Experimental Study On The Improving Effect Of Soft Foundations Through Vacuum Combined With Surcharge Preloading

Vacuum preloading and vacuum combined with surcharge preloading are effective methods to improve the soft foundation. These two methods have been used widely in freeway, dock, building and airport, etc. But the theory of vacuum preloading develops more slowly than its engineering practice. There are more questions to solve, such as the effective improvement depth by vacuum preloading, difference between vacuum preloading and surcharge preloading, the microscopic structure of the soil after vacuum preloading and vacuum combined with surcharge preloading, and so on.Based on the summary of the formers' study, the mechanism of vacuum combined with surcharge preloading is studied through laboratory test, field experiment and microstructure analysis by the author. The main contents of this paper as following:Firstly, vacuum preloading, surcharge preloading and vacuum combined with surcharge preloading are simulated through laboratory test. The mechanism and effect between the vacuum preloading and surcharge preloading of the soil in different depth are analyzed. The strengthening result between vacuum preloading and vacuum combined with surcharge preloading under the same preloading is studied.Secondly, some useful conclusions are drawn through the analyses of the field experiment data, such as, the effective improvement depth under vacuum combmed with surcharge preloading is 4 meters deeper than the depth of drain pile in Zhujiang. The basin-curve of settlement has relation not only to the condition of boundary's airproof, but also to the condition of soil and the magnitude of preloading. The pore water pressure in different depth fall down fast during vacuum-pumping before fill. The pore water pressure increases slowly during filling because of the vacuum negative pressure, and it will decrease deeply when the fill finishes.Finally, through the analyses of the microstructure photo from the original soil sample, vacuum preloading soil sample, surcharge preloading soil sample and vacuum combined with surcharge preloading soil sample, some useful conclusions are drawn: The strengthening result from vacuum preloading and surcharge preloading can be added in mathematics. The shape of soil grain after vacuum preloading, surcharge preloading and vacuum combined with surcharge preloading becomes long and narrow. The order of the soil grain after preloading boost up and the order of the pore space after preloading grows lower. The distributing area in one plane of the soil grain after vacuum preloading, surcharge preloading and vacuum combined with surcharge preloading gets larger and the distributing area in one plane of the pore space becomes smaller, i.e. the soils grains gets more dense after preloading.