造成盛土地盤中および直下のトンネル掘削: 和歌山巿道中平井線 ふじとトンネル

摘要

Fujito Tunnel that was built with NATM is a two-lane road tunnel of 733m in length and approx. 50m~2 in inner cross-section area. The tunnel is on the Wakayama City Planning Road Nakahirai Road (2,190m). which connects National Route 26 and the newly constructed ramps on the 2nd Hanwa National Highway / Wakayama-Misaki Road. The condition of construction in the range to 111.3m from the starting portal was to excavate the ground of gravelly unconsolidated embankment with N value between10 and 20 under 7-16m (0.7-1.6D) from surface. In addition, the condition in the range between 350m and 500m from the starting portal is to excavate directly under embankment which had filled a valley with overburden of approx. 50m (minimum overburden of one meter in former valley surface). For these reasons, there was concern that it would not be possible to ensure sufficient ground arches and that cutting faces would be unstable during tunnel excavation. This report mainly contains information on measures against above-mentioned conditions.%ふじとトンネルは，国道26号と第二阪和国道·和歌山岬道路の新設ランプを結ぶ和歌山巿の都巿計画道路中平井線2,190mのうち，延長733m，内空断面約50m~2のNATMによる2車線道路トンネルである.起点側坑口から111.3mは，N値10～20程度の礫主体の未固結な造成盛土地盤中を土かぶり7～16m程度(0.7～1.6D程度）でトンネル掘削する条件となり，また，起点側坑口より350～500mの区間は土かぶり約50mで谷を埋戻した造成盛土直下（旧谷地形の最小土かぶり1m程度）を掘削するという特殊な施工条件である.そのため，トンネル掘削時には切羽の不安定化や十分なグラウンドアーチの確保ができないことが懸念された.本稿では，おもに上記について施工時の対応を報告する.