SUSTAINABILITY | INSIGHT


SCOPE OF THE RESEARCH The scope of the study, was designed to enable effective comparison of microtunnelling as an isolated process. Work required onsite prior to tunnelling will vary


hugely dependent on the location and the nature of the project. Similarly, construction activities following the completion of tunnelling will heavily depend on the final application of the pipeline, for example as a gravity sewer or as a carrier for other utility infrastructure. To negate these differences project to project, only


the ‘core’ processes of microtunnelling were considered, namely construction of the shafts and the tunnel itself. Calculations considered the following operations: raw material supply; transport of raw materials to manufacturing facilities; manufacturing processes; transport to site; and, onsite activities.


CASE STUDY RESULTS The aggregated results for the three case studies analysed are presented in Figure 1. The projects created a total of 870m of new pipeline, the creation of which produced a total of 1005tCO2


e (tonne of


CO2 equivalent, known as ‘embodied carbon’). This represents an average of 1.16tCO2e per metre tunnelled.


To contextualise the scale of these emissions, driving a petrol car for 10,000 miles ~3tCO2


e (only considering


fuel use; IStructE 2020). Figure 1 shows that both tunnels and shafts make


a significant contribution to the embodied carbon of microtunnelling projects. Materials are shown to dominate emissions, making an especially large contribution to the embodied carbon of the tunnels – despite the total volume of concrete used in the


pipes being lower than in the shafts. This is due to the reinforced concrete pipes using C50/60 concrete (with high cementitious content) and dense reinforcement, demonstrating improvement in pipe design could be a key area for enabling reduced embodied carbon. The embodied carbon from transport to site is


also much higher for the tunnels – this is due to the precast facilities being much farther from site than in-situ concrete suppliers for the shafts, demonstrating the significant impact of establishing local suppliers wherever possible. It’s also worth noting the on- site emissions are considerably higher than in other construction sectors - e.g. IStructE (2020) suggests this may be as low as 1%-2% for buildings. All the projects were carried out by powering the TBM


using diesel generators. If mains power was to be used, analysis based on current UK electricity supply (DEFRA 2022) suggests that the embodied carbon due to TBM operation could be reduced by 39%.


STRUCTURAL MATERIALS: INFLUENCE OF STEEL Materials accounted for 68.5% of embodied carbon across all three projects. Concrete accounted for the majority (58%) of these emissions. This will hardly surprise readers, given the environmental impact of concrete is so well publicised. Steel, despite also being carbon intensive, rarely receives as much limelight. Steel products, including both reinforcement and steel


plate fabrications, accounted for 35.9% of embodied carbon from materials across all three projects. For the present projects, the reinforcement was


Worst case steel Project specific steel Best case steel


1200 19.9% 1000 8.1% 800


sourced from ‘lower embodied carbon’ European suppliers, who tend to produce steel with a high recycled content using electric arc furnaces. In contrast, the steel plate was sourced from ‘higher embodied carbon’ suppliers, who produced the steel plate using traditional basic oxygen furnaces. To explore the impact of steel production techniques,


a ‘worst case scenario’ (where all steel was from ‘higher embodied carbon’ suppliers) and a ‘best case scenario’ (where all steel was from ‘lower embodied carbon’ suppliers) were explored, as shown in Figure 2. The results underline the significant influence the method of steel production can have and underlines the need to lobby for a worldwide shift away from basic oxygen furnaces.


600 400 200


COMPARING CONSTRUCTION METHODS To consider the suitability of microtunnelling, the results of the analysis for the three projects were normalised by the total internal volume of pipeline created, including the embodied carbon from the construction of shafts (see Figure 3). These results were then compared to equivalent values in the literature for the main alternative construction technique for large diameter pipelines, open-cut installation. Only one study, Alsadi and Matthews (2020), was


0


found to have equivalent scope and methodology to enable fair comparison. Both Case Study 1 and Case


November 2024 | 31


Left, figure 2: Sensitivity analysis comparing the overall embodied carbon of the three projects considering: (1) the embodied carbon of the actual steel used in the projects; (2) a hypothetical scenario where all steel was sourced from ‘high embodied carbon’ suppliers; and, (3) a hypothetical scenario where all steel was sourced from ‘low embodied carbon’ suppliers


Embodied carbon (tCO2


e)


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49