Ground Vibration And Vibration Mitigation Considering Train-Track-Soil Interactions

The continuous increase in traveling speed of high speed trains will bring great social and economic benefits to the country.However,it will also inevitably bring a greater dynamic impact on the railway infrastructures,resulting in an elevated vibration level for infrastructures and railway environment.Excessive vibration levels will cause damage to infrastructures,and thereby affect the safety,comfort and stability of the vehicle under the cyclic loading of high speed trains.In addition,the propagation of vibration waves caused by high speed trains can also adversely affect the residents and buildings along the railway corridors with the continuous encryption of the high speed rail networks.Therefore,it is of great practical significance to analyze the ground vibration and mitigate the vibration level induced by high speed trains.Based on a critical literature review related to ground vibration studies all over the world,the critical speed in high speed railways,the saturated ground vibration responses,and the vibration mitigation effects using periodic piles,which are actually a type of phononic crystals,are still needed to do further studies.This study was funded by the National Key Research Development Project ‘Regional Integrated Transportation Infrastructure Security Technology(2016YFC0802203)',the European Commission for H2020-MSCA-RISE Project ‘RISEN: Rail Infrastructure Systems Engineering Network(691135)' and China Scholarship Council.The details of studies related to ground vibration and vibration mitigation considering the train-tracksoil dynamic interactions are as follows:1.The current studies of train-track-soil interaction theory,the critical speed in high speed railways,the saturated ground vibration responses,and the negative effects of ground vibration and mitigation solutions are discussed in detail.Based on the critical literature review,the aim and contents of this study are obtained.2.A three-dimensional train-track-soil dynamic interaction model is developed based on the multi-body simulation principle,finite element theory and perfectly matched layers method.In this model,the wheel-rail dynamic contact model,which considers the random track irregularities,is developed based on the Hertz contact theory.The pile-soil contact model is established by considering the reinforcement of pile foundations in soft soils.The perfectly matched layers(PML),which is an efficient dynamic absorption boundary,is also developed to attenuate the dynamic waves in soils.The software LS-DYNA is used to develop the simulation model by analyzing the element types,mesh sizes,calculation control,and so on.3.The calculations of the dynamic stiffness of rail pads and the dynamic elastic moduli of concrete and CA mortar considering strain rate effect in slab tracks are proposed.The calculation method is considered in the simulation model by using several specific keywords,and the simulation model adopted dynamic material properties of slab tracks is validated with field-test results.The results show that the dynamic stiffness of rail pads is two times the static value under the dynamic high speed train loads.The dynamic elastic moduli of concrete and CA mortar are increased by a maximum of 19% and 75%,respectively,when they are compared with the static values,indicating that the material properties of slab tracks are significantly affected by dynamic train loads.Besides,the vibration level of the train and track are increased when the dynamic material properties of slab tracks are adopted,and the vibrations responses are closer to the field test results.4.A field test was carried out in Shanghai-Nanjing high speed railway,and the vibration responses of the track and subgrade are obtained under the high speed train loads.The simulation model is validated against the field test results.The difference between the field-test accelerations and simulation results is no more than 36%,and the difference for displacement is no more than 11.4%.The simulation spectra of acceleration and displacement are quite close to the field test spectra in the frequency domain.In addition,the simulation model is also verified with the vibration responses of soils along the railway based on the field test results from the Beijing-Shanghai high speed railway.5.The critical speed of high speed railways with slab tracks is investigated based on the train-track-soil dynamic interaction model.The critical speed of soils without-and with-piles is440 km/h and 580 km/h,respectively.The critical speed is much higher than the normal operating train speed.Also,the vibration responses of the track are increased by a maximum of25% at critical speed,indicating that the system of ‘slab track-subgrade-ground reinforced with piles' possesses excellent dynamic performance.When the train speed is close to or higher than the critical speed,the contours of the displacement of soils exhibit the Mach cone phenomenon.The piles can significantly reduce the vibration responses of soils such as displacement,velocity,and dynamic stress at both normal speed and critical speed.Also,the piles will interfere with the propagation of dynamic waves induced by high speed trains,and the wavelength is reduced and Mach angle is increased because of the reinforcement of piles.6.A lab-test of saturated soils was carried out to obtain the dynamic pore water pressure of saturated soils under dynamic loads.The simulation model of saturated soils is validated by comparing the responses with lab-test results.The saturated soil model is developed in the traintrack-soil dynamic interaction model,and the vibration responses of saturated soils under the high speed train loads are investigated.The results show that the maximum pore water pressure of single-layered saturated soils occurs at the soil surface,and the maximum effective stress occurs at around 2 m.In the multi-layered soils,the subgrade and pile foundation can reduce the vibration responses of soils,such as pore water pressure,effective stress,total stress and dynamic displacements.7.The dispersion relation and band gap of periodic piles are obtained based on the dispersion theory of phononic crystals.The dispersion theory proposed in this study is validated by comparing the dispersion relation with lab-test results and other calculation results.The pronounced frequencies(< 9 Hz)of natural soils induced by high speed trains are calculated from the train-track-soil interaction model.The dimension and theoretical band gap of periodic piles are thus determined based on the pronounced frequencies of natural soils,and the theoretical band gap is 0-9.1 Hz.The periodic piles are developed in the train-track-soil interaction model and the vibration mitigation effect using periodic piles is investigated in both time and frequency domains.The results show that the maximum acceleration can be reduced by 96% when the periodic piles are adopted,and the vibration mitigation frequencies are in line with the theoretical band gaps.The vibration assessment index,insertion loss,is calculated,and the maximum insertion loss is more than 40 d B when the periodic piles are adopted in railways.Therefore,as a type of phononic crystals,the periodic piles exhibit excellent vibration mitigation effects in high speed railways.