Ground depression/cave-ins due to subsurface cavities have been occasionally occurred in urban area. To prevent the ground cave-ins, a ground penetrating radar (GPR) method was applied and more than 2,000 subsurface cavities were found in Seoul. For each cavity, excavation was carried out to investigate main cause of the cavity and then the site was restored permanently. It was found that this excavation-and-restoration method was not efficient to repair small size cavities. Instead, grouting methods was used to repair the small cavities temporarily. This study evaluated the field applicability of grouting methods on restoring small cavities. Three types of grouting materials were applied on 12 sections and two non-destructive tests were conducted in field. A falling weight deflectometer(FWD) test was conducted to assess the bearing capacity of the site before and after grouting. Ground penetrating radar (GPR) was used to evaluate the extent of the grouting materials to a cavity. From the FWD test results, the bearing capacity of the Section A and B was enhanced by 7.5% and 13.5% while the bearing capacity of the section C was reduced by 11.5%. It was found from the GPR tests that the grouting materials used in the Section B and C could fill the cavity well and also extended to surrounded area due to injection pressure or selfexpansion. In common, the small cavities could be restored quickly, less than 30 minutes per a cavity by the grouting methods. Hence it can be concluded that the grouting methods can be applicable to restore small cavities. However, it needs to consider the application of grouting methods carefully because the grouting methods can be lost through a pipe link to a cavity. Also, excavations can be conducted occasionally in urban roads so that the stiffness of grouting materials needs to soft enough to be excavated and strong enough to support traffic loads.

This is the abstract section. One paragraph only Road cavities recently in urban are causing collapse of road surface layer due to loss of support bearing capacity. Detecting road cavities with ground penetrating radar(GPR) test, then excavation and backfill are performed in the anticipated cavity area. However sometimes detecting errors are occurred because of the complexity of the GPR test result analysis or interval space between larger gravels. So before unnecessary excavation, verification for detect the cavities results should be needed. The purpose of this study suggest deflection method by the light weight deflectometer(LWD) as a verification way of GPR test results and as a tracking investigation method continuously at the sites having small size cavity. LWD devices has more advantages than larger NDT because FWD has difficulties in a traffic control and entrance of narrow-back road. In this study, LWD tests were conducted on the pavement sections with and without road cavity detected by GPR tests and after excavating the area, the cavity sizes were measured. LWD test results can be applied to verify a subsurface cavity by comparing maximum deflection and deflection ratio between cavity area and non cavity area at the loading center. The higher deflection and lower modulus was measured at cavity sections. Based on the results of the comparative analysis, It is found that deflection method has a possibility of complementary for detecting road cavity. Also cavity size prediction equation was attempted to propose through deflection ratio using a database. Compared with another validation data, the proposed prediction equation is more suitable for detecting cavity existence than size estimation because the average error rate is larger. As a results of the analysis with depth ratio as a factor, it is necessary to improve the cavity size prediction through the normalization using the parameter of road properties.