Study On Mechanical Behavior And Vibration Reduction Performance Of Polyurethane Foam Solidified Ballast In Subway Tunnel

At present,the subway is developing towards high speed,high density,large axle load and large capacity.With the increasing impact of wheel-rail on the track structure,some diseases inevitably appear in the track structure of the subway,such as excessive vibration of ballastless track in a specific section,excessive settlement of ballasted track,frequent maintenance and so on.Therefore,polyurethane foam solidified ballast(PFSB)emerges as the times require.As a new track structure,PFSB has been applied in high-speed railway and heavy-haul railway,with good application effect.However,the research on laying PFSB in urban rail transit is still blank.It is urgent to carry out the related research on the mechanical properties and vibration reduction effect of PFSB in subway tunnel.Based on finite element method,wheel-rail coupling dynamics theory and discrete element theory,the vehicle-track-ballast-tunnel-soil coupling dynamic model and a sleeper-ballast discrete element model were established.The dynamic performance and vibration reduction effect of PFSB in subway tunnel were analyzed,which provided guidance for the design and laying of PFSB in urban rail transit.The main research work and conclusions are as follows:(1)A vehicle-track-ballast-tunnel-soil coupling model was established,and the dynamic characteristics of the common ballastless ballast bed,the common gravel ballast bed and the PFSB were analyzed under the condition of 80km/h.The results show that compared with the common ballastless ballast bed and the common gravel ballast bed,the wheel-rail force and safety index of PFSB change slightly;the displacement of track structure increases slightly;and the vibration acceleration of tunnel wall decreases obviously,which indicates that the track structure of PFSB can reduce the impact of vibration on the environment.Through comparative study of PFSB and common gravel ballast bed,it is found that the maximum noise reduction of PFSB is about 4dB,which indicates that PFSB has a certain noise reduction effect.(2)Based on the established dynamic model,the effects of polyurethane solidified area,fastener stiffness and train speed on the mechanical behaviors of track structure of PFSB were analyzed.The results show that with the increase of solidified area,wheel-rail force and track structure displacements increase slightly;derailment coefficient and wheel load reduction rate change little;and the vibration acceleration of tunnel wall decreases obviously.With the increase of fastener stiffness,wheel-rail vertical force and safety index increase gradually;the vertical displacement of rail decreases gradually;and track structure and tunnel wall vibration increase gradually.With the increase of train speed,the dynamic indexes increase correspondingly.Therefore,in the design and laying of PFSB,the influence of parameters such as polyurethane solidified area,fastener stiffness and train speed should be fully considered.(3)Based on the established refined sleeper-ballast discrete element simulation model,the settlement law and ballast vibration acceleration at different locations of the common gravel ballast,0.85m PFSB under sleeler and full-section PFSB were compared.The results show that the settlement of three types of ballast beds is increasing under cyclic loads.The settlement of common gravel ballast bed is the largest;and the settlement of full-section PFSB is the smallest.The bigger the size of PFSB,the faster the settlement of ballast bed reaches stability rate and the shorter the time to reach stability.The vibration acceleration of full-section PFSB is the smallest;and that of common gravel ballast bed is largest.The acceleration of ballast vibration is the largest;and the acceleration of ballast vibration decreases with the increase of depth.The PFSB attenuates faster than the common gravel ballast bed.(4)A new method of vibration reduction was put forward,which combined PFSB with composite sleeper,sleeper mat and ballast mat.The results show that the combination of PFSB and composite sleeper,PFSB and sleeper mat,PFSB and ballast mat makes the wheel-rail interaction force and safety index slightly increased;the vibration acceleration of tunnel wall structure greatly reduced;and the maximum vibration reduction of vibration acceleration at 1.5m of tunnel wall can reach 10.92dB,8.92dB and 10.60dB,respectively.The method of combined vibration reduction is feasible.PFSB has the advantages of both concrete ballastless ballast bed and common gravel ballast bed,which has the advantage of the smaller overall settlement of ballast bed,longer maintenance cycle and meeting the strength and stability requirements.At the same time,PFSB has a certain effect of vibration reduction,which perfectly meets the requirements of vibration and noise reduction of urban rail transit and becomes a new track structure suitable for underground lines.