Study On The Screening Efficiency Of Trenches In The Saturated Ground By2.5D Finite Element Method

The ground vibrations induced by railway traffic can cause lots of trouble to the people living nearby and some damage to neighboring buildings. As the modern traffic system develops, more and more high-speed railways are built and the running speed of trains becomes faster and faster, which makes the problem of vibration pollution induced by high-speed trains more and more serious. Thus, it’s necessary to carry out the researches about the screening effects of countermeasures to train-induced ground vibration. These researches will help to find out effective and efficient ways to tackle the problem and supply the theoretical guidance for the design of wave barriers and policy making of engineers.It’s well known that FEM is superior to analytical approach at tackling problems with complex model shape and complicated boundary conditions. By assuming the material and geometric properties to be constant along the load-moving direction,2.5D FEM is proposed to improve the calculation efficiency. Besides, a lot of railway lines are built on the saturated soft calys in the coastal area. The saturated soft calys have some special properties, such as high water content, low shear wave velocity and so on, which is easy for the trains to generate excessive ground vibrations in this area. Based on Biot’s theory for the saturated soil, the following researches related to the prediction and isolation of train-induced ground vibrations are conducted by using2.5D FEM:1. Based on the U-W and U-w formulation of Biot’s theory (U is the displacement of soil skeleton, W is the displacement of fluid, w is the relative displacement between soil skeleton and fluid), the corresponding2.5D FEM equations and the viscous-elastic absorbing boundaries for the two fomulations are deduced by introducing Galerkin Method and Fourier Transforms. Furthermore, the ground dynamic responses subjected to the moving load are studied.2. The open trench models in satured ground are built and the screening performances of open trench are studied. Beside, the reduction effects of open trench in saturated ground and in single-phase elastic ground are compared. It is found that the vibration isolation effects of open trench in saturated ground become better as the moving-load speed is larger. Compared with the good performances of open trench in single-phase elastic ground, the screening efficiencies of open trench in saturated ground are not obvious, especially when the moving-load speed is low.3. The study concerning the effect of the underground water level varying on the vibration reduction of open trench in saturated ground is conducted. The variation of the underground water level in saturated ground is simulated by changing the depth of the upper-lying single phase elastic layer. It is found that reduction effects of open trench in saturated ground get improved as the underground water level decends. This trend becomes more significant when the load-moving speed approaches Rayleigh wave velocity. However, when the underground water level is underneath the trench bottom, the variation of underground water level has little influence on the screening efficiencies of open trench in saturated ground.4. In the engineering project, the saturated ground are layered because of the geologic sediments. Then the studies of open trench in the two layered saturated grounds (upper-lying softer layer and upper-lying harder layer respectively) are investigated. The factors including soil layer properties, trench depth and load-moving speed which can affect the performance of open trench in layered saturated grounds are mainly analyzed. It is found that when the load speed is low, the screening performances of open trench in layered saturated ground and in layered elastic ground are similar, but the performances are different when the load speed is high. Open trench performs better in layered saturated ground than that in layered elastic ground when the load speed is high. Besides, the screening efficiencies of open trench in layered saturated ground become better as the trench depth is larger. However, for the layered saturated ground with upper-lying softer layer, trench depth increments have little effect on the screening efficiencies of open trench after the trench penerates into the lower harder layer.5. The model of in-filled trench in layered saturated ground is built, based on which the screening efficiencies of in-filled trench are analyzed and the performances of in-filled trench in layered saturated ground and in layered elastic ground are compared. It is found that for the concrete in-filled trench in layered saturated ground, the trench with a small depth can have good screening effects, and the trench performs better as the trench depth increases. Meanwhile, the vibration isolation effects of in-filled trench improve when the modulus of in-filled materials become larger. As for the natural rubber in-filled trench in layered saturated ground, the trench performs alike the open trench. What’s more, the screening efficiencies of in-filled trench in layered saturated ground and in layered elastic ground are different.From the above, it is concluded that the dynamic mechanic performances of saturated soil and single-phase elastic soil are significantly different. Based on Biot’s theory, the screening effects of vibration isolation trenches in saturated ground are studied in a systematic way by using2.5D FEM. The persent work is of theoretical significance and engineering value in solving the problems of train-induced environmental vibrations.