Thoughts on the usefulness of the USCS and it's derivatives 1

[geotechguy1]

I'm curious what everyone's thoughts are on the usefulness of the USCS and it's derivatives to classify soils for engineering purposes (or more broadly the joys of soil classification)

The advantages I see: Has been used extensively, large availability of correlations

Some points I've been contemplating:

[ul]
[li]Is the A line on the Atterberg chart actually useful? I've found the usefulness of it to largely be: more plasticity = more clay content (or a higher fraction of more active clay minerals) The silt/clay choice being based on being above or below the line has never made any sense to me. The golder version of the USCS calls plastic soils below the A-line 'Clayey silt' rather than 'silt'. I've also worked for employers that just ignore the A-line and call all plastic soils 'clay' with modifiers for varying silt and sand content.[/li]
[li]Is the 50-50 split between 'fine grained' and 'coarse grained' actually useful? I've never found calling a soil that's 55% sand, 20% silt, 25% clay 'clayey sand' to be a useful classification - and contractors and developers often see the word 'sand' and interpret it to mean 'beach sand' and then accuse the geotech of being a moron when it looks like plasticine. [/li]
[li]Does a USCS classification actually tell us anything about engineering properties? Eg. Strength, stiffness, permeability [/li]
[/ul]

I' find completely ignoring the soil classifications and solely looking at shear vanes, SPTs, lab results and CPT results to be substantially more useful.

 

[EireChch]

When you think of silty sand is your mental model a cohesive soil or a non cohesive soil

To me if it has <35% silt and >65% sand, it is a silty SAND. This to me is a cohesionless soil which has no undrained shear strength and immediate settlement only needs to be considered.

I think a better question is:

When you think of sandy SILT is your mental model a cohesive soil or a non cohesive soil

We argued this in the office recently, I will wait for responses before giving my answer.

 

[MTNClimber]

I would say low cohesive soil. Most of my experience has been with non-plastic to low plasticity silts and marine deposited organic silts, so cohesion isn't very high but still present. To add, most charts defining consistency or relative density from blow counts put silts in the plastic soils/consistency side. Not that carries much weight.

 

[EireChch]

Which question are you responding to MTN, mine re sandy SILT or geotech1 re silty SAND?

If my question (sandy SILT), would you consider consolidation settlement and assign an undrained shear strength?

 

[MTNClimber]

I poorly responded to yours, EireChch. Show me on the plasticity chart where the sandy silt plots and I'll give you a better response.

 

[geotechguy1]

What 'elastic silt' particles

I know if you add organics, that will cause soil to plot below the a-line. Likewise, there are certain clay minerals that cause plastic fine grained soils to plot below the A-line. Calling it silt seems bizarre, since silt has nothing to do with it.

 

[MTNClimber]

ML and MH?

 

[geotechguy1]

It plots below the A-line because of the type of clay particle that's in it, it doesn't have anything to do with silt lol that's the point I'm making.

Soils below the A-line: Have certain types of clay minerals, and are called 'silt' because they plot below the A-line
Soils above the A-line: have certain types of clay minerals that plot above the A-line, are called 'clay'

In many practical cases the soil above and below the A-line is majority 'silt' on a size classification basis but it's behaviour is dominated by the percentage and mineralogy of the clay fraction