蛇紋岩を鋼製支保工の降伏を許容した支保構造により掘削: 国道40号　音威子府トンネル

摘要

音威子府トンネルは蛇紋岩区間の土かぶりがおおむね200m以上と大きいことから，塑性変形による著しい土庄が作用して，トンネル支保工が降伏する可能性が予見された．そこで鋼製支保工応力が降伏応力を超えて剛性が低下し，吹付けコンクリートおよび周辺地山に荷重が再分配される状況を模擬するトンネル掘削解析プログラムを開発し，これを用いた掘削解析を通じて土かぶりと地山の変形係数に応じたもっとも合理的な支保パターンを選定できる仕組みを構築した．本稿では開発したプログラムの紹介および構築した支保パターンの選定方法を概説するとともに，蛇紋岩区間の施工結果について報告する．%The excavating works of Otoineppu Tunnel along in the serpentine was predicted that there was a possibility that the tunnel steel supports would yield by considerable earth pressure due to plastic deformation, as the overburden of the section is over 200 m. Accordingly, we developed a tunnel excavation analysis program that simulates situations in which rigidity reduces as the steel supports yield and load is redistributed to the concrete lining and the surrounding ground. We created a framework which makes it possible to select the most rational support pattern that corresponds to overburden depth and deformation coefficient of ground through excavation analysis with the program. This report contains an introduction to the program, an outline of the support pattern selection framework that was constructed as well as the construction results for the serpentine section.