Inspection of the lids of shallowly buried concrete structures based on the propagation of surface waves

• Surface waves are used to assess the surface condition of underground structures.
• The analysis is based on the variation of energy, phase velocity and group velocity of Rayleigh wave.
• The ability of surface waves to assess the conditions of underground structures is demonstrated.
• The degree of degradation of the lid of underground structures may be evaluated from the surface (without borings).

The inspection of underground concrete utility structures can be a challenging task due to their inaccessibility. This article presents a nondestructive inspection technique for the lids of such structures based on the propagation of elastic waves where the variation in soil vertical acceleration following an impact is recorded along a given line at the surface of the soil. The structures investigated are made of reinforced concrete and are located below a shallow homogeneous soil layer which is covered by a pavement. It is shown through finite difference numerical modeling that elastic waves are affected by the state of degradation of the underground concrete structure. It is also shown that the difference in dynamic properties between the soil and the concrete structure causes the latter to act as a waveguide that affects the variation of the vertical acceleration measured at the surface of the model. The propagation of elastic waves within different underground profiles is studied in terms of the variation of their energy and of their group and phase velocity. Theoretical models, computed using the propagator matrix technique, are presented in the appendix to demonstrate the importance of the waveguide effects, caused by the presence of the concrete structure, on the group and phase velocity dispersion curves of Rayleigh waves. Finally, some of the results obtained from the inspection of two different real underground structures are also presented. These results show that the proposed inspection technique, developed based on 1D and 2D numerical testing, is also effective for real structures.