ABSTRACT
This paper summarizes the segment lining’s failure modes that were observed during and after the collapse of 7.08m internal diameter twin bored segment lined tunnels. At the time of the collapse, the tunnels were under construction with the use of EPB TBMs in soft silty sand marine deposits within a confined aquifer. During the construction of a cross passage through a pre-treated grouted block of ground, groundwater and eroded silty sand started coming through the excavation face, and the collapse was triggered by the loss of the supporting medium around the tunnel near the grouted block. Following the water and soil leak, the segmental lining’s radial and circumferential joints started failing, and the tunnel experienced significant distortion. After the collapse, a detailed and careful forensic visual inspection was conducted during the recovery work. The detailed visual inspection data revealed that the joint’s failure mode is different depending on the joint position, and joint slip is a key failure mode that would trigger the global instability of the segment rings.
