Mechanism of soft ground tunnel defect generation and functional degradation

This paper deals with the mechanism of soft ground tunnel defect generation and functional degradation. The subway mileage has increased dramatically worldwide, especially in China, in the past decades and will be continuously increasing in the next decades driven by the demands on underground space usage and the advancement of tunneling technique. Subway tunnels are vulnerable to a variety of defects which, individually or interactively, deteriorate the tunnel function for providing passengers with a safe and comfortable transportation means. Understanding the mechanism of tunnel defect generation and functional degradation and providing effective maintenance measures can slowdown the tunnel defect generation and prevent the tunnel defects from developing into catastrophic structural failure. This paper summarizes typical tunnel defects and major contributing factors to the defect generation based on the findings from an inspection program of 130 km of soft ground tunnels in east China. A detailed example of the tunnel inspection and rehabilitation is presented. A framework is developed for analyzing the tunnel defects. The mechanism of tunnel functional degradation is explored associated with five critical links: environment, structure, components, joints, and materials, within a tunnel operation system. The five links individually deteriorate with time and interactively degrade the tunnel function. The research findings from this paper lay a foundation for developing practice guidance for tunnel maintenance.