Research On Settlement Of Subgrade In Qinghai-Tibet Railway Permafrost Regions Under Dynamic And Static Loads

Railways in permafrost regions in the world have been constructed for nearly a hundred years,but the defect rate of operating lines is still around 30%.Since the opening of the Qinghai-Tibet Railway on July 1,2006,the settlement and deformation of the permafrost subgrade has been plaguing the railway maintenance department.The effects of some defect treatment measures are limited.The contradiction between the plateau frozen soil and the safety of the train entering Tibet has become prominent,so how to maintain the roadbed thermal balance and stability are a scientific problem.In view of these,this paper takes the perfection of the theory of the prevention and control of permafrost subgrade defect as an example,and takes the ordinary subgrade of the warm and ice-rich permafrost site of the Beiluhe,QTR as an example to study the settlement deformation of the warm and ice-rich permafrost subgrade under dynamic and static loads.The research content and results of this article are as follows:(1)A thermo-hydro-mechanical coupled three-dimensional large deformation thaw consolidation model was established to study the thermal stability and thaw consolidation settlement deformation characteristics of ordinary subgrade at the warm and ice-rich permafrost(WIRP)site of Beiluhe,QTR.The influence mechanism of embankment height,ice-rich permafrost layer thickness,initial average annual ground temperature and ballast filling on subgrade settlement was also discussed.The research results are as follows:Permafrost degradation on the Qinghai-Tibet Plateau,exacerbating the thaw consolidation and creep of ice-rich permafrost.The settlement rate and subgrade settlement increase with the increase in embankment height,ice-rich permafrost layer thickness,and initial MAGT.Railway maintenance methods,such as ballast filling and track lifting,increase the weight of the subgrade and aggravate the settlement of WIRP subgrade.(2)Based on the ABAQUS finite element software and the train-track coupling dynamic model ZL-TNTLM program,the train-track-subgrade-site coupling nonlinear dynamic model is established.The model is divided into two subsystems: upper train-track and lower sleeper-ballast-subgrade-site finite element model.First,based on the upper subsystem,study the vibration load characteristics of differently organized Qinghai-Tibet trains under track irregularity excitation,pulse excitation,harmonic excitation,and dynamic excitation;then the vibration load acts on the lower subsystem to study the train load The dynamic pressure stress distribution and propagation characteristics of subgrade in permafrost are analyzed under different seasons,track bed stiffness,train type,train axle load,and train speed.The results show that under different excitations,the influencing factors of sleeper force are different and are closely related to axle load and vehicle speed;under-rail vibration reduction and under-rail cushion stiffness,damping and track bed thickness,diffusion angle,stiffness,density,etc.Closely related;the dynamic pressure stress of the subgrade bed surface is closely related to seasonal changes,track bed stiffness,train speed,train axle load and train type,and is affected by the depth of the subgrade,the bogie action rate,the sleeper action rate and the axle load action rate;comparative study Under these influencing factors,the influence on the dynamic response of subgrade vibration under dynamic deviating stress.(3)Based on the CREEP subroutine of the viscoelastic-plastic constitutive relationship,considering the influence of vehicle types and seasonal changes,a calculation model for cumulative vibration settlement of the Qinghai-Tibet Railway’s permafrost subgrade under the train load is established.The cumulative vibration settlement of the subgrade during the50-year forecast period was studied,and the effects of the embankment height,the ice-rich permafrost thickness,and the initial average annual ground temperature of the site on the vibration settlement of the subgrade were discussed.Finally,based on the previous calculations,the permanent deformation settlement of the permafrost railway subgrade under dynamic and static loads is predicted.The results show that the cumulative vibration settlement of frozen soil subgrade mainly occurs in the shallow layer of ice-rich soil.The cumulative vibration settlement of the top of the subgrade and permafrost increases with the initial annual average ground temperature of the site,and with the height of the embankment.However,the cumulative permanent deformation settlement of the top surface of the subgrade and the permafrost layer increases with the height of the embankment,the thickness of the ice-rich soil layer and the initial annual average ground temperature of the site.Based on this,the prediction formulas for the cumulative permanent deformation settlement on the top surface of the subgrade and the cumulative permanent deformation settlement in the permafrost are established,and suggestions for the design and maintenance measures of the railway subgrade at warm and ice–rich permafrost ground sites are proposed.This article is devoted to providing scientific basis for the treatment of subgrade defects on permafrost regions,perfecting the design theory of the warm and ice-rich permafrost subgrade of QTR under train load,and providing guidance for engineering practice.