Comprehensive Assessment of the Effectiveness of the Application of Foam and Extruded Polystyrene in the Railway Substructure

Abstract

This article presents the authors’ comprehensive evaluation of the application of specific foam thermal insulation materials, namely expanded polystyrene (EPS) and extruded polystyrene (XPS), within railway substructures. The assessment extends to real-world conditions on the tracks of Slovak Railways (ŽSR), which share substantial similarities with other countries’ railway networks. The assessment of structural composition and material selection considers these technical aspects, while technological feasibility and the environmental implications associated with material production, delivery, and incorporation into railway construction do not. Additionally, the thermal insulation materials’ qualities are compared against conventional railway substructure materials. In these conventional setups, the thermal insulation layer often incorporates crushed aggregate of specified fractions and parameters in line with legislative standards. This article complements previous research conducted at the University of Žilina, focusing on the application of various thermal insulation materials within railway substructures. These materials were both experimentally tested (scale of 1:1) and numerically modelled, with results previously published by the authors. The published works detail the utilisation of diverse thermal insulation materials in railway substructures, primarily evaluating two crucial technical parameters: the protection of the subgrade surface against adverse effects of frost and, secondarily, their impact on the deformation resistance of the railway substructure.

1. Introduction

Presently, a diverse array of thermal insulation materials is available on the construction market for potential integration within railway substructure thermal insulation layers. In terms of production method, thermal insulation materials can be divided into different categories:

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Polymer foams—EPS (Expanded Polystyrene) and XPS (Extruded Polystyrene);

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Chemical foams—PUR (Polyurethane Foam), PIR (Polyisocyanurate Foam), and PF (Phenolic Foam);

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Innovative materials—NIM (Nano-Insulation Materials), VIP (Vacuum Insulation Panels), aerogels, and panels from various recycled materials (textiles, PET, etc.);

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Lightweight concrete—FC (Foam Concrete) and AC (Aerated Concrete);

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Artificial aggregates—foam glass, blast furnace slag, expandite, expanded clay, and perlite;

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Natural aggregates—pumice, volcanic tuff or, in general, crushed aggregates (gravel) derived from diverse geomorphological sources with thermal insulation properties largely influenced by the granulometric composition and the resultant intergranularity;

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Composite thermal insulation materials—materials made by combining the above-mentioned or other thermal insulation materials.

The thermal insulation materials listed above can be categorised as suitable, conditionally suitable, or even unsuitable for the track substructure construction. At present, there is no practical experience with their application in the conditions of the ŽSR lines, which consequently hinders the objective classification into the aforementioned categories. These mentioned thermal insulation materials exhibit diverse thermal insulation and deformation parameters alongside varying costs associated with their production, transport and application in the track substructure. Moreover, their use has different quantitative impacts on the environment, encompassing differences in energy consumption, greenhouse gas emission (particularly CO₂) and potential recyclability (thus minimising waste generation after their lifecycle). The comprehensive assessment of these thermal insulation materials implies a qualitative categorisation of their suitability and effectiveness for track substructure applications.

In the conditions of the ŽSR lines, the design procedures for the sub-ballast layers of the track substructure, as outlined in [1], protect the subgrade surface against the adverse effects of frost and water exclusively using a sufficiently thick protective layer of crushed aggregate in combination with geosynthetic elements. These elements, such as geomembranes and sealing geocomposites, serve as standard ŽSR measures to prevent water penetration from the track surface to the subgrade surface. Practical experience with alternative thermal insulation materials in this context remains limited. Challenges pertaining to the protection of frost-susceptible subgrade surfaces against frost effects, as well as monitoring subsequent changes in soil and building material properties, are mainly addressed by railway administrations in regions characterised by high frost indices, notably the Nordic countries within Europe [2,3,4]. Ongoing research abroad efforts predominantly focus on frost heave [5,6,7,8,9], freezing of water in porous materials [10,11,12], and thermo-technical characteristics of soils and building materials [13,14,15,16,17,18,19,20,21]. Regarding the application of thermal insulation materials within the structural layers or the broader track substructure, more comprehensive and enduring practical experience is globally available in the utilisation of foam thermal insulation materials, specifically EPS and XPS [22,23,24,25,26], collectively known as geofoam (GFM) in the professional and scientific literature. PIR, PUR, and PF, which belong to the same category of foam thermal insulation materials, have excellent thermal insulation parameters. On the other hand, they display worse deformation parameters, low diffusion resistance (if they are without a protective layer), and a high ability to bind moisture from the environment.

As a result, their thermal resistance can be significantly reduced in an environment with higher natural moisture, which is characteristic of railway lines in the area of the track substructure. Within the substructure, it becomes essential to shield these thermal insulation materials from moisture penetration, both from above and below. The protection can be achieved using techniques such as a geomembrane (GBR) or a sealing geo-composite (GCO-B). However, while these methods are technologically feasible, they substantially increase the overall construction cost due to the enhanced resistance required against the adverse effects of frost. Consequently, the application of foam thermal insulation materials like PIR, PUR, and PF is limited in this context. Their use would demand more intricate design solutions, both in terms of technology and finances, to effectively prevent moisture infiltration into the thermal insulation layer. It is important to consider that the railway track operates as an open system, constantly exposed to direct contact with atmospheric precipitation and various climatic conditions. With these considerations in mind, this article exclusively examines foam thermal insulation materials, specifically EPS and XPS. These two materials are the most commonly utilised in the construction market due to their suitable mechanical and physical properties. Notably, they are fully recyclable in contrast to PIR, PUR, and PF. Moreover, they appear to be well suited for application within the track substructure. These observations align with both international experience and the outcomes of experimental measurements conducted by the authors as part of their research endeavours. Their research not only builds upon their cumulative experience but also extends it by offering a more comprehensive analysis essential for the practical implementation of EPS/XPS within the track substructure of real railway lines.

The aim of the article is the assess the design of the sub-ballast layers with a built-in thermal insulation layer of EPS/XPS (modified design of the sub-ballast layers) in terms of environmental load (CO₂ production, energy consumption), implemented in Section 3 of the presented article. The following part of the article (Section 2) focuses on the assessment of the different factors and conditions that have an impact and have been considered in the design of the modified design of the sub-ballast layers. This Section also presents the parameters obtained from several foreign sources or previous research activities of the authors.

2. Analysis of Crucial Technical Parameters of EPS/XPS concerning Their Possible Application in the Track Substructure on Actual Railway Lines

As a result, their thermal resistance can be significantly reduced in an environment with higher natural moisture, characteristic of railway lines in the area of the track substructure. In the substructure, these thermal insulation materials would therefore have to be protected from above and below against moisture penetration by, for example, a geomembrane (GBR) or a sealing geocomposite (GCO-B). While these protective measures are technologically feasible, they also increase the overall construction cost due to the need to counter the adverse effects of frost. For this reason, foam thermal insulation materials made of PIR, PUR, and PF have limited use here, or their possible use requires technologically and especially financially more demanding design solutions to prevent moisture penetration into the thermal insulation layer. For a practical and effective application of EPS/XPS, a thorough analysis of the environment and the properties of the track substructure concerning the actual conditions of the railway line operation is essential. In particular, it is necessary to consider the possible mechanical stress of the thermal insulation materials incorporated into the track substructure and the influence of this environment on their thermal insulation parameters in the long term, with a specific focus on their technological and economic viability. In the following Section 2.1, Section 2.2, Section 2.3, Section 2.4 and Section 2.5, the authors define the decisive properties of the environment of the track substructure, from which the minimum required parameters of EPS/XPS for application to the structural layers of the track substructure from the point of view of the traffic (operational) and non-traffic (climatic) load of the lines will be derived subsequently.

2.1. Specification of Critical Environmental Properties of the Track Substructure

The basic characteristic that all thermal insulation materials incorporated in the track substructure must meet is, in particular, their long-term ability to absorb and transfer static and cyclically repeated dynamic loads in the range of geostatic and incremental loads. These loads result in vertical stresses that the structure or its structural layers must withstand without permanent deformation. Thus, it means to withstand a load within a load range that does not exceed the allowable stresses in the individual structural levels of the substructure. The geostatic stresses, possibly initiated in the embedded thermal insulation materials, are induced by endogenous forces, i.e., the self-gravity of the superstructure layers and the embedded structures (in particular, the track skeleton). The incremental stresses, possibly initiated in embedded thermal insulation materials, are induced by exogenous forces, i.e., the gravity of trains, centrifugal and centripetal forces resulting from the movement of rolling stock along the track of different track designs and geometric layout (gradient and direction, track elevation, and magnitude of elimination of centrifugal acceleration), as well as the dynamics of wheel-track interaction due to their natural and normative limiting imperfections for a given track category and its maximum track speed. For example, for a conventional ŽSR corridor track (sleeper distribution u, superstructure 60E1, axle load 225 kN, wheel force 112.5 kN, speed zone SZ4—120 km∙h⁻¹ to 160 km∙h⁻¹ incl.), the above stress interval can be defined for a track bed with a sub-ballast layer (according to [1] referred to as the type of structural layers of the track substructure No. 2, in the case of using geosynthetic element No. 3 at the level of the subgrade surface to approx. 30 to 100 kPa and at the sub-ballast upper surface to approx. 20 to 200 kPa)—Figure 1.

If the EPS/XPS thermal insulation layer, incorporated on the subgrade surface, reduces the thickness of the protective layer only to the necessary protective layer between the aggregate of the ballast bed and the thermal insulation material, the stress redistribution effect will not reach a level identical to that of the subgrade surface. It means that it is necessary to consider a stress interval of approx. <20 to 150 kPa>. Within the defined cyclically repeated stress interval, the thermal insulation materials must function without deformations that would have a major influence on the disintegration of the track geometry. It means that these thermal insulation materials should, while maintaining their thermal insulation parameters as much as possible, retain their surface integrity and achieve only allowable deformations (under ideal conditions, only elastic deformations) while maintaining their separation function. This condition must apply to the entire design life of the railway track structure. At the same time, the thermal insulation materials incorporated in the track substructure cannot technologically restrict the routine line maintenance and repairs, in particular, tamping and dynamic stabilisation of the track skeleton, machine cleaning of the ballast bed, replacement of the ballast bed or individual components of the track skeleton, etc. These technological limitations are the main reason why, in the conditions of the ŽSR railway lines (and not only those), it is possible to consider the application of EPS/XPS foam thermal insulation materials exclusively on the subgrade surface and not on the sub-ballast upper surface level, i.e., immediately under the ballast bed.

The foam thermal insulation material manufacturers usually declare their instantaneous strength or deformation resistance at 2, 5, and 10% compression according to the relevant technical standards and regulations. For example, an XPS or EPS board, with an instantaneous compressive strength of 200 kPa at 10% compression, may compress by up to 15 mm for a thickness of 150 mm in the event of stress initiated in the substructure, but with an unclear proportion of elastic and plastic (permanent) deformations and an unclear long-term evolution of these deformations over time during cyclically repeated loading under the influence of the traffic effects. It is well known that the deformation is lower for short-term force application to an EPS/XPS board than for long-term force exposure. At the same time, long-term stress application significantly increases the plastic deformation over the deformations declared by the manufacturers under short-term loading (creep effect). Considering the permissible limits of the deformation of the absolute and relative top of the line or the rail twist in the most dynamically stressed points of the railway line (switch joints and switch crossings, rail joints, etc.), the dynamic coefficient due to the impact can locally reach significantly higher values compared to a well-maintained conventional wide gauge track, e.g., in comparison to a frictionless track.

In such marginal conditions, the application of thermal insulation materials, with their insufficient deformation resistance, could potentially escalate the operational costs for rectifying the track geometry issues. Concurrently, this could lead to the deterioration of anticipated thermal insulation parameters within the structure, exacerbating the envisaged protection of the subgrade surface against the detrimental impacts of frost. The experimental assessments conducted at the Department of Railway Engineering and Track Management within the Faculty of Civil Engineering at the University of Žilina regarding the integration of thermal insulation materials into the structural layers of the track substructure have solely focused on the influence of non-traffic loads—specifically frost and water effects [27,28,29]. Thus far, there has been no examination of the influence of traffic loads, encompassing the repercussions of plastic deformations on the thermal insulation parameters of these materials. Additionally, a determination of the minimal deformation parameters of EPS/XPS that would ensure endurance against long-term loading in the authentic track substructure has not been established. This should be a focal point for subsequent research within the Department.

In designing the structural layers of the track substructure, incorporating embedded thermal insulation materials with specific deformation characteristics necessitates the consideration of the thermal conductivity coefficients of EPS/XPS at the conclusion of the projected service life of the structure—denoted as the thermal conductivity coefficient λ and stipulated by the manufacturer. This value should be adjusted using a correction factor that takes into account both the intended service life of the structure and the operational conditions on the railway line. These conditions pertain not only to sustained mechanical stresses over time but also to the distinct environment in which the thermal insulation material is applied. An essential determinant influencing the thermal insulation attributes of EPS/XPS is the moisture content of the environment in which the thermal insulation material is implemented. The innate moisture levels of the sub-ballast layers are primarily influenced by their granulometric composition (the ratio of fine fractions to fouling) and the intensity of average precipitation (rainfall and snowfall) or the water regime of the subgrade surface along the railway line. Concerning the application of thermal insulation materials or structural components at the subgrade surface level, a pivotal factor is the sub-ballast protective layer.

According to the normative grain size curve defined in [30] and based on our practical observations, this layer maintains a consistent moisture content of approximately 5 to 7%. Conversely, the subgrade surface composed of fine-grained and frost-susceptible soils, necessitating safeguarding against frost-related harm, can exhibit greater fluctuations in moisture content depending on the water regime and precipitation intensity. Our findings from experimental measurements on the DRETM test platform reveal that this type of soil commonly attains a moisture content of approximately 20%. Consequently, it is advisable for thermal insulation materials, specifically EPS/XPS, to not exceed a sustained moisture content ranging from 5 to 10% (applicable to EPS/XPS manufactured in accordance with current quality standards).

2.2. Required Service Life of EPS/XPS in the Track Substructure

The track superstructure, for instance, is designed to last 40 years, while the substructure and its components are anticipated to endure 80 to 100 years within the ŽSR line conditions. (The attainment of the design life is contingent on faithful adherence to operational maintenance plans.) Pertinent technical standards outline the longevity requisites for foam thermal insulation materials by extrapolating quality parameters over a 50-year span, primarily because these criteria were formulated for civil engineering, where thermal insulation materials are subjected to distinct environmental and mechanical loads. Evidently, the stipulated service life or long-term functionality criteria are roughly half the magnitude of the requirements established for constructing transport infrastructure or railway lines. Consequently, there is currently no normatively prescribed methodology for appraising the service life (with a questionable feasibility of extrapolating to the aforementioned extended timeframes) or residual quality parameters tailored for an 80- to 100-year design life when implementing thermal insulation materials within the structural layers of transport facilities. On one hand, the most extensive practical applications of foam thermal insulation materials in transport structures align approximately with the midpoint of the requisite design lifetime. On the other hand, advancements in these materials have led to technological progress, yielding enhanced quality and durability compared to their earlier iterations used in the construction of transport facilities, which is a viewpoint drawn from our accumulated practical experience.

Accordingly, the selection of thermal insulation materials and their associated parameters for the dimensioning process of the track substructure’s structural layers should be quantified at the culmination of the projected design life of the structure—specifically, between the 80th and 100th year of railway line operation. Presently, the essential data must undergo individual verification, possibly via experimental measurements and simulations. Such analyses could be carried out under specific conditions within the accredited laboratory of the Faculty of Civil Engineering at the University of Žilina (SvF, UNIZA). In light of the aforementioned circumstances, the authors of this article endeavour to expand their research initiatives to encompass the material analysis of heat-insulating materials or structural components. This analysis would scrutinise their mechanical resilience in conjunction with their heat-insulating efficacy within the track substructure throughout its prescribed service life. In this context, vigilant monitoring of the temporal evolution of thermal insulation materials or element deformation subsequent to cyclic loading induced by train operations within real-world intervals, along with the progression of vertical stresses affecting the subgrade surface or sub-ballast upper surface, becomes essential. These concepts must further undergo validation by assessing actual vertical stresses exerted on the structure, utilising integrated stress sensors within the operational track, particularly at its most dynamically strained junctures (such as transition zones and switches). Following the application of stress loads, an examination of the thermal insulation materials will be conducted under varying moisture and temperature conditions. This assessment aims to comprehend the influence of these factors on the thermal insulation parameters, specifically the thermal conductivity coefficient λ.

2.3. Determination of Minimum Deformation Parameters of EPS/XPS for Railway Line Conditions

According to the authors of the article, the most effective application of thermal insulation materials for railway track conditions in terms of protection of the frost-susceptible subgrade surface against non-traffic loads is at the level of the subgrade surface, which is the result of experimental measurements and numerical modelling presented in, for example, refs. [27,28,29], to which this article is related in terms of content. In case of application of EPS/XPS in the track substructure at the level of the subgrade surface, it is possible to define an expected interval of vertical stresses at the level of approximately <20 to 150 kPa> by applying a protective sub-ballast layer between the ballast bed and the thermal insulation material. These stresses must be resisted by the EPS/XPS in the long term, ideally only in the elastic deformation mode, which does not have a negative effect on the quality of the track or the change in the thermal insulation properties of the EPS/XPS as a result of mechanical stresses. An example of the compression and deformation behaviour of EPS of different bulk densities from 15 to 46 kg·m⁻³ (uniaxial loading with an unclear long-term contribution of plastic and elastic deformation behaviour, i.e., creep) is provided in Figure 2 [31].

The elastic part of the load and deformation curve can be inferred from the shape of the hysteresis curve that demonstrates a linear dependence only in the range of 0 to 2% of the deformation, while its further development (creep) due to cyclic loading is unclear. In general, however, the higher the bulk density of EPS/XPS, the higher the resistance to long-term load effects. For the stress conditions at the sub-ballast upper surface, Figure 2 indicates that the required EPS bulk density should be at least 35 kg·m⁻³ (the stress state reached 5% deformation), which is only valid for the short-term load. The ideal EPS selection design should be exclusively based on the elastic part of the hysteresis curve, i.e., for up to 2% deformation, where the required EPS bulk density at the sub-ballast upper surface should be at least 43 kg·m⁻³ and at the level of the subgrade surface a minimum of 33 kg·m⁻³. Since the critical load component in the track substructure includes dynamic, cyclic, and repetitive forces, the stress and deformation behaviour should be monitored under dynamic and cyclic load, which may exhibit behaviour different from that of static load [32].

Figure 2. Example of the influence of the EPS bulk density on the stress/deformation curve [31] and on the coefficient of thermal conductivity [33].

Figure 2. Example of the influence of the EPS bulk density on the stress/deformation curve [31] and on the coefficient of thermal conductivity [33].

As the EPS/XPS are deformed by load, their bulk density naturally increases, which may also lead to some change in the thermal-isolation parameters or to their disruption and an increase in the influence of moisture (especially for XPS). An example of the effect of the EPS bulk density on the thermal conductivity coefficient is depicted in Figure 2, right [33].

The investigation into the long-term deformation of EPS, where the stress state was initiated at stress levels of 30%, 50%, and 70% in relation to the EPS stresses attained at 5% deformation (as depicted in Figure 3), elucidates that when EPS stress reaches 50% of its strength at 5% deformation, there exists an inconsequential influence of creep on the prolonged EPS deformation [33]. These observations underscore that, in order to effectively negate a substantial creep-induced impact on EPS deformation, the EPS stress should be constrained to a maximum of approximately 2.5% deformation. This implies that when employing EPS at the sub-ballast upper surface (with a maximum stress of 200 kPa), the requisite minimum EPS bulk density must not fall below 39 kg∙m⁻³, as illustrated in Figure 2. Similarly, at the subgrade surface level (with a maximum stress of approximately 150 kPa), the minimal EPS bulk density should adhere to a minimum of 30 kg∙m⁻³, also delineated in Figure 2. The case study [4] presents practical experience with applying XPS boards under the ballast bed in the climatically difficult conditions of Finnish railway lines. Due to the high contact stress of the ballast bed grains on the XPS layer, XPS strengths of min. 450 kPa for a 10% strain is recommended on Finnish lines for axle loads of 225 kN (min. 500 kPa for an axle load of 250 kN and min. 600 kPa for an axle load of 300 kN). The higher requirements for XPS under the ballast bed with the fraction of 31.5/63 mm are related to the fact that the bearing component of the contact stresses is generated by the interaction between the coarse grains and the XPS layer and not by the uniform theoretical stress at the contact between the ballast bed surfaces and the XPS board, which exhibits significantly lower values. According to the recommendations of the authors of the article, the establishment of a protective layer on top of the XPS layer of crushed aggregate fraction 0/31.5 mm is considered, while the XPS will also be protected by a protective geotextile (GTX-P). That means that the contact interaction of the grains is neglected, and it is possible to determine the components of the initiated stresses between two uniform, perfect surfaces.

As a general guideline, it is recommended in [4] that the enduring compressive deformation of the XPS board at the sub-ballast upper surface should not surpass 5% under cyclically repeated loading of 2 × 10⁶ cycles, utilising a load impulse of 200 kPa. Notably, the boundary of the plastic part of the stress/deformation hysteresis curve for XPS remains within 5% deformation, indicating a modulus of elasticity that is more than twice that of EPS. Drawing from practical insights gained from Finnish railway lines, it becomes feasible to extrapolate prerequisites for XPS boards implemented at the subgrade surface level. This approach, which we believe holds value not only for ŽSR conditions but also for railway systems in general, involves accommodating the conditions of various railway administrations. As stresses at the subgrade surface level dwindle to approximately 150 kPa due to stress redistribution—occasioned by the adoption of a protective thermal insulation layer instead of the sub-ballast layer—it becomes viable to establish a minimum required strength for XPS boards. This minimum strength, calculated for an axle load of 225 kN at 150 kPa for a 5% XPS deformation, equates to approximately 250 kPa for a 10% deformation.

In parallel, leveraging the aforementioned experiential data, it is possible to define essential mechanical parameters for EPS boards deployed at the subgrade surface level. According to Figure 2 and Figure 3, the EPS strength should align with the actual stress at the designated structural tier when 2% deformation is attained. This choice ensures not only resilience to cyclically repeated loads but also considers the effects of creep over time. Specifically, an EPS strength of 150 kPa at 2% deformation translates to an estimated strength of roughly 250 kPa for a 10% deformation. However, due to the sustained preservation of mechanical and thermal insulation attributes, it is imperative to establish an EPS/XPS strength threshold of at least 300 kPa for a 10% deformation.

2.4. Determination of Calculated Thermal Insulation Parameters of EPS/XPS

In scenarios where the bulk density of EPS/XPS material undergoes alterations due to the impact of long-term loads, a concurrent shift in thermal insulation parameters occurs. This shift is contingent upon the initial bulk density at zero-load application, as illustrated in Figure 2. However, the extent of these alterations is also subject to the particular product and the intricacies of its production process. For instance, in the context of grey EPS containing powdered graphite, numerous attributes experience positive enhancements, including a reduced susceptibility of thermal conductivity coefficient to changes in bulk density. Conversely, in the realm of applying XPS boards, it is essential to acknowledge the potential for damage to the cellular structure of the XPS owing to significant deformation. Such damage can lead to a considerable elevation in water absorption beyond the anticipated laboratory-verified values. This, in turn, results in a marked deterioration of the originally envisaged thermal insulation parameters.

A considerably more influential factor, which substantially diminishes the thermal insulation attributes of EPS/XPS material, is the inherent moisture content of the surrounding environment where these insulation materials are situated. Additionally, the average temperature at the level of the insulation material exerts an influence, given that the coefficient of thermal conductivity λ is moderately impacted by environmental temperature variations. Given that the conventional railway track functions within an open system, it remains exposed to direct climatic elements like rainfall, snow, and fluctuating groundwater levels. These environmental conditions invariably impact the natural moisture levels within the structural layers of the substructure and, by extension, influence the embedded thermal insulation materials. For EPS material, its moisture absorption capacity is profoundly influenced by production techniques and quality—specifically, the adequate cohesion of styrene beads. Enhanced quality in this regard reduces the size of interstitial gaps between expanded EPS beads, thereby curtailing water absorption and water vapour diffusion potential. As a general trend, the moisture absorption capacity of EPS/XPS material decreases with an increase in bulk density. Therefore, in situations involving the minimal bulk density of EPS material, the effect of moisture on thermal insulation parameters should be accounted for alongside mechanical efficiency considerations.

An exemplification of the relationship between the moisture content of EPS/XPS material and its consequential impact on thermal insulation parameters is visually presented in the corresponding figure to the right—Figure 3.

Thermal insulation materials stipulate values for the thermal conductivity coefficient λ in Declarations of Parameters (DOP). These values are typically determined according to procedures outlined in relevant standards and regulations, often confined to controlled laboratory conditions. Consequently, the values are ascertained under circumstances that diverge from the actual conditions following the incorporation of thermal insulation materials into the railway track structure. Moreover, they do not consider the enduring impact of various adverse factors that influence both mechanical and thermal insulation parameters. As an illustration, within Finnish railway legislation, as detailed in [4], the designated thermal conductivity coefficient for XPS stands at λ_XPS = 0.050 W·m⁻¹·K⁻¹ for a 40-year service life. This stands in contrast to the manufacturer-provided value in the DOP, which typically hovers around λ_XPS = 0.032 W·m⁻¹·K⁻¹. Empirical observations suggest that over a 40-year lifetime, the moisture content of XPS (specifically, higher quality XPS produced with CO₂) can elevate to 10–12%. Consequently, structural design considerations should account for the empirically determined thermal conductivity coefficient λ, rather than relying solely on the values supplied in the DOP. Furthermore, it was determined that compression-induced deformation of XPS, up to 7% deformation and under cyclic loading emulating railway traffic, did not significantly impact the thermal conductivity of the dry XPS board. However, at the aforementioned level of permanent deformation, water absorption may undergo a pronounced escalation due to the disruption of the enclosed cell structure of XPS caused by resultant mechanical damage.

The outcomes of the research project outlined in [4] also played a pivotal role in establishing technical conditions governing the supply of XPS thermal insulation boards for Finnish railways. The Finnish technical regulations exclusively permit the application of XPS at the sub-ballast upper surface. The analysis of the impact of incorporating XPS layers, featuring the specified value of λ_XPS = 0.050 W·m⁻¹·K⁻¹, on railway track freezing has been extensively examined in references [27,28,29].

The above publications characterise an experimental stand consisting of 6 measurement profiles built at a scale of 1:1. The individual measurement profiles present the conventional construction of the sub-ballast layers or a modified one (with a built-in thermal insulation layer of extruded polystyrene, liapor, composite foam concrete, or liapor concrete). To monitor the thermal regime of the railway track structure, a sufficiently dense network of temperature sensors was built into each measuring profile. To monitor the moisture regime, the measuring profiles were equipped with protective tubes to measure the moisture of the individual sub-ballast layers using a non-destructive TDR method. Based on the evaluation of several winter seasons, the thermal effect of the built-in thermal insulation materials was demonstrated as higher temperatures were determined at the level below the thermal insulation material compared to the same level in the measurement profile with the sub-ballast layers of conventional materials (crushed aggregate only). The experimental measurements provided the necessary input parameters for the subsequent numerical modelling. Based on these parameters, a numerical model was created in the SVOffice software version 2.4.29. It achieved a temperature agreement of up to ±0.5 °C compared with the actual embedded temperature sensors. The model was progressively optimised and allowed us to monitor the influence of various factors on the railway structure freezing. The entered climatic load values in the numerical models were also based on actual values acquired in cooperation with the Slovak Hydrometeorological Institute. (The necessary range of climatic load used in the numerical modelling was not recorded in the experimental stand area). The provided data allowed us to input realistic temperatures and frost periods even for high values of the frost index. Figure 4 demonstrates an example of the implemented numerical modelling, which indicates the differences in the thermal regime of the trackbed resulting from the different positions of the thermal insulation elements. Figure 4 also demonstrates that when designing a railway track for regions characterised by unfavourable climatic conditions (where the frost index (I_F) exceeds 1200 °C, day), a preferable approach involves situating the XPS thermal insulation layer at the level of the frost-susceptible subgrade surface. In contrast, if the placement is executed at the sub-ballast upper surface, the track substructure becomes susceptible to pronounced freezing along the embankment-shaped earthwork slopes and across the periphery of the track (stretching beyond the ballast bed’s width—red circles in Figure 4). In cases where the XPS thermal insulation layer is positioned at the sub-ballast upper surface, this material is concurrently subjected to substantial contact stress, attributed to the ballast particles pressing against its surface due to the operational track load. Hence, to safeguard against its deterioration, it is prudent to contemplate installing this thermal insulation material at the subgrade surface level. This recommendation holds true even for regions characterised by more favourable climatic conditions (where the frost index I_F is less than 1200 °C, day).

Implementing the thermal insulation layer at the subgrade surface level presents an opportunity for the utilisation of EPS boards, notwithstanding the adverse experiences recounted in [4]. During the 1970s, EPS boards were commonly employed in Finnish railway systems. However, their usage was discontinued after 1980 due to negative encounters arising from inadequate resistance to mechanical damage effects when positioned beneath the ballast bed—specifically, the elevated contact stress between coarse aggregate particles and the EPS board. Hence, considering the foregoing factors, the conditions prevalent in the Slovak Republic (pertaining to tracks within the High and Low Tatras regions, where frost indices reach I_F values exceeding 600 °C, day) prompt a concerted effort to restrict the application of EPS/XPS thermal insulation boards solely to the subgrade surface. This approach enhances the efficacy of safeguarding the subgrade surface, extending to the lineside path area, while simultaneously maintaining lower or acceptable mechanical stresses on this type of thermal insulation layer. This strategy not only aligns with the goal of achieving superior protection but also contributes to the reduction in deformation parameter requirements for EPS/XPS. Consequently, this approach bears the potential to curtail economic expenses and alleviate the environmental impact associated with the structure.

Given that EPS exhibits approximately 20% inferior thermal insulation parameters compared to XPS, primarily due to its internal structure, coupled with a higher water absorption capacity—observed when comparing undamaged EPS and XPS boards—our theoretical projections warrant considering a thermal conductivity coefficient λ for EPS boards that is 30 to 40% higher than that attributed to XPS boards. Consequently, for a 40-year lifespan, a calculated value of λ_EPS = 0.07 W∙m⁻¹∙K⁻¹ should be taken into account. However, owing to the requirement for the track substructure’s design lifespan to span at least 80 years within ŽSR conditions, these indicated thermal conductivity coefficients should be appropriately adjusted for this extended timeframe. This signifies that the design thermal conductivity coefficient for XPS should not be below approximately λ_XPS = 0.07 W·m^−1·K⁻¹, and for EPS, it should not dip below approximately λ_EPS = 0.10 W·m^−1·K⁻¹. The specified design thermal conductivity coefficients for EPS and XPS, acknowledging the heightened expectations for the thermal insulation layer’s longevity under practical track operation circumstances, simultaneously factor in the relatively reduced negative repercussions of mechanical damage at the subgrade surface level—compared to potential outcomes if the XPS/EPS structural thermal insulation components were placed at the sub-ballast upper surface. Nonetheless, ensuring the requisite 80-year design lifespan within ŽSR conditions necessitates adherence to the technological and design prerequisites outlined in Section 2.5. These stipulated requirements can profoundly impact the quality parameters and lifespan of the constructed substructure.

2.5. Design Principles of the EPS/XPS Layer Embedded in the Track Substructure

The establishment of technical and technological normative principles for the integration of EPS/XPS foam thermal insulation components into track substructure layers seeks to achieve cohesive and high-quality implementation of these structural layers. The ultimate goal is to ensure enduring safeguarding of the subgrade surface, spanning from mildly to significantly frost-susceptible conditions against non-traffic loads, namely the adverse impacts of frost and water. Simultaneously, this approach guarantees the stability and requisite service life of the railway track structure. One example of technological principles directly influencing the qualitative integration of the thermal insulation layer or element pertains to the treatment and placement of EPS/XPS boards. Notably, edge treatment involving a semi-groove, as opposed to boards without such treatment, yields a qualitatively elevated level of thermal insulation protection. This elevation is attributable to factors such as natural shrinkage (0.2 to 0.4%), expansion resulting from temperature fluctuations (thermal expansion coefficient ranging from 5 × 10⁻⁵ to 7 × 10⁻⁵ m·m⁻¹·K⁻¹), or board displacement due to overlaying layers’ implementation. Additionally, it addresses precision in board placement and the formation of dilatation/contraction joints. By minimising these factors, which otherwise foster the creation of thermal bridges, the thermal efficiency and protection of the frost-susceptible subgrade surface are enhanced.

Consequently, it is advisable to establish a universal requirement within the regulations of railway authorities, including Slovak Railways, stipulating that geofoam thermal insulation materials, in board form, must feature half-grooves at mutual contact edges to be eligible for use within the track substructure. Applying a sealant or adhesive at points of contact between two geofoam boards with half-grooves can ensure enhanced bonding and more effective prevention of rainwater penetration into moisture-sensitive layers. Furthermore, geofoam boards should be positioned at a transverse slope consistent with the subgrade surface’s 4–5% gradient. This strategic placement facilitates more efficient rainwater drainage above the thermal insulation layer and provides enhanced protection against the detrimental influences of water and frost on the frost-susceptible subgrade surface. Leveraging insights from experimental measurements and numerical simulations, a nomogram depicting the correlation between the design thickness of the protective layer for the frost-susceptible subgrade surface (comprising crushed aggregate layer) and the non-traffic or climatic load (frost index; average annual air temperature) has been developed, as illustrated in Figure 5.

For the conditions of ŽSR lines, when it is possible to consider a maximum frost index I_F of approximately 750 °C per day and a minimum average annual air temperature of approx. 5 °C, it is sufficient to design 30 mm thick XPS boards and an approx. 200 mm protective layer of crushed aggregate. However, for practical reasons, the proposed thicknesses of both XPS and EPS boards should not fall below 50 mm because of their possible damage (breakages) during the implementation of the overlaying (protective) layers. In the case of higher frost indices, it is a necessary condition from the point of view of the trackbed thermal resistance. In calculating the thickness of the EPS/XPS thermal insulation layer, it is necessary to consider the values of the thermal conductivity coefficient λ reduced from the ones provided by the manufacturers, as justified above (see Section 2.4). Also, at the level of the subgrade surface, where the use of a thermal insulation layer is expected, the minimum values of EPS/XPS strength must meet the values specified above (see Section 2.3).

From the point of view of ensuring the designed lifetime of the track substructure with the applied thermal insulation material for 80–100 years, it is necessary to secure specific structural requirements. The subgrade surface formed by fine-grained, i.e., mildly to dangerously frost-susceptible and poorly permeable to impermeable soils, according to [1], must be protected against the penetration of zero isotherms. It prevents the freezing of soil-bound water and, subsequently, a change in its volume (an increase in water volume after freezing by approx. 9%), which can harm the track geometry. As the fine-grained soil naturally allows moisture to migrate into the overlying structural layers, it must be suitably separated from the thermal insulation layer to prevent or limit its influence on the effectiveness of the thermal insulation layer. In addition to the separation, it is also necessary to provide drainage for rainwater, which, after penetrating the track substructure, will penetrate down to the level of the subgrade surface, susceptible to water and frost. Based on the above requirements, the authors do not recommend direct placement of EPS/XPS boards on the subgrade surface, as this would increase the probability of their moisture increase and deterioration of thermal insulation parameters beyond the expected values or the values considered in the dimensioning procedure of the track substructure, and thus reduce the service life of the entire structure. The EPS/XPS thermal insulation boards must therefore be separated from the subgrade surface either by a drainage geocomposite (GCO-D) or by a drainage layer of fine crushed aggregate or sand. This method is a more technologically demanding solution. In addition, in the case of applying a crushed aggregate or sand drainage layer, either the Ter-Zaghi filtration criterion must be met, or the layer must be separated from the subgrade surface by a filter geotextile (GTX-F) to prevent natural colmatisation and degradation of the drainage layer. The minimum thickness of the crushed aggregate or sand drainage layer should be 50–100 mm. The thickness, i.e., its drainage capacity, is influenced by the filtration coefficient determined from the grain size curve, the length of the subgrade surface at the required slope, the location and the average rainfall data, and the method of laying the EPS/XPS boards. (The longer side of the boards should be placed in the transverse direction so that a substantial part of the rainfall is drained above the thermal insulation layer).

The influence of the load progression or vertical stress acting on the thermal insulation materials must also be monitored during construction, i.e., during the laying of the overlying layers (Figure 6). Here, loads significantly higher than the future service load in the completed track structure can be generated.

For example, the General Technical and Qualitative Conditions (GTQC) of the ŽSR lines [35] stipulate that, due to technological constraints, the track substructure layers should be implemented in maximum thicknesses of 150 to 300 mm in cases of using a heavy vibratory soil compactor weighing at least 16 t. A standard soil compactor of the above weight and performance category achieves a maximum under-tread force of 250 to 400 kN at a compaction frequency of 26 to 36 Hz for a tread width of 2.13 m. If a protective structural layer of 200 mm thick crushed aggregate, implemented by successive hauling from the face reducing the work productivity, was applied to a thermal insulation layer of geofoam or other material (foam concrete, foam glass, etc.), the structural stress between the protection layer and the thermal insulation layer would reach a value of approx. 250 to 400 kPa. (In the case of a fully loaded lorry with an axle force of 11.5 t, it would be only about 150 kPa.) The above stresses would exceed by more than a factor of two the stresses initiated at this level by the effect of the operation of the railway line. If the EPS/XPS mechanical parameters were set to the conditions of stress generated on the operated track, their strength would be exceeded in the technological procedure of establishing the protective layer, with an unclear development of its plastic deformations and with a questionable influence on the durability and quality. It is thus necessary to design the protective layer as thick as possible, which, however, for technological reasons, reduces the benefits of the thermal insulation layer in the track substructure.

An alternative strategy entails using a lighter soil compactor (max. 10 t static soil compactor) without activating the vibration, even though this deviates from the prescribed regulations. However, adopting this approach not only jeopardises the attainment of the necessary deformation resistance and compaction rate as specified by regulation [1] but also extends the time for its establishment. These technological limitations must be clearly defined in the relevant regulations, or the technical requirements for EPS/XPS must be modified by increasing their strength to more than double the level, which would not be an economically and environmentally efficient solution.

As pointed out in the introduction of this Section, in the technological procedure of the construction of the track substructure with the applied thermal insulation layer of EPS/XPS, it is necessary to establish a protective layer (preferably of 0/31.5 mm fraction) under the ballast bed, with a minimum thickness of 200 mm. In the most unfavourable case, the design of the thermal insulation layer must ensure an equivalent R-value of EPS/XPS concerning a 500 mm thick crushed aggregate layer, i.e., for a design value of the thermal conductivity coefficient of crushed aggregate of λ_CA = 1.75 W·m⁻¹·K⁻¹ (case of slightly contaminated ballast bed), the minimum required thickness of the thermal insulation layer would be EPS with a design value of thermal conductivity coefficient λ_EPS = 0.10 W·m⁻¹·K⁻¹ in a thickness of 30 mm and using XPS with a design value of thermal conductivity coefficient λ_XPS = 0.07 W·m⁻¹·K⁻¹ in a thickness of 20 mm. Considering the design requirement for setting the minimum thickness of EPS/XPS in the value of 50 mm, respecting the technological feasibility of the overlaying crushed aggregate layers, the practical application of thermal insulation materials from EPS/XPS in the conditions of the railway lines of ŽSR and Lithuania appears to be economically and environmentally inefficient, i.e., with a low degree of use of this structural layer. However, it is not globally applicable, as some railway lines are built in conditions where significantly higher values of the frost index I_F are achieved, e.g., in Scandinavia, far East countries, on railway lines built at high altitudes, etc. The increased thermal insulation protection achieved by EPS/XPS boards thus has its justification and use.

If the principle of the minimum thickness of the EPS/XPS thermal insulation layer at the level of 50 mm is observed, sufficient safety in terms of long-term protection of the subgrade surface against adverse effects of frost and permissible stress from the operational load of the line are achieved. The lifetime guarantee here can reach 80 to 100 years, but at the same time, economically unnecessary overpricing or an increase in the environmental impact of the construction so implemented can occur.

3. Analysis of the Application of EPS/XPS in the Track Substructure from the Point of View of the Environmental Impact (LCA—Life Cycle Assessment)

The construction industry is responsible for a significant share of energy consumption and carbon emissions (approx. 40%). The 2030 Climate and Energy Framework set by the European Commission [36] foresees a reduction in greenhouse gas emissions (compared to 1990 levels) of 40%, a share of renewable energy production of at least 32% and an improvement in energy efficiency of at least 32.5%. By 2050, the EU’s long-term environmental strategy aims to achieve climate neutrality [37]. Climate neutrality by 2050 means achieving zero greenhouse gas emissions for the EU countries by reducing emissions, investing in green technologies and protecting the natural environment. As part of the European Green Deal [38], the Commission proposed on 4 March 2020 the first European Climate Law [39], which regulates the above objectives and strategies.

The mentioned documents imply requirements for the design and implementation of construction works in such a way that there is a significant elimination of energy consumption (or the introduction of environmentally neutral energies) and the production of carbon emissions, in line with EU environmental policy. Critical environmental impacts arise during the sourcing of raw materials, their processing, production and transport to the site for their application in the designed structure and considering its entire life cycle, including recycling. Relevant data on the environmental impact of a specific material or product can be obtained from the Environmental Performance Declaration (EPD) according to EN 15804+A2 [40] and the Type III Environmental Statement according to ISO 14025 [41], developed by LCA. These EPDs and type III declarations are still not available for materials produced in the Slovak Republic, Lithuania, or even in some other EU countries due to the absence of legislation regulating these requirements for manufacturers in the country concerned.

3.1. LCA of Input Materials of the Track Substructure’s Layers

The environmental properties of materials suitable for the construction of railway substructures vary depending on several factors, namely the following:

-

Type of raw materials;

-

The percentage of recycled raw materials introduced into the production chain;

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The origin and distance of the input materials;

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The production methods and technologies used to process raw materials and produce materials;

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The energy mix of the countries where the production process takes place;

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The distance of production from the construction site;

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The complexity of the technology of application of materials to the structure, etc.

In LCA calculations aimed at objectively determining the energy consumption and CO₂ emissions associated with the production of input materials, a significant consideration arises regarding the energy mix of countries or individual manufacturing plants. This pertains to the sources of energy production and their proportions utilised by manufacturing plants. These plants may employ distinct strategies to balance energy supply and demand throughout their operational cycle. Issues related to LCA are presented, e.g., in refs. [42,43,44,45].

Given the objective of the Section of this article, which is to compare EPS/XPS foam thermal insulation materials with conventional materials used in track substructures (primarily crushed aggregate, occasionally gravel sand), and due to the lack of Environmental Product Declaration (EPD) documents for EPS/XPS thermal insulation materials produced in, for instance, the Slovak Republic, LCAs from articles [46,47,48,49,50,51] will be utilised. These will be supplemented by data from the most widely sold EPS/XPS products in the Slovak Republic, for which EPD documents are publicly accessible (EPS—Isover [52] and XPS—BASF [53]). From the aforementioned articles, average LCA assessment values for EPS/XPS per functional unit (R = 1 m²·K·W⁻¹) and per volumetric unit (1 m³) are derived and presented in

Table 1. LCA of EPS/XPS by different authors and from EPD documents from the most sold EPS and XPS manufacturers in the Slovak Republic, converted to one functional unit 1FU = R = 1 m²∙K∙W⁻¹ and one volumetric unit 1 m³.

Source of Data	EPS		XPS
	Energy Consumption per 1 FU (MJ)	CO2 Consumption per 1 FU (kg CO2 eq)	Energy Consumption per 1 FU (MJ)	CO2 Consumption per 1 FU (kg CO2 eq)
Casini [47]	73.50	2.76	72.00	2.76
Su et al. [48]	85.00	6.25	75.00	5.45
Schiavoni et al. [49]	127.31	5.05	127.31	13.22
Biswas et al. [50]	100.87	4.18	100.97	6.11
Pargana et al. [51]	74.31	3.25	98.11	5.21
Saint Gobain Isover [52] and BASF [53]	100.41	4.39	91.28	6.32
Hill et al. [54]	85.80	-	47.30	-
Average values per 1 FU	92.46	4.31	87.42	6.51
Values converted to 1 m3	2498.84	116.58	2732.01	203.49

. These resulting average values closely align with those found in the EPDs of the most commonly used EPS/XPS materials in the Slovak Republic. This alignment establishes a solid basis for conducting an objective LCA of the entire structure incorporating an EPS/XPS layer.

The comparison of energy consumption and CO₂ consumption for EPS/XPS production, transport, application and disposal (for the entire structural life cycle) indicates a considerable data variance. For assessing the average data converted per functional unit, it is possible to state that EPS thermal insulation materials exhibit approx. 6% more energy-intensive life cycle, and XPS thermal insulation materials have, on average, 50% higher CO₂ production. To state the LCA of the whole track substructure, the average data from Table 1 were applied and converted to a volume of 1 m³. To convert from one functional unit to 1 m³ of EPS/XPS thermal insulation material, the average data of thermal conductivity coefficients reported by the manufacturers in the Declaration of Performance (DoP) were applied, i.e., for EPS λ_EPS = 0.037 W·m⁻¹·K⁻¹ and for XPS λ_XPS = 0.032 W·m⁻¹·K⁻¹.

In Slovakia and Lithuania, the authors have not identified any aggregate production plant that possesses a documented environmental declaration for producing the required fraction of crushed aggregate suitable for use in the structural layers of railway substructures. As a result,

Table 2. LCA calculation of crushed aggregate in the quarry nearest to the location of the corridor line with the most unfavourable climatic conditions and for the conditions of the railway lines (also agrees with the average data in the conditions of the Slovak Republic).

Process	Activity	Diesel Consumption per Unit of Work in	Unit of Work	Quantity Processed per Unit of Work	Diesel Consumption for the Production 1 t of Crushed Aggregate	CO2 Consumption for the Production of 1 t of Crushed Aggregate	Energy Consumption for the Production of 1 t of Crushed Aggregate
		(l)	(Sh, km)	(t)	(l)	(kg)	(MJ)
Production	drilling for blasting raw material	12 L/Sh	30 Sh	10,000	0.04	0.10	1.27
	loading of raw material	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	transport in the quarry to the intermediate storage	45 L/100 km	2 km	20	0.05	0.12	1.58
	loading the raw material into the crusher	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	crushing in the jaw crusher	36 L/100 km	1 Sh	80	0.45	1.19	15.84
	loading the semifinished material for transport	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	transport in the quarry to the intermediate storage	45 L/100 km	1 km	20	0.02	0.06	0.79
	Loading into the crusher with classifier	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	secondary crushing and sorting	40 L/100 km	1 Sh	80	0.50	1.32	17.60
	loading of crushed aggregate for hauling	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	transport in the quarry to the intermediate storage	45 L/100 km	1 km	20	0.02	0.06	0.79
	Total				1.83	4.82	64.28
Transport	loading for transport to the construction site	12 L/Sh	1 Sh	80	0.15	0.40	5.28
	crushed aggregate hauling to the construction site	35 L/100 km	35 km	16	0.77	2.02	26.95
	Total				0.92	2.42	32.23
Placement	levelling of the crushed aggregate layer	20 L/Sh	10 Sh	500	0.40	1.06	14.08
	compaction of the crushed aggregate layer	10 L/Sh	10 Sh	500	0.20	0.53	7.04
	Total				0.60	1.58	21.12
Recycling/ recovery	layer extraction and loading	15 L/Sh	1 Sh	70	0.21	0.57	7.54
	transport of crushed aggregate to the storage	35 L/100 km	35 km	16	0.77	2.02	26.95
	levelling of crushed aggregate in the storage	20 L/Sh	1 Sh	200	0.10	0.26	3.52
	Total				1.08	2.85	38.01
Total LCA of crushed aggregate per 1 t of production						11.67	155.64
Total LCA of crushed aggregate calculated per 1 m3 of production						24.51	326.84

presents an indicative calculation of energy consumption and CO₂ emissions for a production plant utilising diesel-powered crushing and screening lines. This includes a two-stage crushing process (primary crushing for a mono-fraction of 0/90 mm and secondary crushing followed by screening to obtain fractions of 0/31.5 mm for the sub-ballast or protective layer and 4/8 mm for the drainage layer). The calculation also encompasses aspects such as production, transportation, storage, site preparation, and recycling at the end of the product’s life cycle.

The considered distribution distance from the aggregate production site to the disposal site is 35 km, which aligns with the average distribution distance between quarries and construction sites (based on data from the Main Mining Office of the Slovak Republic). The placement of crushed aggregate within the construction layer is executed using a wheel grader for levelling, followed by compaction using a 16-ton vibratory compactor. After a projected lifespan of 80 to 100 years, the plan involves removing the crushed aggregate from the railway substructure and transporting it 35 km to a quarry near the construction site. There, it will be utilised as part of the land reclamation process. This framework for crushed aggregate production, transportation, incorporation, and eventual recovery post-use can be generalised to a majority of aggregate production scenarios in the Slovak Republic and Lithuania. Looking ahead, as part of efforts to reduce carbon emissions, aggregate production plants may consider shifting energy sources for crushing and screening operations to carbon-neutral fuels or transitioning to green electricity.

To enable a potential comparison between the Life Cycle Assessments (LCAs) of track substructures incorporating an EPS/XPS thermal insulation layer and those composed conventionally, it becomes imperative to ascertain the LCA values pertaining to the filter geotextile (GTX-F) positioned atop the subgrade surface and the protective geotextile (GTX-P) placed over the EPS/XPS layer. In the interest of simplicity and due to relatively comparable and minimal energy and CO2 consumption figures for both GTX-F and GTX-P across manufacturers, this study focuses on LCA data from a prominent manufacturer with a substantial market presence within the Slovak Republic. Detailed information is provided in

Table 3. LCA of filter geotextile (GTX-F) and protective geotextile (GTX-P).

Source of Data	Filter Geotextile (GTX-F)		Protective Geotextile (GTX-P)
	Energy Consumption per 1 m2 (MJ)	CO2 Consumption per 1 m2 (kg CO2 eq)	Energy Consumption per 1 m2 (MJ)	CO2 Consumption per 1 m2 (kg CO2 eq)
Naue GmbH and Co. KG [55]	39.995	2.639	72.173	3.549

.

3.2. LCA of the Railway Substructure under the Rail Skeleton

Based on the determined LCA values of the materials listed in Table 1, Table 2 and Table 3, necessary for the construction of the track substructure, it is possible to assess its structural composition in the place under the track skeleton. This assessment specifically targets the climatically most unfavourable conditions on the ŽSR corridor lines, with the considered low deformation resistance of the subgrade surface at the level of E_0r = 15 MPa. (E_0r is the static modulus of deformation in the climatically most unfavourable period, the so-called spring deformation resistance). It represents a subgrade surface of fine-grained soils with elevated moisture content, susceptible to freezing in the case of zero isotherm penetration.

The design of the structural composition in

Table 4. Design and comparison of the composition of the track substructure for the most unfavourable climatic conditions on the modernised ŽSR corridor line (SZ4) with the subgrade surface of deformation resistance E_0r = 30 MPa and the required value of the equivalent modulus of deformation on the sub-ballast upper surface E_sub = 50 MPa.

Structure (A)—Conventional Substructure	Structure (B)—Substructure with an EPS Layer	Structure (C)—Substructure with an XPS Layer
subgrade surface frost-susceptible to dangerously frost-susceptible soils + filtration geotextile (GTX-F)
sub-ballast layer 500 mm thick of crushed aggregate fr. 0/31.5 mm to ensure deformation resistance + protective layer of 200 mm of crushed aggregate fr. 0/31.5 mm to provide additional protection against frost	drainage layer 100 mm thick of crushed aggregate fr. 4/8 mm (1)	drainage layer 100 mm thick of crushed aggregate fr. 4/8 mm (1)
	thermal insulation layer of EPS 20 mm thick (50 mm) (2)	thermal insulation layer of XPS 14 mm thick (50 mm) (2)
	protective geotextile (GTX-P)
	protective layer 250 mm of crushed aggregate fr. 0/31.5 mm
ballast bed 350 + 200 mm thick fr. 31.5/63 mm

The considered thermal conductivity coefficients are λ_CA = 1.75 W·m⁻¹·K⁻¹, λ_EPS = 0.10 W·m⁻¹·K⁻¹ and λ_XPS = 0.07 W·m⁻¹·K⁻¹; ⁽¹⁾ the filtration geotextile (GTX-F) with a drainage layer of crushed aggregate fr. 4/8 mm can be replaced by drainage geocomposite (GCO-D) with the required drainage capacity; ⁽²⁾ due to structural reasons, the EPS/XPS thickness shall be set to a min. of 50 mm and subsequently rounded up in multiples of 10 mm.

applies to the case of upgrading corridor lines on the existing earthwork under ŽSR conditions (the specified value of static modulus of deformation on the sub-ballast upper surface is E_sub = 50 MPa). The considered category of the corridor line is for SZ4, i.e., for an operating speed of 120 to 160 km·h⁻¹. In case of insufficient deformation resistance of the subgrade surface (E_0r < 30 MPa), it is possible to either stabilise the subgrade surface or apply a reinforcing geosynthetic (preferably a rigid geogrid) to the sub-ballast layer. These boundary conditions correspond to the actual situation on the railway lines. While stabilising the subgrade surface presupposes its favourable water regime (low water table and or low capillarity of the underlying soil) and longer working sections, using reinforcing geosynthetics has no limitations.

Thus, for comparison, we will use a case whose application is more versatile, i.e., reinforcement of the sub-base layer. Based on the dimensioning of the structural layers for traffic loads according to the legislative document [1], the thickness of the subbase layer for the given boundary conditions is t_SL = 500 mm + 1 geogrid, e.g., Tensar SS20 type. For the case of dimensioning for non-traffic loads (frost action) and for the most unfavourable climatic conditions, in which some sections of the modernised Slovak lines are located (namely in the area of the High and Low Tatras, located in the trans-European corridor No Va), the thickness of the protective layer is t_PL = 700 mm. According to the principles of dimensioning the structural layers of the railway substructure according to the methodology given in [1], the higher structural thickness is decisive, i.e., in our case, 700 mm.

Table 4 indicates that to achieve the required value of the modulus of deformation at the sub-ballast upper surface of the railway substructure E_sub, it is necessary to design the thickness values of the sub-ballast layer that prevent the penetration of the zero isotherms into the subgrade surface. It means that the required thickness of the thermal insulation layer of EPS/XPS is minimal, and due to its structural rounding to a minimum of 50 mm, its complex efficiency (economic, environmental, and technical) will be relatively low. For the above-suggested compositions of the structural layers of the railway substructure (structural compositions (A), (B) and (C), the following

Table 5. LCAs of railway substructures for newly constructed and modernised ŽSR corridor lines in climatically most unfavourable conditions.

Structural Layer	Railway Track Modernization for SZ4 E0r = 15 MPa, Esub = 50 MPa, IF = 700 to 750 °C, Day
	Conventional Structure (A)	Structure (B) with an EPS Layer	Structure (C) with an XPS Layer
Filtration geotextile (GTX-F)	2.64	2.64	2.64
Sub-ballast layer	8.58	-	-
Drainage layer	-	2.45	2.45
Thermal insulation layer of EPS/XPS	-	5.83	10.17
Protective geotextile (GTX-P)	-	3.55	3.55
Protective/sub-ballast layer	8.58	6.13	6.13
TOTAL (kg CO2 eq.)	19.80	20.60	24.94
Filtration geotextile (GTX-F)	40.00	40.00	40.00
Sub-ballast layer	114.39	-	-
Drainage layer	-	32.68	32.68
Thermal insulation layer of EPS/XPS	-	124.94	136.60
Protective geotextile (GTX-P)	-	72.17	72.17
Protective/sub-ballast layer	114.39	81.71	81.71
TOTAL (MJ)	268.78	351.50	363.16

presents their respective LCAs.

Table 5 indicates that the environmental impact of implementing the track substructure layers is significantly higher if it contains thermal insulation layers of EPS/XPS than when using conventional natural materials (crushed aggregate or gravel–sand). In the case of modernising the existing railway lines, the CO₂ consumption for implementing the track substructure with an EPS thermal insulation layer is only 4% higher and with an XPS thermal insulation layer nearly 26% higher compared to conventional natural materials. These differences are caused by the minimal effect of the EPS/XPS thermal insulation layer on the increase in the deformation resistance of the structure apart from the thermal insulation benefit, i.e., it does not affect the reduction in the structural thickness of the sub-ballast layer to ensure the required deformation resistance of the structure at the sub-ballast upper surface. At the same time, there are usually no climatic conditions in the Slovak Republic where the use of EPS/XPS would have a significantly positive impact economically and environmentally compared to conventional structure. Therefore, the article authors will concentrate future research activities on using other, especially innovative thermal insulation materials, which, in addition to providing high thermal resistance, can also provide sufficient deformation resistance of the structure. These materials would achieve a higher reduction in the structural thickness of natural materials (e.g., crushed aggregate) and, at the same time, economically and environmentally positive benefits. A significantly higher design lifetime would be achieved compared to that using EPS/XPS foam thermal insulation materials. Such materials, according to our experience so far, seem to include composite foam concrete [28], which will also be investigated, and the obtained results will be analysed and subsequently published.

3.3. LCA of the Railway Substructure under the Rail Skeleton

The research conducted by the University of Žilina [27,28,29], which assesses various railway substructure materials in terms of protecting the frost-susceptible subgrade surface against the adverse effects of frost, indicates that the most critical place with the risk of subgrade surface freezing is the place of the sideline path. This area is situated along the embankment, where the sub-ballast upper surface is no longer shielded by the ballast bed. The sideline path serves for installing the necessary technical equipment for the railway line, such as the foundations of the traction line poles and signalling equipment, installation of cable trenches, kilometre markers and other necessary elements of railway operation, as well as noise barriers and screens, among others.

Applying thermal insulation materials at this place thus exhibits design limitations. It should also be noted that subgrade surface freezing via the sideline path has less influence on the quality of the track geometry and service life than its freezing in the place directly under the track skeleton or in the active zone of the traffic load. Given the lack of detailed knowledge of the long-term effects on service life and track geometry quality, the protection of the subgrade surface against the adverse effects of frost over the sideline path is not currently mandated in the current ŽSR legislative documents. The existing procedure for designing the structural composition of the railway substructure from the aspect of protection of the subgrade surface against adverse effects of frost identifies, according to the regulation [1], the critical location of the assessment in the place under the ballast bed, specifically within the track axis. The regulation [1] does not consider the possibility of subgrade surface freezing over the sideline path area. Consequently, from an LCA perspective, an alternative option will be explored where protection against adverse frost effects is required by applying an EPS/XPS layer on the sideline path. This approach could potentially yield a more environmentally favourable outcome compared to beneath the ballast bed. Below,

Table 6. Design and comparison of the track substructures in the place of the sideline path for the railway line SZ4 implemented as a new construction and as an upgraded existing double-track line with a double-sided roof-like slope of the subgrade surface of 4% and a horizontal sub-ballast upper surface.

Sideline Path of Modernised Railway Track for SZ4 E0r = 15 MPa, Esub = 50 MPa, IF = 750 °C, Day, Slope 4%
Conventional Structure (D)	Structure (E) with an EPS Layer	Structure (F) with an XPS Layer
sub-ballast layer 350 mm thick	drainage layer 100 mm thick (1)
	EPS layer 52 (60) mm thick (2)	XPS layer 36 (50) mm thick (2)
	protective geotextile (GTX-P)
layer to protect the subgrade surface 900 mm thick	protective/sub-ballast layer 250 mm thick

The considered thermal conductivity coefficients are λ_CA = 1.75 W·m⁻¹·K⁻¹, λ_EPS = 0.10 W·m⁻¹·K⁻¹ and λ_XPS = 0.07 W·m⁻¹·K⁻¹; ⁽¹⁾ the filtration geotextile (GTX-F) with a drainage layer of crushed aggregate fr. 4/8 mm can be replaced by drainage geocomposite (GCO-D) with the required drainage capacity; ⁽²⁾ due to structural reasons, the EPS/XPS thickness shall be set to a min. of 50 mm and subsequently rounded up in multiples of 10 mm.

outlines the suggested structural layer composition for the sideline path in both the new section and the original earthwork of the upgraded SZ4 line. Likewise,

Table 7. LCA overview of the track substructures in the place of the sideline path for the new section of the modernised line in the climatically most unfavourable conditions on the ŽSR corridor lines.

Structural Layer	Sideline Path of an Existing Section of Modernised Track for SZ4 E0r = 15 MPa, Esub = 50 MPa, IF = 700 to 750 °C, Day
	Conventional Structure (D)	Structure (E) with an EPS Layer	Structure (F) with an XPS Layer
Filtration geotextile (GTX-F)	2.64	2.64	2.64
Sub-ballast layer	8.58	-	-
Drainage layer	-	2.45	2.45
Thermal insulation layer of EPS/XPS	-	6.99	10.17
Protective geotextile (GTX-P)	-	3.55	3.55
Protective/sub-ballast layer	22.06	6.13	6.13
TOTAL (kg CO2 eq.)	33.28	21.76	24.94
Filtration geotextile (GTX-F)	40.00	40.00	40.00
Sub-ballast layer	114.39	-	-
Drainage layer	-	32.68	32.68
Thermal insulation layer of EPS/XPS	-	149.93	136.60
Protective geotextile (GTX-P)	-	72.17	72.17
Protective/sub-ballast layer	294.16	81.71	81.71
TOTAL (MJ)	268.78	351.50	363.16

provides an overview of the LCA values based on identical inputs as those presented in Table 5.

Should the technical legislative documents of Slovak Railways (ŽSR) or Lithuanian Railways (LTG) include a mandate for safeguarding the subgrade surface against the adverse effects of frost within the sideline path area upon demonstrating that prolonged subgrade surface freezing in this region could potentially diminish the lifespan of the railway substructure, the incorporation of an EPS/XPS thermal insulation layer would yield a favourable environmental outcome, particularly during the modernisation of existing corridor line sections. An exhaustive analysis of the efficiency of applying EPS/XPS within the track substructure establishes this as the sole approach for EPS/XPS implementation that carries a beneficial environmental impact. However, in the case of newly established segments along modernised corridor lines, where stringent demands are placed on the deformation resistance of the entire track substructure, including the subgrade surface (thereby precluding subgrade surface vulnerability to frost’s adverse effects), the utilisation of EPS/XPS foam thermal insulation materials lacks a positive environmental impact, even within the sideline path area. Consequently, deploying EPS/XPS in the most climatically challenging conditions of ŽSR or LTG lines appears ineffectual and is not advocated by the article’s authors due to the aforementioned reasons.

Building on the preceding analysis, the authors have set a goal to focus research efforts on exploring innovative or more environmentally conscious materials suitable for the structural constituents of the track substructure. These materials should exhibit high thermal resistance and contribute to an enhancement of structural deformation resistance. In a broader sense, the application of thermal insulation materials should lead to a comprehensive reduction in the thickness of natural material layers (such as crushed aggregate) while ensuring that such implementation offers distinct economic and environmental advantages compared to conventional construction methods.

4. Conclusions

When designing a new structure or undertaking the modernisation or reconstruction of an existing railway line, it is imperative to encompass a range of considerations, including mechanical, environmental, and economic aspects. This entails accounting for construction costs, design activities, future maintenance, and operational expenses, all of which must be comprehensively understood and minimised while adhering to technical and regulatory prerequisites. Consequently, the design of the track substructure or the structural sub-ballast layers necessitates a flexible and variant approach. All undertakings should strike a balance between cost and quality, ensuring multi-modal, interoperable, and safe transportation while striving to achieve the lowest feasible carbon footprint.

The prevailing and inevitable trend of minimising environmental impact in the construction sector finds its strong justification, particularly in the context of the most ecologically sound mode of transportation—rail transport—operating on high-quality infrastructure. Such a mode can significantly contribute to realising the objectives outlined in the framework of the European Union’s climate strategy. The authors embarked on an endeavour to ascertain whether the utilisation of EPS and XPS foam thermal insulation materials can viably integrate into the design and implementation procedures of structural sub-ballast layers to align with the aforementioned strategies.

The main conclusions of this analysis are as follows:

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From the point of view of long-term maintenance of the mechanical and thermal insulation properties, a strength of at least 300 kPa for 10% deformation should be defined for EPS/XPS. It should withstand the assumed stresses applied at the level of the subgrade surface of 150 kPa (see Section 2.3).

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The EPS/XPS thermal insulation layer has virtually no effect on the increase in the deformation resistance of the structure apart from the thermal insulation benefit, i.e., it does not affect the reduction in the structural thickness of the sub-ballast layer to ensure the required deformation resistance of the structure at the level of the sub-ballast upper surface.

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It is more suitable to place the EPS/XPS thermal insulation layer at the level of the frost-susceptible subgrade surface to ensure its thermal protection (see Section 2.4).

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In the dimensioning procedure for the structural composition of the sub-ballast layers, it would be appropriate to use a design coefficient of thermal conductivity for XPS higher than λ_XPS = 0.07 W·m^−1·K⁻¹ and EPS higher than λ_EPS = 0.10 W·m⁻¹·K⁻¹ for a design life of the sub-ballast layers of 80 years.

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For the currently valid technical legislative documents of the ŽSR lines, applying EPS/XPS to protect the frost-susceptible subgrade surface against the adverse effects of frost cannot achieve an economically and at the same time environmentally positive impact compared to the conventional construction of the railway substructure with the use of natural materials (see Table 5).

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The above conclusion may change in favour of EPS/XPS if there were a fundamental alteration in the energy mix of EPS/XPS production, with a greater share of green energies (non-carbon or carbon-neutral sources).

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In the case of building or upgrading railway lines in areas with significantly more unfavourable climatic conditions than the territory of the Slovak Republic (e.g., the Scandinavian countries), the economic and environmental impact of the assessed structures with an EPS/XPS embedded layer would be more significant. (In the case of a conventional design of the sub-ballast layers, a significantly thicker structural layer of crushed aggregate is required).

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The climate load limit values at which the design of a railway substructure structure with an EPS/XPS thermal insulation layer would become significant are the air frost index I_F = 900 °C, day and the mean annual temperature θ_m = 4 °C. For these climatic conditions, it is necessary to design, in the case of a conventional design of sub-ballast layers, a design thickness of the protective layer of approximately 0.80 m.

Employing a similar methodology, the authors intend to undertake further research endeavours aimed at evaluating the efficacy of other conventional or pioneering thermal insulation materials (e.g., composite foam concrete) within the track substructure. Such materials could potentially supplement the existing legislative framework applicable to ŽSR conditions, thereby facilitating the feasible and comprehensive application of thermal insulation materials within the track substructure. This approach also aligns with an emphasis on the European Union’s climate strategy. Further research activities will also focus on monitoring the influence of dynamic effects from real train traffic on the sub-ballast layers and the design of modified structural compositions of the sub-ballast layers.