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| | Abstract | | We conducted a highly dense survey by GPR system interfaced with a network typed RTK-GNSS to detect buried boulders in roadbeds layer of an embankment under construction. Firstly we buried boulders again, which actually were collected from roadbeds, and carried out experiments to detect the boulders. We confirmed the effectiveness of the detection after rain fall. The condition made a large permittivity difference between the boulders and the roadbeds. After the experiments, we immediately started investigating the area of about 12,000 m2 with survey lines of about 0.3m interval. Although total length of survey lines reached to about 43.4km, it took only 1.5days using two GPR systems. We defined a criterion to discriminate the anomalies caused by buried boulders from others. It based on a pair of GPR diffraction patterns which reflected at the top and bottom of boulders. We extracted 177 anomalies as possible boulder responses. We then conducted the secondary survey immediately to precisely locate the anomalies. Since some anomalies were detected multiple times because of dense survey lines, we finally identified 148 anomalies and marked the locations in the secondary survey. The located points were immediately dug out with a backhoe to confirm whether boulders had been buried or not. As a result, nonstandard sized boulders for embankment materials were excavated out of 146 anomaly points. Owing to the high effectiveness GPR survey within a total of 6.5 days from the primary survey to the excavation of identified boulders, we successfully contributed to minimize the delay in embankment works of the highway under construction. |
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