Slurry Walls – 11th Avenue Railroad Underpass

The 11^th Avenue Railroad Underpass is located in Nampa, Idaho. The project involved replacing an old railroad bridge and narrow highway underpass beneath the main lines and switchyard of the Union Pacific Railroad with a wider, deeper structure. The new underpass extends to depths greater than 9.1 meters (30 feet) below existing grade, and about 7 meters (24 feet) below the estimated high groundwater level. A major constraint was that railroad traffic could not be interrupted through the site during construction. This and other construction considerations largely drove the design and led to the selection of structural slurry walls to provide both temporary and permanent excavation support and abutment walls. The new railroad bridge was preassembled in longitudinal halves on either side of the old bridge, and supported temporarily on structural slurry approach walls. Train traffic was shifted to one half of the old bridge. Construction of structural slurry wall abutments for the opposite half of the new bridge and demolition of the corresponding vacant half of the old bridge were then undertaken simultaneously. When the abutment walls were ready, the adjacent half of the new bridge was jacked laterally into position and placed on the new abutment walls. Train traffic was moved to the completed half of the new bridge, and the process was then repeated to complete the new bridge. After the railroad bridge was completed, the new underpass was constructed, in effect, by tunneling under the new bridge between the abutment slurry walls. This included demolition and removal of the existing underpass.