Project Background:
The Capital Boulevard Bridge bears great historical significance in the development of Boise City, and ACHD wants to ensure that the structure of the bridge will remain a landmark for the future. To accomplish this, proactive and creative engineering was necessary. In 1992, ACHD faced the task of added additional lanes onto the bridge. In doing so, ACHD was very meticulous to ensure that original appearance of the bridge, an Art-Deco design, was not compromised. The casual observer of the bridge is hard pressed to determine if the original structure was a two-lane or a four-lane design.
Today ACHD is now with ensuring that the piers of the bridge are able to withstand a spring run-off event that could see the bridge crumble into the river. The possible culprit; Scouring of the riverbed that has exposed the original eight-foot concrete footings. With the footing exposed, the scouring has created holes under the peers.
Once the scouring was identified, ACHD immediately commissioned an engineering firm to investigate the severity of the damage. The investigation examined the nature and extent of the problem and determined maintenance solutions and monitoring plans that can be feasibly implemented.
ACHD determined that an emergency project was needed to protect the bridge from the spring run-off of 2001, then a long-term project would be necessary to protect the bridge in the future. The emergency project was completed in January of 2001.
The project that is underway now is the long-term solution to this scouring problem. To ensure the bridge remains intact, construction crews are placing pre-cast concrete blocks in a smooth, even layer around the base of the north pier to form a new concrete pad. This new pad will effectively tie into the existing pier and prevent any further scour damage to the piers.
So how was this scouring damage allowed to occur? Modern bridges are designed to withstand scouring by emplacing long lengths of structural steel know as piles. The Capitol Boulevard Bridge was built prior to the to the use of load-carrying structure such as piles, so its piers only have an eight-foot base that sit directly on the riverbed without any additional support.