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I-405 / SR-167 Direct Connect - Sanitary Sewer Relocation
Washington State Department of Transportation is constructing the I-405 / SR-167 Direct Connector project. As they expand the highway limited access right-of-way south, the public utilities
that serve the residential neighborhood in the Talbot Hill area need to be relocated. WSDOT is responsible for the relocation of the utilities. Because of timing and coordination
issues, the State needs the sanitary sewers relocated prior to their contractor mobilizing onto the site. WSDOT is contracting with the City of Renton to relocate the sewers.
The general location of the sewer relocation project is a triangular area bounded by I-405, Smithers Ave S, and S 15th St. As a sub work of the WSDOT project, the sanitary sewer relocation
will be done under WSDOT’s permits.
The land use designation for the area Residential – Medium Density and is currently zoned Residential - 6 DU/AC and Residential - 8 DU/AC. The area where the new sewers (and other utilities)
will be located will be City right-of-way and the area north would become State Limited Access Right-of-Way. There are not any plans to change the land use or zoning designations in
the area as part of these projects.
There are steep slopes in the area that will be dealt with under the permitting for the parent WSDOT project.
There is one wetland identified in the sanitary sewer construction zone. This is identified in WSDOT’s JARPA permit as Wetland 2.6R which is 6,424 sqft category III. WSDOT’s proposal
is that there will be permanent impact to the wetland due to a proposed earthen berm. The sewer main will be installed through the southern edge or just to the south of the wetland.
In the area, subsurface soils consist of loose to medium dense silty-sand and poorly graded sand with silt and variable amounts of gravel. Drainage in the area appears to be free flowing
except in the vicinity of the wetland discussed above.
As part of our project, WSDOT is having the City of Renton set the basic grade of the new S 14th St / utility corridor which will entail some cut and fill. This is part of the overall
regrade WSDOT is planning to do for their project. The fill needs to go in after the sewer is installed but prior to the WSDOT construction as another utility has to relocate into
the corridor prior to the State’s contractor coming on site to complete the road. For our portion of the project, we are estimating approximately 1600 cy of cut & 3400 cy of fill.
Total estimated cost for the sanitary sewer relocation is $5,000,000
The sewer relocation contractor may have a job shack on site.
As part of the sanitary sewer relocation, there will be two portions of the project that will not utilize typical open cut construction practices.
There will be approximately 800 feet of sewer main installed in an east-west direction (from Whitworth to the west) parallel with and along the southern boundary of the new I-405 right-of-way
utilizing horizontal directional drilling. This method was chosen over open cut because the sewer line will be 40’-50’ deep.
There will also be about 240 feet of sewer main installed under I-405 to the north that will be installed by guided pipe ram in lieu of open cutting the highway.
The remaining portions of the sewer relocation will be accomplished by standard open cut method.
The only part of the project that we are considering as might needing night work is the 800 feet of sewer main being installed by the horizontal directional drill.