Failure analysis of time-delayed collapse triggered by time-dependent fracturing in a deep TBM tunnel

As the underground engineering advances into the deep, high geostress and complex geological conditions increasingly trigger various time-delayed failures. Recently, a new time-dependent surrounding rock failure, named time-delayed collapse, has been encountered multiple times during the construction of a deep TBM tunnel. This time-delayed failure started from the potential unstable rock mass cut by structural planes and was manifested as the heavy blocks detaching from the surrounding rock eventually due to continuous time-dependent fracturing. The examination of the evolution process indicates that intersecting structural planes in the surrounding rock are critical prerequisites for time-delayed collapse. After excavation, under high geostress and gravity, continuous time-dependent fracturing serves as the intrinsic driver of failure development, leading to significant spatial–temporal lag characteristics. Field monitoring also showed that different geological conditions would result in distinct fracturing characteristics during development. However, compared to rockbursts, the subtle time-dependent deformation or weak microseismic activities in time-delayed collapse areas pose challenges for in prediction or prevention in field monitoring.