| The better part of regions of our country's southeast coast and the perimeter regions of Inland River, lake and sea are fluvial facies, marine facies or lagoonal facies sedimentary stratum. On the geology, these stratums belong to Quaternary Period Holocene Epoch Q4 soil layer, and most of these soils are saturation normal soil compaction or lack of compaction soft clay. Along with the development of industry and agriculture construction, a great deal of buildings, such as the industrial factories, wharves, airports, warehouses, storage yards, superhighways etc. need to be constructed on this kind of soft soil foundation, so that the soft soil must be reinforced.At present, the vacuum preloading method is the best economic and effective methods of foundation treatment for this soft soil layer. The method was at the earliest stage put forward by Professor W.Kjellman at Royal Geology College in Sweden in 1952. It is a kind of soft soil foundation reinforced methods, belongs to preloading concretion methods. It needs the sand well or plastics draining board is set inside the soft foundation. Then sand cushion is laid in the ground and one layer airproof sealed films are covered at its top to isolate the atmosphere. The vacuum plant pumps the air by the slotted liner way set in the sand cushion, makes differ air pressure inside and outside of the films, results some boundaries of the foundation generating negative pressure source, accelerates pore water drained out of the soil, so as to make soil mass gradually concreted and the strength elevated.The vacuum preloading reinforcement technique already carried out abroad. According to incompletely statistics, the soft soil foundation reinforced are more than 1500m2 using this method in our country. Although the vacuum preloading method has been extensively applied and the former people had respectably work on the study of reinforce mechanism and the aspects of theory calculation, its some reinforce mechanism is not quite clear and still exist some dispute on the theories field. Such as some problems: Whether the ground water level declines or not. Whether is this method validity in reinforcing thick soft clay or not. How much the maximum reinforcement depth is? In addition, the former research is concentrated in reinforced area, yet there is lack of study on interference of the vacuum reinforced area and its boundaries, range of influence of the lateral deformation around soil mass and its influential factors. These limit the expansion and application of this method in a way. Therefore studying the range of influence of vacuum preloading reinforcement mechanism and lateral deformation are propitious to realize this construction method completely. There is important realistic meaning to improve this method theory and protection perimeter construction. Thus, this method can better service in the engineering construction.This thesis summed up the research actuality of using vacuum preloading method to reinforce soft soil foundation and thoroughly analyzed its reinforcement mechanism. Considering this method improves the effective stress of soil mass by charging the unbalance of its boundaries and inside pore-water pressure. Through the stress analysis, this kind of foundation treatment method makes soil mass generate equal-direction concretion without shearing stress, which further indicates the reinforcement mechanism of this method. The soil mass does not generate instability breakdown even if vacuum preloading is loaded one time. This outstanding advantage can reduce the secondary consolidation after construction, shorten the construction period and speed the construction progress. The variety situation of the underground water level at pumping vacuum is continuously declining without flushing, and then keeps its balance and stabilization to a certain degree. The underground water level declines fast near the draining board and slowly farther the draining board, as a result it become the hopper shaped form. When the underground water level becomes basic stabilization at the vacuum preloading evening, the soil mass can be divided three regions by the climax depth line and vacuum bailing depth line. These soil mass that inside these three regions show different reinforcement mechanism in the process of concretion. The simple calculation formula of the maximum reinforcement depth is propounded in vacuum preloading method.Considering the maximum reinforcement depth of this method is not limited to vacuum bailing depth. That relates to the physical property of soil mass, the resistance of draining water channels, the construction technology and the vacuum degree. The reinforcement effect of this method is obvious within 20m by analyzing engineering dates. The research of the range of influence is prepared to compare the results of existing empirical formula with measured data and study on the range of influence currently by FEM method. It discovers that the calculated values of the range of influence were partial small compared with its measured data. Therefore, the actual protection distance is considering that the calculation distance multiply two as in the actual engineering application. Bigger lateral deformation ranges are mostly apart from the reinforcement areas sideline 20m, especially the dehiscence and deformation of soil mass inside 10m are obvious. The ranges outside 10m seldom generate dehiscence, so the usage of building is generally uninfluenced. The ultimate lateral deformation of soil mass using vacuum preloading method reinforced depends on the spacing and depth of plastics draining board, the vacuum degree, the property of soil mass etc. Among these factors, the coefficient of consolidation (Ch) influences the lateral deformation greatly, but the areas of vacuum preloading consolidation are basic no influence.Finally, it is adopted eight nodal points equal parameter element as formal parameter function, taken advantage of statistical weight parameter method, combined the theory of Biot consolidation, established FEM equation and calculation program, calculated and analyzed the ultimate settlement and the lateral displacement in vacuum preloading reinforcement. The calculation results of Tianjin harbor container article circulating center east district indicate that the FEM calculation procedure can be better analog the actual process of consolidation and the calculated results of ultimate settlement and lateral displacement accorded with the measured value in situation. The lateral displacement of soil mass is along with reinforcement time augmentation, reduced with the depth augmentation. The lateral deformation direction of soil mass is same and towards negative pressure source. From the changing curve of lateral displacement with depth, the displacement is bigger in the soil mass of upper 10m range and is less in under part soil mass. It indicated that the range of influence is mostly within 10m and the max of lateral displacement occurs at the periphery surface of reinforcement area. The soil mass outside reinforcement area is only settlement in vertical direction, is not shear deformation in horizontal direction. The whole foundation of reinforcement area presents shrinkage tendency, which makes the reinforcement soil mass is more compact and averaging and the reinforcement effect is better than the positive pressure in the same condition. These corking explain that the theory of Biot consolidation is rational applied to the theory calculation in foundation treatment using vacuum preloading method and the reinforcement mechanism of soft soil foundation using vacuum preloading method, provide precious experience to study further on these aspects.
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