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| | Abstract | | 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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