an excavation location on a construction site

How To Classify Soils in Construction

Here at Bear Iron Works, we specialize in soil separating equipment. That being so, we need to know to classify soil so we can give our customers the product that is right for them. Today, I want to go over some of the most basic types of soil classification in the construction industry, and give some additional resources to help you understand a little bit more about the soils you are working with.


In the construction industry you will commonly hear many different terms. When discussing soils we typically refer to the individual grains of soils as granules or clasts. These individual granules are what we use to identify what a soil is made up of. There are six different sizes of granules that we utilize in soil testing. Largest to smallest is: boulders, cobbles, pebbles, sand, silt, and clay.

A sizing chart of soil granuals used to classify soil.


To determine what granule sizes you have in your soil, you must complete a sieve analysis test. A sieve analysis test involves an 8 chamber test, each chamber, except for the bottom, has a different sized screen in it. These screens remove larger granules and let the smaller ones fall through. This is very similar to how our Grizzly rock screens work. By dividing the weight of the retained soil in each sieve by the total weight of the soil, you can calculate what percentage of each size of granules makes up your soil.

A chambered sieve analysis test that separates soil by size. Used by AASHTO.

(Image Source: )


Now that we have discovered what percentage of granules make up our soils, we can utilize two different classification systems to give our soil a name. These systems are the Unified Soil Classification System (USCS) and the American Association of State Highway and Transportation Officials (AASHTO). Neither one is more preferred than the other, but it is important to check with your project spec book on what classification to use.

I would like to state that to have a fully accurate classification of a soil, plasticity limits and liquid limits may be required. These limits are the range of when the soil will act like a solid (plasticity limit) or act like a liquid (liquid limit). This is only relative to classifying different silts and clays.


To classify a soil using the USCS we will want to start at the top left of the chart with Gravels. Is 50% or more of the soil larger than the No. 4 sieve? If it is yes then we will use this chart to further classify the soil. We will then see if it has less than %5 fines or more than 12% fines to further our classification. What if it is between 5% and 12%? Then will will utilize a mixture between two classifications, such as GP-GM.

If our soil is less than 50% no. 4 sieve then we will move down to sands and ask is 50% or more of my material smaller than the no. 200 sieve, which is the smallest sieve in our analysis. If it is larger than the 200 sieve, then we will do the same process for the sand as we did for the gravel. If it is not larger than the 200 sieve then we do not have sands, we have fines. For fines we will need to utilize the plasticity index (plastic limit and liquid limit) to determine our soil type. Fines are generally the worst soil to be working with for structural purposes.
Unified soil classification system classification chart for soils.
(USCS Chart Source: )


AASHTO works in a similar fashion, but utilizes slightly different classification techniques. For this chart we will again start in the upper left of the chart. We will begin with group A-1 and determine if our soil matches any of these descriptors. We need to know what percentage of the soil is passing through the sieve. We can calculate this by adding up all the percentages of soils below the sieve size in question. For the no. 10 sieve, we will add up any sieve size that has a higher number. Sieves work similarly to how gauges work, the higher the number of sieve means the smaller the screen size. If none of our numbers match anything in the A-1 category, we will then move right and see if our classification meets A-3. We will continue this until we find a classification that our soil holds true on all values. Once we have determined our category, we can give our soil a name.

For AASHTO, the plasticity index is more important to larger granule soils than the USCS system. With a basic sieve analysis, however, we can determine a base class for our soils.

AASHTO Soil Classification Chart

 (AASHTO Chart Source:

I hope that I was able to teach you some of the basics of soil testing and soil classifications. I am going to leave some links at the bottom with some more references on soil testing and how it is done. Please follow our blog for more educational writings.
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