Deformation and failure mechanism of segmental tunnels induced by side unloading of adjacent excavation

• A sophisticated numerical model of segmental ring considering the interaction between tunnel and ground. • Residual bearing capacity of the concrete segments is investigated. • The deformation control value is proposed to ensure the tunnel in a good service state. Shield tunnels subjected to side unloading of adjacent excavation will deform under complex interaction between the structure and the ground. To understand the deformation and failure mechanism under adjacent excavation, previous study was conducted using physical test with acting given point loads, which fails to reflect the dynamic constraint of the ground. This paper established a sophisticated numerical model that incorporates the tunnel taking precast RC segments, bent bolts, and rebars into account, and the surrounding ground. The deformation behaviors associated with the degradation of the overall stiffness, and the failure mechanism were investigated. The results indicate that with side unloading the tunnel deformation tends to change from a symmetrical horizontal ellipse to an asymmetrical horizontal ellipse. The load on the tunnel significantly decreases at the side adjacent to foundation pit, while the magnitude of bending moment of the tunnel significantly increases. The development of transverse deformation is accompanied with the degradation of the overall stiffness, which is related to the yielding of joints. The deformation of the segmental ring as side excavation can be divided into four stages: elastic stage, elastic-plastic stage, plastic stage, and failure stage, and the critical deformation between stages is 30.1 mm, 51.3 mm, and 81.3 mm, respectively. When the deformation reaches 81.3 mm, 5 yielding sections forms and the equivalent stiffness coefficient degrades to 0.064. To ensure the tunnel in a good service state in engineering practice, the transverse deformation is recommended to be controlled within 51.3mm, exceeding which the contour measure of structural reinforcement is recommended to be conducted.