Void Contamination Index Research Articles

Elastan-Polyurethane Treatment for the Stability Improvement of Coal-Fouled Ballast

This study assesses the performance of coal-fouled ballast upon treatment with a hydrophobic elastomeric polyurethane, Elastan [Elastan-stabilized fouled ballast (ESFB)] and compares it with ballast that was initially clean but became fouled after its treatment during the track service life [fouled Elastan-stabilized ballast (FESB)]. The cyclic test results revealed that the performance of ESFB was significantly poorer compared with FESB for similar levels of fouling. The ESFB had undergone higher extent of deformations and had exhibited lower damping and resilient modulus values when compared with FESB. However, the extent of breakage of ballast particles was almost similar in both ESFB and FESB samples. Further, the extent of shear strains developed in ESFB samples, especially at higher void contamination index (VCI) values and loading frequencies, exceeded the shear strain at which breakage of bonds occurred under direct shear loading conditions. Accordingly, the critical loading frequency and fouling content up to which the polyurethane-based treatment is effective is proposed in the current study. In addition, the deformations of both stabilized and unstabilized ballast are seen to be directly influenced by the interface shear strength of ballast.

Effect of clay fouling on track drainage capacity of railway Ballast: Experimental and numerical study

Ballast fouling is a key contributor to the instability of ballasted railway tracks due to its detrimental influence on the permeability of the ballast layer. Predicting the track drainage condition in a ballasted track segment frequently affected by fouling is vital for planning track maintenance activities. However, only limited studies have been conducted to assess the hydraulic conductivity and track drainage capacity of fouled railway ballast. Hence, full-scale permeability tests were performed in this study using an in-house custom-built large-scale permeability apparatus at the laboratory to attain hydraulic conductivities for 5 different fouling levels of ballast fouled with sandy lean clay material. Based on the experimental results, a relationship between the Void Contamination Index (VCI) and hydraulic conductivity was obtained. The results elucidated that the fouling material reduced the hydraulic conductivity of ballast by more than 90% when VCI was 25% or higher. In addition, using the hydraulic conductivity values obtained from experimental results, a detailed 2D numerical seepage analysis of a rail track was carried out to represent the track drainage capacity at various fouling levels. Based on the results, the significance of maintaining a clean top ballast layer and shoulder ballast was highlighted and a criterion for estimating the drainage capacity of a clay-fouled railway track was proposed to initiate track maintenance operations.

Behaviour of coal-fouled elastomeric polyurethane-stabilized ballast under cyclic loading

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.

Investigating particles migration caused by mud pumping in ballasted track subgrade under cyclic loading-wetting coupling

To study the mud pumping mechanism in the conventional Chinese ballasted track subgrade, a physical modelling system which can simulate the cyclic loading-wetting coupling process was developed. The model specimen is compacted in a transparent cylinder with an inner diameter of 500 mm, including a 350 mm subgrade soil and a 200 mm ballast layer. The test system was equipped with four types of sensors to monitor the load, displacement, volumetric water content, and pore water pressure, respectively. A high-definition camera was installed to monitor the particle migration phenomenon. The results indicate that as the volumetric water content increases, the specimen gradually changes from strain hardening to strain softening. Dynamic pore water pressure is the key factor for particle migration. As the volumetric water content increases, the amount of migrated particle caused by dynamic pore water pressure gradually increases. In the saturated state, it will cause a large number of particles to migrate, the mud pumping phenomenon is significant. The void contaminant index (VCI) [1] of the ballast finally reached 38 %. In practice, such value implies the necessity to carry out subgrade replacement for the normal operation of the railway.

Shear Strength and Drainage Characteristics of Elastomeric Polyurethane Treated Coal-Fouled Ballast

The shear behavior and drainage characteristics of coal-fouled ballast when treated with elastomeric polyurethane are assessed by means of large-scale direct shear and permeability tests. The results from direct shear tests confirmed that the shear strength of both stabilized and unstabilized coal-fouled ballast was highly influenced by the extent of fouling (VCI: void contamination index). The performance index (PI) of elastomer-stabilized coal-fouled ballast (ESFB), determined as the fraction of shear strength of fouled ballast to the shear strength of fresh and unstabilized ballast, lies in the range of 1.23 to 0.84. Moreover, the performance of ESFB having VCI ≥30% was found to be either similar to or poorer than that of clean ballast without any treatment, thus indicating that the elastomer treatment may be provided only to ballast with VCI ≤30%. The results from constant head permeability tests indicate that the hydraulic conductivity of ballast ( k) is highly influenced by the presence of fouling materials but is only slightly reduced as a result of the elastomer stabilization. The k of ballast decreased from 43 to 0.18 mm/s as the VCI increased from 0 to 75%. For VCI ≥ 45% the k of ballast was found to be lower than that recommended for sub-ballast. On the other hand, the k of ballast reduced slightly from 43 to 37 mm/s because of the elastomer stabilization. Furthermore, an empirical relationship is established between k and e to determine the k of both stabilized and unstabilized fouled ballast.

A semi-empirical dilatancy model for ballast fouled with plastic fines

ABSTRACTIn the era of high speed trains, it is very important to ensure the stability of rail tracks under adverse conditions including the fouling of ballast. Fouling of ballast from unstable and saturated soft subgrade soil is one of the major reasons for track deterioration. The reported results of a number of large-scale laboratory experiments on the shear behaviour of ballast and fouled ballast are analysed, herein. It was observed that fines have a significant influence on the shear behaviour of ballast. Shear strength increases and dilatancy decreases with the addition of fines. In this paper, a semi-empirical mathematical model has been proposed to capture the dilatancy of ballast fouled with fines during shearing. The empirical constants a, b and c proposed in the model are a function of the fines content Void Contamination Index (VCI). The results of the model have been compared with the laboratory experiments and are found to be in good agreement.

Micromechanics-Based Investigation of Fouled Ballast Using Large-Scale Triaxial Tests and Discrete Element Modeling

AbstractRailway ballast comprises unbounded discrete grains that are often used to form a load-bearing platform for tracks. Ballast degradation as trains pass over the tracks and infiltration of external fines including slurried (pumped) fine subgrade soils are two of the main reasons for ballast fouling. Fouling causes tracks to settle and also reduces the load-bearing capacity, which is associated with a reduction in internal friction and increased lateral spreading of the ballast layer. This paper presents a study of mobilized friction angle, volumetric behavior, and associated evolutions of contact and fabric anisotropy of fouled ballast subjected to monotonic triaxial loading using a series of large-scale triaxial tests and discrete element modeling. Monotonically loaded and drained triaxial tests were carried out on ballast with levels of clay fouling that varied from 10 to 50% void contamination index (VCI) subjected to three confining pressures of 10, 30, and 60 kPa. The results showed that an inc...

Modelling geogrid-reinforced railway ballast using the discrete element method

Abstract Rail ballast is an unbounded granular material that spreads laterally when subjected to train loading. Railroads can be reinforced by geogrids to reduce lateral movement and to optimize track performance. This paper presents a study of the behaviour of geogrid-reinforced ballast subjected to monotonic and cyclic loading using a large-scale direct shear box and a novel Track Process Simulation Apparatus (TPSA). The shear stress–strain response of fresh and fouled ballast reinforced by geogrid was investigated using large-scale direct shear tests subjected to normal stresses from 15 kPa to 75 kPa, where the levels of fouling varied from 20% to 95% Void Contamination Index (VCI). Cyclic tests for fresh and fouled ballast were conducted using the TPSA to realistically simulate real track conditions. The experimental results showed that a geogrid provides extra internal confinement and interlocks the aggregates in its apertures, hence reduces ballast deformation. The discrete element method (DEM) was used to model geogrid-reinforced fresh and fouled ballast subjected to monotonic and cyclic loading. Irregularly-shaped particles and geogird were simulated by clumping spherical balls together, while the coal fines were simulated by adding 1.5 mm diameter spheres into the pore spaces of ballast. The predicted stress-displacement responses obtained from the DEM were in good agreement with those measured in the laboratory, where the peak shear stress of fouled ballast decreased and the dilation of fouled ballast increased with an increasing level of fouling. The contact force distributions and the orientations of normal and shear force were analyzed to provide more insight into the behaviour of ballast subjected to shearing.

DEM simulation of the behaviour of geogrid stabilised ballast fouled with coal

Geogrids are commonly used in railway construction for reinforcement and stabilisation. When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface behaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together 10–20 spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures. Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15kPa to 75kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress–displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed.

Deformation of Coal Fouled Ballast Stabilized with Geogrid under Cyclic Load

This paper presents the results of laboratory investigations into the deformation of coal fouled ballast stabilized with geogrid at various degrees of fouling. A novel track process simulation apparatus was used to simulate realistic rail track conditions subjected to cyclic loading, and the void contamination index (VCI) was used to evaluate the level of ballast fouling. The experimental results show that coal fines act as a lubricant, causing grains of ballast to displace and rotate, and as a result, accelerate its deformation. However, coal fines also reduce ballast breakage because of a cushioning effect, that is, by reducing interparticle attrition. The inclusion of geogrid at the interface between the layer of ballast and subballast provides additional internal confinement and particle interlocking via geogrid apertures, which reduces deformation. A threshold value of VCI=40% is proposed to assist practitioners for conducting track maintenance. If the level of fouling exceeds this threshold, the geogrid reinforcement significantly decreases its effectiveness and the fouled ballast exhibits a premature dilation. Based on the experimental results, an equation incorporating the VCI was proposed to predict the deformation of fresh and fouled ballast. This equation improves track design and assists in favorable decision support for track maintenance. Additionally, the discrete element method (DEM) was also used to capture the deformation of fouled ballast subjected to cyclic loading, whereas the DEM results were compared with experimental observations.

Behavior of Fresh and Fouled Railway Ballast Subjected to Direct Shear Testing: Discrete Element Simulation

AbstractThis paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behavior of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behavior of fresh and fouled ballast were simulated and compared with the experimental results. Clump logic in particle flow code in three dimensions (PFC3D) incorporated in a subroutine was used to simulate irregular-shaped particles in which groups of 10–20 spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with a various void contaminant index (VCI) ranging from 20 to 70% VCI was modeled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behavior of fresh and fouled ballast as observed in the laboratory, showing that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing VCI. Furthermore, th...

The Role of Ballast-Fouling Characteristics on the Drainage Capacity of Rail Substructure

Abstract The ballast layer is designed to be free draining, but when the voids of the granular medium are wholly or partially filled because of the intrusion of fine particles, the ballast is considered to be “fouled.” To ensure acceptable track performance, it is necessary to maintain good drainage within the ballast layer. This paper critically examines the current methods commonly used for evaluating the degree of ballast fouling and, because of their limitations, a new parameter, “void contaminant index” is introduced. A series of large-scale constant head hydraulic conductivity tests were conducted with different levels of fouling to establish the relationship between the void contamination index and the associated hydraulic conductivity. Subsequently, a numerical analysis was executed to simulate more realistic two-dimensional flow under actual track geometry capturing the drainage capacity of ballast in relation to the void contamination index. In the context of observed test data, the drainage condition of the track could be classified into different categories together with a classification chart capturing the degree of fouling. The contents of this paper have already been considered in track maintenance schemes in the states of Queensland and New South Wales.

Behavior of geogrid-reinforced ballast under various levels of fouling

This paper presents a study of how the interface between ballast and geogrid copes with fouling by coal fines. The stress-displacement behavior of fresh and fouled ballast, and geogrid reinforced ballast was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% Void Contamination Index (VCI), at relatively low normal stresses varying from 15 kPa to 75 kPa. The results indicated that geogrid increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. However, when ballast was fouled by coal fines, the benefits of geogrid reinforcement decreased in proportion to the increasing level of fouling. A conceptual normalized shear strength model was proposed to predict this decrease in peak shear stress and peak angle of shearing resistance caused by coal fines at a given normal stress.