Current situation and development trend of design methods for subgrade structure of high speed railways

Purpose–This method will become a new development trend in subgrade structure design for high speed railways.Design/methodology/approach–This paper summarizes the structural types and design methods of subgrade bed for high speed railways in China,Japan,France,Germany,the United States and other countries based on the study and analysis of existing literature and combined with the research results and practices of high speed railway subgrade engineering at home and abroad.Findings–It is found that in foreign countries,the layered reinforced structure is generally adopted for the subgrade bed of high speed railways,and the unified double-layer or multi-layer structure is adopted for the surface layer of subgrade bed,while the simple structure is adopted in China;in foreign countries,different inspection parameters are adopted to evaluate the compaction state of fillers according to their respective understanding and practice,while in China,compaction coefficient,subsoil coefficient and dynamic deformation modulus are adopted for such evaluation;in foreign countries,the subgrade top deformation control method,the subgrade bottom deformation control method,the subsurface fill strength control method are mainly adopted in subgrade bed structure design of high speed railways,while in China,dynamic deformation control of subgrade surface and dynamic strain control of subgrade bed bottom layer is adopted in the design.However,the cumulative deformation of subgrade caused by train cyclic vibration load is not considered in the existing design methods.Originality/value–This paper introduces a new subgrade structure design method based on whole-process dynamics analysis that meets subgrade functional requirements and is established on the basis of the existing research at home and abroad on prediction methods for cumulative deformation of subgrade soil.

Analysis of temperature field characteristics based on subgrade site measurements of Harbin-Qiqihar High-speed Railway in a deep seasonal frozen soil region

Recent years have seen a large number of high-speed railways built and will be built in seasonal frozen soil regions ot China. Although high-speed railways are characterized by being fast, comfortable and safe, higher standards for defor- mation of the railways＇ frozen subgrade are required. Meanwhile, changes in subgrade soil temperatures are the main factors affecting the deformation of frozen subgrade. Therefore, this paper selected typical test subgrade sections of the Harbin-Qiqihar Line, a special line for passenger transport built in the deep seasonal frozen soil regions of China, to monitor field temperatures. Also, the temperature changing laws of railways＇ subgrade in this region was analyzed by using testing data, the aim of which is to provide a technical support for future design and construction of buildings and structures in a deep seasonal frozen soil region.

Numerical simulation on flow field,wind erosion and sand sedimentation patterns over railway subgrades

The railway subgrades in the sandy areas act as an obstacle interfering wind-blown sand,causing sand erosion and sedimentation,which can disrupt the safe and stable operation of the railway system.Most previous studies mainly focus on the flow field around railway subgrades,however,the real erosion and sedimentation patterns are rarely studied.This study aims to analyze the erosion and sand sedimentation patterns of wind-blown sand over the subgrades with different heights and steel rails using the ratio of the wall shear stress to the critical value of erosion shear stress.Results show that wind erosion near the top of the upwind slope of the embankment and the shoulder on the upwind side are more severe,and the severity increases with an increase in the height of the embankment.With the increase of wind velocity,sand sedimentation both on the windward and leeside of the subgrade decreases and wind erosion by reverse flow occur.This study indicates that railways in sandy areas should be constructed with a moderate subgrade height(4 m).

Review of research on high-speed railway subgrade settlement in soft soil area

Construction issues of high-speed rail infrastructures have been increasingly concerned worldwide,of which the subgrade settlement in soft soil area becomes a particularly critical problem.Due to the high compressibility and low permeability of soft soil,the post-construction settlement of the subgrade is extremely difficult to control in these regions,which seriously threatens the operation safety of high-speed trains.In this work,the significant issues of high-speed railway subgrades in soft soil regions are discussed.The theoretical and experimental studies on foundation treatment methods for ballasted and ballastless tracks are reviewed.The settlement evolution and the settlement control effect of different treatment methods are highlighted.Control technologies of subgrade differential settlement are subsequently briefly presented.Settlement calculation algorithms of foundations reinforced by different treatment methods are discussed in detail.The defects of existing prediction methods and the challenges faced in their practical applications are analyzed.Furthermore,the guidance on future improvement in control theories and technologies of subgrade settlement for high-speed railway lines and the corresponding challenges are provided.

Effect of granite gravel content on improved granular mixtures as railway subgrade fillings

The improved granular mixtures are widely used as the fillings of railway 8ubgrade, and in order to investigate the effect of coarse grain content on granular mixtures, a series of field tests were conducted. The experimental results indicate that the permeability coefficient increases significantly with the increment of granite gravel content, especially in the range of 60%-70%. Thcrc exists a coarse grain content limit defined as 53%-58.5% to reform the permeable granular skeleton. Beyond this limit, the permeable granular skeleton is efficiently formed, and the macro pores between the separate gravels are partially filled, which is the explanation lbr the permeability increase. The investigations indicate the subgrade resistance modulus （ks0, Ev2, and Evd） depends on the granite gravel content, and the resistance modulus increases significantly beyond granite gravel content of 50%. The skeletons of granitc gravel clayey sand mixture change in the long-term deformation objected to the train-induced dynamic load, which involves three main repeated and circular deformation stages. Generally, the long-time deformation is explained as the gravel crushing and filling the internal porous space with crushed gravel fragments. Through these investigations, the C40-G60 or C30-G70 is recommended as an optimum soil mixture for the good permeability and high resistance modulus.

Analysis of railway subgrade frost heave deformation based on GPS

In order to analyze the connection between the railway subgrade frost heave deformation and temperature variation, five GPS stations＇ data were used to monitor the deformation on a certain section of railway subgrade in northeast China. GAMIT software is used to process the data, providing daily solution, daytime solution and nighttime solution. Vertical trends of these five stations were analyzed to investigate frost heave effect on railway subgrade deformation. The results show that the temperature difference between daytime and night induces stations, significant vertical displacement, and the temperature difference between seasons causes settlement of station which appears linear trend.

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.

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.

Numerical simulation of vibrational response characteristics of railway subgrades with insulation boards

This study presents a numerical method based on the surface temperature data and the ground temperature increase in Daqing for predicting temperature field distribution in the Binzhou Railway subgrade and analyzing the temporal and spatial distribution of freeze−thaw status of railway subgrade.The calibrated numerical method is applied to simulate the temperature field distribution and roadbed vibrational response of the railway subgrade with a thermal insulation layer at different seasons.The results show the following:(1)The thermal insulation layer can remarkably increase the soil temperature below it and maximum frost depth in the subgrade.(2)Thermal insulation can effectively reduce the subgrade vibration and protect it from frost damage.(3)Given that the strength requirements are met,the insulation layer should be buried as shallow as possible to effectively reduce the subgrade vibration response.The research findings provide theoretical support for the frost damage prevention of railway subgrades in seasonally frozen regions.

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.

Development of Subgrade Technical System for High-speed Railway

There are two phases in the development of China's high-speed railway after the1990s:the research and experiment phase during the Eighth and the Tenth Five-Year Plan periods;the construction and operation phase during the Eleventh and the Thirteenth Five-Year Plan periods.This paper regards the Beijing-Shanghai High-Speed Railway as the corner stone in the development of high-speed railway subgrade system,and proposes suggestions and solutions to the problems which may reduce the service life of subgrade.These solutions and methods can be referred to in future railway projects.

A full-section asphalt concrete waterproof sealing structure for the high-speed railway subgrade

Purpose–This study aims to investigate the service performances of a new full-section asphalt concrete waterproof sealing structure(FSACWSS)for the high-speed railway subgrade through on-site tracking,monitoring and post-construction investigation.Design/methodology/approach–Based on the working state of the waterproof sealing structure,the main functional characteristics were analyzed,and a kind of roller-compacted high elastic modulus asphalt concrete(HEMAC)was designed and evaluated by several groups of laboratory tests.It is applied to an engineering test section,and the long-term performance monitoring and subgrade dynamic performance testing system were installed to track and monitor working performances of the test section and the adjacent contrast section with fiber-reinforced concrete.Findings–Results show that both the dynamic performance of the track structure and the subgrade in the test section meet the requirements of the specification limits.The water content in the subgrade of the test section is maintained at 8–18%,which is less affected by the weather.However,the water content in the subgrade bed of the contrast section is 10–35%,which fluctuates significantly with the weather.The heat absorption effect of asphalt concrete in the test section makes the temperature of the subgrade at the shoulder larger than that in the contrastive section.The monitoring value of the subgrade vertical deformation in the test section is slightly larger than that in the contrastive section,but all of them meet the limit requirements.The asphalt concrete in the test section is in good contact with the base,and there are no diseases such as looseness or spalling.Only a number of cracks are found at the joints of the base plates.However,there are more longitudinal and lateral cracks in the contrastive section,which seriously affects the waterproof and sealing effects.Besides,the asphalt concrete is easier to repair,featuring good maintainability.Originality/value–This research can provide a basis for popularization and application of the asphalt concrete waterproof sealing structure in high-speed railways.

Land cover change along the Qinghai-Tibet Highway and Railway from 1981 to 2001

Based on the NOAA AVHRR-NDVI monthly data from 1981 to 2001, the spatial distribution and dynamic change of land cover along the Qinghai-Tibet Highway and Railway were studied. The results of the analytical data indicate that the NDVI values in July, August and September are rather high during a year, and a linear trend by calculating NDVI of each pixel computed based on the average values of NDVI in July, August and September were obtained. The results are as follows： 1） Land cover of the study area by NDVI displays high at two sides of the area and low in the center, and agriculture area 〉 alpine meadow 〉 alpine grassland 〉 desert grassland. 2） In the study area, the amount ofpixels with high increase, slight increase, no change, slight decrease and high decrease account for 0.29%, 14.86%, 67.61%, 16.7% and 0.57% of the whole area, respectively. The increase of land cover pixels is mainly in the agriculture and alpine meadow and the decrease pixels mainly in the alpine grassland, desert grassland and hungriness. Grassland and hungriness contribute to the decrease mostly and artificial land and meadow contribute to the increase mostly. 3） In the area where human beings live, the changing trend is obvious, such as the valleys of Lhasa River and Huangshui River and area along the Yellow River; in the high altitude area with fewer people living, the changing trend is relatively low, like the area of Hoh Xil. 4） Human being＇s behaviors are a key factor followed by the climate changes affecting land cover.

Dynamic behavior of new cutting subgrade structure of expensive soil under train loads coupling with service environment

Expansive soil is sensitive to dry and wet environment change. And the volume deformation and inflation pressure of expansive soil may induce to cause the deformation failure of roadbed or many other adverse effects. Aimed at a high-speed railway engineering practice in the newly built Yun-Gui high-speed railway expansive soil section in China, indoor vibration test on a full-scaled new cutting subgrade model is carried out. Based on the established track-subgrade-foundation of expansive soil system dynamic model test platform, dynamic behavior of new cutting subgrade structure under train loads coupling with extreme service environment(dry, raining, and groundwater level rising) is analyzed comparatively. The results show that the subgrade dynamic response is significantly influenced by service conditions and the dynamic response of subgrade gradually becomes stable with the increasing vibration times under various service environment conditions. The vertical dynamic soil stress is related with the depth in an approximate exponential function, and the curves of vertical dynamic soil stress present a "Z" shape distribution along transverse distance. The peak value of dynamic soil stress appears below the rail, and it increases more obviously near the roadbed surface. However, the peak value of dynamic soil stress is little affected outside 5.0 m of center line. The vibration velocity and acceleration are in a quadratic curve with an increase in depth, and the raining and groundwater level rising increase both the vibration velocity and the acceleration. The vertical deformations at different depths are differently affected by service environment in roadbed. The deformation of roadbed increases sharply when the water gets in the foundation of expansive soil, and more than 60% of the total deformation of roadbed occurs in expansive soil foundation. The laid waterproofing and drainage structure layer, which weakens the dynamic stress and improves the track regularity, presents a positive effect on the control deformation of roadbed surface. An improved empirical formula is then proposed to predict the dynamic stress of ballasted tracks subgrade of expansive soil.

Analysis of ground vibrations induced by high-speed train moving on pile-supported subgrade using three-dimensional FEM

The pile-supported subgrade has been widely used in high-speed railway construction in China.To investigate the ground vibrations of such composite foundation subjected to moving loads induced by high-speed trains(HSTs),three-dimensional(3D)finite element method(FEM)models involving the pile,pile cap and cushion are established.Validation of the proposed model is conducted through comparison of model predictions with the field measurements.On this basis,ground vibrations generated by HSTs under different train speeds as well as the ground vibration attenuation with the distance away from the track centerline are investigated.In addition,the effects of piles and pile elastic modulus on ground vibrations are well studied.Results show that the pile-reinforcement of the subgrade could significantly contribute to the reduction of ground vibrations.In particular,the increase of elastic modulus of pile could lead to consistent reduction of ground vibrations.However,when the pile elastic modulus is beyond 10 GPa,this benefit of pile-reinforcement on vibration isolation can hardly be increased further.

Influence of Golmud-Lhasa Section of Qinghai-Tibet Railway on Blown Sand Transport

The Qinghai-Tibet Railway(QTR) passes through 281 km of sandy land, 11.07 km of which causes serious sand damage to the railway and thus, the control of blown sand is important for the safe operation of the railway. Construction of the railway and sand prevention system greatly changed the blown sand transport of the primary surface. Effective and feasible sand-control measures include stone checkerboard barriers(SCBs), sand fences(SFs), and gravel coverings. This study simulated the embankments, SCBs and SFs of the QTR in a wind tunnel, and analyzed their respective wind profile, sand deposition, and sand-blocking rate(SBR) in conjunction with field data, aiming at studying the influence of Golmud-Lhasa section of the QTR and sand prevention system on blown sand transport. The results of wind tunnel experiments showed that wind speed increased by 67.7%–77.3% at the upwind shoulder of the embankment and decreased by 50.0%–83.3% at upwind foot of embankment. Wind speed decreased by 50.0%–100.0% after passing through the first SF, and 72.2%–100.0% after the first row of stones within the first SCB grid. In the experiment of sand deposition, the higher the wind speed, the lower the SBR of SCB and SF. From field investigation, the amount of sand blocked by the four SFs decreased exponentially and its SBR was about 50.0%. By contrast, SCB could only block lower amounts of sand, but had a higher SBR(96.7%) than SF. Although, results show that SFs and SCBs along the Golmud-Lhasa section of the QTR provide an obvious sand blocking effect, they lead to the deposition of a large amount of sand, which forms artificial dunes and becomes a new source of sand damage.

Porosity of crushed rock layer and its impact on thermal regime of Qinghai-Tibet Railway embankment

It has been proven that crushed rock layers used in roadbed construction in permafrost regions have a cooling effect. The main reason is the existence of large porosity of the rock layers. However, due to the strong winds, cold and high radiation conditions on the Qinghai-Tibet Plateau(QTP), both wind-blown sand and/or weathered rock debris blockage might reduce the porosity of the rock layers, resulting in weakening the cooling effect of the crushed rock layer(CRL) in the crushed rock embankment(CRE) of the Qinghai-Tibet Railway(QTR) in the permafrost regions. Such a process might warm the underlying permafrost, and further lead to potential threat to the QTR's integrity and stability. The different porosities corresponding to the different equivalent rock diameters were measured in the laboratory using water saturation method, and an empirical exponential equation between porosity and equivalent rock diameter was proposed based on the measured experimental data and an important finding is observed in our and other experiments that the larger size crushed rock tends to lead to the larger porosity when arbitrarily packing. Numerical tests were carried out to study impacts of porosity on permafrost degradation and differential thaw depths between the sunny and shady shoulders. The results show that the decrease in porosity due to wind-blown sand or weathered rock debris clogging can worsen the permafrost degradation and lead to the asymmetric thermal regime. In the traditional embankment(without the CRL within it), the largest differential thaw depth can reach up to 3.1 m. The optimized porosity appears in a range from 34% to 42% corresponding to equivalent rock diameter from 10 to 20.5 cm. The CRE with the optimized porosities can make underlying permafrost stable and 0 ℃ isotherms symmetric in the coming 50 years, even under the condition that the climate warming can lead to permafrost degradation under the CRE and the traditional embankment. Some practical implications were proposed to benefit the future design, construction and maintenance of CRE in permafrost regions.

Influence of proximity to the Qinghai-Tibet highway and railway on variations of soil heavy metal concentrations and bacterial community diversity on the Tibetan Plateau

An understanding of soil microbial communities is crucial in roadside soil environmental assessments.The 16S rRNA se quencing of a stressed microbial community in soil adjacent to the Qinghai-Tibet Highway(QTH)revealed that the accu mulation of heavy metals(over about 10 years)has affected the diversity of bacterial abundance and microbial community structure.The proximity of a sampling site to the QTH/Qinghai-Tibet Railway(QTR),which is effectively a measure of the density of human engineering,was the dominant factor influencing bacterial community diversity.The diversity of bacterial communities shows that 16S rRNA gene abundance decreased in relation to proximity to the QTH and QTR in both alpine wetland and meadow areas.The dominant phyla across all samples were Actinobacteria and Proteobacteria.The concentration of Cr and Cd in the soil were positively correlated with proximity to the QTH and QTR(MC/WC sam pling sites),and Ni,Co,and V were positively correlated with proximity to the QTH and QTR(MA/WA sampling sites).The results presented in this study provide an insight into the relationships among heavy metals and soil microbial commu nities,and have important implications for assessing and predicting the impacts of human-induced activities from the QTH and QTR in such an extreme and fragile environment.

Summary of research on frost heave for subgrade in seasonal frozen ground

The building of railways on seasonally frozen ground is inevitable as China pursues economic development and the improvement of its citizens'living standards.However,railway construction in seasonally frozen soil areas is often faced with frost heave,leading to uneven subgrades which seriously threaten traffic safety.This article summarizes extant research results on frost heave mechanism,frost heave factors,and anti-frost measures of railway subgrades in seasonally frozen soil areas.

A system for automated monitoring of embankment deformation along the Qinghai-Tibet Railway in permafrost regions

At present, the monitoring of embankment deformation in permafrost regions along the Qinghai-Tibet Railway is mainly done manually. However, the harsh climate on the plateau affects the results greatly by lowering the observation frequency, so the manual monitoring can barely meet the observational demand. This research develops a system of automated monitoring of embankment deformation, and aims to address the problems caused by the plateau climate and the perma- frost conditions in the region. The equipment consists of a monitoring module, a data collection module, a transmission module, and a data processing module. The field experiments during this program indicate that （1） the combined auto- mated monitoring device overcame the problems associated with the complicated and tough plateau environment by means of wireless transmission and automatic analysis of the embankment settlement data; （2） the calibration of the combined settlement gauge at -20 ℃ was highly accurate, with an error rate always 〈0.5%; （3） the gauge calibration at high-temperature conditions was also highly accurate, with an error rate 〈0.5% even though the surface of the instrument reached more than 50 ℃; and （4） compared with the data manually taken, the data automatically acquired during field monitoring experiments demonstrated that the combined settlement gauge and the automated monitoring system could meet the requirements of the monitoring mission in permafrost regions along the Qinghai-Tibet Railway.