Two aging structures will be replaced with new bridges spanning their respective waterways.
The Idaho Transportation Department is planning construction of two crossings on US-20. The crossings are located over Willow Creek in Camas County at milepost 164 and Rock Creek in Blaine County at milepost 173.
In the 1950s culverts were placed at each location and the area was built up from the pipeline to the existing highway. At Willow Creek, a 14’ diameter metal corrugated pipe with approximately 41’ of earth fill was installed. At Rock Creek, an 11’ diameter metal corrugated pipe with approximately 20’ of earth fill was installed.
Due to age and corrosion of the pipelines, both culverts are in need of replacement. ITD analyzed multiple options regarding these crossings. A bridge design was chosen for both Willow Creek and Rock Creek as they were determined to have the best long term value. As a result of choosing a bridge structure, ITD will also be able to restore the stream to a more natural condition, which in turn should have a positive effect on the surrounding ecosystem.
The projects involve construction of two bridge crossings at both Willow Creek and Rock Creek. The respective culverts at each location will be removed along with the existing earthwork.
Due to the natural surrounding rock, the structure spanning Willow Creek will be a 170’ steel girder bridge. The structure spanning Rock Creek will be a 159’ concrete girder bridge. Removal of the pipes will also aid in the migration efforts of fish and wildlife in the region as well.
The new structures will tie into the current highway with a width of approximately 41’ shoulder to shoulder. Both the Willow Creek Bridge and the Rock Creek Bridge will support one lane in each direction.
Bridge construction will take place in three stages. During the first phase, a temporary roadway will be created that will allow for traffic to travel around the work zone. During the second phase, traffic will be routed on the partially constructed bridge. Removal of the earth fill and restoration of the channels will occur during the final phase.
Traffic will be reduced to one lane in the construction zone during the first two phases with width restrictions anticipated. Temporary traffic lights will be in place throughout the duration of the project. Reduced speed limits will also be in effect.
Construction on each crossing will occur simultaneously. Each work zone is anticipated to encompass one mile with approximately seven miles between each zone.