| Recent Japan has many torrential rains to cause flooding. Typhoon developed in October 2019 caused heavy rain to break 142 parts of river levees. Additionally, many parts of seepage were found in the area where levees were not breached. Any parts of levee were not broken in the surveyed area shown in this report, but flooding was caused near the area. In the surveyed area, sand was boiled at the locations over 100 points and large seepage was caused. A part of slope was collapsed in the area, but the damage did not develop to break the levee. Ground-penetrating radar (GPR) and electric survey were applied in this area and measurement with a soil strength probe using a thin metal rod was also applied. Measurement using the soil strength probe provides the depth distribution of a permeable layer constructed of sand with gravel. GPR sections can extend the distribution of soil and permeable layers to wide area. A section of GPR insists that points of seepage are located at the edge in the closest zone between the permeable layer and the ground surface. Revealing the distribution of soil classification in shallow zones can identify locations of seepage near the levee. Also, strong reflection events on a GPR section are found in a high resistivity zone on the result of electric measurement. A reflection event of GPR is located at a boundary of resistivity, which indicates to be a soil boundary. Applying more than one method of geophysical exploration expands the range of interpretation in shallow ground near river levees. |
