Critique of TBM Lining Design Defects: Origin, Characterization, Measurement, and the Role of the Installation Phase

This paper is motivated by the experience of measurement and characterization of tunnel lining defects occurring in a real case involving four tunnels excavated by tunnel boring machines (TBMs). This experience has shown that often the causes of these defects were not well understood and not well covered by current design practices. The paper focuses on five defect types: uncontrolled cracking of the lining, ovalization, joint openings, misalignments, and edge ruptures. It is found that deficient gap filling is the cause of excessive cracking, ovalization and joint openings, whereas misalignments and edge ruptures are caused by deficient placing of the segments. Edge ruptures could also be related to lack of confinement of the stresses at the joint section in the longitudinal direction. The possibility of deficient gap filling is not contemplated in design practices, and perhaps it should be considered, in terms of having enough minimum reinforcement to control cracking when no significant axial force is present. Ovalization is interpreted as being caused by service loads, whereas in fact this paper shows that it is also due to deficient gap filling and its structural consequences are, therefore, not significant. The experience recounted in this paper is a good instance of the importance of understanding the underlying causes of the observed phenomena to properly treat them.