Mechanical Characteristics Of Foam Concrete Subgrade Before And After Freezing-Thawing Under High Speed Railway Load

Foam concrete is widely used in various fields of engineering construction because of its advantages such as light weight,good self-supporting and strong liquidity.The application of foam concrete in high-speed railway subgrade filling can effectively improve the uneven settlement of subgrade in soft soil areas and the frost heave and thaw settlement in seasonally frozen soil areas.However,there is still a lack of research on the mechanical properties of foam concrete under the coupling action of freezingthawing cycle and high-speed railway load,which is not conducive to the design and application of foam concrete in the filling of high-speed railway subgrade.In this thesis,the indoor dynamic triaxial experiment and numerical simulation were used to study the macro mechanical properties and micro damage mechanism of foam concrete under the coupling of freezing-thawing cycles and high-speed railway load,and the dynamic response of foam concrete high-speed rail subgrade under the coupling of freezingthawing cycles and high-speed railway load was analyzed.The main research contents include the following three aspects.(1)Based on the indoor dynamic triaxial tests,the hysteresis curve of foam concrete with different densities under the coupling action of freezing-thawing cycles and high-speed railway loads was studied,and the influence of freezing-thawing cycle temperature,freezing-thawing cycles,loading frequency and dynamic stress amplitude on dynamic elastic modulus,damping ratio and axial cumulative plastic strain was studied.Under the action of different freezing-thawing cycle temperatures,different freezing-thawing cycles,different loading frequencies,and different dynamic stress amplitudes,the shape characteristics and evolution laws of the hysteresis curves of the foam concrete with two different densities are basically the same,and there is no significant difference.As the number of cyclic loading increases gradually,the area and unclosed degree of the hysteresis curve first decrease and then increase and finally stabilize.With the decrease of freezing-thawing cycle temperature,the increase of freeze-thaw cycles,the increase of loading frequency and dynamic stress amplitude,the damping ratio and axial cumulative plastic strain of foam concrete show a trend of increasing gradually.(2)Through the scanning electron microscope(SEM)test,the change law of the micro pore structure of foam concrete before and after the coupling freezing-thawing cycles and high-speed railway load was qualitatively analyzed.With the decrease of freezing-thawing temperature and the increase of freezing and thawing cycles,the damage degree of hole wall gradually increases.Inaddition,the P-A fractal theory was used to quantitatively analyze the influence of freezing-thawing temperature,freezingthawing cycles,loading frequency and dynamic stress amplitude on the P-A fractal dimension of foam concrete,and explore the internal relationship between the irregularity of the micro pore structure and the macro dynamic parameters.With the decrease of freezing-thawing cycle temperature,the increase of freezing-thawing cycles,the increase of loading frequency and dynamic stress amplitude,the P-A fractal dimension of foam concrete gradually increases,and the degree of pore irregularity gradually increases.(3)The finite element numerical simulation software ABAQUS was used to analyze the dynamic response of foam concrete high-speed railway subgrade under the coupling action of freeze-thaw cycle and high-speed railway load.A three-dimensional finite element numerical calculation model of track subgrade foundation was established.Through single factor tests,different freezing-thawing cycle temperatures,different freeze-thaw cycles,different thickness of subgrade surface layer and different thickness of subgrade bottom layer were conducted to study,the influence of different train speeds on the vertical dynamic stress,vertical dynamic displacement and vertical dynamic acceleration in each structural layer of the subgrade.In addition,the sensitivity of different influencing factors on the vertical dynamic displacement of the surface layer of the high-speed railway subgrade bed was analyzed by orthogonal tests,through the range analysis and variance analysis.The sensitivity of each factor from high to low is the train speed,the number of freezing-thawing cycles,the thickness of the bottom layer of the subgrade bed,and the thickness of the surface layer of the subgrade bed.This paper has 80 figures,35 tables and 129 references.