In risk modeling, any observed defect can be assumed to be a
failure event. For demonstration purpose, a tunnel segment between
two subway stations is defined as a defect unit if at least
one defect event is observed at least one joint along the ring section,
as shown in Fig. 11. Otherwise, the ring is defined as a suspension
unit (i.e., the specific defect is not observed at the time of
inspection, but subject to defect sometime later). The numbers of
defects and suspensions are given in Table 2. These data are used
as failure points in the risk modeling and defect indicator in damage
accumulation modeling