Consolidation 2

[geoeng22]

Greetings,

I am working to calculate consolidation settlement for a proposed embankment and have one consolidation test. The test was performed on a sample from a depth of 4-6 ft. Soil unit weight is 0.120 ksf. I have interpreted a preconsolidation pressure based on the curve and had a few questions.

1. Assuming homogenous soil, is the interpreted preconsolidation pressure from my test only valid for the soil at that depth of 4-6 ft? Would the preconsolidation pressure be different for a sample at 10 ft or is that preconsolidaiton pressure the same for the homogeneous soil at 5 ft 10 ft 20 ft.....etc?

2. If the the preconsolidation varies, would preconsolidation pressure vary with depth (increase linearly). i.e. assume preconsolidation pressure at a depth of 5 ft is 3 ksf. With no additional test data, would it be safe to assume preconsolidation pressure at 10 feet be 3ksf + (0.120ksf*5ft) = 3.6ksf?

3. Can overconsolidated soil overlay normally consolidated soil? If a soil near the surface is overconsolidated and has experienced a higher stress in the past than it is currently experiencing in situ, I would think the soil below that would have experienced that same past pressure or greater in its past. Therefore, if a soil sample is overconsolidated, everything below that sample is also overconsolidated.

I hope those questions make sense. Any help understanding this is greatly appreciated! Thanks

geoeng22

 

[EireChch]

1- technically pre consolidation pressure (sig'p) can change but only when there has been deposited material , then removal and then deposited again and due to other reasons. This is not very common, I would say 99% of time your pre consolidation pressure with be applicable to your full CLAY layer. As such, sig'p is valid for your hole layer, it does not change, but your OCR does change (reduces) with depth (due to sigma'v increasing)

2-no, this is fundamentally wrong sig'p is a fixed pressure.

https://www.youtube.com/watch?v=tJSJhR_qPrE

checkout prof kitch online lectures

3- yes, if a soil has felt sig'p then everything below it has to have felt it too.

 

[geoeng22]

Hello EireChch. Thank you for your response. Also, that was an excellent video you shared. I think I will check out some more of his videos in the future.

Regarding my questions, I don't think I phrased them well. But I think in the video at around the 10 minute mark, some of my questions get answered.

That is, the preconsolidation pressure applies to a specific sample at the depth it was taken. As you said, sig'p is fixed, but I think only at that depth. If you take a second sample in the same stratum, but say 25 ft deeper, that soil sample will have a different preconsolidation pressure (higher) then the upper sample. For example, samples at 5 feet and at 30 ft feet would have two different preconsolidation pressures, and I think the sample at 30 feet would be higher. That's where in 2) I was asking if preconsolidation pressure would increase linearly with depth by about the average unit weight of the soil.

 

[EireChch]

Hi Geoeng22 , yes his videos are excellent.

Sorry I have made a confusing mistake in my answer above. I was thinking of over consolidation margin sig'm.

The reason for over consolidation generally comes from a layer of soil that is deposited and then removed. For example if it was 4m of material with unit weight of 20kn/m3, then your sig'm is fixed at 80kPa.

This does not change, the sample at 25 ft should see 80kPa plus, its existing effective stress, sig'v. This should give the same OCR, as you were getting at. please excuse the brain fart above you, i will edit, you were correct, long week!

Your pre consolidation pressure sig'p, should be sig'm + sig'v. So it will increase, not linearly but almost.

 

[geoeng22]

I totally understand, EireChch, no problem.

The material supporting the proposed embankment is terraced alluvium. It is between a meandering river. So based on my consolidation test, I have interpreted the data as showing the soil to be overconsolidated. A phenomenon like you mentioned likely occurred where material has washed away. I only have one sample tested from depth 4-6 ft. That's why I was asking about the preconsolidation pressure varying with depth. A sample at 5 ft will have a lesser sig'p than a sample at 15 ft. And the sample at 15 ft will have a lesser sig'p than a sample at 25ft.

Back to my question 3) is it possible for a overconsolidated soil to be underlain by a normally consolidated soil? I wouldn't think so. If my sample at 5 feet is overconsolidated, then I think everything below it would also be overconsolidated whether at 20 ft 50 ft or 150 ft.