Active scour monitoring using ultrasonic time-domain reflectometry to detect a soil interface

Abstract

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.

Active scour monitoring using ultrasonic time domain reflectometry of buried slender sensors

“…Previous Lamb wave UTDR work by Funderburk et al [27] validated that an applied metal interface produced a detectable reflected wave response that varied according to the location of the interface. This previous study, however, was performed using an aluminium strip, which is lower in stiffness and could be more sensitive to localized pressure from to a weaker pressure interface, such as soil.…”

“…UTDR for scour monitoring will be unique in that the propagating medium does not need to have any permanent damage, but rather impedance changes will stem from where the surrounding soil interface is in contact with the buried sensing strip. Funderburk et al [27] showed that Lamb waves are sensitive to external pressures that interact with the surface. To detect scour, Lamb waves are propagated down the length of a thin strip, which is essentially a plate that is much longer than its width.…”

“…In this study, the UTDR sensing mechanism based on propagating Lamb waves in a long, slender, sensing strip was used to determine the location of the soil interface for scour monitoring purposes. Previous work by Funderburk et al [27] utilized macro-fiber composites (MFCs) to generate first-order symmetric (S0) Lamb waves for detecting pressurized locations with various interfaces, including sand. However, this earlier test was short-range and did not demonstrate detection of a buried soil interface, as would be seen by a buried UTDR sensor undergoing in situ scour.…”