King County’s initiative for improving infrastructure to increase system reliability from combined sewer overflows and to accommodate population growth continued with replacement of the North Creek Interceptor Sewer, a 2-mile pipeline serving the Bothell area since 1970.
This project was originally awarded to another contractor in 2014 who was terminated for default on the contract in 2017. The County awarded the project to JWF via an emergency declaration waiving the competitive procurement process. A core compensation agreement to complete the project was negotiated by JWF and the County.
It included construction of 3,443 feet of new sewer line, along with connecting this new line to previously constructed pipe. The project also included asphalt paving, traffic control, landscaping and wetland restoration. JWF assumed responsibility for the first tunnel drive midway through completion – this included repairs to and utilization of the existing TBM.
The majority of the pipeline was constructed using open faced shield tunneling with a 72-inch casing and 42-inch and 48-inch Hobas carrier pipe in deep sections, including crossing the state highway. There were four drives of 1,002 feet, 547 feet, 1,103 feet and 881 feet. The open cut included completing work started by the prior contractor with 85 feet of 42-inch Hobas pipe, 155 feet of 36-inch Hobas pipe, sliplining of 85 feet of 12-inch PVC and 250 feet of 8-inch ductile iron pipe and miscellaneous 8-inch laterals that tied into the main sewer pipeline. After all of the pipe was in place, the annular space between the pipe and the excavation was grouted with cellular concrete.
Construction took place in both the City of Bothell and unincorporated Snohomish County. The County provided materials and equipment including an open face shield tunnel machine, shored shafts, and some of the casing and carrier pipe needed to finish the job. A construction dewatering system was partially installed by the previous contractor.
The geotechnical characteristics of the area included cobbles and mixed face conditions, high water table, artesian and multiple layered aquifers that required extensive dewatering.
The project required compliance with environmental requirements to eliminate impacts from construction activities to nearby wetlands and North Creek. JWF was able to successfully treat and discharge the output from dewatering activities to North Creek with no impacts to water quality.
There was also extensive public outreach and property owner coordination required for over 30 easements and rights of entry to access the project site.