Soil Classification - Fine Materials 1

[ONENGINEER]

The fine soil materials classifications are unusually made based on A-line in Casagrande Chart. I have seen some engineers classify the fine soil based on the result of hydrometer test only and call it silty clay if the percentage of clay from hydrometer test is more than silt or clayey silt if the percentage of silt is more than clay. Assuming that hydrometer and Atterberg limit tests are the same price with similar accessibility and the duration of testing is not an issue, which test is more reliable/standard to classify and name a soil sample. Thank you.

 

[geotechguy1]

If you locked 1,000 geotechnical engineers in a room on the condition that they can't leave until they have a consensus on the answer to this question they would all die in the room debating it.

 

[ONENGINEER]

Thanks, geotechguy1. That was relieving and conveyed the message well.
My gut feeling is that clays are sheet or needle minerals with surface chemistry rather than an assumed equivalent spherical grain in hydrometer's Stoke law measurements. As such Atterberg limit test might better reflect the strength related behavioral aspects of fine-grained soils, while hydrometer results could explain permeation behavior of a soil sample. However, not having hands on experience in clay laboratory testing and mineralogical studies, I wanted to know what experienced geotechnical engineers believe and practice.

 

[geotechguy1]

My opinion:

A hydrometer tells you how roughly how many particles you have approximating plate / needle shaped particles as a sphere (but - what if the plate / needle shaped particles clump together and don't fully separate with the deflocculant?)

The atterberg tells you something about behaviour



A controversial opinion follows:

The division between 'silt' and 'clay' based on where a sample plots on the Casagrande chart is a load of absolute, total bollocks and 'ML' and 'MH' are nonsense. Soils plot below the A-line either because they contain organics, or because of the mineralogy of the clay particles (i.e. what combination of allophane, halloysite, kaolinte, illite, montmorrilonite,

 

[TigerGuy]

Usually, it comes down to local knowledge and experience. ASTM D2487 and D2488 have extensive (in my opinion) steps to take to classify a soil. I'm not aware of any operation that follows either procedure completely. We usually look into the archives and see what was encountered on nearby sites, before we ever start an investigation.

 

[geotechguy1]

Everywhere I've worked (both city / region and country) has had tacit / customary variations to what is written down in the standards. Usually the classification is almost a heuristic of local custom / conditions for particular soils perhaps with some local nomenclature applied. For example, a soil might be called a 'Clay Till' in a particular region (which isn't in any standard) despite being more than 50% coarse grained and the clay fraction plotting below the A-line.

In NZ most clays have high contents of mineralogies that plot below the A-line that aren't common in North America and the UK where all the standards were written, and the tendency is to call soils 'Clayey silty to silty clay' while focusing primarily on a geological group name or some such and avoid doing Atterbergs entirely.

 

[TigerGuy]

Exactly, that's why it can be entertaining to read a soil classification from somewhere else.

 

[ONENGINEER]

Thank you for the helpful comments.