Active Scour Monitoring using Ultrasonic Time-Domain Reflectometry to Detect a Soil Interface

Local scour is arguably the most pressing issue regarding the safety and longevity of overwater civil infrastructure. Many modern scour detection techniques do not provide continuous scour depth measurements, nor can they function under extreme flow conditions, which is when scour monitoring becomes most critical. Thus, the objective of this study was to develop scour depth monitoring sensors using ultrasonic time domain reflectometry (UTDR). The scour sensor was based on an aluminum strip with two piezoelectric macro fiber composites (MFCs) bonded at one end. The aluminum strip or rod-like sensor is intended to be driven and buried at the location where scour depth measurements are desired. The two MFCs were used to either generate or sense ultrasonic Lamb wave pulses propagating in the aluminum strip. During scour, as sediment is eroded from around the base of the strip, the distance (i.e., scour depth) between the MFCs and the soil interface would increase. The hypothesis was that increasing scour depth would change the mechanical impedance of the system to cause measurable and unique signatures in the residual Lamb wave signals. To test this hypothesis, different interfaces (i.e., metal-metal, polymer-metal, and soil-metal) were applied at different locations along the aluminum strip and MFC system. The MFC sensor-actuator pair was actuated to propagate and measure the corresponding Lamb waves during each test. The results showed clear changes in the residual signal, which were well-correlated to the changing locations of the artificial interface. In particular, the time-of-flight of the response pulse within the residual signature could be used to accurately determine the location of the soil interface or scour depth. Overall, this study demonstrated feasibility of an UTDR sensor for scour monitoring.