An innovative, quick and accurate investigation techniques for detecting cavity under pavement concrete slab becomes significant in order to avoid the concrete slab pavement problems during its service-life. Problems of concrete pavement can be due to two basic causes, i.e, deterioration or deficiency of the pavement, e.g., cracks; and the structural adequacy of the pavement and base structure e.g. cavities in concrete slab and deflection causing settlement. In this research, an improved technique for detecting cavity under concrete box tunnel based on nondestructive Flexural-Rigidity Assessment Technique of Concrete Tracks by Anti-symmetric Lamb Waves (FRACTAL) was proposed. The research was carried out at an concrete box tunnel in Seoul. An appropriate non-destructive technique for investigating at the pavement under concrete box tunnel on the existing pavement should meet the requirement of speed and reliability in field testing. Therefore, flexural rigidity was chosen as a key parameter to assess the structural integrity of a concrete box tunnel, rather than material stiffness. The testing results indicate that the FRACTAL technique is able to detect cavities under the concrete box tunnel.