Soils are classified into three (3) main types. These are...
Let's start with Class A...
Per OSHA (Occupational Safety and Health Administration), this material type is cohesive and has a highly confined compressive strength (1.5 tons p/sq ft or greater) and is the most stable for excavation and backfill purposes. This would include clay, and other clay attributes such as silty clay, sandy clay, and clay loam. Class A generally “sticks” together quite well, does not break apart easily, and is the most ideal soil type as it is most effective in holding back water from your foundation, as well as minimal expansion due to moisture, frost, etc.
Class B soil is similar to Class A, but cohesion is not as good and will break apart easier. Compression strength is between 0.5 and 1.5 tons p/sq ft. Materials is this class include angular gravel, silt, silt loam. Moisture can be a factor, but is generally good material.
Class C is the least stable of the three soil types. There is no cohesion of this type, and has a compression strength of 0.5 tons p/sq ft or less. Material types for this include gravel and sand. Due to the instability of Class C soil, water seeps through it quite easily and can cause issues with a foundation, including water seepage and excess pressure.
Knowing each soil type and it's triats will help you choose the right type of material you need for your own indidvidual application and/or project(s). If you are uncertain, consult with your excavating professional or qualified personnel.
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