Effect and control of foundation pit excavation on existing tunnels: A state-of-the-art review

Foundation pit excavation near existing tunnels has become increasingly common in cities. Excavation-induced unloading can inevitably affect existing tunnels and threaten their operational safety. This paper reviews the state-of-the-art developments and future trends in this issue. First, we discussed the technical requirements for tunnel protection and the necessity for predicting and controlling tunnel displacement. Thereafter, the classification and mechanical mechanisms of the effects of foundation pit excavations on existing tunnels are introduced. Further, the determination methods used for the influence zone of foundation pit excavation on existing tunnels are discussed, aiming at solving the question of 'how far is the influence?'. In addition, the prediction methods used for the excavation-induced tunnel displacement are discussed, aiming at solving the question of 'how big is the influence?'. Finally, the control methods used for the excavation-induced tunnel displacement are discussed, aiming at solving the question of 'how to deal with the influence?'. Consequently, we clarified the research gaps and outlined future directions. Thus, this study is expected to function as a reference for engineers to guarantee the safety of existing tunnels affected by foundation pit excavations.