Groundwater Impacts on Sewer Relocation using Trenchless Technologies Near and Under I-405 in Renton, Washington

The greater Seattle area is one of the fastest growing regions in the US. As the growth of the suburban communities surrounding Seattle has continued, the Washington State Department of Transportation (WSDOT) has continuously worked to improve the freeway system connecting the local communities. One such project includes the installation of High Occupancy Vehicle (HOV) flyover freeway ramps between State Route 167 (SR167) and Interstate 405 (I-405). To accomplish this, WSDOT needs to relocate the remaining freeway travel lanes further to the south to accommodate the new bridges and transitions. Moving the freeway lanes further south required the relocation of existing utilities within Talbot Hill. The existing City of Renton gravity sewer lines in South 14th Street and under I-405 at Shattuck Avenue S will be exposed with the removal of Talbot Hill for the new HOV ramps. Relocation of the existing gravity sewer lines for the City of Renton required the use of multiple construction techniques including both conventional open cut and trenchless technologies due to the depths required for the new gravity sewers and the new crossing of I-405. Geotechnical explorations in the project area identified soils consisting of silty-sand overlaying weathered sandstone bedrock with groundwater ranging from 10 to 35 feet below the ground surface. This paper will look at the alternative construction methods analyzed that would allow for the line and grade installation of gravity sewer. The paper will also discuss the consideration of the depths and slopes required for the gravity installation along with the soil and groundwater conditions encountered. Finally, it will examine the effects of the localized groundwater on the installation of the newly relocated gravity sewer mains within Talbot Hill using both horizontal directional drilling (HDD) and guided-auger bore technology to cross I-405.