformation is significantly affected by the soil stress state when the vacuum pressure is removed. At low deviator stress level, the vacuum removal will cause upward deformation. When the deviator stress exceeds certain critical value, the vacuum removal will cause vertical deformation or even plastic failure in the soil. It is also found from laboratory tests that the soil settlement increase due to the vacuum removal will only occur in the vacuum and surcharge combined preloading projects. Keywords: Anisotropy, Vacuum preloading, Soil consolidation, Soil improvement, Stress path. 1. INTRODUCTION For infrastructure developments such as highways, roads, ports, harbors, and airports, effective and economic soil improvement methods are often useful to accelerate construc- tion and stabilize the existing soil foundations, especially for the soft clay deposits in the coastal regions. Vacuum preloading with combination of fill surcharge over prefabricated vertical drains (PVD) is a common and effective ground improvement method and has been widely used in different countries and regions. Many field monitoring and laboratory tests were reported to study the soil consolidation and ground deformation under vac- uum preloading and surcharge preloading methods (Qian et al., 1992; Cognon et al., 1994; Proceedings of the International Conference on Ground Improvement and Ground Control Edited by Buddhima Indraratna, Cholachat Rujikiatkamjorn and Jayan S. Vinod Copyright © 2012 by Research Publishing Services. All rights reserved. ISBN: 978-981-07-3559-3 :: doi:10.3850/978-981-07-3559-3 01-0106 377