先進ボーリングを用いた切羽前方の湧水調査技術「T-DrillPacker®（水圧圧送方式）」の開発
山岳トンネル施工中に坑内に流入する湧水の発生箇所や流量・水圧を事前に把握することは，工事を安全・確実に進めるために重要である。筆者らは，施工中の切羽から先進ボーリングを行い，削孔途中で削孔管やビットを回収することなくパッカーを設置し，湧水発生箇所の流量・水圧を迅速・確実に測定できる調査技術「T-DrillPacker®」を開発してきた。これまでは，ロッドを継足してパッカーを挿入していたため，パッカーの設置・回収作業に時間を要していた。そのため，トンネル工事を一時中断して行う先進ボーリングでは，地質や湧水状況から複数箇所での調査が必要と判断される場合でも，時間の制約から湧水測定の実施箇所を限定する必要があった。そこで，測定作業を更に高速化するため，パッカーをロッドの代わりに水流で高速圧送し，短時間でボーリング先端に設置する「水圧圧送方式」を新たに開発した。本稿では技術の概要と動作確認試験の結果について報告する。
キーワード：山岳トンネル，先進ボーリング，湧水量，湧水圧，パッカー
＊１ 技術センター 社会基盤技術研究部 地盤研究室
Development of "T-DrillPacker / Pressure-driven Type" for Measurement of Groundwater Inflow Rate and Pressure in Advance Boring
Determining the location, flowrate, and pressure of groundwater ingress during mountain tunnelling is important for achieving safe and steady construction progress. It is therefore important to locate the flow paths and estimate the flowrate and hydraulic pressure (e.g., in fractured zones) ahead of the tunnel face by borehole logging. The authors have developed a measurement system called "T- DrillPacker®" that can install a packer system without pulling out the drilling strings and bit and can measure the flowrate and hydraulic pressure at the positions of groundwater quickly without causing risk of borehole collapse. In the conventional method it took time to install and retrieve the packer because rods must be added when the packer was inserted. As a result, when logging at multiple locations was required depending on geology and/or the flowrate, it was sometimes necessary to reduce the measurement locations due to time constraints in the borehole logging conducted while tunnel construction was interrupted. Therefore, to further speed up the measurement operation, a new "pressure-driven installation type" has been developed in which the packer is pumped to the tip of the borehole in a short period of time with high water pressure, instead of by adding rods. In this paper, the concept is described and an overview of the new measurement system is presented, together with preliminary results of a validation test.
Keywords: Mountain tunnel, advance boring, groundwater inflow rate, hydraulic pressure, packer
*1 Geotechnical Research Section, Infrastructure Technology Research Department, Taisei Advanced Center of Technology