Experimental Study On Mechanism And Improvement Effect Of Vacuum Preloading

Vacuum preloading and vacuum combined with surcharge preloading are new methods to improve the soft foundation. These two methods have been used widely in construction, obtaining prominent economic profit and social benefit, which have become effective and general soft foundation improvement methods. But there are still many problems on research of improvement mechanism, such as the effective improvement depth by vacuum preloading, whether groundwater level changes during vacuum preloading, unloading standard in vacuum preloading, the effect of vacuum combined with surcharge preloading to surrounding environment. These problems have restricted their popularization.Based on the summary of the formers' study, the mechanism of vacuum preloading and the effect of vacuum location to the improvement of subsoil are studied by laboratory test and theory analysis. The main contents of this paper as following:Firstly, groundwater level during vacuum preloading is discussed through laboratory test and theory analysis. Based on different reasons of pore water's drain, drainage flow is divided into three parts, and which are analyzed about their effect on groundwater level. At last, the result of laboratory test is proved: groundwater level don't change basically during vacuum preloading at the top.Secondly, vacuum preloading at the top and vacuum preloading at the bottom are simulated by laboratory test. Difference of two methods and the effect of vacuum location to the improvement of subsoil are analyzed, two methods have great effect on subsoil improvement, but vacuum preloading at the bottom is better than vacuum preloading at the top, these results are explained by the principle of effective stress. To the project, such as wharf stack yard, reclamation engineering, vacuum preloading at the bottom is preferential considered to improve the soft foundation.Finally, based on the results of laboratory test, the improvement mechanism of vacuum preloading is more analyzed. Vacuum preloading is explained with new idea from three pore pressures and action of three pore pressures. From the physical characteristic of vacuum, the law of vacuity transfer is the course of air molecule moving from high density to low density, air molecule must overcome resistance and abide by basic principle - minimum potential energy theorem. Formation mechanism of crust layer in vacuum preloading is analyzed, primary reasons are the loss of vacuity during transfer and the change of saturation degree in the capillary zone, don't have reason of unsaturated area caused by fall of groundwater level. Comparing the mechanism of vacuum preloading and surcharge preloading, current stress calculation method of vacuum preloading by transforming vacuum load to surcharge load don't actual reflect vacuum preloading's characteristic, a new calculation method is put forward.