The Ground Penetrating Radar(GPR) is a typical non-destructive test equipment which is widely used in seeking a cavity or underground facility. Test results are generally expressed 2D monochrome or color images, distribution of the parabolic waveforms are used to determine the existence of cavity and facility. (Fig. 1) But, an analysis method of image may cause errors depending on the knowledge and experience of analyst. In this study, we analyzed the coefficient of correlation between A-Scan data of GPR to judge the existence of cavity located under the pavement layer. The correlation analysis was performed based on the assumption that the relationship of correlation between a number of A-Scan data passing through a non-cavity section is larger than a small number of A-Scan data passing through a cavity section, and relationship of correlation was visualized using Surfer Program. (Fig. 2) In addition, apart from the correlation analysis, we compared the Power spectrum of the A-scan data for the cavity section and non-cavity section. In other words, assuming that the size of the energy changes depending on the existence of the cavity, PSD (Power Spectrum Density) is obtained for all the B-Scan data, and the tendency of the energy size is confirmed using the 3D wireframe map of the Surfer program. (Fig. 3) As a result, the correlation coefficient shows a small tendency in the cavity section and the PSD shows a large tendency, which is intuitively recognized that the energy attenuation in the cavity section is smaller than other material. But, there are some ambiguous sections to judge the tendency clearly, this is estimated to be noise on the underground facility and it is necessary to take measure of mitigating this.

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.

PURPOSES: The purpose of this study is to evaluate different types of Ground Penetrating Radar (GPR) testing for characterizing the road cavity detection. The impulse and step-frequency-type GPR tests were conducted on a full-scale testbed with an artificial void installation. After analyzing the response signals of GPR tests for detecting the road cavity, the characteristics of each GPR response was evaluated for a suitable selection of GPR tests. METHODS: Two different types of GPR tests were performed to estimate the limitation and accuracy for detecting the cavities underneath the asphalt pavement. The GPR signal responses were obtained from the testbed with different cavity sizes and depths. The detection limitation was identified by a signal penetration depth at a given cavity for impulse and step-frequency-type GPR testing. The unique signal characteristics was also observed at cavity sections. RESULTS: The impulse-type GPR detected the 500-mm length of cavity at a depth of 1.0 m, and the step-frequency-type GPR detected the cavity up to 1.5 m. This indicates that the detection capacity of the step-frequency type is better than the impulse type. The step-frequency GPR testing also can reflect the howling phenomena that can more accurately determine the cavity. CONCLUSIONS : It is found from this study that the step-frequency GPR testing is more suitable for the road cavity detection of asphalt pavement. The use of step-frequency GPR testing shows a distinct image at the cavity occurrences.

PURPOSES : The objective of this study is to evaluate the potential risk level of road cave-ins due to subsurface cavities based on the deflection basin measured with falling weight deflectometer (FWD) tests. METHODS: Ground penetrating radar (GPR) tests were conducted to detect road cavities. Then FWD tests were conducted on 13 pavement test sections with and without a cavity. FWD deflections and a deflection ratio was used to evaluate the effect of geometry of the cavity and pavement for road cave-in potentials. RESULTS: FWD deflection of cavity sections measured at 60 cm or a closer offset distance to a loading center were 50% greater than more robust sections. The average deflection ratio of the cavity sections to robust sections were 1.78 for high risk level cavities, 1.51 for medium risk level cavities, and 1.16 for low risk level cavities. The relative remaining service life of pavement with a cavity evaluated with an surface curvature index (SCI) was 8.1% for the high level, 21.8% for the medium level, and 89.8% compared to pavement without a cavity. CONCLUSIONS : FWD tests can be applied to detect a subsurface cavity by comparing FWD deflections with and without a cavity measured at 60 cm or a closer offset distance to loading center. In addition, the relative remaining service life of cavity sections based on the SCI can used to evaluate road cave-in potentials.

PURPOSES : Road subsidence occurs owing to road cavities, which cause many social and environmental problems, especially in cities. Recently, road cavities were detected by various ground radars and repair works were carried out against the detected cavities. The condition assessments related to the road cavities are necessary to understand the potential risk of the cavities. Therefore, in this study, a numerical study was performed to assess the various conditions of road cavities. METHODS : The numerical method adopted in this study is the discrete element approach, and it is suitable for analyzing the condition because it can consider the movement of the soil particles in the surrounded cavity areas. In addition, the triaxial test was modeled and performed under various cavity conditions inside the specimens. RESULTS: The conditions of different cavity locations and shapes were analyzed to identify the effect of cavity state. Three general cases of particle size distributions were formulated to identify the effect of surrounding ground conditions. As a result, the degree of decrement and volumetric strain were varied depending on the locations and shapes of the cavity. Only minor changes were observed when the particle size distributions were altered. CONCLUSIONS: The strength reduction was higher when the cavity formed was larger and located in the upper zone. Similar to the cavity shape, strength reduction and volume deformation are more influenced by the width than the length of the cavities. There is an influence from ground conditions such as the particle size distribution, especially on the wide cavity.

For the evaluation of the road cavity, GPR method is usually used widely. But the technique of image process is very difficult and confused owing to the similar signals. In this study, the deflection analysis is proposed for the supplement method of the GPR.