Sustainable ports & Life Cycle Assessment

1. Introduction

A case study comparing construction methods for a 200-meter cruise ship terminal in Antwerp, Belgium, was conducted by Tractebel engineering. The study analyzed steel sheet pile walls (SSP), diaphragm walls (D-Wall), and deck on piles, considering construction costs, speed of execution, return on investment, and end-of-life scenarios. The SSP wall, designed with standard steel sections, emerged as 20% more cost-effective than the D-Wall. Consequently, the life cycle analysis (LCA) focused on SSP and D-Wall solutions, using the Dutch monetization method to assess sustainability, with a focus on Global Warming Potential (GWP). The LCA, conducted by ArcelorMittal's R&D department and peer-reviewed, concluded that variations in key parameters had limited impact on results, confirming the initial findings.

2. Goal, scope and assumptions

Goal of the study

The study, conducted in compliance with ISO 14040 and ISO 14044, evaluates the CO2-eq. emissions impact of two quay wall solutions through a life cycle assessment (LCA). The primary focus is on total life cycle cost, including factors like demolition, recovery, reuse, recycling, and landfill considerations. Targeted at private investors, public authorities, engineers, and architects, the report presents complex LCA findings in a simple, clear format.

Infrastructure Description and Assumptions

The structure design adheres to European standards, with geotechnical, steel sheet pile, and concrete wall designs based on relevant EN standards. The execution employs land-based equipment to control costs. Assumptions include a 50-year service life without major maintenance, considering parameters like corrosion, corrosion protection, and end-of-life scenarios.

Environmental Indicators

Environmental impacts are characterized based on EN 15804 using CML 2001 methodology. GWP is used as the primary indicator for CO2-eq. emissions, calculated according to EN 15804 based on IPCC 2007. A physical allocation method is applied for steel data. The study serves as a carbon footprint assessment, focusing on GWP to quantify emissions.

Functional Unit

Covering the entire 200m quay wall structure over a 50-year horizon, the LCA addresses retaining wall and bearing foundation requirements.

3. Methodology

Data Collection

The study prioritized recent, relevant sources and utilized Environmental Product Declarations (EPDs) compliant with EN 15804 and registered in IBU, along with the Gabi Database 2018 for transportation and construction site processes. Steel sheet pile data was sourced from ArcelorMittal’s EcoSheetPiles™ EPD, with values adapted to project-specific assumptions. Rebars and other steel elements were sourced from European mills, while concrete data came from plants near the port of Antwerp.

Transportation

Environmental impacts of transport modes were sourced from the Gabi database 2018, with assumptions for distances traveled by rail and truck for various materials.

End of Life Practices

Steel sheet piles were assumed to be fully recoverable, with scenarios for recycling and landfill. Diaphragm walls were partially recoverable, leading to different end-of-life scenarios.

Bill of Materials

The bill of quantities included various items like equipment mobilization, material quantity, structural works, and disposal, detailed in the LCA report. While total mass differs between retaining walls, it's not considered an environmental criterion.

System Boundary

Environmental impacts were calculated across different phases: production, transportation, construction, demolition, and end-of-life, excluding phase B. Site preparation and installation were separated in phase A5. Some elements like diesel consumption for equipment installation were not included due to a lack of reliable data.

Monetization

Monetization, though not compliant with ISO standards, is used in Belgium and the Netherlands to reflect economic actors' positions on global warming and ecological issues. This approach assigns a monetary value to CO2-eq. emissions and considers various parameters to assess environmental impacts.

4. Results

The primary focus of the study is on Global Warming Potential (GWP). In the base scenario, the sheet pile wall demonstrates the lowest environmental impact, with a significant 44% difference compared to the diaphragm wall (concrete).
The largest gap between the two solutions is observed in phases A1-A3, favoring the EcoSheetPile™ quay. The burden in module D of the EcoSheetPiles EPD is due to the manufacturing process of steel in an Electric Arc Furnace (EAF), which requires more scrap than the recycled material available at the end of the life cycle.
Furthermore, phases A1-A3 contribute more than 70% to the whole life cycle, highlighting their significance in both cases. Additional indicators such as Acidification Potential and Abiotic Depletion Potential Elements were analyzed, showing similar trends to GWP.
The comparison of indicators reveals a clear difference between the two alternatives, justifying the statement that "the environmental impact of steel sheet piles is lower than that of the diaphragm wall." With a minimum 10% difference observed for the analyzed indicators, the study confirms the environmental advantage of steel sheet piles over concrete diaphragm walls.

5. Sensitivity analysis

Concrete Carbonation at Use Phase

Carbonation in Module D is considered, but excluded in Module B1 due to the concrete being submerged, making carbonation improbable during use. However, an assessment showed a reduction in the gap between steel and concrete solutions from 44% to 41%.

Concrete EPD

Various concrete EPDs were assessed, showing that higher strength concrete widens the gap between steel and concrete solutions, while lower strength concrete reduces it. Comparison with CEM III cement was not analyzed due to slag allocation inconsistencies.

End-of-life Scenario

The assessment ignored deconstruction for each alternative, modifying system boundaries. This decision was influenced by the negative "Net Scrap Value" for steel and the beneficial carbonation for concrete at end-of-life.

Corrosion Losses

Predicting precise steel loss from corrosion is challenging. Using standard EN 1993-5, an estimated total loss of 136 tonnes led to adapted reuse and recycling rates. Even in the worst-case scenario, steel's GWP difference remains lower than the base scenario.

Conclusion

The sensitivity analysis reaffirms the base scenario findings: concrete structures have significantly higher GWP compared to steel sheet pile solutions, ranging from +19% to over +76% in extreme scenarios.

6. Conclusions, limitations & general comments

Conclusions The LCA demonstrates that a quay wall constructed with steel sheet piles has a lower carbon footprint compared to an equivalent concrete diaphragm wall. The base scenario shows a 44% difference, subject to parameter variations. 

Compared to the concrete solution (D-Wall), the carbon footprint of the EcoSheetPile™ solution (SSP) is by far lower. In the base scenario the difference is 44 %.

Limitations Both steel (SSP) and concrete (D-Wall) solutions are technically equivalent and designed to perform at similar safety levels throughout their service life. Results are specific to this case study and may not be applicable to other situations without further analysis. The LCA focuses on GWP, but other indicators or technical aspects may yield different conclusions. Local conditions and transport factors can significantly influence results in other contexts.

General Comments While EPDs are useful for comparing products, variations in quality and transparency exist. EPDs should be developed by environmental experts with industry specialization to ensure accuracy. Generic EPDs facilitate feasibility and design comparisons, but specific EPDs are crucial for tender comparisons. Products with significant impacts but lacking specific EPDs should be appropriately assessed.