An investigation of the changes in the natural frequency of a pile affected by scour

Abstract

Publication informationJournal of Sound and Vibration, 332 (25): 6685-6702Publisher Elsevier Item record/more information http://hdl.handle.net/10197/6581 Publisher's statementThis is the author's version of a work that was accepted for publication in Journal of Sound and Vibration. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Sound and Vibration (VOL 332, ISSUE 25, (2013) AbstractScour around bridge foundations is one of the leading causes of bridge failure. Up until recently, the monitoring of this phenomenon was primarily based around using underwater instrumentation to monitor the progression of scour holes as they develop around foundation systems. Vibration-based damage detection techniques have been used to detect damage in bridge beams. The application of these vibration based methods to the detection of scour has come to the fore in research in recent years. This paper examines the effect that scour has on the frequency response of a driven pile foundation system, similar to those used to support road and rail bridges. The effect of scour on the vibration characteristics of the pile is examined using laboratory and field testing. It is clear that there is a very clear reduction in the natural frequency of the pile as the severity of scour increases. It is shown that by combining state-of-the-art geotechnical techniques with relatively simple finite element modelling approaches, it is possible to accurately predict the natural frequency of the pile for a given scour depth. Therefore, the paper proposes a method that would allow the estimation of scour depth for a given observed pile frequency. Scour Monitoring: The accurate assessment and monitoring of existing structures to progressive scour has come to the fore of research in recent years [7,8]. A myriad of current technology aims to detect the existence and severity of scour around the foundation systems of existing bridge structures. Most of these technologies comprise underwater instrumentation that aim to monitor the evolution of scour holes over time. A brief summary of available instrumentation is given herein. The use of float-out devices [9] and Tethered Buried Switches [10] positioned in the soil near a bridge pier can provide a simple method of scour monitoring. These devices float out when the scour depth reaches their position and are programmed to send a remote signal to provide a warning. However, they require reinstallation upon floating out, which is a distinct disadvantage. Time Domain Reflectometry (TDR) aims to detect changes in the dielectric permittivity constants between materials that would occur at a water-sediment interface and thus has been used to detect scour [10 -12]. Devices using this method work very well at detecting the depth of scour i...

A comparison of initial stiffness formulations for small-strain soil–pile dynamic Winkler modelling

“…Similar examples of this phenomenon can be found in [5,12]. Filtering the signal has the effect of removing the high frequency oscillations at the beginning of the unfiltered signal and exposing the Published in Soil Dynamics and Earthquake Engineering, 2016 (81C) pp.…”

“…In the case of bridges, most of the work to date has focused on monitoring the superstructure. More recent research has begun to focus on using these damage SoftSoft detection methodologies on sub-structural elements (see [5][6][7]). In these cases, the analyses can be quite sensitive to the soil stiffness assumed in the design where the dynamic oscillations typically remain in the small-strain region.…”

“…This test site has been developed over the past eleven years and has been used for a number of model, prototype and full scale foundation experiments [32][33][34][35]. A full description of the geotechnical properties of the site can be obtained in [5,32,36]. The site is comprised of very dense, fine sand with a relative density between 90% and 100%.…”

Isolating the location of scour-induced stiffness loss in bridges using local modal behaviour

“…The study concluded that accelerometers located on the model bridge pier performed very well at measuring the change in natural frequency as scour severity increased. Using full-scale field testing Prendergast et al [18] validated a method to detect the change in the natural frequency of a single pile affected by scour. Moreover, they suggest a method for creating soil-springs using correlations to geotechnical site data such as shearwave velocities and cone penetration tests.…”

Structural Health Monitoring and Design Code compliance for performance assessment of bridges under scour and seismic hazards

“…The compromised stability of a bridge foundation due to adverse scour hole formation gives rise to a change in the response features of a bridge such as increased pier tilting, pier settlement, changes to dynamic characteristics, increased strain at deck level, crack formation due to differential settlement among other indicators. Primarily, the change in support conditions gives rise to a change in the dynamic characteristics (Doebling et al 1996;Prendergast et al 2016b;Prendergast et al 2016a;Foti & Sabia 2011;Elsaid & Seracino 2014;Briaud et al 2011;Prendergast et al 2013), and this may become a primary indicator of performance. Vibration-based scour detection is a significant advance over the instruments discussed in Table 1, in that it focuses on measuring the response of the structure itself to the formation of scour in lieu of inferring the bridge stability from a measured scour depth.…”