May 26, 2017

Highway and transportation
Under $5 million

Photo courtesy of Granite Construction Co.

Granite had to contend with a high water table to install new fish-passage culverts.

State Route 121 Blooms Ditch fish barrier removal

Location: Olympia

The project’s primary goal was to improve fish passage within the Black River Watershed.

The existing conduit for the stream crossings along state Route 121 were old and undersized metal culverts. These culverts were identified as impingements for fish migration and required improvements to allow access to additional habitat for salmonids in the watershed.

Part of the work involved raising the roadway grades to limit closures due to flooding. Safety was improved by increasing sight distances and installing additional safety features.

This project had very challenging geotechnical conditions consisting of sands and silts in conjunction with a very high water table. Granite led a team of experts including a hydrologist, project engineer and drilling contractor to find a solution to the groundwater issue.

The team came up with an innovative groundwater-control plan composed of a deep well system and utilized adjacent forested areas for the infiltration of water. This system functioned well, drawing down the groundwater, allowing for installation of the culverts. It minimized impacted sensitive areas and the cost for treating water.

The new precast box culverts were designed to be installed within the same alignment as the existing stream channel. This being the case, a bypass of the existing stream was required.

Granite devised a bypass plan conforming to all permit and contractual requirements. During development of the bypass plan the team discovered that the design requirements were excessive. Granite was able to reduce the amount of flow that the bypass system had to be designed for, cutting costs and shrinking the footprint bypass system by over 33 percent.

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