The Idaho Transportation Department’s program to identify bridges that are threatened by subsurface erosion and its plan to remediate action called “scouring” received national prominence last month when it was included on the Federal Highway Administration’s Web site.
The most common cause of bridge failure is from floods
that “scour” bed material from around the piers and columns.
“Scour is the engineering term for the water-induced erosion of
the soil surrounding bridge foundations (piers and abutments),”
according to ITD’s action plan published on the FHWA site:
ITD’s Office Manual Plans of Action for Scour Critical Bridges was posted March 15 on the agency’s engineering Internet pages.
“This manual is intended for use by those responsible for the safety of Idaho bridges that may be threatened by scour,” according to an FHWA announcement. “…The added cost of making a bridge less vulnerable to scour is small when compared to the total cost of a failure, which can easily be two to 10 times the cost of the bridge itself.”
The manual divides the ITD scour critical bridges into four categories based on lifetime risk of failure and annual probability of failure. It describes the minimum monitoring and/or countermeasure response required for each category.
The required responses vary from simple high-flow monitoring plans and bridge closure protocols to a full action plan that covers both monitoring and countermeasure development.
In 2002, ITD used an innovative approach to remediate scour damage by placing “A-Jacks” at the base of piers and abutments. The concrete blocks are shaped like the children’s toy jacks. The blocks trap sediment suspended in the water and deposit it at the base of the columns to replace material that had been washed downriver.
The A-Jacks were installed as part of bridge rehabilitation projects on the Tilden Bridge where Ferry Butte Road crosses the Snake River south of Blackfoot, and the Snake River Bridge west of Shelley. The bridges were severely scoured in the 1997 Snake River Flood, leaving their columns vulnerable to further damage.