Measuring the area and number of ballast particle contacts at sleeper/ballast and ballast/subgrade interfaces

The number of railway ballast particles in contact with a sleeper may be relatively small. The discrete and non-uniform nature of these contacts may cause breakage and wear. This article explores the use of pressure paper to record the loading history of sleeper to ballast particle contacts over >3 million loading cycles in full size tests. The results demonstrate that the actual contact area may be less than 1% of the total, and that the number of individual contacts is in the hundreds. Under sleeper pads, a finer ballast grading, a shallower shoulder slope and changes to the sleeper material are found to increase the number and area of contacts

A behavioural framework for fibre reinforced gravel

The mechanical behaviour of granular materials is known to be influenced by the addition of fibres. However, most previous research has been carried out on materials with relatively small grains (sands), and its application to larger grains is not well documented. This paper reports an investigation into the mechanical behaviour of a fibre-reinforced granular material with a relatively large grain size corresponding to one-third and one-fifth scale railway ballast. The investigation was carried out by means of triaxial tests incorporating a full-field, image-based deformation measurement technique; this enabled detailed observations to be made of each triaxial test specimen during shearing. The test data demonstrate the benefits of random fibre reinforcement for aggregates that have a relatively large grain size. Analysis taking into account the effect of fibre tension on the effective stresses experienced by the granular skeleton provides new insights into the mechanisms of reinforcement in larger sized granular materials

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

Evaluating railway track support stiffness from trackside measurements in the absence of wheel load data

It is generally accepted that track support stiffness is a major factor controlling rates of track geometry deterioration, particularly where the track support stiffness changes abruptly. There is, therefore, considerable potential benefit in being able to quantify and detect changes in the track support stiffness. In recent years, trackside techniques using various types of transducer have been developed to determine track deflections as trains pass. However, deducing the track support stiffness from these measurements requires assumptions to be made concerning train loading and track behaviour, and scope for different interpretations remains. For example, loads from moving trains vary dynamically and it is not usually feasible to measure their exact values at any given point along the track. This paper presents new methods of analysis, which can be applied to frequency spectra of track displacement, velocity or acceleration generated as trains pass to calculate the track support stiffness for trains of known axle intervals, without needing to know the actual loads applied. The approach is demonstrated with reference to theory and measured data from a range of field sites

A review and evaluation of ballast settlement models using results from the Southampton Railway Testing Facility (SRTF)

Many of the world’s railways run on ballasted track, which has for nearly 200 years provided a stable support for train operation. However, with trafficking the geometry of the track deteriorates, mainly as a result of the development of differential settlement of the track-bed (ballast and sub-base). When the geometry defects become too severe, maintenance is needed to realign the track to enable the continued safe running of trains. Maintenance is a major cost associated with ballasted railway track, which usually takes the form of tamping. However, tamping damages the ballast, resulting in a diminishing return period between maintenance interventions until eventually the track-bed requires full renewal. A major component of the differential settlement can be attributed to the ballast layer. However, differential settlement of lengths of track cannot easily be modelled or predicted either computationally or experimentally. Thus the total plastic (permanent) settlement is often used as a proxy for the potential for the development of differential settlement along a length of track in the field. Many empirical models have been developed to predict ballast settlement, usually as a function of the number of train axle passes and/or the cumulative load. However, these models may produce very different results, perhaps indicating that the input variables have not been adequately formulated. This paper describes some current empirical ballast settlement models, and evaluates them using experimental data generated using the Southampton Railway Testing Facility (SRTF). This apparatus represents a section of track consisting of a single sleeper bay 650 mm wide, confined by rigid sides that enforce plane strain conditions. The paper summarises the strengths and weaknesses of the existing models, and suggests variables that could be taken into account to improve them

Scaling relationships for strip fibre reinforced aggregates

Previous research on random fibre-reinforced granular materials has shown that the relative dimensions of the grains and fibres significantly affect the macromechanical behaviour of the mixture. However, quantitative data are scarce and most previous work has focused on fine to medium sands, leaving uncertainties regarding the applicability of current knowledge to larger size aggregates such as railway ballast. In this paper, triaxial test data on 1/3 and 1/5 scale railway ballast are used to develop scaling relationships for the size and quantity of fibres needed to achieve the same reinforcing effect in granular materials of differing grain size. It is shown that, to maintain consistency across scales, fibre content should be quantified as a numerical (i.e., number of fibres per grain) rather than a volumetric ratio. It is further shown that increasing the fibre length increases the resistance of the mixture to deviator stress if the fibres are wide enough; and that provided an allowance is made for the effect of fibre tension, the changes in the stress–strain–strength behaviour of the granular matrix resulting from the changes in void ratio associated with the addition of the fibres are consistent with conventional soil mechanics theory across scales

Remediation of Mud Pumping on a Ballasted Railway Track

AbstractMaintenance of ballasted railway tracks is a major cost for railway infrastructure owners. In many developed countries, much of the railway infrastructure is mature and was built for service requirements long since superseded. The increased demands on historic infrastructure can lead to the development or exacerbation of localised trackbed problems that require disproportionate levels of maintenance. Identifying these and applying cost effective remediation has the potential to reduce maintenance spend in the long term. However, it is not always clear what the most cost effective remediation will be. One type of localised maintenance issue is the development of wet beds or wet spots, which can occur where saturated clayey subgrade soils are overloaded and result in the development of mud pumping as trains pass. This leads to the migration of fines into the ballast bed and a deterioration in local track performance. Over time the track overlying the wet bed settles disproportionately more, sleepers become progressively more voided, and train ride quality deteriorates. Maintenance of the wet bed may involve locally digging out and replacing the ballast; however, unless the underlying cause is addressed the problem is likely to recur, requiring repeated localised maintenance interventions. This is costly, reactive and ultimately an ineffective approach to managing the problem. This paper presents a study of a wet bed in the UK, both prior to and after a full track renewal. Transient track deflections during train passage were monitored using sleeper mounted geophones and high speed filming techniques. Loaded track geometry data were obtained from a track recording vehicle. It is shown that local maintenance interventions were generally ineffective, but that a renewal of the top 200mm of the trackbed including placement of a geotextile filter and geogrid appears to have been successful in remediating the problem, at least in the short term

Error in target-based georeferencing and registration in terrestrial laser scanning

Terrestrial laser scanning (TLS) has been used widely for various applications, such as measurement of movement caused by natural hazards and Earth surface processes. In TLS surveying, registration and georeferencing are two essential steps, and their accuracy often determines the usefulness of TLS surveys. So far, evaluation of registration and georeferencing errors has been based on statistics obtained from the data processing software provided by scanner manufacturers. This paper demonstrates that these statistics are incompetent measures of the actual registration and georeferencing errors in TLS data and, thus, should no longer be used in practice. To seek a suitable replacement, an investigation of the spatial pattern and the magnitude of the actual registration and georeferencing errors in TLS data points was undertaken. This led to the development of a quantitative means of estimating the registration- or georeferencing-induced positional error in point clouds. The solutions proposed can aid in the planning of TLS surveys where a minimum accuracy requirement is known, and are of use for subsequent analysis of the uncertainty in TLS datasets

Remediation of mud pumping on a ballasted railway track

Maintenance of ballasted railway tracks is a major cost for railway infrastructure owners. In many developed countries, much of the railway infrastructure is mature and was built for service requirements long since superseded. The increased demands on historic infrastructure can lead to the development or exacerbation of localised trackbed problems that require disproportionate levels of maintenance. Identifying these and applying cost effective remediation has the potential to reduce maintenance spend in the long term. However, it is not always clear what the most cost effective remediation will be. One type of localised maintenance issue is the development of wet beds or wet spots, which can occur where saturated clayey subgrade soils are overloaded and result in the development of mud pumping as trains pass. This leads to the migration of fines into the ballast bed and a deterioration in local track performance. Over time the track overlying the wet bed settles disproportionately more, sleepers become progressively more voided, and train ride quality deteriorates. Maintenance of the wet bed may involve locally digging out and replacing the ballast; however, unless the underlying cause is addressed the problem is likely to recur, requiring repeated localised maintenance interventions. This is costly, reactive and ultimately an ineffective approach to managing the problem. This paper presents a study of a wet bed in the UK, both prior to and after a full track renewal. Transient track deflections during train passage were monitored using sleeper mounted geophones and high speed filming techniques. Loaded track geometry data were obtained from a track recording vehicle. It is shown that local maintenance interventions were generally ineffective, but that a renewal of the top 200 mm of the trackbed including placement of a geotextile filter and geogrid appears to have been successful in remediating the problem, at least in the short term