| Due to the demands for coastal city construction and the opening to the outside world, the reclamation project has become an important part in coastal engineering construction and coast exploration project. In Tianjin, Shenzhen, Guangzhou, etc., the coastal soft soil areas therein are usually reclaimed through blowing one layer of dredger fills on the original ground and using vacuum preloading to fix the dredger fills and accordingly form the inland areas which can be used as building foundation. Along with the extension of the reclamation project to sea area, the thickness of the needed fills are continuously increased, thus making the traditional vacuum preloading process unable to meet engineering requirements and proposing the demand for new process. Meanwhile, the vacuum preloading process has different functional effects for dredger fills and naturally deposited soil layer, and the consolidation degree of the dredger fills under the effect of the vacuum pressure, which is calculated on the basis of the traditional consolidation theory, is significantly different from the actual consolidation degree of the dredger fills, so it is necessary to establish a new method for calculating the consolidation degree of the dredger fills under the effect of the vacuum preloading. The traditional vacuum preloading process needs a large amount of medium-coarse sands, but such sand source is lack in Tianjin area, so it is necessary to develop new vacuum preloading process which needs a small amount of sands or does not need sands. In order to solve the above problems, systematic research is carried out in this article, with the main search contents and achievements as follows:(1) The new and thin dredger fills have low bearing capacity, the working layer of the machinery placed on the deep and thick dredger fills for arranging and establishing drain boards is the main factor which restricts the vacuum preloading period. In this article, the method of blowing a certain thickness of silty-fine sands on the surface of the dredger fills to form a hard shell is put forwards according to laboratory model experiments and Vesic’s formula for calculating the bearing capacity of the double-layer foundation, and such hard shell can be quickly formed and the bearing capacity thereof can meet the bearing capacity requirement for arranging and establishing drain boards, thus to solve the corresponding problem in this project. This process and the recommendable thickness of the hard shell(0.6~0.8m) have been widely applied in the reclamation project for Tianjin coastal newregion, thus greatly shortening the construction period, reducing the construction cost and quickening the consolidation progress of the dredger fills.(2) The settlement process of the dredger fills with high water contents and mud during water-soil separation, splitting and vacuum preloading periods has been simulated through model experiments in order to accurately calculate the settlement volume of the dredger fills at the water-soil separation stage, the splitting stage and the vacuum preloading stage, thus to provide rational dredger fill elevation for reclamation project and avoid the waste and the engineering problem caused by improper amount of dredger fills.(3) The model experiment and the observation data of the field practical project show that the consolidation degree of the dredger fills calculated according to traditional consolidation theory is greatly different from the actually measured value. In this project, the problematical mechanism is firstly researched: the soil body near the drain boards and the earth surface has low permeability due to the short drainage path and the quickly increased effective stress, thus blocking the drainage passage for the soil body far away and meanwhile reducing the overall consolidation rate of the soil body. Subsequently, the consolidation equation for the consolidation coefficient is established on this basis, and the numerical method of the equation is also proposed. Such achievement has important significance for correctly evaluating the consolidation degree and the nonuniformity of the dredger fill foundation processed by numerical method.(4) The process of using the secondarily blown mud containing small amount of sands to form self-sealing vacuum preloading and accordingly consolidate the dredger fills is put forwards in this article. This method can not only solve the problem of sand source shortage in Tianjin region, but also make the vacuum preloading technology more economic and environmentally friendly. Through the research on the directly-vented type vacuum preloading model experiment for mud seal, we can find that this process can realize the purpose of vacuum preloading consolidation from the aspect of consolidation effect and also has good consolidation effect, and the field pilot plant test can be also carried out in order to perfect the vacuum preloading construction process for a large area. Moreover, the soil body cracking problem during the consolidation process is also analyzed in this article from the micro perspective according to the concept of the shrink film in unsaturated soil mechanics. Through the analysis for the equilibrium condition of the shrink film under the existence of matric suction, water film tension and the drawing force transferred from the surrounding boundary, we have established the mechanical model of the shrink film; through the tension between the water film and the attachment, we have stimulated the stretch-draw effect of the shrink film during the soil body cracking process; through differently-sized and differently-shaped model groove experiment, we have researched the cracking occurrence, development and termination processes of the saturated soil under dried condition and evaporation condition. As a result, we can not only deepen our recognition on the soil body cracking mechanism, but also apply the research achievement in the directly-vented type vacuum preloading process.(5) In this article, ABAQUS software is used to simulate the consolidation process of the dredger fills with complicated stress path, analyze the stress change of the dredger fills in the whole construction processes including manually board-plugging at shallow layers, primary vacuum preloading, sand-blasting film pavement, board-plugging at deep layers, secondary vacuum preloading, fill preloading, vacuum preloading, etc., and stimulate the stress in the soil body through such loading modes as even distribution of load, excess pore pressure and vacuum degree in finite element, as well as calculate the consolidation process of the soil body under the action of external load. Moreover, the actually measured result coincides with the calculation result. During the numerical calculation, we not only consider the influence of the void ratio change on the permeability coefficient of the soil body, but also objectively simulate the gradient change of vacuum load and preload along with the time, thus to meet the actual condition as much as possible. Finally, through the analysis for the influence of the operation time of different working conditions on the settlement of the dredger fills, we can find that the prolonged board-plugging time cannot change the overall settlement of the soil body, but can reduce the vacuum pumping time of the deep layers, thus to save energy.(6) Through the research on the dredger fill self-weight consolidation test, we can find that the evaporation rate of the soil body is basically equal to that of pure water at the early period of natural drying of dredger fills. Under the condition of no rainwater, the soil body with the thickness of 20 cm at the surface layer can consolidate within two months, and the water content of the soil body in the surface layer is reduced below 50% at that time, and afterwards the evaporation rate and the settlement rate of the soil body will be both reduced. Through the application of soil mechanics three-phase property relationship, we can find the relation between the water content change of the soil body and the settlement of the soil body; through one-dimensional self-weight consolidation field test, we can find the change of the water contents of the soil bodies with different depths along with the time, thus to perfect the research on the water content change in the one-dimensional consolidation theory. The change of the water content of the soil body along with the depth at a certain future time can be predicted according to the field data fitting. The obtained research achievement which has been applied in the field project of Tianjin Grand Harbor Surrounding Technology Area can provide theoretical basis for calculating dredger fill elevation and is favorable for arranging construction progress. |
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