Management of the Road Cave-In Risk after a Large Earthquake Using Subsurface Cavity Survey Technology

Shortly after the earthquake that struck Niigata prefecture in Japan on October 23, 2004, the subsurface cavity survey on the national highways was carried out to manage the risk of cave-ins that could become a secondary disaster. The accumulated lane length of 814km was surveyed with the ground penetrating radar technology and more than 400 subsurface cavities were detected. As a result of the study on this actual and rare data, the following knowledge that can be useful to manage one of the secondary disasters after large earthquakes was acquired. (1)To minimize the cave-in risk the subsurface cavity survey should be carried out as soon as possible after an earthquake. (2) The area where the seismic intensity is 5 or more should be covered by the survey. (3) Great care should be taken at locations adjacent to subsurface structures. (4) Even where the seismic intensity is less than 5, the survey is required if the ground condition is not very good. (5) Hazard maps showing the degree of cave-in risk can be a risk management tool. It is utilized to make the most efficient and effective rehabilitation plan.