If you’ve ever driven or ridden in an automobile,
there’s a near 100% chance you’ve hit a bump in the road as you transition onto or off of a
bridge. In fact, some studies estimate that it happens on a quarter of all bridges in the US!
It’s dangerous to drivers and expensive to fix, but the reason it happens isn’t too complicated to
understand. It’s a tale (almost) as old as time: You need a bridge to pass over
another road or highway. But, you need a way to get vehicles from ground level
up to the bridge. So, you design an embankment, a compacted pile of soil that can be
paved into a ramp up to the bridge. But, here’s the problem. Even though the bridge
and embankment sit right next to each other, they are entirely different structures with
entirely different structural behavior. A bridge is often relatively lightweight and supported on
a rigid foundation like piles driven or drilled deep into the ground. An embankment is - if the
geotechnical engineers will forgive me for saying it - essentially just a heavy pile of dirt.
And when you put heavy stuff on the ground, particularly in places that have naturally soft
soils like swamps and coastal plains, the ground settles as a result. If the bridge doesn’t settle
as much or at the same rate, you end up with a bump. Over the years, engineers have come up with
a lot of creative ways to mitigate the settlement of heavy stuff on soft soils, but one of those
solutions seems so simple, that it’s almost unbelievable: just make embankments less heavy.
Let’s talk about some of the bizarre materials we can use to reduce weight, and a few of the reasons
it’s not quite as simple as it sounds. I’m Grady and this is Practical Engineering. In today’s
episode, we’re talking about lightweight fills. This video is sponsored by
HelloFresh. More on them later. The Latin phrase for dry land, “terra firma,”
literally translates to firm earth. It’s ingrained in us that the ground is a solid entity
below our feet, but geotechnical engineers know better. The things we build often exceed the
earth’s capacity to withstand their weight, at least not without some help. Ground
modification is the technical term for all the ways we assist the natural soil’s ability
to bear imposed loads, and I’ve covered quite a few of them in previous videos, including
vertical drains that help water leave the soil; surcharge loading to speed up settlement so it
happens during construction instead of afterwards; soil nails used to stabilize slopes;
and one of the first videos I ever made: the use of reinforcing elements to create
mechanically stabilized earth walls. One of the simplest definitions of
design engineering is just making sure that the loads don’t exceed the strength
of the material in question. If they do, we call it a failure. A failure can
be a catastrophic loss of function, like a collapse. But a failure can also be a loss
of serviceability, like a road that becomes too rough or a bridge approach that develops a major
bump. Ground modification techniques mostly focus on increasing the strength of the underlying soil,
but one technique instead involves decreasing the loads, allowing engineers to accept the
natural resistance of a soft foundation. Let me put you in a hypothetical situation
to give you a sense of how this works: Imagine you’re a transportation engineer working
on a new highway bridge that will replace an at-grade intersection that uses a traffic
signal, allowing vehicles on the highway to bypass the intersection. This is already a busy
intersection, hence the need for the bypass, and now you’re going to mess it all up with a
bunch of construction. You design the embankments that lead up to the bridge to be built from
engineered fill - a strong soil material that’s about as inexpensive as construction gets. You
hand the design off to your geotechnical engineer, and they come back with this graph: a plot
of settlement over time. Let’s just say you want to limit the settlement of the
embankment to 2 inches or 5 centimeters after construction is complete. That’s a pretty
small bump. This graph says that, to do that, you’ll have to let your new embankment sit and
settle for about 3 years before you pave the road and open the bridge. If you put this up on
a powerpoint slide at a public meeting in front of all the people who use this intersection on
a daily basis, what do you think they’ll say? Most likely they’re going to ask you to find
a way to speed up the process (politely or otherwise). From what I can tell from my inbox,
a construction site where no one’s doing any work is a commuter’s biggest pet peeve. So, you
start looking for alternative designs and you remember a key fact about roadway embankments:
the weight of the traffic on the road is only a small part of the total load experienced by
the natural ground. Most of the weight is the embankment itself. Soil is heavy. They teach us
that in college. So what if you could replace it with something else? In fact, there is
a litany of granular material that might be used in a roadway embankment instead of
soil to reduce the loading on the foundation, and all of them have unique engineering properties
(in other words, advantages, and disadvantages). Wood fibers have been used for many years as
a lightweight fill with a surprisingly robust service life of around 50 years before
the organic material decays. Similarly, roadway embankments have been seen as a popular
way to reuse waste materials. In particular, the State of New York has used shredded
tires as a lightweight fill with success, so far avoiding the spontaneous combustions that
have happened in other states. There are also some very interesting materials that are manufactured
specifically to be used as lightweight fills. Expanded shale and clay aggregates are formed
by heating raw materials in a rotary kiln to temperatures above 1000 celsius. The
gasses in the clay or shale expand, forming thousands of tiny bubbles. The aggregate
comes out of the kiln in this round shape, and it has a lot of uses outside heavy civil construction
like insulation, filtration, and growing media for plants. But round particles like this don’t work
well as backfill because they don’t interlock. So, most manufacturers send the aggregate through
a final crushing and screening process before the material is shipped out. Another manufactured
lightweight fill is foamed glass aggregate. This is created in a similar way to the expanded shale
where heating the raw material plus a foaming agent creates tiny bubbles. When the foamed glass
exits the kiln, it is quickly cooled, causing it to naturally break up into aggregate sized pieces.
You can see in my graduated cylinders here that I have one pound or about half a kilogram of
soil, sand, and gravel. It takes about twice as much expanded shale aggregate to make up that
weight since its bulk density is about half that of traditional embankment building materials.
And the foamed glass aggregate is even lighter. All these different lightweight fills can be used
to reduce the loading on soft soils below roadways and protect underground utilities from damage,
but they also have a major advantage when used with retaining walls: reduced lateral pressure.
I’ve covered retaining walls in a previous video, so check that out after this if you want to learn
more, but here’s an overview. Granular materials like soil aren’t stable on steep slopes, so
we often build walls meant to hold them back, usually to take fuller advantage of a site
by creating more usable spaces. Retaining walls are everywhere if you know where to
look, but they also represent one of the most underappreciated challenges in civil
engineering. Even though soil doesn’t flow quite as easily as water does, it is around
twice as dense. That means building a wall to hold back soil is essentially like building a
dam. The force of that soil against the wall, called lateral earth pressure, can be
enormous, and it’s proportional both to the height of the wall and the density of
the material it holds back. Here’s an example: When Port Canaveral in Florida decided to expand
terminal 3 to accommodate larger cruise ships, they knew they would need not only a new passenger
terminal building but also a truly colossal retaining wall to form the wharf. The engineers
were tasked with designing a wall that would be around 50 feet (or 15 meters) tall to allow the
enormous cruise ships to dock directly alongside the wharf. The port already had stockpiles
of soil leftover from previous projects, so the new retaining wall would get its
backfill for free. But, holding back 50 feet of heavy fill material is not a simple
task. The engineers proposed a combi-wall system that is made from steel sheet piles supported
between large pipe piles for added stiffness, in addition to a complex tie-back structure to
provide additional support at the top of the wall. When the design team considered using lightweight
fill behind the retaining wall, they calculated that they could significantly reduce the size of
the piles of the combi-wall, use a more-commonly available grade of steel instead of the specialty
material, and simplify the tie-back system. Even though the lightweight fill was
significantly more expensive than the free backfill available at the site, it still
saved the project about $3 million dollars compared to the original design. The fill at
Port Canaveral (and all the lightweight fills we’ve discussed so far) are granular materials
that essentially behave like normal soil, sand, or gravel fills (just with a lower density). They
still have to be handled, placed, and compacted to create an embankment or retaining wall backfill
just like any typical earthwork project. But, there are a couple of lightweight fills
that are installed much differently. Concrete can also be made lightweight using some
of the aggregates mentioned earlier in place of normal stone and sand, or by injecting foam
into the mix, often called cellular concrete. On projects where it’s difficult or time
consuming to place and compact granular fill, you can just pump this stuff right out of a hose
and place it right where it needs to be, speeding up construction and eliminating the need for lots
of heavy equipment. There are a few companies that make cellular concrete, and they can tailor the
mix to be as strong or lightweight as needed for the project. You can even get concrete with
less density than water, meaning it floats! This test cylinder was graciously provided
by Cell-Crete so I could give you a close up look at how the product behaves. Of course
we should try and break it. Let’s put it under the hydraulic press and see how much force it
takes. The pressure gauges on my press showed a force of just under a ton to break this
sample. That is equivalent to a pressure of around 200 psi or 1.4 megapascals, much
stronger than most structural backfills. You’re not going to be making skyscraper frames
or bridge girders from cellular concrete, but it’s more than strong enough to hold
up to traffic loads without imposing tons of weight into a retaining wall or
the soft soils below an embankment. The last lightweight fill used in heavy civil
construction is also the most surprising: expanded polystyrene foam, also known
as EPS and colloquially as styrofoam. When used in construction, it’s often
called geofoam, but it’s the same stuff that makes up your disposable coffee cups,
mannequin heads, and packaging material. EPS seems insubstantial because of its weight,
but it’s actually a pretty strong material in compression. About 7 years ago I used my car
to demonstrate the compressive strength of mechanically stabilized earth. Well, I still
have that jack and I still drive that car, so let’s try the experiment with EPS foam. This is probably around 5 to 600 pounds, and there is some deflection, but the
block isn’t struggling to hold the weight. In an actual embankment, the
pavement spreads out traffic loads so they aren’t concentrated like
what’s shown in my demonstration to the point where you would never know
that you’re driving on styrofoam. EPS foam has some cool benefits, including how
easy it is to place. The blocks can be lifted by a single worker, placed in most weather conditions,
don’t require compaction or heavy equipment, and can be shaped as needed using hot wires. But
it has some downsides too. This material won’t work well for embankments that see standing water
or high groundwater, because of the buoyancy. The embankment could literally float away. They’re
also so lightweight that you have to consider a new force that most highway engineers don’t think
about when designing embankments: the wind. Also, because EPS foam is such a good insulator,
it creates a thermal disconnect between the pavement and the underlying ground, making
the road more susceptible to icing. Finally, EPS foam has a weakness to a substance that
is pretty regularly spilled onto roadways: it dissolves in fuel. If a crash, spill,
or leak were to happen on an embankment that uses EPS foam without a properly designed
barrier, the whole thing could just melt away. Even with all those considerations, EPS foam is a
popular choice for lightweight fills. We even have a nice government report on best practices
called Guideline and Recommended Standard for Geofoam Applications in Highway Embankments
(if you’re looking for some lightweight bedtime reading). It was used extensively in Seattle on
the replacement of the Alaskan Way Viaduct to avoid overstressing the landfill materials that
underlie major parts of the city. Thousands of drivers in Seattle and millions of people around
the world drive over lightweight embankments, probably without any knowledge of what’s below
the pavement. But the next time you pass over a bridge and don’t feel a bump transitioning
between the deck and roadway embankments, it might just be lightweight aggregate,
cellular concrete, or geofoam below your tires working to make our infrastructure as
cost-effective and long-lasting as possible. Speaking of lightweight, one of my goals in
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