A nondestructive inspection method to estimate the contact condition o
f soil on the surface of an underground pipe, utilizing the resonance
of a transverse Lamb wave circulating along the pipe wall is proposed.
The Q factor of the resonance is considered and measured under some c
ontact conditions by sweeping the vibrating frequency in a 150-mm-inne
r diameter Fiberglass Reinforced Plastic Mortar (FRPM) pipe. it is con
firmed that the Q factor shows a clear response to the change in the c
ontact conditions. For example, the Q factor is 8.4 when the pipe is i
n ideal contact with the soil plane and goes up to 19.2 when a 100-mm-
diameter void is located at the contact surface of the soil. The spati
al resolution of the proposed inspection method is also measured by mo
ving the sensing point along the direction of laying the length of the
pipe into a 85-mm-diameter void. The resolution of the proposed metho
d is estimated at about 50 mm.