Inconsistent effect of dynamic load waveform on macro-and micro-scale responses of ballast bed characterized in individual cycle:a numerical study

Cyclic load is widely adopted in laboratory to simulate the effect of train load on ballast bed.The effectiveness of such load equivalence is usually testified by having similar results of key concerns of ballast bed,such as deformation or stiffness,while the consistency of particle scale characteristics under two loading patterns is rarely examined,which is insufficient to well-understand and use the load simplification.In this study,a previous laboratory model test of ballast bed under cyclic load is rebuilt using 3D discrete element method(DEM),which is validated by dynamic responses monitored by high-resolution sensors.Then,train load having the same magnitude and amplitude as the cyclic load is applied in the numerical model to obtain the statistical characteristics of inter-particle contact force and particle movements in ballast bed.The results show that particle scale responses under two loading patterns could have quite deviation,even when macro-scale responses of ballast bed under two loading patterns are very close.This inconsistency indicates that the application scale of the DEM model should not exceed the validation scale.Moreover,it is important to examine multiscale responses to validate the effectiveness of load simplification.

Study on the particle breakage of ballast based on a GPU accelerated discrete element method

Breakage of particles will have greatly influence on mechanical behavior of granular material(GM)under external loads,such as ballast,rockfill and sand.The discrete element method(DEM)is one of the most popular methods for simulating GM as each particle is represented on its own.To study breakage mechanism of particle breakage,a cohesive contact mode is developed based on the GPU accelerated DEM code-Blaze-DEM.A database of the 3D geometry model of rock blocks is established based on the 3D scanning method.And an agglomerate describing the rock block with a series of non-overlapping spherical particles is used to build the DEM numerical model of a railway ballast sample,which is used to the DEM oedometric test to study the particles’breakage characteristics of the sample under external load.Furthermore,to obtain the meso-mechanical parameters used in DEM,a black-analysis method is used based on the laboratory tests of the rock sample.Based on the DEM numerical tests,the particle breakage process and mechanisms of the railway ballast are studied.All results show that the developed code can better used for large scale simulation of the particle breakage analysis of granular material.

Study on Influence of Ballast Spring Modulus on Track Structure Based on Finite Element Method

The finite element method is used to simulate the orbital structure,and the finite element model of"rail-sleepers-ballast"can be established.The model of the elastic modulus of different ballast and sleeper is calculated,and the rail displacement,the sleeper stress and the fastening force are deduced.The results show that the elastic modulus of the ballast can be increased to reduce the displacement of the rail and the supporting force of the fastener,but the stress of the sleeper will be increased.When the modu-lus of elasticity increases,the rail displacement,small.

Macro-microscopic dynamic characteristics of the ballast bed induced by the variation of moisture content

Due to the excellent drainage performance of the ballast,existing studies mainly focus on the dynamic response of ballast under field capacity or saturation.Attention has rarely been paid to dynamic changes in moisture content and potential influences.In this article,we firstly conduct a model test to determine the variation of ballast moisture content under artificial rainfall.After that,a full-scale model test with cyclic loading is carried out to study the effect of moisture content variation on the macro-microscopic response of the ballast bed,where several wireless particle sensors are installed to obtain ballast motion characteristics at strategic locations.The results show that the moisture content increases gradually and stabilizes at a flat peak under rainfall,despite the excellent drainage performance of ballast bed.After halting rainfall,the moisture content drops back to field capacity,which indicates dynamic flowing surface water on ballast particles under rainfall.Such flowing surface water brings changes to the original dynamic equilibrium of ballast bed:macroscopically,the deformation rate of stabilized ballast bed increases significantly,reaching a local peak under field capacity;microscopically,the x-and z-angular accelerations of the ballast show positive correlation with rainfall intensity.The multiscale responses indicate that field capacity is a critical moisture content.

Rheological Properties of Self-compacting Concrete Paste Containing Chemical Admixtures

Self-compacting concrete （SCC） was used for the filling layer of CRTSⅢ plate ballastless track, which needs excellent workability. The rheological properties of SCC cement paste containing chemical admixtures （CA） such as polycarboxylate-based superplasticizer （PCE）, air-entraining agent （AE） and defoamer （DF） were investigated using a Brookfield R/S SST2000 soft solid tester with a vane geometry spindle. The cementitious materials were designed as one, two and, three components systems by addition of ordinary portland cement （OPC） with these chemical admixtures. The rheological properties of one-component system （PCE paste） were improved with increasing the content of PCE. For two components systems of PCE-AE and PCE-DF, yield stress and plastic viscosity reduced firstly and increased afterward with the increasing of AE content. And the plastic viscosity reached the optimum when the content of AE is 0.004wt%. In general, the trend of yield stress and plastic viscosity decreased with the increasing of the DF content. For three components systems, PCE-AE-DF systems, the rheological properties were improved compared with the sample with AE or DF, which attributed to mixes of the active components mentioned above （CA） which could have a synergetic effect.

Analysis of dynamic stresses in ballasted railway track due to train passages at high speeds

Repeated train passages bring detrimental effects on train operations,especially at high speeds.In this study,a computational model consisting of moving train vehicles,track structure,and track foundation is used to investigate the stress distribution in the track substructure and underlying soil,particularly when the train speed approaches the critical speed via 2.5D finite element method.The numerical model has been validated by in-situ test results from a ballasted high-speed railway.The computational results reveal that the substructure is shown to be effective in reducing the stresses transmitted to the ground;however,a simple Boussinesq approximation is proved to be inaccurate because it cannot properly take account of the effect of multi-layered substructures and train speeds.It is acceptable to assume a simplified smooth track in the analysis model for determining the maximum stresses and displacements for a low-speed railway(≤100 km/h)but,for a high-speed one,the dynamic amplification effect of track irregularities must also be considered in subgrade design.Analysis of the stress paths revealed that the load speed and track irregularity increase the likelihood of failure for the subgrade;track irregularity can induce many times of principal stress rotations even under a simple single moving load.