Bridge scour monitoring using accelerometers placed on bridge piers - a numerical investigation

Bridge scour has been identified as one of the leading causes of bridge failures in recent years, and caused the collapse of the Malahide Viaduct, a mainline railway bridge on the TEN-T network, in North Dublin in 2009. Scour is the term given to the removal of supporting soils from around foundations as a result of increased hydraulic stresses, a process exacerbated in times of heavy flooding. Scour is notoriously difficult to predict due to its dependence on many uncertain hydraulic parameters and difficulties in determining the exact condition of underwater foundations. This is a particular problem for rail bridges which may have been built more than 150 years ago. Protecting structures against scour generally involves relatively expensive installations that may not prove economically viable for widespread usage. Therefore, monitoring of scour-critical infrastructure is an attractive solution for infrastructure managers. The most common method for scour monitoring is to undertake visual inspections using divers to obtain the depth of scour around a foundation. This can prove time consuming and be particularly dangerous in times of heavy flooding, when scour is most critical. This paper charts the development of a new method of scour monitoring that uses the bridge structure itself to indicate the severity of the scour problem. Scour causes a reduction in foundation stiffness by removing soil from around foundation elements. This can manifest itself as a change in the natural frequency of bridge piers, due to the increase in effective length and reduction in soil stiffness. This paper presents a numerical examination of the effect of scour on the frequency response of a range of bridge pier geometries. The purpose is to show the effectiveness of the technique at remotely monitoring scour using accelerometers placed above the waterline.