Analysis On Dynamic Performance Of A Short Bridge-tunnel Transition Subgrade With A Pile-plank Structure And Its Influence Factors

With the development of the high-speed railway to mountainous and hilly areas,the distance between bridges and tunnels is too short to set up a conventional inverted trapezoidal transition section,so it is urgent to introduce a new type of transition section.For the first time on a high-speed railway,a deep buried pile-plank structure subgrade was adopted as a bridge-tunnel transition section,so its dynamic characteristics and transition performance deserved further study.Based on the engineering background of the bridge-tunnel transition section with a deep buried pile-plank structure,the dynamic performance and its influence factors of the bridge-tunnel transition zone with a pile-plank structure transition section were studied by field tests and numerical analysis.The main work and conclusions are as follows:(1)The vertical vibration acceleration and dynamic displacement of five test sections on tunnel,transition section of pile-plank structure and abutment excited by the CRH380A type train were analyzed in time domain.The distribution of dynamic response in the transition zone along the longitudinal direction of the line under train loads was obtained.The effects of three different driving conditions of the line and the adjacent line,driving directions and axle loads on dynamic response in the transition zone were compared and analyzed,show that the stiffness of the structure where the test section is located is the main factor affecting the dynamic response of the subgrade surface.(2)According to engineering conditions of the test section,a numerical analysis model which considers vehicle-track-transition zone coupling vibration was established by the ABAQUS.The vibration acceleration and velocity time history curves of the measuring points on the inside of the base slab of the three test sections arranged on the transition section were extracted,find that their distribution law is consistent with the measured curves,and the effect of the wheelset is clear.Comparing the vibration acceleration and velocity amplitudes of the measuring points on the inside of the base slab on five test sections with the field measured results,it shows that the relative error of the amplitudes is basically less than 30%.Therefore,the rationality of the modeling method and calculation parameters in this paper was verified.(3)A inverted trapezoidal transition section was set between the bridge and tunnel with reference to the Code for Design of High speed Railway,and a corresponding numerical model was established.Calculation results show that the maximum vibration acceleration of the base slab in the transition zone is 10.970m/s~2,which is much more than the specification limits of 5.0m/s~2,indicating that the inverted trapezoidal transition section is inappropriate when the distance between the bridge and tunnel is too short to satisfy the length of the transition section regulated by the code.(4)Based on the univariant analysis,the parameter sensitivity analysis of design parameters of the deep buried pile-plank structure(pile diameter,pile spacing and plank thickness)was carried out by the verified modeling method and calculation parameters.Results show that within the variation range of design parameters set in this paper,the longitudinal distribution of the vertical vibration acceleration amplitudes of the vehicle,rail,track slab and base slab does not change,and the distribution range of the dynamic stress attenuation rate in the subgrade is 68.45%～80.38%.While the pile diameter decreases,or the pile spacing increases,or the plank thickness decreases,the change rate of the maximum vertical dynamic displacement on the rail in the transition zone increases slightly.Except that the pile diameter is 0.5m,the maximum dynamic displacement of the subgrade surface on the S2 section is 0.224mm,which exceeds the limit of0.22mm in the Standard for Repair of High speed Railway Subgrade,the dynamic displacement of the subgrade surface under other conditions conforms to the requirements.(5)The dynamic adaptability of a deep buried,shallow buried or non buried pile-plank structure as a bridge-tunnel transition section was discussed by numerical analysis method.By comparing the change rate of the maximum vertical dynamic displacement on the rail in the transition zone,the vertical vibration acceleration amplitude on the inside of the base slab and the vertical dynamic stress amplitude of the surface of soil or fillers under the bearing plank in the middle of the span,it shows that the dynamic adaptability of a deep buried or non buried pile-plank structure is better than that of a shallow buried pile-plank structure.