Study On Long-term Settlement Of Soft Soil Foundation Under Fast Rail Subway Train Load

Due to the typical engineering characteristics of “three high and two low” of saturated soft soil,the subway shield tunnel constructed in the coastal soft soil area is prone to uneven settlement after construction under the long-term repeated action of subway train load during the operation period,which brings great hidden danger to the traffic operation and tunnel structure safety.In this paper,the saturated soft clay in Tianjin area is taken as the research object.Aiming at the fast-speed subway train load,the indoor experimental study on the dynamic response of saturated soft clay under train load is systematically carried out.The mechanical response characteristics of soil are analyzed from three aspects:plastic strain-vibration,dynamic stress-strain and dynamic pore water pressure.At the same time,the three-dimensional dynamic numerical model is established by using the finite element software ABAQUS,and the evolution law of plastic deformation of the surface,tunnel waist and bottom soil is analyzed.In addition,the relevant parameters of the long-term settlement prediction model are obtained by fitting the indoor test data,and compare with the actual monitoring settlement data of the line,and predicted the settlement value that this line can reach in the future operation period.The specific research results are as follows:(1)The generation mechanism and calculation method of metro train load are defined.According to the artificial excitation force function and Metro A-type vehicle parameters,the expression and time history curve of metro train load under fast speed are determined.On this basis,the indoor dynamic triaxial test scheme is determined.(2)The indoor cyclic dynamic triaxial test method was used to study the dynamic response characteristics of soft soil under different speed subway train loading conditions.The results show that the plastic cumulative strain and dynamic pore water pressure of soft soil under fast speed are less than those under average speed and normal speed.In the ten stage vibration stop test,the accumulated plastic strain and pore water pressure in the next stage are smaller than those in the previous stage.(3)The indoor cyclic dynamic triaxial test method was used to analyze the change law of intermittent train load vibration on the dynamic characteristics of soft soil.The cumulative plastic strain during the vibration stop period was gradually reduced to a relatively stable value after the vibration period was stopped.The dynamic pore pressure during the stoppage period increased on the basis of the first five vibration stages of the test,and gradually decreased and stabilized during the five stoppage periods after the test.(4)The numerical simulation method is used to study the settlement law of the soft soil foundation and the surface of the tunnel under different speed conditions,and the influence range of soil settlement and surface settlement at the bottom of the tunnel under the load of a fast speed subway train is determined.The soft soil foundation of the tunnel is obtained.The post-construction settlement decreases with the increase of subway train speed,the surface settlement range at faster speeds is about 3.5 times the tunnel diameter on each side of the tunnel axis,and the vertical settlement range of the soil below the tunnel is about 4 times the tunnel diameter.The prediction model fitted with experimental data combined with the layered sum method is used to calculate the long-term settlement of the soft soil foundation under the tunnel in the next 20 years to reach 29 mm.Compared with the actual settlement monitoring data,it can be seen that the model predicts better results.