| Building high speed railway in the soft soil, especially in the east coastal area of China, high requirements are put forward to foundation treatment for controlling settlement rigidly. Vacuum preloading is an effective method for soft foundation improvement. However, the engineering experience in design using such treatment is relative poor, it must be studied further. Nowadays, as the construction of high speed railways has stepped into great development period, further understanding of the vacuum preloading have theoretical and practical significances. In this paper, with the case of the Beijing-Shanghai high speed railway in China, the soil deformation behavior, numerical simulation and consolidation mechanism of the vacuum preloading for soil treatment were completely studied by applying the field and laboratory tests, theoretical and numerical analysis. Central works were as followed:(1) The regional soil characteristics of the undisturbed soil samples taken from field were obtained by various laboratory tests. The consolidation simulataneous tests between vacuum preloading and surcharge preloading were carried out with patent apparatus. Permeability tests under vacuum preloading were also carried out. The tests showed the vacuum preloading had better effects to decrease the water ratio and void ratio of the soil as comparison to surcharge method. Conductivity coefficient has some fluctuation in initial stage afer vacuum preloading. Later it declined slowly and declining velocity became slower and slower. However, the consolidation effects under the two different loads were almost accordant.(2) Through field tests, the soil behavior, settlement velocity and pore water pressure distributing, etc. were statistically analyzed and the mechanisms of vacuum preloading and surcharge preloading were also studied. Both of them are aimed to cause pore pressure differences for draining and then increase the effective stresses with the pore pressure dissipation to reach soil consolidation.(3) Through the analysis of measurements of the excess pore water pressure, consolidation process of soil in vacuum preloading was compartmentalized three phases for the first time. The three phases were:â‘ seepage consolidation in shallow soil and " surcharge loading" effect in deep soil by vacuum suction;â‘¡consolidation stabilization in shallow soil and transformation of "surcharge loading" effect to "negative pressure" effect in deep soil;â‘¢seepage consolidation in deep soil under "negative pressure" effect and to balance. The soil in vacuum preloading was divided into shallow soil and deep soil. Seepage consolidation happened in shallow soil and the superposition of "surcharge loading" and "negative pressure" caused deep soil consolidation. Such proposed theory could explain the phenomenon that the excess pore water pressure in some survey points became positive or maintained constant in first several days after picking up the suction.(4) Maximum deepness (depth of shallow soil) lmax by vacuum preloading in the first phase was brought forward. And momentary "surcharge loading" P resulted in by groundwater decline was also developed. lmax could be obtained from variation curves of excess pore water pressure of all measurement points. p could be approximately calculated through decline depth of groundwater. The essential cause of soil behavior by vacuum preloading was not negative pressure difference solely caused by seepage field, but rather the working together of "positive pressure" and "negative pressure" during different phases as well as the especial phenomenon:groundwater decline. Meanwhile,"positive pressure" would weaken intensity of seepage field.(5) A one-dimension vacuum consolidation model including "two springs" was established and applied to analyze soil consolidation. The excess pore water pressure was obtained by solving the consolidation equation under the given initial and boundary conditions, and a basically reasonable agreement existed with the measurements.Mechanisms of vacuum preloading and surcharge loading were analyzed in effective stress path and strain. Considered radial consolidation of sand drain, analytical solutions of excess pore water pressure were analyzed. Based on large deformation consolidation theory and unsaturated soil theory, mechanisms of vacuum preloading were discussed. (6) The model of the vacuum preloading considering Biot consolidation theory was established using finite element method (FEM). The results showed that:â‘ In numerical calculation, surcharge preloading was not identical with vacuum preloading.â‘¡Vertical drainage channels were applicable to be treated as the medium of strong permeation during the settlement calculation of roadbed and applicable to be treated as negative pressure boundary conditions for calculating horizontal displacement at the slope toe of roadbed.â‘¢It was very important to define negative pressure distribution mode of vertical drainage channels.(7) Based on the grey theory and Genetic Algorithms, settlement forecast model were established and applied to forecasting roadbed settlements under vacuum preloading. Based on one-dimension consolidation theory, the consolidation settlement calculation formulas method of both vacuum preloading and that combined surcharge preloading were deduced under some simplifications. By comparing the theory solution and the measurements, some correction parameters taken account into the instantaneous settlement and lateral deformation were proposed for both vacuum and surcharge preloading.(8) The influences of vacuum intensity and groundwater decline on soil consolidation effect were discussed. Increasing the quantity of jet pumps or declining the groundwater would make better reinforcement effect, but the cost would rise simultaneously. At last, the treatment effect was appraised by conventional criterions.
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