Dynamic Response And Cumulative Settlement Of Foundation In Subgrade-Tunnel Transition Section On Heavy Haul Railway In Loess Region

Recent advances in machinery,material and stiffening measures have facilitated rapid growth in railway wagon axle load and velocity.As a result,inevitably,dynamic effect on subgrade,foundation and tunnel is more significant than high-speed trains and traditional trains.The subgrade-tunnel transition section subjected to above dynamic effect generate greater differential dynamic displacement,differential dynamic stress and differential cumulative settlement in loess region.Firstly,the work status and service performance deteriorate.Secondly,differential cumulative settlement of foundation structure will result in significant track irregularity,which can induce greater wheel-rail interaction force.And then,the differential settlement increases sharply,accelerating the failure of subgrade and impelling railroad train derail.In summary,the differential dynamic response and differential settlement is directly related to the operation safety and other key technical problems,such as train speed and axle load raise,so it has already become an important major aspect of foundation designing for subgrade-tunnel transition section.Therefore,based on "key construction technology of heavy hual railway" program,and "Detection and reinforcement technology of heavy hual railway bridge and subgrade"(863 Program),this present thesis investigates the static and dynamic performance of unsaturated loess,the vibration response of underlying natural and composite foundation,and accumulative settlement generated by perennial train dynamic load.The technological measures including laboratory experiment,in-situ monitoring,theoretical analysis and numerical simulation are performed to implement the above investigation,taking into account the background of dynamic behavior and accumulative settlement of subgrade-tunnel transition section on the heavy haul lines(HHL)in loess region.The main research work and achievements are concluded in details as follows.(1)Based on static triaxial test,the effect of moisture capacity,compacting coefficient and ambient pressure on the deformation and strength performance of unsaturated loess is examined,and then emphasis is given to the initial elastic modulus.Specifically,a practical calculation model for failure strength of loess is built.Based on dynamic triaxial test,the influence of moisture capacity and ambient pressure on dynamic deformation of natural and disturbed loess is analyzed,and a further research of the dynamic deformation modulus and damping ratiounder various moisture capacity and ambient pressure is conducted.Through Cyclic-Dynamic triaxial tests,the physical deformation features of loess is studied,and a special attention is paid to the effect of dynamic stress,dry density and moisture capacity on physical deformation.Specifically,a relation model between accumulative deformation and parameters such as loading cycles,dynamic stress and static failure strength is established,taking the proportion of dynamic stress in static failure strength as normalization factor.(2)In view of the characteristics of unloading-loading in subgrade-tunnel transition section construction,a nonlinear elasticity constitutive model is developed for construction simulation,and an extra introduction about model parameters and UMAT is presented.An integral subgrade-tunnel transition section FEM for construction is established,and considering the stress field transfer from construction model to dynamic model,the track-subgrade-tunnel-composite foundation dynamic system model is also built,in both of which,the pile-soil interaction based on Coulomb friction theory is taken into consideration.The rationality and accuracy of dynamic model is guaranteed by comparing the calculation results with in-situ monitoring data.(3)The construction process of excavation-supporting-back filling is simulated within the construction model,and the influence of nonlinear elasticity parameters on transverse distribution of ground settlement is examined.Compared with the in-situ monitoring data and similar research,the rationality and accuracy of the construction model is verified.According to the stress spatial distribution,a Block-Increase model is established to describe dynamic deformation behavior of loess subjected to the vibration induced by heavy haul train loads,which can improve the computational efficiency of complex model under the premise of accuracy.(4)The spatial distribution characteristics of dynamic deformation and stress induced by train in natural foundation and composite foundation is explored,and a further study of the influence of primary parameters such as velocity,axle load on the vibration response of subgrade-tunnel transition section is carried out.(5)The transverse and longitudinal distribution of the dynamic deviatoric stress in loess foundation are investigated,and combined with the above proposed accumulative plastic strain prediction model,the accumulative settlement and differential settlement is calculated.Also,the accumulative settlement longitudinal distribution is specifically studied.And the effect of primary parameters such as velocity,axle load,pile length and pile spacing on the accumulative settlement and differential settlement is compared.