Study On Mechanism Of Saturated Soft Soil Foundation Improved By Vacuum Preloading And Compaction

In our country, the technology of vacuum preloading has been well developed. In recent years, due to the higher requirements of construction period and subgrade bearing capacity, methods of joint reinforcement is widely used for soft ground treatment, such as vacuum preloading combined with dynamic consolidation method. A lot of engineering practice has accumulated valuable engineering experience, but in the related mechanism research, it is necessary to make further exploration. Some points of view were expressed that foundation can be compacted after vacuum preloading treatment, but there are different opinions. This article will test whether the soft soil can be compacted after vacuum preloading treatment through experiments. In addition, some points of views were expressed that vacuum preloading method has a finite effective reinforced depth(typically 20 ~ 25m), but there are different opinions too. Based on the model simulation experiments, this article will use similarity theory to analyze the effective reinforced depth of vacuum preloading method. Solving these problems is of great significance to explain the related mechanism and improve the technological level of the related projects. Based on the review of previous studies, experimental exploration and engineering practice, this paper analyzes the related problems by model experiments and ABAQUS finite element analysis:(1) Models with different lateral stiffness are designed to simulate the deformation characteristics of the edge and center of the ground reinforced by vacuum preloading method. It is found that the vertical settlement will be smaller when the lateral deformation is large, and vice versa. And as a result, in the process of vacuum preloading treatment, vacuum degree in the soil, excess pore water pressure, lateral displacement, settlement and water discharge have good synchronization, shows that the effective transfer of the vacuum degree in the drainage channel and the soil is the key to obtain success in vacuum preloading treatment.(2) Through the indoor experiment, it is found that the water content of the soil is decreased significantly, but the soil is still in the saturated state, and the saturated soft soil after the vacuum preloading treatment can’t be compacted. On the basis of this, the mechanism of vacuum preloading combined with dynamic consolidation is analyzed. The main reinforce effect of the saturated soft soil is caused by vacuum preloading. Dynamic compaction method mainly compact filling layer, drainage layer, and the unsaturated soil layer above the underground water level, and cause shear deformation by exerting positive pressure to the foundation. The settlement caused by dynamic compaction is mainly produced by the two parts, not by the compaction of saturated soft soil. The dynamic consolidation effect on the saturated soft soil foundation after vacuum preloading treatment is very limited.(3) According to the similarity theory, the effective reinforced depth of vacuum preloading is studied by model experiments. It is pointed out that the vacuum degree downward transfer is a dynamic process, there isn’t a limited transmission depth. As long as the flow-through and ventilation performance of the prefabricated drains is not decreased and the pumping time is long enough, vacuum degree can be delivered to a great depth.(4) Finite element analysis shows that, to achieve better preloading effect, the permeability coefficient of prefabricated drains shouldn’t be less than 5×10-3cm/s, and should be greater than 1×10-2cm/s, if the permeability coefficient is larger than 5×10-2cm/s, that the prefabricated drains can be used for reinforcement of a soft soil foundation of 45 m depth, and vacuum degree at the bottom of the prefabricated drains barely losses. Then an "economical effective reinforcement depth" of vacuum preloading method is explored.