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):
$$ PI = LL – PL $$ Click here to continue reading