Track Settlement Research Articles

In-situ investigation on mud pumping in ballastless high-speed railway and development of remediation method

Mud pumping induced by intensive precipitation and traffic loading is a typical distress in high-speed railway either for ballasted or ballastless tracks. In this study, in-situ investigations, numerical simulation and laboratory tests were conducted to reveal the formation of mud pumping and to explore an appropriate method to remediate mud pumping in ballastless railway. Field investigations and numerical simulation indicated that there was a whipping effect in the dynamic responses of the slab track due to train passages, which induced detachments at the end of concrete base from the roadbed surface. This detachment provided infiltration routes for rainwater into the roadbed layer and initiated the mud pumping. An evaluation method was proposed to analyze particles stability of roadbed materials taken from the investigating site, and the results indicated that instable finer particles promoted an increasing instability of coarse particles. Based on the characteristics of particle stability of roadbed materials and the formation process of mud pumping, an optimized polyurethane chemical injection (PCI) method was proposed to remediate mud pumping and tentatively applied to a mud pumping reproduced ballastless slab track in the full-scale physical model test. Both static and dynamic performances of the remediated ballastless track were restored to their initial values before the occurrence of mud pumping. The accumulative track settlement was less than 2.2 mm even under more than one million train loads, indicating that the PCI method is effective in mud pumping remediation of ballastless slab track.

An efficient physical-based method for predicting the long-term evolution of vertical railway track geometries

The dynamic wheel-rail contact forces resulting from the interaction between vehicle and track are responsible for the local track settlement. If these local settlements vary along the track, geometric irregularities develop further amplifying the dynamic loading of the track caused by the interaction between the vehicle and track. In this work, an efficient vehicle-track interaction (VTI) model is presented for predicting the long-term evolution of vertical track settlement during operation. The VTI model has two interacting components – vehicle and track. The vehicle model describes the vertical dynamics of an 8th of a car. The track model considers an elastic rail on discrete (sleeper) supports. Each sleeper location can have its own stiffness, relative height and settlement characteristics. Dependent on the distribution of stiffness and settlement behaviour along the track together with the initial track geometry, each sleeper settles dependent on the number of load cycles (vehicle passes). The track model is initialized with measured vertical track geometry data and static track deflection data at the beginning (day 0) for two types of track sections in the field, a track section where concrete sleepers with Under Sleeper Pads (USP) are used and a track section where only concrete sleepers are used. Using the same settlement model parameters (constant along the track) for the two tracks, the physical-based VTI model can predict the different track geometry quality evolution for both tracks over 350 days. Finally, the VTI model is used to assess the track geometry deterioration when the track/vehicle properties are changed. The prediction strength of the fast VTI model based on the physical understanding can assist in designing and optimizing tracks and in supporting of maintenance activities.

A whole system model framework to predict damage in turnouts

This paper presents a whole system model framework that enables a holistic prediction of accumulated track damage in railway turnouts. A modular approach allows for insight into the interaction of different damage mechanisms such as rail profile change due to the plastic deformation and wear as well as track settlement. The methodology focuses on the two most critical areas of the turnout in terms of damage: switch and crossing panels. It is shown that at the beginning of the service life of the turnout the running surface of the rails in these areas change significantly due to plastic deformation, which in turn increases the dynamic impact forces from passing vehicles. These impact forces cause vertical track settlement that is most pronounced in the crossing area. Track settlement introduces additional dynamics into the system leading to self-reinforcing behaviour. It is shown that the proposed methodology can predict accumulated track damage accounting for the coupling between different damage modes such as track settlement, rail profiles plastic deformation and wear. It can therefore be used for holistic assessment of turnouts e.g. in design optimisation studies.

Using large datasets for finding the correlation between the rate of track settlement and changes in geometry indices

The allowable speed limit of railroad tracks is dependent upon maintaining track conditions that will ensure the safety and ride quality. Settlement of railway tracks amplifies the dynamic loads of trains, which results in high maintenance costs and poor ride quality. Direct settlement measurement in the field is costly due to the requirement for installing many gauges along the many kilometers of track. Different datasets from the US passenger tracks consisting of more than 300 million elements were employed to propose a novel method for measuring the rate of settlement from the reported space curve. In this method, Cross Power Spectral Density (CPSD) was employed for determining the quality of data and choosing reliable observations for calculating the settlement. After calculating the rate of settlement for two railway tracks with a total length of 287 miles, its correlation with changes in different geometry indices was investigated. Equations were proposed for predicting the rate of settlement by changes in geometry indices. Results indicate that standard deviation index has the best correlation with the rate of settlement. Track quality index (TQI) is sensitive to high-frequency noise and does not yield highly accurate results when there is noise in the dataset.

Numerical Analysis of Additional Stresses in Railway Track Elements Due to Subgrade Settlement Using FEM Simulation

The prediction of additional stresses in ballasted track due to subgrade deformation is the main objective of the present paper. In this context, a 2D finite element model of ballasted railway track was built using the ANSYS Workbench program. Based on this model, an investigation of stresses and deformation values of track elements was conducted in three cases with different contact types. It was found that the case introducing the status of a new track, which has frictional contacts between sleepers and ballast with bonded contacts between other elements, has lower stresses in most of the track elements. Moreover, this case was applied for studying the effect of the settlement on track elements. It was found that stresses increased with increasing the settlement value. The average percentages of increased stresses are 4.18%, 5.85%, and 7.21% in railhead, tie plate, and sleeper, respectively, due to a 1 mm increase in the settlement. Finally, a second-degree polynomial equation was derived to predict the additional stresses in each element due to track settlement. It is expected that this study would help to decrease the maintenance costs and extend the service life of the track elements by predicting the additional stresses in them.

The influence of dynamic loading and thermal conditions on tram track slab damage resulting from subgrade differential settlement

To guarantee the operation safety of the tram in typical soft soil area, the damage characteristics of tram track slabs due to subgrade differential settlement, dynamic train loads and environmental issues are investigated in this work. A three-dimensional (3D) finite element analysis (FEA) model is utilized to calculate the additional track vertical profile irregularity due to the subgrade differential settlement. The overall rail vertical profile irregularities are employed as excitations of a tram vehicle-track-subgrade coupling model to calculate the dynamic pad forces. After that, a sequentially coupled thermal-stress analysis procedure is proposed to evaluate the initial stress-damage field of the slabs resulting from environmental temperature variation. The plastic damage theory is adopted to investigate the damage characteristics of tram track slabs due to multiple issues. According to the damage factors calculated from this model, the criterion of subgrade differential settlement of the tram track is proposed. The damage state of the tram track is suggested to be a key factor of the subgrade differential settlement control system.

Simplified geotechnical rheological model for simulating viscoelasto‐plastic response of ballasted railway substructure

AbstractA proper understanding of the mechanical behaviour of the substructure layers is crucial for optimising the design and performance of a ballasted railway track. The recent advent of high‐speed trains and heavy haul freight wagons has heightened this need more than ever. The accurate prediction of the long‐term performance of the railway tracks under increased speed and loads still remains an intriguing challenge for researchers and design engineers. In this context, the present paper proposes a simplified geotechnical rheological model to evaluate the viscoelasto‐plastic response of the track substructure layers. The proposed approach combines plastic slider, elastic springs and viscous dampers to predict the transient response during a train passage, and the irrecoverable deformation accumulated in the track substructure over an operational period. The model simulates tri‐layered substructure (ballast, subballast, and subgrade) in comparison with existing rheological approaches employing either single or dual‐layered substructure. The model is validated against the field data published in the literature. An acceptable agreement between the predicted results and the field data verifies the accuracy of the model. Parametric investigations are conducted to study the influence of train and track parameters on the cumulative track deformation. The results demonstrate the enhanced capability of the rheological model to adequately capture the crucial effects of axle load, train speed and thickness of granular layers on the accumulation of track settlement. The proposed method can provide an effective tool for the practising engineers for quick prediction of changes to the geometry of railway tracks over their operational periods.

Study on Deformation Law of Subway Construction under Passing Existing Line in Short Distance

With the large-scale development and utilization of underground space projects, the subway-led Rail Transit Project is developing vigorously, and the safety and stability of subway under passing existing projects have become a key issue for the future. In this paper, the deformation law of subway tunnel under the existing track is studied by using the method of ROCSCIENCE software and field measurement, which is based on the construction of a new double-tunnel shield-driven subway tunnel under the existing city of pile rapid track project. In the crossing area, the distance between the top of the tunnel and the pile foundation is 6.2 m and the distance between the tunnel and the existing track surface is 19.6 m. It is required that the settlement of track caused by the new tunnel construction should not exceed 2 mm. The results show that the settlement of the existing track is greatly influenced by the new tunnel without reinforcement and grouting, and the maximum settlement is more than 9.92 mm. The settlement of the existing track can be controlled to 1.93 mm by adjusting the construction parameters of the shield machine and grouting reinforcement at different stages during construction.

Large-scale testing facility for heavy haul track

Given the substantially increased demand for increased axle loads of heavy haul trains, there is an imperative need to develop sustainable track infrastructure. When subjected to heavy axle loading, ballast aggregates rapidly break down, compromising the particle friction and associated load bearing capacity. Therefore, understanding the deformation and degradation (breakage) of ballast subjected to various boundary and loading conditions is crucial for improved track design and performance monitoring. Ideally, field testing should be carried out in real-life tracks to avoid laboratory scale and boundary effects, but field tests are often expensive, time-consuming and may disrupt rail traffic, hence not always feasible. A prototype test facility that can simulate appropriate axle loading and boundary conditions for standard gauge heavy haul tracks is presented in this paper. In collaboration with more than a dozen Universities and Industry organisations, Australia's first and only National Facility for Heavy-haul Railroad Testing (NFHRT) has recently been constructed and is now fully operational. This new facility enables a real-size (1:1 scale) instrumented track section to be subjected to continuous cyclic loading simulated via two pairs of dynamic actuators in synchronized operation. The results of a typical test are presented in this paper including the measured track settlement and lateral deformation, transient vertical and lateral stresses, rail and sleeper accelerations, resilient modulus and breakage of ballast. The test results show that an average track settlement of about 14 mm and lateral displacements up to 9 mm are recorded after 500,000 load cycles. Subjected to a 25-tonne axle load, the maximum vertical stress measured at the sleeper-ballast interface is about 225 kPa and this attenuates with depth. The test results of this iconic facility are generally consistent with actual field measurements obtained in heavy-haul tracks located in the towns of Singleton and Bulli in the state of New South Wales, Australia.

Improving the Railway Stability in the Joint Zone for Modern Operating Conditions

Increasing axial loads require the best solutions for maintenance of the railway rack structure. The study is devoted to improving the railway stability in the joint zone for modern operating conditions. The relevance of the issue is substantiated according to analysis of domestic and foreign practices. The analysis shows that using under-sleeper pads reduce vibrations transferred from concrete sleepers to crashed stone, including the joint area. The possibility of solving the issue by using of under-sleeper pads for straightening the track in the joint zone is considered. The paper contains the results of studies conducted in the Experimental ring of JSC VNIIZhT in order to select durable materials for ensuring long-term track stability. In order to assess the implementation effectiveness of under-sleeper pads and to ensure the track stability in the joint zone, the accumulation of track settlements for pads with different stiffness was evaluated, as well as the accelerations in the vertical plane of the sleepers.

The Effect of Traffic on Ballast Settlement

The main factor that contributes to the deterioration of track components is traffic load. Explanations on how the speed, load and repetition of traffic influence the long-term settlement of ballast in a ballasted track are very scarce. Having in mind that tracks subjected to the same load show different settlement behaviors, explanations of track settlements in accordance with the speed, load and repetition are needed. This study is motivated to show how traffic affects the long-term settlement of ballast and how the traffic parameters (speed, load and repetition) contribute to the process. Using finite element modeling software, the three-dimensional track is modeled and analyzed for different values of speed, load and number of repetitions. Drucker Prager plastic model is applied for the ballast and sub-ballast materials of the track. To get the parameters for the Drucker Prager plastic model of granular materials, triaxial test simulation is performed using discrete element software. Hertz’s contact theory is used to model the contact between the wheel and rail. The long-term settlement behavior of ballast material is analyzed by applying 8000 cycles of moving axel load. According to the analyses, a change in the speed and the number of repetitions of train movement changes the permanent settlement of ballast more than the variation in load. Increase in the speed of train movement by 20 km/hr. will increase the stress transferred to the subgrade by up to about 1000 kPa. Speed of train movement is the most contributing parameter in the degradation of ballast material more than the load and number of repetitions. In a conventional ballasted track after about 6000 repetitions of train movement ballast material will start to decrease its performance. The comparison of the effects of the variation of individual traffic parameters speed, load and number of repetitions on the settlement of ballast which is not touched by former researches is well addressed by this study which is very helpful for designing a new railway track and monitoring existing railway tracks. Based on the analysis of the model with the parameters from Ethio-Djibouti standard gauge railway track, the possible maintenance period of ballast material is predicted.

Numerical simulation of the effect of moving loads on saturated subgrade soil

The substantial increase in excess pore pressure and plastic strain in subgrade soil due to faster and heavier trains is a major cause of track instability. Stresses that occur in track substructure during the passage of trains have not often been captured accurately due to insufficient rigour in dynamic analysis. This paper presents a numerical investigation into the plastic/dynamic characteristics of a saturated porous medium (capping and subgrade) subjected to moving axle loads. In this study, a detailed numerical analysis is described, whereby a coupled fluid-dynamic framework is developed for the saturated porous medium in conjunction with a generalized plasticity model, in order to examine the cyclic loading response of a soft subgrade soil. The constitutive model is calibrated by undrained cyclic triaxial testing carried out on a soft foundation soil retrieved from a coastal railroad site having a historical record of mud pumping. Kelvin elements are attached to the transmitting boundary to absorb wave energy and to prevent any back-propagation of waves into the saturated subgrade domain. This approach is then validated against a semi-analytical solution and wayside measurements. The results show that the stress path corresponding to the applied moving load exhibits a series of heart-shaped envelopes along which the deviator stress and mean effective stress gradually decrease with the successive loading cycles. The build-up of pore water pressure and the associated soil deformation (e.g. accumulated settlement) are exacerbated when the train speed approaches a critical velocity of the saturated subgrade. A relationship between the train speed, track settlement and drainage capacity of a sub-ballast (capping) layer is also established. As a practical guide, the influence of permeability of the capping layer is highlighted in relation to controlling track settlement for a given operational train speed.

Experimental study on cyclic settlement of piles in silt soil and its application in high-speed railway design

Pile-supported embankment is popularly used for constructing high-speed railways on soft soils. With train running at high speed, the dynamic load on the pile top generated by the train passages increases noticeably. To keep the long-term settlement of the railway track within a reasonable range, it is necessary to investigate the settlement of piles in soils simultaneously subject to the monotonic load caused by the weight of the track structure and embankment as well as the cyclic load caused by the train passages. In this paper, model tests on piles in silt soil under combined monotonic and cyclic loading were conducted. The behavior of a pile in saturated silt was investigated by measuring pile settlement, soil pressure on the pile surface, and pore water pressure in the soil adjacent to the pile. Based on the model testing results, the ratio of the mean value of cyclic load to the ultimate bearing capacity of the pile (Pm/Pu) and the ratio of the half amplitude of the cyclic load to the ultimate bearing capacity of the pile (Pcyc/Pu) were identified as the main factors dominating the development of pile settlement. A higher Pcyc/Pu leads to faster accumulation in the pile settlement, and a higher Pm/Pu facilitates this process. A chart with three zones determined by the combination of Pcyc/Pu and Pm/Pu has been proposed to distinguish the settlement trends of the piles: stable zone, metastable zone and unstable zone. The combination of Pcyc/Pu and Pm/Pu located in the stable zone generates very small settlement. Moreover, for the loading combination in the metastable zone, an analytical model was proposed to predict the cyclic pile settlement subject to cyclic load with large cycles. The advantage of the model is that the coefficient of friction at the pile surface and the cyclic strain degradation parameter are incorporated. Finally, a case study is presented to demonstrate the application of the proposed chart and the prediction method on cyclic pile settlement in the design of piled embankment of high-speed railways.

Evaluation of Shape Array sensors to quantify the spatial distribution and seasonal rate of track settlement

Rail track geometry irregularities can lead to ride discomfort for passengers and redistribution of wheel loads potentially causing derailments. Current techniques for monitoring track settlements involve the use of discrete sensors or vehicle mounted sensors, which make it difficult to capture either spatial or temporal variations in settlement. Shape Array sensors (SAA) can potentially be used to capture temporal variations in distributed track settlement profiles to monitor and investigate potential track geometry irregularities and inform track maintenance programs. In this study, a site with known ground deformation issues (i.e. soft spots) was monitored with both an SAA and intermittent LIDAR scans. The objectives of the research were to investigate the accuracy of the SAA for measuring track settlements based on a comparison with LIDAR data, use those measurements to assess track irregularities at the site, evaluate temporal and seasonal changes in rail deformation, and to gain insight into the underlying causes of the ground deformation issues. The SAA was found to provide comparable settlement measurements to those from the LIDAR with the added advantage that the data could be used to assess settlement rates. At this particular site, long-term rail settlements were found to be a function of seasonal/climatic conditions with slower settlement rates in the winter and higher rates in the summer. In addition, the measurements indicated that the deformation issues are potentially caused by an asymmetric bearing failure.

Quantification of Railway Ballast Degradation by Abrasion Testing and Computer-Aided Morphology Analysis

AbstractBallast degradation (particle breakage and abrasion) induced by repeated train loads may lead to insufficient track stiffness and excessive track settlement. For safe operation and effectiv...

Quantification of the influence of rolling stock failures on track deterioration

This study focuses on the quantification of the influence of rolling stock failures (RSFs) on railway infrastructure. Taking the wheel flat, a common RSF, as an example, we introduce four quantification indexes to evaluate the influence on the following four deterioration mechanisms: track settlement (TS), track component fatigue (TCF), abrasive wear (AW), and rolling contact fatigue (RCF). Our results indicate that TS, TCF, and AW increase sharply with the increase of the wheel flat length and the vehicle speed, and this increasing trend becomes more acute with the increase of the wheel flat length and the vehicle speed. At low speeds, RCF increases gradually as the wheel flat length increases; at high speeds, it increases sharply at first and then decreases gradually. The influence of the wheel flat on TCF and AW is the most obvious, followed by TS and RCF. These findings can help infrastructure managers (IMs) to better understand infrastructure conditions related to RSFs and can aid them in managing problems with vehicle abnormality in track access charging.

Field Test Optimization of Shield Tunnelling Parameters Undercrossing an Existing High-Speed Railway Tunnel: A Case Study

An existing high-speed railway tunnel undercrossed by shield tunnelling is a high-risk project. Selecting reasonable shield tunnelling parameters is key in controlling the deformation of existing high-speed railway tunnels. In this paper, a Metro Line 3 undercross of an existing high-speed railway tunnel project is taken as the research background. First, a shield section with similar geological conditions as the undercross section is selected as the test section. Then, reasonable shield tunnelling parameters are determined by monitoring the settlement values of surface and ground structures in the test section. Finally, combined with the monitoring results of vertical soil displacement, the reliability of the shield tunnelling parameters is verified. After the completion of shield tunnelling in the test section, there is no void behind the shield tunnel wall, and the monitoring results of the vault settlement and segment posture in the tunnel meet the control values in the specification, which further support the rationality of the tunnelling parameters. In the process of shield tunnelling the undercross of the existing high-speed railway tunnel, the monitoring results of overall tunnel settlement, bottom uplift point settlement, track settlement, and track height difference meet the control values in the specification, which effectively avoids the adverse impacts of the shield tunnelling undercross of the existing high-speed railway tunnel and ensures the safety of the existing tunnel.

Modelling railway ballasted track settlement in vehicle-track interaction analysis

The geometry of a ballasted railway gradually deteriorates with trafficking, mainly as a result of the plastic settlement of the track-bed (ballast and sub-base). The rate and amount of settlement depend on a number of factors, and for various reasons are difficult to predict or estimate analytically. As a result, various empirical equations for estimating the rate of development of plastic settlement of railway track with train passage have been proposed. A review of these equations shows that they (i) do not reproduce the form of settlement vs number of load cycles relationships usually seen in the field; (ii) do not reflect current knowledge of the behaviour of soil subgrades in cyclic loading; and (iii) are often critically dependent on the curve fitting parameters used, which in turn depend on the circumstances in which the calibration data were obtained. To address these shortcomings, this paper develops a semi-analytical approach, based on the known behaviour of granular materials under cyclic loading, for the calculation of plastic settlements of the trackbed with train passage. The semi-analytical model is then combined with a suitable vehicle-track interaction analysis to calculate rates of development of permanent settlement for different initial trackbed stiffnesses, vehicle types and speeds. The model is shown to be able to reproduce recursive effects, in which a deterioration in track geometry causes an increased variation in dynamic load, which feeds back into a further deterioration in track geometry. The new model represents a significant improvement on current empirical equations, in that it is able to reproduce observed aspects of railway track settlement on the basis of the known behaviour of soils and ballast in cyclic loading.

Analysing the effect of principal stress rotation on railway track settlement by discrete element method

Principal stress rotation induced by moving loads from trains significantly influences railway track settlement accumulation. The stationary cyclic loading commonly adopted to study railway ballast behaviour under repeated train loading cannot fully represent the effects of principal stress rotation, which needs to be properly considered in both laboratory tests and numerical simulations for a better understanding of ballast deformation behaviour. This paper focuses on studying railway ballast deformation behaviour with an emphasis on particle scale interactions under two different loading scenarios – namely, stationary cyclic and moving wheel loading. A ballasted track model consisting of five sleepers was established based on the discrete-element method (DEM) with realistic polyhedron-shaped elements. The numerical model was validated first based on the testing results from a full-scale high-speed railway testing facility at Zhejiang University. Numerical results clearly indicated that moving wheel loading induced larger principal stress rotation than stationed cyclic loading did. Larger principal stress rotation mobilised higher particle rotation and displacement, which further increased particle rearrangements through individual particle rolling and sliding, and potentially could cause accelerated ballast degradation. It is recommended to consider principal stress rotation in ballast settlement predictions to prevent possible underestimation by stationary cyclic loading and its limitations.

Discrete element modelling of two-layered ballast in a box test

It has been recently reported that ballast comprising differently graded layers helps to reduce track settlement. The main goal of this paper is to provide micro mechanical insight about how the differently layered ballasts reduce the settlement by employing DEM and thus propose an optimum design for two-layered ballast. The DEM simulations provide sufficient evidence that the two-layered ballast works by preventing particles from moving laterally through interlocking of the particles at the interface of the different layers in a similar way to geogrid. By plotting the lateral force acting on the side boundary as a function of the distance to the base, it is found that the walls in the region of 60–180 mm above the base always support the largest lateral forces and therefore this region might be the best location for an interface layer. However, considering the weak improvement in performance by increasing the thickness of the layer of scaled (small) ballast from 100 to 200 mm, it is suggested that it is best to use the sample comprising 100 mm scaled ballast on top of 200 mm standard ballast as the most cost effective solution.