Emission transition of greenhouse gases with the surrounding rock weakened - A case study of tunnel construction

High energy consumption, high resource consumption and high greenhouse gas (GHG) emission are characteristics of tunnel construction. Different from ground engineering, tunnel is highly influenced by surrounding rock. However, surrounding rock has never been mentioned in previous published studies on GHG emissions in tunnels. In order to evaluate the effect of surrounding rock conditions on GHG emissions, this study expounded the relationship between tunnel designs and rock mass classifications. Besides, five different surrounding rock conditions and tunnel lining designs of a real tunnel in China were introduced in detail. Life cycle assessment (LCA) was used to analyze the GHG emissions in five tunnels with different surrounding rock. The tunnels with worse rock conditions generate more GHG emissions in construction, while the emissions are between 6220 t- 17,010 t CO2 eq. More than 60% of the GHG emissions are from materials. For the first time, the importance of surrounding rock conditions to GHG emissions in tunnels was clarified in this study. It has been found that emissions increased sharply in many cases after comparing the same construction for different surrounding rock conditions. Based on the defined relative contribution indexes, transition paths of GHG emission were ascertained. Rock mass and casting & lining are the major two emission transition processes when the rock mass conditions get weak. Over 69% of emission increment were from cement and steel. Three emission characteristics of tunnel construction affected by surrounding rock were summarized as: correlation of surrounding rock conditions, non-continuity of emission and non-consistency of emission transition path. (C) 2018 Elsevier Ltd. All rights reserved.