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  An automated and rapid measuring method for cross-section deformation of shield tunnels

  Due to the high cost and continuously aging deterioration, the inspection and maintenance of structures have become increasingly important in the tunneling field over the past decades [1]. Shield tunnel consists of precast concrete segments that form rings by the connection of steel bolts. Unlike tunnel constructed by cast-on-site concrete linings, the rigidity of shield tunnel is relatively small, which is prone to appear large deformation under accidental loads or aggressive environment. In the maintenance of shield tunnels, the geometry of cross-section profiles is important information to assess the current service condition [2] and make predictions of structural performance [3]. For shield tunnels using as subway structures, to avoid the occurrence of collapse due to large deformation, the cross-section deformation of every ring should be surveyed to ensure the safety of operation. Also because of the high traffic pressure in metropolis, subway tunnel structures are only available for inspection during everyday midnight after the subway operation ceased. So there are thousands of shield tunnel rings to be measured in limited hours. It is really a challenge for tunnel operators. Actually, according to some first-hand survey data [2], most defects including cross-section deformation are not severe in shield tunnels, and only several severely deteriorated sections in subway network need high intensity inspection. Thus the inspection strategy will have an important influence on the efficiency and cost of maintenance. In general knowledge, it is an effective way to do two-step inspections, which are preliminary screening of whole structure and detailed inspection in specific sections. In the preliminary screening, the accuracy could be deregulated to acceptable level by cutting down the cost of device and improving the speed of inspection. After screening out the possible areas of severe defects, detailed inspection with high

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