Ultrasonic Guided Wave Inspection of Anchor Rods Embedded in Soil

Ultrasonic guided wave nondestructive testing for small-diameter cylindrical steel anchor rods embedded in soil was experimentally studied by using piezoelectric probes attached to the sides of the rods. The 60-kHz longitudinal [L(0,1)] mode was chosen as the guided wave for pulse-echo measurements on anchor rods with a diameter of around fifteen millimeters mainly because of its low attenuation due to energy leakage into the surrounding soil. Pulse-echo measurements were performed by using four transmitting and four receiving probes attached to the sides of anchor rods protruding above ground. Appropriate procedures for the transmission and reception can selectively generate the L(0,1) mode going in only the downward direction and can selectively extract the signal of the L(0,1) mode coming from only the lower direction, which considerably improves the signal-to-noise ratio (SNR). An experimental study for detecting defects of artificial cross-sectional loss on embedded anchor rods was conducted to examine the fundamental performance of the measurements by means of evaluating the SNRs. An on-site field experiment on actually used equipment was carried out to examine the potential of estimating degrees of real corrosion wastage on embedded anchor rods. The results show that the attenuation rate of the L(0,1) mode in the buried part of the rod and distance amplitude correction (DAC) curves taking account of the attenuation rate are effective in estimating the degrees of corrosion wastage on embedded anchor rods.