Risk mitigation with measurements while constructing second tunnel adjacent to existing tunnel

Summary

Abstract

In Japan, expressway construction projects have been progressed in which the temporary operated two-lane road is converted into four lanes with a two-lane of each direction. The main purposes are eliminating chronic traffic congestion during tourist seasons, reducing significant accidents caused by face-to-face traffic, and securing emergency transportation routes in serious disasters. Those projects often require the second tunnel close to and parallel to the first tube. In the Tokai-Hokuriku Expressway, the second tube of Johana tunnels with 3,221 m in length is now being enlarged the existing small tunnel used for an evacuation route. The 2nd tube is excavated by a conventional drill and blast method parallel to the first tube with the representative elongation of 30 m. To investigate the influence on the 1st tube of ground displacement and oscillation caused by face progresses of 2nd tube, the lining soundness evaluation (cracks investigation) has been carried out in advance throughout the entire of 1st tube. In addition, tunnel convergence, lining strain for stresses evaluation, and vibration of 1st tube have been monitored during excavation. Some of connection drifts presently a small cross-sectional area between 1st tube and the escape tunnel are going to be re-profiled for contributing to vehicles evacuation after the completion. Those re-profiling procedure requires the excavation proximity to the 1st tube in service, which leads to special consideration on ground conditions and deformation characteristics between two tunnels in advance and applying auxiliary measures to ensure ground stability if necessary. The authors have planned to monitor horizontal and vertical underground displacement with an optical fiber sensing method to investigate ground deformability and to monitor ground settlement ahead of the connection drifts’ face during enlargement. In this paper, we report those measurement methods, measurement results, and construction results of the two evacuation connection drifts constructed so far.