Boundary Dam Spillway No. 1 TDG Modifications Removal

Metaline Falls, Washington

Best Small Project

Submitted By: IMCO General Construction

Owner: Seattle City Light

Lead Design Firm/Civil/Structural Engineer: Hatch Associates Consultants Inc.

General Contractor: IMCO General Construction

Subcontractors: Proline Concrete; Sunset Co.; Post Tensioning Solutions

The Boundary Dam, located on the Pend Oreille River in northeastern Washington state, is as visually striking as it is geographically remote. The dam’s pair of unique sloped spillways are cut into rock 275 ft above the river. When operating, water from the spillways pours out from both sides, meeting in midair to dissipate the falling-water energy that would otherwise erode canyon walls. The project goal was to remove total dissolved gas (TDG) modifications made in 2016 in order to mitigate impacts to local fish populations. The existing features of Spillway No. 1 were removed and the surface was completely restored. To minimize environmental impacts from the worksite, any leaks, spills or drops had a direct drainage path into the river. To mitigate risks, engineering controls included standard inlet protections, secondary containment on equipment and fluid containers and wattles in parking areas. The technical challenges of the job were complicated by the worksite location. The spillway was only accessible by foot or helicopter. All materials were hoisted by crane from 95 ft above the work area. A small parking lot connected to an access road served as the staging area for the crane, materials and equipment. The 200-ton crane was delivered in ten separate loads and required the entire staging area to assemble 240 ft of boom.

“Our team is incredibly proud of this recognition. It was a fun and challenging project,” says Tyler Kimberley, IMCO Construction president and CEO. “The camaraderie and commitment to excellence by our project team made this a success and allowed the project to overcome great challenges, working in such a remote location, working on the spillway of an operating dam, and meeting the difficult specifications.”