With the advancement of tunneling technique, currently, soft ground tunnels are primarily constructed by shield driven. Soft ground shield driven tunnels are vulnerable to a variety of defects which deteriorate tunnel function for providing passengers with a safe and comfortable transportation means. It has practical significance to understand the mechanism of tunnel defect generation and provide timely maintenance to slowdown the tunnel defect generation and to prevent the tunnel defects from developing into catastrophic structural failure. This paper deals with the defect analysis of soft ground shield driven tunnels. The paper summarizes typical tunnel defects and major contributing factors to tunnel defect generation based on findings from an inspection program for 130 km of subway and roadway tunnels in the eastern part of China. A detailed example of the tunnel defect inspection and rehabilitation is presented. A framework is developed for analyzing the causes of tunnel defects. The problems associated with the current practice for tunnel inspection and maintenance are discussed. The research findings from this paper lay a foundation for future tunnel maintenance and rehabilitation.
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, the advancement of tunneling technique and the government policy. Currently there are 96 subway lines with a total mileage of 2800 km in 19 cities in China. The subway lines in China were expected to reach 177 with a total mileage of 6100 km by the year of 2020 and 290 with a total mileage of 11740 km by the year of 2050 . The subway tunnels are constructed in the densely populated cities, mostly located in east China where the subsurface soils mainly consist of soft clays and silts. The soft ground tunnels are primarily constructed by shield driven and supported by prefabricated concrete lining.