Study On The Dynamic Stress In Ground Induced By Train Moving Load And Cyclic Properties Of Soft Saturated Clay

The South-East China is the most developed in China most area of which is covered with deep soft clay. Many important traffic fundamental projects have been built during the 9th five year plan. They have done a lot for the economic fast development. At the same time some technological problems occurred: considerable settlement developed during the long term operation of these projects. To solve this problem is a significant technological support for the safe operation of these projects in use and the building of future projects and is greatly meaningful. It does be the foundation of predicting long term settlement of ground induced by traffic load to study the complex stress in ground induced by train and dynamic properties of soft clay under this kind of stress.On the base of the important advances in the field of train-induced vibration and cyclic properties of soil under cyclic loading, some subjects as following were investigated on theoretical analysis and cyclic triaxial tests:(1) The solutions of displacement and contact force of Timoshenko beam lying on an elastic half-space under a moving load were obtained. The critical velocities of moving load were studied and the displacement of Timoshenko beam on an elastic half-space and Euler-Bernoulli beam on an elastic half-space was compared. And it is found that Euler-Bernoulli beam is precise enough for most practical projects. As proves the rationality of using Euler-Bernoulli beam on an elastic half-space to analyze train-induced vibration.(2) Some in-situ measurements were carried out to analyze characteristics oftrain-induced vibration. And based on these data the rationality of the presented theoretical model was proved.(3) A theoretical model of Euler-Bernoulli beam on an elastic half space under moving loads was presented in order to calculate stress in ground induced by train. The spatial distribution, time histories, stress path and principal stress axes rotation were studied. It is found train-induced stress is characteristics of low amplitude, low frequency, non-symmetric cyclic loading, which fluctuates from a deviated stress. The principal stress axes rotate continuously and stress state of soil element changes from simple shear to triaxial shear and back to simple shear repeatedly.(4) The influence of train speed, embankment stiffness and soil shear modulus on stress was analyzed. It is found that the static solution is precise enough to calculate stress when train speed is low. But stress amplitude increases greatly when train speed is high. Stress decreases with the embankment stiffness increasing but varies little when the embankment stiffness increases to some extent. The variation of stress with soil shear modulus is complex, which is related to train speed and the location of soil element.(5) Cyclic triaxial tests were carried out using the lOHz GDS bi-directional triaxial system and the characters of low amplitude, low frequency, long acting time, deviated stress and bi-direction of train-induced stress were simulated. The law of the deformation, pore pressure and cyclic strength of Xiaoshan nature soft clay was studied. It is shown that the deviated stress promotes the development of strain and pore pressure, lowers the cyclic strength curve but has small effect on the minimum limited cyclic strength. Due to the anisotropy and localization of soil, dynamic confining pressure can also induce plastic deformation accumulation and residual pore pressure development. And the velocity of plastic deformation accumulation and residual pore pressure development is higher than that under dynamic deviator stress of the same amplitude.The bigger the dynamic stress amplitude is, the bigger the difference is. (6) At last the principle of train-induced long-term settlement is analyzed. It is found that softening of soil, plastic deformation accumulation, and soil structure destruction within the critical boundary induced by train are the reason of long-term settlement.The presented study has important theoretical value for soil dynamic mechanics and will provide important technological support for the construction of future key traffic fundamental projects.