One Tunnel, Two Pipelines

Pipe Jacking Under I-5 Near Seattle Nets Savings for Owner

By Laura Anderson

In August, a Washington State trenchless contractor completed an installation of 540 lf of steel casing to house new gravity water and sewer lines under an active 10-lane interstate in Lynnwood, Washington; without this solution, the crossing would otherwise be improbable.

The I-5/164th Martha Lake Gateway Sewer and Water Improvement project was completed for the Alderwood Water and Wastewater District (AWWD). The steel casing housed a 30-in. ductile iron water line and a 20-in. HDPE sewer pipe, and when centered and appropriately positioned, correct alignment required a maximum allowable clearance of 2.6-in. on either side. Implicit in the design was that accurate, uniform and constant tolerances throughout the alignment were crucial.

Interstate 5 is a north-south route through the State of Washington, extending into Oregon and British Columbia. Lynnwood is located approximately 20 miles north of Seattle and considered a bedroom community to Seattle proper. Residential housing sandwiched the alignment on the west and east sides before it crossed under I-5’s 10 lanes of traffic.

The Martha Lake Gateway Sewer and Water Improvement project scope of work included the installation of 4,300 lf of open-cut 8- to 14-in. sanitary sewer; 1,900 lf of open cut 6- to 8-in. ductile iron water main along with connections; dewatering, erosion control and site restoration. TITAN Earthwork LLC of Sumner, Washington, was AWWD’s general contractor on this project. Trenchless subcontractor Northwest Boring Co. Inc. of Woodinville, Washington, pipe jacked 540 lf of 66-in. steel casing under I-5 with an open-face, man-entry TBM system and 202 lf of 42-in. auger bored steel casing under 164th Street SW. Total project costs were $5 million, which included the two trenchless runs tallying $650,000.

Steve Greiling, project manager with TITAN Earthwork explained, “From the onset of the client’s diligent pre-award review process, Northwest Boring’s statement of qualifications meshed well with TITAN’s underground capabilities and ultimately enabled AWWD to confidently execute the contract.

“With the work going under 10 lanes of live freeway traffic, which also serves as Washington’s primary north-south throughway, one can imagine that our contract had very specific performance expectations with tight tolerances enforceable by the AWWD, Snohomish County and the Washington State Department of Transportation. These demands necessitated precision, engineered QA/QC controls and accuracy to ensure TITAN’s trailing infrastructure within the casing would function perfectly.”

Northwest Boring has overcome many challenges during its 60 years in the trenchless construction industry. The company has performed this particular type of installation previously and found with the right set of project variables, installing two carrier pipes in one casing can present significant savings.

Don Gonzales, president of Northwest Boring stated, “We’ve done the multiple-carrier-pipe installation bore several times and it can save the owner lots of money. The I-5 crossing was over a half million dollars, so to be able to tunnel once, rather than twice, makes a big difference in the bottom line.”

The contractor used an all-in-one Akkerman TBM 540, a 5000 Series Pump Unit to power the TBM and jack the casing, a 524 haul unit for muck removal, and a laser stand for guidance from its fleet of equipment. The TBM was outfitted with an open cutter-face, dressed with bullet teeth in order to contend with the anticipated glacial geology and potential obstructions, which included wood and/or large boulders.

Northwest Boring crews mobilized in May 2014 to begin shaft preparations. A 12-ft wide by 32-ft long jacking shaft was constructed for the TBM launch at 24-ft deep and was shored with steel trench boxes. Attainment of the gravity alignment required the launch at a 1 percent downward grade to jack the steel casing from east to west under I-5.

A pipe jacking system was used to advance the TBM and 20-ft. lengths of pipe, with a compact jacking frame outfitted with a built-in yoke on a skid base, and an in-shaft power unit situated on the end of the skid base near the thrust block. Spoil removal was conveyed to the haul unit’s dirt bucket and hoisted to the surface for removal. The TBM is a manned-machine, where a crew member controls the machine’s advancement on grade from the interior. Due to the open face, this person was simultaneously responsible for keeping an eye out for obstructions, then halting operations for their removal at the face of the bore as necessary. Additional crew members maintained the spoil removal process on the haul unit, controlled the jacking frame hydraulics and directed the crane operator to receive additional pipe lengths in the shaft. Together the crew moved in unison to orchestrate these responsibilities in a seamless approach.

Operations became hindered when project adversity was encountered. Soil conditions on the baseline report were identified as consolidated glacial till with cobbles and boulders. Gonzales commented that the soil was, “some of the most difficult dirt in America that I’ve seen,” and furthered that they came across “blow counts up to 50 with 3-in. embedded cobbles and boulders,” although they experienced average jacking forces of approximately 70 tons throughout. Additionally, the downward grade of the alignment made for pooling water, requiring continuous dewatering during the boring process. The combination of extremely hard, consolidated soils with embedded cobbles and boulders was very demanding on the equipment. Gonzales reported that, “the pipe jacking equipment held up well even when the boulders were met. It managed to maintain virtually perfect line and grade throughout the soil variations.”

When Northwest Boring completed the 66-in. crossing at the end of June 2014, TITAN crews assumed the installation of the 30-in. ID water and 20-in. sewer pipeline. Northwest Boring then moved to 164th street to complete the 202-ft, 42-in. steel casing installation across six-lanes of traffic with an auger bore machine, which concluded their portion of the work.

The general contractor was also impressed with the bore. Greiling reported, “Northwest Boring flat out delivered, resulting in final inverts that deviated from design by only 2/100ths – an outcome that confirms that Northwest Boring has the sound leadership, expertise to implement the best tunneling equipment for the given conditions, high-caliber field talent, and is an overall outstanding option.”

Gonzales pointed out that, “The equipment worked wonderfully in this ground condition. Having face access was key because without it, removal of the boulders may not have been possible.”

Laura  Anderson is director of marketing and communications for pipe jacking and tunneling manufacturer, Akkerman of Brownsdale, Minnesota.