Big Chem-EZ: How are potholes made?

Editor’s Note: In 2020 (and 2021…and 2022…you get the point), there have been a lot of unanswered questions about life and living, so in our partnership with the Chemical Engineering Service Learning Class at Tulane University, taught by Dr. Julie Albert, we made it our aim to find questions we could answer. The series is called “Dear Big Chem-EZ” (think “Dear Abbey” but with less about “Why does my partner ignore me?” and more about “Can I actually drink my tap water?” and “What’s that smell outside my house?”).

You can look for new pieces every first Tuesday of the month because we love science, we love answers, and we love knowing what’s going on with all these potholes! Let’s take a look! If you have questions you’d like answered, send them to thebigchemez@gmail.com.

Dear Big Chem-EZ,

Driving down some of the roads in New Orleans can feel like a real-life game of Mario Kart^TM. However, instead of swerving for coins or special items, drivers swerve to prevent damaging their cars. It seems like every road you turn down on, there is always a pothole waiting for you, seemingly appearing out of nowhere. It makes you wonder. What causes these potholes to form? 

There are a variety of factors that play into the formation of potholes. Potholes are a product of cracks, water, freezing weather, and harsh sunlight. To understand how potholes form, it is first important to understand how roads are built.

There are three main components that make up roadways, as shown below. The first and bottom layer of roadways is the subgrade layer, which is the foundation and most important component of the roadways. This layer typically consists of the native soil that has been compacted to withstand heavy traffic loads [4]. The next layer is the subbase layer, which typically consists of crushed aggregate, gravel, or recycled materials, such as oyster shells. The main role of the subbase layer is to evenly distribute the load over the subgrade layer [4]. The last and top layer of roadways is the asphalt layer. The main role of the asphalt layer is to prevent water penetration into the pavement and provide extra protection from abrasion and weathering effects [4].

The formation of potholes happens in steps.

• First, the asphalt layer begins to degrade. Despite the strength of the asphalt layer, factors such as constant traffic, harsh sunlight, rain, and freezing temperatures causes the asphalt to weather [3]. When there’s extreme heat, such as 90ºF, the asphalt expands, and as the temperature drops at night or it starts to rain, the asphalt contracts [2].
• Overtime, the asphalt forms cracks, which create an entrance for water to penetrate through the asphalt and aggregate layers and into the subgrade layer. This causes the soil in the subgrade layer to become waterlogged, which weakens the foundation of the roadway.
• Then as the weight of traffic passes over the roadway, the water is pushed out of the cracks, carrying some of the soil used in the subgrade layer.
• Overtime, this process repeats itself, and the roadways continue to lose more and more soil in the subgrade layer, resulting in less support for the asphalt layer. Eventually, the weight of traffic causes the asphalt and aggregate layers to cave inward, and a pothole is born.

– Big Chem-EZ

 

References:

[1] “Dangers of Potholes in New Orleans: Who Is Liable for Your Damages?” Law Office of John W. Redmann, LLC, 10 May 2021, https://www.redmannlaw.com/dangers-of-potholes-in-new-orleans-who-is-liable-for-your-damages/.

[2] “How Does Summer Heat and Weather Affect Asphalt?” JR Paving & Construction Co Inc, https://www.jrpavingandconstruction.com/how-does-summer-heat-and-weather-affect-asphalt.

[3] “How Pavement Reacts to Temperature Changes.” Mr Pavement, 27 Apr. 2018, https://www.mrpavement.com/about-us/blog/general-information/how-pavement-reacts-temperature-changes/.

[4] “Pavement Design.” Asphalt Pavement Design Services | Twin Cities, MN, https://bitroads.com/Customer-Education/Pavement-Design.