6-Min Solutions

[Daisy]

In the solution, for change in vertical pressure, where does (15-3)*Gamma water come from? Thanks.

 

[Civ_Guy]

I believe this is incorrect. When calculating the primary consolidation, you are using the 'effective pressure'. As per the CERM; 10th ED., "Only the effective pressure causes consolidation." Therefore, the addition of the pore water pressure is incorrect.

I had the same issue. My answer to the problem was 25" (I did it quick, it may be off.)

 

[IlPadrino]

Not incorrect... and it's true that effective pressure (effective stress) is what causes the consolidation.

In this problem, there are two things happening which increase the effective pressure: addition of the embankment (very large, so we don't have to worry about influence diagrams) and lowering of the water table.

The calculation of the initial effective stress (aka preconsolidation stress) subtracts out the pore pressure... essentially, the clay is buoyed (is that a word? I mean sort of float) by the water table.

When calculating the change in stress (Δpv') you have to add the pressure of the fill and increase in effective stress because the water table is lowered. The increase in stress due to lowering of the water table is adding back in 12ft of the pore pressure taken out in the initial effective stress calculation (the clay is less bouyed).

If you don't add back in the (12ft x 62.4pcf) you'll get too small an answer (20in)...

 

[Civ_Guy]

While I will defer to IlPadrino (thank you for your comments) in Coduto's book (Geotechnincal Enginnering - Principles and Practices) he states that ". . .consolidation can only occur as some of the water is squeezed out of the voids." (page 372, 2nd paragraph). Does this not infer that the water (and the water pressure) cannot be included in the calculations?

Also, my answer (25") includes the different in the the dry effective soil pressure effective and the saturated soil pressure (92.89 - 58.36) .

 

[IlPadrino]

While I will defer to IlPadrino (thank you for your comments) in Coduto's book (Geotechnincal Enginnering - Principles and Practices) he states that ". . .consolidation can only occur as some of the water is squeezed out of the voids." (page 372, 2nd paragraph). Does this not infer that the water (and the water pressure) cannot be included in the calculations?
Also, my answer (25") includes the different in the the dry effective soil pressure effective and the saturated soil pressure (92.89 - 58.36) .

I'm not a geotech guy by trade... so I wouldn't defer to me! Still, it's the discussion that's valuable so this is all good.

For me, the crux of this problem is to understand that effective vertical stress causes the consolidation... and pore pressure reduces the effective vertical stress. When the pore pressure is reduced (i.e. the water table is lowered), the effective vertical stress INCREASES.

I think we're sort of saying the same thing... just that you're arriving at a different conclusion. Pore pressure is "included" in the calculations because it reduces the effective vertical stress of the load. However, the weight of the water (yeah, another way of saying pore pressure!) is not included in the vertical stress as a load which, in and of itself, adds to the consolidation.

But in a different way, for lateral earth pressure, the weight of the water *IS* included in the total horizontal pressure (it's a load that acts the same in all directions). And, still, to get the effective vertical stress, you have to reduce the weight of the soil by the pore pressure.

Not sure what you're getting at with the dry vs. saturated density... you need to use the saturated density for calculating effective vertical stress.

There must be some geotech gurus here that can weigh in on this "pressing" question!

 

[Civ_Guy]

I agree, we are discussing it from different perspectives. I had stated the different stresses to show I had a different number than your 20" and hopefully not confuse the person who had asked the initial question.

I too hope someone another GEO guru can pipe in. I know this discussion is helping me.

Thanks IlPadrino.

 

[Daisy]

Sorry to pipe in so late in the discussion, I'm just now getting back to the problem after busy week at work, sick kid, yada yada.

I understand that effective (ie taking out water pressure) is what causes consolidation, but I'm not sure I'm following why you add it back in. Ok, let's see if I can 'talk' this out. To find the inital effective pressure, you take Gamma-total minus Gamma-water, times the height of the midpoint of the clay layer. Check...I get this.

Next step, find the change in vertical pressure by adding the very large fill. Gamma-total (fill) times the height of the fill = change in vertical pressure. Check...I get this.

Lowering the water table to 12'. I'm still not getting this part of it. I know that by lowering the water table, as the clay is consolidating, the effective stress is changing. I guess I just need to accept, if, on the exam there is a problem like this, that you have to add back in the pressure from the water...unless someone can further try to help me understand this.

Thanks for your help so far guys, I appreciate it!

 

[IlPadrino]

Lowering the water table to 12'. I'm still not getting this part of it. I know that by lowering the water table, as the clay is consolidating, the effective stress is changing. I guess I just need to accept, if, on the exam there is a problem like this, that you have to add back in the pressure from the water...unless someone can further try to help me understand this.

Ask yourself this: all other things being equal except for a changing water table, which case would have a greater vertical stress... the initial case with a higher water table or the final case with a lower water table?

The answer is the final case... you can calculate the buoyant unit weight of soil given the water table using the general equation γb = γ - γw where γb is the buoyant density, γ is the total density of the soil and γw is the density of water. Do you see that without the buoyant effect of the water table, the vertical stress is more?

I don't think this issue is "as the clay is consolidating"...

 

[Daisy]

YES! I get it, I get it!! I understand...I think you said it in a previous post, that you took out the water pressure initially, so you have to add it back in!! All it took was a week to get this problem...should be no problem now on the exam! (Wish I had a week to take the exam!) Thank you so much!!

 

[IlPadrino]

YES! I get it, I get it!! I understand...I think you said it in a previous post, that you took out the water pressure initially, so you have to add it back in!! All it took was a week to get this problem...should be no problem now on the exam! (Wish I had a week to take the exam!) Thank you so much!!

You're welcome... working hard now to understand what's happening in the problem will pay big dividends when exam time comes!

Civ_Guy: How about you? Does it make sense yet?