| With the development of high-speed railway, the structure of railway has broken through the traditional forms composed of track-ballast-soil subgrade. In addition to the old ballast track, the new ballastless track has appeared in recent years. The ballast track has abandoned the old design method laying the ballast layer on the soil subgrade directly, instead of using the multi-layer structure system. The increasing train axle loads and speed, and changes in railway structure not only increase substantially the dynamic effect on the system of train and line, but also make the dynamic characteristics more complex, so a higher demand is put forward for the line system. As the base of railway, the dynamic characteristics of subgrade may determine the stability and security of the overall railway system, so its dynamic response (dynamic stress, dynamic deformation and acceleration, etc.) naturally becomes a critical research.The analysis of dynamic response is particularly complex, which may be affected by such many factors as train type, axle load, operation, line structure and the lower part. Many scholars conducted a great deal of research work on the models of track and foundation, and the vibration response characteristics, and made some important results of theoretical studies. However, the subgrade models are often simplified due to its property is still not grasped, which resulting in the calculation model is imperfect, and there are greater access between theoretical results and measured datas.Testing methods, especially the field tests, have always been a direct and most effective way in the analysis of subgrade dynamic property. But the fact that the pilot test also has its own drawbacks, for example, there are difficulties to simulate the infinite boundary of subgrade and size effect in model test, and field test cost is very high, and is subject to the testing environment and test equipment. Using numerical simulation to conduct the dynamic analysis of railway system has been widely applied, this method can quickly calculate for different conditions. However, the simulation results depend entirely on the model parameters, once the parameters are unreasonable, then the results are very different from the actual situation. The defects of test and simulation could be avoided if we combine the two ways organically, such as to provide key parameters for the numerical simulation by testing methods, this is undoubtedly a new way to analyze the dynamic behavior of railway.The dynamic characteristics of subgrade filling are related to the strength, fatigue properties, accumulative deformation and dynamic stability of subgrade, and directly affect the design, use and maintenance of the high-speed railway. All existing subgrades use high standards filling to meet the requirements of high-speed railway, such as graded broken stone, and A, B group filling, etc, which led to the high project cost. In order to save the project investment and protect the environment, and expand the selection range, it is necessary to develop new filling.This paper studied the dynamic characteristics of subgrade bed through ballastless track model tests and ballast track cyclic loading tests in Dazhou-Chendu railway. Because wider distribution of the red mudstone in our country, this paper studied the dynamic characteristics and pavement performance under cyclic loading, in order to provide a reference to the subgrade design of red mudstone. The methods of dynamic simulation for the two track structures were studied respectively relying on test parameters. On this base, the systematic evaluation of the factors which may affect the dynamic response was conducted, through the introduction of orthogonal experimental design and statistical theory. Main contents and conclusions are as follows.(1) The field ballast track cyclic loading testsIn the test section of red mudstone subgrade, the field ballast track cyclic loading tests of subgrade bed were carrid out by the ZSS50equipment which can simulate the dynamic effect of different axle loads. The following conclusions can be drawn from the tests.①The dynamic stress has a saddle-shaped distribution along the subgrade cross section, and the value under rail position is maximum. Along the depth direction of subgrade, dynamic stress gradually decreases. The dynamic stress in the surface layer of subgrade bed decays faster than that in the base layer. Dynamic stress at the bottom of subgrade bed has been decayed by80%, it indicates that subgrade bed bears most of the dynamic action. The distribution law of displacement and acceleration is similar with that of dynamic stress.②After the18ton Axle load excited3million times, the total deformation of subgrade surface was3.22cm. The deformation in the first1million times developed faster, accounting for about70%of the total deformation, then the development gradually stabilized. The deformation of the base layer of subgrade bed was0.48cm, and that of the surface layer was0.12cm in the stage of18ton Axle load excitation. Axle load increased to25ton and excited3million times, the total deformation of subgrade surface was4.07cm. The deformation of the base layer of subgrade bed was1.0cm, and that of he surface layer was0.22cm. It can be seen that the train axle load has a greater impact on the subgrade deformation(2) The ballastless track dynamic model tests Large proportion of the ballasted track dynamic model test was carried out, the model was composed with the surface layer (graded broken stone) and the base layer(A, B group filling) of subgrade bed. The following conclusions can be drawn from the tests.①The dynamic stress has a saddle-shaped distribution along the subgrade cross section, and the value under rail position is maximum. Along the depth direction of subgrade, dynamic stress gradually decreases. Dynamic stress at the bottom of subgrade bed has been decayed by70%, same as the ballast track, subgrade bed bears most of the dynamic action. The distribution law of displacement and acceleration is similar with that of dynamic stress.②With the load frequency improves, both the dynamic stress, displacement and acceleration values are increasing. Acceleration is most affected by the frequency, followed by dynamic displacement, and dynamic stress is the minimum affected. The decay law of dynamic stress along the depth of subgrade is basically unchanged.③With the increase of dynamic load, the saddle-shaped distribution of the dynamic parameters in subgrade surface is more and more obvious along the subgrade cross section. However, the effect of dynamic loads to the dynamic parameters along the depth direction is very small.(3) Research on dynamic characteristics of red mudstone subgrade soilDue to red mudstone particles are easily broken, low strength, easy disintegration and softening in water, there is a big controversy for the applicability of using red mudstone to fill railway subgrade in engineering field. But red mudstone are widely distributed in China’s Southwest, Northwest, Central South and Southeast, if it can be used in the construction of high-speed railway, then a lot of engineering investment may be saved.①For the Jurassic Suining group of red mudstone, after its basic physical and mechanical properties are grasped, this paper studied the dynamic parameters (modulus, damping ratio), critical dynamic stress, the dynamic strength and accumulative deformation by British GDS triaxial testing system, and hoped to provide the necessary reference for the design of red mudstone subgrade.②Considering main factors such as soil type, soil stress state, soil physical state and the number of cyclic loads applications, this paper put forward a prediction model to calculate the accumulative strain of red-mudstone soils subjected to cyclic loads. The validity of this model was verified through the field excitation tests.③From the point of the critical dynamic stress of red mudstone, meanwhile, the characteristics should be taken into consideration is that red mudstone is softened easily in water, it concludes that compacted red mudstone can be used to fill the base layer of subgrade bed, but can not meet the requirement of the surface layer of subgrade bed, the thickness of surface layer of subgrade must be ensured not less than0.6m. At the same time, the results of accumulative deformation in the field cyclic loading tests demonstrate that the red mudstone filling is applicable in high-speed railway.(4) Dynamic simulation analysis of track-subgrade systemThe model of track-subgrade system was built in the finite element software ABAQUS, associated with the hysteretic damping of FLAC3D which can simulate soil non-linearity, the two types of track structure were simulated using reasonable boundary conditions (visco-elastic artificial boundary).①Based on the prototype of vibration test section in Dazhou-Chendu railway, the numerical simulation of ballast track-subgrade system was conducted. The force and deformation characteristics of the soil subgrade were mainly discussed in different conditions, including train speed, axle load, the stiffness and thickness of each layer of system structure. This could provide a reference for the design of ballast track subgrade and the choice of mechanical parameters.②Based on the prototype of the subgrade in Suining-Chongqing railway, the numerical simulation of ballastless track-subgrade system was conducted. The force characteristics of slab track on soil subgrade, and the dynamic response of each layer of system structure were mainly discussed in different conditions, including train speed, axle load and material properties. This could provide a reference for the design of slab track-subgrade and the choice of mechanical parameters.③The calculation results show the vertical distribution of dynamic stress in subgrade surface. For ballast track, the force range is about7sleepers spacing (about3.6m), for ballastless track, the force range is a track unit (about5m). In subgrade surface, for ballast track, dynamic stress has a saddle-shaped distribution along the subgrade cross section. For ballastless track, the distribution of dynamic stress is more uniform. The maximum value appears at the edge of the base, and there is a little change within the base. The dynamic stress of the ballast track decays faster than that of the ballastless track.(5) Comprehensive evaluation of the impact parameter to dynamic responseBased on the results mentioned above, through the introduction of orthogonal experimental design and statistical theory, this paper analyzed the dynamic response of ballast track and ballastless track respectively, dynamic response the systematic evaluation of the factors which may affect the dynamic response was conducted. The sensitivity of the response of track and subgrade to the impact of the structural dimensions and mechanical properties was evaluated systematically.①For ballast track, the main factors affecting the dynamic stress of ballast include the ballast stiffness and the stiffness of the surface layer of subgrade bed. The thickness of ballast has a significant impact on the dynamic stress in the subgrade surface.②For ballastless track, the main factors affecting the dynamic stress of CA mortar layer include the CA mortar stiffness and base thickness. Meanwhile, base thickness has a significant impact on the dynamic stress in the subgrade surface, and fastener stiffness is a major factor in the influence of rail displacement. |
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