Bulk Density: Definition & Calculation

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  • 0:04 Bulk Density
  • 0:30 Calculating Bulk Density
  • 1:49 Compaction
  • 2:42 Bulk Density vs…
  • 3:13 Lesson Summary
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Lesson Transcript
Instructor
Hassan Alsaud

Earned my B.S. in Civil Engineering back in 2011. Have two years of experience in oil and gas fields and two year as a graduate research assistant. Earned my Master degree in Engineering from Tennessee State University in 2016.

Expert Contributor
Kathryn Boddie

Kathryn earned her Ph.D. in Mathematics from UW-Milwaukee in 2019. She has over 10 years of teaching experience at high school and university level.

In this lesson, we'll discuss bulk density, a property of soils. You'll learn the definition of bulk density as well as how to calculate it. We'll also explore the differences between bulk density and other types of density.

Bulk Density

Bulk density, or dry bulk density, is a property of soils and other masses of particulate material. It's the weight of the particles of the soil divided by the total volume. Thus, it should be noted that the unit of bulk density is the unit of weight over the unit of volume, for example kg/m3 for the metric system and lb/ft3 for the English system.

Calculating Bulk Density

Bulk density is given by the following equation:


bulk density


Let's look at an example:

A dry sample of soil has a volume of 0.5 m3 and weighs 800 kg. What's the bulk density of the sample?

Remember that:

Bulk density = mass of soil / total volume

Let's plug in our figures:

800 kg / 0.5 m3 = 1600 kg/m3

So, the bulk density of our soil sample is 1,600 kg/m3.

Here's another example:

A soil sample has a mass of solids equal to 400 lbs. and a water content of 20%. What is the bulk density of the soil given that the volume of the sample is 5.0 ft3?

To obtain the mass of water of the soil sample, we should multiply the water content and the solid mass:

Mw = 20% * 400 lb = 80 lb

Therefore, the total mass is:

400 lb + 80 lb = 480 lb

We can obtain the bulk density of the sample of the soil with our formula:

480 lb / 5.0 ft3 = 96 lb/ft3

Compaction

Have you ever emptied a container of spices to mix them in a bigger bowl, only to find that you couldn't place the spices back in their original container? The reason for that is that the spices were compacted by the production factory. When you emptied the container, the spices maintained their weight but increased in total volume because the voids' volume increased.

To calculate the total volume of soil, we add the volume of solids plus the volume of water plus the volume of air. The volume of voids is found by adding the volume of water and the volume of air. When a sample of soil is compacted, its volume of voids decreases which decreases the total volume of the soil sample. This increases the bulk density of the soil. So as depth increases, bulk density increases.

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Additional Activities

Additional Examples of Bulk Density and Wet Bulk Density

In the examples to follow, students will be able to use the formula for bulk density of soil to determine the bulk density or wet bulk density of various soil samples. Students will also manipulate the formula to solve for the mass of the soil, given the total volume and bulk density of the soil, or for the total volume of the sample, given the mass of the soil sample and bulk density of the sample. After completing the beginning examples and understanding the results, students will be ready to try the challenge examples, which involve comparisons of different soil samples and their respective bulk densities.

Beginning Examples

  • Find the dry bulk density of a soil sample which weighs 470 kg and has a volume of 0.25 cubic meters.
  • Find the wet bulk density of a soil sample which has solid mass of 375 kg and a water content of 15%, given that the volume of the sample is 0.75 m^3.
  • If the dry bulk density of a soil sample is 2400 kg/m^3 and the volume of the sample is 0.25 m^3, what is the mass of the soil sample?
  • If the dry bulk density of a soil sample is 1500 kg/m^3 and the sample weighs 300 kg, what is the total volume of the sample?

Solutions

  • Using the formula
    we have that the bulk density is 470/0.25 = 1880 kg/m^3.
  • The wet bulk density will be the total mass divided by the total volume. The solid mass is 375 kg and the mass of the water is 0.15 * 375 = 56.25 kg, for a total mass of 431.25 kg. So the wet bulk density is 431.25/0.75 = 575 kg/m^3.
  • Using the formula
    and solving for the mass, we have that the mass is 2400 * 0.25 = 600 kg.
  • Using the same formula but solving for the volume, we have that the volume is 300/1500 = 0.2 m^3.

Challenge Example

  • What volume would you need to compact a soil sample of mass 400 kg to so that its bulk density is equal to that of a soil sample with mass 850 kg and total volume 0.5 m^3?

Solution

  • First, find the bulk density of the heavier soil sample. The density is given by 850/0.5 = 1700 kg/m^3. Then find the volume needed in order to have the same bulk density for the other sample. Using our formula and solving for volume, we have that the volume is 400 / 1700 or about 0.235 m^3

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