Environmental impact analysis for the construction of subway stations: Comparison between open-excavation and underground-excavation scheme

The excessive environmental impacts from the construction of subway infrastructure have become a concern for operators who are keen on continuing to lower the environmental footprint, but the factors affecting the impacts are yet to be fully identified. Excavation scheme is the governing factor in subway construction, not only because it prescribes the ensuing construction procedures, but it also epitomizes the design philosophy towards the interaction between the subway structure and city environment. In this paper, a comparative assessment is made among the fifteen mid-point environmental impacts from the two major excavation schemes in subway construction, i.e. the open-excavation (OE) and underground-excavation (UE) scheme. The case station provides a perfect example for such a comparison as it consists of both OE and UE section at the same buried depth. GHG emissions are identified as the largest contributor after normalization. However, GHG emissions are not representative for the overall impacts. The UE section is responsible for more impacts per area than that of OE section in all the investigated categories. The gap between the two sections is attributable to the extra reinforcement procedures that protect the surrounding infrastructure from adverse settlement. Though the underground environment limits the use of construction machines, the UE construction involves more on-site impacts because it requires 11.79 times the labor used in the OE section. Based on the results, the adaptability of the two methods was discussed, considering the life-cycle performance and their interaction with the city environment.