Louis Le Pen

Contribution of base, crib and shoulder ballast to the lateral sliding resistance of railway track: a geotechnical perspective

The sleeper to ballast interface plays a crucial role in the stability of ballasted railway track, transferring both vertical and lateral loads safely from the superstructure to the sub-base. However, current conceptual models for the behaviour of the interface are incomplete and too simplistic to assess the response of the track system to the loads exerted by modern trains. For example, the increased curving speeds associated with tilting trains introduce potentially significant combinations of vertical, lateral, and moment loading which are not explicitly considered in current assessment procedures. Also, the relative contributions of the base, crib, and shoulder ballast to lateral sliding resistance are at present poorly quantified. The behaviour of the sleeper to ballast interface was investigated in a series of tests in an apparatus capable of applying combinations of load representative of real trains. This article presents test data quantifying the relative contributions to total sliding resistance of the base, crib, and shoulder. New calculations are presented, which enable the resistance from the crib and various sizes of shoulder ballast to be quantified. The results of the experiments and calculations are compared with each other and with the literature, and reasonably consistent patterns of behaviour are identified.

Sleeper end resistance of ballasted railway tracks

This paper describes model tests used to investigate how ballast shoulder width and height contribute to a railway sleeper’s resistance to lateral movement for a range of shoulder widths and heights. Deflection and resistance were measured and photographs taken during the tests.The photographs were analyzed using a digital image correlation technique to identify the zones of ballast surface disturbance, which demonstrated that a bulbed failure volume was mobilized at the ultimate limit state. An idealized three-dimensional failure mechanism is proposed, and resistances are calculated using the limit equilibrium approach. The calculation provides a reliable estimate of the measured resistance. The work identifies the optimum shoulder width and height. The calculations are extended to demonstrate that when a number of sleepers are moved simultaneously, the sleeper end resistance may be one-third less per sleeper than that indicated in tests on an isolated sleeper. Image analysis and limit equilibrium calculations show that this is caused by overlapping of mobilized failure volumes from adjacent sleepers.

The effect of enhanced curving forces on the behaviour of canted ballasted track

The performance of railway ballast depends not only on the characteristics of the ballast itself but also the magnitude, frequency and number of axle loads imposed by the trains running on the track it supports. Recent trends towards more frequent and faster trains have highlighted some potential limitations of ballasted railway track. In particular, the introduction of tilting trains on the UK West Coast Main Line, which allowed train operating speeds to be increased over much of the route, has coincided with the sporadic occurrence of localized areas of ballast migration. This phenomenon involves ballast particles moving from the high side of a canted section of track to the low side, exposing the sleeper ends on the high (outer) side of the curve, reducing the lateral resistance of the track and therefore increasing maintenance requirements. This paper reports the results of an investigation carried out to try to identify the underlying mechanism responsible for this behaviour. Measurements of sleeper end displacements under the passage of trains were made at two locations which had experienced ballast migration. Analysis of the measured data together with numerical modelling of the forces imposed on the track structure suggest that the increased speed and typical axle loads of the tilting trains result in larger lateral forces and larger rotation of the sleeper (in the vertical plane) than for a train consisting of a locomotive plus trailing vehicle, as was previously used on the route. It is hypothesized that this leads in turn to a slow but progressive movement of the ballast down the slope of the canted track. However, given the highly localized nature of the ballast migration sites, other factors such as an irregular sub-base stiffness and ballast depth may also contribute to the phenomenon and further work is required to determine the combinations of circumstances that may trigger this effect.

Improving the performance of railway tracks through ballast interventions

Maintenance and eventual renewal of a ballasted track constitute major operational costs for a railway network. Thus, significant benefits would accrue from a more robust track design having a longer service life and reduced maintenance requirements. This paper presents the results from a laboratory study and explores the potential to achieve this through improving the ballast grading and reducing the ballast shoulder slope. Cyclic loading tests were carried out on a section of track representing one sleeper bay in plane strain, in the Southampton Railway Testing Facility. A cyclic load representing a 20 tonne axle load was applied at 3 Hz for at least 3 million cycles, during which measurements of permanent and resilient vertical deflection were made. Certain interventions are found to result in lower rates of permanent settlement and different resilient ranges of movement. Supplementary measurements to determine longitudinal pressure, ballast breakage and attrition, and shoulder slope movement were used to explore the mechanisms responsible for the observed improvements in ballast bed performance. It is concluded that the use of finer ballast gradings and a shallower shoulder slope have the potential to reduce maintenance requirements.

Automated processing of railway track deflection signals obtained from velocity and acceleration measurements

Measurements of low-frequency vibration are increasingly being used to assess the condition and performance of railway tracks. Displacements used to characterise the track movement under train loads are commonly obtained from velocity or acceleration signals. Artefacts from signal processing, which lead to a shift in the datum associated with the at-rest position, as well as variability between successive wheels, mean that interpreting measurements is non-trivial. As a result, deflections are often interpreted by inspection rather than following an algorithmic or statistical process. This can limit the amount of data that can be usefully analysed in practice, militating against widespread or long-term use of track vibration measurements for condition or performance monitoring purposes. This paper shows how the cumulative distribution function of the track deflection can be used to identify the at-rest position and to interpret the typical range of track movement from displacement data. This process can be used to correct the shift in the at-rest position in velocity or acceleration data, to determine the proportion of upward and downward movement and to align data from multiple transducers to a common datum for visualising deflection as a function of distance along the track. The technique provides a means of characterising track displacement automatically, which can be used as a measure of system performance. This enables large volumes of track vibration data to be used for condition monitoring.

Behaviour of under sleeper pads at switches and crossings – Field measurements:

Major growth in rail traffic in many parts of the world in recent years has brought railway networks close to capacity and restricted the time available for track access to carry out maintenance work without costly temporary route closures. There are, therefore, significant benefits in designing or modifying ballasted track systems to reduce maintenance and associated access requirements. Under sleeper pads (USPs) offer the potential to extend ballasted track system life and to extend the intervals between routine maintenance. This paper presents and evaluates field measurements, made using geophones and high speed filming with digital image correlation (DIC), of the performance of a renewed section of track incorporating two switches and crossings (S&C) over a period of two years. One S&C was fitted with two types of USP (categorised as medium and soft), while the other had no USPs and acted as a control. Measurements demonstrate that the bearers with USPs fitted showed less variability in movement than bearers without USPs fitted. The provision of soft USPs caused large increases (>40%) in vertical bearer movements relative to bearers without USPs, although the medium USPs showed little difference. Increased movements of elongated bearers supporting both tracks fitted with soft USPs led to increased bearer rotations towards the loaded track. This effect was aided by the rigid steel collar fixing in the middle of the bearer used in this design of S&C, and raises questions concerning the desirability of this feature. DIC measurements showed that the at rest position of the elongated bearers rotated towards the track on which a train had most recently passed.

Data set for "Modelling the effects of trafficking and tamping on scaled railway ballast in triaxial tests"

Data for the paper Aingaran, S., Le Pen, L., Zervos, A., & Powrie, W. (2018). Modelling the effects of trafficking and tamping on scaled railway ballast in triaxial tests. Transportation Geotechnics.

Data to accompany "A Review and Evaluation of Ballast Settlement Models using Results from the Southampton Railway Testing Facility (SRTF)"

Data to support: Abadi, Taufan et al (2016) A review and evaluation of ballast settlement models using results from the Southampton Railway Testing Facility (SRTF). Procedia Engineering, 143, 999-1006.