Illabot Creek Alluvial Fan Restoration

Illabot Creek is a tributary of the Skagit River system in Skagit County, Washington. Like many rivers, the creek’s historic alignment was heavily modified from its natural state. What was once a meandering natural stream and alluvial fan had been straightened into an artificial channel to flow beneath a single bridge on Rockport-Cascade Road during the 1970s.

This half-mile alteration damaged the creek, which had historically supported large populations of salmon and trout species as one of the most productive Skagit River tributaries. To try to restore fish habitat, Skagit County officials joined the Sauk-Suiattle and Swinomish Indian Tribes and Skagit River System Cooperative (SRSC) in an effort to reconnect the creek’s historic channels and build two new 100-foot span bridges to improve habitat conditions and fish passage.

GeoEngineers worked as a geotechnical subcontractor for KPFF on this important project. The Illabot Creek project included both environmental restoration and infrastructure components and required a diverse project team. R2 Resources designed the new stream alignment and environmental improvements, KPFF handled structural design and Interwest Construction and Tiger Construction were the contractors.

Geotechnical engineering services focused on the two new bridges over Illabot Creek. Work included borings, test pits, lab testing and recommending design and foundation parameters for the structures and temporary by-pass road. Final design and construction of the upstream channel and two bridges was completed within a 12-month window.

Approach

• GeoEngineers team reviewed existing geologic maps to get an understanding of the overall subsurface conditions in the region, which they determined was primarily layers of fill overlying alluvial fan deposits.
• During the preliminary geotechnical investigation, GeoEngineers drilled three boreholes within the roadway to a depth of about 30 feet. Subsurface soil and groundwater conditions were also evaluated by excavating six test pits. The soil conditions encountered during site explorations were generally consistent with the mapped geology.
• Based on the geotechnical investigation, GeoEngineers recommended using conventional shallow footings to support the bridge structures—saving the client money compared to deep foundations. Bridge design was completed in accordance with AASHTO LRFD Bridge Design Specifications.
• The team also evaluated seismic risks to the site and concluded that the soil was not susceptible to liquefaction and provide recommendations for stormwater infiltration.
• GeoEngineers collaborated with the design team to evaluate options for a temporary detour road to facilitate efficient construction. A WSDOT wrapped face geotextile wall constructed with onsite backfill material was utilized for rapid construction and to minimize cost.
• The site contained a large amount of oversize cobble and boulder material. GeoEngineers provided earthwork recommendation to help the contractor prepare for excavation and reuse of onsite material as backfill to achieve suitable bridge bearing and settlement performance.

Results

Based on the geotechnical investigation, GeoEngineers recommended using conventional shallow footings to support the bridge structures—saving the client money compared to deep foundations. Thanks to this award-winning project, Illabot Creek is now reconnected to its historic floodplain. Logjams, riparian planting and engineered side channels provide the habitat needed for fish spawning and will support this important state industry sustainably into the future.