Steps & Tips For Successful Proctor Soil Compaction Tests

Steps & Tips For Successful Proctor Soil Compaction Tests

11th Oct 2018

Of all the material tests that take place at a construction site, the Proctor soil compaction test is one of the most important. Performing accurate testing in-situ or in the lab can determine the success or failure of a structure. Keep these steps and tips in mind as you prepare to perform testing at your site.

What Is The Proctor Soil Compaction Test?

The Proctor soil compaction test is used to determine the maximum dry density for any soil you may be building on. This testing is necessary to determine how much soil must be compacted to ensure the stability of a structure and prevent damage or failure from settling.

While standard soil compaction tests can be performed in the field for preliminary results, the Proctor compaction test is generally performed in the lab. Laboratory testing in addition to field tests is best-practice, as lab testing is a standard of regulatory bodies like the American Association of State Highways and Transportation Officials (AASHTO) and the American Society for Testing and Materials (ASTM). Additionally, laboratories provide access to a wider array of test types, such as static and vibration testing, for the most accurate results possible.

What Is The Purpose Of The Proctor Soil Compaction Test?

The Proctor soil compaction test reveals a soil sample's moisture-density relationship — in other words, how its density changes with moisture content. It also reveals the maximum density achievable for that particular soil type. The moisture level when the soil is at peak density is called the optimum moisture content. The information gathered from the soil compaction tests is then used as reference points for field testing, generally before groundwork begins.

The Proctor soil compaction test is essential because of how much soil types vary by site and region. Grain size, distribution and shape, along with the presence of clay minerals and the soil's initial moisture content can change dramatically from one location to another. These variables are the reason that the Proctor test is an ASTM and AASHTO standard.

Materials Needed For The Standard Proctor Soil Compaction Test

There are two versions of the Proctor soil compaction test, but today we'll be focusing on the standard version. Modified versions of the test can be performed for soils with especially high densities.

For the standard test, you will need:

  • Sieves
  • Heavy-duty zip bags or sealed metal cans for moisture conditioning of soil specimens
  • Soil compaction mold(s)
  • Straight edge for leveling soil in molds
  • 5.5 lb. soil compaction hammer/dropping weight
  • 12" drop
  • 3-5 layers (lifts) of soil
  • Scale/balance
  • Drying oven
  • Pans for drying and mixing

Steps For Performing The Standard Proctor Soil Compaction Test

Before you start, a note: Determining particle size is an important precursor to the Proctor test. Soil particles larger than 2 mm will require a modified version of the Proctor soil compaction test to produce accurate results. As you sift the soil, the coarser minerals can be used to determine particle size. They can then be added back to the final test specimens to get the most accurate representation of what the soil is like at a site.

Once you've confirmed that the particle size is 2 mm or smaller, you can proceed with the standard Proctor soil compaction test by following these steps.

Step 1: Prepare About 5 Soil Samples

To prepare the samples, sift the particles and gradually air-dry them to a range of moisture contents starting around 10% or more below your anticipated optimum moisture content, up to that much above it. Among your samples, include one that is representative of your approximate/estimated optimum moisture content. Then, seal them up for 16 hours for proper moisture conditioning.

Step 2: Compact The Soil

Press the soil into the Proctor soil compaction mold in three to five layers (lifts) using the straight edge and soil compaction hammer/drop weight. Then weigh the sample. Oven dry the mold and weigh the sample again.

Step 3: Evaluate Your Results

Use wet mass/weight and dry mass/weight to calculate the exact moisture content of each sample. The moisture content of the sample with the highest dry weight reveals the optimal moisture content of the location's soil for maximum dry density. You now also have enough information to calculate the maximum dry density (the density you'll want at your construction site) using the following formulas:

  • Bulk Density = weight of the compacted soil in grams/volume of the compacted soil in milliliters
  • Dry Density = Bulk Density/(1 + % optimal moisture content, expressed as a decimal)

Tips For An Easier, More Effective Soil Compaction Test

While much of the Proctor soil compaction test can be performed manually, there are several machines that can automate the process and put less strain on you. Use these tips the next time you perform a Proctor test.

Tip 1: Easy Reference For Optimum Moisture Content

There is a simple trick for estimating optimum moisture content in your soil. The optimum moisture level should be roughly equivalent to the moisture level in a soil sample that stays put when you manually lump it together, but breaks apart cleanly if you try to "bend" the top. It's an easy tactile and visual cue to help you know you're getting close to the right water content.

Tip 2: Expedite Initial Air-Drying

If you are preparing cohesive soil samples, the initial air-drying process can be expedited if you break down large clumps in the soil. Just be sure not to crush the individual particles.

Tip 3: Use A Soil Ejector

A soil ejector can be used to get your specimen out of soil compaction mold more easily and completely. Standardizing this process ensures that you get the most accurate dry weight measurement possible.

Tip 4: Invest In An Automatic Soil Compactor

Instead of repeatedly dropping a soil compaction hammer or drop weight to manually compact your soil specimens, let automatic soil compactors do it for you. On top of reducing how much effort you have to put into the test, an automatic compactor will enhance repeatability and accuracy to give you the best possible results.

Find Everything You Need For The Proctor Soil Compaction Test At Certified MTP

Certified Material Testing Products has the equipment you need to perform any kind of soil testing at your construction site. Let us help you run accurate tests and make structures built to last.