Why Some Roads Are Made of Styrofoam

Summary

This video explores the issue of bumps that often occur at the transition between roads and bridges, a common problem due to differences in how embankments and bridges settle over time. The video discusses various lightweight fill materials, such as expanded shale, foamed glass aggregate, and EPS foam, which are used to reduce the load on soft soils and prevent settlement. The video also highlights the advantages and challenges of using these materials in construction, emphasizing the importance of innovative solutions in civil engineering.

Takeaways

• 🌉 The bump often experienced when driving on or off a bridge is a common issue affecting nearly a quarter of US bridges and is caused by differential settlement between the bridge and the embankment.
• 🚧 Engineers have developed various ground modification techniques to manage the weight of structures on soft soils, with one approach being the use of lightweight fills to reduce the load on the ground.
• 🌲 Wood fibers are a type of lightweight fill that can last up to 50 years before decaying, offering a robust solution for reducing the weight of embankments.
• 🔥 Shredded tires have been successfully used as a lightweight fill in New York, reusing waste material and avoiding issues like spontaneous combustion seen in other states.
• 🏗️ Expanded shale and clay aggregates, as well as foamed glass aggregate, are manufactured lightweight fills that are created by heating materials to form tiny bubbles, reducing their density.
• 🛑 Retaining walls often require the use of lightweight fills to reduce the lateral pressure exerted by the backfill, which can be significant and proportional to the wall's height and the material's density.
• 🚧 The use of lightweight fill in the Port Canaveral expansion project saved approximately $3 million by reducing the size of the required piles and simplifying the structure.
• 🏗️ Cellular concrete, or lightweight concrete, can be pumped directly into place, speeding up construction and reducing the need for heavy equipment, making it suitable for difficult or time-consuming projects.
• 📦 Expanded polystyrene foam, or geofoam, is a lightweight fill material that is easy to handle and place but must be used with caution due to its buoyancy, susceptibility to wind, and reaction to fuel.
• 🌿 Lightweight fills like geofoam are used in various construction projects worldwide, contributing to cost-effective and long-lasting infrastructure, often without the drivers' awareness.

Q & A

What is the common issue faced by drivers when transitioning onto or off of a bridge?

The common issue is hitting a bump in the road, which can be dangerous and expensive to fix. This bump is often caused by the differential settlement between the bridge and the embankment.

Why do bridges and embankments settle differently?

Bridges and embankments settle differently because they are structurally distinct. Bridges are relatively lightweight and supported on a rigid foundation, while embankments are essentially heavy piles of soil that can cause the ground to settle, especially on soft soils.

What is one simple solution engineers have proposed to mitigate the settlement of embankments?

One simple solution is to make embankments less heavy by using lightweight fills, which reduces the load on the foundation and can help prevent the formation of bumps at bridge transitions.

What are some examples of materials used as lightweight fills in embankments?

Examples of lightweight fill materials include wood fibers, shredded tires, expanded shale and clay aggregates, foamed glass aggregate, cellular concrete, and expanded polystyrene foam (EPS or geofoam).

How do lightweight fills benefit retaining walls?

Lightweight fills reduce lateral pressure on retaining walls, which can simplify the design and construction of these walls, and potentially reduce the size of the required structural elements.

What is the advantage of using cellular concrete as a lightweight fill?

Cellular concrete can be pumped directly to where it needs to be, speeding up construction and eliminating the need for heavy equipment. It is also strong enough to handle traffic loads without imposing excessive weight.

What are the potential drawbacks of using expanded polystyrene foam (EPS) in embankments?

EPS foam may not be suitable for areas with standing water or high groundwater due to buoyancy issues, it is affected by wind due to its light weight, and it can be susceptible to damage from fuel spills due to its solubility in fuel.

How does the use of lightweight fills impact the cost and effectiveness of infrastructure projects?

While lightweight fills can be more expensive than traditional materials, they can save money on infrastructure projects by reducing the size and complexity of structural elements, thus making the infrastructure more cost-effective and long-lasting.

What is the significance of the graph that the geotechnical engineer provides in the hypothetical situation?

The graph represents the settlement of the embankment over time and is used to determine how long the embankment must settle before paving the road and opening the bridge, which is crucial for planning construction timelines and minimizing disruptions.

Why might a transportation engineer consider using lightweight fills for a new highway bridge project?

A transportation engineer might consider using lightweight fills to reduce the settlement time of the embankments, allowing for a quicker construction process and less disruption to the existing traffic and community.

Outlines

00:00

🌉 The Bridge Bump Problem

The paragraph discusses the common issue of encountering bumps when driving on or off bridges, which affects nearly a quarter of all bridges in the US. The root cause is the differential settlement between the bridge and the embankment leading up to it. Bridges are typically lightweight and supported by rigid foundations, while embankments are heavy structures made of compacted soil. When these two structures settle at different rates, especially on soft soils, bumps form. Engineers have devised various methods to mitigate this issue, one of which is to reduce the weight of the embankment by using lightweight fills. The video introduces the concept of lightweight fills and sets the stage for discussing various materials and techniques used to address this problem.

05:02

🏗️ Innovative Lightweight Fills in Civil Engineering

This paragraph delves into the use of lightweight materials as fills in civil engineering to reduce the load on soft soils and prevent the formation of bumps at bridge transitions. It mentions wood fibers and shredded tires as examples of organic and waste materials that have been repurposed as lightweight fills with a service life of around 50 years. The paragraph also introduces manufactured lightweight fills such as expanded shale and clay aggregates, and foamed glass aggregate, which are created by heating materials to form tiny bubbles that reduce density. These materials are used to lessen the load on foundations, protect underground utilities, and reduce lateral pressure on retaining walls. A case study of Port Canaveral's expansion project in Florida illustrates how using lightweight fill can lead to significant cost savings despite the higher upfront cost of the material.

10:08

🚧 Advanced Lightweight Fills and Their Applications

The paragraph explores advanced lightweight fills like cellular concrete and expanded polystyrene foam (EPS or geofoam), which are used in heavy civil construction. Cellular concrete, made by replacing traditional aggregates with lightweight ones or by injecting foam, can be pumped directly into place, speeding up construction and reducing the need for heavy equipment. EPS foam, commonly known as styrofoam, is strong in compression and easy to handle, making it a popular choice for lightweight fills despite its susceptibility to buoyancy, wind, thermal disconnection, and dissolution in fuel. The paragraph highlights the use of these materials in real-world applications, such as the Alaskan Way Viaduct replacement in Seattle, emphasizing their role in creating cost-effective and long-lasting infrastructure.

15:14

🍽️ HelloFresh Sponsorship and Practical Engineering

The final paragraph shifts focus from civil engineering to the sponsorship of the video by HelloFresh, a meal kit delivery service. It discusses the personal experience of the video's host, Grady, and his wife using HelloFresh, turning cooking into a date night activity. The paragraph promotes HelloFresh's new 'Fast & Fresh' line, which offers quick and easy meal preparation, and provides a discount code for viewers to try the service. The sponsorship is woven into the video's narrative, highlighting the practicality and convenience that both the engineering solutions and HelloFresh's meal kits provide in their respective domains.

Keywords

💡Settlement

Settlement in the context of the video refers to the process of the ground or soil compacting or sinking over time, often due to the weight of structures or fill materials placed on it. This is a critical concept as it leads to the formation of bumps at bridge transitions, which can be dangerous and costly to repair. The video discusses various methods to mitigate settlement, particularly in areas with soft soils, to ensure the stability and longevity of infrastructure.

💡Embankment

An embankment is a man-made mound of earth and rock, typically used to support roads, railways, or bridges at a particular elevation. In the video, embankments are highlighted as areas where the ground settles, causing bumps when the bridge does not settle at the same rate. The discussion revolves around how to design embankments to minimize this settlement, especially when built on soft soils.

💡Geotechnical Engineering

Geotechnical engineering is the branch of civil engineering concerned with the engineering behavior of earth materials. In the video, geotechnical engineers are mentioned as professionals who understand the complexities of soil and its ability to support structures. They play a crucial role in designing solutions to prevent settlement and ensure the stability of embankments and bridges.

💡Lightweight Fills

Lightweight fills are materials used in construction that have a lower density than traditional soil or gravel, reducing the load on the ground and thus minimizing settlement. The video explores various types of lightweight fills, such as wood fibers, shredded tires, expanded shale, and foamed glass aggregate, which are used to reduce the weight of embankments and improve the performance of infrastructure.

💡Retaining Walls

Retaining walls are structures that hold back soil or rock to create level areas for construction, such as building pads, terraces, or walkways. The video discusses how lightweight fills can reduce the lateral pressure exerted by the soil against these walls, making the construction process more feasible and cost-effective.

💡Lateral Earth Pressure

Lateral earth pressure is the force exerted by soil on retaining walls or structures, which can be significant and is proportional to the wall's height and the material's density. The video explains how using lightweight fills can reduce this pressure, simplifying the design of retaining walls and making them more stable.

💡Cellular Concrete

Cellular concrete, also known as lightweight concrete, is a construction material made by replacing some of the stone and sand with lightweight aggregates or by injecting foam into the mix. The video mentions that this material can be pumped directly into place, speeding up construction and reducing the need for heavy equipment, making it an efficient choice for certain projects.

💡Expanded Polystyrene Foam (EPS)

Expanded polystyrene foam, or EPS, commonly known as styrofoam, is a lightweight material used in construction as a geofoam. The video highlights its use in embankments for its ease of placement and塑形能力, but also discusses its limitations, such as susceptibility to buoyancy, wind, and damage from fuel spills.

💡Ground Modification

Ground modification techniques are methods used to improve the load-bearing capacity of soil, such as vertical drains, surcharge loading, and soil nails. The video emphasizes that while most of these techniques aim to strengthen the soil, using lightweight fills is a strategy to reduce the load itself, accepting the natural resistance of soft foundations.

💡Serviceability

Serviceability in engineering refers to the ability of a structure to perform its intended function without significant distress or discomfort to users. The video discusses how excessive settlement can lead to a loss of serviceability, such as a bumpy road or bridge approach, which is undesirable and can be mitigated through the use of lightweight fills.

Highlights

Nearly 100% of drivers experience a bump when transitioning onto or off a bridge due to settlement issues.

About a quarter of all bridges in the US have settlement issues at the approaches.

Bridges and embankments, despite being adjacent, behave differently structurally, leading to differential settlement.

Embankments are essentially heavy piles of dirt, which can cause the ground to settle, especially on soft soils.

Engineers have developed creative methods to mitigate settlement, including using lightweight materials for embankments.

Lightweight fills can reduce the load on soft soils, preventing bumps and improving road stability.

Wood fibers are used as a lightweight fill with a service life of around 50 years before decay.

Shredded tires have been successfully used as a lightweight fill in New York, avoiding spontaneous combustion issues.

Expanded shale and clay aggregates are manufactured lightweight fills created by heating raw materials to form tiny bubbles.

Foamed glass aggregate is made by heating raw material with a foaming agent to create bubbles, then cooled to form aggregate.

Lightweight fills reduce lateral pressure on retaining walls, simplifying design and construction.

Cellular concrete is a lightweight concrete that can be pumped and placed, speeding up construction.

Expanded polystyrene foam (EPS or geofoam) is used as a lightweight fill, despite its susceptibility to buoyancy and wind.

EPS foam is strong in compression and can support significant weight, such as a car.

Lightweight fills like EPS foam are popular for their ease of placement and cost-effectiveness in certain applications.

Despite potential issues with buoyancy, wind, and chemical sensitivity, EPS foam is used extensively in major projects like in Seattle.

Lightweight fills contribute to cost-effective and long-lasting infrastructure, improving the driving experience.

Transcripts

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  2023 is to keep eating healthy meals for dinner.  

That’s a lot easier with HelloFresh,  sponsor of this channel since 2019,  

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something fun and new for the new year. Thank  you for watching, and let me know what you think.