  One of the most prevalent causes of bridge failure around the world is scour,
the gradual erosion of soil around a bridge foundation due to fast-flowing
water. A reliable technique for monitoring scour would help bridge engineers
take timely countermeasures to safeguard against failure. Although
vibration-based techniques for monitoring structural damage have had limited
success, primarily due to insufficient sensitivity, these have tended to focus
on the detection of local damage. High natural frequency sensitivity has
recently been reported for scour damage. Previous experiments to investigate
this have been limited as a result of the cost of full-scale testing and the
fact that scaled-down soil structure models tested outside a centrifuge do not
adequately simulate full-scale behaviour. This paper describes the development
of what is believed to be the first-ever centrifuge-testing programme to
establish the sensitivity of bridge natural frequency to scour. For the
fundamental mode of vibration, these tests found up to a 40% variation in
natural frequency for 30% loss of embedment. Models of three other types of
foundation, which represent a shallow pad foundation, a deep pile bent and a
deep monopile, were also tested in the centrifuge at different scour levels.
The shallow foundation model showed lower frequency sensitivity to scour than
the deep foundation models. The level of frequency sensitivity (3.1 to 44% per
scour depth equivalent to 30% of embedment of scour) detected in this
experiment demonstrates the potential for using natural frequency as an
indicator of both local and global scour of bridges, particularly those with
deep foundations.
