Where does the dirt go from Seattle's big construction projects?

As a general contractor who does small house remodels in Seattle, Chris Spott knows how to get rid of a pickup truck load of dirt.

But when it comes to mega-projects like Seattle’s waterfront tunnel – or the excavation for Amazon’s new headquarters – Spott had no idea. So he submitted his question to KUOW's Local Wonder: Where does the dirt go from Seattle’s big construction projects? Reporter Joshua McNichols went to find an answer.

I started in Seattle’s South Lake Union neighborhood where Amazon has been building its headquarters. It’ll have three towers and a building that looks like a giant glass golf ball.

Cool stuff. But all that architecture is just noise to Garry Horvitz. He doesn’t get paid to think about that. He gets paid to think about dirt.

“Whenever we’re doing a project, if you’re dealing with an architect, they’re concerned with the glory of how high above ground the building will be,” Horvitz said. “The first question I ask is: How deep in the ground is it going?”

Of course, when you take out dirt, you make a hole. As a geotechnical engineer with Hart Crowser, Horvitz’s job is to make sure holes like that don’t collapse in on themselves.

He’s not working on this project, but he does this kind of work on other big buildings all the time.

We met up at the Amazon site, at a section they just started digging. I asked him what kind of hole we might see there.

“For a building of this size, they’re probably going to go down 40 or 50 feet and the amount of soil that might be generated might be on the order of a quarter of a million cubic yards,” Horvitz said.

I asked Horvitz how a person wraps their head around a number like that. “A typical dump truck you see going down the road holds 10 cubic yards,” Horvitz said. “So if you’re dealing with 250,000 cubic yards, that’s 25,000 dump trucks full of soil.”

This is not the first time this soil has been moved. It used to be part of Denny Hill. During Seattle’s early years, the city flattened out the hill. Much of the dirt flowed into a river valley right on Westlake Avenue. “Westlake used to be a stream valley, the headwaters of which were about where the Sheraton Hotel is right now,” Horvitz said, “and so this area here along the spine of Westlake used to be about 30 to 40 feet lower than it is now.”

But you can’t just dump your extra dirt into a stream bed anymore. You need a place with a permit to take your dirt.

“So where does all that dirt go?” I asked.

“Well, it goes a number of places,” Horvitz told me. Valuable soils – soils that can take a structural load – may go through a soil broker who matches buyers and sellers. Contaminated soil may be treated. In some cases, the contaminants are cooked out of it at high temperatures. But once you get down a story or two, most dirt is clean and without value.

“There are gravel pits or mines that are scattered around Puget Sound,” Horvitz told me. “And as you can imagine, these mines wind up being very large holes in the ground, and at some point when those mines are played out, they need to be filled back in, as part of environmental reclamation.”

One of those mines is in Monroe. I headed out to the Fiorito mine in the Cascade foothills to meet Zak Fiorito. He runs Monroe Rock, one of many quarries you can find around Monroe.

There are gravelly mines in the valley formed by the Skykomish River. And there are rock mines like this one up in the hills.

The top of one of those hills was taken away. The rock was used in construction.

“Mining companies often get a bad rap because people think, Oh, you’re cutting down all the trees and you’re wrecking the environment and basically ruining the earth,” Fiorito said. “What people don’t see is that you drive on the road. And you have to have the rock products from somewhere. So if you don’t mine, then you don’t have roads to drive on.”

The miners dug until there wasn’t anything worth selling. Fiorito said after all the good rock was gone, they started filling up the hole left behind with dirt – lots of dirt.

That hole is now a hill. And that hill is growing.

“That hill used to be about 60 feet below where we’re standing. That’s how much fill is in there now,” said Fiorito. It’s mostly dirt, but also concrete and asphalt from all over King and Snohomish counties.

Fiorito said sometimes contractors try to hide garbage in their loads – plastic pipes, for example, or other debris. Fiorito and his crew watch carefully for stuff like that. Drivers who dump illegal stuff have to pick it up and take it away to a landfill. That’s not a mistake you make twice.

Fiorito offered me a ride to the top of the hill in his truck. It’s not an ordinary truck. It felt like something out of Star Wars. The wheels were tall as a grown man. He handled it like a race car over the quarry’s bumpy roads.

“Is there anything this thing can’t drive over?” I asked.

“It’ll get stuck in the mud if the mud’s soft,” Fiorito told me.

As we’re driving, dump truck drivers from all over the region radio in. They announce their arrival at the quarry: “American Pride, D16, coming up off Al’s scale.”

A woman’s voice from Monroe Rock’s work shack answered: “Yep, Nelson, he’s going to follow you up. And don’t forget to pick up your dropped axle, please.”

This mine takes dirt from 300 trucks a day. At $100-150 per truck, that’s good money. But it wasn’t always so good.

“During the recession it was super slow here,” said Fiorito, with only about 30 trucks showing up each day. “We were hardly getting any material. Whereas now, we’re getting a lot.”

“Forty four inbound for Mercer Island,” squawked the radio.

“How long did it take to build this mountain up?” I asked.

“We started filling it in probably 1999, a little bit,” said Fiorito. “My best guess is that there’s probably 5 million tons of fill here. And there’s approximately 3 million left that can go in.”

We arrived at the top of the hill: a broad, flat, muddy plane, about the size of a couple football fields. Around us, a 360 degree view of mountains and river valleys.

Three or four at a time, the dump trucks dropped their burdens on the ground. A bulldozer came and smooshed them out, forming another thin layer of this growing hill.

And all over the region, many picturesque places like this have been shaped – and are still being shaped – by our region’s big projects.

Seattle’s waterfront tunnel, for example, is sending its dirt across Puget Sound by barge. It comes to rest in an old quarry on a stunning little peninsula near Port Ludlow, on the Olympic Peninsula.

Eventually, that land will become something completely different. It could become a housing development. Or a park.

That future offers Zak Fiorito something to feel good about.

“I get to put it all back again,” Fiorito said. “I get to finish the slopes, I get to plant trees. I get to say, Okay, I’m making it nice again. I mean, yes, it’s been a mess for however long the mine’s been here. Decades. But there’s a certain amount of gratification being able to make it, you know, making it all nice again. It’s like, yes, you’re building a park.”

So that’s where the dirt goes from Seattle’s big construction projects.

It’s part of a cycle.

We dig holes in the county because we need materials so we can build things in the city.

And then we use the dirt from the city to fill those holes back up again.

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