Location: Seattle, WA
Owner: Washington State Department of Transportation

OCI was under contract with HNTB, part of Seattle Tunnel Partners, and developed site specific and construction phase specific TESC Plans and Narrative for the project. Major areas of concern for erosion and sediment transport on this project were the south and north portals, particularly the cut and cover section of the tunnel which involved pile construction, deep excavations, and concrete handling activities associated with viaduct demolition, muck transport, and emergency storage.

OCI completed the engineering analysis needed to obtain the NPDES Construction Stormwater General Permit and the King County Wastewater Discharge Authorization for the project. OCI determined temporary and permanent discharge locations and designed connections to the existing drainage systems. In order to determine discharge capacity of existing outfalls, a backwater analysis for outfall pipes from the regulator discharging to Elliott Bay at the south portal area was conducted. MGSFlood and SBUH design software were used to determine the maximum stormwater flows and volumes expected from the south and north portals based on the maximum footprint of construction activities. This information was then used to determine temporary storage tanks and treatment plant size required to meet discharge standards.

OCI coordinated with the geotechnical engineer for dewatering volumes and assisted in reviewing compliance plans for discharge volumes and water quality. This project involved coordination with the Contractor, design-build permitting team, and various stakeholders on the project such as Seattle Public Utilities, King County, City of Seattle, WSDOT, and Port of Seattle.

OCI worked as an extension of the HNTB drainage design team and assisted with stormwater design and calculations. OCI’s engineers performed QC of the stormwater report and plans.