Dynamic Characteristic And Operation Safety Assessment For Ballasted Track-subgrade Of High-speed Railway In High Latitude Cold Region

In recent years,with the continuous improvement of railway construction in cold regions,the problem of subgrade frost heave has become a key factor restricting the speed of high-speed trains.Subgrade frost heave will cause geometric deformation of the track structure,which affects the dynamic performance of the vehicle-railway-subgrade coupling system,and,even more seriously,affects the safe operation of high-speed railway vehicles.Domestic and foreign scholars have widely investigated the dynamic characteristic of coarse-grained soil,frost heaving theory of soil,dynamic response of subgrade,and the evaluation of track structure.However,few studies have shown the interaction between frost heave subgrade,track irregularities,and high-speed trains of high-speed railways in cold regions.Moreover,the corresponding safety operation evaluation methods are rarely reported.The coupling effect of subgrade frost heave and vehicle vibration is analyzed,taking the Harbin-Jiamusi Rapid Railway as the application background.Laboratory tests,field monitoring,theoretical analysis,and numerical simulation were conducted to study the characteristics of high-speed railway base course materials.A series of dynamic triaxial tests were performed to reveal the dynamic characteristics of frozen coarse-grained materials under the effect of temperature and frequency.The frost heaving mechanism of unsaturated coarse-grained materials was explored based on the experimental tests.The dynamic finite element model of the vehicle-track-frost heaving subgrade system was developed.The influence of frost heaving deformation on the dynamic response indexes of the whole system was systematically studied.A multistage operation safety assessment method for the high-speed railway was established.The main content and understandings obtained are as follows:(1)The low-temperature dynamic triaxial tests for base course materials of highspeed railway subgrade were carried out,and the dynamic parameters were obtained.The effects of temperature,confining pressure,and vibration frequency on dynamic stressstrain relationship,dynamic shear modulus,damping ratio,and threshold dynamic shear stress were analyzed.A modified Hardin model was proposed to describe the stress-strain relationship of frozen coarse-grained soil.The dynamic shear modulus attenuation and damping growth models were established considering temperature and frequency effects.In addition,the Mann-Kendall method was innovatively introduced to determine the threshold dynamic shear stress of frozen coarse-grained soil.(2)A typical section along the Harbin-Jiamusi Rapid Railway was selected for field monitoring.The dynamic responses of vertical acceleration in time and frequency domains during the freezing and unfreezing period were studied.The effect of different seasons on subgrade vibration characteristics was found.By comparing the vibration response of the ballastless track of the Harbin-Dalian high-speed railway,the influence of the track structure on the vibration characteristics of the high-speed railway subgrade in the cold region was expounded.(3)The requirement of a high-speed railway is stringent.Even slight frost heave deformation will lead to profound influence.The evolution law of the temperature field,frost heave deformation,and water vapor migration were analyzed through frost heave tests.The results showed that there existed a mixed migration phenomenon of liquid water and vapor during the freezing process of coarse-grained materials.Based on the frost heave test results,combined with the water-heat-vapor coupling theory,a frost-heave model of unsaturated coarse-grained soil was established,considering water-vapor migration.Furthermore,the heat transfer and water-vapor migration process in coarsegrained soil during freezing were revealed.(4)Based on the vehicle-subgrade coupling dynamics theory,a dynamic analysis model considering vehicle,track,and frost heave subgrade was established.The effects of frost heave wavelengths,amplitudes,frost heave wave distances,and train speeds on dynamic indicators were analyzed.The dynamic response law of vehicle-track-subgrade under multi-factor coupling influence was revealed.The results showed that the dynamic response of vehicle-track-subgrade involved many dynamic indexes,and each influencing factor had different influences on dynamic indexes.(5)There are too many evaluation indicators,and they are difficult to be unified.Based on the numerical simulation results,a method to calculate the "safety degree" of the vehicle-track-subgrade system was proposed.This method was developed by combining the analytic hierarchy process and the fuzzy comprehensive evaluation method.So multiple evaluation indicators of different parts were unified into a safety degree using this approach.A multistage operation safety assessment model of the high-speed railway with the ballasted track was established.The relationship between various indexes and the safety degree of the system was revealed.In addition,a safety degree prediction model on the operation state of high-speed railway using multi-layer perceptrons neural network was established.The suggested control value of subgrade frost heave was preliminarily discussed.The results proposed by this paper can provide references for the maintenance work of the frost heave subgrade of high-speed railway.