The Norimoto Tunnels are twin tunnels about 750 m in length in Shin Tomei Expressways that is under construction by the Central Nippon Expressway Company at Shinshiro City in Aichi prefecture. An existing aqueduct tunnel was constructed in 1960 directly below Norimoto Tunnel at about 17 m offset. Before excavation of the Norimoto Tunnels, there were concerns regarding impact on the existing tunnel, both dynamic impact by vibration associated with blasting and static impact by load of loosened rock mass, and displacement associated with tunnel excavation. In this paper, specific actions taken during excavation of the section crossing the existing tunnel and the results will be discussed.
Before actual excavation of the section crossing the existing tunnel, a section 208 m in length was set up for monitoring both dynamic and static impact and the impact on the existing tunnel was estimated in advance. To improve the accuracy of numerical analysis of the impact prediction, a test excavation section was set up prior to excavation of the crossing section. The results of numerical analysis were verified based on the measured ground displacement during the test excavation to improve the accuracy of prediction analysis. Excavation of the crossing section was continuously monitored automatically using an embedded 3-directional compound vibrometer for dynamic impact and 3-dimensional extensometer for static impact.
As a result of the excavation of the crossing section, since both dynamic and static impacts were observed to correspond approximately to the prior predicted value, the impact on the existing tunnel was reduced to the minimum. Furthermore, no deformation due to the tunnel excavation was observed during visual inspection of the existing aqueduct tunnel. The specific actions taken during this tunnel excavation will certainly contribute to future similar neighboring construction projects.
