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Pikes/Pines | Hitting the pavement for a geological trip through the natural history and present day questions of the Capitol Hill streets and sidewalks beneath our feet

Ready for shipping from CalPortland’s Ready Mix and Pioneer Aggregates Plant in DuPont (Image: Cal Portland)

I don’t know about you, but I rarely consider the streets and sidewalks I travel over unless they’re an impediment. Biking around Seattle I know where to veer past a specific pothole. I’ve found myself in a groggy rage having spilled coffee down my sleeve, tripped by a tree root uplifted section of sidewalk. My car is old and I know when a road is equally as pocked by time.

And yet, it’s easy to just feel as if roads happen (if one ignores the traffic revisions that we endure for years). A good number of folks reading this do not remember a time when new roads were built on or adjacent to the Hill. They were just there and unless you are a civil engineer, an urban planner, or a mass transit or bicycling advocate, you might not have considered them either.

Sometimes roads take us in directions we hadn’t considered. When I first pondered the natural history of roads, I had this quaint idea of delving into what grows in the cracks of the concrete. There are surely compelling stories here, but really, you can figure it out: roads are made of earthly materials and plants grow in and wear at said medium with their roots, which combine forces with other types of weathering. We’ve all probably seen a bokeh image of a tree growing out of rock in some misty locale. Give some seeds a few years without any bother and our streets and sidewalks would quickly begin weathering away, all manner of vegetation sprouting from the cracks.

Ultimately, I realized I didn’t really know what roads are made of. Where did the material come from? What are the environmental costs of putting in and maintaining roads? How long does a road last? These are all questions I recently put forward to folks at the Seattle Department of Transportation who endured such infantile questions about our city streets with grace.

The DuPont plant is not exactly pretty to look at (Image: Cal Portland)

Civil servants who work on our thoroughfares have a lot to say about all aspects of questions pertaining to roads. For instance, they are happy to stretch on about what they are made of, which turn out to be two main products. Asphalt, which is a crude oil derivative, and Portland cement, which can simplistically be viewed as glued together rocks. However, we live in a world where consumers are disconnected from resources, so as users of the product, we should also probably know the source.

Happily Steven Read, of the Seattle Public Utilities Materials Laboratory, had answers.

The vast majority of raw material for roads come from the Cal Portland Pioneer Gravel mine in DuPont, Washington, or Texada Island in British Columbia. According to Read, the glacial heritage of our region is a blessing for roads, giving us easy access to a perfect key ingredient for making portland cement: gravel created and deposited by the ice sheets that once sat a mile above your head. Both these gravel pits are cost effective to source from, both monetarily and environmentally, because you can barge directly to Seattle from either one (instead of say trucking it all).

Read was quick to point out that gravel mines are not nice to look at (a casual observer on Google Maps will quickly pick them out), and that they don’t have great environmental track records (see the history of the Maury Island gravel mine for some context). As he put it, “the mines we have are the mines we’ll get to keep using.” They are problematic even as they are a necessity for how we are currently doing things.

It’s easy to over-intellectualize the environmental impacts of roads and pick at details. That’s exactly what the public pays folks at SDOT to do (and it’s an increasingly enormous, underfunded job in our growing region). But the bottom line is that roads are there for cars, trucks, and other carbon emitting vehicles, many with single occupants, (and some dashing about the Hill delivering single cans of Coke to people who just can’t be bothered to step away from their screens). And then there’s the actual materials being used.

Even if asphalt is readily recycled (every industry website quickly points this out), it’s made of the twisted, time crunched bodies of ancient life, aka crude oil. The closest and most numerous deposits that contribute the crude oil to make asphalt are the Canadian tar sands, which if you are even slightly aware should set off alarm bells. Concrete production is one of the most environmentally impactful industries in the world; both in terms of energy production, (according to Read producing a pound of concrete emits about a pound of carbon) and the physical disruption of mining. And yet, even if you don’t drive, those busses you rely on have a heavy impact on roads, and only walking or biking still rely on those surfaces and their upkeep.

And as it turns out roads are expensive to maintain and don’t last forever. When I asked about the lifespan of a typical Seattle road, civil engineer Ben Hansen said it was around 50 years for concrete pavement. That is longer than I’ve been alive, but that doesn’t sound terribly long for something that seems so durable nor so environmentally costly. This has pressed the people who build roads to try using recycled concrete and other materials, which can have mixed results. From the point of view of someone like Read, recycling might not always be worth the effort. If you end up with a less durable surface that needs to be repaired more often or entirely replaced, the numbers might not add up. As with most things I start to ponder, roads are a lot easier to just drive down without a care in the world.

Emily Burns, SDOT Pavement Engineering Management Section Manager, made a point along these lines. Roads and sidewalks are publicly funded assets that can be taken for granted. There’s complexity in how they get made, who is supposed to take care of them, and how we get trees, pedestrians, bicycles, mass transit, cars, trucks, private businesses, single family homes, apartment complexes, and other government entities to play nice together. As Hansen put it, all these entities are vying for space that we aren’t making any more of. No one is making more space.

Which brings me back around to my initial thoughts around plants growing in the cracks. This is my comfort area because this column likes weeds but not necessarily getting lost in the tall, complex public policy kind — even if they are important to the wellbeing of all.

After talking with the folks at SDOT, I started my commute home from work by walking down a busy Seattle street. As I walked, I looked down at material deeply tied to our geological heritage.

Looking down at what grows between the cracks. (Image: Brendan McGarry)

My feet were pounding over a mix that likely included gravel crushed and thrust into piles by the leading edge of continental ice sheets. And in the chinks between this armoring were chickweed, dandelion, and speedwell. Welcome or not, they were trudging down the endless path of trying to root where they fall as seeds, in this case into a seemingly unyielding medium. And yet when the glaciers receded, this part of the world was probably just as bleak, just as hard and cold and life sprang back readily. The seeds of street trees overhead were down there too, nestled amidst lichen and moss, waiting for soil to be built by decaying sluff settled in the cracks. Though it’s often overstated, nature is everywhere, we just have to apply the right lens and travel down the right road, even if it leads to some uneasy answers.


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6 months ago

I’m curious about the original brick pavement in Seattle. Is it all still there, for the most part, underneath the asphalt roads?

6 months ago
Reply to  rebeccabush

As society transitioned from horses and wagons to automobiles in the first half of the 20th century, brick (and cobblestone/sett) roadways were found lacking for ride and traction. Structurally, though, they were still adequate. As paving materials technology developed, the bricks and cobbles were surfaced over with a smooth layer of asphaltic concrete more suitable to modern vehicles.

Many streets in Seattle still have their original brick base covered over by a layer of asphalt. Parts of Madison St and 12th Ave are examples streets on the Hill that retain large portions their original brick under the asphalt.

However, many old brick based streets are now reaching the end of their structural life with heavy bus and truck loading. When that happens, the pavement must be rebuilt from the ground up. An example of that process is 23rd Ave in the Central District, which was recently reconstructed in in Portand cement concrete.

Julie by the park
Julie by the park
6 months ago

Thanks for enlightening me on something I take for granted daily. An excellent read!

Concrete Jungler
Concrete Jungler
6 months ago

Thank you for this! Insightful and poetic, great way to start my Monday morning. Would love to see more of this content!