抄録
Recently, old narrow tunnels are reused by widening their sections for economic efficiency. New design concept will be required in their construction because excavation damaged zone (EDZ) might be already formed around old tunnels. Especially, estimation of EDZ extent is necessary for safe construction and stability assessment of a widening tunnel. In this study, EDZ around an old tunnel constructed in the Neogene soft rock seventy years ago was estimated based on P-wave velocity and observation of the face during widening. Deformation caused by widening the old tunnel was also measured. It was found that EDZ estimated by reduction in P-wave velocity was in good agreement with that estimated by observation of the face. It was also found that deformation of the widening tunnel was much less than that of either the old tunnel or a new tunnel constructed near the widening tunnel. In the widening tunnel, most of EDZ had been excavated and non-damaged zone outer side of the EDZ had been little excavated. It can be expected that deformation of a widening tunnel is little when the section size of that is almost equal to extent of EDZ.
