Advances in design theories of high-speed railway ballastless tracks

The design theories of the ballastless track in the world are reviewed in comparison with the innovative research achievements of high-speed railway ballastless track in China.The calculation methods and parameters concerning train load,thermal effect,and foundation deformation of high-speed railway ballastless track,together with the structural design methods are summarized.Finally,some suggestions on the future work are provided.

Parameters studies for rail wear in high-speed railway turnouts by unreplicated saturated factorial design

Rail wear is one of the main reasons for reducing the service life of high-speed railway turnouts in China. The rail wear characteristics of high-speed railway turnouts are influenced by a large number of input parameters of the complex train-turnout system. To reproduce the actual operation conditions of railway turnouts, random distributions of these inputs need to be considered in rail wear simulation. For a given nominal layout of the high-speed railway turnout, 19 input parameters for rail wear simulation in high-speed railway turnouts are investigated based on orthogonal design of experiment. Three dynamic responses(wheel-rail friction work, normal contact force and size of contact patch) are defined as observed values and the significant factors(direction of passage, axle load, running speed, friction coefficient, and wheel and rail profiles) are determined by two unreplicated saturated factorial design methods, including the half-normal probability plot method and Dong 93 method. As part of the associated rail wear simulation, the influence of the wear models and the local elastic deformation on the rail wear was separately investigated. The calculation results for the wear models are quite different, especially for large creep mode. The local elastic deformation has a large effect on the sliding speed and rail wear and needs to be considered in the rail wear simulation.

Structural analysis and design of frost resistance function for subgrade of high-speed railway ballasted track in cold regions

According to the technical characteristics of short fixed wheelbase of a high-speed carriage, a subgrade-track integrated space mechanical response analysis model is proposed for trains under the action ofbiaxial load after the comparison of the stress distribution characteristics of the ballast track subgrade bed structures for high-speed railway under the action of uniaxial load and biaxial load. The loading threshold value （high-cycle long-term dynamic strength） under the circum- stance where the cumulative deformation of subgrade structure gradually develops and finally reaches the convergent state, and its relationship with the foundation coefficient K30 were deduced, based on the characteristics of cumulative defor- mation evolution obtained from the unit structure filling model test under the action of cyclic loading. In view of structure stability and frost resistance requirements of the railway subgrade in cold regions, technical conditions to maintain good service performance of subgrade structure of high-speed railway ballasted track are discussed and analyzed. Study results show that the additive effect manifests itself obviously for railway train bogies under the action of biaxial load than uni- axial load, which has a significant dynamic effect on the subgrade bed bottom and a slight effect on the surface layer. Thus, the adoption of a biaxial load model in the design of a high-speed railway subgrade accurately reflects the vehicle load. Pursuant to the structure design principle, the design method of the subgrade structure of high-speed railway ballasted track is proposed to meet the technical requirements such as structural strength, bearing stiffness and high-cyclic and long-term stability. Technical indicators are obtained for the variation of thickness of the surface layer of reinforced sub- grade bed in the double-layer subgrade mode along with the change of K30 at the subgrade bed bottom. The double-layer structure mode of ＂closure on the upper layer and drainage on the lower layer＂ was proposed in order to meet the water- proofing and drainage requirements of the upper layer of the subgrade bed in cold regions. A dense-framework graded gravel filler with weak water permeability at a coefficient of 10 4 cm/s is used on the upper layer and the void-framework graded gravel filler at the water permeability coefficient of 10 2 cm/s is adopted on the lower layer.

Full-scale multi-functional test platform for investigating mechanical performance of track–subgrade systems of high-speed railways

Motivated by the huge practical engineering demand for the fundamental understanding of mechanical characteristics of high-speed railway infrastructure,a fullscale multi-functional test platform for high-speed railway track–subgrade system is developed in this paper,and its main functions for investigating the mechanical performance of track–subgrade systems are elaborated with three typical experimental examples.Comprising the full-scale subgrade structure and all the five types of track structures adopted in Chinese high-speed railways,namely the CRTS I,the CRTS II and the CRTS III ballastless tracks,the double-block ballastless track and the ballasted track,the test platform is established strictly according to the construction standard of Chinese high-speed railways.Three kinds of effective loading methods are employed,including the real bogie loading,multi-point loading and the impact loading.Various types of sensors are adopted in different components of the five types of track–subgrade systems to measure the displacement,acceleration,pressure,structural strain and deformation,etc.Utilizing this test platform,both dynamic characteristics and long-term performance evolution of high-speed railway track–subgrade systems can be investigated,being able to satisfy the actual demand for large-scale operation of Chinese high-speed railways.As examples,three typical experimental studies are presented to elucidate the comprehensive functionalities of the full-scale multi-functional test platform for exploring the dynamic performance and its long-term evolution of ballastless track systems and for studying the long-term accumulative settlement of the ballasted track–subgrade system in high-speed railways.Some interesting phenomena and meaningful results are captured by the developed test platform,which provide a useful guidance for the scientific operation and maintenance of high-speed railway infrastructure.

Application of Continuous Compaction Technology in the Quality Control and Acceptance of High-Speed Railway Subgrade

On the basis of the theory of continuous compaction technology, we put forward the vibratory compaction value （VCV） index measured by a roller compactor, in order to evaluate the quality of continuously compacted subgrade. The relationships between the VCV index and the currently used indexes K30, Evd, Ev1 and Ev2, were explored by analyzing the experimental data collected from two test sections on the Beijing - Shanghai High-Speed Railway. The result shows that the VCV index has a linear relationship with the currently used indexes.

Damage tolerance of fractured rails on continuous welded rail track for high-speed railways

Broken gap is an extremely dangerous state in the service of high-speed rails,and the violent wheel–rail impact forces will be intensified when a vehicle passes the gap at high speeds,which may cause a secondary fracture to rail and threaten the running safety of the vehicle.To recognize the damage tolerance of rail fracture length,the implicit–explicit sequential approach is adopted to simulate the wheel–rail high-frequency impact,which considers the factors such as the coupling effect between frictional contact and structural vibration,nonlinear material and real geometric profile.The results demonstrate that the plastic deformation and stress are distributed in crescent shape during the impact at the back rail end,increasing with the rail fracture length.The axle box acceleration in the frequency domain displays two characteristic modes with frequencies around 1,637 and 404 Hz.The limit of the rail fracture length is 60 mm for high-speed railway at a speed of 250 km/h.

Snow disaster characteristics in Palongzangbu River Basinand mitigation countermeasures for road engineering

The Palongzangbu River Basin contains the highest number of maritime province glaciers in China. There are 130 glacial lakes, 64 snow avalanche sites and 28 glacial debris flow gullies distributed within the basin. Snow disasters play a controlling role in the Sichuan-Tibet Highway construction, due to the terrain’s special characteristics of high altitude and large height differential. Segmentation mitigation countermeasures for the Sichuan-Tibet Highway are presented based on snow disaster severity level and damage mode of the road. In the Ranwu to Midui section, snow avalanches are regional disasters, so the line should be placed in sunny slopes. In the Midui Gully to Yupu section, the line should be placed in shady slopes and at higher elevations to reduce the risk of glacial lake outburst. In the Yupu to Guxiang section, all three snow disasters are minimal. In the Guxiang to Tongmai section, glacier debris flows are the major threat, thus the road should be placed in shady slopes.

Laboratory investigations of earthquake-and landslideinduced composite surges

Seismic surges and landslides are both major secondary mountain hazards during an earthquake. This paper investigates earthquake-and landslide-induced composite surges through largescale shaking table water tank model experiments. A series of tests were conducted for various initial water depths, peak ground accelerations, slide impact velocities, and slide volumes. Based on the results of the tests, the effects of these parameters on the maximum wave heights of the earthquake-and landslide-induced composite surges were analyzed. An amplification coefficient of seismic surges was defined, and the prediction equation for the amplification coefficient was developed through nondimensional multiple linear regression analysis. Then, an empirical equation for the maximum wave heights of the composite surges was developed based on the amplification coefficient and Demirel's method. This equation provides a calculation method for earthquake-and landslide-induced composite surge waves.

Reasonable compensation coefficient of maximum gradient in long railway tunnels

This paper deals with the theory and calculation methods for compensation of the gradient in railway tunnels through theoretical analysis, numerical calculation, and statistic regression methods. On the basis of the principle that the resultant force is zero, the formula of the maximum calculated gradient was derived for the freight and passenger line and high-speed passenger special line. The formula of aerodynamic drag in tunnel is provided using the domestic and foreign relevant experimental investigations, and revised with modem train and engineering parameters. A calculation model of aerodynamic drag when the train goes through a single-tracked tunnel was built. Finally, the concept of maximum calculated gradient was adopted to revise the formula for compensation of the gradient in railway tunnels.

System reliability evaluation of long railway subgrade slopes considering discrete instability

This paper develops a dual-indicator discrete method(DDM)for evaluating the system reliability performance of long soil subgrade slopes.First,they are segmented into many slope sections using the random finite element method,to ensure each section statistically contains one potential local instability.Then,the k-out-of-n system model is used to describe the relationship between the total number of sections n,the acceptable number of failure sections m,the reliability of sections R_(sec),and the system reliability R_(sys).Finally,m and R_(sys)are jointly used to assess the system reliability performance.For cases lacking spatial data of soil properties,a simplified DDM is provided in which long subgrade slopes are segmented by the empirical value of section length and R_(sec)is substituted by that of crosssections taken from them.The results show that(1)DDM can provide the probability that the actual number of local instabilities does not exceed a desired threshold.(2)R_(sys)decreases with increasing n or decreasing R_(sec);that is,it is likely to encounter more local instabilities for longer or weaker subgrade slopes.n is negatively related to the horizontal scale of fluctuation of soil properties and positively related to the total length of subgrade slopes L.(3)When L is sufficiently large,there is a considerable opportunity to meet local instabilities even if R_(sec)is large enough.

Stress distribution in roadbeds of slab tracks with longitudinal discontinuities

Stress concentration occurs in the foundations of railway tracks where discontinuous components are located.The exacerbated stress under the expansion joints in slab tracks may trigger foundation failures such as mud pumping.Although the higher stress due to the discontinuities of track structures has been discussed in past studies,few focused on the stress response of roadbeds in slab tracks and quantitatively characterized the stress pattern.In this paper,we performed a dynamic finite element analysis of a track-formation system,incorporating expansion joints as primary longitudinal discontinuities.The configurations of CRTS Ⅲ slab tracks and the contact conditions between concrete layers were considered.Numerical results show that longitudinal influencing length of induced stress on roadbed under wheel load relates to the contact conditions between concrete layers,increasing nonlinearly at a larger coefficient of friction.Given a measured coefficient of friction of 0.7,the calculated longitudinal influencing length(9.0 m) matches with field data.The longitudinal influencing length is not affected with the increasing velocity.As stress concentration arises with expansion joints,the worstcase scenario emerges when double-axle loads are exerted immediately above the expansion joints between concrete bases.A stress concentration factor Cvon the roadbed is proposed;it increases with the increasing velocity,changing from 1.33 to 1.52 at velocities between 5 and 400 km/h.The stress distribution on roadbeds transforms from a trapezoid pattern at continuous sections to a triangle pattern at points with longitudinal discontinuities.An explicit expression is finally proposed for the stress pattern on roadbed under expansion joints.Although structural discontinuities induce stress raiser,the extent of concentration is mitigated with increasing depth at different velocity levels.

Numerical investigation on wheel-turnout rail dynamic interaction excited by wheel diameter difference in high-speed railway

目的:车轮型面磨耗和加工误差导致轮对两侧车轮的轮径不同。本文旨在探讨不同幅值和分布形式的轮径差对道岔区轮轨接触几何、轮轨法向接触性能和车辆通过道岔动力响应的影响规律,提出保证车辆通过道岔时的安全性和舒适性的轮径差限值。创新点:通过数值仿真,分析轮径差对道岔区轮轨接触性能和轮轨动态相互作用的影响。方法:1.基于迹线法,揭示轮径差对道岔区轮轨接触几何的影响。2.通过建立轮轨接触有限元计算模型,探讨轮径差对轮轨法向接触性能的影响。3.通过建立车辆-道岔耦合动力学模型,综合考虑在不同幅值和分布形式的轮径差激励下,车辆通过道岔的轮轨动态相互作用、运行舒适性和磨耗指数评价指标,提出轮径差限值。结论:1.轮径差加剧了道岔区固有结构不平顺。2.轮径差通过改变轮载过渡位置,对尖轨上的轮轨法向接触性能有较大影响。3.可根据轮径差幅值将轮径差对道岔区轮轨动力响应的影响划分为三个区域:轮径差小于1.5 mm时,轮缘与尖轨提前接触使轮轨横向力快速增大;轮径差在1.5~2.5 mm时,等值同相轮径差使车辆通过道岔失稳;轮径差大于2.5 mm时,轮缘与尖轨的持续接触增强了车辆稳定性,但增加了轮轨磨耗。4.建议将轮径差控制在2.5 mm以内,且应控制同相分布轮径差小于2 mm。

各向异性黏土中的深基坑开挖单撑失效数值模拟研究

近年来,随着基坑建设深度需求的不断增大,多道支撑式支护体系的使用大幅增长,而结构局部连接失效导致内支撑体系渐进失效破坏的案例时有发生。针对该工程问题,以往的研究通常将土体理想化,忽略了土体力学特性存在各向异性的特征,导致计算结果存在一定的偏差。本文基于天津某地铁车站基坑项目,采用NGI-ADP土体本构模型考虑土体的各向异性,建立三维有限元模型模拟不同初始位置单撑失效(OSF),通过荷载转移率和荷载增加率这两个指标,量化单撑失效后基坑支撑系统的受力变化。分析结果表明:土体的各向异性对支撑轴力的影响不容忽视;同一道支撑的不同初始位置单支撑失效后,基坑支撑的轴力响应是相似的;单撑失效后,在失效支撑附近荷载传递存在就近现象,主要在竖直方向上传递大幅荷载。

Temperature field test and prediction using a GA-BP neural network for CRTS Ⅱ slab tracks

The CRTS Ⅱ slab track, which is connected in a longitudinal direction, is one of the main ballastless tracks in China, with approximately 7365 km of operational track. Temperature loading is a very vital factor leading to slab track damages such as warping and cracking. While existing research on temperature distribution rests on either site tests in special environments or theoretical analysis, the long-term temperature field characteristics are not clear. Therefore, a long-term temperature field test for the CRTS Ⅱ slab track on bridge-subgrade transition section was conducted to analyze the temperature field. A GA-BP(genetic algorithm optimized back propagation) neural network was trained on the test data to predict the temperature field. The vertical and lateral temperature distributions in four typical days were carried out. We found that the temperature along the track was distributed in a nonlinear manner. This was particularly distinct in the vertical direction for depths of less than 300 mm. The highest and lowest daily temperatures and the daily range of the temperature were analyzed. With the increasing depth, the daily highest temperatures and range of the temperature were smaller, the daily lowest temperatures were higher, and the time corresponding to this peak value appeared later in the day. Both the highest and lowest daily temperature could be predicted using the GA-BP neural network, though the accuracy in predicting the highest temperature was higher than that in predicting the lowest temperature.

Design theories and maintenance technologies of slab tracks for high-speed railways in China:a review

The durability and reliability of slab track structures are essential for the long-term safety and stable operation of high-speed railways.In order to provide a solid theoretical basis and technical reference for the advancement of high-speed railway quality,this paper comprehensively discusses design theories of slab track structures,service performance evolution and maintenance technologies,and reviews the innovation happening in the industry.On top of that,the damage evolution,fatigue features and durability of slab tracks,which are highly relevant to serviceability,are summarized,and the future research trend of slab track service behaviours is pointed out.In addition,this paper summarizes the rules of establishing standards for damage maintenance,typical solutions for repairing damage and methods of evaluating the maintenance outcomes that combine field tests and numerical simulations.It also envisions a future direction where advanced testing technologies would assist the evaluation of maintenance effects.

Influence of unsupported sleepers on the dynamic stability of ballasted bed based on wheelset impact tests

The unsupported sleeper can change the load characteristics of ballast particles and thus affect the dynamic stability of a ballasted bed.In this work,a laboratory test was constructed on a ballasted track containing unsupported sleepers.The ballasted track was excited by a wheelset,and the influence of unsupported sleepers on the dynamic stability of a ballasted bed was studied.The results show that the main frequency of the sleeper vibration appeared at 670 Hz,and the first-order rigid vibration mode at the frequency of 101 Hz had a significant effect on the condition without the unsupported sleeper.When the sleepers were continuously unsupported,the vibration damping effect of ballasted bed within the frequency range of 0–450 Hz was better than that at higher frequencies.Within the frequency range of 70–250 Hz,the vibration damping effect of the ballasted bed with unsupported sleepers was better than that without the unsupported sleeper.Owing to the excitation from the wheelset impact,the lateral resistance of the ballasted bed with unsupported sleepers whose hanging heights were 30,60,and 90 mm increased by 37.43%,12.25%,and 18.23%,respectively,while the lateral resistance of the ballasted bed without the unsupported sleeper remained basically unchanged.The unsupported sleeper could increase the difference in the quality of the ballasted bed between two adjacent sleepers.In addition,test results show that the hanging height of the unsupported sleeper had little effect on the lateral resistance of a ballasted bed without external excitation,but had an obvious effect on the rate of change of the lateral resistance of a ballasted bed and the acceleration amplitude of the sleeper vibration under the wheelset impact.

An oversize correction method of dry density for non-cohesive soils filling the embankment of high-speed railway

Non-cohesive soils have been widely used in construction of high-speed railway for their excellent physical and mechanical properties,and the determination of maximum and minimum dry density of such soils containing oversize particles is an important topic.In this study,the influence of oversize particles on dry density of non-cohesive soils is investigated by packing tests.Test results show that oversize particles will make"extra"voids around their surfaces,which increase significantly if the size ratio of oversize material to matrix material is not very big.The dry density of the total material will be overestimated by Elimination Method due to the omission of the"extra"voids.Thus,a geometric model is proposed by which the"extra"voids can be taken into account,and a new oversize correction method for non-cohesive soils is developed.Test results confirm the applicability of this method on the condition of oversize fraction being less than 40%by mass.

Theoretical and experimental investigation on bridge-borne noise under moving high-speed train

Bridge-borne noise pollution caused by train-induced bridge vibration has attracted more and more attentions due to its low-frequency characteristic.In order to investigate the numerical simulation technique of bridge-borne noise and noise reduction methods,a simply supported prestressed concrete (PC) box-girder bridge is adopted for study.Based on train-track-bridge interaction theory,the dynamic response of the bridge under a moving high-speed train is calculated in time-domain and assumed as the sound source of bridge-borne noise.Then bridge-borne noise is estimated according to boundary element method (BEM) in frequency-domain.The time-frequency transform is conducted by fast Fourier transformation (FFT).The validity of the numerical simulation technique is verified through comparison with field measurement results.Furthermore,noise reduction methods are proposed and corresponding effects are discussed.Results show that the proposed numerical simulation method is feasible and accurate in assessing bridge-borne noise.The dominant frequencies of bridge vibration and bridgeborne noise range from 40 Hz to125 Hz and from 31.5 Hz to 100 Hz,respectively.The peak frequency of bridge-borne noise near the bottom plate is 63 Hz.Increasing the thickness of deck plate,adjusting the inclination of webs to 0°-12°,strengthening the boundary constraints and adding a longitudinal clapboard are very effective noise control measures.

Analysis of Earthquake-Triggered Failure Mechanisms of Slopes and Sliding Surfaces

Earthquake-induced landslides along the Dujiangyan-Yingxiu highway after the Ms 8.0 Wenchuan earthquake in 2008 were investigated.It was found that:(1) slopes were shattered and damaged during the earthquake and open tension cracks formed on the tops of the slopes;(2) the upper parts of slopes collapsed and slid,while the lower parts remained basically intact,indicating that the upper parts of slopes would be damaged more heavily than the lower parts during an earthquake.Large-scale shaking table model tests were conducted to study failure behavior of slopes under the Wenchuan seismic wave,which reproduced the process of deformation and failure of slopes.Tension cracks emerged at the top and upper part of model,while the bottom of the model remained intact,consistent with field investigations.Depth of the tension crack at the top of model is 32 cm,i.e.,3.2 m compared to the prototype natural slope with a height of 14 m when the length scale ratio(proto/model) is 10.Acceleration at the top of the slope was almost twice as large as that at the toe when the measured accelerations on shaking table are 4.85 m/s2 and 6.49 m/s2,which means that seismic force at the top of the slope is twice the magnitude of that at the toe.By use of the dynamic-strength-reduction method,numerical simulation was conducted to explore the process and mechanism of formation of the sliding surface,with other quantified information.The earthquake-induced failure surfaces commonly consist of tension cracks and shear zones.Within 5 m from the top of the slope,the dynamic sliding surface will be about 1 m shallower than the pseudo-static sliding surface in a horizontal direction when the peak ground acceleration(PGA) is 1 m/s2;the dynamic sliding surface will be about 2 m deeper than the pseudo-static sliding surface in a horizontal direction when the PGA is 10 m/s2,and the depths of the dynamic sliding surface and the pseudo-static sliding surface will be almost the same when the PGA is 2 m/s2.Based on these findings,it is suggested that the key point of anti-seismic design,as well as for mitigation of post-earthquake,secondary mountain hazards,is to prevent tension cracks from forming in the upper part of the slope.Therefore,the depth of tension cracks in slope surfaces is the key to reinforcement of slopes.The depth of the sliding surface from the pseudo-static method can be a reference for slope reinforcement mitigation.

Experimental study of temperature gradient in track slab under outdoor conditions in Chengdu area

Temperature is one of the important loads for designing slab track. The characteristic of slab track tem- perature varies greatly with different regional climates. In this work, a bi-block slab track model was built under outdoor conditions in Chengdu area; the statistical characteristic of temperature gradient in track slab and the relationship between temperature gradient and surface air temperature were tested and analyzed. The results show that the track slab temperature gradient will vary periodically according to the surface air temperature, and show a clear nonlinearity along the height direction. The temperature gradient distribution is extremely uneven： the temperature gradient in the top part of the track slab is larger than that in the bottom part; the most frequently occurring temperature gradient of the track slab is around -3.5 ℃/m and more than 75 % locates in the level -10 to 10 ℃/m; concrete with a relatively good heat exchange condition with the surrounding air has a narrower band distribution. In addition, the frequency distribution histogram should exclude the time zone from 00：00 to 06：00 because there is almost no traffic in this period. The amplitude of track slab temperature variation is obviously lower than that of the air temperature variation, and the former is approximately linear with the latter.