Inspection Method for Contact Condition of Soil on the Surface of Underground Pipe Utilizing Resonance of Transverse Lamb Wave

A nondestructive inspection method to estimate the contact condition of 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 contact conditions by sweeping the vibrating frequency in a 150-mm-inner diameter Fiberglass Reinforced Plastic Mortar (FRPM) pipe. It is confirmed that the Q factor shows a clear response to the change in the contact conditions. For example, the Q factor is 8.4 when the pipe is in 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 spatial resolution of the proposed inspection method is also measured by moving the sensing point along the direction of laying the length of the pipe into a 85-mm-diameter void. The resolution of the proposed method is estimated at about 50 mm.