Soil Classification

[ONENGINEER]

Can someone explain the difference between Modified Unified and Unified soil classification systems. What are the pros and cons of one against the other.

Why some used the A line on Casagrande Chart and others use A and U line with little differences near the PI less than 4?

My search did not yield convincing answers. Many thanks for clarifications on the above.

 

[geotechguy1]

The Modified ones usually just add more things which may vary from firm to firm and place to place. There is no intermediate plasticity in the original USCS, and I don't think the original USCS had all the qualifiers that have later been added.

In truth, no one follows these systems in entirety. There are always local terms that creep in and aren't written down or tacit knowledge. I don't think 'Clay Till' is a term used in any official classification system but it is commonly used to describe soils in Alberta, many of which are technically on the coarse grained side of the 50/50 split in the USCS.