Experimental study and numerical simulation of the impact of under-sleeper pads on the dynamic and static mechanical behavior of heavy-haul railway ballast track

Laying the under-sleeper pad(USP)is one of the effective measures commonly used to delay ballast degradation and reduce maintenance workload.To explore the impact of application of the USP on the dynamic and static mechanical behavior of the ballast track in the heavy-haul railway system,numerical simulation models of the ballast bed with USP and without USP are presented in this paper by using the discrete element method(DEM)-multi-flexible body dynamic(MFBD)coupling analysis method.The ballast bed support stiffness test and dynamic displacement tests were carried out on the actual operation of a heavy-haul railway line to verify the validity of the models.The results show that using the USP results in a 43.01%reduction in the ballast bed support stiffness and achieves a more uniform distribution of track loads on the sleepers.It effectively reduces the load borne by the sleeper directly under the wheel load,with a 7.89%reduction in the pressure on the sleeper.Furthermore,the laying of the USP changes the lateral resistance sharing ratio of the ballast bed,significantly reducing the stress level of the ballast bed under train loads,with an average stress reduction of 42.19 kPa.It also reduces the plastic displacement of ballast particles and lowers the peak value of rotational angular velocity by about 50%to 70%,which is conducive to slowing down ballast bed settlement deformation and reducing maintenance costs.In summary,laying the USP has a potential value in enhancing the stability and extending the lifespan of the ballast bed in heavy-haul railway systems.

Refined model analysis of basement rock degradation mechanism of heavy-haul railway tunnel

This study investigated the degradation mechanism of the surrounding rock of a heavy-haul railway under a water-rich condition,based on the construction of the Taihangshan tunnel for the Wari Railway,a heavy-haul railway that used standard construction practices for axle loads of 30 t.Remote monitoring demonstrated that the coupling effect between the dynamic load of a heavy-haul train and the groundwater leads to the deterioration and hollowing of the surrounding rock.This study clarified the void evolution process and deterioration mechanism of the basement rock under the comprehensive influence of the groundwater–train dynamic load using a refined discrete element numerical simulation.The results revealed that the groundwater was the primary influencing factor in the deterioration of the lower part of the heavy-haul railway tunnel.Rock particles were gradually lost under the effects of long-term erosion due to groundwater and heavy-haul trains,which inevitably damaged the basement rock after the construction was completed.Based on this observation,the critical conditions for the deterioration and attenuation law of the physical parameters of the basement rock were obtained.The results of this study can provide ideas and serve as a reference for the forecasting and disaster treatment of basement rock damage in heavy-haul railway tunnels.

Fatigue damage analysis of ballastless slab track in heavy-haul railway tunnels

A ballastless slab track,which is commonly used in the track structures of heavy-haul railway tunnels,was analysed based on field measurement data of the Fuyingzi Tunnel on the Zhangtang Railway.In accordance with the measured data,the dynamic load thresholds and distributions on the surface and bottom of the ballastless slab track were investigated.A fatigue damage analysis of the ballastless slab track was performed based on the dynamic load time–history curve.The results show that the ballastless slab track can accomplish train load attenuation and reduce the dynamic load from heavy-haul trains by 47.22%from the surface to the bottom.In addition,the distribution at the bottom of the ballastless slab track exhibited a triangular shape,and the dynamic load threshold at the line centre accounted for 78.67%of that at the track position.Meanwhile,the distribution at the surface was saddle-shaped;the dynamic load threshold at the track position accounted for 79.55%of that at the line centre position.The fatigue damage of the ballastless slab track was analysed effectively by combining the measured data and the linear fatigue damage theory.Moreover,the accuracy of the calculation results was verified based on the measured dynamic stress of the ballastless slab track structure.The dynamic action of the train load led to more-concentrated damage to the track bed,and the damage occurred earlier than that in the ordinary line.The axle load was the primary influencing factor of the track bed fatigue damage,and the damage mainly occurred in the track position.These results provide a theoretical basis for performing stress analysis and designing parameters for ballastless slab tracks in heavy-haul railway tunnels.

Design and implementation of a coal-dust removal device for heavy-haul railway tunnels

The Daqin Railway is China’s first electrified double-track heavy-haul railway line dedicated to coal transportation.Over the years of operation,the tunnels have been covered with a thick layer of coal dust,which also pervades in the air.Worse is that the coal dust has even buried the rail joints in some sections,making the repair and maintenance of the line difficult.This paper introduces a coal-dust removal device for railway tunnels that integrates pipeline transportation with dust-collection techniques.The device is mainly composed of a power system,a conveying system,a dust-filtration and collection system,and a control and protection system.The key technical elements of the system,such as the dust-extraction method and dust-filtration and collection parameters,are optimized based on the characteristics of the coal dust in the tunnel(obtained via field trials),which greatly enhances the adaptability of the device.Coal-dust removal efficiency reached over 20 t/h,which improves the working environment,reduces the intensity of manual work required and solves the problem of coal-dust removal from the most polluted area—within 500 m of the tunnel entrance.

Point cloud-based environment reconstruction and ray tracing simulations for railway tunnel channels

Radio propagation environment plays a critical role in the performance of wireless communication systems,and understanding channel characteristics is vital for ensuring reliable communication links and optimizing system performance.Ray tracing is an effective method to investigate propagation characteristics in a complex environment,and how to quickly and accurately obtain environmental information needs to be solved.This paper presents dynamic environment reconstruction and ray tracing simulation in railway tunnel environment based on Simultaneous Localization and Mapping(SLAM)algorithm and Poisson reconstruction algorithm.Accurate channel parameters are obtained and analyzed based on ray tracing simulation.Both straight and curved tunnels are considered and investigated,and the results show the channel characteristics in complex railway tunnel environments.

Test and application of 60 kg·m^(−1) bainite rails for heavy-haul railway

Purpose-In order to develop high-strength,high-toughness and high-wear-resistance rails suitable for the development and application of heavy-haul railways.Design/methodology/approach-Based on the trial production of 60 kg·m^(−1) bainite rails,the Zeiss inverted optical microscope,transmission electron microscope and static hydraulic universal tester were used to test the microstructure and property of rail base metal and welded joints.Meanwhile,a trial laying of rails,wing rails of frogs and switch rails for turnouts was performed to systematically analyze their strength,toughness and wear resistance.Findings-The results show that the base metal of 60 kg·m^(−1) bainite rail is of a uniform microstructure,with a carbide-free bainite matrix,a few of stable residual austenite and M/A islands,and it features high hardness,good wear resistance and good strength-toughness balance.The welded joint is of a uniform microstructure and has good properties.Originality/value-A bainite rail,laid in a curve section of heavy-haul railway is able to serve for 48 months with a gross traffic tonnage of nearly 600 million tons,whose service life is more than one time longer than that of pearlite rail;the service life of the wing rail of frog and the switch rail for turnout with 60 kg·m^(−1) bainite rails is 3-4 times longer than that with U75V rails,and no serious damage occurs.The bainite rails also have strong peeling and spalling resistance.

Deformation characteristics and safety assessment of a high-speed railway induced by undercutting metro tunnel excavation

Stratum deformation(settlement) is a challenging issue in tunnel engineering, especially when construction of metro tunnels has to undercut high-speed railway. For this purpose, we used the FLAC30 software to analyze the stratum settlement characteristics of high-speed railway at different crossing angles intersected by metro tunnel, in terms of ground settlement trough, stratum slip line and irregularity of ballastless tracks. According to the evolution of the stratum settlement at different angle regions, an optimized angle is proposed for the actual project design. In order to reduce the influence of stratum settlement on the safety of high-speed railway, an approach of safety assessment is proposed for the shield engineering undercutting high-speed railway, as per Chinese specifications using numerical results and on-site conditions. A case study is conducted for the shield tunnel section crossing the Wuhan-Guangzhou High-speed Railway between the Guangzhou North Railway Station and the Huacheng Road Station, which represents the first metro tunnel project passing below a high-speed railway in China. A series of measures is taken to ensure the safe excavation of the shield tunnel and the operation of the high-speed railway. The results can provide a technical support for performing a safety evaluation between high-speed railways and metro tunnels.

Model test on dynamic characteristics of invert and foundation soils of high-speed railway tunnel

A dynamic model test(CL = 4) at different velocities of train,namely different loading frequencies,is carried out to study the dynamic characteristics of a high-speed railway tunnel invert and its foundation soils.Not only are the accelerations,dynamic coefficients,dynamic stresses of the invert and foundation soils emphatically analyzed,their relationship with the velocity of the train are discussed in detail.Through laboratory testing,the attenuation of vibration propagating from up the rails is obtained and the calculation formula of the speed influence coefficient of the tunnel invert is preliminarily established.The depth of the foundation soils influenced by vibration is also determined in this study.It is shown that the responses of the tunnel invert and foundation soils to vibration are slightly increased with the velocity of the train;circumferential stresses in the bottom of the invert are tensile stresses and maximum stresses appear under the foot of the rails;the dynamic soil pressures of the foundation decrease quickly with the distance away from the tunnel invert and an exponential relationship exists between them.

Deterioration of concrete in railway tunnel suffering from sulfate attack

For the sake of understanding the deterioration behavior of concrete in actual railway tunnel structures subjected to aggressive sulfate medium in practice,detailed field investigations and tested analysis on sprayed concrete linings of approximately 40-year-old railway tunnels in environments containing sulfate ion were carried out,respectively.The results show that the deterioration of concretes in the investigated area is serious,which involves complicated physicochemical process between the sulfate salt and concrete.Among them,the secondary sulfateminerals such as gypsum formation under very high concentration sulfate ion condition by accumulating and evaporation process dominate,followed by the crystallization of sulfate salt and formation of thaumasite.

Railway tunnel concrete lining damaged by formation of gypsum,thaumasite and sulfate crystallization products in southwest of China

The railway tunnel concrete lining exposed to sulfate-bearing groundwater beyond 40 years in southwest of China was investigated. Field investigation, sulfate ions content and corroded products analysis, macro/microscopic test were carried out. Results show that under the tunnel concrete lining structure and its served environmental conditions, sulfate solutions permeate concrete lining and accumulate on windward-side of concrete lining, resulting in the increase of sulfate ions content on windward-side and the diffusion of sulfate ions from windward-side to waterward-side, which cause the concrete lining of windward-side damaged seriously but the waterward-side of concrete lining is still in perfect condition. It is confirmed that structural characteristic of tunnel and environmental conditions lead to physical attack with the leaching of concrete and sodium sulfate crystallization as well as chemical corrosion with formation of gypsum in high sulfate concentration and formation of thaumasite in proper temperature rather than formation of ettringite. These achievements can provide academic and technical supports for understanding the deterioration mechanism of concrete lining as well as constructing railway tunnel under sulfate attack.

Aerodynamic effects on a railway tunnel with partially changed cross-sectional area

The present study numerically explored the aerodynamic performance of a novel railway tunnel with a partially reduced cross-section.The impact of the reduction rate of the tunnel cross-section on wave transmissions was analyzed based on the three-dimensional,unsteady,compressible,and RNG k-εturbulence model.The results highlight that the reduction rate(S)most affects pressure configurations at the middle tunnel segment,followed by the enlarged segments near access,and finally the exit.The strength of the newly generated compression wave at the tunnel junction where the cross-section abruptly changes increases exponentially with the decrease of the cross-sectional area.The maximum peak-to-peak pressureΔP on the tunnel and train surface for non-uniform tunnels is reduced by 10.7%and 13.8%,respectively,compared with those of equivalent uniform tunnels.Overall,the economic analysis suggests that the aerodynamic performance of the developed tunnel prototype surpasses those conventional tunnels based on the same excavated volume.

Finite-State Markov Wireless Channel Modeling for Railway Tunnel Environments

In recent years,high-speed railways(HSRs)have developed rapidly with a high transportation capacity and high comfort level.A tunnel is a complex high-speed rail terrain environment.It is very important to establish an accurate channel propagation model for a railway tunnel environment to improve the safety of HSR operation.In this paper,a method for finite-state Markov chain(FSMC)channel modeling with least squares fitting based on non-uniform interval division is proposed.First,a path loss model is obtained according to measured data.The communication distance between the transmitter and receiver in the tunnel is non-uniformly divided into several large non-overlapping intervals based on the path loss model.Then,the Lloyd-Max quantization method is used to determine the threshold of the signal-to-noise ratio(SNR)and the channel state quantization value and obtain the FSMC state transition probability matrix.Simulation experiments show that the proposed wireless channel model has a low mean square error(MSE)and can accurately predict the received signal power in a railway tunnel environment.

Experimental investigation on the invert stability of operating railway tunnels with different drainage systems using 3D printing technology

In recent years, the invert anomalies of operating railway tunnels in water-rich areas occur frequently,which greatly affect the transportation capacity of the railway lines. Tunnel drainage system is a crucial factor to ensure the invert stability by regulating the external water pressure(EWP). By means of a threedimensional(3D) printing model, this paper experimentally investigates the deformation behavior of the invert for the tunnels with the traditional drainage system(TDS) widely used in China and its optimized drainage system(ODS) with bottom drainage function. Six test groups with a total of 110 test conditions were designed to consider the design factors and environmental factors in engineering practice,including layout of the drainage system, blockage of the drainage system and groundwater level fluctuation. It was found that there are significant differences in the water discharge, EWP and invert stability for the tunnels with the two drainage systems. Even with a dense arrangement of the external blind tubes, TDS was still difficult to eliminate the excessive EWP below the invert, which is the main cause for the invert instability. Blockage of drainage system further increased the invert uplift and aggravated the track irregularity, especially when the blockage degree is more than 50%. However, ODS can prevent these invert anomalies by reasonably controlling the EWP at tunnel bottom. Even when the groundwater level reached 60 m and the blind tubes were fully blocked, the invert stability can still be maintained and the railway track experienced a settlement of only 1.8 mm. Meanwhile, the on-site monitoring under several rainstorms further showed that the average EWP of the invert was controlled within 84 k Pa, while the maximum settlement of the track slab was only 0.92 mm, which also was in good agreement with the results of model test.

New technologies for high-risk tunnel construction in GuiyangGuangzhou high-speed railway

Based on the construction of high risk tunnels in Guiguang-Guangzhou high-speed railway, several new technologies were developed for high-risk tunnel con- struction. First, an integrated advanced geological predic- tion was developed for tunneling in karst area. Then, a new system of ventilation by involving the dedusting technol- ogy was proposed and used in the field, which received a good air quality. Finally, a method to minimize the dis- tance between the working face and the invert installation was proposed by optimizing the invert installation and adopting the micro bench method. Applying the method to the project obtained an excellent result. The achievement obtained for this study would be able to provide a valuable reference to similar projects in the future.

Prototype test study on mechanical characteristics of segmental lining structure of underwater railway shield tunnel

Based on the first unde rwater railway shield tunnel, the Shiziyang shield tunnel of Guangzhou Zhu- jiang River, the prototype test was carried out against its segmental lining structure by using ＂multi-function shield tunnel structure test system＂. And the mechanical characteristics of segmental lining structure using straight assembling and staggered assembling were studied deeply. The results showed that, the mechanical characteristics of segmental lining structure varied with the water pressures; especially after cracking, the high water pressure played a significant role in slowing down the growing inner force and deformation. It also testi- fied that the failure characteristics varied with straight assembling structure and staggered assembling structure. Shear thilurc often occurred near longitudinal seam when using straight assembling.

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.

Viscoplastic solutions of time-dependent deformation for tunnels in swelling rock mass considering stress release

Excavation and control of tunneling responses in swelling soft-rock tunnels of Sichuan-Tibet railway under seepage conditions were studied.For this,a fractional viscoplastic(FVP)model for swelling soft rocks was established by introducing Abel dashpot and unsteady viscosity coefficient,considering additional swelling deformation and damage of rock caused by humidity effect.In view of the FVP model,the viscoplastic deformation solutions for rock mass surrounding tunnel under seepage conditions were derived and long-term mechanical responses of swelling rocks upon tunnel excavation were analyzed.Next,a stress release coefficient considering seepage and creep was proposed,based on which control responses considering stress release and failure mechanism of stress release measures were analyzed.The results showed that:(i)The one-dimensional(1D)FVP model has a good application for swelling rock and the three-dimensional(3D)FVP model could well describe the whole creep process of rock mass despite a much higher creep attenuation rate in the first stage of creep;and(ii)An appropriate stress release and deformation of surrounding rocks could effectively reduce the supporting resistance.However,upon a large stress release,the radius of plastic region could increase significantly,and the strength of the surrounding rock mass decreases greatly.The proposed solution could provide a theoretical framework for capturing the excavation and support responses for tunneling in swelling rock mass in consideration of time effect.

Finite element analysis on deformation of high embankment in heavy-haul railway subjected to freeze-thaw cycles

Finite element simulations are increasingly providing a versatile environment for this topic. In this study, a two-dimensional finite element analysis is conducted to predict the deformation of high embankment in Bazhun heavy-haul railway, China. A recently developed nonlinear softening-type constitutive model is utilized to model the be- havior of subgrade filling materials subjected to freeze-thaw cycles. For the convenience of practical application, the dynamic loading induced by a vehicle is treated as a quasi-static axle load. The deformation of this embankment with different moisture content under freeze-thaw cycles is compared. The results show that when subjected to the first freeze-thaw cycle, the embankment experienced significant deformation variations. Maximum deformation was usually achieved after the embankment with optimum moisture content experienced six freeze-thaw cycles, however, the em- bankment with moisre content of 8.0% and 9.5% deforms continuously even after experiencing almost ten freeze-thaw cycles. Overall, this study provides a simple nonlinear finite element approach for calculating the deformation of the embankment in changing climate conditions.

Structural mechanism and construction method of mud and water inrush in Xiangyun tunnel of Guangtong-Dali railway

Mud and water inrush in tunnel is a common problem in the construction process. Nowadays, the research and classification on this are mostly focused on karst situations. According to the characteristics of the surrounding rock and damage forms of the tunnel in the studied area, the author analyzed the geological and structural characteristics of mud and water inrush in tunnel and obtained their construction type. Meanwhile, the advanced water detection under the complex geological conditions was studied by using induced polarization method, transient electromagnetic method and three-dimensional seismic method, it can be concluded that the water-rich fracture zone exists within the detection range with a risk of large mud and water inrush disaster. The concrete construction treatment measures are put forward:①cement-water glass binary slurry is selected as the material for ground and hole grouting, its advantage is that the gel time can be controlled, and it has certain grout ability in the strata with large permeability coeffcient, which is conducive to excavate construction immediately after grouting.②applying the mature retrograde grouting construction can reduce grouting time and improve the excavation efficiency.

Distribution characteristics and the evolution law of excavation damage zone in the large-span transition section of high-speed railway tunnel based on microseismic monitoring

Purpose–The microseismic monitoring technique has great advantages on identifying the location,extent and the mechanism of damage process occurring in rock mass.This study aims to analyze distribution characteristics and the evolution law of excavation damage zone of surrounding rock based on microseismic monitoring data.Design/methodology/approach–In situ test using microseismic monitoring technique is carried out in the large-span transition tunnel of Badaling Great Wall Station of Beijing-Zhangjiakou high-speed railway.An intelligent microseismic monitoring system is built with symmetry monitoring point layout both on the mountain surface and inside the tunnel to achieve three-dimensional and all-round monitoring results.Findings–Microseismic events can be divided into high density area,medium density area and low density area according to the density distribution of microseismic events.The positions where the cumulative distribution frequencies of microseismic events are 60 and 80%are identified as the boundaries between high and medium density areas and between medium and low density areas,respectively.The high density area of microseismic events is regarded as the high excavation damage zone of surrounding rock,which is affected by the grade of surrounding rock and the span of tunnel.The prediction formulas for the depth of high excavation damage zone of surrounding rock at different tunnel positions are given considering these two parameters.The scale of the average moment magnitude parameters of microseismic events is adopted to describe the damage degree of surrounding rock.The strong positive correlation and multistage characteristics between the depth of excavation damage zone and deformation of surrounding rock are revealed.Based on the depth of high excavation damage zone of surrounding rock,the prestressed anchor cable(rod)is designed,and the safety of anchor cable(rod)design parameters is verified by the deformation results of surrounding rock.Originality/value–The research provides a new method to predict the surrounding rock damage zone of large-span tunnel and also provides a reference basis for design parameters of prestressed anchor cable(rod).