Preconsolidation pressure for clay 2

[Geosubhtech]

Greetings to All,

i am working on some project, for retaining wall i need to calculate settlement. So, for settlement calculation i need to check is it pre-consolidated clay or over-consolidated clay. so hence pre-consolidation pressure is required to check OCR. so the bore log i have there is no Consolidation test.
I checked from the book "Cone Penetration Testing in Geotechnical Practice, BY-T. Lunne, J.J.M. Powell, P.K. Robertson" (graph which i used is attached).
i am getting OCR of 15-18 at shallow depth having SPT values less than 19.
So, could you please help me to get any reference for reliable co-relation, to arrive at the pre-consolidation pressure.

 

[oldestguy]

If you run Atterberg Limit tests, there are relationships between those results and pre-consolidation, in general. Currently I can't find this in my references, but there may be others here than can. At least that would "put you in the ball park". These tests, along with moistue content should be run on boring samples any way to give a feel for what is there.

 

[LRJ]

The method you have looks fairly like the SHANSEP approach, albeit different curves for different plasticity index values have been plotted (i.e. (s_u/σ'_v0)_NC has been related to plasticity index in the SHANSEP equation). I think your biggest problem when using the data you have is in trying to correlate SPT blows to undrained shear strength. SPTs just aren't that reliable. How have you determined s_u?

Also, while values of OCR do tend to be quite large near to the surface, they should be limited by the passive earth pressure. Zhang et al. (2017), in a paper titled 'Effects of scour-hole dimensions and soil stress history on the behaviour of laterally loaded piles in soft clay under scour conditions', outlined one method to predict the limiting OCR: essentially they substituted the passive earth pressure coefficient (K_p = (1 + sinΦ')/(1 - sinΦ')) for the at-rest earth pressure coefficient (K₀) in the well-known equation proposed by Mayne and Kulhawy (1982) which relates K₀ and OCR (K₀ = (1 - sinΦ').OCR^sinΦ'). After substituting and rearranging, the following equation is derived:

OCR_limit = [(1 + sinΦ')/(1 - sinΦ')²]^1/sinΦ'

For example, if your Φ' angle for clay was 20° then your OCR_limit would be approximately 27. The values you state are lower than this, which is reassuring.

 

[oldestguy]

OK, I found the chart in the US Navy Design Manual for soil mechanics. With the lab tests you ought to be able to see where you stand.

See the file attached.

 

[Geosubhtech]

Thank you LRJ,
The S_u we found by conducting UU - Triaxial test. So phi - drained is not available for my case.

by the way thanks for introducing SHANSEP MODEL. it's interesting i am going through it.

 

[Geosubhtech]

Thank you OLDESTGUY,

but finding preconsolidataon pressure and OCR are quite interdependent. so i am in search for some method like co-relating overburden pressure and Su and atterberg limits.(which i have in bore log).

thanks for the help.

 

[LRJ]

If s_u is from UU then that should be alright - certainly better than a correlation with SPT blow counts. The reason for the high OCR is likely due to the shallow depth (i.e. low σ'_v0). What depth are you calculating these values at?

I think it would be good to check against the SHANSEP approach I mentioned, providing that the SHANSEP approach is appropriate for your soil (the appropriateness of SHANSEP depends on how the strata was deposited - SHANSEP isn't appropriate for glacial till, for example).

 

[Geosubhtech]

As it is a continuous retaining wall, 8.39m width and considered as strip footing, the influence zone is going upto 4.19m. the values i am considering about is a Undisturbed soil sample collected at 2.5 m depth. The UU test also conducted at that sample. Next UDS collected at 5.5m depth.

 

[aeoliantexan]

A mentor of mine taught me that the preconsolidation pressure is approximately 2 times the unconfined compression strength. That is probably a little conservative. As I would seldom choose an allowable bearing pressure greater than the unconfined compressive strength, That's close enough.

 

[Geosubhtech]

Actually i found some standard method for finding preconsolidation pressure.
"Nagaraj, T. and Srinivasa Murthy, B., "Prediction of the Preconsolidation Pressure and Recompression Index of Soils," Geotechnical Testing Journal, Vol. 8, No. 4, 1985, pp. 199-202,

https://doi.org/10.1520/GTJ10538J.

ISSN 0149-6115"

 

[fattdad]

Normal consolidation means that there is no stress history other than the current stress. This often yields natural water contents at or near the liquid limit. The liquidity index tells that story, hence correlation to Ip.

When soils are normally consolidated, we expect the undrained shear strength to increase by a factor of 0.2 to 0.3Su/P, where Su is the undrained shear strength and P is the current effective stress.

If you are in the upper 10 ft and the soil is unsaturated, you have some measure of preconsolidation. You have to! Heck, just the loss of water from complete saturation (i.e., the depositional environment) has caused capillary tension, increase in effective stress and some measure of preconsolidation!

If the water table is at the surface and the clay is saturated, that's a different story. Now we can gauge how the undrained shear strength SHOULD exist if the soils are normally consolidated. So, at the depth of 10 ft (assuming buoyant unit weight of 50 pcf), the P would be 500 psf and the returning Su would be 100 psf (for Su/P of 0.2). Wow! That's low!

You have N=15 or thereabouts. An N-value of 9-15 correlates to an unconfined compressive strength of 1 to 2 tsf, which would correlate to an undrained shear strength of 1 to 2 ksf (1,000 to 2,000 psf). Any chance that you will exceed this pressure? You obviously have some measure of preconsolidation. At this point; however, we have no idea about your water contents, soil classification or the proposed loading of the engineering improvements.

Good luck,

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

Maybe this is a bit too "simple" - but for a normally consolidated clayey material, Su/sigma' is approximately 0.23 (range of 0.2 to 0.3 max). If you know the Su, value, you can determine the estimated maximum sigma'. This value divided by the current sigma'(current) will give you an approximate guess as to the OCR.

Say for example that, as fattdad points out - your undrained shear strength is in the order of 1500 psf (75 kPa), then sigma' is about 300+. Is your loading higher than this? If not, then settlements should be small.