Over-consolidation in soft clay soils 1

[ONENGINEER]

The SPT values in muddy clay soils are about 1-3 or sometimes WOH. The clay soils in the area is known to be over-consolidated. A pre-consolidation pressure of 4 tsf can be constructed on the oedometer test curves.

Question: How could a very soft muddy clay soil have been under a preconsolidation pressure of 4 tsf. Conceptually the preconsolidation pressure should be zero for such a soft clay.

Thanks for shedding some lights on this.

 

[geotechengineer1]

There are two possible pathways to answer this question:

One is that the preconsolidation-overconsolidation concept is just a manipulation of data using log scales and the idea that engineers can elucidate a 'preconsolidation pressure', at least in that the idea that you can plot some oedometer data on a log graph and somehow divine the maximum past consolidation pressure, is false.

The other way to look at is that consolidation is caused by increase effective stress. You can change the effective stress by increasing the load - adding a large stockpile of fill, a glacier, or a foundation. Or in a long enough time scale, if erosion or a landslide peels away a meter of soil from above your soft clay deposit. But you can also increase the effective stress by lowering the groundwater table - it's readily possible that the groundwater table might have fluctuated quite substantially over the geological life of the deposit. Likewise, seepage forces cause a change in effective stress. If the seepage is upward, as is the case in soil traveling along a flow path under a shoring seepage, the effective stress could decrease to zero. If it's downward, as in the case of seepage forces caused by say, constant seepage of water into the ground due to rainfall or large rain events, there will be an increase in the effective stress. Another possibility is matric suction - we assume, for convenience sake, that the soil is either saturated or dry, but in reality the majority of soil involved in geotechnical projects is partially saturated and is currently or has in the past been exposed to matric suction, which could generate quite substantial increases in effective stress. Finally, tree roots and plant vegetation also cause suction forces which can change the effective stress.

So it's possible that:

[ul]
[li]The soil in the past was exposed to a higher effective stress due to higher overburden pressure, and the soil or glacier or whatever was causing this higher load has been eroded away or melted[/li]
[li]Effective stresses have been higher in the past due to fluctuations in the groundwater table[/li]
[li]Effective stresses have been higher in the past due to seepage forces[/li]
[li]Effective stresses have been higher in the past due to matric suction / partially saturated soil mechanics[/li]
[li]Effective stresses have been higher in the past due to tree roots / plant vegetation[/li]
[/ul]

 

[ONENGINEER]

GeotechEngineer1. Thank you for explaining the reasons for a previous higher effective stress conditions. As the soil stands now, is incorporating a preconsilidation pressure applicable for settlement calculation? Or should it be assumed that the soft clay (SPT 0-3) is normally consolidated, disregarding the preconsolidation pressure obtained from oedometer test. The problem is using the Cc or Cr for consolidation calculation.

 

[geotechengineer1]

Well, you need to get advice from a senior engineer at your firm for a calculation like this.

Personally I do not like using Cc and Cr.

Rather, I would use the results of the oedometer to determine a constrained modulus that is appropriate for the stress range of the current loading + the applied loading. Since you say it is a soft clay you should also think about secondary consolidation - do you know the organic content or anything about it's geological history? This is very important information. The past effective stress history (and the increases in stiffness as a result) are accounted for in the oedometer test.

 

[ONENGINEER]

Actually the senior engineer of the project advised that the area is known to have a preconsolidation pressure of 2.5 tsf and signed off himself.

The question here is for exploring further for future cases. I am not much familiar with the clays in the US East Coast area but cannot feel that a clay that has been under a 2.5 tsf (preconsolidation) pressure, would show SPT readings of WOH or would appear as a muddy clay. Unless as said (by geotechengineer1) the preconsolidation factor has been removed long time ago and in effect we would be presently dealing with a normally consolidated clay.

Also did you mean by constraint modulus for settlement calculation as the same method that uses mv values? Thank you again.

 

[Mad Mike]

ONENGINEER,

My only comment is that you may be "cherry-picking" the WOH reference.

You've stated initially blow counts 1-3, sometimes WOH- now keep in mind that even with WOH, the difference between a slow drop and a rapid drop of hammer may be considerable in terms of undrained shear strength/consolidation potential.

Let's just speculate that your current 1-3 N blow count material was once WOH material...that pre-consolidation pressure signed off by your senior Engineer doesn't look so bad then!!!

I say this because I've had a fair bit of experience with testing very soft clays that have been surcharged for decades and sometimes, despite knowing the consolidation history, the field data don't fully support what I know to be accurate. It's tricky knowing where to place your faith- if I had a site where I lacked extensive experience with the materials, I would always calculate settlement using the most severe data. Perhaps I'd check it against a plausible range and make minor adjustments, but don't get too brave!!!

All the best,
Mike

 

[fattdad]

jam a cone in the ground.
it's a sensitive formation and N-values don't tell the whole story.
your boss is ignoring first principles.

Hard to tell. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoEnvGuy]

To echo Fattdad's point of first principles. You are investigating a deposit with soft clays, turn a field vane.

 

[geotechguy1]

What you need to do to solve an engineering problem is know: What is the stiffness of the material (for soil, and both undrained and drained loading), and what is the strength of the soil (in shear). You then need to know under what stress range the stiffness value applies (since soil is non-linear) and ensure that it is applicable to your problem.

Pre-consolidation pressure is a construct created by engineers who wish to linearize a non-linear problem by plotting it on a log scale (and even then, if you look at oedometer results on a log scale you should see that it is still not linear).

 

[ONENGINEER]

Thank you for all valuable feedback, which were educative to me.

In this specific project, there were a few historical boreholes in the project area drilled for other reasons in which SPT (every 3-5 ft) were carried out to characterize soil for deep footings at the time. Now they want to use the same borehole logs to find soil parameters for a a shallow foundation design. Using CPT or a vane was not on their agenda with their budgeting. (A practice that unfortunately may not necessarily follow standards).

I have not done oedometer tests for the last 20+ years. However, I would like to learn more if it would theoretically be possible for an over-consolidated clay (with preconsolidation pressure of 2.5-4 tsf) to appear as a soft muddy clay of SPT values of <=3 even if assumed over consolidated, after the elimination of the factors that at one time increased the effective stresses (preconsolidation). My feeling is that a muddy soft clay should be dealt as NC not OC but am not sure if this is supported by laboratory investigation. Thanks again.

 

[geotechguy1]

You need to do more investigations and testing. If it really is soft clay and your client is cheap, grab a hand auger and a shear vane kit and get to work. Your client might think you're the first geotechnical engineer to ever do anything!

 

[GeoEnvGuy]

I find it hard to believe that you do not have budget to perform field vanes. You take the split spoon off the rods and you put on a field vane, but instead of hitting it with a hammer you put on a 1/2 inch drive torque wrench which you can buy at a hardware store. Granted the astm says to do something a little more controlled in rotation speed and ensuring the rods go straight.