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  Lining crack evolution: a qualitative and quantitative index for degradation of tunnels in operation

  Pertinent evaluation of structural integrity and safety of tunnels in operation is a prospect that tunnel engineers are striving for. In the meanwhile, clarifying the cause of tunnel anomalies is also essential to effective tunnel maintenance. Researches indicated that the pattern of lining cracks can be characterized to suggest possible candidates of anomaly causes. Nevertheless, quantitative description of lining variation is potent in defining chronological changes of a tunnel. Thus, possessing both qualitative patterns and quantitative values, lining crack evolution can be a satisfactory index for degradation of tunnels in operation. The lining images of a highway tunnel located on a creeping slope in south-eastern Taiwan is recorded from Feb. 2008 to Apr. 2012 every six months or so, while maximum displacement be hundreds of millimetres. Inspected via a novel image-mosaic technology, the high resolution layout images of tunnel wall surfaces were employed to interpret and depict lining crack distributions. The configuration of existing lining cracks at Feb. 2008 contains transverse crack, enclosed cracks and inclined cracks. New cracks nearby transverse cracks develop in transverse pattern, and cracks around enclosed cracks tend to complete the close shape. The incremental inclined cracks stretch over more than four construction joint based on the original inclined cracks. The total incremental crack length during Feb. 2008 and Apr. 2012 is 526.7 m, with average velocities varying between 7.5-43.1 m/month and average 11.1 m/month. Lining crack evolution reveals that the degradation of structure varies with spatial location and time. Take cracking rate as the degradation velocity of case tunnel, it went down from Feb. 2008 to Mar. 2010, rose till Aug. 2010, and reduce again until Apr. 2012.

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