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Behavior Evaluation of Geogrid–Polyurethane Composite-Stabilized Ballast
Abstract
A new method of ballast stabilization wherein a layer of geogrid is placed toward the bottom of the ballast layer, and the top zone
of ballast beyond the geogrid influence zone is treated with polyurethane, was investigated in this study. The deformation and degradation behavior of geogrid–polyurethane composite stabilized ballast (CSB) was assessed based on large-scale cyclic tests conducted at a frequency (f) of 32 Hz. The test results indicated that the CSB had significantly reduced vertical and lateral deformations, by about 63% and 65.9%, respectively, compared with those of unstabilized ballast (USB), and by 38.8% and 26% compared with those of geogrid-stabilized ballast
(GSB). Furthermore, the composite stabilization helped to improve the resilient modulus from 216.75 to 280.83 MPa, the damping ratio from 0.230 to 0.340, and the track stiffness from 41.33 to 68.13 MN/m compared with the values for USB. The breakage of ballast also was reduced significantly owing to ballast–polyurethane bonds that remained intact even after 250,000 load cycles. It was found that the geogrid placement location and its corresponding influence zone play a significant role in the overall performance of CSB. The test results further indicated that the geogrid when placed 76 mm from the ballast–subballast interface and a polyurethane treatment depth of 152 mm measured from the sleeper soffit exhibited significantly improved performance under cyclic loading.
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The major drawbacks of a railway track include noise, vibration, and aggravated track degradation. Resilient mats and asphalt have been increasingly used in recent years to mitigate this noise and vibration. However, these materials are quite expensive. Conventional asphalt is very stiff and brittle, making it more prone to cracking. The present work aims to develop a novel material that can be used as a base layer in ballasted and slab tracks. The current research proposes a sustainable and resilient base course layer comprising ground rubber (GR) and polyurethane foam adhesive (PFA). In this study, the performance of GR embedded in the sand is investigated. The use of PFA-treated sand with and without GR is then explored. The optimum dosage of PFA for soil and GR for treated and untreated soil is recommended based on static direct simple shear (SDSS) and cyclic direct simple shear (CDSS) tests. SDSS tests were performed to evaluate the monotonic performance of all mixtures. CDSS tests were performed to assess the long-term performance of these different mixes under repeated cyclic loading (50,000 load cycles) and varying cyclic shear stress amplitude. It is shown that PFA helps reduce the settlement and enhance soil shear strength, while GR increases the damping ratio of the soil. The optimum dosage of PFA is recommended 10%. The optimum GR content for untreated and PFA-treated soil is recommended 5 and 10%, respectively.
The dynamic performances of a polyurethane-reinforced ballast bed and an unreinforced ballast bed at an operation site were tested by using the impact excitation technique. The influences of the ballast beds with and without polyurethane reinforcement on the natural frequency were investigated, along with the vertical and longitudinal vibration transmission characteristics of the track structure. The results show the following. Compared to the ballast bed, the polyurethane ballast bed has an inhibitory effect on the rail vibration in the range of 200–400 Hz, but there is an increased rail vibration amplitude in the higher frequency range. The polyurethane-reinforced ballast bed significantly increases the amount and amplitude of the dominant vibration frequencies of the sleeper. As the frequency increases (> 90 Hz), the superstructure of the polyurethane track rapidly transfers from being flexible to rigid, and the track thus absorbs and dissipates the impact load, quickly damping the vibration in the range of 90–470 Hz. The calculation results of the displacement transmission ratio (DTR) proposed in this study also support this finding, suggesting the use of DTR to more intuitively evaluate the vibration damping effect of each component of the track. The polyurethane material reinforces the ballast bed rather than the damping vibration, thereby improving the stability of the track structure.
In order to investigate the characteristics of the lateral resistance of the polyurethane-mixed ballast with different bonding areas, a sleeper–ballast model was established using the discrete element method. The lateral resistance of the polyurethane-mixed ballast with single and combined bonding modes was analyzed. The results show that the lateral resistance increases with the increase of the bonding depth of the shoulder ballast. After the bonding depth of the crib ballast and the base ballast reaches 5 cm and 15 cm below the bottom of the sleeper, respectively, there is no significant development of the lateral resistance. The lateral resistance cannot be improved efficiently by bonding the slope ballast. The combined bonding of the crib ballast and the base ballast is an effective method to improve the lateral resistance. With the gradual failure of the bonding contact points between the sleeper and the ballast, the average value of the lateral resistance decreases linearly and the lateral resistance presents a greater randomness under the more complex bonding mode.
As train speeds and heavy haul axle loads constantly increase due to market demands, so do the stresses and strains experienced by track structures. This is especially true for track transitions that generate high dynamic forces on both the track and vehicles because of poor vertical track geometry and/or differing track stiffness values on either side of the track transition. Reducing differential settlement between the two track structures at a track transition is one method of improving the life of the track, and increasing maintenance intervals. In this study, rigid polyurethane foam was used to reinforce ballast. Tests were conducted using a dynamic hydraulic load frame and a single sleeper in a large ballast box subjected to heavy haul axle loads. Unreinforced, reinforced and 50% reinforced ballast layers of 300 mm depth were tested to approximately 5 000 000 load cycles. The results showed that rigid polyurethane foamreinforced ballast exhibited in the order of 60% less settlement for a fully reinforced layer, and 42% less settlement for a half reinforced layer. The use of rigid polyurethane foam (RPF) to reinforce ballast has a number of benefits that could result in better track geometry and longer maintenance cycles, in turn resulting in lower life cycle costs.
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.
There are many issues surrounding the performance of critical assets on high-speed ballasted railway lines. At assets like switch & crossings and bridge transitions high track forces can be produced resulting in higher ballast settlements and hence track misalignments. The latter result in higher track forces and hence more settlement, leading to the need for increased track maintenance to ensure comfort and safety. Current technologies for solving issues like ballast movement under high-speed loading regimes are limited. However, a technique that has been well used across the UK and now increasingly overseas to stabilise and reinforce ballasted railway tracks is the application of in-situ polyurethane polymers, termed XiTRACK. This paper discusses how this technique can be used to solve these types of long-standing issues and presents actual polymer application profiles at two typical critical sites, namely a junction and a transition onto concrete slab-track.
Large-scale models comprising a single tie/ballast system were constructed over artificial subballast-subgrade supports having variable compressibility ranging from rigid to very flexible (with California bearing ratio of 1). A 920-mm-long by 250-mm-wide by 150-mm-deep steel footing was used to model the bearing area of a typical tie. Each rail seat was subjected to a program of repeated loading equivalent to a cumulative axle tonnage of 2 to 20 million tonnes in track. The performance of test sections reinforced with a single layer of geogrid at variable depths below the footing (tie) was compared against unreinforced test configurations. The results showed that geogrid reinforcement in ballast over compressible ballast support can be effective in reducing the rate of permanent deformation associated with lateral ballast spreading.
Railway ballast breaks down and deteriorates progressively under train cyclic loading, and soft formation soil fails due to repetitive stress, leading to costly rail track maintenance. Using geosynthetics, track conditions can be improved and maintenance costs can be reduced. This paper addresses the potential use of geosynthetics for improving the deformation characteristics of rail ballast and formation soil. The prospective use of different types of geosynthetics was investigated using a large-scale prismoidal triaxial rig, and a plane strain finite element analysis (PLAXIS) of the rig was carried out to obtain the optimum location of geosynthetics in rail track substructure. A large-scale consolidometer was also employed to determine the effect of prefabricated vertical drains (PVDs) in optimising the accelerated primary consolidation of track soft formation. This paper also includes a section where recommendations are made on how to prepare the stability of rail tracks on surface formation soils considerably disturbed/remoulded by the Asian tsunami in Sri Lanka. The research findings reveal that geosynthetics have a good potential for resilient track construction and for reducing the cost of track maintenance.
In this paper the author presents a general method of obtaining approximate solutions of the steady forced vibration of a damped system of one degree of freedom for the case of sinusoidally varying disturbing forces.
The approximation consists in expressing all the damping terms of the original differential equation by a single equivalent damping term, proportional to the first power of the velocity of motion.
In the case of a system influenced by a centrifugal disturbing force and damped by constant friction and by friction proportional to the first power of the velocity, experimental evidence is in good agreement with the approximate solution.
While the paper deals solely with a system of one degree of freedom, there is no reason aside from complexity why the approximate solution cannot be applied to systems of several degrees of freedom.
The deformation and degradation behavior of unstabilized ballast and that stabilized with elastomer and geogrid was evaluated using process simulation test (PST) apparatus at various loading frequencies (f). The results indicated that elastomer has significantly reduced the extent of both vertical and lateral deformations of ballast. For example, elastomer reduced the extent of vertical settlement (Sv) in ballast from 22.76 to 9.83 mm, and the lateral deformation (ld) from 8.14 to 1.95 mm (f=15 Hz). It was also seen that the deformation and degradation of both stabilized and unstabilized ballast increased nonlinearly with the increase in the applied loading frequency (f). Further, the beneficial effect of elastomer in restraining the lateral flow of ballast was seen over the entire depth of treated ballast, unlike the geogrid, whose efficiency was maximum at its placement location and then reduced at farther locations. Moreover, elastomer has significantly enhanced the resilient modulus (Mr) and damping ratio (D) of ballast when compared with geogrid-reinforced ballast. For example, the elastomer increased the Mr and D of ballast by 25.9% and 66.6%, respectively, in comparison with an increment of only 15.1% and 25.3% in the case of geogrids (f:15 Hz). The Mr and D of ballast were found to be influenced by the effectiveness of the ballast stabilization technique in reducing lateral displacements. Additionally, elastomer has significantly reduced the vertical stress (σv) at the ballast–subballast interface by 34%. Further, elastomer was found to be more efficient than the geogrid in reducing the dynamic amplification factor (DAF) at any loading frequency. Similarly, elastomer reduced the breakage of ballast by 71% compared with the 40% of geogrid reinforcement (f=15 Hz).
The use of scrap rubber in ballasted tracks has shown promising results in terms of vibration reduction, enhancement of damping ratio, and so on, but its use in slab tracks has not been explored yet. The present study investigates the use of scrap rubber in the form of ground and granulated rubber mixed with soil and treated with Polyurethane Foam Adhesive (PFA). The optimum content of rubber and PFA were obtained from static direct simple shear test. This optimum rubber and PFA dosage were then used to prepare triaxial specimens and tested cyclically to assess the performance of the treated and untreated soil. A novel base layer for a slab track comprising soil, scrap rubber and PFA is proposed. The novel base material shows superior performance in terms of the reduced settlement, decreased excess pore water pressure and enhanced damping ratio.
In current study the effect of treatment zone on the performance of polyurethane stabilized ballast layer was examined under cyclic loading conditions. Test results reveal that the deformation and degradation response of ballast is significantly dependent on the depth of ballast treated with polyurethane. For a given treatment depth, the ballast samples treated only beneath the sleeper and those with treatment all along the track length has undergone similar extent of deformations and degradation. On the other hand, with the increase in treatment depth from top 75 mm of ballast (i.e. one-fifth of ballast depth) to 380 mm (full depth of ballast layer), the lateral spread reduction index (LSRI) was found to increase significantly. Further, it is evident that the polyurethane has significantly reduced the vertical stresses and improved resilient and damping characteristics of ballast. It is seen that the track stiffness (k) of all PSB samples was significantly higher than that of unstabilized ballast (USB) sample. The track life enhancement factor (Lef) defined the ratio of number of load cycles (N) required to attain the designated strains in case of stabilized ballast and unstabilized ballast is found to vary from 2000 to 60 for 0.4 % volumetric strain. The performance ranking of different PSB samples is given based on the track stiffness per unit of polyurethane used. The current study recommends the treatment of top half of ballast layer but only beneath the sleeper as it was found to be effective in improving the performance of ballast.
A series of cyclic loading tests were performed to establish the effect of particle breakage on the damping, resiliency and serviceability of railroad ballast. Five geogrids of triangular, square and rectangular apertures with different sizes were used to stabilize the ballast having the mean diameter (D50) of 42 mm. It was established that the particle breakage that occurs during cyclic loading helps in energy dissipation and also enhances the track resiliency. Further, the quantum of particle breakage is governed by the extent of lateral and vertical strains in ballast. Accordingly, empirical relationship has been established between breakage reduction index, BRI with the lateral and vertical spread reduction indices, LSRI and VSRI. The damping ratio (Dr) and resilient modulus (Mr) is shown to increase with the increase in particle breakage (Bg). Furthermore, the volumetric (εvol) and shear strains (εs) in ballast at the end of testing was found to be influenced by the particle breakage (Bg). Further, the track life enhancement factor (Lef), defined as the ratio of number of load cycles (N) required to attain a given extent of strains in ballast in case of geogrid-reinforced ballast (Grb) to that of unreinforced ballast (Urb), is found to vary from 500 to 1 for 0.3 and 0.5% lateral strain and 12.50 to 1 for 2 and 3 % vertical strain. The Lef was found to be highest in case of geogrid G1. In addition, a unique relationship is established between Bg and Lef wherein it was found that as the particle breakage increases, the service life of track reduces.
The ballasted tracks require periodic maintenance to retain the track alignment and restore the track geometry. In an effort to overcome these challenges, various track stabilization techniques like introduction of geosynthetics in track substructure and the treatment of ballast with bitumen and polyurethane is adapted by several railway organizations. The geosynthetic insertions provide additional lateral confinement to ballast owing to the interlocking of particles within the geogrid apertures. On the other hand, the bitumen and polyurethane stabilization creates a bond between the individual particles thus helping in improving the track performance. This paper critically reviews the performance of ballast stabilized with different techniques. The practical benefits and the associated challenges with each of the stabilization methods are also highlighted in the paper.
A brief description is made of the rockfill laboratory of the Comision Federal de Electricidad in Mexico. Equipment is available for large-scale (specimens 113 cm in diam) triaxial and compressibility testing of granular materials with maximum particle size up to 20 cm, under lateral pressures up to 25 kg per sq cm. Results of triaxial tests run with three different rockfill materials are presented. The influence of gradation, confining pressure and particle breakage are discussed.
Laboratory investigations were carried out using process simulation test (PST) apparatus to assess the role of geogrid on the deformation response and resilient modulus of railroad ballast at different loading frequencies (f) varying from 10 to 40 Hz. The materials used for testing involved fresh granite ballast and subballast having mean particle sizes (D50) of 42 mm and 3.5 mm, and geogrids with different aperture shapes and sizes. The results from the laboratory investigations reveal that the degradation and deformation behavior of ballast is influenced by the loading frequency (f). The use of geogrids is shown to reduce the extent of deformations in ballast (both lateral and vertical) and its degradation (Bg: ballast breakage), but the effect of geogrid diminishes with the increase in frequency (f). The assessment of the lateral strain profiles along the ballast depth indicates that the geogrid influence zone (GIZ) is dependent on both the type of geogrid used and the loading frequency (f). The GIZ is found to vary from 38 mm (0.90 D50) to 237 mm (5.64 D50), with it being the lowest at f = 40 Hz and for geogrid having relatively larger apertures. Further, the geogrids are found to significantly enhance the resilient modulus (Mr) of ballast. In addition, the Mr is established to increase with f and Bg. The inclusion of geogrids also reduced the extent of vertical stress (σv) including the dynamic amplification factor (DAF).
A series of large-scale cyclic tests was conducted using process simulation test ( PST) apparatus to capture the influence of loading frequency ( f) on the deformation and degradation behavior of ballast with and without geogrids. Fresh granite ballast and subballast having mean particle diameter ( D 50 ) of 42 mm and 3.5 mm, respectively and five geogrids of different aperture shapes and sizes were used in this study. The tests were conducted at f ranging from 10 to 40 Hz and up to 250,000 load cycles. The test results from the laboratory investigations confirmed that the deformation and degradation behavior of ballast is highly influenced by loading frequency ( f). Moreover, the results showed that geogrid reinforcement stabilizes ballast by reducing the extent of lateral spreading ( l d ) and vertical settlement ( S v ) and by minimizing particle breakage ( BBI). The inclusion of geogrids also reduces the extent of vertical settlement in subballast layer ( S vsb ). In addition, the extent of volumetric ( ε v ) and shear strains ( ε s ) including void ratio ( e f ) and density ( γ bf ) were found to be highly influenced by f. It is further seen that lateral spread and vertical settlement reduction index ( LSRI and VSRI) of all the geogrids decreases with the increase in f. Moreover, it is revealed that with the decrease in LSRI, the extent of S v and BBI increases. A correlation is developed between the deformations ( l d and S v ) and interface efficiency factor ( α). The effectiveness of geogrid in keeping the ballast in place is evaluated in terms of a new parameter ‘geogrid efficiency factor ( G ef )’. The value of G ef for geogrids used is found to vary from 0.02 to 0.27. It is further shown that G ef under cyclic loading conditions is a function of the interface efficiency factor ( α) evaluated under direct shear conditions.
During the development for railway, ballasted track is dominant structure and makes up more than 95% of the whole track modes. However, its shortage is considerable in high speed railway and heavy haul system. Regarding to the ballasted railway track's defects as particle breakage, settlement, and geometry irregularity which lead to enormous maintenance and cost. Polyurethane reinforced ballasted track has shown great application prospect. This reinforcement method can settle several problems, including stiffness adjustment, ballast flight prevention, and stability in specific zones, such as curve, tunnel line. This paper presents a comprehensive review of polyurethane research and application within ballasted track system. Besides, according to different usage, varies of bonding methods are also introduced in this paper. However, some challenges still exist such as maintenance and cost, potential solutions are put forward for further investigated and validated, consequently. Accordingly, an overall prospect of polyurethane reinforcement in railway system is presented.
Ballasted rail tracks form one of the most essential worldwide transportation modes in terms of traffic tonnage, serving the needs of bulk freight and passenger movement. High impact and cyclic loads can cause a significant deformation leading to poor track geometry. Regular impact loads at crossings, switches and insulated joints are often observed to cause rapid densification and degradation of ballast apart from the induced noise and vibrations. Moreover, the transfer of high dynamic and impact loads to the underneath subgrade sometimes causes its liquefaction and subsequent fouling of the overlying granular media. In order to mitigate these problems, the concept of the inclusion of geosynthetics in rail tracks is introduced. This paper presents the current state-of-the-art knowledge of rail track geomechanics, including results obtained from laboratory testing, field investigations and numerical modelling to study the load-deformation behaviour of ballast improved by geosynthetics. The shear stress-strain and deformation behaviour of geosynthetic-reinforced ballast is investigated in the laboratory using a large-scale direct shear test device, a Track Process Simulation Apparatus (TPSA) and a Drop-weight Impact Testing equipment. Computational modelling using the discrete element method (DEM) is employed to simulate geosynthetic-reinforced ballasted tracks, capturing the discrete nature of ballast aggregates when subjected to various types of loading and boundary conditions. Discrete element modelling is also used to conduct micromechanical analysis at the interface between ballast and geogrid, providing further insight into the behaviour of ballast subjected to cyclic loadings. These results provide promising approaches to incorporate into the existing track design routines catering for future high speed trains and heavier heavy hauls.
Ballasted railway tracks, despite their benefits, present some limitations and drawbacks, mainly associated with geometry degradation due to ballast settlement and particle breakage. Periodic maintenance interventions are thus required as well as renewal processes, which lead to the significant consumption of natural materials and energy whilst causing frequent interruptions to traffic. This is made more problematic when aggregates with appropriate characteristics for ballast are not available in the proximity of the construction/maintenance site, which is becoming increasingly common due to restrictive environmental guidelines. In this context, this paper presents a review of the effectiveness of the major conventional techniques/materials for track design and maintenance as well as innovative solutions that are being developed to reduce track degradation, whilst also analysing their main parameters to optimize track behaviour and durability, depending on its design and the required changes in its mechanical performance. The aim is to then provide a set of recommendations and guidelines for the use of such technologies to improve track response and durability as well as highlighting possible further research associated with both the development of innovative solutions and the improvement of conventional techniques.
The paper reports on the study of variation of horizontal stability of the sleeper base of a ballast section depending on the degree of ballast strengthening with a polyurethane-based two-component RT-CS-001 binder. The studies are performed on an acting section of the railroad. The thickness of the strengthened ballast is determined as a function of the binder flow rate. After GeoComposite thus formed gained full strength, modulus of elasticity of the structure is measured. It is shown that the strengthening of ballast with the binder increases Young’s modulus by a factor of four, allowing creating the track sections of varied rigidity both in the construction process and during the current maintenance of railways. Measurements of the lateral resistance of unloaded sleepers in the static mode against the flow rate of the binder fixing the shoulder of the ballast prism are performed. Strengthening of the ballast prism shoulder increases 2.7 times the static lateral resistance to the shift of the sleeper upon application of lateral load without application of vertical load; it does not prevent the use of the machines for deep cleaning of the ballast material during repair and modernization of railway tracks. It is shown that on long railways sections, the flow rate of binder material and the thickness of the resulting GeoComposite can be monitored by the ground penetrating radar (GPR).
The two main functions of ballast are train-load distribution and track-water drainage. Repeated train passage subjects ballast aggregates to abrasion and, eventually, breakage. Mixing of bonding materials such as polyurethane into aggregates can effectively mitigate ballast degradation. In the present study, large-scale triaxial tests were carried out to investigate the characteristics of polyurethane-mixed ballast materials. The stiffness and strength of the materials could be predicted in terms of the polyurethane contents, and the linear relationships could be established. According to those relationships, the performance of polyurethane-mixed aggregates as enhanced railway ballast materials can be simply but reasonably estimated.
Large repetitive wheel loads from heavy haul and passenger trains can cause significant track deformation that leads to poor track geometry and safety issues. The inclusion of geosynthetics and rubber mats (i.e., shockmat) in critical sections in the track for reducing these adverse effects was further examined through an extensive field trial in the town of Singleton, New SouthWales (NSW), Australia. Four types of geosynthetics and a shockmat were installed below the ballast layer in selected sections of track constructed on three different
subgrades (soft alluvial clay, hard rock, and concrete bridge), and the performance of the instrumented track was monitored for five years under in-service conditions including tamping operations. The measured stress-deformation response indicates that the geosynthetics effectively control the long-term and transient strains in the ballast layer, with the obvious benefit of reducing maintenance costs. The study also showed that the aperture size of geogrids in the range of 1.1 times the mean particle size of the ballast was most effective. The placement of shockmat on a concrete bridge contributed to reduced ballast breakage. The dynamic amplification of stresses induced by moving trains was observed, and it became more pronounced at higher axle loads and train speeds. The dynamic track modulus was evaluated adopting the concept of modified beam on an elastic foundation (BOEF), and this approach was found to be largely influenced by the axle load, train speed, placement of synthetic inclusions, and type of subgrade.
Understanding the complex load transfer mechanism and the subsequent accumulation of deformation in ballast and subballast layers under repeated wheel loading is essential to design resilient rail tracks. Large-scale cyclic tests have been conducted on railroad ballast instrumented with optical based fiber Bragg grating (FBG) sensors, LVDTs, pressure plates and the settlement pegs to explore the role of geogrid and its interaction with ballast in improving the track performance. Latite basalt and geogrids with different aperture sizes were used for the investigations. The laboratory experimental results indicate that the geogrid inclusions enhance track performance by arresting the lateral spreading of ballast and thereby significantly reducing the extent of its vertical settlement. In contrast, the reinforcement of ballast with geogrid has only a marginal effect on reducing the settlement in the subballast layer. The results also show that geogrid minimises the amount of particle breakage, the effectiveness of which is governed by its placement position, with lowest breakage occurring when the geogrid is placed at a location 130 mm above the subballast. In addition, geogrids also reduce the extent of vertical stress in the subgrade soil. The laboratory test results establish beyond doubt the effectiveness of FBG sensing system in capturing the ballast movement under cyclic loading.
This paper presents preliminary findings of a new technology currently being tested in a research project at the University of Illinois. The effectiveness of elastomer polyurethane coating of ballast is evaluated for its ability to reduce aggregate breakage and resulting ballast fouling. Railroad ballast degradation and fouling related to aggregate breakdown under heavy axle loads, poor drainage, mud pumping, and water/ballast pockets are among the most commonly encountered track substructure (ballast, subballast, and subgrade soil) problems. The structural integrity of seriously fouled ballast can be compromised leading to track instability and ultimately, train derailments. Because of this serious consequence, costly ballast maintenance activities, such as undercutting, tamping, and shoulder cleaning, are routinely performed by railroads especially on tracks serving the heavy axle load unit trains. In the research project, clean AREMA No.4 aggregates along with the polyurethane coated particles were subjected to realistic field loading conditions in a large shear box test apparatus used for strength testing of ballast at full gradation. The urethane coated ballast was allowed to set for 1, 3, 7, and 14 days prior to subjecting the samples up to 10 shear passes. Shear and normal stress data were gathered during testing; and the fines generated by all tested samples were collected and analyzed. Early findings show a major increase in the shear strength gained with the polyurethane coating, a decrease in the breakdown of the coated ballast, and a decrease in particle reorientation which could lead to a reduction in ballast settlement.
This paper presents the measured results of full-scale testing of railway track under laboratory conditions to examine the effect on the track stiffness when the ballast is reinforced using a urethane cross-linked polymer (polyurethane). The tests are performed in the GRAFT I (Geopavement and Railways Accelerated Fatigue Testing) facility and show that the track stiffness can be significantly enhanced by application of the polymer. The track stiffness is measured at various stages during cyclic loading and compared to the formation stiffness, which is determined prior to testing using plate load tests. The results indicate that the track stiffness increased by approximately 40 to 50% based on the measured results and from the previously published GRAFT I settlement model. The track stiffness was monitored during loading for a maximum of 500,000 load cycles. The paper concludes by presenting and commenting on, the application of the technique to a real site where the Falling Weight Deflectometer was used before and after polymer treatment to determine the dynamic sleeper support stiffness. The very challenging site conditions are highlighted, in particular the water logged nature of the site, and comment made on the effect of the water on polymer installation. The results of the FWD measurements indicate that a good increase in overall track stiffness was measured. These results are consistent with the laboratory tests which are performed on a different soil and use a different measurement technique and hence confirm that regardless of the soil and measurement system track stiffness increases are observed using this technique.
The paper presents a study of the effect of geosynthetic reinforcement on the (cumulative) plastic settlement, of a repeatedly loaded plane strain footing on a thin layer of granular aggregate overlying different compressible bases. Results were obtained using aggregate particles of both rounded and angular (freshly crushed) shapes. Past work has reported results using different geosynthetic reinforcement types, different ratios of footing width to reinforcement width and different reinforcement depths. Herein results for a surface footing, mainly 300mm wide, placed on a unreinforced or geosynthetic reinforced aggregate are presented for a series of plane strain tests. Most tests were conducted on a 50mm depth of aggregate that was placed on a 12.5mm depth of compressible base. Three base types are used; rigid plus 12.5mm of aggregate; 12.5mm thick base of equivalent elastic modulus (Em) of 4.40MN/m2; and similarly 12.5mm base of equivalent elastic modulus (Em) of 0.625MN/m2. The repeated load was returned to zero at the end of each cycle that is typical of a vehicle loading on a track or pavement support. The results demonstrated the importance that the compressible base and the reinforcement has on aggregate behaviour (settlement and breakdown). The practical significance to ballasted gantry crane and railway tracks is briefly discussed.
Ballast being an unbounded granular medium spreads laterally when subjected to high-frequency cyclic loading. To reduce lateral movement of ballast and to optimize track performance, rail tracks can be reinforced with geogrid. In this study, a novel large-scale process simulation test (PST) apparatus that can capture the lateral strain variation upon loading is described. Laboratory tests were conducted to explore the deformation and degradation response of both unreinforced and reinforced ballast under high-frequency cyclic loading. Fresh Latite basalt having an average particle size (D50) of 35 mm, and geogrids with different aperture sizes were tested. The laboratory experimental results reveal that the ballast deformation (both lateral and vertical) and the breakage during cyclic loading are influenced by the geogrid type and its placement location. Moreover, the lateral strain profiles along the ballast depth have been measured and the geogrid influence zone (GIZ), defined as the distance to which the effect of geogrid in arresting the lateral displacement of ballast exists, has been determined. The GIZ is found to vary from 160 mm (4.60D50) to 225 mm (6.45D50) depending on the location of the geogrid. In addition, the optimum geogrid position in the track has been identified to be 65 mm above the subballast. The test results also exemplify the ability of geogrid to arrest lateral displacement of ballast, reduce settlement and minimize particle degradation under high-frequency cyclic loading.
This paper presents the measured settlement results of full-scale testing of unreinforced and polymer reinforced ballast railway track under laboratory conditions, using the GRAFT I (Geopavement and Railways Accelerated Fatigue Testing) facility to show the significant reduction in track settlement that can be obtained using in-situ ballast reinforcement techniques. The measured settlement profile of each track specimen is monitored up to a maximum of 500,000 load cycles at different load levels and formation conditions, ranging from a subgrade Young’s modulus of 25–51 MPa. The results of a 3-dimensional polyurethane ballasted track reinforcement technique called XiTRACK, at an equivalent axle load of 44.4 tonnes, are then directly compared to the unreinforced settlement profile using developed settlement models from the tests. The polymer treated track is observed to reduce permanent track settlement by 99%, effectively giving slab-track like performance. The free draining properties of the formed GeoComposite are also highlighted. The paper concludes by presenting and commenting on, the application of the technique to real UK Switch & Crossing sites on the West Coast Main Line and at Clapham Junction. In the former case, at Points 215D, the application of the technique eliminated the need for track maintenance over the 10 year operating cycle at axle loads of 25 tonne and main line speeds of 125 mph confirming the measured laboratory observations.
Understanding the complex mechanisms of stress transfer and strain accumulation in layers of track substructure under repeated wheel loading is essential to predict the desirable track maintenance cycle as well as the design of the new track. Various finite element and analytical techniques have been developed in the past to understand the behavior of composite track layers subjected to repeated wheel loads. The mechanical behavior of ballast is influenced by several factors, including the track confining pressure, type of aggregates, and the number of loading cycles. A field trial was conducted on an instrumented track at Bulli, New South Wales, Australia, with the specific aims of studying the benefits of a geocomposite installed at the ballast-capping interface, and to evaluate the performance of moderately graded recycled ballast in comparison to traditionally very uniform fresh ballast. It was found that recycled ballast can be effectively reused if reinforced with a geocomposite. It was also found that geocomposite can effectively reduce vertical and lateral strains of the ballast with obvious implications for improved track stability and reduced maintenance costs.
This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10-40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20 Hz≤f≤30 Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel approach was used to model a two-dimensional (2D) projection of real ballast particles. Clusters of bonded circular particles were used to model a 2D projection of angular ballast particles. Degradation of the bonds within a cluster was considered to represent particle breakage. The results of DEM simulations captured the ballast behavior under cyclic loading in accordance with the experimental observations. Moreover, the evolution of micromechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading was presented to explain the mechanism of particle breakage. It has been revealed that particle breakage is mainly due to the tensile stress developed during cyclic loading and is located mainly in the direction of the movement of ballast particles.
A series of experiments are described involving the full-scale simulation of geogrid reinforcement for railway ballast, which allowed the key parameters influencing the reduction in vertical settlement (permanent deformation) under repeated loading to be studied. The results demonstrated that grid geometry, stiffness, rib cross-sectional shape and junction strength are all influential. The research data was applied as part of a wide ranging study to improve the effectiveness of ballast reinforcement and understanding of the fundamentals of grid/aggregate interaction.
The XiTRACK three-dimensional track reinforcement technique has been very successful in overcoming difficult, long-standing track problems on the railway. This success led to the specification of the technique for use at Deep Wharf level crossing, Purfleet. This was the first time that the technique had been used to reinforce the track structure at a level crossing. The main technical challenge was the application of the technique to stabilise the track over very poor ground, namely the very soft alluvial soils present at the site. A review of other proposed methods, such as concrete foundations and/or piles, illustrated the cost-effectiveness of the reinforcement method: in particular, the rapid installation of the system (the polymer cured within 15 s) minimised track downtime and thus enabled considerable cost savings to be achieved. The design process adopted was used to predict track behaviour before and after treatment, which enabled the design of the most appropriate polymer rheology and polymer distribution and loading levels to achieve optimum performance and ensure that the technique worked. This paper describes the work performed at Purfleet using the technique, including measured acceleration time histories before and after treatment.
Results of large-scale laboratory model tests conducted to determine the permanent settlement due to cyclic load of the railroad bed for a proposed high-speed train route extending from Seoul to Pusan in South Korea are reported. The possibility of using geogrid layers as reinforcement to reduce settlement of the subbase layer was investigated. Based on the present model test results, it appears that practically all permanent settlement due to cyclic load is completed after application of 105 cycles of load. The most beneficial effect of reinforcement is derived when one layer of geotextile and one layer of geogrid are placed at the interface of the subgrade soil and subbase course.
Improvement of track, highway and runway unbound aggregate behaviour using geogrids is researched. Geogrid reinforcement into unbound aggregate in most cases will improve the performance of the unbound aggregate portion of a transportation support. Unfortunately, the optimal location and number of geogrid layers have not been established. Presented are experimental results for three different construction possibilities of geogrid reinforcement in the unbound aggregate layers. The aggregate layers were subject to both repeated loading and static loading. The advantages of the different construction methods are studied and field applications are discussed. Finally, conclusions are made regarding the optimal position of the geogrid reinforcement.
A review of track design procedures-sleepers and ballast
- T Jeffs
- G Tew
- Jeffs T.
Elastomeric revetments-An innovative solution for coastal protection and erosion control
- S D Hicks
- M Bower
- Leberfinger
Resilience characteristics of subgrade soils and their relation to fatigue failures in asphalt pavements
- H B C K Seed
- C E Chan
- Lee