Underground car parks. Part 2 | Life Cycle Assessment

Cities have grown substantially, resulting in higher building densities and a scarcity of space. In many European cities mobility still relies predominantly on individual cars and the associated inherent problem: traffic jams and scarcity of parking slots. Mindsets are changing, but currently, underground car parks (UCP) seem to be the adopted solution in densely populated urban areas.

A first market study performed by Royal Haskoning DHV, a Dutch consulting engineering firm, in 2018 compared several alternatives to build the surrounding retaining walls of an UCP in typical Dutch soil conditions. Their conclusion was that for a 2 to 3 level UCP, a permanent steel sheet pile wall is up to 50% more cost-effective than the second best in class, a secant pile wall, and even more compared to other alternatives (cutter soil mix, diaphragm wall). Besides its execution time is also significantly faster.

Later in 2019, ArcelorMittal contracted Witteveen & Bos (W+B), another Dutch engineering firm, to dive deeper into this topic and to prepare a more detailed analysis of four alternatives for building the retaining wall of a standard UCP. The results of the technical and financial analysis are detailed in Part 1 of this brochure.

This second part deals with the environmental impact of the alternatives, focusing on the Global Warming Potential (GWP), which can be seen in this case equivalent to the carbon footprint. The environmental impact is determined through a Life Cycle Assessment (LCA) based on the bill of quantities prepared by Witteveen & Bos and was performed by ArcelorMittal’s R&D department. It was peer-reviewed by the Dutch research organisation TNO, acting as an independent expert.The objective was to compare the Total Life Cycle Cost, including the burdens or benefits of the end of life phase, which are dismantling and recycling of the building elements. No reuse was considered.

EPD & LCA

In order to provide a sound comparison of a steel sheet pile wall with alternative solutions a simple but realistic case study was carried out. The case study is based on a hypothetical geometry of a 250 meter long by 30 meter wide underground car park (UCP), with two levels below grade that would be built in the city of Amsterdam in the Netherlands. 

Following retaining structures were analysed 

• steel sheet pile wall (SSP);
• cutter soil mix wall (CSM), also known as deep soil mix in other countries;
• secant pile wall;
• diaphragm wall (D-Wall). 

The cross section of the steel sheet pile solution is shown in Figure 1. The length of the retaining walls slightly varies by alternative. The strutting system and other elements (slabs,…) are quite similar for all the solutions, so that as a simplification, Witteveen & Bos selected the same elements for the four construction methods. Hence, only the retaining walls differ in this study.

Goal of the study

The study was conducted to be compliant with ISO 14040 and ISO 14044. The material data are based on EPDs compliant with EN 15804 and the infrastructure global LCA, although in principle not applicable, is inspired by the EN 15978 methodology. 

The main objective of the study is to evaluate the influence of a structural solution on CO₂-eq. emissions considering the life cycle of an underground car park structure. It proposes a comparison of four alternative solutions through LCA.

The total life cycle cost is the main indicator, hence after the service life demolition, recovery of structural elements, reuse, recycling and landfill are to be considered whenever technically feasible. 

The target group of the report include private investors, public authorities, engineers and architects that may not be familiar with the complexity of a LCA approach. The report was therefore written on purpose in a quite simple and clear form. More technical details on the background information and data can be obtained from ArcelorMittal’s experts.

Circular economy

Data

Preference was given to the most relevant and recent sources. The database was built on the following elements 

• Environmental Product Declarations (EPDs), following the standard EN 15804 and registered in IBU. Those data are public and peer reviewed;
• Gabi Database 2018 for transportation as well as construction site and in-use processes. 

Representativeness and consistency of the data was checked, and whenever possible, German or European EPDs and databases were used. Note that some inherent cut-off might be done in the data, but all the data in the EPDs are compliant with European standards.

The selected steel sheet piles are manufactured in ArcelorMittal’s mill in Belval, Luxembourg. The data for steel sheet piles was extracted from ArcelorMittal’s EcoSheetPiles™ EPD. Note that since this is an LCA being performed for a specific project, the values form the EPD were adapted to fit the project specificities. Therefore, an Excel tool was developed by the R&D department.

Rebars could be delivered from any mill in Europe, hence the difficulty in choosing a specific mill. In that case, the best option is to consider one EPD and to calculate an average distance from mills covered in the EPD to the jobsite. 

Structural steel such as the H-beams used to reinforce the CSM wall and the secant pile wall would be fabricated in one of the mills in Luxembourg (Belval or Differdange). An EPD for steel beams manufactured in Differdange was selected.

Concrete is assumed to be fabricated in a plant close to the city of Amsterdam. Specific German EPDs for concrete, with and without module D, were used. Using a Dutch EPD might introduce a bias because Dutch EPDs included in the official Dutch database (Nationale Milieu Database, NMD) are generally elaborated with a different software and based on a different life cycle inventory (ecoinvent).

Monetization

Monetization is a commonly and politically approved approach to reflect the economical actors’ position to global warming and ecological issues. This approach is not compliant with ISO 14040-44 but is applied in Belgium and the Netherlands. This methodological process can address the issue of evaluating a fair and appropriate equilibrium between environmental impacts and costs. 

For instance, the default value for 1 tonne of CO₂-eq. is taken as 50 € in the Netherlands, and within a range in Belgium (up to 100 €). This weighting factor is used to multiply the calculated CO₂-eq. content.

In the Netherlands, the method leads to a global index called MKI (ECI in English). It considers a total of 11 environmental indicators, including some that are not required in standard European EPDs (i.e. toxicological indicators) and weighting factors for each indicator. Additionally, the method subdivides the environmental data for the LCA in three different categories. The first category corresponds to a specific EPD for a specific product (usually from a single manufacturer), whereas the third corresponds to generic data (average values from available databases or manufacturers) and is penalized by an additional weighting factor to take into account the averaging and spreading of generic data.

In the Netherlands, the penalization factor is 30 %. Consequently, manufacturers that want their products to be part of Category 1 data must develop a specific EPD for the Netherlands. Category 2 data contains for instance collective EPDs, i.e. data for one specific product but produced by several manufacturers, or an EPD covering several products produced by a single manufacturer. Note however that this approach has a weakness: in specific cases it might be more favourable to use generic data (category 3) than a specific EPD which has a very high environmental impact!

Bentonite and Transport distance for concrete

The water volume used to create the bentonite to support the temporary trenches / holes of the D-Wall and secant pile wall is considered in the phase A5 (installation). Due to the lack of data and information, the water treatment on site after extraction of bentonite was not included in the current calculations. The results were reassessed without considering the water volumes, and lead to a reduction of the gap between the CO₂-eq. emissions of less than 1 %.

The transport A4 module contributes to less than 1 % of the total value on the considered perimeter. Although 10 km of delivering distance for the concrete may be on the low side, doubling or even tripling the distance would not have changed the conclusion between the concrete solutions and the steel solution.

End-of-life scenario – excluding modules C3 & D

The assessment of the influence of the end-of-life scenario is performed by ignoring the deconstruction / demolishing of the structure for each alternative. Hence the system boundaries are modified by removing phases C3 and D. Fact is that for the concrete solutions and the CSM, the basic scenario already disregarded modules C3 and D because these structures would not be demolished. 

Figure 6 shows an increase of the difference between the steel solution and the three other alternatives, for instance rising from 88 % to 144 % compared to the CSM wall. This is mainly due to the fact that for the sheet piles EcoSheetPile, module D leads to a burden, whereas for the other solutions, it was already ignored.

End-of-life scenario – corrosion losses

A precise loss of steel mass due to corrosion is hardly predictable because the corrosion phenomenon differs by exposure zone and by location. Several effects during the use phase may have a significant influence on this parameter. When accurate longterm local measurements are not available, it is usual to adopt the corrosion rates proposed in Chapter 4 of the standard EN 1993-5. However, in the Netherlands it is standard practice to use the values recommended in the CUR 166 manual. For steel buried in the soil, the corrosion loss per face can be estimated as 0.012 mm / year, leading to a loss of approximately 137 tonnes of steel over the service life of 100 years (no corrosion occurs on the surface protected by the painting).

Hence, the reuse and recycling rates of the EPD EcoSheetPiles were adapted to 0 % reuse, 84.2 % of recycling, and consequently 15.8 % landfill. This worst-case scenario for the steel sheet pile wall (SSP-wall) reduces the gap in the GWP between the steel sheet pile wall and the alternatives, i.e. falling from +88 % (basic scenario) to +35 %, see Figure 7.

End-of-life scenario – deconstruction and recycling of the D-Wall and secant pile wall

This scenario considers that the portion of the concrete wall and its reinforcement above the excavation level (-10.5 m) will be demolished after the service life, and that the portion completely embedded (3.5 m) will be left in place. Currently this scenario is technically improbable but might be imaginable in the future. It applies to the diaphragm wall and to the secant pile wall. The cutter soil mix wall cannot be demolished since it consists in strengthening the existing soil; although a technique to extract the reinforcing steel beams may be developed in the future, it was not considered as an option in this study. 

Considering the end of life and the recycling modules of the EPDs of the concrete solutions decreases the gap between the sheet pile wall and the concrete structures by roughly 5 % to 10 %, see Figure 8.

Influence of the steel sheet pile profile

This scenario analyses the influence of the chosen sheet pile section on the results. Currently, several equivalent profiles from different sheet pile series can be used for the same retaining structure. Wider profiles from the AZ-800 range are lighter and can usually be installed faster, but the choice needs also to consider driveability criteria, which depend mainly on the soil conditions, length of the pile, etc. The study compared two profiles, the AZ 20-800 and the AZ 20-700 (original choice in the report of Witteveen & Bos). The following Table 1 shows the bill of materials for both profiles. Note that the AZ 20-800 is 800 mm wide compared to 700 mm for the AZ 20-700, and has a different height; this leads to a difference in seal-welding length and coating area (and its mass).

The gap between the steel sheet pile wall and the other solutions shrinks by less than 10 %, confirming the global trend of the basic scenario.

Influence of the fire protection painting

This scenario analyses the influence of the chosen fire protection painting on the results. The basic scenario uses PROMASPRAY® P300, and FIBROFEU® is a spray coating with an equivalent fire protection performance. The spray coating is also applied to the CSM, so that the impact on both solutions is quite similar, see Figure 10. 

The gap between the steel sheet pile wall and the other concrete solutions shrinks by 30 % to 40%. Despite the quite higher carbon footprint of this alternative product, the global trend of the basic scenario would not be altered.

Influence of the different scenarios on the GWP of the sheet pile wall

This section summarizes the variation of the GWP of the steel sheet pile wall with the assumptions of the different scenarios. In Figure 11, it is clearly visible that for the type of structure under assessment, the loss of steel thickness due to corrosion has the major influence. The increase of the GWP due to corrosion amounts to almost 40%. 

Note: for steel manufactured through the secondary route (recycling of steel scrap in an Electric Arc Furnace), omitting the stage beyond End-of-Life (module D) can lead to a significant reduction of its carbon footprint (-23% in this case study)! This is a drawback of some LCA models.

Conclusions

The sensitivity analysis that was conducted confirms the robustness of the models and of the assumptions made in the basic scenario. In all the cases, the carbon footprint (expressed as the GWP) of the steel sheet pile solution is always lower. The difference varies by alternative and depends also on the scenario. 

Compared to the steel sheet pile walls, the range of the increase in GWP for the extreme scenarios is shown in Table 2.

EPDs are currently a tool that in a certain way enable a quite fair and transparent assessment of the environmental impact of a specific product or service. As a manufacturer, we believe that if it is used in a correct way, an LCA based on peer-reviewed EPDs according to EN 15804 is an excellent method to compare different products and/or several alternatives. Investors can undoubtedly encourage manufacturers to improve their competitiveness (i.e. productivity, new production technologies) in order to reduce their environmental impact by awarding a financial incentive to the solutions with the lowest environmental impact for each project. This approach is already implemented in some European countries for public procurement.

Unfortunately, we have also noticed that not all EPDs have been drafted with the quality and fair assessment that can be expected from such a document, and even worse, in some cases it can be misleading. EPDs should be worked out by environmental experts that are also specialized in the industry for which the EPD is applicable to avoid wrong assumptions or to miss some key processes.

Generic EPDs are a nice tool to compare alternatives, like in this UCP case study, a steel structure versus concrete structures and a soil mix structure, at a feasibility stage or design stage for instance. But when it comes to the comparison of alternatives at the tender stage, a specific EPD from the manufacturer of the product is the most appropriate and should be required. A product that has a major impact on the LCA result, but which is not covered by a specific EPD, should be penalized, for instance by using a weighting factor on its environmental impact indicators relative to a specific EPD from a product produced with the same process, or relative to the best-in-class product.