King County Embarks on Large Conveyance System Upgrade that includes Challenging HDD

摘要

The North Mercer Island Interceptor and Enatai Interceptor Upgrade Project (henceforth the North Mercer Enatai Project) is a large conveyance upgrade project for King County, in Seattle, WA. The project includes the upgrade of a pump station on Mercer Island and open cut pipeline, crossing of the East Channel of Lake Washington, and the installation of a 36-inch HDD beneath Enatai hillside to convey flows to the Sweyolocken Pump Station located in South Bellevue. The HDD is designed in complex geotechnical conditions that include recessional outwash with zones of large gravel and small cobbles. More challenging than the site geology is the layout of the bore which begins within the Washington State Department of Transportation (WSDOT) ROW parallel to the Interstate-90 (I-90) Bridge where it crosses Lake Washington, as seen in Figure 1. This segment of I-90 is extremely sensitive due to very soft soils at the foundation that have resulted in movement of the foundation piles for the bridge. These circumstances resulted in extensive coordination with WSDOT and the requirement for a specialty HDD analysis showing the area of influence around the HDD and the potential impacts to the I-90 bridge piles. In addition, the coordination with WSDOT addressed the installation of the HDD beneath I-90 on and off ramps on the east end of the bore. This paper will focus on the project factors that led to the choice of using trenchless techniques, the selection of the preferred trenchless method, and the specialty analysis that was required by the governing agencies to get approval for construction within a Federal Highway ROW.