Monitoring interaction of shallowly buried, large-section, and long-distance twin box jacking tunnels with small spacing under the Beijing-Hangzhou Grand Canal

Field tunnel monitoring of stresses and strains is of critical importance for understanding the interaction behavior of twin tunnels, but it is rarely done for projects of twin box jacking tunnels. In this study, the optical frequency domain reflectometer (OFDR) sensing technique was employed to conduct field monitoring for a project of twin box jacking tunnels with a small spacing (5.4 m), a shallow minimum cover depth (4.39 m), a large cross-section (9.8 m × 5.9 m), and a long jacking distance (215.9 m) crossing the Beijing-Hangzhou Grand Canal. A scheme of laying circumferential and axial “W” shaped optical fibers was proposed to monitor the accumulated vertical misalignment and rotation of tunnel. Results indicate that small spacing could cause more significant compressive strains in the earlier constructed tunnel, and the maximum compression positions were in the crown and the wall near the second tunnel. In addition, the rotation angle decreased with the increment of tunnel spacing, when the spacing was lower than one times the box width. In the end, an axial monitoring scheme was discussed to evaluate the tunnel opening, and a horizontal misalignment scheme was derived to accomplish the full monitoring of tunnel misalignment in three directions.