Precast concrete ramp panels feature distinctive leaf patterns that complement the adjacent wooded foothills. Photo courtesy of Kiewit.

Ask any Eastside commuter about the morning backups from downtown Issaquah onto Interstate 90 and you're bound to hear moans about sitting in traffic for 40 minutes to drive no more than 2 miles. Eastside communities groaned even more when Microsoft announced plans to build a new campus on the Sammamish Plateau north of the Front Street exit.

Luckily, the Washington State Department of Transportation followed Microsoft with an announcement of its own. It planned to build a $176 million interchange to ease area congestion overall. The new Sunset Interchange was designed to incorporate bridges and flyovers to shuttle traffic on and off the freeway more smoothly.

After two years and tens of millions of dollars in change orders, Sunset is set to open on schedule in August.

"The project didn't come in on budget, but it's a real testament to the people working on it that we were able to get the job done on time," said Ann Hegstrom, project manager for the Pacific Building Division of Omaha- based Kiewit Construction Co., which serves as general contractor on the job.

The interchange will span nearly 25 acres, including more than 2 miles of new exit ramps, 104,000 cubic meters of earthwork and four new bridge structures connecting Issaquah to the south with the Sammamish Plateau and Issaquah Highlands to the north.

The bridges are the highlight of the project, including a 925-ft. twin steel-tub flyover, a precast, post-tensioned concrete-tub undercrossing, another twin steel tub structure with a girder span that has the distinction of being the longest structure ever hauled on a Washington State highway and a 1440-ft. concrete box girder structure.

The interchange also boasts a number of MSE structural walls, as well as soldier pile walls with tiebacks constructed to allow for on and off-ramps on either side of the highway. All of the MSE walls have precast concrete panels with a distinctive floating-leaf pattern.

The leaves though a bit unusual, stand out from the freeway corridor like a bas-relief painting and give the interchange an artsy appearance.

Mark Hinthorne, Issaquah's planning director, said the interchange will improve things fairly soon after it opens. "People will shift their driving patterns to take advantage of it, and in the end, that's all we can hope for," he added.

Project Challenges

From its beginning in April 2001, the Sunset job presented a number of challenges.

First, Hegstrom said was the need to plan and schedule installation of the undercrossing and steel-tub flyover spanning I-90. This made it necessary to close the freeway to traffic. Because of heavy volumes, closures could only be from 11:00 P.M. to 5:00 A.M.

Next, during earth moving and drilling operations, Kiewit and its subcontractors discovered unforeseen site conditions. Among them was a preexisting landslide and car-sized underground granite boulders that precluded normal drilling. Overcoming these hurdles cost the project contractor six months and led to nearly 100 change orders totaling nearly $25 million.

In order to drill shaft foundations, Seattle-based Malcolm Drilling Co. brought in European oscillating drill rigs.

Changing the shaft required changing the column size, which in turn changed the hammerhead crossbeams, the bearings and the connections for the steel-tub girders.

In the end, Kiewit and the subcontractors were successful; in some cases drilling column shafts of up to 100 ft. deep and 12 ft. wide. However, the project had been delayed six months.

The job also has presented logistical challenges for the Washington Department of Transportation in that it has had six funding partners. In addition to WSDOT, they are:

• Port Blakely Communities
• King County
• City of Issaquah
• Federal Highway Administration
• Transportation Improvement Board
• Sound Transit

WSDOT project engineer Matt Preedy said that although this kind of cooperation and teamwork is difficult to pull off, doing it successfully "has given us a good look into what the future holds." He was referring to future projects involving widening of Interstate 405 and the floating-bridge replacement on state route 520.

"All of these large-scale jobs will require interagency coordination, and this job is a good case of showing that we can deliver these sorts of projects with all these involvements," Preedy added.

Practical Challenges

The waters of Issaquah Creek also had to be protected.

The salmon-spawning stream runs along the southern boundary of the Front Street off-ramp, then turns north and passes beneath the bridge and flyover of the interchange.

Permitting conditions allow access to the creek only from June 15 to Sept. 15. The creek is inaccessible at other times in order to preserve the fragile ecosystem.

Hegstrom said the project spent nearly $1.5 million on temporary erosion control.

This was a complicated contracting task. There were 11 material suppliers and subcontractors in all, including Tacoma-based Concrete Technology Inc. for the pre-stressed girders; San Diego-based L.B. Foster Co. for the MSE wall panels; Seattle--based Harvard Steel for the rebar; Vancouver-based USI for steel girders; Redmond-based Watson Asphalt Paving Co. Inc. for the asphalt paving and Kent-based Signal Electric.

Looking ahead

Project engineer Preedy and his colleagues said the project is far from over.

After the interchange opens in August, planning currently underway will continue. First priority: a City of Issaquah $40 million southeast bypass that establishes a direct route connecting I-90 to Issaquah-Hobart Road to the south, circumventing downtown.

There is also work planned to the north, in and around the future 150-acre Microsoft campus. This work should be finished by 2007 or 2009, and in addition to new office space, should consist of 3,200 homes and other retail locations.