Bore excites engineers

MARYSVILLE – This is a boring story.

It’s such a big bore that engineers from throughout the Puget Sound region were practically salivating over it alongside Marysville’s wastewater treatment plant on Wednesday.

The boring they were gushing over was a horizontal drilling operation so large it’s one of the biggest of its kind.

Putting sewer pipes underground is nothing new, but carving out a tunnel 4 feet in diameter and half a mile long is, said Paul Godlewski, vice president of Seattle-based Shannon &Wilson Inc., a geotechnical and environmental consulting firm.

It’s especially fascinating when it’s a plastic – and not steel – pipe that’s being dragged through the hole.

“I don’t think there have been 10 projects in the world that have gone this far and have been this wide,” Godlewski said.

He was one of about 15 engineers who watched a contractor start the 12-hour process of dragging a snakelike 2,800-foot plastic pipe through the hole. The unbroken pipe – 3 inches thick – is 3 feet in diameter.

Tossing out words such as “reamer,” “frak out” and “hole opener,” the engineers held a bit of a pep rally before heading out to watch.

“This is kind of the culmination of this $35 million project,” Jeff Massie, Marysville’s assistant engineer, said minutes before the tour. “Most engineers find (this) kind of thing neat.”

The city is spending $35 million to connect its wastewater treatment plant to a pipeline Everett uses to send its effluent into the waters of Commencement Bay.

When finished this fall, the pipe will allow Marysville to stop dumping its wastewater into Steamboat Slough, something that could become a problem, since all of the growing cities on the Snohomish River continue to increase the amount of effluent they release into the river.

Most of the nearly five miles of sewer pipe will be buried using the tried and true method of digging a ditch and connecting the pipe piece by piece. But to avoid sensitive environmental wetlands and the cost of digging under highways, the city decided to bore two half-mile sections of tunnel, Massie said.

The first section, which goes under Steamboat Slough and I-5, was finished Wednesday. The second, which will go under Highway 529 and Union Slough, should be completed by Labor Day weekend.

To accomplish the uncommon tunneling feat, the route first had to be mapped out with special stakes, said Pete Weber, senior engineer and project manager for Hammond Collier Engineers of Seattle.

Then a pilot tunnel was dug along the route, with the stakes used to control the route and depth of the tunnel. Depths ranged from 40 feet to 80 feet.

Using progressively larger reaming heads, the tunnel was then gradually enlarged to 48 inches, Weber said. A special mix of mud and water was pumped into the tunnel to make sure it didn’t collapse.

For those scratching their heads over terminology, the engineers also were trying to make sure there wasn’t a frak out. Those occur when a tunnel is so close to the surface that the mix of mud and water that’s supposed to support the tunnel forces its way to the surface, creating an unwanted breach that can cause a collapse.

One of many unusual pieces of equipment used at the site included a hole opener, a device like a spinning top that is used to clean out the tunnel before the pipe is dragged through.

Reporter Lukas Velush: 425-339-3449 or lvelush@heraldnet.com.

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