As much as I love infrastructure and the urban
environment, it definitely has its downsides. Cities represent a remarkable transformation
of the landscape from natural to human made. We change almost everything: cut down trees,
level the ground, and slice and dice the land into individual plots. But one of the most significant changes to
the landscape that comes with urbanization is impervious cover. I’m talking about anything that prevents
rain from soaking into the subsurface: buildings, sidewalks, driveways, and the biggest culprits
- streets and parking lots. Impervious cover is a big issue. When it rains, that water has to go somewhere. If it can’t soak into the ground, it washes
off into creeks and rivers. That means increasing the magnitude of floods
and the amount of pollution in waterways. It also means less water goes to recharge
groundwater resources. When you pave paradise to put up a parking
lot, you cause a pretty significant disruption to some really important natural processes
in a watershed. But, not all cover has to be impervious. Hey, I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about
permeable pavement. This video is sponsored by HelloFresh, America’s
#1 Meal Kit. More on that later. Management of stormwater in urban areas is
a vast field of study. Pretty much since humanity started building
stuff, we also started building ways to keep that stuff dry. Traditional engineering had a single goal
in mind - get stormwater off of the streets and property and into a creek, ditch, or river
as quickly as possible. It’s not hard to see the problem with this
strategy. Every new road and building means a higher
volume of runoff in the waterways during a storm event. As cities grew, flooding problems became more
severe and more frequent, streams were eroded, and receiving waterways were polluted. So, over time, municipalities adopted rules
to try and curb these problems, focusing primarily on flooding. Now, in nearly every large city (at least
in the U.S.), land developers are required in some way or another to make sure their
projects won’t worsen downstream flooding. The traditional solution to this is control
of flood peaks through onsite detention: essentially having a small pond to store runoff during
a storm, allowing gradual release to mitigate flooding. Detention and retention ponds have a lot of
complexity and deserve their own separate video. They definitely help reduce flooding, but
they don’t really replace the other functions of the natural landscape: the filtration and
reduction of runoff volume that comes from water infiltrating into the ground. Also, these basins are usually pretty ugly
and kind of gross, since they concentrate polluted runoff in one mucky area, and beauty
is already in short supply in many urban areas. For all these reasons, cities are encouraging
(and sometimes requiring) developers to take even greater responsibility for impacts on
the natural landscape through a process called Low Impact Design, or just LID. LID practices are ways to integrate stormwater
management as a part of land development and mimic natural hydrologic processes. There is a considerable variety of LID strategies
that help manage urban stormwater, reduce erosion, minimize pollution, and help with
flooding. These are things like rain gardens, green
roofs, and vegetated filter strips. If you live in a big city, there’s a good
chance your municipality has a manual describing the strategies that work best for your area. One of my favorites of these addresses the
problem at its root: just make the cover less impervious. Pavement serves a vital role in a city. A quick glance at the condition of dirt roads
after a good rain is all you need to understand this. Pavement equals accessibility. In most places, the soil making up the ground
isn’t a stable, durable surface for people to walk, roll, scoot, or drive. Particularly when the earth gets wet, it loses
strength and turns to muck. You can see why we normally prefer pavement
to be impermeable to water. Pavements protect against erosion and weakening
of the soil. A poorly designed pervious pavement works
about as well as if it wasn’t paved at all, since it doesn’t provide any protection
against water. If you watched my previous video on potholes,
you know the cruel fate of pavement that inadvertently lets water through. So, how is it possible to achieve the good
parts without the bad, to allow water to infiltrate into the subsurface through a pavement without
softening and weakening its foundation? Luckily we have a pretty good example to help
understand how this works. Some might even call it the OG permeable pavement. I’m talking about steel grating. You’ve almost certainly seen grating used
on roads, sidewalks, or other surfaces to allow water in while keeping most everything
else out. We can do precisely the same thing with traditional
pavement as well. I have the ingredients for concrete laid out
here in my shop, or at least a type of concrete. If you’ve seen my previous videos on this
topic, you’ll know that concrete is a mix of cement, rocks, sand, and water. If you leave out the sand, you get something
really cool: a material that behaves almost exactly like regular concrete, but that is
full of voids and holes that can let water pass through. This is a really cool effect that is almost
an optical illusion. Our brains are so used to seeing water runoff
a paved surface, they almost can’t make sense when it flows straight through. This has led to quite a few viral clips of
water disappearing into parking lots or roadways. And this isn’t just possible with concrete. Asphalt can be made similarly porous, along
with different kinds of pavers. The permeability of the pavement isn’t the
end of the story, though. Going back to our permeable pavement proxy,
steel grates don’t just sit directly on the ground. Look through, and you’ll see, the water
passing through has to have somewhere to go. Soil usually can’t absorb 100 percent of
the water when it rains. If it could, we’d never have any runoff
and hardly ever any floods. That means, even if we can get rain to percolate
through pavement, it needs somewhere to go after that. The pavement itself gets all the glory, but
the real workhorse of a permeable pavement system is the reservoir below. This is generally made from a layer of stones
of uniform size to create voids that temporarily store water coming through the surface pavement. The design of the stone reservoir is just
as crucial as the pavement above because it depends on how much water must be stored and
how quickly that water can infiltrate into the ground. Both of these require careful engineering. For certain types of impermeable soils, like
clay, it may not be feasible to try and get all that water to infiltrate, so some permeable
pavements work like detention ponds, where the water is stored temporarily and released
gradually over time through drains. Whether it soaks into the ground or is discharged
into a waterway little by little, the permeable pavement has made a considerable improvement
over the alternative of having rainwater wash right off the surface. This is a really helpful strategy to address
stormwater in urban areas, but it’s not without challenges. Most importantly, permeable pavement isn’t
that strong. If you make concrete or asphalt with a bunch
of holes and voids, it makes sense that it probably can’t hold up the weight of traditional
mixes. That’s why we really don’t use these systems
in areas with heavy traffic. Permeable pavements are mainly relegated to
parking lots and road shoulders. But we also need to keep them away from buildings
where you don’t really want a lot of water soaking into the foundation soils. And we can’t use them on slopes either,
because the stored water would just flow along the slope through the reservoir and eventually
back out, rather than staying in storage. The pavement itself can be clogged by dirt
and leaves over time, so it has to be swept or washed regularly to remain permeable. Finally, although they help snow and ice melt
faster naturally, using porous pavements in colder climates requires special consideration
to avoid damage from freezing water and deicing salts. Even given its simplicity and use over the
past few decades, permeable pavement is still a fairly new and innovative way to manage
urban stormwater. There’s still a lot to learn about how to
implement it effectively and efficiently. It’s a great example of using engineering
to try and bring more harmony between constructed and natural environments. It’s time again for everyone’s favorite
segment of watching me try to cook while my wife tries to capture that on video. And we’ve got another helper in the kitchen
now! We’re not visiting a lot of restaurants
right now (for obvious reasons) so, we’re really thankful for HelloFresh, the sponsor
of this video, for converting cooking from a chore into our favorite thing to do on date
night. “Do you think my camera’s going in slow
motion or is that how slow you chop shallots?” HelloFresh delivers seasonal recipes and pre-measured
ingredients right to your door. We get the vegetarian meals, even though we
aren’t vegetarians, just to get some exposure to new recipes we’ve never tried before. They’re always a delicious way to break
out of recipe ruts. “Seems really dry in there.” “You have to toast it” The pre-portioned ingredients mean there’s
less prep and less food waste, and the packaging is mostly recyclable or already recycled content. HelloFresh also helps us get dinner on the
table quickly on the days we just don’t have time for planning, prep, and shopping,
especially now that we have this one to feed as well. It’s always great even when I don’t focus
as hard on the presentation. “I believe the directions said to drizzle
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details. “Tell me when you want to start recording…” That’s HelloFresh.com and use code 80PRACTICAL. Thank you for watching, and let me know what
you think.
I did my capstone report on porous concrete v asphalt for a bus turn around. Long story short: it is prone to ravelling and most of all hella expensive. I live in Canada as well so the freeze/thaw cycles aren’t really conducive for this. It’s cool but largely impractical.
We did a bit of research into this material for parking lots and came up with the following issues.
Compressive strength is significantly lower than normal concrete typically use 4ksi but this stuff was sub 2ksi so it would have issues with cracking, gouging and tear out. Also due to being porous the rebar quickly rusted away by end of year 1.
The advantage that it permits water to flow through it quickly results in washout of the paver base which can lead to sink holes under the concrete.
The fact that the material is porous makes cleanup of spills impossible. Add to that, power washing of this material is destructive even at low pressures for this process.
Frost heave and fracture will happen if the material is wet and it freezes. Our 4x4 test pad was in chunks after one year in the NW Indiana area.
One good rain/freeze is all that stands between a highway and a gravel road.
I work for a company that actually places this material. We call it cement treaded permeable base (CTPB). We place it on asphalt, then the permeable material, and then pave concrete over it. Since it is a base, it doesn’t see any traffic, can’t get dirty, and conveys the water that penetrates the concrete as desired.
Maybe not great for roads but I put down a permeable paver system in my backyard patio instead of pavestone and it's great! Also cheaper. Also, didn't kill my trees.
Well it's a cool idea my coworker installed some of this and said it was a pain. Maintenance cost is also high because it has to be vacuumed regularly.
Woow cool!
1min of reading comments later...
Alright it sucks
Practical Engineering isn't as entertaining as Smarter Every Day, but damn is it more informative. Seriously great channel.
I do long term planning of roads as my job. And we're pretty skeptical of all these "innovative" materials. Plastic and other recycled materials seem to be a popular idea. I honestly have never heard of this, but on low strength subgrades I am dubious as to how it performs.
Myself and my colleagues generally just favour sticking with regular asphalt and more proven pavement designs. We already have a difficult time trying to get a solid 20 years out of road surfaces, and corporately they reckon we can get 80 to 120 years out of our pavements, although few are older than 60 years. This design I think has very limited situations which he did hint at.