Research On The Design Theory And Method For Ballastless Track On Passenger Dedicated Line

With the large scale construction of Passenger Dedicated Lines(PDLs), the ballastless track has got rapid development and extensive application. However, the design methods of different ballastless tracks were different, a uniform design theory for ballastless track did not exist. Based on the systematical summarization of ballastless track and corresponding projects home and aboard, a uniform ballastless track theoretical system is established through academic and experimental research in this thesis. The theory was successfully used in several projects, such as the ballastless track synthetically test section on Suining-Chongqing line(Suiyu line), the ballastless track test section on Wuhan-Guangzhou PDL, as well as the design of bi-block ballastless track and slab track for 250km/h reference blueprint design. The research work and main conclusion were divided into following areas:(1)Develop the stress calculation method of ballastless track under train loadConsidering the structure character of ballastless track, the stress under the train load should be calculated using the beam-shell model on elastic foundation. Rails are simulated by Euler beam. Bearing layers are simulated by elastic thin shell. The elasticity of the fastener, CA motar and subgrade are simulated by spring combination. The model is actualized using Finite Element Method. The plain concrete or hydraulic bounded layer should be modeled by using their reduced elastic modulus to reflect the influence to their bending stiffness when cracked. The greater the elastic modulus, or the larger the thickness of the supporting layer, or the stronger the interlaminar connection status, or the less the interval between cracks, the smaller the reduced elastic modulus of the cracked support layer will be resulted in. The calculation model and parameters were verified by the comparison between different models and field test data on Suiyu line.A parameter study about slab track and bi-block ballastless track was carried out using the beam-shell model. The results show the thickness of the track slab and base slab have great influence to the stress in bearing layer. The thickness of the track slab should be reduced and that of the base slab should be increased in structural optimization. The width of the base slab should be ascertained according to the 45°load dispersion angle. The length of the base slab should be extended and dowels should be set at the base slab ends to improve the subgrade mechanical condition. In bi-block ballastless track, the stress level in combined structure is less than that in separated structure. A smaller track slab width should be used when the embedded width constraint of the bi-block sleeper is satisfied.(2)Establish the thermal stress calculation method of ballastless trackThe thermal stress, cracks interval and crack width calculation formula were derivated according to the crack develop character of continuous ballastless track. A parameter study was carried out using these formulas. The crack type should be controlled to unstable cracks to limit the crack width in continuous ballastless track. The maximum stress in rebar is controlled by concrete tensile strength and reinforcement ratio. The maximum crack width is related to the bond strength between rebar and concrete, the concrete tensile strength and reinforcement ratio. The maximum rebar stress and crack width have no relation to the temperature drop range. The reinforcement ratio should be larger than 0.73% to limit the crack width within 0.5mm when using C40 concrete. The diameter of the rebar should be selected between 18mm and 25mm. The reinforcement ratio should be enhanced when using high-grade concrete in track slab or low-grade concrete in support layer or coated rebar or slip form construction method.Considering the temperature gradient value in ballastless track aboard, pavement and the test data on Suiyu Line comprehensively, the maximum temperature gradient value of our ballastless track was proposed. Taking slab track for example, the warp stress was analyzed under different constraint conditions and elastic modulus of CA motar. As a conclusion the warp stress of the ballastless track can be calculated using the infinite plate formula.(3)Study the foundation deformation influence to the ballastless trackThe subgrade uneven deformation and the deflection of the bridge are assumed to a sine or half-wave sine curve. The additional moment of the ballastless track bearing layer was calculated using the beam-shell finite element model considering the foundation deformation, a simplified rigid foundation and an elastic foundation model separately. As a conclusion the additional moment due to sine type foundation deformation can be calculated using rigid foundation method. The uneven deformation limit should be raised to guarantee no suspending under self-weight in short high stiffness track slab.The displacements at bridge end have great influence to the fastening system, especially for step height, bridge end rotation and pad stiffness. Considering the influence of the train load and step, a limit for bridge end rotation was proposed from the viewpoint of fastening system protection. The bridge end displacements have a certain effect to the uplift stability of the ballastless track, especially when using large uplift resistance fastening system. The interlaminar connection between ballastless track and bridge should be strengthened at bridge end.(4) Establish the design theory and method of ballastless track preliminarilyThe ballastless track design is divided into two parts: the function design and structure design. The track components and construction method are determined in the function design to satisfy the high evenness and stability requirements. The reinforcement in the bearing layer is determined to satisfy the strength and durability requirements in the structure design according to external loads and their interaction. The external loads include train loads, influences of temperature and the foundation deformations.A function analysis about ballastless track and their main components was carried out based on the summarization and classification of ballastless tracks home and aboard. The high evenness and stability of the ballastless track on PDLs is guaranteed by the function design conception. The ballastless track structure should always be in elastic stage because of its 60 years life span requirement, so the structure design method should be based on allowable stress method. The reinforcement concrete structure will crack due to load action and the bending stiffness will decrease which result in the change of the moment. A structure coefficient was introduced to reflect its influence. Taking bi-block ballastless track for example, the structure designs of ballastless track with single slabs and continuous slab on subgrade and bridge were carried out. The structure design examples show that the reinforcement for ballastless track with single slabs is controlled by the moment under train load, the reinforcement for ballastless track with continuous slab is controlled by the temperature drop and concrete shrinkage.(5)Establish the simulation model of the ballastless track falling wheelset experiment to estimate and evaluate the dynamic character of the ballastless trackBased on the beam-shell model on elastic foundation, a ballastless track falling wheelset experiment simulation model was established. The dynamic character of the slab track was analyzed. The results show the fastener stiffness has great influence to the acceleration of each component. Lower fastener stiffness should be used to reduce the system vibration level. The elastic modulus of the CA motar has great influence to the track slab and base slab acceleration. The acceleration and the frequency of the base slab are significant lower than that of the support layer in bi-block ballastless track, which reflect that CA motar can isolate vibration to a certain degree. The subgrade stiffness mainly influences the acceleration of the base slab, but the degree is relatively small. The thickness of the track slab and the base slab should be about 0.2m and 0.3m to reduce the system vibration level.