| Study on vacuum preloading has never been stopped since it was proposed. Recently, vacuum-surcharge preloading has been widely used in various types of soft foundation treatment project because of its significant advantages, such as short construction period, good economic benefit, handling large area, and avoiding foundation destabilization which result from the excess load. With rapidly developing in recent two decades, great progress has been made in vacuum preloading and vacuum-surcharge preloading. However, theoretical research still lags behind engineering practices.Based on the consolidation mechanism and the numerical simulation, this paper studies and discusses the mechanism and simple computation method of vacuum combined with surcharge preloading. First of all, based on analysis of field measurement and negative pressure consolidation test data, some new viewpoints of the mechanism of vacuum preloading are discussed from the angle of macrostructure and microstructure. In the process of vacuum preloading, the subsoil can be divided into three districts vertically, and every district has different consolidation characteristics and mechanism. Then the viewpoints are validated by explaining and analyzing of some problems and phenomena such as effective improved depth, groundwater table variation and pattern of pore water pressure variation in the actual case; Secondly, based on analyzing the measured data of Chenjiagang Power Plant soft foundation, this paper studies the variation of pore water pressure and excess pore water pressure, then it reveals the microscopic strengthening mechanism of vacuum-surcharge preloading; the pore water pressure of shallow soil layer reduces rapidly as the vacuum preloading begins, while the pore pressure of deep soil layer reduces a lot in the vacuum-surcharge preloading period. This paper discusses the soil deformation pattern of vacuum-surcharge preloading, the settlement rate changes rapidly and vacuum degree quickly increased up to 80kPa as the vacuum preloading begins, while the settlement rate changes slowly in the vacuum-surcharge preloading period. Thirdly, based on the principle of consolidation degree equivalent, the actual and complex sand well ground is turned into the sand wall ground for the purpose of simplifying the calculation. This paper derives the equivalent permeability coefficient of the sand wall ground. Combining with the consolidation characteristic of vacuum-surcharge preloading, this paper proposes a simplified numerical calculation method, which is derived by plane stain BIOT consolidation FEM and modified Cambridge model. Compared to the conventional FEM analysis, the simplified method has many advantages, such as convenient grids division, simple parameters and less calculation. This paper applies the simplified method to an actual project case, which shows that the results of the implified method are accordant to the project measured data. This indicates that the simplified method is feasible and reliable, and can be more convenient for the engineering practice and designing. |
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