Dynamic Interaction Of Slab Track Structure-subgrade System And Accumulative Settlement In High-speed Railways

Train loads induced dynamic responses of the track structure and subgrade and the accumulative settlement of subgrade soil in high-speed railways is a very complicated issue of the dynamic interaction between track structure and subgrade. Settlement will accumulate under the long-term cyclic loading of trains. Vibrations caused by high-speed trains will not only jeopardize the performance of railway itself, but also disturb the environment. Unfortunately, relative studies on the theory and the testing of the dynamic behaviors of high-speed railways are limited around the world. Since the theory, testing and the operation experience of high-speed railways are poor, the design code of high-speed railways is mainly based on the experience of traditional ballasted railways.According to the characteristics of high-speed train loads and the structure of ballastless slab track-subgrade system, this paper focuses on the dynamic interaction between the track structure and subgrade and the accumulative settlement based on the full-scale model testing, numerical analysis and theoretical calculation. The main contents and conclusions are listed as follows:(1) A full-scale experimental platform of the slab track-subgrade system, consisting of the sequential loading system and the full-scale slab track-subgrade system(15m×5m×6m high), was established to explore dynamic performance and long-term durability of the geotechnical infrastructure in ballastless high-speed railways. The feasibility and reliability of the full-scale model testing were verified based on numerical methods and field measurements. The full-scale experimental platform had the ability of simulation of30Hz cyclic loading and360km/h train moving loading.(2) Based on the vehicle-rail-subgrade coupled analysis model, parametric studies showed that the load sharing ratio of fasteners is highly dependent of the stiffness of fasteners. With high fastener stiffness, more loads were supported by the middle fastener under the loads, while the load sharing ratios of fasteners on both sides decreased. Results also showed that the stiffness of CAM layer and subgrade had relatively little influence on the load sharing ratio of fasteners, and the influence of damping ratios of the fasteners, CAM layer and subgrade could be neglected.(3) The test on the load sharing of fasteners under rails showed that a train wheel axle load acting on the rail surface was shared by five pairs of neighboring fasteners, and the load sharing ratios of these five pairs of fasteners were approximately9%,24%,34%,24%, and9%, respectively. Based on the validation with the test results, the verified vehicle-rail-subgrade coupled analysis model was used to calculate the fastener loads. In addition, the load sharing ratios of each fastener under a train wheel axle load were found to approximately follow a Gaussian function distribution. Then a formula was proposed to determine the load sharing ratio of each fastener based on the test results. For smooth track, this formula can describe the distributions of equivalent train loads on fasteners for a train wheelset, a train bogie, or a whole train.(4) The dominant frequencies of the dynamic responses were controlled by the train parameters. The dynamic responses of the displacement and the soil stresses were dominated by the train carriage, while the vibration velocities of the track structure were dominated by the two bogies in adjacent train carriages.(5) The model testing showed that vibration intensities of the track structure and underlying soils presented a monotonically ascending tendency with train speeds for a well-built ballastless track. The roadbed effectively reduced vibrations transmitted from slab track to underlying soils. Dynamic soil stress measured at the roadbed surface in the ballastless railway varied in the range of13-20kPa, which was only about a quarter of that in ballasted tracks (50～100kPa). However, the attenuation rate of dynamic soil stress of a ballastless track was much slower than that of a ballasted track. Test results of track vibrations and dynamic stresses inside underlying soils (roadbed, subgrade and subsoil) were found strongly dependent on train speed when the speed was within150～300km/h. The amplification factor of softer subsoil was much larger than that of the shallow roadbed. An empirical formula was proposed to determine the dynamic soil stress beneath the ballastless track, taking into account the effect of train speed and soil depth.(6) Water level rising resulted in the reduction of the first natural frequency, from16Hz of the normal subgrade, to15Hz of the saturated subsoil,12Hz of the saturated subgrade, and15.5Hz when the water level fell back to the subsoil surface. The magnification factor of track displacement remained around1.0until the loading frequency reached about50%of the first resonant frequency for all the four cases. The dynamic responses of the displacement at the track surface can be considered static when the loading frequency was lower than50%of the first resonant frequency. However, when the loading frequency was higher than50%of the first resonant frequency, higher water level resulted in larger magnifications of the displacement. The transverse distribution of contact pressure under the track structure changed significantly with the variation of water level. The maximum contact pressure moved from the edges toward the track center with the increase in water level. Moreover, water level rising lead to faster development of accumulative settlement. For saturated silty soil, the excess pore water pressure increased larger than that in the subgrade due to its lower permeability. Therefore, the development of accumulative settlement in silty soil was related to the excess pore water pressure caused by train loads. While for the subgrade with higher permeability, the development of accumulative settlement was mainly attributed to the loss of effective confining pressure.(7) The effective stress path and the strength failure criterion in the p-q plane were proposed for both saturated and unsaturated soils. Considering the residual stress caused by the deformation incompatibility when the load was removed, a computational algorithm was programmed to determine the accumulative settlement of the field soil. The stress components in six directions and the corresponding stress path can be obtained with the help of the model testing and the numerical model. Then the accumulative settlement for different layers in the full-scale physical model were calculated and compared with the experimental results.(8) The control criterions of high-speed railway subgrade, i.e., the elastic deformation, dynamic stability of the subsoil soil and the accumulative settlement, were evaluated based on the results of the full-scale model testing. For the normal subgrade constructed according to the design code, all of these three control criterions can be satisfied. For saturated subsoil, the elastic deformation of the subgrade and the dynamic stability of the subsoil soil also satisfied the control criterions, while the accumulative settlement caused by the saturated subsoil was beyond the control criterion. For the saturated subgrade, although the elastic deformation remained small compared to the control criterion, the subgrade was not stable anymore, and very large accumulative settlement developed in a fast rate. Therefore, special drainage system should be designed and constructed to let water drain out.