| In China,modern trams are mainly used in subways or light rails that are not covered by first-and second-tier cities.They are an effective supplement to public transportation and have been opened in many cities such as Shanghai and Nanjing.Many cities such as Chengdu,Wuhan,etc.also started the planning of modern trams.However,most of the under-trunk structures of modern trams are currently designed with reference to the national railway technical standards.The few local codes that have been promulgated also draw on the relevant achievements and requirements of the national railways,and have not yet formed a technical specification or standard that closely combines the characteristics of the trams.There are significant differences between the trams in terms of axle load,axle load distribution,driving speed,and operating environment.The relevant standards for copying the national railway may not only cause technical incompatibility,but may also cause economic irrationality.Therefore,it is of great engineering significance to carry out relevant technical research on the foundation structure of modern trams,and it is also an urgent need for the development of trams.Since most of the trams are operated in urban areas,it is necessary to share the right to control with municipal roads while strictly controlling environmental noise.The embedded track structure formed by the polymer material with continuous support and elastic locking function in the overall concrete track bed,which can improve the contact relationship between the wheel and the rail,has been widely used in modern trams.The trolley axle is transmitted and diffused to the under-floor foundation through the embedded track structure.As the operation time increases,problems such as the stiffness change and the deformation increase of the foundation may be caused,which may adversely affect the ride comfort of the upper rail structure,thereby causing driving comfort is reduced,and in severe cases,it may cause a driving accident.Therefore,it is of great significance to improve the design and construction technology of modern trams by accurately grasping the load characteristics of the trams under the embedded track structure and exploring the underlying infrastructure.To this end,the paper is based on the two typical embedded ballastless track structures involved in the Technical Regulations for Embedded Continuous Support Ballastless Tracks in Sichuan Province(reported draft),using the Winkler spring foundation superimposed beam model and semi-infinite theory of space elasticity discusses the distribution of load diffusion to the longitudinal direction of the foundation plane under biaxial loading,and further discusses the distribution pattern of the load distribution along the top surface of the foundation and its load along the depth.For the embedded slab ballastless track structure and according to the vehicle-orbit coupling,dynamics theory was constructed to construct a vertical coupling dynamics analysis model of "tram-electric train-embedded slab track-underline foundation",and the top surface of the under-line foundation is analyzed from 20km/h?100km/h.Based on the dynamic stress characteristics,the mathematical influence correlation method was used to discuss the value of the dynamic influence coefficient.On the basis of grasping the load acting characteristics of the underlying foundation of the tram line,and referring to the railway bed structure design theory,the technical scheme of the underfill replacement of the tram line was discussed.The conclusions of the paper analysis and research are as follows:1)By analyzing two embedded track structures,namely cast-in-place embedded track structure and plate-type embedded track structure,the longitudinal stress of the base surface was analyzed by Winkler spring foundation composite beam model theory,and the two track modes were respectively divided into 4 layer,3 layer calculations,and consider local stratification.The basic surface load characteristics were analyzed and studied,and then the basic load conditions were analyzed and calculated.Combined with the load distribution model of ballastless track,the Boussinesq solution was used to analyze the variation law of vehicle load underneath the foundation depth.The analysis showed that the vertical distribution of embedded and cast-in-place rail loads was not much different when the local base coefficient was 40KPa/m,which was 10.21?14.74m and 11.32?14.41m respectively.The foundation coefficient had a certain influence on the change of the load length,and as the foundation coefficient increases,the change becomes smaller and smaller.When the foundation coefficient increased by 7 times,the length was reduced to 65.10%,65.%,and 66.7%,respectively,which was about 66%.The foundation coefficient had a certain influence on the change of the load length,and with the increase of the foundation coefficient,the change was smaller and smaller,and the two gradually decay along the depth and the change law was similar.2)Based on the dynamics theory and method of the vehicle-orbit coupling system,the dynamic model of the vertical coupling system tramcar-embedded track-foundation vertical coupling system was established.Under the condition that the orbital excitation applies the uneven orbit spectrum,that is,the US 6-level spectrum,the vehicle running speed was 80km/h,the established model analysis showed that the fundamental surface dynamic action guarantee rate was 97.72%(2 times mean square error).Corresponding conditions,the corresponding fundamental surface dynamic coefficient was the limit dynamic coefficient was?dj=1.25.When the base surface dynamic force guarantee rate was 65.54%(0.4 times mean square error),the corresponding state was the normal power coefficient ?dc=1.07.When the vehicle speed was 80km/h,the ultimate dynamic coefficient ?dj and the frequently encountered dynamic coefficient ?dc were compared with the general knowledge.3)Taking the plate-type embedded track as an example,the maximum value of each statistical point is a statistical parameter.Considering different speeds,the calculation speed of the vehicle in the simulation was 20 km/h,40 km/h,60 km/h,80 km/h,100 km/h analysis and calculation.As the speed increased,the basic dynamic stress increased gradually from 1.01 to 1.20.The basic limit dynamic stress also gradually increased from 1.09 to 1.38.The constant dynamic stress and the ultimate dynamic stress gradually decrease with the increase of the foundation depth and the attenuation speed decreases gradually.4)Under the dynamic action of the modern tram at 80km/h,that is,when the constant power coefficient was 1.07,the foundation thickness was determined according to the stress ratio method,and the range of the vehicle load is determined according to the ratio of the base load dynamic and static stress to 0.1.After analysis and calculation,the vehicle load area of the plate-type embedded track structure and the cast-in-place track structure was 2.05m and 1.85m,respectively.5)Analysis of two modern tram tracks,ie,plate-type embedded tracks and cast-in-place embedded tracks.Under the condition that the foundation coefficient was 40 MPa/m,the design technology of the replacement thickness of the embedded track foundation of modern tram was discussed.It suggested that the thickness of the embedded track of the embedded track of modern tram would be two kinds of embedded track replacement.The thickness was 1.25 m and 0.91 m,respectively.6)Based on the conclusion of the replacement of the foundation under different bearing capacity conditions,the embedded track replacement thickness scheme for the tram was obtained.For slab rails,the local base bearing capacity was 50 kPa,60 kPa,70 kPa,80 kPa,90 kPa,100 kPa,110 kPa,120 kPa,125 kPa,130 kPa,135 kPa,140 kPa,and the thickness of the replacement.They ware:3.84m,3.18m,2.66m,2.25m,1.89m,1.57m,1.29m,1.05m,0.85m,0.65m,0.39m,0m.For cast-in-place rails,the local base bearing capacity was 50 kPa,60 kPa,70 kPa,80 kPa,90 kPa,100 kPa,110 kPa,120 kPa,125 kPa,and the replacement thicknesses ware:3.51 m,2.89 m,2.39 m,1.96 m,1.61 m,1.28 m,0.95 m,0.51 m,0 m.Therefore,as the bearing capacity of the foundation increased,the thickness of the replacement layer gradually decreased until it was not replaced. |
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