Scouring evaluation of river-crossing bridges using resistivity image profiling techniques

Bridge scour is one of the leading causes of bridge failure. As climate change brings increasing rainfall intensity that leads to higher water levels and accelerated rates of water flow in rivers, the danger that scouring poses to bridges also increases. As the piles and foundations of a bridge are exposed through scouring, the integrity of the bridge is at risk and the end result may be a partial or total bridge collapse. Conventional underwater inspection methods used around the world often fail to accurately assess the conditions caused by bridge scour. To obtain a more accurate riverbed profile and underwater imagery for scour detection and monitoring, engineers have employed sonar, laser and radar technologies. This paper proposes using resistivity image profiling (RIP), an advanced geophysical surveying method, to evaluate bridge scour conditions. RIP measures the electrical resistivity field of a target area to detect bodies of anomalous materials and helps in estimating the depths of layers composed of various materials. This paper presents a case study that demonstrates excellent results from an RIP survey using Schlumberger electrode arrays performed on a 37-year-old bridge, which was severely damaged by scour. By placing as-built drawings over RIP mappings, the existing riverbed with regard to the bridge piles shows a 2.3 m scour during an 18-year period (from 1997 to 2015). A regular 2D RIP survey is recommended for individual bridges to examine current overlaying conditions and to predict future scouring. Also, a 3D RIP survey is relatively cost effective in indicating general and local scouring.