Research On Track Dynamic Characteristics And Optimal Design Of Subgrade-bridge Transition Zone In High-speed Railway

For a long time, bridge-subgrade transition zoneis a very tough difficult point to the railway traffic, which has been plaguing the railway designers and maintenance crews. Early due to low vehicle speed, the smoothness requirement of transition zone is lower. Meanwhile, ballasted track is used in most of the lines, thus in bridge-subgrade transition zone repair and maintenance work have been done after the problems arose. There are rarely any design measures to bridge-subgradetransition zone.With the use of various ballastless tracks and increase of vehicle speed, the settlement and stiffness changes of transition zone are more and more serious. A lot of elevated structures bring frequent transition of bridge and subgrade. The alternate use of ballasted track and ballastless track causes the superposition problem of ballasted track and ballastless track. The anchorage system behind abutment of CRTS II slab track brings a certain degree of ambiguity to transition zone stiffness design, and transition zone stiffness change becomes more complex. The transition zone has become one of the problems to be solved in the design of high-speed railway.This thesis focuses on the high-speedbridge-subgrade transition zone, railway ballasted-ballastless transition zone, and new CRTS II slab track anchorage system transition zone to establish a detailed vehicle-track spatial dynamic model. Comprehensive analysis is carried out on dynamic impact response of transition zone, and comprehensive evaluations on different processing measures of transition zone are given, which provide theoretical support for dynamic impact. Design suggestions of different types of transition zone are put forward. The main research contents are as follows:(1) A summary is made on different types of transition zone and application status of measures for different stiffness transition.The types of domestic high-speed railway transition zone and track transition zone are summarized. The bridge-subgrade transition zone and ballasted-ballastless transition zone are made as typical research object. The disease situations, different transition zone processing measures and processing effects are summed up. The current theoretical research is concluded, and the deficiency of study is analyzed. Aiming at the current study, the research contents can be drafted.(2)Complete and detailed vehicle-track-transition zone coupling systematic dynamic analysis models for high speed railway are established.According to three dimensional finite element method, meanwhile, through analysis and search of materials and structural parameters, reliable dynamic analysis models for transition zone are established. The simulation models contain vehicle-ballasted/ballastless track transition zone-bridge model, vehicle-ballasted/ballastless track transition zone-subgrade model and vehicle-ballastless track-bridge/subgrade transition zone model.The models with high degree of coupling and comprehensive factors, get reliable calculation results.(3)Analysis is made on dynamic response of different measures for ballasted-ballastless track transition zone in high-speed railway.The operating safety, travelling comfort and track vibration indexes are assessed when the vehicle is running through transition zone at different speed. The dynamic indexes are analyzed and contrasted on ballasted-ballastless track transition zone(on bridge and subgrade) and bridge-subgrade junction. The effects of auxiliary rail, base plate elongation, sleeper elongation, sleeper widening and ballast bonding are assessed. Quantitative assessment is laid on comprehensive processing measures for ballasted-ballastless track transition zone. Based on this, the concept of dislocation classification of ballasted-ballastless track transition zone is put forward, which provides theoretic reference for design of ballasted-ballastless track transition zone in high-speed railway.(4)The impact effects caused by stiffness difference and differential settlement of bridge-subgrade transition zone are analyzed respectively.Assessment work is carried out on impact effects caused by stiffness difference and differential settlement of bridge-subgrade transition zone under different operation speed. It is indicated that the uneven settlement of the bridge-subgrade is the main factor of impact. Aiming at the uneven settlement behind abutment, the differential settlement values are selected to study the limit value of differential settlement. It is pointed out that the differential settlement value should be within20mm in the range of20m.(5)The impact effects of transition zone caused by end beam behind abutment of CRTS I slab track and anchorage system behind abutment of CRTS II slab track are analyzed.The dynamic response is analyzed and compared on end beam and anchorage system of two types of ballastless tracks when the vehicle is in operation. From the analysis results, it is indicated that the impact is slight when the end beam is used, but a certain extent of impact is caused when anchorage system is used because of large stiffness at the position of the primary pile tip. As a result, twice impacts occur behind the abutment and at the primary pile tip, and the extent of impacts is similar.(6)Aiming at the twice impacts to the invertedT-shaped and Π-shaped anchorage system, reasonable approaches are researched to eliminate the twice impacts of primary pile tip.Through the analysis of impact extent of twice impacts behind the primary pile tip, assessment is done on twice impacts of anchorage system when the subgrade filling is replaced and approach plate is laid behind the primary pile tip. The advantage of subgrade replacement behind the primary pile tip is pointed out, and the reasonable thickness of approach plate behind the primary pile tip is analyzed. It is indicated that the thickness of approach plate behind the primary pile tip should be within300mm.(7) In the transition zone of silty clay subgrade and bridge, invertedT-shaped anchorage system is used. The long-term cumulative plastic deformation is calculated under train cyclic loading, and the dynamic characteristic of track structure above the subgrade is also analyzed.The three dimensionaldynamic interaction model of track and subgrade is builtto obtain the distribution of dynamic deviator stress. The existing prediction models of subgrade cumulative plastic deformation under train cyclic loading are summarized. Using the method LiD. Q. and Selig put forward, the corresponding dynamic deviator stress, static strength and other parameters are determined. Analysis work is carried out on subgrade cumulative plastic deformation in vertical, lateral and longitudinal direction. At the same time, the deformation of subgrade bed, ballastless track and rail are analyzed.