•  2003 Project Highlights
•  Evaluation Criteria

I-90, Sunset Interchange

King County

** Here's what you had to say

Design: Construction Management: Schedule: Cost: Engineer’s estimate: $42,245,682 Contractor’s bid amount:  $39,100,844 Estimated cost for completion:  $71,347,590 Site exploration conducted during design did not reveal large underground boulders and poor soils.  Later discovery of these conditions forced numerous change orders, construction delays, and cost overruns. The contract included nearly 500 tracked cost issues and over 150 change orders. The amount paid to the contractor doesn't include an estimated $4.5 million to add a retaining wall.  This wall is required to mitigate for a slide area that wasn't identified during design and  to complete a portion of the trail impacted by the slide area.  A proactive approach to environmental problems minimized, but didn't prevent, permit violations. Good communication between the contractor and WSDOT lessened impacts on the traveling public and surrounding community.

The new Interstate 90 Sunset Interchange is located on the east edge of Issaquah. It provides additional access from I-90 to Issaquah, the Sammamish Plateau and Issaquah Highlands. Crews expanded an existing half-interchange into a full interchange and built roads to connect the new interchange to planned commercial and residential developments. Crews also built water treatment ponds, bicycle trails, creek enhancements, wetland mitigation, and landscaping and installed traffic loops and freeway cameras.

This project reduces congestion and enhances safety by reducing traffic on nearby Front Street and SR 900 interchanges and connecting city and county streets. The interchange helps commuters today and supports jobs and housing for the future.

The project opened on-schedule.  Cost overruns were due largely to unexpected boulders the size of Volkswagens and poor soils. WSDOT took innovative steps to minimize the cost overruns, including using a new method to drill shafts and using nearby WSDOT-owned sites to obtain gravel and dispose of poor soil.

Bridge foundations were blocked by hundreds of large boulders that stymied all conventional attempts at removal. WSDOT directed the use of a new, expensive drilling technique using specialty equipment. At the same time, earthwork crews experienced wet soils filled with large boulders and numerous landslides. The plan to use on-site soil to build the project was scuttled. WSDOT authorized mining of gravel at an adjacent abandoned pit site and hauled poor soils to waste berms in the median of I-90 near the junction of State Route 18. Once the adjacent pit was depleted, remaining gravel and soil was imported from a commercial pit at great expense.

The year-round work schedule needed to meet tight deadlines meant that earthwork progressed into winter months. This caused extensive environmental risk to the East Fork of Issaquah Creek, a habitat for endangered Chinook salmon. Over $1.5 million was spent to protect the creek and salmon habitat, including use of a first-of-its-kind portable "chitosan" sand filter system to treat and clean dirty construction runoff water.

Site exploration is standard practice for WSDOT construction projects.  However, extensive site exploration that would have been required to discover the underground boulders is not feasible because it is very  expensive.  Spending money to conduct extensive site exploration on every project to find rare problems would be wasteful.  

Similarly, WSDOT accepts risks resulting from limited site exploration.  If we required contractors to cover these project risks they would increase their bids on every project dramatically, even though most projects don't encounter these problems. 

After the soil and boulder problems were revealed, there was serious discussion of stopping and redesigning this project. The project was completed largely due to extraordinary WSDOT and Kiewit Construction Company teamwork.

WSDOT Contact:
Matt Preedy, WSDOT Project Engineer
Phone: 425-649-4436
Email: preedym@wsdot.wa.gov

Contractor:
Kiewit Construction, Renton, WA

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