Ballast Aggregates Research Articles

Ballast Settlement Ramp to Mitigate Differential Settlement in a Bridge Transition Zone

Differential settlement in railroad track transitions, often associated with differences in track stiffness, may apply considerable impact load and may cause rapid deterioration of track geometry. Such differential settlement commonly seen in bridge approaches may lead to problems in ride comfort, track safety, and reliability. Ballast and subballast layers have been identified as a major cause of differential settlement related to the particulate nature of the aggregate deformation behavior causing degradation and breakdown associated with increased track usage. This paper describes an innovative field approach that successfully demonstrated the use of engineered ballast materials for reducing or mitigating the differential settlement problem in a bridge transition zone. Discrete element method (DEM) simulations were used to predict full-scale track deformations of four ballast materials having different gradations and aggregate shape properties. An imaging device was used to create accurate particle size distributions and shapes for measuring and quantifying by indexes the ballast aggregate shape properties, that is, particle flatness and elongation and angularity. On the basis of the results of the DEM modeling study, a bridge approach settlement ramp was constructed and tested at the Transportation Technology Center's Facility for Accelerated Service Testing. The results showed that the settlement ramp approach could be achieved by constructing ballasted track with proper selection of materials in each section of track transition zone. This study also demonstrated that the DEM simulation approach combined with image analysis could be used as a quantitative tool for predicting the ballast performance and providing help in design of track transitions.

Dynamic characterisation of a ballast layer subject to traffic impact loads using three-dimensional sensing stones and a special sensing sleeper

Spectral analyses, based on field measurement of dynamic response of the ballast layer subject to traffic loads, evaluated the dependency on frequency regarding attenuation effect and strength of ballast layer. The measurement was conducted by using high-performance three-dimensional acceleration sensors and a special sensing-sleeper, which was assembled to assess the dynamic response loads acting on the sleeper bottom at a wide frequency domain. The measured results confirmed that for impact-load components over 100Hz, the ballast layer resists because of its high rigidity and can reduce the impact loads substantially. However, the ballast layer is almost non-resistant to the low-frequency load components, which are not reduced enough. The finite element vibration analysis of the ballast aggregate, whose ballast stones are expressed as a polyhedron model using a three-dimensional discrete element method, examines frequency characteristics up to high-frequencies and clarifies the stress distribution of the ballast and the wave propagation of the ballast stone.

Characterizing Ballast Degradation through Los Angeles Abrasion Test and Image Analysis

Ballast fouling, often associated with deteriorating railroad track performance, refers to the condition in which the ballast layer changes its composition and develops a much finer grain size distribution. Fouling is commonly caused by degradation or breakage of ballast aggregates under traffic loading, although other fine materials including but not limited to coal dust, fine-grained subgrade soils, and sand can also contaminate a clean and uniformly graded ballast layer. An experimental approach is described to characterize stages of railroad ballast degradation studied through Los Angeles abrasion testing in the laboratory. An aggregate image analysis approach is used to investigate ballast particle abrasion and breakage trends at every stage through detailed quantifications of individual ballast particle size and shape properties. The experimental study indicated that the fouling index (FI) commonly used by practitioners was indeed a good indicator of fouling conditions, especially when all voids created by larger particles were filled by fine materials as FI values approached 40. Image analysis results of ballast particles larger than 9.5 mm (3/8 in.) scanned after a number of turns of the Los Angeles abrasion drum showed good correlations between percentage changes in aggregate shape properties, that is, imaging-based fatness and elongation, angularity and surface texture indexes, and the FI. The establishment of such relationships between in-service track fouling levels and ballast size and shape properties with similar field imaging techniques would help to understand field degradation trends better and as a result improve ballast serviceability and life-cycle performance.

Influence of Size and Shape Properties of Railroad Ballast on Aggregate Packing

Railroad ballast is uniformly graded coarse aggregate placed between and immediately underneath the ties to provide drainage and structural support for the loading applied by trains. In the United States, several ballast gradation recommendations are suggested by the American Railway Engineering and Maintenance-of-Way Association (AREMA). Most of these recommended uniform gradations have somewhat wide ranges at their control sieves; this range often creates significantly different grain-size distributions within the same gradation band. In this analysis, AREMA No. 24 gradation was studied for controlled changes in grain-size distributions within its band. Gradations were created by using three control sieves within the AREMA No. 24 gradation band. In addition, to emphasize the significance of particle shape, three sets of particles having high-, medium-, and low-angularity indexes quantified by image analysis were also considered in the packing simulations carried out with an image-aided ballast aggregate assembly modeling approach with the discrete element method (DEM). The significance of passing each control sieve as well as the importance of particle angularity on number of contacts, coordination number (average number of contacts that one particle makes with its neighbors), and porosity was analyzed by using analyses of variance and regression in the DEM packing simulations. The results indicated that increasing the number of particles between ballast nominal maximum sieve size and its half size would increase the number of particle contacts created in the granular assembly. When the imaging-based angularity indexes of the aggregate particles increased, the number of contacts also increased to give a higher coordination number. The ballast DEM simulations proved to be a powerful tool to study and optimize ballast gradations.

Triaxial testing and discrete-element modelling of geogrid-stabilised rail ballast

Geogrids have been found to improve the performance of unbound aggregate layers in transportation applications by providing confinement and arresting movement through interlock between individual aggregate particles and their apertures. Geogrid reinforcement offers an effective remedial measure when railway structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength and permanent deformation behaviour of geogrid-stabilised railroad ballast. Geogrids with triangular, rectangular and square apertures were tested in the laboratory experiments. Cylindrical ballast specimens were prepared and tested with geogrids placed at different heights within the specimen using a large-scale triaxial apparatus. An imaging-based discrete-element-method approach was developed to model triaxial test results and investigate geogrid-reinforcement mechanisms. With the capability to create actual ballast aggregate particles as three-dimensional polyhedron elements having the same particle-size distributions and imaging quantified average shapes and angularities, the modelling was able to capture the ballast behaviour with and without geogrid reinforcement reasonably accurately.

Investigation of Aggregate Properties Influencing Railroad Ballast Performance

Class I freight railroads nearly exclusively use premium, high-quality ballast aggregates such as granite, trap rock, and quartzite for their mainline ballast needs. These natural virgin aggregates are crushed stones with certain geologic origins, and their properties may vary significantly depending on quarry sources and production techniques. Findings are presented from a comprehensive laboratory study on ballast aggregates undertaken at the University of Illinois to investigate differences in engineering behavior of premium railroad ballast materials obtained from 13 quarries across the Union Pacific Railroad system. Laboratory research tasks consisted of gradation analysis; quantifications of shape, texture, and angularity properties with the University of Illinois aggregate image analyzer; shear strength evaluation by using large direct shear (shear box) tests; and ballast degradation and durability analyses with Los Angeles abrasion tests conducted at 400 and 1,000 turns and evaluations of subsequent ballast fouling. As the compacted ballast voids ratio increased, aggregate strength typically decreased because of lower solid densities obtained from shear box testing. Aggregate shape properties quantified through imaging-based shape indices were shown to be linked to the strength properties of different ballast density groups. One of the ballast aggregates with flat and elongated particles but very strong mineralogy had quite high strength properties despite the tendency for particles to break during shearing. Higher angularities that generally resulted in higher strength properties could also yield higher fouling and increased breakdown potential of the aggregates.

Determination of dynamic ballast characteristics under transient impact loading

Train-related impact loads acting on a sleeper bottom were directly measured and spectral analysis was performed to determine the dynamic characteristics of ballast layer under traffic impact loads. The results indicate that broadband vibration components act on the ballast layer, which exhibits two different types of behavior depending on dynamic stiffness characteristics. For vibration components over 100 Hz, the layer has high rigidity, resists dynamic loading sufficiently and absorbs impact loading adequately. However, in the low-frequency range, it easily deforms and is hard to absorb load components. FE vibration analysis of ballast aggregate was also performed to examine related characteristics. The mechanisms by which wave propagation velocity inside a ballast layer is reduced and wave motion inside it is greatly reduced were numerically simulated using only elastic body analysis without consideration of material nonlinearity.

Field investigation on variation of rail support modulus in ballasted railway tracks

Rail support modulus is an important factor in safety of railway track. This parameter is defined as support force imposing on rail length unit per rail unit displacement in vertical direction. Rail support modulus is important because it affects track performance and maintenance requirements. Both low and large modulus is undesirable. Low modulus of the rail support has been shown to cause differential settlement that then increases maintenance needs. On the other hand, higher value of rail support causes axle load to be distributed on fewer sleepers and therefore received dynamic loads for any sleeper and rail-sleeper fastening will be increased. Sandy desert areas are critical regions about the contamination of ballast. In these areas, flowing sand grains influence between ballast aggregates and increase the stiffness of ballast layer and the rail support modulus. In this paper, the results of a field investigation about the effect of ballast contamination on the values of the rail support modulus in sandy desert areas are presented.

Field investigation of variation of loading pattern of concrete sleeper due to ballast sandy contamination in sandy desert areas

Sleeper is one of the most important components in a railway track system. Thus, for accurate analysis and design of concrete sleepers, knowledge of imposed loading pattern is necessary. Sandy desert areas are critical regions where loading status of concrete sleeper is different from other areas. In these areas, the influence of flowing sand grains between the ballast aggregates increase the stiffness of ballast layer; consequently, rail support modulus increases, so the received share of total axle load subsequently increases on the sleeper which is placed under the wheel load. On the other hand, the pressure distribution underneath the sleeper changes considerably. In this paper, the results of a field investigation about the variation of rail support modulus and also variation of loading pattern of concrete sleeper followed by the variation of bending moment of sleepers in sandy desert regions were presented.

Discrete Element Modeling for fouled railroad ballast

Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with relatively finer materials or fouling agents commonly from the ballast aggregate breakdown, outside contamination such as coal dust from coal trains, or from subgrade soil intrusion. This paper aims to investigate the effect of fouling on ballast strength and stability. Fouling mechanism was first explained followed by investigation of mechanical properties of fouling agent. Coal dust was chosen as the fouling agent due to its poor mechanical properties and it had been reported to mostly decrease aggregate assembly strength comparing to other fouling agents. An image-aided Discrete Element Modeling (DEM) approach was introduced to simulate the coal dust fouled track field settlement performance. To that end, DEM model parameters for clean and coal dust fouled ballast were validated by matching the DEM direct shear box simulation results to the laboratory shear box testing results for both samples. By assigning laboratory calibrated model parameters to the “half-track” ballast sample generated in DEM, effects of different fouling percentage as well as different fouling locations on ballast settlement performance are studied. Results from the “half-track” DEM simulation revealed that fouling could lead to unfavorable track distresses such as “hanging tie”. Further, shoulder fouling scenario was proven to be critical in track maintenance.

Data Analysis Techniques for GPR Used for Assessing Railroad Ballast in High Radio-Frequency Environment

Railroad ballast supports heavy rail loading, prevents track deformation, and provides drainage of water from the track structure. However, over time, ballast is fouled by the breakdown of ballast aggregate and/or the infiltration of fines, which undermine the ballast functions and affect the railroad track structural capacity. Ground penetrating radar (GPR) provides a rapid, effective, and continuous way to assess railroad track substructure condition; especially ballast. However, the GPR system faces some challenges during field surveys including high radio-frequency interference from railroad communication and automation, and strong reflections from rails. In this study, appropriate techniques were used to remove the interference and reduce the strong clutter from rails to obtain clear GPR data of railroad substructure. A time-frequency method, short-time Fourier transform, was then applied to extract ballast fouling condition over depth. A field survey using multiple sets of 2-GHz air-horn antennae was conducted during summer 2007 at the Transportation Technology Center, Inc. in Pueblo, Colo. Compared to ground-truth excavation and ballast gradation analysis results, GPR was found to be an effective technique to assess railroad track ballast substructure condition.

Effect of Grading on Degradation of Crushed-Rock Railway Ballast and on Permanent Axial Deformation

A series of large-scale repeated-load triaxial tests on a crushed metasandstone railway ballast is described. The effects of different gradings on degradation and accumulation of permanent strain are investigated. The material tested was a mechanically strong ballast aggregate with grading curves similar to the envelope curves in the Norwegian railway ballast specification (EN13450, Category E) and the finer grading in the American Railroad Engineering and Maintenance of Way Association's (AREMA) ballast (AREMA 4) commonly used in the United States on railway mainline tracks. The scope of the research was to assess the influence of grading on ballast breakage and permanent vertical deformation. Breakage and production of fines influence ballast service life, and permanent strain from tamping affects the duration of track alignment. As an approach to simulate the impact from ballast tamping, each test specimen was loaded totally with 4 million load repetitions but dismantled and rebuilt after each 1 million load applications. The results of the study indicate that an increase in average grain size to a single-graded ballast material gives more permanent strain, more ballast breakage, and slightly more production of fines less than 0.063 mm. The recompaction after each tamping operation is the main cause of ballast breakage.

Effects of Dynamic Excitations Due to Very Fast Trains on the Instability of Railway Rock Ballast

It is well known that during the passage of very fast trains (VFT), ballast aggregates can vibrate violently resulting in ballast instability. A simple two dimensional model of the railway track system was proposed, consisting of a sleeper, loose ballast rocks in contact under Coulomb dry friction, compacted rocks and soil foundation. The dynamic behaviour of the system caused by the transient axle load of a single bogie was investigated by explicit finite element method (FEM). It was found that the sleeper exhibits a residual settlement due to irreversible displacement of the ballast rock aggregates. The settlement is maximum at train speed around the Rayleigh wave speed. Similar behaviour was observed for transient force due to two and four closest set of bogies. The simple model shows promising results in investigating this non-linear, irreversible phenomenon and should be extended to more complicated representation of the ballast aggregates under transient forces due to the whole train.

Laboratory Characterization of Fouled Railroad Ballast Behavior

Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with relatively finer materials or fouling agents, which commonly come from breakdown of the ballast aggregate, outside contamination such as coal dust from coal trains, or subgrade soil intrusion. Effects of different fouling agents on ballast aggregate shear strength were studied at the University of Illinois. Through the use of a large direct shear (shear box) device, the strength properties of both clean and fouled ballast samples were determined when three types of fine materials–-coal dust, plastic clayey soil, and mineral filler–-were added to clean ballast samples at various percentages by weight of ballast under both dry and wet [mostly optimum moisture content (OMC)] conditions. Realistic sample preparation procedures were conducted to closely simulate field-fouling scenarios. Test results showed that when the coal dust fouling percentage increased, the ballast shear strength steadily decreased. Wet fouling was found to exacerbate this trend. Results of ballast samples fouled with clay and mineral filler also showed decreasing trends in strength properties; however, coal dust was by far the worst fouling agent for its impact on track substructure and roadbed. Approximately 15% coal dust fouling by weight of ballast was statistically significant to cause considerable strength reductions. In the case of ballast fully fouled with wet coal dust at 35% OMC, the friction angles obtained were as low as the friction angle of coal dust itself.

Time-Frequency Approach for Ground Penetrating Radar Data Analysis to Assess Railroad Ballast Condition

Railroad ballast plays an important role in supporting heavy rail loading, preventing the deformation of track, and providing drainage of water from the track structure. However, over time, ballast is fouled by the breakdown of ballast aggregate and/or the infiltration of fines, which undermine ballast functions. This may result in damage to the rail system, such as track settlement. Ground penetrating radar (GPR), a nondestructive method, can be used to rapidly, effectively, and continuously assess railroad track substructure conditions. Ballast under various fouling conditions generates various electromagnetic (EM) scattering patterns. In this study, air-coupled 2 GHz antenna was found to be sensitive to the scattering pattern change. Appropriate data processing was used to remove the effects of ties and rails to obtain clear GPR images of the subsurface layers. Then, the amplitude envelope and time-frequency approaches were implemented to characterize the signal in time and frequency domains simultaneously. Using these techniques, non-fouled ballast thickness can be assessed and trapped water can be detected, along the track.

Testing of Hungarian aggregates for railway ballast according to MSZ EN 13450: 2003

In June 2003 Hungary accepted the European Standard for Aggregates for Railway Ballast (MSZ EN 13450: 2003). The European norm, compared with the Hungarian Standard, also includes a new examination method called the micro-Deval method for railway ballast aggregates, which was previously not used in Hungary. This paper presents the test results of aggregates for railway ballast-producing quarries, according to newly accepted Standard. The test materials consist of andesite, basalt, dolomite and limestone. The results cover a significant range. The study was aimed at finding a relation between the testing methods, in order to reduce the testing procedure; however, no clear relationship was found.

Track and support rehabilitation for a mine company railroad

The initial support and tie foundation problems of a railroad track built in 1974 to carry axle loads 21% greater than presently (1998) permitted under interchange laws at a speed of 88 km/h are presented. Some of the concerns related to and affecting the final rehabilitation that resulted in a final satisfactory performance are documented. The foundation engineer's interest is directed to the ballast, the tie, and to a lesser extent the rail's behaviour. Cobble-sized (75-150 mm) river gravel was used as the source for the ballast in the rehabilitation. The ballast's satisfactory performance has permitted the establishment of guidelines for future use of this type source of material where suitable quarried rock is not available. The railroad's satisfactory performance after rehabilitation has proven that the design concepts involving 36 t (40 tons) axle loads operating on 60 kg/m (119 lb/yd) continuously welded rail, concrete ties, and ballast manufactured by crushing from a river aggregate were achievable.Key words: railway track, concrete ties, ballast aggregate, aggregate, crushed face, rail, 36 t (40 tons) axle loads.

Repeated-load response of aggregates in relation to track quality index

Track quality rating systems are briefly introduced as a background for undertaking a study of the repeated-load response of ballast aggregates. The results from a number of different laboratory investigations are reviewed and this data interpreted in terms of track quality behaviour. The paper reviews selected results from repeated-load triaxial tests used to investigate the repeated-loading response of different granular railway ballasts at stress levels comparable with those below North America heavy freight axle loads. These results are used to establish an aggregate index to predict deformation and breakdown of ballast under repeated loading. Large-scale test programs are reviewed that relate aggregate quality to simulated ballasted track formation response using a 0.9 m long by 250 mm wide tie to represent a typical loaded rail seat. The large-scale testing was restricted to one aggregate that was subjected to a range of load levels and artificial subgrade stiffness. Similar related work on coarse, single-sized aggregates recommended for drainage layers in highway pavements is reviewed to illustrate the trade-offs between support compressibility and quality of aggregate defined by the aggregate index value. The laboratory tests and model performance are compared with published data to establish a laboratory performance rating in the form of an aggregate index for prediction of those track quality indices that relate to deformation as a function of aggregate type. Key words : ballast, aggregate, hardness, toughness, railway, track quality.