報告書番号

N10013

タイトル（和文）

地上からの調査に基づく坑道建設性評価（その１）－難工事事象の地質要因の分析と山はね予測評価法の提案－

タイトル（英文）

Rock mass evaluation for predicting tunnel constructability in the preliminary investigation stage - Phenomena causing difficult tunneling and rockburst prediction -

概要 （図表や脚注は「報告書全文」に掲載しております）

高レベル放射性廃棄物の処分場の選定に向けて行われる概要調査においては，坑道内空が膨張したり（膨張性地山），岩が破壊して飛び出す（山はね）などの難工事事象への遭遇可能性を事前に評価することが重要な課題の１つである。そこで難工事に遭遇したトンネルなどの施工事例280について資料約500件を収集して地質要因を整理した。高圧・大量湧水や膨張性地山の難工事は、第四紀火山分布地域、グリーンタフ地域、付加体堆積岩地域の混在岩の劣化部に多い。また、山はねの発生と地圧などとの関係を分析した。水圧破砕により測定した水平最大地圧SHと水平最小地圧Shの差が30MPa以上のときに山はね遭遇が示唆されることがわかった。これらの知見より、概要調査段階に得られる情報から山はねへの遭遇可能性を評価する手法をまとめた。

概要 （英文）

For the selection of the Detailed Investigation Areas for HLW disposal, predicting the tunnel constructability is one of the requirements together with assessing long-term safety. This report is the 1st of the three papers dealing with the evaluation of tunnel constructability. This paper deals with the geological factors relating to difficult tunneling such as squeezing, rockburst, and others. Also it deals with the prediction of rockburst. The 2nd paper will deal with the prediction of squeezing. The 3rd paper deals with the engineering characteristics of rock mass through rock mass classification.
This paper about difficult tunneling has been based upon analysis of more than 500 tunneling reports about 280 tunnel constructions. The causes of difficult tunneling are related to (i) underground water, (ii) mechanical properties of the rock, or (iii) others such as gas. The geological factors for excessive water inflow are porous volcanic product of Quarternary, fault crush zone and hydrothermally altered zone of Green Tuff area, and degenerated mixed rock in accretionary complex. The geological factors for squeezing are solfataric clay at Quarternary volcanic zone, fault crush zone and hydrothermally altered zone of Green Tuff area, mudstone and fault crush zone of sedimentary rock of Neogene and later.
Information useful for predicting rockburst has been gathered from previous reports. In the preliminary investigation stage, geological survey, geophysical survey and borehole survey from the surface are the source of information. Therefore rock type, P-wave velocity from seismic exploration and in-situ rock stress from hydrofracturing have been considered. Majority of rockburst events occurred at granitic rock, excluding coal mine where different kind of rockburst occured at pillars. And P-wave velocity was around 5km/s at the rock of rockburst events. Horizontal maximum and minimum stresses SH and Sh have been tested as a criterion for rockburst. It has been inferred that SH – Sh > 30MPa is a good criterion for rockburst occurrence. When rock stress data is not available, continuous occurrence of borehole breakout is also a good indicator of rockburst.

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