| For road tunnels in Japan, the New Austrian Tunneling Method (NATM) was introduced in 1989 as a technical standard. Since then, defects on permanent lining due to external force have considerably decreased in newly-constructed tunnels. However, flaking and cracks due to material degradation are still common defects, while new-devised techniques in order to improve quality of permanent lining have adopted in some recent tunnels. In this paper, inspection records, mainly obtained by periodical close-up visual inspection and hammer striking test, are analyzed. The analysis is conducted on 22 tunnels constructed by NATM with total length of 19.7 km including 1,927 spans of permanent lining. Major conclusions include: Delamination of fragments of permanent lining mostly occur around construction joint and tunnel crown. In addition, some of delamination occurs along cracks, even though they are fine hair cracks. An increase tendency of delamination and cracks is found over time. These results are reflected in the periodical tunnel inspection guideline in Japan 2019 as a range of hammer striking test. |
