Determining the Presence of Scour around Bridge Foundations Using Vehicle-Induced Vibrations

Abstract

Bridge scour is the number one cause of failure in bridges located over waterways. Scour leads to rapid losses in foundation stiffness and can cause sudden collapse. Previous research on bridge health monitoring has used changes in natural frequency to identify damage in bridge beams. The possibility of using a similar approach to identify scour is investigated in this paper. To assess if this approach is feasible, it is necessary to establish how scour affects the natural frequency of a bridge and is it possible to measure changes in frequency using the bridge dynamic response to a passing vehicle. To address these questions, a novel Vehicle-Bridge-Soil Interaction (VBSI) model is developed. By carrying out a modal study in this model, it is shown that for a wide range of possible soil states, there is a clear reduction in the natural frequency of the first mode of the bridge with scour. Moreover, it is shown that the response signals on the bridge from vehicular loading are sufficient to allow these changes in frequency to be detected.

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

“…However, accounting for this effect is not trivial and can be neglected in bridge natural frequency analysis. Prendergast et al [16] describe a numerical study into monitoring scour around the central pier of a two-span integral bridge using vehicle-induced vibrations. The work focusses on detecting changes in the first natural frequency of the bridge using a single 'sensor' (virtual node point in numerical model) located on the pier and also investigates the influence of foundation soil stiffness on the method.…”

“…The work of authors [6,[14][15][16][17][18][19][20] shows that there is a general agreement that changes in natural frequency offer potential as a method to remotely detect and monitor the presence of scour around a foundation element. However, to date no research has been undertaken into using changes in natural frequency as a method to locate a scour hole (i.e.…”

“…An integral bridge is purposefully chosen as (1) the horizontal response may be more readily excited by a passing vehicle due to the moment connections between the substructure and the deck, (2) various modes of an integral bridge are affected independently by scour at different locations and utilising knowledge of these modal changes can isolate scour hole location and (3) integral bridges are becoming much more commonly constructed due to the ease of maintenance and lack of expansion joints. The analysis builds on work presented in Prendergast et al [16], which proposed a method to identify the occurrence of scour at some part of the substructure using a single sensor, and expands the approach to locating scour using distributed sensors. Challenges related to bridge spectra being contaminated by vehicle-induced frequencies, where to locate 'sensors' and required time-length of signals, are also investigated in this paper.…”

“…The soil is modelled using Winkler springs [24] with stiffness corresponding to a uniform medium dense sand (see [16] for a study on soil density effects). The method for deriving spring stiffness coefficients is from the American Petroleum Institute [30,31] design code, which specifies nonlinear relations for spring stiffness with strain.…”

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.…”