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Development of Passive and Active Piezoelectric Scour Depth Monitoring Sensors

Abstract

Local scour is arguably the most pressing issue regarding the safety and longevity of overwater civil infrastructure. Scour is the leading cause of bridge collapse in the United States and therefore poses a great threat to public safety, disrupts local commerce, and costs millions of dollars in repairs. In the next century, the effects of climate changes will make more bridges susceptible to scour failure more than ever before. 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. This research aims to address the significant drawbacks of existing scour monitoring techniques by validating three piezoelectric driven-rod scour monitoring sensors developed using: hydrodynamic scour testing, localized velocity modeling, mathematical relationships and structural adaptations to mitigate environmental influences, as well as a novel, active soil interface sensing mechanism.

The first proposed scour sensor, or piezo-rods, feature continuous piezoelectric polymer strips embedded within and along the length of slender cylindrical rods, which could then be driven into soil where scour is expected. When scour erodes away foundation material to reveal a portion of the piezo-rod, ambient fluid flow excitations would cause the piezoelectric element to output a voltage response corresponding to the dynamic bending strains of the sensor. The voltage response is dependent on both the structural dynamic properties of the sensor and the excitation fluid’s velocity. By monitoring both shedding frequency and flow velocity, the exposed length of the piezo-rod (or scour depth) can be calculated. Hydrodynamic testing of the sensor system in a flume is discussed. Each rod was installed using a 3D-printed footing designed for ease of installation and stabilization during testing. Two series of experimental flume tests were conducted: (1) the piezo-rod was driven into sediment around a mock pier to collect scour data, and (2) the piezo-rod was used to monitor its own structural response by collecting vortex-shedding frequency data in response to varied flow velocities to establish a velocity-frequency (V-F) relationship.

The sensors were installed in a layout designed to capture symmetric scour conditions around a scaled pier. In order to analyze the system out of steady-state conditions, water velocity was increased in stages during testing to induce different degrees of scour. As ambient water flow excited the portion of the exposed rods, the embedded piezoelectric element outputted a time-varying voltage signal. Different methods were then employed to extract the fundamental frequency of each rod, and the results were compared. Further testing was also performed to characterize the relationship between frequency outputs and flow velocity, which were previously thought to be independent.

The results from soil-free velocity testing showed that the piezo-rod successfully captured structural vortex-shedding frequency comparable to state-of-the-art testing. A one-dimensional numerical model was developed using the V-F relationship to increase the accuracy of voltage-based length predictions of the piezo-rod. Two-dimensional flow modeling was also performed for predicting localized velocities within a complex flow-field. These velocities, in conjunction with the V-F relationship, were used to greatly improve length-predictive capabilities of the piezo rod. Higher mass ratio rods are known to be less susceptible to the influences of flowing-water excitations. Therefore, the second generation of the piezo-rod design aimed to increase the mass ratio of the scour sensor while also introducing more frequency features into the signal signature that can be used to determine length. Two variations of large-scale piezo-rods were developed and then optimized to have greater participation of high-order frequencies under free-vibration conditions using weights as localized masses. The results showed that a vibrational sensor could be made to output up to five modal frequencies, which are related to the number of additional participating weights.

As a result of the velocity influence on the piezo-rod, and other passive sensors exited by flowing water, an active scour depth monitoring sensor using ultrasonic time domain reflectometry (UTDR) was developed. To make the third generation UTDR sensor, a long, slender plate is coupled with two flexible piezoelectric devices – macro-fiber composites – that propagate Lamb waves along the length of the plate to form the scour sensor. 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. The sensor was tested for sensitivity to external pressure using metal weights and was able to detect the position of the pressure up at a length of up to ~ 20 ft. The sensor was tested under simulated scour conditions, being buried in sand at various depths. The results showed that the Lamb wave scour sensor was capable of reliably detecting a soil interface at 1 ft intervals. The scour sensor was also able to detect uncompacted soil interfaces, which is important considering the issue of scour hole refill following an extreme event. Overall, the Lamb wave UTDR sensor was demonstrated to be a feasible sensing mechanism for scour depth monitoring.

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