Infrastructure Technology Institute
Scour, the process by which moving water removes soil and rock from the bed of a waterway around the foundations for bridge piers and abutments, is by far the leading cause of bridge failures in the United States. Because it is a serious and widespread problem, much research has been devoted to methodologies and devices for monitoring scour along bridge foundations and abutments, with varied degrees of success.
In the late 1990s and early 2000s, the Northwestern University Infrastructure Technology Institute (NU-ITI) developed a method for measuring structural response to scour based upon tilt measurements. Rather than attempt to measure the scour holes themselves, the NU-ITI system uses a network of electronic tilt meters to indirectly measure scour by measuring the long-term tilt of the bridge piers. During high water, the tilt sensors are read at least once an hour --- typically more often --- and the data are examined by state highway engineers at regular intervals. Before electronic monitoring, an engineer was typically stationed at the bridge 24 hours a day to watch for signs of distress. Should distress occur – for example, a noticeable bend in the bridge railing – the brbridge would be closed immediately. With electronic monitoring, the same level of diligence in observation can be attained without personnel on site. The tilt-based scour measurement technique was subsequently adopted by the California Department of Transportation (Caltrans) for scour-critical bridges statewide. Currently, at least six bridges on the California highway system are monitored in this way; several others were monitored for years before the bridges were replaced as programmed.
One major barrier to wider application of tilt-based scour monitoring is the cost of wiring power and communication links to sensor nodes on the bridge. Wireless sensor networks provide an attractive alternative since they eliminate the need to run power and communication cables; this is especially important on long spans that may exceed one-quarter mile in length or on spans where access is difficult. In response, NU-ITI is developing and evaluating two wireless sensor network systems for tilt-based scour monitoring of highway bridges. Following laboratory evaluation, both systems were deployed for field trials on a bridge in California in September 2011.
The first system is a commercial off-the-shelf wireless sensor network paired with the same tilt sensors used in previous bridge monitoring projects. The second system was developed in-house from wireless components based on the ZigBee standard. The wireless data acquisition devices are paired with a small, low-power electrolytic tilt sensor. Both systems are scalable to many wireless nodes. Repeater nodes may be deployed as necessary to extend the range of the system.
Both systems send data to a base station located on a nearby utility pole. The base station aggregates the data from the two wireless sensor systems and transmits it to NU-ITI over a cellular modem. From there, the data are presented on a password-protected Web site for interpretation.
ITI engineers will return to the field site in early summer 2012 to make adjustments to the systems according to field performance lessons learned thus far. It is expected that these field trials will lead to development of a practical low-cost wireless sensor network for scour that could be deployed on bridges nationwide.