US 90 Interchange at LA 85 Highway

A design-build project model and creative geotechnical solutions reduced project costs.


Iberia Parish, Louisiana

GeoEngineers was part of a design-build team led by Gilchrist Construction Company that was awarded the State of Louisiana’s first completed design-build highway project. GeoEngineers provided geotechnical engineering design services for the technical design proposal to the Louisiana Department of Transportation and Development (LA DOTD).

The 1,900-foot overpass of two, two-lane bridges cost the state approximately $24.5 million. According to LA DOTD, the project, which opened in June of 2011, was entirely funded by federal stimulus funds received from the 2009 Recovery and Reinvestment Act (ARRA).

GeoEngineers completed engineering analyses and provided recommendations for design and construction of driven-pile foundations for the two bridge structures. The foundations were designed to meet AASHTO LRFD and interstate highway system standards so that the interchange can be upgraded to an interstate, which will be named Interstate 49 at LA85.

Geotechnical services for the project included review of project geology, explorations previously completed and explorations and laboratory testing for foundation, embankment, pavement and communication tower design.

After the team won the contract, GeoEngineers’ Baton Rouge office oversaw the geotechnical services contract, providing geotechnical design to the bridge and road contractor team as needed throughout the 17-month-long construction project.


  • Soft soil conditions at the Louisiana site presented challenges. GeoEngineers evaluated the soil to resolve what type of foundations it could bear and to reduce post-construction settling. Our engineering analysis led us to recommend driven-pile foundations for the highway overpass bridges, which provided the necessary stability in the soft soil.
  • Using sophisticated Pile Driving Analyzer (PDA) equipment, the project team monitored the pile driving in order to gauge stresses in the piles, check for damage and estimate load capacity.
  • The team provided engineering analysis and recommended wick drains, which sped up the exit of water from the consolidating soil in the approach embankments; and surcharge, which added weight to speed settlement of the embankments. Both were designed to reduce post-construction settlement and prevent down-drag loads on the proposed adjacent bridge foundations.


  • The unique design-build method made the project more quickly “shovel-ready,” enabling the LA DOTD to secure the ARRA funding necessary to construct the project.

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