Big Wood River Bridge

Geotechnical services for an award-winning bridge redesign.

The Idaho Transportation Department (ITD) needed to upgrade the SH-75 bridge over the Big Wood River. The aging two-lane bridge located just outside Sun Valley, Idaho, was too small to accommodate ITD’s road-widening plans. ITD hired GeoEngineers to complete subsurface investigations and make geotechnical design recommendations for retaining walls adjacent to the new bridge and provided designs for a pavement section.

Designers planned for a four-span concrete girder bridge with two 90-foot interior spans, plus two 30-foot “jump” spans to allow wildlife to pass under the bridge on both sides of the river. The new bridge also required new approaches on either side, amounting to more than 1,250 feet of pavement reconstruction in total.


GeoEngineers’ geotechnical engineers provided the following services:

  • A Phase I Materials Report, geological reconnaissance and life-cycle analysis for the proposed pavement sections. Engineers walked the site and reviewed available data on the site to estimate subsurface soil conditions. The preliminary assessment supported early design efforts of the project.
  • Further subsurface investigation consisting of borings and a Phase II Soils report. Representative soil samples were collected from borings and tested in GeoEngineers’ AASHTO-certified laboratory to evaluate their pertinent engineering properties. Five borings were completed along the proposed SH-75 mainline as part of the Phase II subsurface exploration.
  • Prepared a Phase III Pavement Estimating Report. This report includes recommendations for pavement type and surface smoothness for the new sections of approach roadway. GeoEngineers recommended Superpave Hot-Mix Asphalt (HMA) concrete pavement.
  • Foundation investigation and recommendations for mechanically stabilized earth (MSE) retaining walls. This included additional subsurface exploration (9 borings), soil sampling, field and laboratory testing, engineering analyses and preparation of a Phase IV Foundation Investigation Report.
  • Analysis of surface runoff and design recommendations for water management and discharge that protects the site from erosion.


After 18 months of construction the new Big Wood River Bridge opened in September 2017. GeoEngineers’ geotechnical investigations and recommendations for the retaining walls and pavement design were critical to completing ITD’s wider and more environmentally friendly bridge, capable of carrying a much higher volume of traffic.

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