| The high-speed railway requires the track structure with high smoothness and stablity. Controlling the post-construction settlement of subgrade is the necessary prerequiaite that keeps high smoothness and stablity of track, which is also the key factor of the high-speed railway maintaining good long-term service performace. The cumulative plastic deformation under numerous cyclic loading is an important component of the post-construction settlement for the upper structure of subgrade. Starting with the controlling of the cumulative plastic deformation, the design method of subgrade structure was found which not only is the development and improvement of the existing subgrade structure design theories, it would be significant for assessing the service perfprmance of high-speed railway subgrade which has constructed and put into sevice in it’s design working life, about100years.Summing up the results of research at home and aboard, the paper circled around to construct the subgrade structure design method based on the long-term cumulative defornation evolution state controlling, the characteristics of subgrade bearing train load and the controlling parameters of subgrade filling and the indoor large scale dynamic model testing of subgrade and the test technique of the coefficent of subgrade reaction were studied. The main work and conclusions are as follows:1. Analysis on the characteristics of the subgrade bearing train loadAs ballastless track structure has bigger stiffness, better Stability and better capacity for diffusing load, the two axle loads between the bogie wheelbase have obvious superimposed effect. The dynamic stress time history curve which is measured on the subgrade surface shows that one loading and unloading process of the dynamic stress in ballastless track subgrade is completed by two axle loads of the same bogie. While for ballasted track, the process is completed by single axle load.The relation between the changing regularity of dynamic stress with time and the movement of the train axle load is established which based on the the dynamic stress time history curve which is measured on the subgrade surface of ballastless track. And the calculation method of longitudinal impact range of train load which diffuses to the subgrade surface through the track structure is determined b. What’s more, by using that calculation method, the calculated longitudinal impact range is between9m and10m, which is consistent with the theoretical calculation result of Winkler spring foundation composite beam theory. Based on these theories and methods, the load model of ballastless track and the load distribution can be simplified to be trapezoidal distribution in the longitudinal of the lines and to be uniform distribution in the transverse direction, is proposed. In the existing design, the load model of ballasted track supposes the longitudinal impact range of train load is2.8m. Here, Sleeper spacing, thickness of ballast and other factors are not concerned. Aimed at the above shortcoming and based on the assumption that single wheel load is shared by the5sleeper and diffusion model of the train load is in broken line form, the relationship formula among the longitudinal impact range of dynamic stress on the subgrade surface in ballasted track, the distance of sleepers and ballast thickness is established. So, the ways to determine the impact range of dynamic stress on the subgrade surface in ballasted track is furture perfected. By establishing the equilibrium relationship between the train axle load and dynamic stress on the subgrade surface, analysing the comprehensive coefficient values of dynamic effect and considering the correction of transversely uneven effect of dynamic stress on the subgrade surface in ballasted track, the calculation method of dynamic stress on the subgrade surface and the load level for checking load-bearing capacity and durability in the roadbed design are got.The amplitude-frequency characteristic analysis for the dynamic stress time history curve,which is measured on the subgrade surface of ballastless track, shows that the good capacity for diffusing train load in ballastless track makes the frequency response characteristics, which is corresponding to the bogie wheel base, not obvious. The frequency of subgrade under dynamic load is sharply reduced. And the main frequencies are1,2,3,4times of the frequency value which is calculated by using the train length as the basic disturbance wavelength.2. The status features of the long-term cumulative deformation of the geotechnical filling under the cyclic loading and the study of the cyclic loading research of the filled earth unitThe analysis of the deformation law of the geotechnical filling under cyclic loading and the study of the development status of the cumulate plastic deformation has obtained results. Based on the existing research results, with the start point that whether the cumulate plastic deformation converge or not and whether the speed of convergence is fast or slow, the paper put forward that there are four status of the cumulate plastic deformation development. They are fast settling, long-term settling, long-term breaking and fast breaking. What’s more, the paper establish the criterion based on the development of the cumulate plastic deformation speed, which is expressed by the negative power function:f(N)=CN-P. WhenP≥2, it’s called the fast settling status; when1<P<2, it’s called the long-term settling status; whenO<P≤1, it’s called the long-term settling; whenP≤0, it’s called the fast broken status.It constructs the model of the graded broken stone with the different compacting factor of0.9,0.95,1.0and conducts the cyclic loading experiment which can set out the ground coefficient K30with the rigidity loading board. Then, it obtains the three threshold value of the four status of the graded broken stone which called fast settling, long-term settling, fast broken and long-term broken based on the "p-convergence". The experiment indicates that the K30value of the graded broken stone of the different compacting factor of0.9,0.95,1.0is137MPa/m,214MPa/m,380MPa/m respectively. The transition threshold value ε11from the fast settling status to the long-term settling is140-202μs, the mean value ε11is64με. The transition threshold value ε12from the long-term settling status to the long-term broken is443-650με, the mean value ε12is526με. The transition threshold value ε13from the long-term settling status to the long-term broken is1339-2172με, the mean value ε13is1734με. Moreover, it establishes relationship between the threshold value of the cyclic strain, dynamic bearing capacity and the K30value.3. The design method for subgrade of the high-speed railway based on the deformationstate controlling and the key technical indexsBased on established load pattern of bed structure design and bed filing accumulated deformation evolution status control parameters, In order to satisfy different track structures on bed deformation state control requirements, Using cyclic strain as the core control indicators、K30values as the core design index, Using the principle of mechanics analysis, Constructed based on the long-term cumulative deformation evolution state control of high speed railway bed structure design method and design process, Put forward ballast track and ballastless track bedding structure of cyclic strain corresponds to the level state of filling parameter selection principle.Using long-term cumulative deformation evolution state control bed structure design method, Discussed under the condition of different thickness of bed surface, ballast track and ballastless track structure at the bottom of bed the minimum control K30values. For ballastless track structure, Design K30values for190MPa/m the surface layer of bed thickness in0.2m-1.2m, at the bottom of bed the minimum control values of K30about108MPa/m-102MPa/m; The same under the condition of bed surface ballast track structure, at the bottom of bed the minimum control values of K30is about 100MPa/m～68MPa/m. Thus it can be seen ballastless track structure need the higher at the bottom of bed fill compaction standards to meet the long-term cumulative deformation fast and stable control requirements, while ballast track structure at the bottom of bed the control standards of K30significantly influenced by the thickness of the surface layer of bed.4. The large scale dynamic model testing of subgrade in the laboratoryThe large-scale made ground mode is set up indoor, the large dynamic test is taken. This model, which is consisted of made ground and boundary constraint,could simulate the mechanical response simulation of local structure in elastic half space under cyclic loading. And the made ground has a surface of3m×3m and a thickness of2.2m,the boundary constraint is provided by brick walls and sand bag stack. The model structure is applied by a series of load level of20kPa,40kPa,60kPa,80kPa,100kPa,120kPa, giving a total of2600000times of cyclic loading. During the test, the elastic deformation,cumulative plastic deformation, dynamic stress of different depth of the model structure, as well as the lateral elasticity and the cumulative plastic deformation from the top surface of40cm will be tested.When the load level is less than60kPa, the maximum cyclic strain is166με, the cyclic strain along the depth is less than the cyclic strain threshold εt1which is corresponding to the compaction state of the made ground, being in a fast steady state; When the load level is100kPa and120kPa, the cyclic strain of the range from the model surface about50cm thickness is bigger than the strain threshold, infering that the evolution of the cumulative plastic deformation within this structure layer is in a long steady state.This conclusion is verified well by the evolution trend of real measured deformation cumulative plastic. The test results show the rationality of the subgrade design concept based on the control of the long-term evolution deformation and the feasibility of the cyclic strain as the key control index.5. The test technique of the coefficient of subgrade reaction in the compaction quality detection of subgradeBased on the model test of laboratory fill, analyze and compare the characteristics of the two mini-plate loading test through the indoor testing of deformation modulus and coefficient of foundation K30, and particularly focusing on the influence and efficiency of the loading mode upon the testing results. Results show that, the ground coefficient can better reflect the compaction quality of subgrade than the strain modulus. The time interval of loading has significant influence on the testing results. The testing will take a long time to satisfy the steady criterion of deformation1%. And the testing time will reduce about30%-50%if the deformation steady criterion changed from1%to2%, or the time interval of loading was about6min, and the K30error will within10%. On the basis of analysis experimental results of the loading mode, the load-time controlling loading method was proposed, considering load is in positive correlation with the settlement stability time, namely the time interval of loading, Δt1=2min, is equal to load grade. The method improves test efficiency nearly50%, and has advantage of easy operation and small errors, which is especially suitable for the compaction quality detection of railway subgrade being made of gravel soil. |
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