| We conducted a detailed GPR survey to detect buried boulders in a highway under construction. The size of boulders were observed against the regulation for embankment subgrade materials. The illegal-size boulders should be removed because they might cause undulations of road surfaces. The survey area was 540 m in length and 20 m in width. First, two parties surveyed the area by using the same GPR tools combined with network RTK-GNSS positioning system. Next, we processed the acquired datasets by using a criterion that we established. The criterion was established based on GPR diffraction patterns to extract anomalies generated from buried boulders. A total of 148 anomalies were extracted. Then, we marked all locations of the anomaly on the road surface, and immediately dug at the extracted points to confirm whether illegal-size boulders were buried or not. As a result, boulders which was bigger than 20 cm in size were removed at 146 points. It took 6.5 days for this survey from a preliminary survey to excavations at all anomaly points. Such a quick handling contributed to minimize the delay of construction. This high hitting ratio demonstrated the effectiveness of dense GPR survey for detecting buried boulders. |
