Tonquin Avenue Bridge Replacement

GeoEngineers’ shallow-foundation solutions lessened the cost and environmental impact of replacing the bridge.

The Tonquin Avenue Bridge is located in a coastal town in southwestern Washington State. In 2006, the City of Ocean Shores closed the 40-year-old timber bridge over man-made Lake Minard Canal because its pilings were failing and its timbers were rotting. In 2008, the City decided to replace the old bridge, which had been a part of the coastal community’s tsunami escape route. The City’s bridge replacement plans called for removing the existing timber structure, installing a new 148-foot span and adding new pedestrian and bicycle access.

GeoEngineers was subcontracted by AECOM to provide geotechnical engineering and environmental permitting services for the project. In the process, GeoEngineers furthered AECOM’s objectives of minimizing project costs and lessening impacts to the environment and public during bridge construction.

The new bridge was opened with a ribbon-cutting ceremony on December 8, 2011.


Geotechnical Services

Our geotechnical team assessed the feasibility of using shallow and deep bridge foundations and developed preliminary recommendations to support AECOM’s type, size and location study for the new structure. GeoEngineers also developed final design recommendations for the selected design.

A deep foundation system supporting a multiple span bridge typically would have been used for a bridge of this length in order to meet the required navigational clearances and bridge settlement tolerances. We recognized that the piles would need to go deep because of downdrag loads associated with liquefiable soils at the bridge site. Our team developed a ground-improvement system that controlled the liquefaction at each abutment, making a shallow-spread foundation system feasible for the abutments. In order to use a simple span bridge and achieve the required navigational clearance, the structural engineer specified pre-cambered girders, which also provided a more aesthetically pleasing bridge profile.

Environmental Permitting Services

GeoEngineers provided comprehensive environmental permitting compliance with National Environmental Protection Act (NEPA) by coordinating with WSDOT Highways and Local Programs and the Federal Highways Administration. In addition, we provided State of Washington environmental compliance that included shoreline and fish and wildlife hydraulic permit approval.

The bridge replacement plan called for impact-pile-driving, which caused some concern, given the potential presence in the area of coastal shorebirds listed under the Endangered Species Act (ESA), as well as the sport fish stocked in Lake Minard Canal.

GeoEngineers conducted a detailed construction-noise evaluation to address increased noise levels as a result of pile driving in the area. We also assessed the distribution and possible occurrence of ESA-listed species. In our evaluation, the team used current WSDOT methodology to analyze the effects of noise on species. In addition, the team collected extensive first-hand accounts regarding the historical and recent distribution and habitat use of potential species in the area. The team determined that temporary construction-related in-air noise would not have any impact on regulated habitats.


  • Bridge was completed and re-opened in late 2011, re-linking neighborhoods and reconnecting coastal residents with an important safety route
  • GeoEngineers’ ground improvement system design allowed for a shallow foundation solution, avoiding the cost and environmental impact of using in-water foundations and a deep foundation system
  • Permitting was completed on a streamlined timetable as a result of GeoEngineers’ adept environmental work with state and federal agencies
  • The bridge-replacement project was one of the first pilot projects of the Greenroads Foundation, a non-profit organization that manages certification reviews for roadway and bridge projects, using its Greenroads™ Rating System, a sustainability-rating system for roadway design and construction.
Photo 5 courtesy of Greenroads.

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