Study On Pressure Decay And Boosting Effect In Vacuum Preloading Process

Clogging effect arising in the vacuum preloading for the dredged fill could reduced the vacuum pressure in the soil,which brings the consolidation pressure applied on the soil down and has an impact on the soil improvement.Nowadays,pressurized method is adopted by some engineering to solve the clogging effect in the vacuum preload.This method means to inject pressured gas into soil by a pressured pipe so as to increase the consolidation pressure and improve the permeability of clogging soil.This paper carries series model tests to achieve the vacuum preloading of dredged soil.Transmission and attenuation of vacuum pressure in soil are measured.Improvement of soil permeability induced by pressured method is analyzed from micro and macro prospects.The main work results are summarized as follows:(1)Measured system for the attenuation of vacuum pressure is established by integrating vacuum pump,conical bottle and vacuum pipes.Variations in vacuum pressure as consolidation time are recorded by vacuum gauges for different water content,soil particles gradation and soil depth.Test results show that vacuum pressure in the soil decreases as decreasing water content,increasing soil depth or increasing soil permeability.(2)Pressured vacuum preloading test is conducted.Permeability of soil sample after different pressured time is measured.Micro structure of soil is analyzed by scanning electron microscopy(SEM)and image processing software.It is concluded that pressurization could increase the soil permeability obviously by about 1.5 times.The effect of pressurization on the soil is greater when the degree of consolidation of soil is 40%.200-times SEM picture of soil shows that the soil cracks increases significantly after the pressurization process.However,the effect of pressurization on the soil porosity is not obvious indicated by more times SEM picture of soil(3)Based on the Hansbo consolidation theory,a solution for PVD induced consolidation of soil under the vacuum preloading is proposed considering the increasing soil permeability and consolidation pressure in the pressurization process.The proposed solution is confirmed by comparing to the existing test result.Analyzed results show that the pressurization process applies effect on the soil permeability and consolidation pressure at the same time.The soil permeability increases by 1.5?2.0 times and consolidation pressure grows by 40%?80% of applied pressure.