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The effect of polyurethane content on the behavior of ballast was assessed based on large-scale direct shear and cyclic loading tests. The large-scale direct shear tests were performed at normal stresses (σn) of 40 to 120 kPa at a shearing rate (Sr) of 9 mm/min , while cyclic tests were performed at loading frequencies (f) of 16 to 32 Hz. The direct shear test results indicated that the increase in polyurethane content from 1.5% to 3.75% by weight of ballast has significantly improved the shear strength of ballast. However, the increment in shear strength is found to be insignificant for polyurethane contents greater than 2.25%, thus indicating 2.25% to be optimum for the stabilization of ballast. Further, the cyclic load tests were performed on unstabilized ballast and ballast stabilized with 2.25% polyurethane (PSB2.25%). The test results indicated that PSB2.25% has exhibited significantly reduced extent of vertical and lateral deformations, and had shown enhanced resilient modulus and damping ratio in comparison to unstabilized ballast (USB). Further, the vertical settlement of PSB2.25% at 2 million load cycles was observed to be less than the settlement of USB at 250,000 load cycles, which implies that the stabilized ballast continues to perform effectively even after higher load cycles. [ABSTRACT FROM AUTHOR]

In the current study, we explored the performance of steel slag ballast (SSB), granite ballast (GB), and a slag–granite ballast mixture under cyclic loading conditions using a large-scale track simulation apparatus. In addition, the single-particle crushing strength and toxicity characteristics of steel slag were evaluated. The test results indicate that the SSB particles exhibited higher crushing strength (σo) than conventional GB. In addition, the toxicity characteristics leaching procedure (TCLP) test results indicate that the leaching concentrations of all the heavy metals in the slag were below the recommended limit, except for lead, which exceeded the drinking water limit. The cyclic test results indicate that the SSB exhibited lower vertical (Sv) and lateral deformation (ld) than the GB, and that, with the increase in the proportion of SSB (PSSB) in the GB, both the Sv and ld were reduced. It was additionally found that, with an increase in PSSB in the GB, the resilient modulus (Mr), damping ratio (D), track stiffness (k), and track modulus (u) increased, whereas particle breakage (Bg/BBI) decreased. It was further observed that the extent of the sleeper (avs) and ground acceleration was lower in the SSB samples than the GB samples. Nonlinear regression models were developed to predict the vertical strain (ε1), lateral strain (ε3), Mr, and D of the slag–granite ballast mixture. A detailed analysis of the performance of the slag–granite ballast mixture, in terms of the toxicity characteristics, volumetric strain (εv), shear strain (εs), Mr, D, avs, Bg/BBI, and k, revealed that a blend of 58% SSB and 42% GB (SSB58) can be used for optimal performance. Practical Applications: The problem of the excessive demand for conventional ballast is addressed in the current study by utilizing steel slag as railway ballast. The benefits of utilizing steel slag ballast (SSB) over conventional granite ballast (GB), in terms of lesser vertical deformation and particle degradation, are highlighted. The lower degradation exhibited by the SSB helped in reducing the frequency of ballast replacement operations, and hence the track maintenance costs. Moreover, owing to the higher specific gravity of the SSB, its lateral displacement was found to be lower compared with conventional GB, and thereby the lateral stability of the track is improved when using steel slag as railway ballast. In addition, the SSB exhibited a higher resilient modulus compared with conventional GB, implying better passenger riding comfort where the track has steel slag as the railway ballast. In addition, in bridge approaches, SSB can be mixed with conventional ballast to allow for a gradual transition from soft to stiff and stiff to soft zones. [ABSTRACT FROM AUTHOR]

Rubber-coated ballast (RCB) is a new innovative ballast aggregate obtained by coating natural aggregates with recycled crumb rubber by using polyurethane binder. The objective of RCB is to reduce the ballast degradation, thereby reducing the deterioration of rail track geometry and also maintenance costs. Natural ballast; crumb rubber of sizes 0.4, 0.8, and 2 mm; Elastan polyurethane; and geogrid (triangular, 69 mm aperture size) were used in this study. The results of abrasion, impact value, and soundness tests show that RCB has a higher resistance to abrasion, impact, and weathering compared to natural ballast. Subsequently, the shear behavior and particle breakage of natural ballast, RCB, unreinforced, and geogrid-reinforced RCB–natural ballast mix was explored using the large-scale direct shear apparatus at different normal stresses and shearing rates (Sr). The test results indicate that due to rubber coating the peak friction angle (φ) of RCB samples reduces as compared to natural ballast. The reduction in φ of RCB samples is attributed to a change in surface texture. Moreover, 0.4-RCB (RCB made with rubber crumbs of size 0.4 mm) shows the least reduction in φ compared to 0.8-RCB and 2-RCB (RCB made with rubber crumbs of size 0.8 and 2 mm). It was further observed that with the increase in proportion of 0.4-RCB in geogrid-reinforced natural ballast both the peak friction angle and particle breakage (Bg) of ballast decreased significantly. The current study established the optimum content of 0.4-RCB to be mixed with geogrid-reinforced natural ballast to be in the range of 44%–53%. The problem of excessive degradation of ballast particles and the associated changes in the track geometry owing to the repeated application of heavy axle loads is addressed by coating the natural ballast with crumb rubber generated from waste tires. The rubber-coated ballast (RCB) is found to offer better abrasion, impact, and weathering resistance when compared to natural ballast. Moreover, it undergoes negligible particle breakage under direct shear conditions. The decrease in the extent of particle breakage helps in reducing the frequency of ballast replacement operations and hence tracks maintenance costs. Furthermore, RCB, when reinforced with geogrid, offers better shear strength than that of natural ballast alone. These observations imply that the tracks with geogrid-reinforced RCB can effectively reduce the risk of track instability. [ABSTRACT FROM AUTHOR]

The current study assesses the deformation and degradation behaviour of geogrid and elastomer-stabilized coal fouled ballast under cyclic loading with frequencies (f) ranging from 15 to 35 Hz. Different percentages of coal were added to ballast to have void contamination index (VCI) in the range of 0–45%. Test results confirmed that the deformation and degradation behaviour of ballast was highly affected by the presence of coal fines. The elastomer-stabilized ballast (ESB) exhibited lower deformations and degradation when compared to geogrid-stabilized (GSB) and unstabilized ballast (USB). The effectiveness of geogrid/elastomer in arresting the spread of ballast along its depth is evaluated in terms of the stabilization efficiency factor (S ef, cyc ). The S ef, cyc of clean GSB is computed to be 0.107, whereas the S ef, cyc of clean ESB is 0.811. The S ef, cyc of GSB decreased significantly with the increase in VCI but in case of ESB the reduction is only marginal. Further, the resilient modulus (M r ) and damping ratio (D) of ballast reduced due to coal fouling. The elastomer has significantly improved resilient modulus (M r ) and damping ratio (D) of ballast when compared to GSB. Moreover, the extent of lateral and vertical deformations was found to be influenced by the interface shear strength as obtained under direct shear conditions. In addition, empirical models were developed to predict deformation, breakage and resilient modulus of ballast as functions of loading frequencies and VCI. [ABSTRACT FROM AUTHOR]

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 (D50) 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 (ld) and vertical settlement (Sv) and by minimizing particle breakage (BBI). The inclusion of geogrids also reduces the extent of vertical settlement in subballast layer (Svsb). In addition, the extent of volumetric (εv) and shear strains (εs) including void ratio (ef) and density (γbf) 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 Sv and BBI increases. A correlation is developed between the deformations (ld and Sv) 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 (Gef)'. The value of Gef for geogrids used is found to vary from 0.02 to 0.27. It is further shown that Gef under cyclic loading conditions is a function of the interface efficiency factor (α) evaluated under direct shear conditions. [ABSTRACT FROM AUTHOR]

In the recent times, railway organizations across the world have resorted to the use of geosynthetics as a low-cost solution to stabilize ballast. In this view, extensive studies have been conducted worldwide to assess the performance of geosynthetic-reinforced ballast under various loading conditions. This paper evaluates the various benefits the rail industry could attain because of the geosynthetic reinforcement. A review of literature reveals that geogrid arrests the lateral spreading of ballast, reduces the extent of permanent vertical settlement and minimizes the particle breakage. The geogrid was also found to reduce the extent of volumetric compressions in ballast. The overall performance improvement due to geogrid was observed to be a function of the interface efficiency factor (α). Moreover, studies also established the additional role of geogrids in reducing the differential track settlements and diminishing the stresses at the subgrade level. The geosynthetics were found to be more beneficial in case of tracks resting on soft subgrades. Furthermore, the benefits of geosynthetics in stabilizing ballast were found to be significantly higher when placed within the ballast. The optimum placement location of geosynthetics has been reported by several researchers to be about 200-250 mm below the sleeper soffit for a conventional ballast depth of 300-350 mm. A number of field investigations and track rehabilitation schemes also confirmed the role of geosynthetics/geogrids in stabilizing the tracks thereby helping in removing the stringent speed restrictions that were imposed earlier, and enhancing the time interval between maintenance operations. [ABSTRACT FROM AUTHOR]