Vacuum Preloading: Experiment And Theory

Soft clay is distributed in the vast area of China with poor engineering property. In recent years, vacuum preloading is extensively used to treat soft clay ground, whereas the development of the theoretic study in vacuum preloading cannot follow the progresses in engineering, which has limited the progress and application in engineering practice. Therefore, it is necessary to systematically study the mechanics of vacuum preloading. In this paper, some aspects of vacuum preloading were studied as follows.Vacuum consolidation test and load consolidation test were performed and compared for different kind of soil samples. After analyzing the results of tests, the four major mechanisms of vacuum consolidation were defined as: variation of capillary potential, drainage of sealed bubbles, vaporization and action of atmosphere pressure, in which, the action of atmosphere pressure is the most important factor. The distribution of negative pore-water pressure, the action of atmosphere pressure and the restraint of deformation determined the consolidation status of vacuum consolidation. In the same consolidation status, similar properties of consolidation were observed for either vacuum consolidation or load consolidation.The mechanics of vacuum preloading was discussed by contrast of vacuum preloading and heaped preloading. The factors affecting the result of vacuum preloading for ground consolidation were as follows: the geological conditions of site, the intensity of vacuum pumping, the condition of underground water and installation of vertical drainage line. Through analysis of these factors, some conceptions and design principles were proposed for vacuum preloading. A site topographic coefficient was defined. The design principles were suggested. The total pump numbers installed should be determined according to soil volume to be treated. How many pumps were turned on should be based on the dynamic design according to project target. And how many pumps should be in operation in standard shut off procedure. It was approved that although the theoretical boundary of decline of groundwater level is 10m, the actual boundary is only about 6m. A method to back analysis the distribution pattern of negative pore-water pressure along vertical drainage line was proposed. Some practical vacuum preloading projects were analyzed. The results showed that distribution patterns of negative pore-water pressure along the vertical drainage line were uniform and the magnitude was about half of the vacuum degree under the membrane.Based on the results from both laboratory test and field application of vacuum preloading it can be concluded that soil is water-saturated in the processes of vacuum consolidation or vacuum preloading. Thus, the consolidation properties in negative-pressure area cannot becalculated by using the theory based on non-saturation soil. Negative pore-water pressure and suction were specified. Stress and strain variations during vacuum consolidation were analyzed and the effect of Mandel in vacuum preloading was evaluated. Some solutions for vacuum preloading consolidation were derived on the basis of previous research findings.Fundamental equations of seepage and consolidation analysis of vacuum preloading were generalized. Basic process of vacuum preloading analysis using FEA method was introduced. Some key subjects of numeric analysis were discussed such as: selecting principles of constitutive equations of soft clay, the equivalent theory of vertical drains ground, the definition of load step, the determination of vertical drains as well as the treatment methods of the decline of groundwater level, sealing wall and nearby water source.On the basis of layered method, a method to calculate the consolidation settlement of vacuum preloading was deduced. The elements affecting total settlement during vacuum preloading consolidation were discussed. The principle that defined the coefficient of total settlement correction by the magnitude of combined loadings was suggested. The mechanism and compone...