Argonne Road Bridge
Spokane Valley Job Requires Big Equipment

A scour problem at the Argonne Road Bridge's existing footings has required the installation of six massive support shafts placed deep into the Spokane River.

by Sheila Bacon

Things are moving and shaking in the Spokane Valley.

Inland Foundation Specialties is driving 8-ft.-diameter caissons into the Spokane River as support for the rebuilt Argonne Road Bridge. The caissons are driven 120 ft. deep, then filled with concrete and removed. Image courtesy of Inland Foundation Specialties

Crews are using a mammoth vibratory driver to push six caissons deep into the Spokane River; the beginnings of a new support system for a bridge that had begun to give in to Mother Nature.

The Argonne Road Bridge, north of Millwood, is actually two merged spans, one built in the 1920s and one in the 1970s. Swift river flows had caused erosion around the footings of the bridge, and a study commissioned by the owner, Spokane County, found that it would be more expensive to repair and mitigate the scour damage than to completely replace the bridge.

To minimize future scour problems and protect against the failure of the bridge in a major flood event, the plan called for deeply drilled shafts to replace the footings as the bridge's support system.

Crews from Inland Foundation Specialties of Boise, Idaho - a subcontractor to general contractor Harcon Inc. of Spokane - are using a model 400B King Kong vibratory driver/extractor from American Piledriving Equipment of Kent, Wash., to drive the 8-ft.-diameter shafts 120 ft. beneath the riverbed.

The vibration of the driver essentially liquefies the earth, and the weight of the 61-ton caisson and pressure of the driver help the caisson burrow into the ground, said Mark Woodall, business development manager for Inland Foundation Specialties of Boise, Idaho. Once the caissons are in place and the riverbed material extracted, crews install a rebar cage, fill the caisson with concrete and remove the casing. Pier caps, girders and deck construction follow.

The 28,500 lb. driver clamps onto the caisson in four places - a testament to its large size. On smaller jobs, drivers that use a single clamp to attach to smaller caissons are more frequently used.

The job is one of Inland Foundations Specialties' most unique projects to date.

"We've gone that deep and done holes that big in diameter before, but never both on the same job," Woodall said.

Bridge Remains in Use

Another driving option was the use of an oscillator instead of a vibratory driver. An oscillator uses a twisting motion - similar to that of twisting the lid of a jar on and off - to advance a caisson.

Oscillators are a quieter method, but typically more expensive, and the Argonne Road Bridge's location in a nonsensitive area didn't warrant the expense of an oscillator, Woodall said.

A 400B King Kong vibratory driver/extractor from American Piledriving Equipment is used to drive the caissons into the riverbed. Image courtesy of Inland Foundation Specialties

The bridge spans the backwaters of the Upriver Dam, which was built in 1894 to provide Spokane with water from the river. When the head gates are opened in the spring, the river's flow is increased, and over time the swiftly moving river has contributed to the erosion around the bridge's footings, said Paul Lennemann, construction engineer for Spokane County.

The bridge has remained in use during the construction process, said Bob Anderson, project superintendent for Harcon Inc, the Spokane general contractor. The west half of the bridge was replaced first, while traffic was diverted to the east half. Now, drivers are channeled onto the completed west half while construction is under way on the east half.

The bridge is actually three spans: two box girder abutments with a precast span in between.