Study On The Dynamic Characteristics Of Foamed Concrete Subgrade Under High-speed Railway Load

In recent years,China’s high-speed railway has developed rapidly.With the wider distribution of high-speed railway lines in the country and the faster and faster operation speed of high-speed railway,the diseases of high-speed railway subgrade caused by various complex geological conditions and moving loads of high-speed railway trains are becoming more and more frequent,which threaten the operation safety of highspeed railway trains to a certain extent and affect the operating benefit of high-speed railway network.The settlement control of high-speed railway subgrade is an important measure to ensure the safe and effective operation of high-speed railway line.As a new lightweight roadbed material,foamed concrete has broad application prospects in controlling the settlement of high-speed railway subgrade.Its low material cost,green and environmental protection,is also in line with the current social advocacy of energy conservation and emission reduction.In this thesis,a series of experiments were carried out on the foamed concrete specimens using the indoor GDS dynamic triaxial apparatus.The dynamic equivalent stiffness,damping ratio and cumulative axial strain of the foamed concrete under high speed rail load were investigated.Scanning electron microscope(SEM)was used to observe the internal microstructure of the foamed concrete before and after the test.Qualitative and quantitative analysis of the change rule and mechanism of the internal microstructure of the foamed concrete before and after the test were made.The three-dimensional finite element model of track subgrade foundation is established by ABAQUS finite element software.The dynamic response of foamed concrete subgrade under high speed rail load is studied.The conclusions drawn will help further understand the performance and internal structure characteristics of foamed concrete,and provide a theoretical basis and reference for the related design of high-speed railway.The main research results are as follows.(1)The dynamic characteristics of foamed concrete specimens under different conditions are studied by means of confining pressure,dynamic stress amplitude and dry and wet state of materials.The dynamic equivalent stiffness increases first and then decreases with the increase of dynamic stress amplitude.The trend of dynamic equivalent stiffness and experimental vibration times is similar.Different dynamic stress amplitudes will affect the rate of change of dynamic equivalent stiffness with test vibration times,and the greater the dynamic stress amplitude,the greater the dynamic equivalent stiffness of foam concrete will be.The damping ratio of foam concrete increases with the increase of dynamic stress amplitude,and the rate of growth gradually accelerates.The damping ratios of foamed concrete specimens increase with the increase of test vibration times under different dynamic stress amplitudes.When the dynamic stress amplitude is 30 k Pa,the damping ratio increases fastest with the test vibration times and the damping ratio is the largest.When the dynamic stress amplitude is 10 k Pa,the damping ratio fluctuate with the fitting curve of test vibration times.However,the overall damping ratio basically increases with the increase of test vibration times.Under the condition of different confining pressures and different dynamic stress amplitudes,the axial cumulative strain of foam concrete specimens showed two stages of rapid growth and steady slow growth with the increase of test vibration.The increase of dynamic stress amplitude or higher water content of specimens can increase the axial strain accumulation rate and ultimate strain of foamed concrete specimens,and increase confining pressure will reduce them.(2)Scanning electron microscopy(SEM)is used to describe the pore structure characteristics of foamed concrete with different multiplier images.The micropores in foamed concrete can be classified into aggregate pores,connected pores and complete independent pores.The internal structure of foamed concrete specimens is relatively loose,and the influence of dynamic triaxial test on the pore morphology is not obvious,but it may be the cause of partial hole wall rupture.Image-Pro Plus(IPP)software was used to quantitatively analyze the distribution and shape characteristics of the micro pore structure of the foamed concrete specimens.The analysis results show that the pores of the foamed concrete specimens are mainly small(pore diameter <400 μm)pores,and the shapes of these small pores are very close to the circle.The arithmetic average of the shape coefficient of the foam concrete pore under all test conditions is close to 1.The dynamic triaxial tests under different conditions have squeezing and damaging effects on the pore shape of the foamed concrete specimens,and the internal structure of the specimens are more disturbed under dynamic loading with water.The fractal dimension of the microscopic pores of foamed concrete is studied by using fractal theory.The fractal dimension of the surface of the foam concrete specimen is approximately 1.03～1.07,showing a good correlation with the dynamic triaxial test parameters and the dynamic characteristics of the material.(3)A three-dimensional finite element model of track subgrade foundation is established by using ABAQUS finite element software.The dynamic response of foamed concrete subgrade under high speed rail moving load is studied.The results show that the dynamic response(such as dynamic displacement and acceleration)of high-speed railway subgrade will increase with the increase of train speed.Foamed concrete can effectively reduce the rail bending caused by uneven settlement in the transitional section of road-bridge.With the increase of density of foamed concrete,the effect of improvement is gradually enhanced.From the abutment to the subgrade,the acceleration increases first and then decreases along the longitudinal direction,and maintains at a certain level,reaching the maximum at the junction of the transition section and the ordinary subgrade section.The dynamic acceleration of the subgrade gradually decreases along the depth direction,and the attenuation velocity in the foamed concrete is much less than that in the ordinary soil roadbed.There are 62 figures,12 tables and 96 reference papers in this thesis.