Performance of Silt

[UP830]

General question, how do you know if a Silt will perform as a cohesive or non cohesive soil?

Is it dependent on Plasticity? Non plastic silt, with PI less than 10, will perform as a cohesive soil?

I feel that the topic is not well discussed in textbooks. If some one has a good reference please shout.

 

[oldestguy]

Depends on content of clay and clay types. Same question can go for sand, say with 10% montmorillonite. Probably too general a term to get exited about for actual jobs.

 

[Ron]

Performance may include both cohesion (which for silts is generally low) and adhesion (mostly from "suction" in wet silts). It depends on what your performance expectations are. If you are expecting cohesion, you probably won't get much.

 

[UP830]

Thank you oldestguy and Ron, my question is related to settlement calculations primarily at this stage.

I am dealing with a Silt with sand which comprises the following:

6% - Gravel
18% - Sand
76% - Silt

Moisture content 32%
LL - 41%
PL - 27%
PI - 14%

On the borehole the material is logged as a silt, however I have just noticed when responding to your comments, when you plot the Atterberg data it is a CL - Lean Clay.

 

[Mad Mike]

It is rare to get that much silt in a sample with 0% clay- check that the grading analyses were run down to the clay particle size (2um)- it may be that the silt and clay have been lumped together in the -75um fraction.

If those results are accurate and there is no significant clay present, I would expect it to behave as a granular material for the purpose of settlement analyses- rapid settlement, mostly upon first loading.

Lastly, make sure the silt is not organic- this would reflect in a very low dry density and the material would usually be black in colour. There are some special considerations for settlement in that black organic silt.

All the best,
Mike

 

[UP830]

I have spoken with the lab regarding the material plotting above the A line and they have since changed it to a Lean Clay with Sand. It was an error apparently. Sample at 8m was the topic. From the image below, there is less than 3% clay based on 2micron sieve. However i understand that ASTM is based only on atterberg data when it comes to more than 50% of the material passing the 200 sieve. On this basis, as the material is a clay as PI>7, I should assess consolidation.

Assuming the material plotted below the A line and PI <7, it would be a Silt, it is at this stage I get stuck on whether it will undergo consolidation or not. I get your argument on the amount of Clay fraction and it is logical to me. Is there any reference or guidance you could point me to to learn more? I take it your assessment is based on engineering judgement?

A PI of 7 seems to be the deciding factor for if it is clay or silt, however this seems to simple to decide whether consolidation needs to be assessed.

Thanks again for the replies.

 

[maxim22]

My gut feeling is the limits don't correlate well with particle distribution, PI is similar for all samples tested. I would expect sample from 8m to be less plastic. Need more information to check something else is going on like the log and the geology type. Is there only one error? Then how relevant is 8 m sample to loading condition? Where is the water level?

 

[fattdad]

it's your project!

Run a consolidation test and determine Cv, Cc, Cr, Po, etc. Then you'll have data to know how to do the calculations, which will guide your decisions.

I'd say the same thing if it was an actual silt too.

f-d

ípapß gordo ainÆt no madre flaca!