Abstract: For the internal damage of hollow cavity, water content, cracks and other structures of tunnel bottom plate. To improve the detection efficiency of traditional contact type impact echo method, the air coupling impact echo response characteristics of internal defects in tunnel floor structure is conduced from the perspective of gassolid coupling, and regarding the air as the stress wave transmission coupling medium between floor structure and detection system. The results indicate follows. (1) The defeat depth in floor structure determined by air coupled impact echo method is basically consistent with the theoretical one. (2) The detection accuracy is related to the size and depth of defects, and the detection accuracy gradually decreases as the size and depth of defects increase. (3) The detection effect of this method for defects of hollows and water contain is better than that for defect of cracks.

Key words: tunnel, floor structure, defects, air coupling, impact echo