Curved Microtunnel Drive: Seattle’s Ship Canal Water Quality Project

By Dr. Chris D. Breeds, P.E., Dennis Molvik and Don Gonzales

Northwest Boring completed the Ship Canal Water Quality Project Curved Microtunnel beneath the Washington Ship Canal in Seattle, Washington in July 2023. This project was part of the Ship Canal Water Quality Project (SCWQP) designed to provide offline storage for the combined sewer overflow (CSO) flows from five Seattle Public Utilities (SPU) and two King County Department of Natural Resources and Parks (DNRP) CSO basins to meet parts of the City of Seattle’s and King County’s Consent Decrees.

Overall Project

The overall Ship Canal Water Quality Project (SCWQP) Storage Tunnel Contract for Seattle Public Utilities (SPU) is comprised of 13,939 lf of 18-ft, 10-in. ID segmentally lined tunnel using a 21.5-ft diameter pressurized face tunnel boring machine (TBM) with tunnel depths ranging from 35 to 85 ft to top of tunnel, and the 646-ft, 94-in. diameter conveyance pipe casing under the Ship Canal installed by Northwest Boring. Lane Construction was hired as the general contractor for the $255 million project.

When completed, the tunnel project will capture and temporarily store more than 29 million gallons of untreated stormwater and sewage until the treatment plant is ready for it. The tunnel will improve water quality regionally by keeping more than 75 million gallons of polluted stormwater (from rain) and sewage each year from flowing into the Lake Washington Ship Canal, Salmon Bay, and Lake Union.

Mining on the main tunnel was completed in June 2023, and work on the contract was expected to continue through 2024.

Microtunneling the Ship Canal

For the microtunnel alignment, baselined ground conditions included glacially over consolidated, dense to very dense silt, fine sandy silt, silty fine sand and gravel (Till) with scattered cobbles and boulders. Northwest Boring Co. was hired as the microtunneling subcontractor.

Northwest Boring was founded in 1951 and has established itself as a premier contractor providing trenchless and tunneling services. Northwest Boring was an early adapter of microtunneling in the United States, and has completed numerous innovative and challenging projects.

Microtunnel work involved jacking a 96-in. inside diameter, 118-in. outside diameter RCP, equipped with Jackcontrol hydraulic joints, along an alignment that included: a 67-ft long straight section, a 367-ft long curved section excavated along a 650-ft radius curve, followed by a 182-ft long straight section. Northwest Boring selected a Herrenknecht AVN 2000AB MTBM to excavate the alignment with the RCP casing pipe, aforementioned hydraulic joint system and Intermediate Jacking Station (IJS) designed by the Swiss firm Jackcontrol in accordance with the project specifications. The MTBM was christened Helene in honor of Northwest Boring vice president Dennis Molvik’s Norwegian immigrant mother.

The 90-ft deep jacking shaft at the Fremont site accommodated the curved microtunnel system components, thrust block, entry seal, pipe brake and jacking frame. The embedment ring was recessed in the final shaft lining to facilitate installation of a microtunnel plug that was dowelled into the end of the casing pipe during the carrier pipe installation process.

Early Progress

Northwest Boring launched the MTBM in August 2021 and progress was uneventful until Sept. 2, 2021, when large inflows of water and lubrication were experienced between the joint between Pipe 8 and Pipe 7 as Pipe 9 was being jacked into place. Microtunneling operations were stopped, and worker safety was assessed.

Northwest Boring immediately recognized that inflows could flood the shaft and using hydraulic pressure against the MTBM cutter face moved the failed joint along with the entire MTBM and other pipe joints backward about 15 in. to a position inside the inflatable seal.

Northwest Boring immediately engaged with the owner and started a program of root cause analysis, risk assessment, forensic investigation, as well as parallel insurance notifications and a myriad of other interactions all in concert with Northwest Boring’s Casualty Management Plan. This plan had previously been developed for such an occasion and paved the way through a very complicated process of problem resolution and the inevitable problems and conflicts that develop when a microtunnel project encounters a catastrophic failure.

Preparations for Completing the Microtunnel

A plan to remove and replace the failed Pipe 8 was developed after development of a recovery plan and risk analysis that was shared with all interested parties and approved by the owner. It was decided that the pipe string would again be pushed backward using the same process initially used to move the joint between Pipes 8 and 7 safely within the inflatable seal. This was successfully performed and the failed Pipe 8 was removed.

At this stage, it was determined that the remaining uninstalled pipe would need to be inspected and subsequently repaired if needed. Northwest Boring subsequently evaluated, using risk analysis, whether the first eight RCP pipes that had already been microtunneled would need to be removed or if it would be less risky to proceed leaving them in place. The risk analysis determined that the already installed RCP should be left in place but recommended risk mitigation using Miller Pipeline’s WEKO-SEAL across the joints and the utilization of joint restraints if needed to constrain joint movement should a gasket failure occur.

Project Completion

Required equipment was designed and procured or constructed, RCP repair procedures were developed and reviewed and RCP repairs were completed, all of which occurred over an approximate 12-month period. On Oct. 25, 2022, Northwest Boring performed a test of the MTBM system, ensuring that the entire pipe string that had sat in place for approximately 14 months could still progress forward. After this test, microtunneling then recommenced.

With only 15 more pipe segments to go until break in to the receiving shaft, Northwest Boring encountered another problem in that at two of the joints, the joint between Pipes 2 and 3 and the joint between Pipes 4 and 5, the WEKO-SEALs that were covering those joints were bulging. This meant that up to 2 bar of groundwater pressure was now acting on the inside face of the bell and the spigot of these pipe joints thereby complicating the recovery of the MTBM. This necessitated the installation of joint restraints across those joints in order to tie the first five pipe segments together and keep them from pushing apart when the MTBM was removed.

On Jan. 26, 2023, the MTBM broke into the 3RS receiving shaft and on Feb. 8, 2023, the MTBM was lifted out of the receiving shaft and set on a trailer, for her next journey.

Dr. Chris D. Breeds, P.E., is microtunnel engineer at SubTerra Inc. Dennis Molvik is vice president and Don Gonzales is president at Northwest Boring Co. Inc.

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