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| | Abstract | | After the seasonal rain front which had become more active by typhoon number 8 approaching brought the localized heavily rain at Nagiso town, Nagano prefecture in July 9, 2014, the disaster of debris flow occurred at Nashizawa valley. When the debris flowed along Nashizawa valley, Nashizawa Sabo dam where located in upstream area of the valley received it and lost a upper concrete block of dam body.The remained dam body was pushed and was partially displaced to the front, and several open cracks appeared on the face of the dam body. The authors surveyed the internal fracture in the concrete body of the Sabo dam by GPR to understand destroyed process of the Sabo dam. We scanned about the area of 10 m by 13m on the front face of the Sabo dam by suspending dual antennas of central frequency 300/800MHz from the crest of the dam. As a result of GPR measurement, we found that the cracks inclined with low angle to the wall surface of the dam and distributed near surface to the depth of about 0.8m. We also estimated the deformation of the front face by analyzing digital photographs. The result showed that upper block of remained Sabo dam body was pushed about 13cm to the front. We considered that the detected reflections were lower boundary of separated thin concrete block by following the block which was pushed to the front. |
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