Lesson How Dense Are You?

Learning Objectives

After this lesson, students should be able to:

• Relate the physical properties of soils to foundation failures in streets, homes and other structures.
• Describe how soil constituents affect the foundations of all structures.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Introduction/Motivation

Have you ever seen cracks above a doorway or along the outside of a house? Have you ever tried to open a door or window that seemed to be stuck? Have you ever seen cracks in a concrete floor? Why do you think these problems occur? (Have students brainstorm ideas. Tell students that all of these issues are caused by foundation problems.) (Show students Figure 1.) Have you ever seen parts of a road that are heaved?

All of these issues have a common cause. Can you guess what it might be? (Listen to student ideas.) Each of these situations is most likely caused by poor soil compaction or other soil problems that result in damage to the concrete in a house foundation or to road pavement. As you will see today, geotechnical engineers who are responsible for identifying soil constituents can prevent, identify and repair these problems!

(Introduce students to geotechnical engineering and this lesson with a 10-minute video, an introduction to geotechnical engineering, available at: https://www.youtube.com/watch?v=xZZ2O1fOBVs)

Lesson Background and Concepts for Teachers

In certain areas of the U.S., the soil contains clay materials (see Figure 2). Clays in soil are responsible for most of the foundation failures affecting buildings, streets and other structures because clays can shrink or swell depending on the soil moisture content. This shrinking/swelling of clay-containing soils, referred to as expansive soils, is responsible for many of the observable signs of foundation damage and/or failure. In regions with these clays, geotechnical engineers are employed to determine the constituents of soils before construction projects are initiated.

One of the tests geotechnical engineers use to determine soil constituents is bulk density. Knowing the bulk soil density helps to determine the stability and suitability of a given soil as a foundational support. Bulk density soil measurements are exactly what they sound like: to determine the bulk density of a given soil, a geotechnical engineer calculates the density of the soil. The bulk density of a soil is measured as the mass of the soil per unit volume in g/cm³. Soil texture and structure determines soil bulk density (see Figure 3). Refer to the associated activity How Dense Are You Lab to have students investigate the desity of soils to determine the most suitable soil for a foundational support building. Fine textured soils such as silts and clays have less total pore space and lower bulk density than coarse soils such as sand. In general, the higher the bulk density of a soil the greater the support it provides for a foundation. Sandy soils are very stable and do not heave or consolidate with increased or decreased moisture. Sandy soil has the greatest bulk density of all soil components. Sandy soil with a small quantity of silt provides the greatest structural support for foundations.

Texture is one of the first things determined when a soil is examined. Texture can be identified in the field by feeling the sand particles and estimating silt and clay content by flexibility and stickiness. No field mechanical-analysis procedure is as accurate as the fingers of an experienced person. To guide students in identifying soil constituents, refer to the attached Soil Identification Flow Diagram to teach texture by the feel analysis. This requires students to perform tests to handfuls of soil, such as adding water, squeezing and rubbing, and then identify the soil type based on answers to questions about the tests performed. Possible soil types include: sand, loamy sand, sandy loam, sandy clay loam, sandy clay, silt loam, silty clay loam, silty clay, loam, clay loam, and clay.

Soils can be treated with lime, lime and concrete, fly ash and other materials to reduce the potential of the soil to heave, shrink, and/or swell. Lime treatment has been demonstrated to increase the stability, strength and reliability of problematic soils. Geotechnical engineers analyze soil materials to determine what percentage of lime is required to eliminate the difficulties observed with clay containing soils. This soil analysis increases observable success in foundation construction.

Associated Activities

• How Dense Are You Lab - Students determine the mass and volume of soil samples and calculate the density of the soils. They use this information to determine the most suitable soil for a foundational support building.

Vocabulary/Definitions

density: Mass per unit volume of a substance.

expansive soil: Soils that show volumetric changes in response to changes in their moisture content.

geotechnical engineering: Branch of civil engineering concerned with the engineering behavior of earth materials; geotechnical engineering includes investigating existing subsurface conditions and materials; determining the physical/mechanical and chemical properties that are relevant to the project considered, assessing risks posed by site conditions; designing earthworks and structure foundations and monitoring site conditions, earthwork and foundation construction.

mass: The amount of matter in an object.

physical property: A property of a substance that can be measured or perceived without changing its identity.

silt: A sedimentary material consisting of particles smaller than sand and larger than clay. Silt is often found at the bottom of bodies of water where it accumulates slowly by settling through the water.

soil pore space: The total space not occupied by soil particles in a bulk volume of soil.

volume: The amount of space occupied by a three-dimensional object or region of space, expressed in cubic units.

Assessment

Concluding Discussion: Before proceeding to conduct the associated activity, ask students the following questions to assess their understanding of the lesson:

1. What is a common cause of cracked foundations that we talked about today? (Possible answers: Poor soil compaction, soil problems.)
2. What type of engineer would help to identify, prevent and resolve foundation problems? (Possible answers: Geotechnical engineer, civil engineer, environmental engineer.)
3. What does the term "expansive soil" refer to? (Answer: Soil that shrinks or swells, depending on its moisture content.)
4. What soil constituent is most responsible for foundational problems? (Answer: Clay)
5. What type of test do engineers often use to determine soil constituents? (Answer: Bulk density testing.)
6. What is an example of units that could be used for density? (Possible answers: g/cm³, kg/m³, g/mL, kg/mL)
7. What does density measure, and what is the equation for density? (Answer: mass per unit volume, d = m/v)
8. What type of soil provides the best foundational support? (Answer: Sandy soil)

References

Copyright

Contributors

Supporting Program

Acknowledgements

The contents of this digital library curriculum were created through the University of Texas at Arlington's College of Engineering NSF RET grant. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.