光ファイバケーブルセンサを用いた切羽前方変位計測手法の開発
TN-Monitor®による3次元切羽前方変位計測の実現
切羽前方変位計測手法「TN-Monitor」では，これまでトンネル前方における鉛直変位とトンネル軸方向変位が計測対象であった。しかし，大土被りトンネルで鉛直方向応力よりも水平方向応力が卓越する場合や，岩盤物性の異方性が顕著で，トンネル横断方向の剛性が小さい場合には，切羽通過後変位と同様に切羽前方変位も，沈下よりも水平方向変位が支配的になる場合がある。そこで本研究では，多点でケーブル延長方向のひずみを計測することが可能な光ファイバケーブルセンサAGF（All Grating Fiber）と高い位置分解能を達成できるOFDR（Optical Frequency Domain Reflectometry）測定方式を採用することで，鉛直および水平変位量を同時に評価できる計測手法を開発した。本成果に伴い，TN-Monitorは3次元切羽前方変位計測手法（鉛直・水平・トンネル軸方向変位）に拡張された。
キーワード：山岳工法，切羽前方変位，計測，光ファイバ計測，OFDR
＊１ 技術センター 社会基盤技術研究部 地盤研究室
Tunnel Pre-displacement Monitoring by Fiber Optic Sensing
3-D Displacement Monitoring Ahead of Tunnel Face by Newly Evolved TN-Monitor
“TN-Monitor” is a tunnel pre-displacement monitoring method, for measuring vertical and tunnel longitudinal displacement ahead of the excavation face. As in the case of displacement after the face passage, ground behavior ahead of the face may be more dominant laterally than vertically and longitudinally in deep tunneling where in-situ horizontal stresses pre-dominate over vertical stresses, or anisotropy of the rock mass properties may be so significant as to make the rock mass stiffness in the tunnel cross-sectional direction relatively small. The authors therefore developed a displacement measurement method using fiber optic sensing, which can simultaneously evaluate vertical and lateral displacements ahead of the tunnel face. The method uses special fiber optic cables named All Grating Fibers (AGF) capable of evaluating the longitudinal expansive and contractive strains along the cables with high positional resolution by the Optical Frequency Domain Reflectometry (OFDR) method. Thanks to this research achievement, TN-Monitor has now been updated into a 3-dimensional tunnel pre-displacement measurement method.
Keywords: conventional tunneling, tunnel pre-displacement, measurement, fiber optic sensing, OFDR
*1 Geotechnical Research Section, Infrastructure Technology Research Department, Taisei Advanced Center of Technology