青藏铁路高温多年冻土区列车行驶路基长期永久变形数值模拟研究

Numerical Simulation of Permanent Deformation of Embankment Induced by Train Load in Permafrost Regions Along the Qinghai-Tibet Railway

摘要:

基于动三轴试验建立的冻结、融化状态青藏铁路粉质黏土的累积塑性应变模型，考虑路基土体动应力的作用，二次开发适用于青藏铁路高温极不稳定多年冻土区路基长期永久变形的蠕变法则，并在既有的黏弹塑性本构模型将其引入，计算列车荷载作用下青藏铁路路基的永久变形。分析结果显示：（1）列车荷载作用下距离路基顶面不同埋深土体的累积动力永久应变随着等效振次的增加而增加，并且先期的增长速度较快，最终趋于稳定，长期沉降在路基中心处最大，最终在路基距离轨道最远处几乎为零；（2）随着列车速度的增加，路基中心永久变形逐渐增大；（3）随着列车轴重的增加，路基中心永久变形逐渐增大，并且随着埋深的增加，列车轴重对路基永久变形的影响逐渐变小；（4）随着冻结深度的增加，路基表面累积塑性变形呈降低趋势；随着融化深度的增加，路基表面累积永久变形增加。

Abstract:

With the completion of the Qinghai-Tibet Railway （QTR），several types of damage induced by traffic loads have occurred in the permafrost embankment；therefore，the safe operation demand and speed increasing scheme of the railway will be severely affected.Permafrost is an important engineering characteristic of the QTR and is the main reason for the settlement deformation of the embankment.The embankment deformation induced by trains includes elastic and plastic deformation.Although the residual plastic deformation will ultimately become accumulative permanent deformation with an increase in load number，plastic deformation might be only a small proportion of the total deformation in the embankment induced by a single train load.Uneven deformation in an embankment is caused by excessive post-construction settling and creates a severe negative influence on the service period of a train and on the safety and comfort of its passengers.Therefore，the accumulated settlement of embankment under the traffic load is an important influence factor of embankment stability.Making an accurate estimate of permanent embankment deformation is of great significance for guaranteeing the stability of trains and the design of railway embankments.In this study，a Duo test section is analyzed，and on the basis of dynamic tri-axial tests with samples taken from the QTR，an empirical accumulative plastic strain model is created in frozen and molten states.The CREEP subroutine is used to provide a theoretical foundation for calculation of permanent deformation of the QTR embankment.The results indicate that when the depth is 2.5 m or less，the accumulative permanent deformation of the embankment increases with an increase in train speed.However, when the depth is 8.2 m，the accumulative permanent deformation decreases as the train speed increases.The cumulative plastic strain increases gradually and maintains a constant level with an increase in depth to the top of the embankment under traffic loading and increases to maximum at the embankment center.Moreover，permanent deformation increases with an increase in train speed.Further，the maximum deformation of the embankment increases from 8.9 mm to 47 mm when the axle load increases from 8 t to 25 t.Because axle load has a significant influence on embankment deformation，controlling the axle load of a train may be effective in slowing the uneven deformation of the embankment.Permanent deformation increases with an increase in train axle load，and the influence of the train axle load is reduced with an increase in depth.In addition，long-term settlement in the embankment center increases with an increase in the equivalent vibration.Permanent deformation increases from 31.8 mm to 35.8 mm，or 13%，when the equivalent vibration of the train is 50 000 times and 100 000 times.After that，however，accumulative permanent deformation increased by 5.9%，3.7%，and 2.5% for each respective 50 000 times increase in vibration.The accumulation of permanent deformation of the embankment increases rapidly in the early train run and then gradually slows.Therefore，protection and repair of the embankment should be conducted early.Finally，the freeze thaw condition has a significant effect on the accumulated permanent deformation of the embankment.The cumulative plastic deformation on the embankment surface decreases with an increase in frozen depth and increases with an increase in thawed depth.The work provides experimental data for further research on the dynamic characteristics of frozen soil.