White Paper: Tiltmeter-Based Bridge Scour Monitoring

Scour is by far the primary cause of bridge failures in the United States. Scour and other hydraulic effects are particularly threatening because the deterioration is often invisible , hidden beneath turbid water. Many scour monitoring methods attempt to measure the development of scour pockets themselves. However, this is difficult due to debris — both floating and submerged — and shifting streambeds. Scour holes develop and backfill with loose sediment as flood discharges peak and recede, misleading the engineer to assess that conditions are good. The newly deposited sediments fail to indicate the true loss of ground stability. To address this issue, the Infrastructure Technology Institute at Northwestern University, in cooperation with the California Department of Transportation, developed a scour monitoring system based on direct measurement of the response of the bridge structure to scour. Using sensitive electronic tilt sensors mounted at the top of each pier, the system measures the movement of each pier cap. Observation of the bridge's response to daily and seasonal temperature cycles reveal the normal structural movement patterns, which are used to construct an envelope of typical safe operation. That is, if pier movement during high water periods differs from the typical pattern, further action — up to and including bridge closure — may be required. However, if pier movement during high water is similar to the historic record, the engineer gains confidence that the bridge substructure is not affected by scour. The tilt sensor-based system is composed of commercial off-the-shelf equipment and is fully scalable; it has been deployed successfully on both short and long bridges — from structures a few hundred feet long with only two or three piers to a quarter-mile-long bridge with eighteen instrumented piers. Remote communication and robust Internet-enabled display technology provide convenient access to both real-time and historical data, enabling quick comparison for decision-making. INTRODUCTION Hydraulics, including scour, is by far the primary cause of bridge failures in the United States 1. Scour and other hydraulic effects are particularly threatening because the deterioration is often invisible, hidden beneath turbid water or obscured by loosely re-deposited material. Evaluation of the stream channel, including probing for scour pockets, has been part of the National Bridge Inspection Standards since 1988 2. For many bridges, probing about substructure elements with a soil tile probe during biennial inspections may be sufficient. However, some bridges present particular challenges to scour evaluation due to natural or …