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Solid Ground? Measuring Soil Bearing Capacity

Abstract

Foundations for many types of structures rest on soil. This project shows how you can investigate the bearing capacity of different types of soil.

Summary

Areas of Science
Difficulty
 
Time Required
Short (2-5 days)
Prerequisites
None
Material Availability
Readily available
Cost
Very Low (under $20)
Safety
No issues
Credits

Jeffrey R. Hess, D. E. Harvey Builders, Inc. and Texas A&M University

Andrew Olson, Ph.D., Science Buddies

Objective

The goal of this project is to test the bearing capacity of different types of soil.

Introduction

The bearing capacity of soil is an important consideration in construction projects. Dams, bridge abutments, and temporary support structures (falsework) during construction are all examples of structures that can be supported by underlying soil.

How do engineers know how much weight soil can support? Is the amount of weight different for different types of soil? How does the size of the area in contact with the soil affect bearing capacity? How does the presence of water in the soil affect bearing capacity? Do you think sand or topsoil will have a higher bearing capacity? Why? These are the kinds of questions you can begin to answer with this project.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

More advanced students should also study:

Questions

Bibliography

  • Wikipedia contributors, 2006. Soil Mechanics, Wikipedia, The Free Encyclopedia. Retrieved June 19, 2006.
  • The SI unit for pressure is the pascal (Pa). For a definition of the pascal in base units, and a link to a calculator that converts between different units of pressure, see: PEditor, 2000. Pascal, Sizes, Inc. Retrieved June 19, 2006.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

Experimental Procedure

  1. Do your background research and make sure that you are knowledgeable about the terms, concepts, and questions, above.
  2. You can make a simple experimental apparatus for measuring soil bearing capacity in a plastic tub. You'll need a piece of 1-inch diameter dowel, about as long as your tub is high. Mark off 1-cm increments along the length of the dowel, as shown in Figure 1.

    Drawing of a dowel marked off in 1-cm increments
    Figure 1. 1-inch diameter dowel, marked off in 1-cm increments (not to scale).


  3. Make a platform from a small piece of wood or hardboard and attach it firmly to the top of the dowel with a wood screw and glue (Figure 2). It's a good idea to drill a pilot hole for the wood screw, so the dowel doesn't split. Your platform should be large enough to support the weight you'll be using.

    Drawing of a dowel marked off in 1-cm increments with a platform on top
    Figure 2. Dowel with platform attached (not to scale).


  4. Make a jig to hold the dowel vertically over the soil surface (Figure 3). The dowel should be able to slide freely, so the hole needs to be slightly larger than the diameter of the dowel. You can drill it with a 1-inch spade bit, or a 1-inch hole saw, and then file out the hole to enlarge it enough for the dowel to slide freely. Make the legs from thinner dowels, and glue them to the bottom. You'll push these into the soil, so they need to be a bit longer than your tub is high (tall enough to hold the dowel upright).

    A table with a hole in the center used to fit a dowel snuggly
    Figure 3. Jig for holding the dowel upright. The hole should be slightly larger than the dowel, allowing it to slide freely, but still supporting it. The legs should reach the bottom of the tub, and leave the platform high enough above the soil to keep the dowel from tipping over (not to scale).


  5. Fill the plastic tub with the sand or soil you want to test.
  6. Push the legs of the jig firmly down into the soil, and set the dowel so that it rests on the surface of the soil (see Figure 4).

    A table that has a dowel through a center hole and a flat object on top
    Figure 4. The dowel supported in the jig. Push the legs of the jig firmly down into the soil, and set the dowel so that it rests on the surface of the soil (not to scale).


  7. Load the platform with weight (e.g., container + 500 ml water), and record how far the dowel penetrates into the soil. Keep track of the results in your lab notebook.
  8. Add an increment of weight and record how far the dowel penetrates into the soil. Repeat until the dowel does not penetrate further. Repeat the measurement at least three times, in different locations in the tub. Calculate the average penetration depth for each weight and soil type tested.
  9. For each soil type, make a graph of the penetration depth vs. weight.
  10. What happens to the soil underneath the dowel? If you add more weight to the dowel, does it descend by the same amount for each additional (equal) increment in weight?
  11. The Science Buddies project Get Down and Dirty: How Does Soil Change with Depth? describes how to classify different soil types based on physical properties (soil structure, consistence, and texture). See the Experimental Procedure section, in particular, steps 7, 9, and 10.
  12. More advanced students can compute the pressure applied to the soil by the weighted dowel. Pressure is the force per unit area. The SI unit for pressure is the pascal (Pa), which is newtons per square meter. Calculate the pressure on the dowel for each weight tested, and make a graph of soil penetration depth vs. pressure.
icon scientific method

Ask an Expert

Do you have specific questions about your science project? Our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Variations

  • What happens if you pack the soil down in the container prior to your test? To do this in a repeatable way, you could set a piece of wood (cut to fit your tub) on top of the soil, and load it with a fixed weight (e.g., a bucket full of bricks, or water). Lift the bucket and set it down on the board repeatedly to "tamp" the soil down. Do the same number of tampings, with the same weight, for each soil type. Measure the height of the soil in the tub before and after packing. Test both packed and unpacked soil samples for different soil types. Does packing make a difference for every sample tested?
  • What happens if you add a fixed amount of water to the test container?
  • What happens if you saturate the soil in the test container with water?
  • What happens if you increase the effective diameter of the dowel (attach a fender washer to the bottom of the dowel).
  • Try measuring the bearing capacity of soil samples collected from different depths.
  • If you are interested in learning more about different soil types, see this Science Buddies project in the Environmental Science area: Get Down and Dirty: How Does Soil Change with Depth?

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General citation information is provided here. Be sure to check the formatting, including capitalization, for the method you are using and update your citation, as needed.

MLA Style

Science Buddies Staff. "Solid Ground? Measuring Soil Bearing Capacity." Science Buddies, 20 Nov. 2020, https://www.sciencebuddies.org/science-fair-projects/project-ideas/CE_p009/civil-engineering/measuring-soil-bearing-capacity. Accessed 4 Oct. 2022.

APA Style

Science Buddies Staff. (2020, November 20). Solid Ground? Measuring Soil Bearing Capacity. Retrieved from https://www.sciencebuddies.org/science-fair-projects/project-ideas/CE_p009/civil-engineering/measuring-soil-bearing-capacity


Last edit date: 2020-11-20
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