Simulation Of Soil Dynamic Response Under The Movable Track Traffic Loads

With the growth of the economy, transportation engineering in China is rapidly developing. The development of expressway system and track traffic system is faster than that of other traffic systems. Although the growing track traffic can make material and energy much more easily transport, this system also induces a serial of negative effects, the subsidence in traffic foundation soil is one of the negative effects, and in some places the subsidence induced by traffic vibration has brought serious damages, so, some scholars and engineers have focused their research interests on it.Although many researchers have studied the soil response under the traffic vibration since 1960s, however, this problem is so complicated that some problems should be valuable to be further studied. Based on the analysis of present achievements, the paper presents two new methods based on Finite Element Method (FEM) and Central Circle Displacement Function Method (CCDFM) to simulate the soil response under the moving traffic vibration loads.Firstly,a model of the movable traffic vibration loads is built up based on vibration mechanism analysis, some factors related to the vibration, such as the speed of the train, bump of the train, the weight of the wheels, the disperse effect of the sleeper and so on, is included in this model. Then, a three-dimensional elastic-plastic finite element model of foundation soil is set up in ANSYS. Herein, Drucker-Prager model is chosen to simulate the non-linear characteristics of the soil. The bottom of the model is constrained in horizontal plane. 3-D viscous-elastic artificial boundary is adopted to simulate the bottom of the model and the boundary condition. Under the movable traffic loads, the time-history curves of the displacement and the acceleration in different places in this foundation soil are calculated, and several conclusions are got. When the load is just on, the displacement has an abrupt change, but as the load moving, the displacement will stay at a stable value, the stable value decreased when it is farther or deeper from the track, and the attenuation of this stable value is much as the distance from the track increases. When the load is just on, the acceleration has also an abrupt change, but as the load moves, the value tends to zero. The train speed has little effect on the displacement and the acceleration in the vertical direction, but it has an obvious effect on the displacement and the acceleration along the horizontal direction, a magnitude change takes place as the distance from track in this direction increases.Then, based on discussion the advantage and the disadvantage of Lamb's integral solution for the soil responds under the harmonic vibration, a reconstructive Central Circle Displacement Function Method (CCDFM) is used to analyze responds of the soil under the moving traffic vibration loads, and a exact analytic solution is got. At fast, the speed factor is added in the CCDFM to simulate the load movement, then, the displacement time-travel curve of the foundation soil under the moving harmonic vibration is calculated based on the platform of MATLAB. Further, the relationship of soil displacement and traffic vibration frequency and the moving velocity is analyzed. The results are as follows: The displacement function converge a stable value as the load moves. The stable value decreased as the distance is farther from the track in horizontal direction, which is similar to the result from FEA analysis in ANSYS. The frequency has much effect on this stable value. The higher the frequency is, the larger the stable value is, and its change-tendency is linear. The speed of the train has little effect on the displacement of the vertical direction.The dynamic response of foundation soil under movable traffic vibration loads is complicated. The results in this thesis shows that numerical simulation based on FEA and reconstruction Central Circle Displacement Function Method can be used to explore the subsidence mechanism of foundation soil under traffic vibration in some cases.