Salix Interchange in Woodbury County, IA

This project reconstructed the interchange at I-29 and County Road K-25, along with its entrance and exit ramps. It also included a new bridge near the City of Salix.

The new interchange design:

  • Utilizes over 52,000 cubic yards of Expanded Polystyrene (EPS) fill
  • Meets current requirements for vertical and horizontal clearance over I-29
  • Conforms to current interchange ramp geometry design standards
  • Fits within the existing right-of-way (ROW)

Evaluating Bridge Replacement Possibilities

The original I-29/K-25 interchange was built between 1958 and 1960. Its design included a four-span, 215-feet long bridge to carry K-25 over I-29. A 15-foot deep over-excavation of the existing soils and replacement with sand was planned at each abutment. Problems during construction due to very soft soils resulted in a change to the bridge design. The 4-span bridge became an 18-span, 734-foot long bridge.

In 2012, a bridge replacement concept study was performed. The original bridge had surpassed its design life with low vertical clearance and outdated interchange geometry and ramp terminal sections. Two, four, and eight-span bridges were evaluated. Goals of the study were to:

  • Assess soil impacts on the bridge and embankment design
  • Review possible remedial alternatives
  • Provide preliminary cost information

Preliminary cost estimates indicated an 8-span bridge with an increased interchange footprint would have the lowest cost. However, a 4-span bridge, utilizing EPS fill with an interchange footprint within the existing ROW was selected; the cost of expanded polystyrene (EPS) fill was balanced with the bridge length to maximize economy and maintain required interchange sight distances.

Project design and construction considerations included:

  • Minimizing the superstructure weight
  • Updating the interchange geometry and clearances to meet current standards
  • Constructing a new interchange within the existing ROW
  • Completing construction in one year
  • Maintaining traffic on interchange ramps as much as possible during construction
  • Limiting post-construction settlement at bridge abutments from ¼ to ½ inch

I-29/K-25 Uses A1010 Structural Steel

The new continuous welded plate girder bridge is 403-feet long by 40-feet wide.

It’s the first in the state of Iowa to utilize A1010 stainless structural steel girders and the first in the country to utilize A1010 structural steel in continuous girders.

The two southernmost girders are fabricated from A1010 structural steel while the remaining four girders are fabricated from A709 grade 50 weathering steel.

Shuck-Britson developed the design, material, and fabrication specifications for the A1010 steel, coordinating with the Iowa DOT and leading steel producers and fabricators nationwide.