Undrained and Drained Shear strength for the same clay layer 1

[CEMAB]

Hello all,

I have a question (and maybe not the smartest one in the world) that got me confused.

Let’s say there is a normally consolidated clay layer overlain by sand and gravel deposit. There is a dam constructed on top of the sand and gravel deposit. Dam weight introduces excessive pore water pressure into the clay layer. Theoretically, for let’s say slope stability purposes. one should use undrained shear strength for the clay layer.

Where I got a bit confused is the following. Bulbs of pressure coming from the dam are somehow limited to a portion of substratum below the structure. So basically the same clay layer experiences “excessive pore pressure” in vicinity of the structure, but not away from it. Will it make sense to analyze this problem using both drained and undrained shear strength of the clay? Just to clarify this more, please see the attached sketch.

Thank you

 

[ntschwanz]

You probably want to do both drained and undrained analyses but not for the reason you're thinking. Think of pore-water pressure generation in two ways; (1) from volume changes such as loading from the dam trying to consolidate the underlying clay as evidenced by the bulbs of pressue, and (2) from shear or shape change, such as shear stresses that develop due to the loading. In the second case, the clay is normally consolidated so the likely tendancy is for the clay to contract during shear under drained conditions, so you would expect positive pore-water pressures (PP) to develop during shear due to the embankment loading. Now you have positive PP's develop due to overall stress increase and positive PP's that develop due to shear, so your analysis would represent the clay using undrained shear strengths throughout the clay foundation. You will also want to complete an effective stress analysis using drained strengths and seepage forces. Thinking of this in a critical state soil mechanics frameworks helps. Of course there are other load cases to consider as well.

 

[CEMAB]

Thanks a lot ntschwanz.

How different the situation could be if we had overconsolidated clay instead of normally consolidated?

If I remember correctly, overconsolidated clays tend to lose their strength over time and usually drained conditions are more critical to analyze. What if the clay under the dam becomes normally consolidated with undrained conditions being more critical?

How can I combine two portions of the clay layer (overconsolidated away from the dam with drained conditions being more critical and normally consolidated clay underneath the dam with undrained conditions being more critical) in one analysis.

 

[ntschwanz]

Keep in mind that you don't need to reach failure to develop shear stress, the shear stresses throughout the foundation change as the embankment is constructed. The shear stress in the clay, beyond the pressure bulbs shown in your example, is increasing and as the shear stress increases one would expect the PP to increase (for your normally consolidated clay). The embankment loading is attempting to cause both volume changes and shear changes even beyond the 0.1 pressure bulb. Hope that helps.

 

[CEMAB]

Thank you once again.

I would also appreciate if you could address my question on overconsolidated clay.

How different the situation could be if we had overconsolidated clay instead of normally consolidated?

If I remember correctly, overconsolidated clays tend to lose their strength over time and usually drained conditions are more critical to analyze. What if the clay under the dam becomes normally consolidated with undrained conditions being more critical?

How could two portions of the clay layer (overconsolidated away from the dam with drained conditions being more critical and normally consolidated clay underneath the dam with undrained conditions being more critical) combined together in one analysis.

Thank you once again

 

[ntschwanz]

Sorry, misread your question. I think you're assuming a scenario where the clay is heavily over consolidated prior to placing the embankment but the embankment loading is such that the post construction effective stress exceeds the preconsolidation pressure beneath the embankment but doesn't at some distance away (clay is normally consolidated beneath the emb but remains heavily OC'd away). Now you have compressive stresses tending to positive PP generation and shear stresses that cause positive PP beneath the embankment and negative PP at distance). The undrained and drained analyses still apply but the question of whether the rate of dissipation of the negative PP in the heavily OC'd clay is faster than the rate of dissipation of positive PP in the normally to lightly OC'd clay can be considered. If you believe that's that case then you can make decisions on appropriate shear strengths to use throughout the clay or move to an advanced analysis.

 

[CEMAB]

ntschwanz, thank you once again for your input. So basically it all boils down to Pore Water Pressure (PWP), PWP dissipation (whether water makes its way in or out of soil) and dissipation rate. If we could calculate the exact PWP at any location at any point of time, there wouldn't be a need for undrained shear strength parameters. Correct?

Thanks once again,
I really appreciate your time and effort .

 

[ntschwanz]

Correct, with the emphasis on "accurate". We may have the ability to calculate PWP to use in an effective stress based model, but not with the accuracy or confidence generally needed throughout the entire range of possible loading. So in your embankment dam example you look at the end-of-construction (short term undrained) load case, the steady-state-seepage (long term drained) load case along with others.

 

[CEMAB]

If you would have to review a report where for the situation described above the clay layer was divided (vertically) into two layers: one below the embankment (representing NC clay) with undrained shear parameters and one at the toe and away (representing OC clay) with drained shear parameters, will this analysis look reasonable to you?

Can positive PP generation due to potential dilatant behaviour of the OC clay be good rationale for use of drained (more conservative in this case) rather than undrained strength for OC clay?

 

[moe333]

I think you need to analyze short term and long term conditions separately. Short term would use undrained strengths for both NC and OC soil, and long term would use drained strengths for NC and OC soil.

 

[marc2016]

Use undrained strength for the clay and apply your pore pressures within your material types(if your using limit equilibrium analyses) ending with the phreatic surface at the dam water-level. You can build separate models for pressure depreciation effects, draw-down effects(time dep) if you think it's worth considering.

 

[fattdad]

The problem statement means you need to consider the state of pore pressure in the effective stress analyses. It's not a one or two modes of analyses concern. There is time involved and with the passage of time the pore pressures change, which in turn affects the effective stress condition. Undrained analysis assumes gravity turn on, no dissipation of pore pressures and the state of undrained strength at that singular moment. Your problem statement may need to consider staged construction, the increase in both undrained and effective strength with the passage of time. It's just not as simple as one or the other or both. You have to return to first principals.

f-d

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