Carbon footprint reduction in transit tunnels

by P. Jarast, M. Bakhshi and V. Nasri Carbon footprint analysis is becoming more and more popular in industries due to increasing concerns about global warming and greenhouse-gas (GHG) emissions. The construction industry is a major producer of carbon dioxide (CO2) emissions. Huang et al. (2018) reported that in 2009, the total CO2 emissions of the global construction sector (which includes building and infrastructures) were 5.7 Gt (6.2 billion st), contributing 23 percent of the total CO2 emissions produced by global economic activities. In the United States, the construction sector generates the third highest GHG emissions among industrial sectors. It is therefore important that this sector significantly reduces its consumption of energy and materials.One of the most relevant activities within this sector is tunneling construction. The significant impact of the construction industry largely arises from the embodied carbon in the primary construction materials — cement and steel — and its use of various types of high-energy-consuming equipment. Research shows that more than 80 percent of the CO2 emissions in the construction phase of a tunnel are attributable to the construction materials cement and steel. Thus, reducing the need for cement and steel can make a significant contribution to reducing CO2 emissions. This comes along with a significant reduction in construction costs. Several studies have focused on the calculation of emissions related to tunnel construction (Li et al., 2011; Miliutenko et al., 2012; Huang et al., 2013; Fremo, 2015; Huang et al., 2020; Lee et al., 2016; Xu et al., 2019).This article estimates the portion of produced carbon emissions of a transit project related to tunnel lining. Using a low-carbon concrete mixture instead of a typical one, the CO2 emission saving is evaluated. A baseline concrete mixture with ordinary Portland cement (OPC) and rebar reinforcement is compared with a low-carbon concrete mixture with fiber reinforcement. The results of this study can be used as a reference for CO2 emission calculations of tunnel projects. It also helps provide a clearer quantitative picture of the carbon emissions of the tunnel liner. Project information The project consists of five stations and a tunnel constructed with a tunnel boring machine (TBM). The tunnel is approximately 4 km (2.4 miles) long and 15 m (50 ft) deep on average, excavated through rock.In the baseline design, an arc-shaped section with cast-in-place (CIP) concrete lining using OPC concrete mixture was proposed. To reduce carbon emissions, an alternative case was suggested. In this case, while a similar section was assumed, the CIP lining was replaced with a low-carbon shotcrete lining.A TBM-bored tunnel is another option instead of an arc-shaped CIP section. For this case, an external diameter of 9.3 m (30.5 ft) with a 7+1 precast segmental lining system and 1.8 m (5.9 ft) width was proposed. For this case, changing from an OPC concrete mixture to a low-carbon concrete mixture for prefabricated segments is suggested.In the following, the details of tunnel geometry and concrete mixtures for the baseline and low-carbon cases are presented. Baseline designTunnel geometry. Figure 1 shows the tunnel geometry for the baseline design. It consists of three components: 75 mm (2.9 in.) temporary shotcrete, 40 cm (15.7 in.) CIP lining and 32 cm (12.5-in.) thick invert. Concrete mixture design. The baseline concrete mixture consists of OPC with no slag and silica fume. Table 1 shows the mixture design for the temporary shotcrete, CIP arc and invert. CO2 emissions calculation. The CO2 emission of a material can be calculated using the following formula: Mass of CO2eq = (CO2eqFactor) x Material quantities (1) where CO2eq Factor is the amount of equivalent CO2 per kg of material from environmental product declarations. In this article, the Environmental Protection Agency’s (EPA) report to Congress EPA 530-R-08-007, Life 365 version 2.0 and Bath University’s Inventory of Carbon & Energy were used for reference. Table 2 summarizes the details of CO2 emission calculations for a unit volume of the baseline-concrete-mixture design. The material quantities are taken from Table 1. Based on the tunnel geometry presented in Fig. 1, the volume of shotcrete, CIP arc and invert per 1 m (3 ft) of the tunnel can be calculated. Taking these quantities together with the information in Table 2, the total CO2 emissions per 1 m (3 ft) length of the tunnel can be estimated. Table 3 summarizes the calculation.In the calculation for the shotcrete volume, the thickness of the shotcrete is adjusted for over-excavation tolerance. Considering drill and blast as the excavation method, the over-excavation tolerance would be 0.5 m, and the average adjusted thickness can be calculated as: Low-carbon concrete designAs an alternative solution, the temporary shotcrete and CIP arc were replaced with reinforced shotcrete. In the following, similar calculations as those used for the baseline design are presented. Tunnel geometry. Figure 2 shows the tunnel geometry for the low-carbon design. In this design, the CIP concrete lining is replaced with reinforced shotcrete. The components consist of 50 mm (1.9 in.) temporary shotcrete, 100 mm (3.9 in.) permanent shotcrete lining and 32 cm (12.5 in.) thick invert. The shotcrete was reinforced with 3D 65/35BG fiber, and the invert has the same mixture design as reinforced shotcrete. Concrete mixture design. To reduce carbon emissions while having the same strength properties, the Portland cement was replaced with limestone Portland cement, and 27 percent of the cementitious material was replaced with slag and silica fume (22 percent slag and 5 percent silica fume, which is considered moderate supplementary cementitious materials (SCM). This mixture was proposed for both the shotcrete and invert. Table 4 presents this concrete-mixture design. CO2 emission calculation. Using the same methodology for CO2 emission calculation as in the baseline design, the embodied carbon footprint for a unit volume of SCM concrete mixture is calculated. Table 5 summarizes the details of this calculation. The material quantities are taken from Table 4. Based on the tunnel geometry presented in Fig. 2, the volume of shotcrete, CIP arc and invert per 1 m (3 ft) of the tunnel can be calculated. Taking these quantities together with the information in Table 5, the total CO2 emissions per 1 m length of the tunnel can be estimated. Table 6 summarizes this calculation.In the calculation for the shotcrete volume, the thickness of the shotcrete is adjusted for over-excavation tolerance. Using the road header for excavation together with robotic shotcrete and real- time in situ 3D laser scan, the over-excavation tolerance would be 0.02 m and the average adjusted thickness can be calculated as: Comparing the total CO2 emissions of this case with the baseline, the total saving is 79 percent. TBM tunnel with prefabricated segmental lining designTunnel geometry. The tunnel geometry of the TBM tunnel with segmental lining design consists of 7+1 segments, 1.8 wide. Concrete and grout mixture designs. Concrete design. The baseline concrete mixture consists of OPC with no slag and silica fume. To reduce carbon emissions while having the same strength properties, Portland cement was replaced with limestone Portland cement and 27 percent of the cementitious material was replaced with slag and silica fume (22 percent slag and 5 percent silica fume). Table 7 presents these concrete mixture designs. Backfill/infill concrete. The baseline backfill/infill concrete consists of OPC with no slag and silica fume. In the carbon-reduced case, 50 percent of the Portland cement was replaced with slag. Table 8 presents the backfill/infill concrete mixtures for these two cases. Tail-void-backfill grout. The tail-void-backfill grout design is presented in Table 9. The same design is used for both the baseline case and the carbon-reduced case. CO2 emission calculation. Using the same methodology for CO2 emission calculation as in the baseline design, the embodied carbon footprint for a unit volume of OPC and SCM concrete mixtures and grout mixture is calculated. Tables 10 to 12 summarize the details of these calculations. The material quantities are taken from Tables 7 to 9. Tables 13 and 14 summarize the details of the total embodied carbon footprint of the tunnel based on the geometry of the tunnel and the unit CO2 emissions presented in the previous tables for the baseline-mixture design and SCM-mixture design.Based on Table 14, the total carbon footprints of the two cases — baseline design and carbon-reduced design— are calculated as below: Total CO2eq per 1 m tunnel, baseline design= 6.7 + 2.2 + 1.4 = 10.3 ton (4) Total CO2eq per 1 m tunnel, carbon-reduced design= 3.6 + 1.4 + 1.4 = 6.4 ton (5) As can be seen, by using the carbon-reduced concrete mixture for the segments and backfill/infill concrete, the total carbon footprint per 1 m (3 ft) length of the tunnel is reduced by 38 percent. ConclusionThis article presents the details of embodied CO2 emission calculation. As shown, for CIP tunnel lining, by improving the mix design (using limestone Portland cement and 27 percent supplementary cementitious material) together with using real-time in situ 3D-laser scan, CO2 emissions can be reduced by 80 percent (from 7.9 CO2 kg per 1 m (3 ft) length of the tunnel to 1.7 CO2 kg per 1 m (3 ft) length of the tunnel). For prefabricated segmental lining, by improving the mix design and using steel fibers instead of steel rebars, CO2 emissions can be reduced by 38 percent (from 10.3 CO2 kg per 1 m (3 ft) length of the tunnel to 6.4 CO2 kg per 1 m (3 ft) length of the tunnel).Considering the scale of tunnel projects, these reductions in CO2 emissions are huge and should not be ignored or underestimated. It should be noted that this emission estimation is a part of the total CO2 emissions during the construction phase, and CO2 produced by construction equipment and transportation should be added to it to have a complete estimate. ReferencesFremo, O., 2015. Life Cycle Assessment of the Byasen tunnel in Trondheim, Norway. Master Thesis. Norwegian University of Science and Technology, Department of civil and transport engineering. Trodheim (Norway), 103pp.Huang, L., Bohne, R., Bruland, A., Drevland, P., Salomonsen, A., 2013. Life Cycle Assessment of Norwegian standard road tunnel. In: The 6th International Conference on Life Cycle Management in Gothenburg.Huang, L., Drevland, P., Bohne, R., Liu, Y., Bruland, A., Manquehual, C.J., 2020. The environmental impact of rock support for road tunnels: The experience of Norway. Sci. Total Environ. 712, 136421.Huang, L., Krigsvoll, G., Johansen, F., Liu, Y., Zhang, X., 2018. Carbon emission of global construction sector. Renew. Sustain. Energy Rev. 81(Part 2) pp. 1906–1916.Lee, J., Shim, J.A., Kim, K.J., 2016. Analysis of environmental load by work classification for NATM tunnels. J. Korean Soc. Civ. Eng. 36 (2), 307–315.Li, X., Liu, J., Xu, H., Zhong, P., 2011. Calculation of endogenous carbon dioxide emis- sion during highway tunnel construction: A case Study. International Symposium on Water Resource and Environmental Protection, May 2011, Xian (China), pp. 2260–2264.Miliutenko, S., Akerman, J., Bjorklund, A., 2012. Energy use and greenhouse gas emissions during the Life Cycle stages of a road tunnel— the Swedish case norra lanken. Eur. J. Trans. Infrastruct. Res. 12 (1), 39–62.Xu, J., Guo, C., Chen, X., Zhang, Z., Yang, L., Wang, M., Yang, K., 2019. Emission transition of greenhouse gases with the surrounding rock weakened—A case study of tunnel construction. J. Cleaner Prod. 209, 169–179.

Carbon footprint reduction in transit tunnels

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