Ballasted Railway Track Research Articles

USE OF GEOSYNTHETICS TO IMPROVE SEISMIC PERFORMANCE OF EARTH STRUCTURES

Abstract After reviewing seismic performance of earth structures based on case histories in Japan and relevant model test results, advantages of using geosynthetics in improving their seismic performance are demonstrated. As one of the successful applications, geosynthetic-reinforced soil retaining walls are highlighted, focusing on several influential factors such as facing rigidity, arrangement and properties of reinforcements, and backfill and subsoil conditions. In addition, further applications of the reinforcement method using geosynthetics are introduced, which include combination with other reinforcement methods, application to bridge abutments and piers, and application to ballasted railway tracks.

High Performance Bi-Block Sleeper for Improvement the Performances of Ballasted Railway Track

This article focuses on the performance of an innovative railway sleeper, recently patented in Italy and called “HP-BB” (High Performance Bi-Block sleeper), under different efficiency conditions of the ballast. The research is carried out through a comparative assessment of traditional mono- and bi-block sleepers, evaluating the lateral resistance of the single sleeper thought as independent of the other constituents of the track.

In situ performance of a ballasted railway track with under sleeper pads

An extensive full-scale field test to investigate the influence of under sleeper pads (USPs) on track quality (with respect to magnitudes of vertical track irregularities) and track dynamics has been conducted on the Schweizerische Bundesbahnen test site at Kiesen in Switzerland. Static track geometry data and dynamic track responses in terms of sleeper strain and accelerations of rail and sleeper during train pass-by have been measured and analysed. It is observed that the use of USPs generates higher rail and sleeper accelerations but lower sleeper strains due to bending. The degradation of track geometry appears to slow down when USPs are used.

Measurements of transient ground movements below a ballasted railway line

This paper presents the results of a detailed investigation into the ground deformations that occur under a railway line during the passage of a train. Four horizontal boreholes were installed at different depths below a ballasted railway track. Ground deformations were measured using geophones at set distances from the centreline of the track within each borehole. The results show vertical displacements reducing with depth, from a maximum at the sleeper. Sleeper displacements are dominated by pairs of bogies at the ends of adjacent wagons (which have a frequency of loading 1 Hz), although the effects of individual bogies (2 Hz) and axles (6 Hz) are also apparent. Higher loading frequencies attenuate with depth so that at a depth of 0·780 m below the sleeper soffit no axles are visible within the displacement data and by a depth of 1·98 m only the combined effect of pairs of adjacent bogies is apparent. In contrast, longitudinal horizontal motion is greatest at a depth of 0·78 m below the sleeper soffit, and the longitudinal horizontal displacements at the sleeper and at a depth of 0·78 m are dominated by the individual axles (∼6 Hz). By a depth of 1·98 m, the longitudinal horizontal motion is dominated by the bogie pairs. A dynamic linear-elastic two-dimensional finite element model was developed and validated using the measured displacements.

Dynamic Stress Analysis of a Ballasted Railway Track Bed during Train Passage

Scientific design of a railway track formation requires an understanding of the subgrade behavior and the factors affecting it. These include the effective resilient stiffness during train passage, which is likely to depend on the stress history and the stress state of the ground, and the stress path followed during loading. This study investigates the last of these, by means of a two-dimensional dynamic finite-element analysis. The effects of train speed, acceleration/braking, geometric variation in rail head level, and a single unsupported sleeper are considered. Results indicate that dynamic effects start to become apparent when the train speed is greater than 10% of the Rayleigh wave speed, vc, of the subgrade. At a train speed of 0.5vc, the shear stresses will be underestimated by 30% in a static analysis, and at train speeds greater than vc the stresses due to dynamic effects increase dramatically. Train acceleration/braking may increase shear stresses and horizontal displacements in the soil, and hence the requirement for track maintenance at locations where trains routinely brake or accelerate. For heavy haul freight trains, long wavelength variations in rail head level may lead to significantly increased stresses at passing frequencies (defined as the train speed divided by the wavelength of the variation in level) greater than 15, and short wavelength variations at passing frequencies of 60–70. Stress increases adjacent to an unsupported sleeper occur in the ballast and subballast layers, but rapidly become insignificant with increasing depth.

Two Approaches of Finite-Element Modeling of Ballasted Railway Track

Use of a ballast is still popular in railway engineering due to its resilience, relatively low noise, and convenience in construction and maintenance. The ballast layer was modeled with two modeling approaches in this study–continuous elastic solid and spring-connected elements. Two-dimensional finite element models were built. The parameters of ballast layers were correlated between two models to assure comparability. Three levels of vehicle moving speed were analyzed with the models. Significant differences of rail deflection and ballast were found in all speed levels. Vibration spectra were also compared to reveal the characteristics of different finite element models. It was found that the model with spring-connected discrete elements had higher characteristic frequency than the simple ballast model. Increasing speed may significantly increase rail deflections and ballast vibration levels and result in particles movements in the ballast layer. A thorough understanding of model characteristics and engineering problems is crucial to choose the most appropriate model.

Experimental settlement and dynamic behavior of a portion of ballasted railway track under high speed trains

The study of railway tracks under high speed trains is one of the most important researches in the domain of transport. A reduced scale experiment with three sleepers is presented to study the dynamic behavior and the settlement of ballasted tracks. A large number of trains passing at high speeds are simulated by signals, applied with the help of hydraulic jacks, having the shape of the letter M and representing the passages of bogies on sleepers. This experiment offers results such as displacements, accelerations, pressures and settlements that allow to better understand the dynamic behavior of a portion of a ballasted railway track at reduced scale and to estimate the settlement versus the number of load cycles. It was found that mechanical properties such as the global stiffness of the track can have important variations during the experiment. The settlement was also found to be a function of the acceleration of sleepers and above all it was observed, for accelerations above a critical value, that the increase of settlement per cycle was very high.

OS2 ダイナミクスのモデル化 : バラスト軌道を例として(基調講演)

OS2 ダイナミクスのモデル化 : バラスト軌道を例として(基調講演)

Dynamic response of high-speed ballasted railway tracks: 3D periodic model and in situ measurements

A three-dimensional (3D) model for the soil–railway track system is proposed. It is based on a geometrical periodic hypothesis. The dynamic soil–structure interaction is taken into account. This representation is used for the case of a ballasted railway track subjected to high-speed moving loads and a new formulation of dynamic responses of the system is proposed. Moreover, recent in situ measurements performed in a high-speed line of the North of France are presented. Lateral and vertical accelerations in several locations of the railway track have been measured and the periodic model is confronted with these records.

Field trials for dynamic characteristics of railway track and its components using impact excitation technique

Assessment of condition of railway track is crucial for track design, repair, and effective maintenance operations. In-field dynamic testing in combination with track modelling represents an efficient strategy for identification of the current condition of railway track structure and its components. Field investigations for the dynamic characteristics of a railway track and its components were carried out and are presented in this paper. A non-destructive technique using impact excitation, so-called ‘ modal testing’, was utilized in these trials. Integrated approach combining field measurements, experimental modal analysis, and finite element modelling to evaluate the dynamic parameters of the in situ railway track components are appended. A ballasted railway track site in Central Queensland managed by Queensland Rail (QR) was selected to perform the field tests. Six sleeper-fastening-rail assemblies were selected for dynamic testing. The frequency response functions (FRFs) were recorded by using Bruel & Kjaer PULSE vibration analyser in a frequency domain between 0 and 1600 Hz. The data obtained were best fitted using the least-square technique to determine the dynamic stiffness and damping constants of the tested track components. In addition, the experimentally determined resonance frequencies along with the dynamic properties of the track components can provide an important input for determining the maximum speed and axle load for the future track upgrades. This paper also points out on how to judge the dynamic responses (e.g. FRFs) together with the visual inspection of existing conditions from the field experience. Examples of testing results representing the deficient integrity are additionally highlighted. Based on the results, the impact excitation technique is an efficient method susceptible to the structural integrity of railway track structures.

Stress changes in the ground below ballasted railway track during train passage

The design of railway track formations has traditionally been empirically rather than analytically based, with ballast and sub-ballast layer thicknesses specified mainly on the basis of previous practice. Recent design methods are more scientifically based, and for the most advanced design methods currently in use, input parameters are typically determined from cyclic triaxial testing. The changes in stress experienced by an element of soil below a railway track as a train passes are complex, involving (for example) a cyclic rotation of the principal stress directions. In these conditions, soil element testing in uniaxial compression may lead to the underestimation of vertical strains. Testing in a hollow cylinder apparatus, which can impose the rotations in principal stress direction likely to be experienced by a soil element in the field, may therefore be preferable to triaxial testing. However, there are as yet no data to guide the designer to a rational specification of a testing programme in this more complex apparatus. This article reports the results of finite element analyses carried out to investigate the stress changes experienced by an element of soil beneath a ballasted railway track during train passage. The effects of element location, the initial in situ stress state of the soil, and the elastic parameters used to characterize its behaviour are investigated, and the modelling of the stress paths in a cyclic hollow cylinder apparatus is discussed.

Experimental Investigation on Dynamic Railway Sleeper/Ballast Interaction

In ballasted railway tracks, one of the important components that supports the rails and distributes wheel/rail loading onto the ballast supporting formation is a railway sleeper (sometimes is also called a “railway tie”). This paper presents results of an experimental modal analysis of prestressed concrete sleepers in both free-free and in-situ conditions, incorporating the dynamic influence of sleeper/ballast interaction. Dynamic interaction between concrete sleepers and ballast support is crucial for the development of a dynamic model of railway track capable of predicting its responses to impact loads due to wheel flats, wheel burns, irregularities of the rail, etc. In this study, four types of prestressed concrete sleepers were in-kind provided by the Australian manufacturers. The concrete sleepers were tested using an impact hammer excitation technique over the frequency range of interest, 0–1600 Hz. Frequency response functions (FRFs) were measured using PULSE modal testing system. The FRFs were processed using STAR modal analysis package to identify natural frequencies and the corresponding mode shapes for the sleepers. The conclusions are presented about the effect of the sleeper/ballast interaction on the dynamic properties of prestressed concrete sleepers and their use for predicting railway track dynamic responses.

Achieving sustainability in vibro stone column techniques

As part of achieving environmental sustainability in ground treatment, there is an increasing desire to use recycled materials for vibro stone column techniques. Materials used in stone column construction are required to be free-draining, hard, inert and comply with acceptable criteria in terms of material type, grading, hardness and chemical stability. Spent railway track ballast and crushed concrete probably have the greatest potential for this application in the UK. New European standards for aggregates allow the use of environmentally sustainable materials in a wider range of applications than under the British Standards they replace. However, where such materials are considered for use, as an alternative to natural primary aggregates, it is important that these materials are fit for purpose. Technical requirements for vibro stone column aggregate are discussed. Of particular importance is the fines (clay/silt) content of the aggregate, as this can have a significant effect on the angle of internal friction of the stone column material (and drainage characteristics). This in turn influences both the carrying capacity of the stone column and the settlement characteristics of the stone column-soil composite. Some applications of recycled aggregate in the context of vibro stone columns are presented, including a case history for spent railway ballast and crushed concrete. Also presented is an innovative technique for avoiding pollutant linkages via stone columns using ‘vibro concrete plug’ technology.

Infrared thermographic investigation of railway track ballast

A theoretical study was undertaken to determine if infrared thermography is an appropriate method to identify the condition of railway track ballast. Within this study the optimal conditions for an infrared survey were established. A laboratory experiment was undertaken to identify clean and spent ballast using an infrared camera. By cooling the ballast and watching it heat up to room temperature over time a difference in the rate of heat transfer between the two types of ballast was observed. A field trial of the infrared camera was undertaken over an operational track as part of the work within a normal track maintenance possession. The field trial identified an area of dirty ballast within a section of clean ballast and the findings were also confirmed by using ground penetrating radar and a trial pit. The laboratory and fieldwork are described in detail and sample infrared images as well as visual images of the same area are given — along with calculated values for the emissivity. The findings proved that it is possible to calibrate an infrared camera so that it can determine the condition of the ballast. Also from the success of the field trial it was shown that infrared thermography is a suitable method of identifying the condition of ballast on an operational track

Electromagnetic properties of railway ballast

Laboratory experiments were undertaken to identify and characterise the dielectric properties of railway track ballast using Ground Penetrating Radar (GPR). Better results were obtained with lower frequency antennas. Clear distinctions were obtained between wet and dry and clean and spent ballast. The laboratory experiment is described in detail and sample radar scan plots given — the final analysis of dielectric constants is also given. The implications of the findings for radar velocity are discussed. The application to identifying defects in railway track bed is discussed.

12. Railway track ballast

Railway track formations generally consist essentially of a layer of coarse aggregate, or ballast, in which the sleepers are embedded (see Fig. 12.1). The ballast may rest directly on the subgrade or, depending on the bearing capacity, on a layer of blanketing sand. The layer of ballast is intended to provide a free draining base which is stable enough to maintain the track aUgnment with the minimum of maintenance. The function of the blanketing sand is primarily to provide a filter to prevent contamination of the ballast by fine particles derived from ascending waters (see Fig. 12.2). The fundamental engineering is reasonably well understood. It concerns the transmission of the load from the base of the sleeper on to an area of closely packed ballast beneath. The sleeper bears on to the points of the angular ballast which are, in consequence, liable to fracture or abrade under load, a problem more severe with concrete than with wooden sleepers. If the layer of ballast is not close-packed and of sufficient depth there is a tendency for the sleepers to move up and down as the wheels of the traffic pass in succession, eventually causing the sub-grade to be disturbed. Consequently the ballast layer must be thick enough to hold the track in position and prevent traffic loads from distorting the subgrade, while the aggregate under the sleepers must be tough enough to resist the abrasion and degradation caused by the intermittent traffic loads.

Model Testing of Two-Layer Railway Track Ballast

Railway track ballast requires regular attention to maintain line and level. A recently developed maintenance method, known as pneumatic ballast injection (PBI) or stoneblowing, re-levels and realigns the track by lifting the sleepers and track and blowing smaller size gravel between the ballast and the base of the sleeper. This creates a two-layer ballast bed. Series of large-scale model tests have been carried out to gain a better understanding of the behavior of such a bed under repeated loading. The results confirm that stoneblowing is a better form of track maintenance than the commonly used ballast tamping method, but the size and type of stone and the thickness of the injected layer are of critical importance in determining the postmaintenance behavior.

The application of time domain ground penetrating radar to evaluate railway track ballast

This paper will evaluate Ground Penetrating Radar (GPR) as a Non Destructive Test (NDT) technique for the assessment of railway track bed ballast. Ballast and tracked deterioration is responsible for the majority of differential settlement of tracks, resulting in poor track geometry. Traditionally ballast is only replaced when it has visually deteriorated, resulting in the planning and programming of maintenance and renewals currently being condition driven. There is a significant potential cost saving if ballast deterioration can be detected earlier. GPR can survey large lengths of track ballast in a relatively short space of time. This paper will show how GPR can be used to quickly identify the degree of track bed ballast deterioration and detect the ballast/formation interface.

The sound-absorbing properties of some common out-door materials

The sound absorption coefficients of pure sounds of frequencies from 125 to 4000 cycles per second have been measured at random incidence for specimens of some commonly occurring out-door materials, among them, gravel, turf, sand, ashes, railway-track ballast and snow. They mostly share the common characteristic of increased absorption with rising pitch. Some of the materials (e.g. loose gravel soil and ashes) are highly absorbent, snow is remarkably so, while others (e.g. compressed gravel and wet sand) are indifferent absorbents. Some practical aspects are discussed, and also the influence of nearly grazing incidence, such as often obtains with out-door sounds, in raising the degree of absorption.