Study On Dynamic Response And Long-term Settlement Of Saturated Soft Clay Under Metro Train Vibration

Although the development of the subway system has brought great convenience to people's travel,the excessive dynamic response of the foundation soil caused by the operation of the subway trains and the deformation of the foundation have become important problems during the operation period of the subway system.These two problems are especially serious in the area with widely distributed soft soil.In these area,even the soft soil foundation has been reinforced and went through a long-term consolidation process,the train operation may still cause excessive dynamic response of the soil,and even lead to uneven settlement of the foundation.In order to make a clear understanding of these problems,the soft clay in the Pearl River Delta region was chosen as the research object in this paper,the dynamic response and long-term settlement of the saturated soft clay under metro train vibration were studied by using the dynamic triaxial test and the explicit numerical simulation method.The main research contents and the results achieved include:(1)Based on the dynamic response test of saturated soft clay under the action of metro train vibration,the characteristics of dynamic stress-dynamic strain of soil,the dynamic strength and dynamic modulus of soil were systematically studied.The test results show that the soil hysteresis is a series of non-closed hysteresis loops,and the degree of opening increases with the increase of the amplitude of the dynamic stress.When the dynamic strain is lower than the limit value,the frequency has a little effect on the dynamic backbone curve.Based on the influence of the dynamic stress amplitude and frequency on the cumulative plastic strain of the soil,a new mathematical model of cumulative plastic strain considering frequency and dynamic bias stress was established.The stable and destructive?_d-N curves are inconsistent;the frequency has a little effect on the dynamic elastic modulus in the middle period of vibration.The dynamic elastic modulus of the soil increases with the increase of the dynamic stress amplitude.The new normalized parameters obtained by the Hardin-Drnevich model verification test data are reasonable and feasible.(2)A three-dimensional finite element model of the subway tunnel structure(train-ballastless track-tunnel-ground)was established.The calculation results show that from the rail through the track bed and the tunnel to the soft clay layer at the bottom of the tunnel,the vibration acceleration and the dynamic stress amplitude are continuously attenuated.For the soft clay layer around the tunnel,the dynamic stress amplitude of the soil near the tunnel bottom is the largest while the minimum dynamic stress amplitude appears at the side wall.For the ground soil,the soft clay near the tunnel bottom has the highest dynamic response,and it attenuates with the increase of the depth and the lateral.The modulus of CA mortar(cement asphalt mortar)has a little effect on the structural dynamic response,and the speed of the subway trains has a great influence on the structural dynamic response.(3)A three-dimensional finite element model of the subway depot(train-ballast track-ground)was established.The calculation results show that after the soft soil foundation was strengthened by the piles,the vibration acceleration of the soil in the reinforcement zone attenuates faster,and the modulus of the piles has a great influence on the dynamic stress of the soil.The dynamic stress amplitude of the soft clay attenuates with the depth,indicating that the settlement mainly occurs in the shallow area of the soft soil.(4)Combining the test results and the numerical simulation results,a new model used for the prediction of the additional dynamic deformation of the foundation(long-term settlement)was established.