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Gaps generated underneath railway track slabs may cause unstable conditions. Such gaps can form because of different reasons, including settlement of soft soil, unsuitable construction of a concrete slab on loosely compacted soil, and drastic stiffness change in a transition zone between the bridge deck and embankment. The gaps underneath railway track slabs are not easily detectable by common nondestructive test methods. A nondestructive test (NDT) based on a wavelet time–frequency concept is proposed for the practical purpose of detecting gaps under track slabs. The method uses a microphone sensor to catch leaked Rayleigh acoustic waves in the air and an accelerometer to measure surface Rayleigh waves on the slab. In order to investigate the possibility of developing the test system, a finite element analysis (FEA) was performed to simulate Rayleigh wave generation on the concrete surface and in the air. A test system module composed of a microphone and an accelerometer, data acquisition system (DAQ), and an analyzer program was also assembled for a small backyard pilot test. It was verified that the new NDT test system could be successfully adapted for detecting gaps underneath railway track slabs and track bed soil.

Nondestructive Detection of Gaps between Railway Track Slabs and Soil Foundation Using Leaked Air Waves