Selection of Track Form in Railway Tunnel from a Life Cycle Analysis Perspective

doi:10.21203/rs.3.rs-3734827/v1

The use of greenhouse gas emissions as a criterion for decision-making within the rail industry is increasing. The demand for the consideration of this criterion affects the type of decision models acceptable by railway infrastructure managers in the planning, construction, and maintenance of railway assets. The total amount of greenhouse gas emitted from a track solution in tunnels during its service life depends on the track form (i.e., ballasted track or the ballastless track), the type of construction, maintenance machines used, the current traffic profile, and the length of the tunnel. However, the development in the design of ballastless track systems during recent decades to make them environmentally friendly is a motivation for infrastructure managers to rethink and consider the use of the system. This paper addresses the effect of applying the optimized ballastless track system Rheda 2000® in a railway tunnel (the Hallsberg-Stenkumla tunnel) as part of a new line project in Sweden. The greenhouse gas emissions, represented by life cycle CO₂ equivalent emission is calculated using the climate impact software developed by the Swedish Transport Administration Trafikverket. The result is compared with the estimated emission from the conventional ballasted tracks. The study shows that CO₂ equivalent emissions by a ballastless track during its life cycle is 10% lower than that of the ballasted track. The primary total emission driver for both track form solutions is the emissions generated at the manufacturing of rails. The second important emission factor for the ballasted track solution is the emission from the renewal of the track form during its life cycle. The second important emission factor for the ballastless track solution is concrete manufacturing. The model applied in the study is an integral part of an integrated decision support system for effectively selecting track solutions from a lifecycle perspective.

Ballastless track

greenhouse gas emission

railway tunnel

decision support system

The rail infrastructure is one of the challenging technical systems for which the Swedish Transport Administration implements thorough planning and managerial control. As a vital part contributing to infrastructure effectiveness, rail track assets aim to ensure proper availability and economic profitability, influencing the selection of type and configuration of track solutions. However, the sustainable development challenges and current research have recently brought a deeper understanding of track assets' environmental impact [1]. Ecological friendliness has become an increasingly influencing criterion when selecting track solutions [2], [6]. One of the negative environmental impacts of track assets is life cycle GHG emission, represented by life cycle CO₂ equivalent emission. Despite the problem's complexity, reducing CO₂ equivalent emissions is a recognized challenge humanity must solve shortly [3].

There are two construction types of railway tracks referred to as track forms in this article, i.e., the ballasted track and the ballastless track. When considering the track forms from the structural point of view, the primary difference between them is the presence and the lack of some components in the track structure. The ballast layer and relatively low content of concrete and steel in the ballasted track structure distinguish it from the ballastless track structure with a high content of steel and concrete while lacking ballast.

Researchers have differing views on which of the track forms results in higher life cycle CO₂ equivalent emissions. Zandi et al. argued that the emission from a ballastless track is more significant than from a ballasted track; the main reason for this is the more significant percentage of concrete and steel in the structure of the ballastless track [2].

Milford and Allwood investigated the life cycle CO₂ equivalent emission generated by four types of tracks and revealed that at moderate traffic loads the process and maintenance emissions of double-headed embedded track are lower than that for the double-headed conventional track [4]. Pons et al. evaluated the environmental impact of the track forms in 18 impact categories. They revealed that CO₂ equivalent emissions of ballasted tracks are lower for service life up to 50 and 75, and 100 years [5]. Biancardo et al. pointed out a higher environmental friendliness of the ballastless track than the ballasted one [6]. The authors argued that a higher environmental impact of the ballasted track depends on the less available ballast of proper characteristics and the increased use of raw material compared to the readily available concrete track plate in the ballastless track.

Previous research has not adequately addressed the following issues in the estimation of life cycle CO₂ equivalent emissions in their studies: (i) earthworks and geotechnical conditions affecting the design of track forms, (ii) differences in maintenance and renewal required for track forms in the corresponding line condition, and (iii) recent developments in optimizing the environmental impact of ballastless tracks. The current study aims to address the abovementioned issues in the revealing relationship between the types of track forms, their design, maintenance, and renewal needs, operating conditions, and their CO₂ equivalent emissions during the life cycle. This article presents the result of the study of the life-cycle CO₂ equivalent emissions of the ballastless track solution compared to the ballasted one using the Swedish railway tunnel case.

This article is an extension of the conference paper titled “Selection of Track Solution in Railway Tunnel: Aspect of Greenhouse Gas Emission” [7], that was presented at IAI2023 Congress. Additional content of this study include: a description of the standard LCA approach and its adaptation to create a customised LCA methodology for the current case study. Further, parametric study and sensitivity analysis have been carried out and added to the initial study to improve the understanding of the relationship between track design and life cycle carbon emissions. This provides information about the most significant factors that can influence the selection of track form for the case study.

2.1 Study Approach

The approach implemented is a bottom-up approach previously used in the research related to the life cycle assessment (LCA) [2], [8]. This approach allows for assessing the environmental impact of a system using the deep knowledge of its life cycle demands. Implementing the approach, we focused on one specific asset type of the railway infrastructure, i.e., a track asset in rock tunnel, and aimed to identify the reduction potential for the life cycle CO₂ equivalent emissions.

The life cycle CO₂ equivalent emissions studied are for (i) the transportation of rock material produced by the excavation of a 4.8 km double-pipe railway rock tunnel along the Hallsberg - Stenkumla line in Sweden, (ii) the construction, maintenance, and renewal of the Rheda 2000^® Green ballastless track system during the tunnel operating time. The life cycle CO₂ equivalent emissions from the track system were calculated for the design without the hydraulically bounded layer (HBL) (see the green-contoured area in Fig. 1), and considering the relevant traffic conditions, the estimated service life of the track elements and the complex of works for the construction, maintenance, and renewal of the ballastless track in the tunnel. In this study a similar calculation for a ballasted track solution in the tunnel was performed. The life cycle CO₂ equivalent emissions from the ballastless and ballasted track solutions in the tunnel were compared to reveal the dependence of a track asset's life cycle CO₂ equivalent emissions on the type of its track form.

2.2 Standard LCA Approach

The standard life cycle assessment (LCA) approach is built on the principles described in ISO 14040:2006 [10]. It encompasses six modules of life cycle stages, i.e., (i) pre-design phase, (ii) product phase, (iii) construction phase, (iv) operation phase, (v) disposal, and (vi) other relevant information such as machinery environmental impact, etc. The modules, in turn, are divided into several life cycle stages. which are set depending on the assessment´s character. Table 1 presents the standard LCA structure, including modules and life cycle stages.

The standard LCA approach requires that the criteria for selecting life cycle stages and the models used are described and motivated. The life cycle stages depend on the assessment´s character and can be modified to create an adapted LCA.

Table 1

The elements in the standard LCA in ISO 14040:2006.
Module	Life Cycle Stage
A0 Pre-design phase	A0	Feasibility Study
A1-A3 Product phase	A1	Acquisition of raw materials
	A2	Transportation
	A3	Production
A4-A5 Construction phase	A4	Transportation
A4-A5 Construction phase	A5	Construction and installation
B1-B8 Use phase	B1	Use
	B2	Maintenance
	B3	Refurbishment
	B4	Replacement
	B5	Renewal
	B6	Operating energy
	B7	Other operating processes
	B8	Users ‘energy use
C1-C4 Final phase	C1	Dismantling, demolition
	C2	Transportation
	C3	Recovery of used product
	C4	Disposal
D Operations beyond system boundaries		For instance, lighting and heating

2.3 Data Collection

All data collected for the current study fall into two subcategories: (i) general data required by the climate impact calculation tool (CICT) developed by the Swedish Transport Administration [8] and (ii) system-specific data related to the case study-specific conditions. Input data for the first subcategory include the quantities of geotechnical works, raw materials and track components and the pre-installed CO₂ equivalent emission factors for these factors. Standardized emission-related measures associated with the construction, maintenance, and renewal works adopted by the Swedish Transport Administration was used. The second subcategory encompasses the expected life cycle behaviour of the Rheda 2000® Green ballastless track system in the study case, i.e., the quantitative values of the corresponding design elements and emission-related construction, maintenance, and renewal works required for the Rheda 2000® Green system in the Hallsberg - Stenkumla tunnel.

The data collection method includes gathering basic data and filling data gaps. The primary data were collected by completing the data request in CICT; the basic quantitative values were obtained using the system's selected boundaries. The missing data related to the ballastless track system design was gathered; information from research databases and the experts and designers at RailOne, that is, the developer and manufacturer of the ballastless track system Rheda 2000® Green was obtained.

Computation of CO₂ Equivalent Emission

The computation of CO₂ equivalent emissions was carried out using CICT based on the standard LCA principles and framework. The applied logical order of calculation is as follows.

The emissions in CO₂ equivalent from the transportation of rock material from the tunnel's construction have been calculated. The amount of rock material was estimated as a function of the tunnel cross-section. CO₂ equivalent emissions from tunnelling and excavation are not included.
The CO₂ equivalent emissions from the construction of track solution includes the emissions during (i) the “cradle-to-grave” of the track components, i.e., from the extraction of raw material to the finished product, (ii) the construction of the track system, and (iii) transport of the material and components.
The CO₂ equivalent emissions for the operation phase encompass the emissions from maintenance and renewal of the track during the estimated service life of the tunnel considered by a pessimistic life cycle scenario presented in section 3.4. According to the Swedish Transport Administration's experiences and using the principles of the standard element model developed at the Graz University of Technology [9], the frequencies for maintenance and renewal work for the ballasted track in the tunnel concerning the line traffic conditions were established. A similar set was selected for the Rheda 2000^® Green system. Meanwhile, the system's design was assimilated for the Hallsberg – Stenkumla tunnel conditions. A series of design-adapting workshops with the system designers was conducted.
The CO₂ equivalent emissions from the final stage phase were not included in the computation. The reason for this is the lack of knowledge about demolishing the ballastless tracks.
Within the module of other environmental information, the Hallsberg-Stenkumla project´s conditions influencing CO₂ equivalent emissions were regarded in the computation. To name some, it concerns the line´s traffic conditions, the productivity of the applied machinery, and diesel consumption by this machinery during construction and maintenance.
Emissions in modules for the respective track solutions were combined into a total emission value of the considered track solution using the formula below. Then the main CO₂ equivalent emission drivers for each track form were determined.

$$Total {CO}_{2}equivalent= \sum _{t=1}^{T}\sum _{n=1}^{N}{X}_{tn}$$

Where t is the time period or lifecycle phase, n is lifecycle activities and X_tn is the CO₂ equivalent emissions associated with activity N occurring during time period t. T is the calculation period while N is the total number of life activities for track system.

3.1 Description of the Line

The New double-track line between Hallsberg and Stenkumla lies in Örebro country and connects the Hallsberg railway yard with the new double-track asset in Stenkumla. Along the section Km 7 to Km 9, the tracks go through a double-pipe railway tunnel. The line is dedicated to mixed traffic. Table 1 presents the characteristics of the line´s traffic conditions.

Table 2

The technical specification of traffic conditions on the Hallsberg – Stenkumla line.
Parameter	Unit	Value
Axle load	ton	25
Design speed	kph	200
Track curve radii in the tunnel U2/N2	m	2 880/2 860
Max slope in the tunnel	‰	10
Annual passed tonnage	MGBT	23
Expected annual passed tonnage	MGBT	33

3.2 Characteristics of the Tunnel

The Hallsberg – Stenkumla tunnel is a rock double-pipe railway tunnel with four cross service tunnels on each 500 meters. In accordance with the project´s System Requirement Document, the tunnel construction will generate an amount of rock material on 374 000 m³.

1. Track Solutions

3.1.1 The ballastless track system Rheda 2000® Green

A ballastless track includes rails and rail fastenings that are either directly attached to a prefabricated concrete track slab, also called the track concrete layer (TCL), or lie on prefabricated concrete elements poured into an in-situ TCL alternatively placed on this. The natural modulus of elasticity of the tunnel bottom has influenced the design of ballastless track systems in tunnels. Recent design optimization of the ballastless track system Rheda 2000® has led to the appearance of the environmental-friendly version called Rheda 2000® Green (see Fig. 2). The system adapted for the Hallsberg - Stenkumla tunnel has a construction height of 0.68 m from the tunnel bottom to the top of the rail (TOR). The design of the system is characterized by optimized dimensions, the lack of longitudinal reinforcement in TCL, and its plastic fiber reinforcement. Additionally, CEM II (reduced clinker percentage to 65%) instead of CEM I (clinker percentage 95–100%) in TCL is envisaged. These measures reduced the system's life cycle CO₂ emissions by 31% [11].

3.1.2 Track transitions

A ballastless track solution in the tunnel requires a track transition solution at each tunnel port to connect to the surrounding ballasted track. This transition solution, referred to in this study as track transition, is designed to achieve a controlled stiffness change between the "hard" ballast-free track and the "soft" ballast track. It makes it possible to keep the value of the dynamic forces arising in the track structure and the interfaces within permitted tolerances. In this study, the CO₂ equivalent emission of the transitions is included in the ballastless track calculation.

3.1.3 The ballasted track

The conventional ballasted track consists of a layer of ballast placed on a levelling layer of sub-ballast. On the ballast lay concrete or wooden sleepers to which the rails are attached. The ballasted track design in tunnels is specific due to the natural modulus of elasticity of the tunnel bottom. To reduce the ballast deterioration rate, the track design in tunnels includes an enlarged thickness of the ballast layer and sub-ballast layer. In this study, the ballasted track solution has a construction height of 1.60 m from the tunnel bottom to TOR. This construction height to the current regulation of the Swedish Transport Administration, consists of 0,4 m between TOR and the bottom of the sleeper, 0,4 m of ballast under the bottom of the sleeper, and 0,8 m of sub-ballast.

3.2 Service Life of the System

In the current study, the estimation of the life cycle CO₂ emissions includes the emissions generated by the excavation and transport of rock material gained by the tunnel construction and the life cycle CO₂ emission from the track asset in the tunnel. The system's service life, i.e., the tunnel's load-bearing system and the track asset in that, is estimated by the "worst-case" scenario. The latter means an intentionally shortened service life to estimate the life-cycle behaviour of the system concerning the following:

current and expected traffic conditions, and
relevant infrastructure regulations
common service life risks of the system.

The passed tonnage determines the lifetime of track components. The reference values of the passed tonnage indicating component change are partly taken from the Swedish Transport Administration's experience and partly from scientific sources. Then, the service life duration was recalculated for the critical track components considering (i) the expected traffic volume, (ii) the ratio of the passenger and freight trains, and (iii) the tunnel conditions affecting dynamic behaviour and rate of deterioration of track components. In this study, the critical track component's life determines the track form's life. For a ballasted track, this component is the ballast; for a ballastless track, it is TCL. Further, in this study, the critical track component's life corresponds to the track form's economic service life, and the latter is a period until the optimal balance between maintenance and reinvestment is achieved. The estimated economical service life for the tunnel and the track solutions presents in Table 2.

Table 3

Estimated economical service life of the track solutions in the Hallsberg – Stenkumla tunnel.
	Sub-system
	Tunnel	Ballastless track	Ballasted track
Economical service life, years	100	50	30

3.3 System Boundary

The system boundary for the calculation of life cycle CO₂ equivalent emission is chosen in this study to lie between zone 2 and zone 3 according to the Fig. 3. The length of the track section between the system boundary points at the respective tunnel ports is 2,400 meters (4,800 meters in total).

The computation includes new construction, emission-related maintenance activities, and all renovation measures (incl. all materials) required for a track solution of 4,800 m in a rock tunnel over 100 years. Emission-related maintenance activities in this study refer to the use of larger machines and heavier equipment and involve a significant climate impact. The life cycle CO₂ equivalent emission from the track transition includes in the computation of the ballastless track solution. Besides the track system, the computation does not include any installations in the tunnel.

4.1 Distribution of the life cycle CO₂ equivalent emissions

The calculations show that the CO₂ equivalent emissions from construction of the tunnel and the maintenance and renewal of the track asset are approximately 1,420 tons (approx. 10%) higher for the ballasted track than for the ballastless track, during its lifetime, with the conditions for the life cycle analysis in this study. Figure 4 shows that the life cycle emissions for ballasted track are 15,744 tons of CO₂ equivalents and 14,324 tons of CO₂ equivalents for ballastless track including the track transitions.

The largest ballasted track emission items are the renewal works and excavation and transport of the tunnel rock and removed ballast (illustrated in Fig. 5 as diesel and explosives), manufacture of rails (hot dip galvanized steel), sleepers and reinforcements (steel). Figure 5 presents the distribution of the emissions items for the track solution with ballast.

For the ballastless track solution, the most emissions are generated from the manufacture of rails (visualized in Fig. 6 as hot dip galvanized steel), excavation, and transport of the tunnel rock material (the most significant part of the diesel post) as well as the manufacture of concrete for TCL and the levelling layer.

The study indicates that choosing a track with ballast is more detrimental to the climate from a life cycle perspective than choosing a track without ballast. It is also clear that the materials that make up the respective track form significantly contribute to the number of CO₂ equivalents emitted. These materials' required number of replacements is essential.

The natural modulus of elasticity of the tunnel bottom allows a structural modification of the ballastless track system Rheda 2000® to reduce its life cycle CO₂-equivalent emissions. This is achieved through (i) reducing the concrete content in the track system, (ii) using CEM II with a lower content of clinker, contributing to a lower emission factor of the concrete, and (iii) eliminating the longitudinal reinforcement in TCL.

4.2 Sensitivity Analysis of the Significant CO₂ Emission Factors

In the current study, several significant factors on the life cycle CO₂ equivalent emission were identified. These factors are as follows.

CO₂ emissions related to diesel consumption during
rock excavation for tunnel construction
track installation
track maintenance and renewal
CO₂ emissions related to manufacturing of
rails
sleepers
concrete for TCL and the levelling layer.

To examine the influence rate of these factors on the life cycle CO₂ equivalent emission, a sensitivity analysis was carried out. Firstly, the consumption of diesel during corresponding track works has been estimated. Then, the magnitude of the significant factors with a rising by 10% was calculated. Further, the change in life cycle CO₂ equivalent emission for the track forms and the significant factors has been calculated using CICT. Finally, the difference between the change in life cycle CO₂ equivalent emission for the track forms. The outcome of these calculations is presented in Table 4.

Table 4

Change in the life cycle of CO2 equivalent emissions from the ballasted and the ballastless track solutions depending on changes in the magnitude of the significant CO2 equivalent emission factors by 10%.
10% Change in CO2 Emission Factor	Change in Life Cycle CO2 Emission, ton CO2e		Difference in the Life Cycle CO2 Emission, ton CO2e
10% Change in CO2 Emission Factor	Ballasted Track	Ballastless Track	Difference in the Life Cycle CO2 Emission, ton CO2e
Diesel consumption
Consumption during rock excavation for tunnel construction, m3	157	141	16
Consumption during track installation, m3	43	36	7
Consumption during track maintenance and renewal, m3	134	70	64
Manufacturing of the rails, sleepers and concrete
Rail manufacturing, m	603	611	-8
Manufacturing of the sleepers, pc	155	310	-155
Manufacturing of the concrete for TCL and the levelling layer, ton	0	244	-244

The larges difference concerns the factor “Manufacturing of the concrete for TCL and the levelling layer. The next larges difference is for the factor “Manufacturing of the sleepers”. The authors selected the first factor for further sensitivity analysis.

The calculation of the break-even point allows to determine at which value of the factor´s magnitude, the ballastless track ceases to be more beneficial from the climate impact perspective compared to the ballasted track. The calculation for the factor “Manufacturing of the concrete for TCL and the levelling layer” encompasses the manufacturing of concrete of two types, i.e., C 30/37 for TCL, and C20/25 for the levelling layer, the in-situ plain side concrete, and the in-situ plain filling concrete. The break-even point has been calculated for the given factor separately for each type of concrete. The reason for this is different environmental product declaration factor (EPD) for these two types of concrete. Implementing a rise of the factor`s magnitude on 10% at each stage, the simulation revealed that the original life cycle CO₂ equivalent emission of the ballastless track solution become equal to that of the ballasted track solution in the Hallsberg-Stenkumla tunnel (15 744-ton CO₂e) when the manufacturing of concrete C30/37 rises on 81%. For the factor “Manufacturing of the concrete C20/25 this value is 77%. Figure 7 displays the relationship between the change in the magnitude of the factors and the life cycle CO₂ equivalent emission of the ballastless track solution.

Based on experience, it can be assumed that the increase in the quantity of the concrete C30/37 for TCL by 81% as well as the increase in the quantity of the concrete C20/25 for the levelling layer, the in-situ plain side concrete, and the in-situ plain filling concrete by 77% compared to that calculated in the study is technically infeasible. Consequently, when considering the factor of manufacturing the concrete, the authors of this study conclude that the life cycle CO2 equivalent emission of the ballastless track solution in the Hallsberg-Stenkumla tunnel cannot be higher than that of the ballasted track solution, thus confirming the robustness of the study outcome.

4.3 A Look into the Future

The calculations are made with the "worst-case" scenario, which means a lack of development in the production of fuels, vehicles, machines, or materials about climate performance and energy efficiency. The emission totals reported for each track form are likely overestimated.

With the CO₂ equivalents emission reduction targets agreed upon by the countries of the world in the Paris Agreement [12], intensive climate development work is underway with players using materials, fuels, and vehicles on which the Swedish Transport Administration depends in building facilities. In addition, the Swedish Transport Administration also sets specific climate requirements in its projects to encourage further development. The goal is to have Swedish Transport Administration contracts free of fossil fuels by 2030, and by 2040 the building and maintenance of the authority´s facilities will generate zero net CO₂-equivalent emissions. Machines that perform the work and materials shall be embedded in the object. Exactly how much will be reduced and how quickly this adjustment will occur depends on how quickly production processes and the production of alternative fuels, vehicles, and machines can be adjusted. However, a plausible scenario is that a part of emissions from renewal occurring in the second half of the track asset life cycle can be eliminated.

The bottom-up approach [9] developed for LCA has been applied for estimating the life cycle CO₂ equivalent emissions from the study case using CICT. It is found that for the ballasted track solution, the amount of renewal required for the track forms during its service life is the most critical factor influencing the life cycle CO₂ equivalent emissions; the amount of renewal has a more significant influence on life cycle CO₂ equivalent emissions than the CO₂ equivalent emissions associated with the embedded maintenance emission and component and material emissions. The influence of the modulus of elasticity of the tunnel bottom on the design, renewal, and maintenance of the geometrical quality of the track with ballast is essential.

The most critical factors influencing CO2 equivalent emission in the design of a ballastless track system are the concrete and steel content and concrete type. The sensitivity analysis shows that change in this factor within a feasible range will not affect the study's outcome, i.e., the ballastless track form is preferable from a carbon emission perspective. For this track solution, the modulus of elasticity of the bottom of the tunnel has a decisive influence on modifying the ballastless track, aimed at reducing life cycle CO2 equivalent emissions. Among uncertainties, it is worth naming that for the ballastless track solution, the risk of frost damage in the drainage system shall be minimized by proper design solution. This lies beyond the scope of the current study and is one of the subjects of future investigation.

ACKNOWLEDGEMENTS

The authors thank the Swedish Transport Administration for initiating and supporting this project. We also thank the managers and designers of PCM RailOne AG for their expertise and support in conducting the study.

COMPLIANCE WITH ETHICAL STANDARDS

Sources of Funding

The research work for this article was funded by the Swedish Transport Administration, the R&D project 07830.

Disclosure of Potential Conflicts of Interest

No potential conflict of interest was reported by the authors.

Research Involving Human Participants and/or Animals

This research did not involve any human participants and/or animals.

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Selection of Track Form in Railway Tunnel from a Life Cycle Analysis Perspective

Status:

Journal Publication

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Abstract

Figures

1. INTRODUCTION

2. METHOD

3. USE CASE

1. Track Solutions

4. RESULTS AND DISCUSSION

CONCLUSIONS

Declarations

References

Status:

Journal Publication

Version 1