Robust capacitance sensors for wireless bridge scour monitoring

The risk of bridge scour, or the erosion of bed material around bridge foundations due to flowing water is exacerbated by climatic effects such as flooding and hurricanes. The difficulty in visual inspections of buried and underwater infrastructure elements such as bridge foundations has prompted interest in their real-time monitoring using novel sensory networks. To monitor bridge scour, this study detects the change in dielectric properties of the media near the soil-water interface using an array of discrete capacitance sensors embedded at different locations. The concept was proven using three commercially available capacitance sensors. Two types: stainless steel-type (SS-type) and printed circuit board-type (PCB-type) capacitance sensors were interfaced with an open-source Arduino-based data acquisition unit for real-time monitoring of sediment soil level, sediment scouring, and scour-hole refill in hydraulic flume experiments. Their performance in three different sediment types: uniform sand, silica fume, and gravel was investigated in the flume tests. The results indicate that low-cost low-power capacitive sensors can be used for monitoring scour and scour-hole refill. The sensors were largely immune to changes in sediment type and demonstrated a reliable and repeatable response. Small-scale static tests were also conducted to understand the influence of salinity on sensor response. The PCB-type sensors were susceptible to salinity, whereas the SS-type sensors demonstrated adequate signal differentiation between the salt solution and solution-soil mixtures up to a salinity of 10 ppt. Durability tests were carried out on SS-type sensors, which demonstrated good signal stability for 9 months, even after biofilm development. The SS-type sensors also performed well without any change in signal output after damage/deformation due to simulated debris impact.