Experimental Study on Vacuum Preloading with Combination of Fill Surcharge

Abstract

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 preloading 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 experiments 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.