Environmental impact assessment of high-speed railway tunnel construction: A case study for five different rock mass rating classes

Abstract

Although rail is usually considered the cleanest and most energy-efficient motorized transport, the emissions and energy consumption during the construction stage are commonly overlooked. The purpose of this paper is to estimate the environmental impact of the construction of 1 km of double-track high-speed railway tunnel for five different Rock Mass Rating (RMR) classes using the Life Cycle Assessment (LCA) methodology. The LCA is performed for seven selected environmental impact categories, using a tool developed by the authors. The categories assessed are: Global Warming Potential (100 years), Ozone Layer Depletion, Acidification Potential, Eutrophication, Photochemical Oxidation, Abiotic Depletion of Fossil Resources and Abiotic Depletion of Non-Fossil Resources. A cradle-to-site system boundary is adopted for this study, considering all activities up to construction but not taking into account maintenance, operation or decommissioning stages.The results show that for each of the five different RMR classes, the support, lining and infrastructure works account for at least 70% of the impact in each environmental impact category during the lifecycle of high-speed railway tunnels up to construction. Lining and infrastructure works are the main contributors to all the selected environmental impact categories for RMR classes S-I and S-II, contributing an average of 80% and 70% respectively. Support, lining and infrastructure works represent practically the entire environmental impact for the S-III class, with a total share of 85% in all impact categories. Finally, the support phase has the greatest impact in classes S-IV and S-V, with an average of 43% and 63% for each class respectively, due to the higher material quantities required to support poorer quality rocks. Specifically concrete, diesel and steel are responsible for between 89.3% and 99.9% in each environmental impact category, depending on the RMR class, whereas the impacts of other materials such as aggregates, wood and mortars account for less than 0.1%.A combination of the Life Cycle Assessment methodology considering the RMR index has been shown to provide valid benchmarking criterion when assessing the performance of a railway tunnel during the planning stages. Nevertheless, further research is required to fully understand the environmental impacts of this type of construction, as most of the literature only considers one environmental impact category, namely GWP100 (CO2e emissions).