Soil Classification

There are two main methods of soil classification: AASHTO, and the Unified Soil Classificaiton System (USCS). Both methods involve finding the appropriate soil class given some parameters like passing a certain sieve size, liquid limit, plastic limit, significant makeup, and other ratings.

AASHTO classification returns an A-1 through A-8 rating. A-1 is the best all-round and A-8 is not really suitable for building.

USCS classification returns a two-digit group symbol for the soil such as GW for gravelly well-graded, SM for silty sand, CL for low plasticity clay etc.

You will need to be familiar with the sieve sizes (e.g. #200 allows up to 0.075 mm grains to pass) and how to determine what percent of the soil passes through that sieve, find the charts for this in your reference.

The Plasticity Index (PI) is equal to the Liquid Limit (LL) minus the Plastic Limit (PL):

$$ PI = LL – PL $$

The Plasticity Index is used in both methods, in AASHTO to differentiate between A-7-5 and A-7-6 and in USCS it is the vertical axis on the plasticity chart.

Distinguishing Ambiguous Classifications

There are a couple instances where you will run the numbers and parameters in either classification test but be left with an ambiguous soil. Here are some pointers on where to go from here:

  • M and O classifications require identification by observation, color, and other testing,
  • Organic compounds are usually dark in soil, organics are bad for engineering soils.
  • Clays have plasticity, silts do not as much.

About Conrad

I am a Civil Engineer. I work in San Diego and am preparing to take the PE Exam. I am interested in surfing, business, travelling, and spending time with my wife. Thanks!

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