INSIGHT | SUSTAINABILITY
Ground level 1.38m 1.7 - 2.2m 2.2 - 3.4m ID 1.2/1.5mm ID 1.2m 724 kgCO2e/m3 928 kgCO2e/m3
Case study 1 Total length = 360m No shafts = 4
Case study 2 Total length = 459m No shafts = 3
2253 kgCO2e/m3 ID 1.2m
Alsadi and Matthews (2020) Total length = 122m Prestressed concrete cylinder pipe
Open-cut installation
Case study 3 Total length = 51m No shafts = 2
Installed using microtunnelling with reinforced concrete pipes and open-dug caisson shafts. Embodied carbon values include the contribution of shafts.
Above, figure 3: Embodied carbon results, normalised by the internal volumeof pipeline created, compared to a similar pipeline constructed using open-cut installation. (ID = internal diameter)
5.5 - 6.1m 986 kgCO2e/m3
ID 0.76m
Study 2 were found to have lower normalised
emissions than cut and cover, despite having larger pipeline diameters and greater cover to the pipe. Alsadi and Matthews (2020) did not consider the additional emissions due to road closures, which further favours the microtunnelling results. However, it’s worth noting the low diameter and length of the open-cut pipeline may have impacted the comparison and further investigation is required to provide further objective comparison of the construction methods. Case Study 2 resulted in particularly low normalised
emissions, partly due to over half the pipeline being larger diameter (1.5m), which resulted in a lower
normalised embodied carbon. Having fewer shafts per metre tunnelled also had a significant positive effect on the embodied carbon for Case Study 2. Overall, these results demonstrate the potential benefits of increased drive lengths and reducing the number of shafts. It is also suggested higher diameter pipelines, in place of multiple lower diameter pipelines, is preferable. Case Study 3 had much higher embodied carbon
due to its significantly greater depth, requiring deeper shafts.
DESIGN OPTIMISATION:
Preliminary design Total length of tunnel = 297m
430 tCO2 Total shafts = 6 e
INFLUENCE OF ADDITIONAL SHAFTS During early design stages, the layout of the tunnels in Case Study 1 was changed to optimise construction. This enabled retrospective analysis to consider how these design changes affected the embodied carbon of the project. The change in design involved decreasing the number
Total length of tunnel = 360m Total shafts = 4 378 tCO2 e
of shafts but resulted in an increase in the total length of tunnelling (see Figure 4). Despite the increase in tunnel length, our calculations predicted the design change resulted in a 12.1% decrease in total embodied carbon. This effectively demonstrates how the greatest savings in embodied carbon can often be made through good decision-making in the early design stages of projects. The result also underlines the value of maximising drive lengths and utilising curved tunnel drives to minimise the number of shafts in microtunnelling projects.
Above, figure 4: Embodied carbon reduction in Case Study 1 through minimising the number of shafts
32 | November 2024
DEEPER DETAIL Further details on the research can be found in Swallow & Sheil (2023), which is published in the American Society of Civil Engineers’ (ASCE) Journal of
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