This paper proposes an environmental assessment approach consistent with sustainable development principlesin the design of underground structures. A literature review highlighted the methods applicable at the projectconstruction stage for buildings or road infrastructure that may specifically be intended for the underground. Anunderground assessment was developed and implemented for the urban case study of a multimodal tunnel opento soft modes, such as pedestrians, bikes and buses, as well as to conventional road modes.A Life Cycle Assessment (LCA) was performed using and combining three methods (CED, EDIP, CML), inyielding 17 indicators. Two distinct scenarios were investigated when upgrading the safety of the existing roadtunnel by allowing for evacuations in case of an emergency. Thefirst (Scenario 1) consists of the constructionand operation of a simple safety gallery, while the second (Scenario 2) consists of the construction and operationof a new tunnel dedicated to soft modes that can also be used as a safety gallery.The impact assessment results showed that variations in the set of indicators in Scenario 2, compared to thoseof Scenario 1 were significant enough to allow for a more refined comparison between the two scenarios. Inaddition, the contributions of the operation phase to the total impacts were examined. The relative weight of theconstruction phase was discussed with respect to the underground solution. This study was then able to point outthat the Life Cycle Assessment is a powerful environmental assessment method for a comparative study ofvarious urban development scenarios, including underground facilities, as of the early project stages.