MANHATTAN, Kan. – Soils are a crucial component for agriculture and food production. Soils support all kinds of life. From an engineering perspective, soils are also important, as they provide the groundwork for any type of building or structure.
“No matter if it’s a road, a building, a bridge or a levee, it’s supported on the native soils you have available,” said Stacey Kulesza, a geotechnical engineer who is an assistant professor in the Department of Civil Engineering at Kansas State University.
Engineers must perform soil tests at a building site before construction can begin. This is why all students majoring in civil engineering must complete courses such as soil mechanics—a course Kulesza teaches to undergraduate students at K-State.
Those students who want to specialize in geotechnical engineering, a branch of civil engineering focused on the behavior of earth materials, will further study how to evaluate a building site, collect more data related to soils and design earth-retaining structures, as examples. The relationship between knowledge of soils and the field of engineering fits the May theme for the 2015 International Year of Soils, “Soils Support Buildings and Infrastructure.”
Soils support all projects, big and small
Kulesza said all building projects require a drill rig to take soil samples so they can be classified. The scale of the project determines how deep to drill and how many samples are needed.
“You can’t change the soils you have,” she explained. “You just have to adapt your structure to work with them.”
A small residential structure, for example, might require obtaining one borehole per street. A neighborhood with many houses might require drilling 15 to 20 feet deep to classify the soil and understand where water is located.
The water table affects how a structure will behave, Kulesza said, as will the presence of soils that expand when wet and shrink as they dry out. These expansive soils are typically more clay-like. Clay soils also take many years to properly settle below a structure compared to soils that contain more sand and allow water to escape quickly.
Damages to structures due to expansive soils are similar to damages caused by frost heave, she said.
“In Kansas, we have issues of frost heave, so if you have certain soils that are susceptible to heaving when it gets cold outside, that could affect your house,” Kulesza said. Fine-particle soils of the silty and loamy types are examples.
An even larger project, such as a campus building, would require drilling multiple boreholes, she said, and drilling much deeper to understand the depth and variability of the bedrock below the surface.
“There’s always more information we can gather and more tests we want to run,” Kulesza said. “Typically, geotechnical is a small part of any kind of structure or plans that you have for your building.”
She added that engineers always account for some movement and amount of unknowns in structures to reduce risk.
Support for other types of structures
Levees are other important structures that prevent the overflow of water, and soils are used to build these structures. Kulesza, who studies levees, said many were built long ago with no design in mind.
“How we design a levee today is have an inside where water can’t get through, so use a clay-like soil,” she said. “Then we put a more stable soil on the outside.”
To help prevent soil erosion, levees are often covered with grasses.
“It’s important to remember that your levees are not designed to be over-topped and have water flowing over them,” Kulesza said. “They are designed to temporarily hold back water, and then for (the water) to go away. You can’t have erosion eat away at your levee.”
Kulesza said there is currently more than 100,000 miles of levee in the United States. To help keep levees in good shape, it is best to leave them alone and keep them free of debris.
Roads and bridges are other important structures that must have a stable foundation. Erosion around a bridge foundation, called scour, is often caused by water flow.
Kulesza said large rock, or riprap, is used to pile around the bridge to enhance stability. Another solution is to replace the soil and cover it with a large plastic grid to make it more erosion-resistant.
“Going back and fixing structures for scour is complicated,” she said. “A lot of times we design to accept scour is going to occur. We monitor the bridge for scour, especially in the event of a flood.”
To watch a video interview with Kulesza, log on to the K-State Research and Extension YouTube page (https://www.youtube.com/watch?v=MwW6W_UA3Ks&feature=youtu.be). The Soil Science Society of America has numerous resources for the public, teachers and children about soil and each monthly theme for the International Year of Soils. Log on to www.soils.org.