| 講演要旨(和文) | | 平成26年10月6日、台風18号により小貝川左岸54.6k付近の堤内地盤で漏水が発生した。調査地は浅部に砂礫層が分布しており、表層を粘性土が被覆している。河川水位の上昇により砂礫層が被圧し、一部で被覆層を突き抜けて漏水が発生したものと考えられる。本研究では、RTK-GNSSに基づく稠密GPR探査を実施して地下浅部の反射面を3次元的に検出し、電極間隔0.5mの電気探査6測線および受振間隔0.5mの表面波探査7測線によって検出した反射面下方の物性を把握した。検出したGPRの反射面は高比抵抗・高S波速度を示し砂礫層と考えられた。また昔の堤防のような直線状の構造が水田下に埋没していることが明らかになった。漏水箇所は痕跡調査から多数あることが確認されているが、堤防上の監視カメラ映像によると、特定の場所からの漏水が卓越しており、この場所は砂礫層が浅く分布し盛り上がった箇所であることがわかった。 |
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| | 講演要旨(英文) | | The water level of the Kokai river rose by rain supplied from typhoon number 18 which pass across the Kanto district in October 6 2014, several seepage occurred in town side of levee around 54.6km point. A gravel layer with high permeability is distributed at near surface in the area, and is covered by surface clay layer. As a mechanism of seepage, it was considered that the water leaked through weak area of the surface covered layer under the condition of high water pressure in gravel layer. In the study, we carried out dense GPR survey with aid of positioning RTK-GNSS and detected 3 dimensional reflections near surface. We also investigated physical properties beneath the detected reflections by 2 dimensional electrical surveys with electrode interval 0.5m and by 2 dimensional surface wave methods with receiver interval 0.5m. The analyzed results indicated that detected GPR reflections were harmonic to upper boundary both of high resistivity area and high Vs S-wave velocity area, and it was considered to be gravel layer. The seepage site is characterized with shallow gravel layer and with 220 ohmm resistivity and with 220 m/s S-wave velocity. We also revealed buried linear structure which was possible to be old levee, beneath rice field. |
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