史跡保護のため火山砕屑岩地山をV字形縦断線形で掘削: 九州新幹線西九州ルート 木場トンネル

摘要

木場トンネルは，V字形の縦断線形を有し，火山砕屑岩を主体とした延長約2.9kmの山岳トンネルで，直上に住宅が密集しているほか，送電鉄塔や二級河川，長崎自動車道などの保安物件と交差している.工事は，硬質な巨礫混じりの火山碎屑岩を主体とした複雑な地質条件に対応が可能な，大出力の中折れブーム付き自由断面掘削機を採用した.本稿では，地下水位が高い地山を突込みで掘削するため，突発的なトンネル湧水に対応可能な強制排水設備の計画および路盤泥濘化対策や，保安物件直下の掘削における管理体制の構築，小土かぶり区間における止水，切羽安定対策など，施工における課題を挙げ，その対応についての検討結果および現時点までの施工実賴について報告を行う.%The Koba Tunnel is a mountain tunnel of approx. 2.9km in length which has V-shaped longitudinal alignment and mainly passes through pyroclastic rock ground. In addition, it runs beneath a dense residential area and intersects with some important structures such as electric transmission towers, a second-grade river and the Nagasaki Expressway. We used the high power roadheader with a folding boom which enable responses to complex geological conamons in mainly pyroclastic rock containing hard boulders.This report provides information on construction challenges such as planning of forcible drainage facilities that can respond to unexpected welling water in the tunnel and prevention measures for muddy roadbed according to down grade excavation in the ground with high groundwater level, setting up of a management system in excavation directly underneatn important structures and cutting off water and stabilizing cutting face under small overburden before showing the results of study for these responses and construction results up to present point.