Measuring joint opening displacement between model shield-tunnel segments for reduced-scale model tests

Abstract

Joint opening between lining segments in a shield tunnel is a critical parameter for the tunnel performance under different loading conditions. Large joint opening may be an indicator of concrete crushing in the lining and may lead to water leakage. Therefore monitoring the joint opening is an important issue in the model experiments of shield tunnels. However the shield tunnel models in reduced-scale model tests, including centrifuge tests, are small in size and buried in soils, making it quite difficult to monitor the joint opening. In this study, an approach is proposed to measure such opening in reduced-scale model tests. The approach employs a large-range electric strain gage, which is attached across the joint with a free length. The approach was validated and applied to two loading tests of a model shield tunnel made of plexiglass and sliced into six equal segments, which were connected by steel threads simulating the bolts. It was found that the joint opening was proportional to the voltage output of the strain gage, which was independent of the magnitude of the free length. With an appropriate coefficient that was previously calibrated, the joint opening was accurately measured.